25th anniversary article: organic electronics marries photochromism: generation of multifunctional interfaces, materials, and devices.

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25th Anniversary Article: Organic Electronics Marries Photochromism: Generation of Multifunctional Interfaces, Materials, and Devices Emanuele Orgiu and Paolo Samorì* number of possible candidate molecules/ processes which are probably mature enough to meet the initially envisaged applications.[97–112] However, the initial desire for scaling down the device size is somehow getting to an end for the silicon-based electronics because of limitations coming from the photolithographic steps involved in the production process of the devices. In this regard, organic and polymer electronics, i.e., the electronics based on organic and polymeric semiconductors, cannot complement its inorganic counterpart and the relentless need of miniaturization unless new challenging avenues are pursued. A novel approach can be envisaged where molecular functions are “embedded” or added into organic semiconductors via a controlled combination with an additional molecular component possessing supplementary functions. This is the case of photochromic molecules[113–118] that are small organic molecules able of undergoing reversible isomerization upon light irradiation at definite wavelengths between (at least) two (meta)stable states featuring markedly different properties at the molecule level. In these molecular switches the specific isomeric state is set when the molecule is exposed to a light stimulus with a proper wavelength. The most common families of photochromic molecules, depicted in Figure 1, are azobenzenes,[119–128] diarylethenes[129–140] and spiropyrans/-oxazines.[141,142] In addition, the photochromic family encompasses also fulgides,[143] imidazole dimers[144] and stilbenes that are not considered in this manuscript since they have not yet been exploited in combination with organic semiconductors. Photochromic molecules have been demonstrated in the last two decades to represent key components for both optical data storage[132,145–149] and optical switching.[150,151] The optical readout relies on the possibility of sensing the differences in fluorescence[152–154] and absorption, even in the IR region,[155] of the two isomeric states. However, optical readout is a normally destructive process because the molecule's state is altered when exposed with light. A complementary approach is represented by the measurement of the electrical properties in molecular junctions where the isomerization of the photochromic molecule gives rise to a binary conductance.[156–161] However, it is fair to point out that

Organic semiconductors have garnered significant interest as key components for flexible, low-cost, and large-area electronics. Hitherto, both materials and processing thereof seems to head towards a mature technology which shall ultimately meet expectations and efforts built up over the past years. However, by its own organic electronics cannot compete or complement the silicon-based electronics in integrating multiple functions in a small area unless novel solutions are brought into play. Photochromic molecules are small organic molecules able to undergo reversible photochemical isomerization between (at least) two (meta)stable states which exhibit markedly different properties. They can be embedded as additional component in organic-based materials ready to be exploited in devices such as OLEDs, OFETs, and OLETs. The structurally controlled incorporation of photochromic molecules can be done at various interfaces of a device, including the electrode/semiconductor or dielectric/semiconductor interface, or even as a binary mixture in the active layer, in order to impart a light responsive nature to the device. This can be accomplished by modulating via a light stimulus fundamental physico-chemical properties such as charge injection and transport in the device.

1. Introduction Conventional silicon’s greatest strength is its ability to scale device dimensions down and to generate low-cost functions regardless of eventual relatively high costs/area. Conversely, ‘molecular solids’ relying on weak interactions such as organic semiconductors (OS) have always been thought of complementing this need for low-cost production as well as largearea applications by virtue of their characteristic chemical versatility and ease of processability. For this reason, OS hold a huge potential for the integration in everyday flexible electronics[1–22] as they exhibit a number of properties such as light emission,[23–45] charge transport,[45–74] and photovoltaic response.[75–96] Hitherto, synthetic pathways and processing of organic semiconductors have hugely improved and generated a Dr. E. Orgiu, Prof. P. Samorì Nanochemistry Laboratory, ISIS & icFRC Université de Strasbourg & CNRS 8 allée Gaspard Monge 67000 Strasbourg, France E-mail: [email protected]

DOI: 10.1002/adma.201304695

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fabrication of molecular junctions requires high precision and resolution which necessitates time-consuming processes, bulky equipment, highly controlled processing environments as well as advanced and costly deposition/fabrication techniques. Furthermore, neat films of photochromic molecules could be certainly exploited for large-area electronics via cheap production solutions if they did not exhibit moderate switching ratios[162] or low charge carrier mobilities.[163] These evidences seem to suggest that the huge potential held by photochromic molecules can be better harnessed when they are combined with organic semiconductors which bring in (i) the ease of processability, (ii) the low-cost manufacturing, (iii) a variety of different device geometries and interfaces that can be used all at the same time in order to generate multifunctional devices, and (iv) a proper semiconductor behavior of the materials which is needed in all of the three-terminals device configurations. This article reviews recent approaches that have been pursued aiming at combining the exceptional (opto)electronic properties of organic and polymeric semiconductors with the photo-responsive nature of photochromic molecules, in an effort to fabricate bi-/multi-functional devices.

2. Use of Photochromic Molecules As a Tool for Tuning The Charge Transport in Organic Semiconductors In spite of the still relatively low but increasingly higher reported values of field-effect mobilities,[11,20] organic semiconductors hold huge potential for the generation of a new electronic era owing to an ideally infinite number of possibilities to tune their optical, mechanical, and transport properties.[20] This number is as large as the solutions that synthetic chemistry can offer to the scientific and industrial community. Different side groups have been synthetically attached to organic semiconductors in order to modulate their fundamental properties such as solubility, assembly capacity, and optical properties. Likewise one or more photoresponsive bi-stable photochromic groups have been covalently linked to organic semiconductors in order to induce a bi-functional nature, by adding a selective response to distinct wavelengths. This concept resulted in a number of examples of dyads, multiads or polymers incorporating photochromic units, as presented in section 2.1. However, the above-mentioned approach causes complexity of the macromolecule synthesis implementing several additional synthetic steps, thus it is time-consuming and consequently not really suitable for process up-scaling. In addition, the semiconducting properties of the organic semiconductor as well as the isomerization of the photochromic molecule included in the macromolecule could be altered by the two components influencing each other. In addition, driving the self-assembly of these sophisticated (macro)molecules towards highly ordered supramolecular architectures, which is crucial for efficient charge transport in organic semiconductors, may be hindered by their multiple functionalities. Most likely because of these reasons, to date no examples of combining photochromic molecules together with polymeric semiconductors

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Emanuele Orgiu is Assistant Professor at the Faculty of Chemistry as well as research scientist at Institut de Science et d'Ingénierie Supramoléculaires (ISIS) at the University of Strasbourg. He holds a Ph.D. in physics of organic devices, earned in 2008 from the University of Cagliari (Italy) working under the supervision of Prof. A. Bonfiglio. He was the recipient of a J. W. Fulbright Fellowship from the University of Santa Clara, Santa Clara, CA, in 2007, for fostering entrepreneurship in young researchers. In 2013, he was awarded the Technology Review Award – MIT Under35 for France. He is the coordinator of the European project “UPGRADE” (bottom-UP blueprinting GRAphene baseD Electronics). He works in the fields of materials science and physics, with a focus on interfaces and transport in both graphene and organic semiconductors and fabrication and characterization of nanodevices.

Paolo Samorì is Distinguished Professor (PRCE) and director of the Institut de Science et d'Ingénierie Supramoléculaires (ISIS) of the Université de Strasbourg where he is also head of the Nanochemistry Laboratory. He is also Fellow of the Royal Society of Chemistry (FRSC) and junior member of the Institut Universitaire de France (IUF). He obtained a Laurea in Industrial Chemistry at University of Bologna in 1995. In 2000 he received his Ph.D. in Chemistry from the Humboldt University Berlin (Prof. J. P. Rabe). He was permanent research scientist at the Consiglio Nazionale delle Ricerche (CNR) of Bologna and Visiting Professor at ISIS. His research activity is focussed on applications of scanning probe microscopies beyond imaging, hierarchical self-assembly of hybrid architectures at surfaces, supramolecular and graphene electronics, and the fabrication of organic-based multifunctional nanodevices.

exhibiting both functions at the same time have been presented in the scientific literature. Conversely, the solution processability of most of the organic semiconductors can be exploited for the realization of devices where both functionalities are embedded by either fabricating multilayered devices, where an interlayer of photochromic molecules is sandwiched between the other layers, or by blending


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REVIEW Figure 1. Photochromic molecules encompassed in this review, featuring two independently addressable states lead to reversible structural changes like geometry/steric hindrance and electronic changes such as dipole moments, π-conjugation, HOMO-LUMO gap, and redox potential. All these changes occurring on the molecular level affect the macroscopic properties such as shape, aggregation behavior, conductance of the resulting materials, or interface containing these photoswitches. In particular, the sketch depicts (a) azobenzenes, (b) diarylethenes and (c) spiropyrans.

photochromic molecules and organic semiconductors. Examples of both cases along with advantages and disadvantages of these two approaches are discussed in section 2.2 and 2.3, respectively.

2.1. Dyads, Multiads, and Polymers Incorporating Photochromic Molecules and Organic Semiconductors In the attempt of combining organic (semi)conducting molecules and photochromic molecules, several important studies were carried out demonstrating that these two worlds could be mutually advantageous: dyads, multiads, and polymers decorated with photochromic units were synthesized and tested leading to major changes in conductivity and optical properties upon light irradiation.[150,157,159,164–170] Among the various excellent works, recently Pärs et al. reported on the possibility to covalently tether an organic semiconducting molecule such a perylene bis-imide (PBI) to a dithinylcyclopentene (DCP).[171] The molecular system would act as an optical transistor operating under ambient conditions (Figure 2). In this work, the DCP unit photoisomerizes from the open to the closed form upon exposure to UV light (280–310 nm) whilst the ring-opening occurs under visible light (500–650 nm) exposure. In order to exploit the PBI-DCPPBI triad as an optically gated molecular system in which the fluorescence is modulated, the authors carried out a controlled series of irradiating cycles by alternating light pulses at 300 nm (conversion of the DCP unit to the closed form) and 514 nm (probing of the fluorescence in the PBI units). Thanks to the above-mentioned irradiation scheme, the fluorescence intensity of the PBI units could be modulated by energy transfer from the PBI to the DCP unit in the closed form. In particular, the change in fluorescent intensity (I) can be either ON (ION) or OFF (IOFF) depending on whether DCP is in its open or closed isomeric form, respectively. Clearly, the OFF level of intensity corresponds to a low rather than zero intensity. The switching ratio can be defined as: I final − I initial ΔI = I ON I ON

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(1)

where Ifinal and Iinitial are related to ION/IOFF depending on the specific sequence performed. Furthermore, the fatigue resistance of the triads was probed by illuminating the sample at 635 nm while a wavelength of 514 nm would probe the fluorescence. This sequence was repeated five times per switching cycle. A 300–514 nm sequences was then used to induce the back isomerisation of DCP to the closed form. The outcome of a 3000 cycle-long experiment are displayed in Figure 2 revealing a fluorescence contrast drop from 0.83 down to 0.38. In addition, the two insets show the variation of the fluorescence intensity in the first and last 100 seconds of the experiment where, respectively, a high and then low intensity are measured. Within this work the authors propose an analogy of their optical molecular model to a real transistor where the gate is the conversion beam while the source and drain would be the S1 and S0 states of the PBI units, as they ensure the flow of fluorescence photons. Following their analogy, the sign of the gate “optical electrode” would swing between 635 or 300 nm in such a way that, like in a transistor, they could control the amount of electrons between “source” (S1) and “drain” (S0) as shown in the top inset of Figure 2. Another example of modulation of the fluorescence operated via a diarylethene dyad based on perylene di-imide is reported by Tan et al.[167] In addition to the undoubtedly disruptive examples of integration of photochromic molecules into dyads, a remarkable effort has also been devoted to the synthesis of photochromic conjugated conductive polymers as, in principle, any photochromic switching unit conjugated within a π-electron polymer would lead to a modulable conductivity.[165,166,168,169,172–174] However, despite the notable advances, these materials suffer from a slow or sometimes even irreversible switching mechanism along with low electrical conductivities. It is fair to point out that the above-mentioned dyad/multiad approach often requires complicated and multi-step synthetic paths, hampering their easy production and up-scalability and therefore their actual technological exploitation. By and large, the ability to photochemically tune materials properties like the conductivity or the fluorescence represents an overall advantage of photochromic molecules but their integration in real devices such as transistors requires their combination with suitable semiconductors.



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2.2. The Multilayer Approach

Figure 2. (a) Schematic sketch of the molecular triad consisting of two perylene bisimide (PBI) units that are covalently linked to a dithienylcyclopentene (DCP). Top: Closed form, bottom: open form. (b) Absorption spectra of the PBI–DCP–PBI triad (toluene) for the open (red) and the closed (blue) form of the DCP unit. For comparison the dashed line shows the absorption spectrum of pure PBI in the same solvent and at the same concentration. (c) Contrast ratio related to the modulation of the fluorescence intensity vs. number of switching cycles (2 × 5 conversion/probe sequences). Each point in the plot is an average over 50 switching cycles. Bottom Insets are the intial and the final fluorescence intensity modulation, respectively. Top inset shows the analogy of the optical analogy to a transistor device. In particular, the S1 and S0 states of PBI act as as source and drain whilst the conversion beam can be seen as a gate voltage of positive/negative polarity. The optical pumping represents the external circuit. Reproduced with permission.[171] Copyright 2011.

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The use of photochromic molecules in electronic devices dates back to the late ‘80s when Tachibana et al. demonstrated how azobenzene derivatives integrated in multilayered LangmuirBlodgett structures could tune the conductivity in a bi-stable fashion under light irradiation.[175,176] Unfortunately, this approach revealed a pronounced thermal instability of the lightresponsive compounds which would lead to undesired changes in the device output. It is only in 2001 that a number of experiments carried out by Tsujioka et al. brought to the attention of the scientific community the possibility of using diarylethene derivatives compounds in multilayered photo-switchable bipolar memory devices.[177–181] However, these devices could initially reach ON-to-OFF current ratio below 100 owing to a photostationary state with 90% of closed isomers) was achieved on a different diarylethene derivative. Although the authors did not nail down the reasons for such a remarkably high conversion yield, the superior switching capacity of the new derivative, where the side triphenylamine are replaced by benzothiazole units, could be the reason for the improved perfomances.[182] An insightful analysis of the transport in a multilayer diode incorporating a photochromic layer[183] has been recently reported by Meerholz and co-workers.[184] In this work, a crosslinkable dithienylethene (XDTE) serves as a photomodulable hole injection barrier layer in an organic light-emitting diode (OLED) leading to remarkable changes in both luminance and current density. By means of in-situ reflectance absorption spectroscopy all the molecules in the closed form are monitored so that all the changes in output current can be associated to fractions of closed isomers, X, varying from 0 (all of the molecules are in the open form) to 1 (all of the molecules are in the closed form). Interestingly, the switching can be attained both using either optical or electrical stimuli, leading to two completely different morphologies in the film incorporating the closed molecules. Optical irradiation generates a quasi-isotropic distribution of the molecules in the XDTE film whilst filament-like morphology dominates the assembly when electrical stimuli are applied. The authors introduce the electrical closing of the XDTE in view of the observed electrochemical ring closure achieved in solution which likely creates an alternative charge transport pathway across the film. Figure 3 provides evidence for the two different types of output device current modulation, further underlining the very fine and achievable control. A subtle control over the output current is also attained when a careful combination of electrical and optical stimuli is employed. These findings demonstrate that the photochromic diode can attain a variety of continuous current levels, rendering it a good prototype of a multilevel memory device. Nevertheless, a multilayer approach is demanding in terms of device processing, as it must ensure minimal interlayer


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Figure 3. (a) The cross-linkable dithienylethene (XDTE) used in Ref. [56]. (b) Comparison of the current density (at 8 V) measured as a function of closed isomer fraction in the XDTE interlayer with electrical or optical stimuli. The “electrical closing” transport relies on the formation of electrically induced conductive filaments within the film while in the “optical closing” case the closed isomers are quasi-isotropically distributed in the XDTE layer. Reproduced with permission.[184] Copyright 2013.

penetration. Apart from using orthogonal solvents severely limiting the choice of the molecular components, a viable way of preparing multilayered structures is via cross-linking of successive layers[184,185] or by depositing the photoswitchable layer by means of a PDMS stamp at the top of an already-prepared device.[186] Unfortunately, the former way requires additional synthetic effort to impart to the chosen compounds the capacity to undergo cross-linking. Further, since the switching efficiency can be reduced by the presence of overlayers, a different geometry is desirable where the light/photochromic molecules interaction is maximized and not screened by any absorptive top molecule eventually reducing the number of photons reaching the component that should undergo photoisomerization.

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A viable way of bringing together photo-switching capability of photochromic molecules while acting on the charge transport in organic semiconductors is provided by creating a binary mixture. In 2009 Crispin et al.[187] put forward a model supported by theoretical calculations suggesting the use of a variety of diarylethenes blended with organic semiconductors in order to realize two-terminals optically switchable devices. A few years before, experiments were already carried out by different research groups[188–193] showing the possibility to introduce light-controlled traps in a polymer film in order to modulate the output current in a diode. The bi-component film relied on the use of spiropyran (6-nitro-1′,3′,3′,-trimethylspiro[2H-1benzopyran-2,2′-indoline]) as photochromic molecule and either poly[2-methoxy-5(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV)[189,190,192] or poly[methyl(phenyl)silylene] (PMPSi)[188] as polymer semiconductor. The analysis of the generated trapping levels due to the presence of dipolar species in the blend was tested in the framework of the Space-ChargeLimited Current (SCLC)[188–190] and by simulations and modeling.[192] In the former case, the comparison of the J–V curves before and after irradiation provided different curve shapes which could be ascribed to the spiropyran (SP) being in the high-energy-gap state or in the low-energy-gap state, thus acting or not as trapping centers for charges. The physics behind this experiment is dramatically different than what presented in the previous section where the photochromic molecules would form a separate layer while interacting with the semiconductor mostly changing their resistivity across the device. Light-induced conformational changes of spiropyrans in a semiconductor polymer matrix were employed more recently by Li et al.[194] in order to modulate the channel conductance in an organic field-effect transistor (OFET). In this case, SPs which feature a very large variation in dipole moment in the two distinct isomeric forms, are expected to cause a significant change in the energy landscape of charges transported within the poly(3-hexylthiophene) (P3HT) layer. The device scheme is portrayed in Figure 4a where also a possible, yet unproven, assembly of the two interacting molecules i.e., P3HT and SP, is depicted. Upon UV irradiation for about 3 minutes the low-conductance state of the FET channel was changed gradually into a higher conductance state when an increase in output current is recorded (Figure 4b). The back reaction was performed under white light and it required about 8 minutes for the devices to restore the initial drain current value. Noteworthy, the devices showed long-term stability and they could be tested in air up to 3 hours without giving evidence of degradation. As an alternative to the proposed mechanism, the authors suggest that charge transfer between the P3HT backbone and the phenoxide unit of the SP can occur. The photogenerated phenoxyde group in the SP-open form can act as a charge trap which can generate a decrease in mobility. Ishiguro et al. recently showed that the approach based on the combination of SPs with a polymer semiconductor as a mixed active layer in a FET device can be expanded also to other polymers such as poly(triarylamine) (PTAA).[195] Recently, a different approach has been reported which relies on the introduction of photo-tunable energetic levels in



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2.3. The Blending Approach

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Figure 4. (a) Device layout employed by Li et al.: a top-contact bottomgate transistor with HMDS-functionalized silicon oxide as the dielectric layer and gold source and drain electrodes as the injecting electrodes. The bi-component film is a blend of P3HT and SP. The SP molecules are thought of being randomly dispersed within the interdigitated P3HT side chains and close to the polythiophene backbone. (b) An example of a switching cycle: The drain current vs. time plot highlights how the device is able to respond with a different output upon irradiation with a different wavelength. Reproduced with permission.[194] Copyright 2012, RCS.

the bandgap of P3HT by using diarylethenes (DAE) derivatives.[196] In this work DAEs were chosen as the photochromic molecules because they offer the significant advantages of high fatigue resistance, thermal stability of both switching states as well as featuring radically different electronic properties, including HOMO and LUMO energy levels and redox characteristics. Bi-functional organic thin-film transistors (OTFTs) were fabricated by using blends of DAEs with a polymeric semiconductor acting the electroactive material (see Figure 5a). The dual functionality in these OTFTs was achieved by engineering the energy levels of the blend through the insertion of the DAE phototunable energy levels in the polymer matrix in order to tune the charge transport. Hence, blending the semiconducting material with a suitable DAE should make it possible to selectively control and modulate the charge carriers within the film as a result of light irradiation at different wavelengths. A schematic of the energy level engineering in the above-mentioned bi-component films is depicted in Figure 5b where two DAE derivatives in the open (DAE_1o) or in the closed (DAE_1c) form feature different energy levels which are supposed to act differently on the hole transport by generating deeper (DAE_1c) or shallower (DAE_2c) energy trapping levels in the band gap of P3HT. Dynamic characterization of 1832


DAE_1o/P3HT-based FET devices was performed upon irradiation of the transistor over second-scale cycles (Figure 5c) while switching the light source between two different wavelengths, i.e., UV and visible light, respectively. This experiment demonstrated that a dynamic control of the output current of the device could be performed optically in a reversible fashion on a timescale of seconds without exhibiting fatigue for several irradiation cycles. In addition, the response time of the device, which represents the current variation of the output current in response to a single 3-ns long laser pulse centered at 310 nm, was found to fall within the time scale of a few micro-seconds, a truly relevant result paving the way towards exploitation in real electronics. Inorganic-organic hybrid structures can also be exploited to assemble multicomponent materials incorporating photochromic molecules. Among various systems, metallic nanoparticle can be seen as ordered nanostructured scaffolds that can be easily synthesized with a high precision as monodisperse objects. In this framework, Raimondo et al. decorated gold nanoparticles (AuNPs) with a chemisorbed SAM of azobenzene and studied the interparticle propensity to undergo aggregation as a function of the isomeric state of the azobenzene changed by light irradiation at specific wavelength.[197] In a later set of experiments, the photoresponsive nanoparticles were incorporated in the polymer semiconductor via blending in the attempt to provide optical response to the FET.[198] In particular, P3HT was blended with AuNPs coated with a chemisorbed azobenzene-based SAM, the latter acting as trap charges in the device channel (Figure 6, top panel). Through a light-induced isomerization between the trans and cis forms of the azobenzene molecules chemically tethered to the AuNPs, a variation in the tunneling barrier is expected which sets the efficiency of the trapping process in the semiconductor film. The electrical characterization of the FETs upon irradiation with UV light was investigated on devices having different AuNPs/P3HT ratios; it revealed significant changes in both mobility and threshold voltage between devices with azobenzobenzene coated AuNPs with respect to AuNPs with a non-photoresponsive coating or pristine P3HT both used as a reference (Figure 6, bottom panel). An increase in drain current was observed that reached saturation within a few minutes although bias stress signature started to appear after 10 min inducing a decrease in the ID. By and large, this approach makes it possible to confer a dual functionality to an organic/NPs-based FET through the gating of the drain current both electrically (by means of the gate electrode), as in a conventional transistor, and optically (via optical irradiation control). Overall, multifunctional devices such as photo-switchable transistors and memories were realized via blending of the organic semiconductor with different types of photochromic molecules which makes this approach more appealing for device mass production than a multilayered approach. Yet, numerous challenges must be tackled in the future including (i) control over phase segregation in bicomponent films, (ii) how the crystallization of the organic polymer is affected by the presence of a photochomic molecule in the mixture, (iii) dewetting and miscibility in a common solvent, (iv) extending the approach to small semiconducting molecules interacting with photochromic molecules given that only polymers were considered up to now, (v) how the presence of injected charges within


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Alongside attaining modulation of the charge carriers which are transported in the organic semiconductor, controlling the charge injection is one of the major hurdles for organicbased devices.[199–201] Two possible pathways could be pursued to clear this hurdle: (i) the chemisorption of self-assembled monolayers (SAMs) on the injecting electrodes, or (ii) the introduction of an interlayer between injecting electrode and charge transporting material. The former strategy is a universal method to modify numerous physico-chemical properties of surfaces[202] such as metal work function and surface wettability. This can make it possible to attain a better charge injection at metal/ semiconductor interface by either decreasing the interfacial energy mismatch between these materials or to achieve a better control over the molecular order and degree of crystallinity within the semiconductor layer assembling on top of the SAMs, the latter being crucial for the optimization of the transfer integral as well as promoting the charge transport within the active material. The introduction of a photochromic interlayer between the electrode and the electroactive material is a viable approach to impart a bi-functional nature to such interfaces, i.e., by changing surface properties such as work function or wettability by irradiation at specific wavelengths, as already demonstrated in literature.[156,160,203–205] In this regard, we have recently showed how a photomodulable bi-stability of the charge injection in an azobenzenebased SAM chemisorbed onto gold source and drain electrodes can be achieved.[206] OFETs in bottom-gate bottom-contact configuration employing an air-stable perylene di-imide derivative (PDIF-CN2) as the semiconductor layer were tested in order to provide a proof of concept of the suggested working principle. The thiol-functionalized biphenyl azobenzene derivatives employed in this work were previously found to form highly ordered and tightly packed SAMs on Au(111), with isomerization yields up to 96%.[207] Such a high yield of Figure 5. (a) General scheme of the tested organic thin-film transistors including chemical isomerization could be attained because of the formulae of the molecules employed in the bi-component film, namely the photochromic mol- strong intermolecular interactions between ecules DAE_1 and DAE_2 in their two isomeric states and the P3HT semiconducting polymer. adjacent molecules within the 2D crystalline (b) Energy diagram showing the HOMO levels of the employed components and depicts domains.[208] In Crivillers et al.’s work, this the photo-modulation mechanism occurring in the device. (UV = ultraviolet; vis = visible). (c) Dynamic switching of an OTFT made from DAE_1o (20 wt% in P3HT) under several irra- azobenzene-based SAM (AZO-SAM) is chemdiation cycles with ultraviolet (λirr = 365 nm) or white light (λirr > 400 nm). Reproduced with isorbed on the Au source and drain electrodes of an OFET to optically modulate the injecpermission.[196] Copyright 2012, Nature Publishing Group. tion of charges at the electrode/semiconductor interface (Figure 7a). In such configuration, the device irrathe film may affect the isomerization mechanisms and its yield diation needed to induce the photoisomerization of the AZOgiven that this latter may depend on the specific photochromic SAM has to be performed from the top of the device. Hence, molecule/organic semiconductor pair considered. Adv. Mater. 2014, 26, 1827–1845



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Figure 6. (Top panel) Device scheme employed in the study: Gold nanoparticles with various coatings are blended with P3HT which acts as the semiconductor layer for the bottom-contact bottom-gate transistor. (Bottom panel) Photoresponse cycles vs. time of s-AZONPs/P3HT (in black), OPE-NPs/ P3HT (in red), and P3HT (in blue). For the latter, the ID/ID,min was multiplied by a factor of 100 for a better comparison. (a) 4 cycles, (b) 10 cycles. (VG − VTH = −4 V, VD = −10 V, L = 5 µm). Reproduced with permission.[198] Copyright 2012, US National Academy of Science.

the semiconductor acts as an undesired light-absorbing layer. In light of this, the organic semiconductor was chosen in view of two features: (1) its absorption spectra not overlapping the characteristic absorption bands of the azobenzene derivative, and (2) its electrical properties being very little sensitive to exposure to UV and visible light. The trans azobenzene isomer spectra show an intense π – π* transition band at ∼365 nm and a weaker n– π * band around 450 nm whilst the cis isomer is characterized by a more pronounced n– π * transition band at ∼450 nm. Upon illumination of a trans SAM with UV light the isomerization from trans to cis occurs. The cis-to-trans isomerization can be triggered by irradiation into the n– π * band of the cis form upon white light exposure. However, PDIF-CN2 thin films exhibit reduced absorption features in the 350–410 nm range and an intense absorption band which peaks at ∼ 550 nm. These optical features made PDIF-CN2 an ideal candidate material for the AZO-SAM FET experiment. Initially the AZO-SAM were chemisorbed in its trans form and successively exposed to UV light (365 nm) for 90 min. The thermal back reaction was attained by just keeping the sample in dark for 24 hrs. The photoinduced cis-to-trans isomerization which could be achieved upon exposure to white light was deliberately not carried out due to the photogeneration of charge carriers which may render it difficult to separate the different contributions to the current coming from both the back

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isomerization reaction and the illumination with the white light. The output and transfer characteristics in cis-SAM-based FETs revealed a 20% increase in the maximum source-drain current (ID,max) which is displayed in Figure 7b. This change was accompanied by a decrease in threshold voltage and a ΔVTH (trans-to-cis) of 9.2 V and 7.1 V in the linear and the saturation regime. Such a variation provides evidence for the suggested working principle: The smaller thickness of the SAM in the cis form leads to a reduced tunneling barrier/thickness which results in higher drain current. The optical modulation process was found to be reversible up to four reversible cycles with negligible switching fatigue. In order to nail down the switching mechanisms, two control experiments were performed: a PDIF-CN2 solution were spin-cast on both bare gold and undecanethiol-SAM treated gold electrodes and irradiated with UV light for 90 min. In both cases no changes were measured by means of electrical characterization supporing the hypothesis that the AZO-SAM is the photo-responsive part of the device. Further, PDIF-CN2 shows a weak absorption in the UV, thus discarding a role played by the photo-excitation of the semiconductor. In addition to the difference in tunneling resistance of the cis-AZO and trans-AZO, the change of the major electrical FET parameters could relate to (i) a difference in film morphology


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REVIEW Figure 7. (a) Representation of the working principle of the AZO-SAM FET device. The semiconductor is a perylene di-imide derivative, namely PDIFCN2. The AZO-SAM chemisorbed on gold source and drain injecting electrodes undergoes reversible isomerization upon exposure to UV light (transto-cis) and switches back (cis-to-trans) after thermal recovery. As a result, the charge injection and ultimately the drain current are photomodulated. (b) Transfer (right) and output (left) curves acquired after an in situ switching of the AZO-SAM from trans to cis state. The semiconductor is a spincoated PDIF-CN2 film. Reproduced with permission.[206] Copyright 2011.

at the semiconductor/AZO-SAM interface, and (ii) a change in the work function (WF) of the AZO-SAM owing to the photoisomerization. The trans- and cis-AZO-SAM show different hydrophobic properties, with static water contact angle values of 87.2° ± 1.3° and 70.6° ± 1.9°, respectively. The different wettability may play a major role on the interfacial film microstructure thus making charge injection more or less favorable in the two isomeric state of the SAM. In order to tackle this issue, two additional and separate trans and cis devices were prepared. PDIF-CN2 solution were spin-coated onto two different samples having trans- and cis-AZO-SAM-modified electrodes, respectively. Electrical characterization (output and transfer curves) showed markedly higher drain currents for the cis-AZO transistor, therefore supporting the findings of the in-situ experiment. The ID (cis)/ID (trans) increase amounted to 240% which the authors attributed to the potentially improved interfacial semiconductor/AZO-SAM morphology of PDIF-CN2 films separately spin-coated onto the two isomeric forms of the AZO-

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SAM. AFM imaging performed on regions of PDIF-CN2 films spin-coated over AZO-SAM-functionalized Au electrodes, did not revealed significant variation except for an almost negligible roughness increase. Molecular reorganization as well as the reduced injecting area of the source electrode,[58] where a low number of azobenzenes interact with the organic semiconductor, in a bottom-contact architecture are likely to increase the device fatigue reported in their work. Besides the reduced cyclability issue which usually affects photochromic molecules, in this specific case a molecular rearrangement of the semiconductor at the interface may take place, leading to a more static interface, which was evidenced by a lower current modulation efficiency after repeated cycles. In order to discard a possible work function difference between trans- and cis-AZO SAM, Kelvin probe measurements showed only a subtle difference of about 70 meV between the two isomeric states of the films. Therefore, the observed differences in output current could not be ascribed to a substantial work function shift leading to a more favorable energy level alignment. In spite of the low



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in Figure 8a: 1) incorporation of what the authors call compound 1 (oxetane-functionalized dithienylethene, XDTE, Figure 3a) into electroluminescent polymer, that here we call device of type “blend”; 2) a layer of compound 1 between the anode and the emissive layer, here called devices of type “bi-layered”; 3) a further hole transporting layer sandwiched between PEDOT:PSS and 1, device “tri-layered”. A pronounced decrease in electroluminescence efficiency was observed in device of “blend” type even upon addition of small quantities (0.5 wt%) of XDTE. Furthermore, no significant isomerization could be observed under UV light irradiation. Conversely, “bi-layered” devices would exhibit electroluminescence and ON-to-OFF ratio of ∼ 5. In “tri-layered” devices an extra holetransporting layer such as a triphenylamine dimer (XTPD) derivative was added to the structure. The authors reported on 7 different XTPD derivatives (2–8) with HOMO level spanning from – 5.17 down to –5.56 eV. The highest ON-to-OFF on single XTPD layers amounted to ∼200, obtained when the HOMO level of the XTPD was close in energy to the work function of PEDOT:PSS. On the contrary, the switching ratio was found to decrease with lowering the HOMO level of the XTPD, with ON-to-OFF ∼ 1 in the worst case (lowest XTPD HOMO level). The switching ratio could be further improved by setting the thickness of the XDTE layer. In particular, OLED devices with a double layer formed by XTPD derivative 2 Figure 8. (a) Different device structures tested in Zacharias et al. The Y axis indicates the and 3 (16-nm thick) and a 40-nm thick XDTE energy of the HOMO and LUMO levels of each single component of the multilayered structures layer displayed ON-to-OFF ratio as high as encompassed in this study. In particular, (left) XDTE was blended with the emitting material, device of type “blend”. (center) XDTE was employed as an interlayer, device of type “bi-layered”. 3000. The devices' emission onset was about (right) Various hole injecting materials 2 – 8 and their combinations were used as an interlayer 6 V in the OFF state but was reduced to 2.7 V between XDTE and the ITO/PEDOT electrode, device of type “tri-layered”. (b) Comparative plot by UV irradiation (ON state). Exposure to of an optimized device comprising the following layers: PEDOT (35 nm)/2 (8 nm)/3 (8 nm)/1 monochromatic 590-nm light would make (40 nm)/blue emitting polyspirofluorene (70 nm)/Ba(4 nm)/Al (150 nm), in which absorption the devices switch back to the OFF state. and current density are simultaneously recorded under irradiation with a UV (312 nm, left) and an orange (590 nm, right) light sources. (a) is readapted from Ref. [185]. Reproduced with OLED devices exhibiting electroluminescence as high as 0.8 cd A−1 (λmax = 428 nm) permission.[185] Copyright 2009. and constant with the applied voltage were measured. In order to gain a deeper insight into the working principle values recorded in FET devices, the difference in mobility (μtrans of their OLED devices, the authors have used a reference device – μcis) further reflects an improved charge injection in tranwhere the photochromic layer is replaced by the XTPD 6 which sistor with cis-AZO-functionalized electrodes. features a HOMO energy of ∼5.4 eV, therefore close in energy Another noteworthy approach which uses photochromic molto that of XDTE. By utilizing this configuration, the electroluecules interlayer in order to tune the charge injection into an minescence attained was of 2.1 cd A−1 (λmax = 456 nm). Regardorganic device is presented by Zacharias et al.[185] In their work, they report on a solution-processed multilayered OLED device, less of the reduced efficiency of the electron blocking layer at fabricated by cross-linking of each single layer, where a phothe interface between a the blue emitting polyspirofluorene tochromic layer of a dithienylenethene can be photo-switched and 1, the lower efficiency of the switchable device is owing to reversibly. Interestingly, ON-to-OFF ratio as large as 103 was the blue-shifted emission which stems from a different location of the emission zone. Finally, the electron blocking mateachieved for both electroluminescence and current density. rial as well as the position of the emission area were improved Zacharias et al. tested three different configurations displayed



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4. Photoresponsive Dielectric Interfaces and Bulk It is well-known that the physics of organic-based transistors is governed by interfacial properties of different nature occurring both at the electrode/semiconductor but also at the dielectric/ semiconductor interface.[58,209,210] In these devices, polaron transport occurs in the active layer within the very first few nanometers in the vicinity of the dielectric layer. This implies that even minimal chemical changes at the interface could induce major variations in the overall device performances. Hence, a different yet viable approach to attain a bi-stable nature in an OFET is through the modification of the semiconductor/dielectric interface with photochromic molecules whose different isomeric state would lead to changes in the energetic landscape offered to the charge carriers by introducing phototunable variations in the interfacial dipole.

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In the work by Zhang et al.,[211,212] the interfacial properties are controlled by functional SAMs of spiropyrans that have been often employed to modify the morphology of organic semiconductors.[213,214] In this work, SAMs of SPs confer a multifunctional nature to the OFET given that a significant change in dipole moment occurs when a UV light induces the switching between their neutral form, closed-SP (6.4 D), and the zwitterionic colored form, open-SP (13.9 D) as determined by density functional theory calculations.[186] Pentacene-based OFETs were prepared in bottom-gate top-contact configuration on a heavily doped p-type silicon wafer with a 300-nm thick layer of thermally grown SiOx. SAMs of SP were prepared on silicon wafer substrates by silanizing first the SiOx with APTMS and successively tethering a SP carboxylic acid to the surface through covalent amide bond formation with the aid of DCC (carbo di-imide dehydrating-activating agent). XPS and FTIR measurements were carried out proving the effective immobilization of the SPs on the surface. A clear evidence of the SAM switching was provided through (i) UV-Vis absorption spectra which revealed a large characteristic difference in the absorption band at wavelength λ = 563 nm upon UV (λ = 365 nm) and visible light (λ > 520 nm) irradiation, proving the reversible switching between closed-SP and open-SP; (ii) the calculated percent conversion (indicated with xe) of SP molecules from closed-SP to open-SP at the photostationary state which was of about 84%; (iii) light-induced variation of the water contact angle of 12–15 degrees suggesting that the surface is more polar after UV irradiation, as one would expect when SP-SAM is in the open form. Nonetheless, the about 10-fold lower mobility measured on devices with SP-SAM functionalized oxide with respect to pristine SiOx suggested that some interfacial trapping related to both the presence of SP molecules and unreacted amine groups on the surface has to be invoked to explain such a decrease. In addition, a large (negative) shift in threshold voltage was measured but could not be attributed to any variation in capacitance of the SAM/SiOx structure upon irradiation. Overall, large and reversible variations in drain current in response to UV and white light irradiation were measured over ∼100 SP-functionalized devices. Aiming at providing evidence for the switching reversibility the authors performed a number of short-irradiation-time measurements. The devices consistently exhibited long-term and reversible operational stability. Figure 9a clearly shows several switching cycles of the drain current recorded as a function of time. The measurements were performed over a period of ∼3 hrs and for ∼200 cycles; the devices showed a good switching behavior without appreciable degradation when operated in ambient atmosphere. Furthermore, two important merit figures were used to quantify the response to irradiation: the current change ratios (P) and the responsivity (R) expressed in A/W P=

I light I l − I dark = I dark I dark

(2)

R=

I light I l − I dark = Pill LWI ill

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by employing a further XTPD layer (7) between layer 1 and the blue emitting layer. The switching dynamics were investigated by recording current density and the photochromic layer absorption in a functional device using a reflection set-up. Following LambertBeer law, at a wavelength of 595 nm, the absorption should be directly proportional to the concentration of compound 1c. An exponential absorption increase (with a rate constant or the ring closure amounting to kclosed = 20 J−1 cm2) was observed upon irradiation with a 312 nm light source as displayed in the left part of Figure 8b. The reverse reaction, from ON to OFF state, was obtained by irradiation with an orange light source centered at λ = 590 nm and featured a rate constant kopen = 3.4 J−1 cm2. Noteworthy, the devices incorporating XDTE tested in Zacharias et al. showed a remarkably high resistance to fatigue, indicating that the switching properties of the pristine photochromic layer are almost unchanged once incorporated in the device. By and large, the use of photochromic molecules at the electrode/active layer interface allows a photo-modulable tailoring of the work function of the injecting electrode while changing surface properties such as the wettability. Moreover, the fraction of photochromic molecules involved in the SAM or in the interlayer can be modulated by choosing the right irradiation power and duration at a given wavelength which paves the way to multilevel-injection interfaces where ideally a number of intermediate levels could be separately addressed by optical stimuli. Conversely, a number of open questions are yet to be addressed: (i) how the mechanical constraint exerted on both chemisorbed SAMs and films of photochromic molecules by the presence of an upper layer affects the isomerization yield of single molecules and, ultimately, the ensemble behavior, (ii) the switching fatigue experienced during device operation, (iii) the need for additional chemistry steps in order to modify the structure of the photochromic molecule when it is employed as an interlayer since this function requires solubility in solvents orthogonal to those of the underlying layer or the possibility to cross-link the whole photochromic layer for the deposition of an upper layer on it, and (iv) how the ensemble effect (cooperative or collective effect) could be beneficial for the isomerization yield of photochromic SAMs.

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Figure 9. (a) Plot of ID vs. time for a pentacene-based FET with SP-SAMs functionalized silicon oxide. The switching experiment lasted for about 3 hours (first three cycles expanded for clarity) revealing a reversible photoswitching upon irradiation with UV and visible light. VD = −30 V; VG = −15 V. (b) Responsivity (R) and photosensitivity (P) as a function of the gate voltage (effective irradiance power = 7.4 µW cm−2) for the previously mentioned device. (drain voltage was kept at −100 V). Reproduced with permission.[211] Copyright 2011, ACS.

where Idark and Ilight are, respectively, the drain current measured in dark and under light irradiation, Il is the drain current under illumination, Pill is the illumination power incident on the device channel, Iill is the light power intensity, L and W are respectively the channel length and width of the device. Noteworthy, the device photoresponsivity was found being biasdependent. Figure 9b shows the responsivity data of the device at several gate voltages for a constant drain bias. In the best cases R as high as ∼400 A/W and P up to ∼450 were extracted at very low effective light power density of 7.4 µW cm−2. The reported values were found to exceed those previously measured in most of the organic-based phototransistors and are comparable with those of amorphous silicon (R = 300 A/W and P = 1000) which makes this finding truly relevant for technological exploitation. Regarding the switching mechanisms, the authors considered several possibilities. First, the effect of the molecular dipole change (13.9 D for open-SP and 6.4 D for closed-SP) should be taken into account. If one considers the relationship between electric field across a SAM and its molecular dipole: R=

I light Il − I dark = Pill LWI ill

(4)

where ε is the dielectric constant, N and dmol are the areal density anvd height of the SP-SAM, respectively, it appears that the voltage variation across the monolayer in response to a change in molecular dipole would amount to 1.1 to 1.7 V. An additional change in dielectric constant upon isomerization could be taken into account that would further explain the authors’ findings. Another possibility could be a possible charge transfer mechanism between pentacene and photogenerated phenoxide ion groups (open-SP form). These latter groups could act as trap centers thus leading to a decrease in charge mobility and a threshold voltage shift towards more negative values, which did not occur in the experiments. To rule out the possibility of interfacial charge transfer, a PMMA interlayer was used to cover the SP layer but this attempt resulted in device performances comparable to those without interlayer. Another intriguing approach to attain a bistable nature in OFET was reported by Tseng et al.[215] who integrated both

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aggregated clusters and SAM of azobenzene derivatives at the dielectric/pentacene interface. In their work, SAMs of several substituted azobenzene derivatives indicated with AZO-Sil-R (with R = CF3, H, C12H25, CH3) were chemisorbed onto SiOx (Figure 10a). Each type of SAM features a different dipole moment depending on the head group attached. In addition, discrete clusters of multiple layers of azobenzene-carrying acid molecules, indicated as compound AZO-acid-R (with R = CF3, CH3) could also lead to bistability in the pentacene film surrounding them (Figure 10b). Before evaporation of the organic semiconductor, the SAMs formation was characterized by ellipsometry and attenuated total reflectance infrared spectroscopy (ATR-IR). Further, pentacene films deposited on the SAMs were investigated by both atomic force microscopy and X-ray diffraction. These experiments revealed a polycrystalline nature in all the films grown on the azobenzene monolayers. Interestingly, the diffraction peak intensities were found to slightly differ depending on the specific terminal functional group which could be ascribed to the difference in surface energy. OFETs in bottom-gate top-contact configuration were prepared on the pentacene-azobenzene composite films by thermal evaporation of gold source and drain electrodes through a shadow mask. All the devices exhibited p-type behavior which is expected when pentacene acts as the active layer. Field-effect mobility would fall in the 0.27 − 0.35 cm2 V−1 s−1 range between devices prepared on different SAM-modified substrates, with the exception of the film deposited on the mixed monolayer of AZO-Sil-CF3/C10, which exhibited lower field-effect mobility if compared to those recorded on single component SAM. The mobility was higher in OFET where the pentacene film was deposited onto the C12H25-terminated azobenzene SAM likely owing to the different crystallinity of the active layer, as confirmed by X-ray and AFM data. The threshold voltage showed remarkable shifts to more positive voltages both in devices with AZO-Sil-CF3 monolayer and mixed layer-assembled SiO2 surface whilst shifts to higher negative threshold voltage values were measured in devices bearing AZO-Sil-H and AZO-Sil-CH3 monolayer-assembled surfaces, if compared to the OTS-modified surfaces which were used as a reference. In fact, fluorinated surface bear an intrinsically electronegative nature leading to more positive field-effect threshold voltages, whereas electron-rich surfaces


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REVIEW Figure 10. (a) (left) Chemical structures of several substituted azobenzene derivatives indicated with AZO-Sil-R (with X = CF3, H, C12H25, CH3) chemisorbed in SAM form onto silicon oxide layers Each type of SAM features a different dipole moment depending on the head group attached. In addition, (right) discrete clusters of multiple layers of azobenzene-carrying acid molecules, indicated as compound AZO-acid-R (with X = CF3, CH3) could also lead to bistability when acting within the bulk of the semiconductor (pentacene). (b) Schematics of the switching principle presented in the work by Tseng et al. When the azobenzene SAM undergoes isomerization the dipole of the molecular backbone is changed leading to a different dipolar interaction with the pentacene upper layer. (a) is readapted from Ref. [215]. Reproduced with permission.[215] Copyright 2012, ACS.

Aiming at phototuning the dielectric properties in order to modulate the drain current, Lutsyk et al.[217] integrated spiropyran derivatives in a poly(methylmethacrilate) (PMMA) matrix which serves as the gate insulator in an FET device (Figure 11). Their findings rely on the reversible modification of the fieldeffect mobility owing to phototunable formation of trap states in the organic semiconductor in the vicinity of highly polar molecules such as SP or MC. According to literature, deep trap states can be formed when the dipole moment is as high as 10D.[187,218] An ITO layer acts as the bottom gate for the whole structure. Successively, a 1:0.1 wt.% solution of PMMA and SP (solvent: ethyl acetate) was spin-coated to form the insulating gate layer. The organic semiconductor and then the electrodes were both thermally evaporated. Upon UV irradiation, an increase in drain current is observed already after 1 min exposure of the device to the light source. This effect was attributed to the to the isomerization of SP to MC. The MC isomer switches back to the SP form either by white light irradiation or slow thermal recovery. The former photoreaction is undoubtedly faster than the latter but the authors chose the back reaction in dark in order to rule out the contribution to the drain current coming from the generation of photocarriers in the organic layer upon irradiation with visible light. This would be instrumental to make a proof of principle of their theory. The relaxation time at ambient temperature lasted 16 hours during which the drain current value recovers its initial value. Despite the fact that kinetics is not considered within the manuscript, the authors found that the isomeration process was reversible. The threshold voltage shift which stems from the SP isomerization into MC in the PMMA matrix was of about 5 Figure 11. Sketch of the bottom-gate top-contact FET structure employed in Lutsyk et al.[217] volt and differed of 7 and 12 volt respectively The active layer is an n-type semiconductor (PTCDI-C13H27). The gate is either PMMA or from devices where only PMMA is used as the dielectric. Furthermore, addition of SP to PMMA blended with spiropyrans. Reproduced with permission.[217] Copyright 2011, ACS.

need a more negative bias to compensate charge the higher amount of positive charge accumulated. Upon UV-induced trans-to-cis isomerization of azobenzene derivatives the system undergo a variation in both direction and magnitude of the overall molecular dipole moment of the SAM chemisorbed on SiOx (Figure 10b). The effect of such photisomerisation on the ID-VG curves was evaluated. Noteworthy, the authors performed control experiments in FETs with no azobenzene-SAM which confirmed the photochemical stability of the pentacene films, evidenced through the negligible variations in current upon UV irradiation. Discrete-clusters interfaces led to better memory window and faster responses than those recorded in SAM-functionalized devices, a result that the authors attributed to a better contact and more efficient charge transfer between pentacene and the trapping centers. In addition, the supposedly better contact would lead to a faster discharging of the trapping sites which results therefore in shorter retention times. A similar approach of introducing controlled deep trapping levels by using a layer of thermally evaporated diarylethenes (instead of spiropyrans) at the pentacene/PMMA interface was employed by Yoshida et al.[216] In their work, a rewritable memory is realized by photoswitching the energy levels of a DAE derivative sandwiched between the polymer dielectric and the active layer.

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PMMA led to changes in the device transconductance by ∼8% whereas a further reversible ∼2% increase was measured upon irradiation with UV light. These results apparently contradict previous theoretical prediction[187,193,219] and experimental evidence[188,189,220] where current was found to increase upon SP to MC isomerization in a semiconductor matrix. However, this trend is consistent with a previous experimental observation[221] which used pentacene as the active layer and spiropyrans/PMMA as the dielectric. Nevertheless, the switching mechanism could be explained in two possible ways: (1) formation of charged states originated by dipoles in the vicinity of the dielectric/semiconductor interface, or (2) changes of the dielectric bulk properties of the insulator. The studies presented above represent an impressive improvement for the normal functioning of OFETs because of the added functionalities brought in by the photochromic molecules integrated in the dielectric bulk and interface. Nevertheless, some issues are yet to be further understood. Though several hypotheses such as charge transfer were put forward, a clear understanding of the photochromic SAM/organic semiconductor interface physics still needs to be unraveled. Further, the relationship between the change in steric hindrance of the photochromonic molecule upon isomerization and the mechanical constraint exerted by the dielectric polymer is still an open question.

5. Conclusions and Future Outlooks This review encompasses a number of attempts that aimed at bringing together the world of photochromic molecules and organic electronics for the generation of multi-functional devices. Thanks to the features added by the photoresponsive nature of these bi-stable molecular switches new routes can be mapped out which are likely to drive the present and future organic electronics community towards devices exhibiting novel and multiple functionalities. In particular, optical switching of the output current has been achieved and proven in combination with organic semiconductors by introducing controlled photo-modulable trap levels either in the semiconductor bulk or at the interface between dielectric and active layer leading to a novel generation of fast optical response FETs and multi-level memories with ON-to-OFF ratios approaching 104. Further, by tuning the process of charge injection with light, a different amount of charges can be transferred from an injecting electrode into the active layer of an organic-based transistor or an OLED either by changing the tunneling barrier of a photo-responsive SAM or using an interlayer featuring phototunable energy levels. By and large, organic-based devices’ performances are strongly interface dependent therefore exploiting photochromic molecules to tune not only the bulk transport properties of the active layer but also those of the metal/semiconductor and dielectric/ semiconductor interfaces will open up a Pandora's box of opportunities for the generation of bi- up to multi-functional devices. However, this work also attempts to draw a roadmap while aspiring to put in evidence the most significant issues which are yet to be further understood. In spite of the suggested

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hypothesis of charge transfer, a more nuanced study on the physics of charge transport is needed to disentangle the mutual effect the semiconductor and photochromic component exert in a binary blend or when they share a large area interface. Further, how a charge injected into the semiconductor can affect the isomerization process of the photochromic molecules is still an open question which will require careful spectroscopic investigation as well as modeling/simulation activity. The role of the light power density on the isomerization of the photochromic molecule in the solid state also needs to be properly tackled. To date, the role of myriad of intermolecular interactions on the switching efficiency in each specific semiconductor/ photochromic molecule pair needs to be unveiled. This could be achieved by an in-depth photochemical investigation of the isomerization yield in solution at different concentrations compared to that recorded in films. In case of ordered assemblies, the influence of intermolecular interactions can be reproducible by designing systems and assemblies thereof that can undergo isomerization in processes that are either cooperative or collective in nature. By exploiting these characteristics a high photochemical yield and improved faster time response to the optical stimuli could be certainly achieved. To gain deeper insight into the working principles of molecular systems incorporating both organic semiconductors and photochromic molecules, further fundamental research is required. Optimized materials, a deep and all-embracing understanding of their photophysical and photochemical processes, as well as more nuanced design rules for the devices will pave the way towards a novel generation of multifunctional, lowcost, large-area, and flexible electronics.

Acknowledgments This article is part of an ongoing series celebrating the 25th anniversary of Advanced Materials. We are grateful to Dr. Nuria Crivillers, Dr. Corinna Raimondo, Dr. Andrea Liscio, Dr. Vincenzo Palermo as well as Prof. Stefan Hecht, Prof. Marcel Mayor, Prof. Franco Cacialli, Dr. David Beljonne and Dr. Jérôme Cornil and their research teams for the joint research activity. This work was supported by the EC through the ERC project SUPRAFUNCTION (GA-257305) as well as the Agence Nationale de la Recherche through the LabEx project CSC, and the International Center for Frontier Research in Chemistry (icFRC). Received: September 18, 2013 Revised: January 13, 2014 Published online: February 19, 2014 [1] H. E. Katz, A. J. Lovinger, J. Johnson, C. Kloc, T. Siegrist, W. Li, Y. Y. Lin, A. Dodabalapur, Nature 2000, 404, 478. [2] B. Crone, A. Dodabalapur, Y. Y. Lin, R. W. Filas, Z. Bao, A. LaDuca, R. Sarpeshkar, H. E. Katz, W. Li, Nature 2000, 403, 521. [3] J. A. Rogers, Z. Bao, K. Baldwin, A. Dodabalapur, B. Crone, V. R. Raju, V. Kuck, H. Katz, K. Amundson, J. Ewing, P. Drzaic, Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 4835. [4] E. J. Meijer, D. M. De Leeuw, S. Setayesh, E. Van Veenendaal, B. H. Huisman, P. W. M. Blom, J. C. Hummelen, U. Scherf, T. M. Klapwijk, Nat. Mater. 2003, 2, 678. [5] U. Zschieschang, H. Klauk, M. Halik, G. Schmid, C. Dehm, Adv. Mater. 2003, 15, 1147.


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[36] C. D. Muller, A. Falcou, N. Reckefuss, M. Rojahn, V. Wiederhirn, P. Rudati, H. Frohne, O. Nuyken, H. Becker, K. Meerholz, Nature 2003, 421, 829. [37] C. T. Chen, Chem. Mater. 2004, 16, 4389. [38] S. R. Forrest, Nature 2004, 428, 911. [39] T. D. Anthopoulos, M. J. Frampton, E. B. Namdas, P. L. Burn, I. D. W. Samuel, Adv. Mater. 2004, 16, 557. [40] J. Zaumseil, R. H. Friend, H. Sirringhaus, Nat. Mater. 2006, 5, 69. [41] M. Muccini, Nat. Mater. 2006, 5, 605. [42] F. Cicoira, C. Santato, Adv. Funct. Mater. 2007, 17, 3421. [43] K. Walzer, B. Maennig, M. Pfeiffer, K. Leo, Chem. Rev. 2007, 107, 1233. [44] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Luessem, K. Leo, Nature 2009, 459, 234. [45] R. Capelli, S. Toffanin, G. Generali, H. Usta, A. Facchetti, M. Muccini, Nat. Mater. 2010, 9, 496. [46] A. Assadi, C. Svensson, M. Willander, O. Inganas, Appl. Phys. Lett. 1988, 53, 195. [47] A. Dodabalapur, L. Torsi, H. E. Katz, Science 1995, 268, 270. [48] L. Torsi, A. Dodabalapur, L. J. Rothberg, A. W. P. Fung, H. E. Katz, Science 1996, 272, 1462. [49] Z. N. Bao, Y. Feng, A. Dodabalapur, V. R. Raju, A. J. Lovinger, Chem. Mater. 1997, 9, 1299. [50] H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K. Bechgaard, B. M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J. Janssen, E. W. Meijer, P. Herwig, D. M. de Leeuw, Nature 1999, 401, 685. [51] H. E. Katz, Z. N. Bao, S. L. Gilat, Acc. Chem. Res. 2001, 34, 359. [52] T. M. Pappenfus, R. J. Chesterfield, C. D. Frisbie, K. R. Mann, J. Casado, J. D. Raff, L. L. Miller, J. Am. Chem. Soc. 2002, 124, 4184. [53] N. Stutzmann, R. H. Friend, H. Sirringhaus, Science 2003, 299, 1881. [54] M. Halik, H. Klauk, U. Zschieschang, G. Schmid, S. Ponomarenko, S. Kirchmeyer, W. Weber, Adv. Mater. 2003, 15, 917. [55] M. Halik, H. Klauk, U. Zschieschang, G. Schmid, C. Dehm, M. Schutz, S. Maisch, F. Effenberger, M. Brunnbauer, F. Stellacci, Nature 2004, 431, 963. [56] J. L. Brédas, D. Beljonne, V. Coropceanu, J. Cornil, Chem. Rev. 2004, 104, 4971. [57] C. R. Newman, C. D. Frisbie, D. A. da Silva, J. L. Bredas, P. C. Ewbank, K. R. Mann, Chem. Mater. 2004, 16, 4436. [58] F. Dinelli, M. Murgia, P. Levy, M. Cavallini, F. Biscarini, D. M. de Leeuw, Phys. Rev. Lett. 2004, 92. [59] R. J. Chesterfield, J. C. McKeen, C. R. Newman, P. C. Ewbank, D. A. da Silva, J. L. Bredas, L. L. Miller, K. R. Mann, C. D. Frisbie, J. Phys. Chem. B 2004, 108, 19281. [60] L. L. Chua, J. Zaumseil, J. F. Chang, E. C. W. Ou, P. K. H. Ho, H. Sirringhaus, R. H. Friend, Nature 2005, 434, 194. [61] H. Sirringhaus, Adv. Mater. 2005, 17, 2411. [62] T. D. Anthopoulos, S. Setayesh, E. Smits, M. Colle, E. Cantatore, B. de Boer, P. W. M. Blom, D. M. de Leeuw, Adv. Mater. 2006, 18, 1900. [63] A. L. Briseno, S. C. B. Mannsfeld, M. M. Ling, S. Liu, R. J. Tseng, C. Reese, M. E. Roberts, Y. Yang, F. Wudl, Z. Bao, Nature 2006, 444, 913. [64] V. Coropceanu, J. Cornil, D. A. da Silva, Y. Olivier, R. Silbey, J. L. Brédas, Chem. Rev. 2007, 107, 926. [65] J. Cornil, J. L. Brédas, J. Zaumseil, H. Sirringhaus, Adv. Mater. 2007, 19, 1791. [66] J. Zaumseil, H. Sirringhaus, Chem. Rev. 2007, 107, 1296. [67] C. E. Finlayson, R. H. Friend, M. B. J. Otten, E. Schwartz, J. J. L. M. Cornelissen, R. L. M. Nolte, A. E. Rowan, P. Samorì, V. Palermo, A. Liscio, K. Peneva, K. Müllen, S. Trapani, D. Beljonne, Adv. Funct. Mater. 2008, 18, 3947.



REVIEW

[6] A. C. Arias, S. E. Ready, R. Lujan, W. S. Wong, K. E. Paul, A. Salleo, M. L. Chabinyc, R. Apte, R. A. Street, Y. Wu, P. Liu, B. Ong, Appl. Phys. Lett. 2004, 85, 3304. [7] M. M. Ling, Z. N. Bao, Chem. Mater. 2004, 16, 4824. [8] I. McCulloch, Nat. Mater. 2005, 4, 583. [9] R. A. Street, W. S. Wong, S. E. Ready, I. L. Chabinyc, A. C. Arias, S. Limb, A. Salleo, R. Lujan, Mater Today 2006, 9, 32. [10] J. D. Slinker, J. A. DeFranco, M. J. Jaquith, W. R. Silveira, Y. W. Zhong, J. M. Moran-Mirabal, H. G. Craighead, H. D. Abruna, J. A. Marohn, G. G. Malliaras, Nat. Mater. 2007, 6, 894. [11] A. Facchetti, Mater. Today 2007, 10, 28. [12] H. Klauk, U. Zschieschang, J. Pflaum, M. Halik, Nature 2007, 445, 745. [13] M. Berggren, D. Nilsson, N. D. Robinson, Nat. Mater. 2007, 6, 3. [14] J. H. Cho, J. Lee, Y. Xia, B. Kim, Y. He, M. J. Renn, T. P. Lodge, C. D. Frisbie, Nat. Mater. 2008, 7, 900. [15] H. Yan, Z. H. Chen, Y. Zheng, C. Newman, J. R. Quinn, F. Dötz, M. Kastler, A. Facchetti, Nature 2009, 457, 679. [16] T. Rauch, M. Boberl, S. F. Tedde, J. Furst, M. V. Kovalenko, G. N. Hesser, U. Lemmer, W. Heiss, O. Hayden, Nat. Photonics 2009, 3, 332. [17] D. T. Simon, S. Kurup, K. C. Larsson, R. Hori, K. Tybrandt, M. Goiny, E. H. Jager, M. Berggren, B. Canlon, A. Richter-Dahlfors, Nat. Mater. 2009, 8, 742. [18] J. A. Rogers, T. Someya, Y. G. Huang, Science 2010, 327, 1603. [19] S. C. B. Mannsfeld, B. C. K. Tee, R. M. Stoltenberg, C. V. H. H. Chen, S. Barman, B. V. O. Muir, A. N. Sokolov, C. Reese, Z. N. Bao, Nat. Mater. 2010, 9, 859. [20] A. C. Arias, J. D. MacKenzie, I. McCulloch, J. Rivnay, A. Salleo, Chem. Rev. 2010, 110, 3. [21] R. Pfattner, M. Mas-Torrent, I. Bilotti, A. Brillante, S. Milita, F. Liscio, F. Biscarini, T. Marszalek, J. Ulanski, A. Nosal, M. Gazicki-Lipman, M. Leufgen, G. Schmidt, L. W. Molenkamp, V. Laukhin, J. Veciana, C. Rovira, Adv. Mater. 2010, 22, 4198. [22] M. Cavallini, P. D’Angelo, V. V. Criado, D. Gentili, A. Shehu, F. Leonardi, S. Milita, F. Liscio, F. Biscarini, Adv. Mater. 2011, 23, 5091. [23] J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, A. B. Holmes, Nature 1990, 347, 539. [24] N. C. Greenham, S. C. Moratti, D. D. C. Bradley, R. H. Friend, A. B. Holmes, Nature 1993, 365, 628. [25] M. Berggren, O. Inganas, G. Gustafsson, J. Rasmusson, M. R. Andersson, T. Hjertberg, O. Wennerstrom, Nature 1994, 372, 444. [26] P. E. Burrows, Z. Shen, V. Bulovic, D. M. McCarty, S. R. Forrest, J. A. Cronin, M. E. Thompson, J. Appl. Phys. 1996, 79, 7991. [27] M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, S. R. Forrest, Nature 1998, 395, 151. [28] A. W. Grice, D. D. C. Bradley, M. T. Bernius, M. Inbasekaran, W. W. Wu, E. P. Woo, Appl. Phys. Lett. 1998, 73, 629. [29] J. S. Kim, M. Granstrom, R. H. Friend, N. Johansson, W. R. Salaneck, R. Daik, W. J. Feast, F. Cacialli, J. Appl. Phys. 1998, 84, 6859. [30] R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. Bredas, M. Logdlund, W. R. Salaneck, Nature 1999, 397, 121. [31] H. Aziz, Z. D. Popovic, N. X. Hu, A. M. Hor, G. Xu, Science 1999, 283, 1900. [32] U. Mitschke, P. Bauerle, J. Mater. Chem. 2000, 10, 1471. [33] P. K. H. Ho, J. S. Kim, J. H. Burroughes, H. Becker, S. F. Y. Li, T. M. Brown, F. Cacialli, R. H. Friend, Nature 2000, 404, 481. [34] U. Scherf, E. J. W. List, Adv. Mater. 2002, 14, 477. [35] E. J. W. List, R. Guentner, P. S. de Freitas, U. Scherf, Adv. Mater. 2002, 14, 374.

1841

REVIEW

www.advmat.de

1842

[68] J. Rivnay, L. H. Jimison, J. E. Northrup, M. F. Toney, R. Noriega, S. F. Lu, T. J. Marks, A. Facchetti, A. Salleo, Nat. Mater. 2009, 8, 952. [69] R. Dabirian, V. Palermo, A. Liscio, E. Schwartz, M. B. J. Otten, C. E. Finlayson, E. Treossi, R. H. Friend, G. Calestani, K. Müllen, R. J. M. Nolte, A. E. Rowan, P. Samorì, J. Am. Chem. Soc. 2009, 131, 7055. [70] A. Shehu, S. D. Quiroga, P. D'Angelo, C. Albonetti, F. Borgatti, M. Murgia, A. Scorzoni, P. Stoliar, F. Biscarini, Phys. Rev. Lett. 2010, 104. [71] H. Bronstein, Z. Chen, R. S. Ashraf, W. Zhang, J. Du, J. R. Durrant, P. S. Tuladhar, K. Song, S. E. Watkins, Y. Geerts, M. M. Wienk, R. A. J. Janssen, T. Anthopoulos, H. Sirringhaus, M. Heeney, I. McCulloch, J. Am. Chem. Soc. 2011, 133, 3272. [72] I. G. Lezama, M. Nakano, N. A. Minder, Z. H. Chen, F. V. Di Girolamo, A. Facchetti, A. F. Morpurgo, Nat. Mater. 2012, 11, 788. [73] J. J. Brondijk, W. S. C. Roelofs, S. G. J. Mathijssen, A. Shehu, T. Cramer, F. Biscarini, P. W. M. Blom, D. M. de Leeuw, Phys. Rev. Lett. 2012, 109. [74] J. Cornil, S. Verlaak, N. Martinelli, A. Mityashin, Y. Olivier, T. Van Regemorter, G. D'Avino, L. Muccioli, C. Zannoni, F. Castet, D. Beljonne, P. Heremans, Acc. Chem. Res. 2013, 46, 434. [75] G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science 1995, 270, 1789. [76] J. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglia, R. H. Friend, S. C. Moratti, A. B. Holmes, Nature 1995, 376, 498. [77] L. Schmidt-Mende, A. Fechtenkötter, K. Müllen, E. Moons, R. H. Friend, J. D. MacKenzie, Science 2001, 293, 1119. [78] M. Svensson, F. L. Zhang, S. C. Veenstra, W. J. H. Verhees, J. C. Hummelen, J. M. Kroon, O. Inganas, M. R. Andersson, Adv. Mater. 2003, 15, 988. [79] P. Peumans, S. Uchida, S. R. Forrest, Nature 2003, 425, 158. [80] H. Hoppe, N. S. Sariciftci, J. Mater. Res. 2004, 19, 1924. [81] F. Yang, M. Shtein, S. R. Forrest, Nat. Mater. 2005, 4, 37. [82] D. C. Coffey, D. S. Ginger, Nat. Mater. 2006, 5, 735. [83] J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger, G. C. Bazan, Nat. Mater. 2007, 6, 497. [84] J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T. Q. Nguyen, M. Dante, A. J. Heeger, Science 2007, 317, 222. [85] S. Gunes, H. Neugebauer, N. S. Sariciftci, Chem. Rev. 2007, 107, 1324. [86] P. W. M. Blom, V. D. Mihailetchi, L. J. A. Koster, D. E. Markov, Adv. Mater. 2007, 19, 1551. [87] V. Palermo, M. B. J. Otten, A. Liscio, E. Schwartz, P. A. J. de Witte, M. A. Castriciano, M. M. Wienk, F. Nolde, G. De Luca, J. J. L. M. Cornelissen, R. A. J. Janssen, K. Müllen, A. E. Rowan, R. J. M. Nolte, P. Samorì, J. Am. Chem. Soc. 2008, 130, 14605. [88] B. C. Thompson, J. M. Frechet, Angewandte Chemie 2008, 47, 58. [89] M. Campoy-Quiles, T. Ferenczi, T. Agostinelli, P. G. Etchegoin, Y. Kim, T. D. Anthopoulos, P. N. Stavrinou, D. D. Bradley, J. Nelson, Nat. Mater. 2008, 7, 158. [90] A. J. Moule, K. Meerholz, Adv. Mater. 2008, 20, 240. [91] J. L. Brédas, J. E. Norton, J. Cornil, V. Coropceanu, Acc. Chem. Res. 2009, 42, 1691. [92] J. Roncali, Acc. Chem. Res. 2009, 42, 1719. [93] M. R. Lee, R. D. Eckert, K. Forberich, G. Dennler, C. J. Brabec, R. A. Gaudiana, Science 2009, 324, 232. [94] C. J. Brabec, S. Gowrisanker, J. J. Halls, D. Laird, S. Jia, S. P. Williams, Adv. Mater. 2010, 22, 3839. [95] M. Helgesen, R. Sondergaard, F. C. Krebs, J. Mater. Chem. 2010, 20, 36. [96] A. Facchetti, Mater. Today 2013, 16, 123. [97] Q. Miao, M. Lefenfeld, T. Q. Nguyen, T. Siegrist, C. Kloc, C. Nuckolls, Adv. Mater. 2005, 17, 407.


[98] J. E. Anthony, Chem. Rev. 2006, 106, 5028. [99] R. Schmidt, M. M. Ling, J. H. Oh, M. Winkler, M. Konemann, Z. N. Bao, F. Wurthner, Adv. Mater. 2007, 19, 3692. [100] V. Palermo, P. Samorì, Angew. Chem. Int. Ed. 2007, 46, 4428. [101] S. Sergeyev, W. Pisula, Y. H. Geerts, Chem. Soc. Rev. 2007, 36, 1902. [102] I. McCulloch, M. Heeney, M. L. Chabinyc, D. DeLongchamp, R. J. Kline, M. Coelle, W. Duffy, D. Fischer, D. Gundlach, B. Hamadani, R. Hamilton, L. Richter, A. Salleo, M. Shkunov, D. Sporrowe, S. Tierney, W. Zhong, Adv. Mater. 2009, 21, 1091. [103] C. Li, M. Y. Liu, N. G. Pschirer, M. Baumgarten, K. Müllen, Chem. Rev. 2010, 110, 6817. [104] M. Gsanger, J. H. Oh, M. Konemann, H. W. Hoffken, A. M. Krause, Z. N. Bao, F. Wurthner, Angew. Chem. Int. Ed. 2010, 49, 740. [105] W. Y. Lee, J. H. Oh, S. L. Suraru, W. C. Chen, F. Wurthner, Z. N. Bao, Adv. Funct. Mater. 2011, 21, 4173. [106] G. De Luca, W. Pisula, D. Credgington, E. Treossi, O. Fenwick, G. M. Lazzerini, R. Dabirian, E. Orgiu, A. Liscio, V. Palermo, K. Müllen, F. Cacialli, P. Samorì, Adv. Funct. Mater. 2011, 21, 1279. [107] T. M. Figueira-Duarte, K. Müllen, Chem. Rev. 2011, 111, 7260. [108] Y. Diao, B. C.-K. Tee, G. Giri, J. Xu, D. H. Kim, H. A. Becerril, R. M. Stoltenberg, T. H. Lee, G. Xue, S. C. B. Mannsfeld, Z. Bao, Nat. Mater. 2013, 12, 665. [109] A. Facchetti, Nat. Mater. 2013, 12, 598. [110] N. D. Treat, J. A. Nekuda Malik, O. Reid, L. Yu, C. G. Shuttle, G. Rumbles, C. J. Hawker, M. L. Chabinyc, P. Smith, N. Stingelin, Nat. Mater. 2013, 12, 628. [111] C. B. Nielsen, M. Turbiez, I. McCulloch, Adv. Mater. 2013, 25, 1859. [112] I. McCulloch, Adv. Mater. 2013, 25, 1811. [113] B. L. Feringa, W. F. Jager, B. Delange, Tetrahedron 1993, 49, 8267. [114] H. Bouas-Laurent, H. Durr, Pure Appl. Chem. 2001, 73, 639. [115] M.-M. Russew, S. Hecht, Adv. Mater. 2010, 22, 3348. [116] M. Irie, Chem. Rev. 2000, 100, 1683. [117] M. V. Peters, R. S. Stoll, A. Kuehn, S. Hecht, Angew. Chem. Int. Ed. 2008, 47, 5968. [118] V. I. Minkin, Chem. Rev. 2004, 104, 2751. [119] S. Shinkai, Pure Appl. Chem. 1987, 59, 425. [120] Z. F. Liu, K. Hashimoto, A. Fujishima, Nature 1990, 347, 658. [121] T. Ikeda, O. Tsutsumi, Science 1995, 268, 1873. [122] T. Nagele, R. Hoche, W. Zinth, J. Wachtveitl, Chem. Phys. Lett. 1997, 272, 489. [123] A. Archut, F. Vogtle, L. De Cola, G. C. Azzellini, V. Balzani, P. S. Ramanujam, R. H. Berg, Chem. Eur. J. 1998, 4, 699. [124] N. Tamai, H. Miyasaka, Chem. Rev. 2000, 100, 1875. [125] T. Hugel, N. B. Holland, A. Cattani, L. Moroder, M. Seitz, H. E. Gaub, Science 2002, 296, 1103. [126] M. Alemani, M. V. Peters, S. Hecht, K.-H. Rieder, F. Moresco, L. Grill, J. Am. Chem. Soc. 2006, 128, 14446. [127] J. Zeitouny, C. Aurisicchio, D. Bonifazi, R. De Zorzi, S. Geremia, M. Bonini, C.-A. Palma, P. Samorì, A. Listorti, A. Belbakra, N. Armaroli, J. Mater. Chem. 2009, 19, 4715. [128] K. Smaali, S. Lenfant, S. Karpe, M. Ocafrain, P. Blanchard, D. Deresmes, S. Godey, A. Rochefort, J. Roncali, D. Vuillaume, Acs Nano 2010, 4, 2411. [129] S. L. Gilat, S. H. Kawai, J. M. Lehn, J. Chem. Soc., Chem. Commun. 1993, 1439. [130] G. M. Tsivgoulis, J. M. Lehn, Angew. Chem. Int. Ed. Eng. 1995, 34, 1119. [131] S. L. Gilat, S. H. Kawai, J. M. Lehn, Chem. Eur. J. 1995, 1, 275. [132] F. Stellacci, C. Bertarelli, F. Toscano, M. C. Gallazzi, G. Zotti, G. Zerbi, Adv. Mater. 1999, 11, 292. [133] M. Irie, Chem. Rev. 2000, 100, 1685. [134] M. Irie, S. Kobatake, M. Horichi, Science 2001, 291, 1769. [135] E. Murguly, T. B. Norsten, N. R. Branda, Angew. Chem. Int. Ed. 2001, 40, 1752.


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www.advmat.de

Adv. Mater. 2014, 26, 1827–1845

[176] H. Tachibana, T. Nakamura, M. Matsumoto, H. Komizu, E. Manda, H. Niino, A. Yabe, Y. Kawabata, J. Am. Chem. Soc. 1989, 111, 3080. [177] T. Tsujioka, Y. Hamada, K. Shibata, A. Taniguchi, T. Fuyuki, Appl. Phys. Lett. 2001, 78, 2282. [178] T. Tsujioka, M. Irie, J. Photochem. Photobiol., C 2010, 11, 1. [179] T. Tsujioka, H. Kondo, Appl. Phys. Lett. 2003, 83, 937. [180] T. Tsujioka, K. Masuda, Appl. Phys. Lett. 2003, 83, 4978. [181] T. Tsujioka, M. Shimizu, E. Ishihara, Appl. Phys. Lett. 2005, 87. [182] T. Tsujioka, T. Sasa, Y. Kakihara, Org. Electron. 2012, 13, 681. [183] R. C. Shallcross, P. Zacharias, A. Koehnen, P. O. Koerner, E. Maibach, K. Meerholz, Adv. Mater. 2013, 25, 469. [184] R. C. Shallcross, P. O. Körner, E. Maibach, A. Köhnen, K. Meerholz, Adv. Mater. 2013, 25, 4807. [185] P. Zacharias, M. C. Gather, A. Koehnen, N. Rehmann, K. Meerholz, Angew. Chem. Int. Ed. 2009, 48, 4038. [186] Q. Shen, Y. Cao, S. Liu, M. L. Steigerwald, X. Guo, J. Phys. Chem. C 2009, 113, 10807. [187] F. L. E. Jakobsson, P. Marsal, S. Braun, M. Fahlman, M. Berggren, J. Cornil, X. Crispin, J. Phys. Chem. C 2009, 113, 18396. [188] S. Nespurek, J. Sworakowski, C. Combellas, G. Wang, M. Weiter, Appl. Surf. Sci. 2004, 234, 395. [189] P. Andersson, N. D. Robinson, M. Berggren, Adv. Mater. 2005, 17, 1798. [190] P. Andersson, N. D. Robinson, M. Berggren, Synth. Met. 2005, 150, 217. [191] M. Weiter, J. Navratil, M. Vala, P. Toman, Eur. Phys. J. Appl. Phys. 2009, 48. [192] M. Weiter, M. Vala, O. Zmeskal, S. Nespurek, P. Toman, Macromol Symp 2007, 247, 318. [193] P. Toman, S. Nespurek, Mol. Cryst. Liq. Cryst. 2008, 496, 25. [194] Y. R. Li, H. T. Zhang, C. M. Qi, X. F. Guo, J. Mater. Chem. 2012, 22, 4261. [195] Y. Ishiguro, R. Hayakawa, T. Chikyow, Y. Wakayama, J. Mater. Chem. C 2013, 1, 3012. [196] E. Orgiu, N. Crivillers, M. Herder, L. Grubert, M. Patzel, J. Frisch, E. Pavlica, D. T. Duong, G. Bratina, A. Salleo, N. Koch, S. Hecht, P. Samorì, Nat. Chem. 2012, 4, 675. [197] C. Raimondo, F. Reinders, U. Soydaner, M. Mayor, P. Samorì, Chem. Commun. 2010, 46, 1147. [198] C. Raimondo, N. Crivillers, F. Reinders, F. Sander, M. Mayor, P. Samorì, Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 12375. [199] A. Kahn, N. Koch, W. Y. Gao, J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 2529. [200] N. Koch, A. Kahn, J. Ghijsen, J. J. Pireaux, J. Schwartz, R. L. Johnson, A. Elschner, Appl. Phys. Lett. 2003, 82, 70. [201] S. Duhm, G. Heimel, I. Salzmann, H. Glowatzki, R. L. Johnson, A. Vollmer, J. P. Rabe, N. Koch, Nat. Mater. 2008, 7, 326. [202] J. C. Love, L. A. Estroff, J. K. Kriebel, R. G. Nuzzo, G. M. Whitesides, Chem. Rev. 2005, 105, 1103. [203] N. Crivillers, S. Osella, C. Van Dyck, G. M. Lazzerini, D. Cornil, A. Liscio, F. Di Stasio, S. Mian, O. Fenwick, F. Reinders, M. Neuburger, E. Treossi, M. Mayor, V. Palermo, F. Cacialli, J. Cornil, P. Samorì, Adv. Mater. 2013, 25, 432. [204] N. Crivillers, A. Liscio, F. Di Stasio, C. Van Dyck, S. Osella, D. Cornil, S. Mian, G. M. Lazzerini, O. Fenwick, E. Orgiu, F. Reinders, S. Braun, M. Fahlman, M. Mayor, J. Cornil, V. Palermo, F. Cacialli, P. Samorì, Phys. Chem. Chem. Phys. 2011, 13, 14302. [205] R. Klajn, Pure Appl. Chem. 2010, 82, 2247. [206] N. Crivillers, E. Orgiu, F. Reinders, M. Mayor, P. Samorì, Adv. Mater. 2011, 23, 1447. [207] M. Elbing, A. Blaszczyk, C. von Haenisch, M. Mayor, V. Ferri, C. Grave, M. A. Rampi, G. Pace, P. Samorì, A. Shaporenko, M. Zharnikov, Adv. Funct. Mater. 2008, 18, 2972.



REVIEW

[136] H. Tian, S. Yang, Chem. Soc. Rev. 2004, 33, 85. [137] S. Kobatake, S. Takami, H. Muto, T. Ishikawa, M. Irie, Nature 2007, 446, 778. [138] H. Tian, S. Wang, Chem. Commun. 2007, 781. [139] R. Arai, S. Uemura, M. Irie, K. Matsuda, J. Am. Chem. Soc. 2008, 130, 9371. [140] S. Yagai, K. Ishiwatari, X. Lin, T. Karatsu, A. Kitamura, S. Uemura, Chem. Eur. J. 2013, 19, 6971. [141] F. M. Raymo, M. Tomasulo, Chem. Soc. Rev. 2005, 34, 327. [142] I. Yildiz, E. Deniz, F. M. Raymo, Chem. Soc. Rev. 2009, 38, 1859. [143] Y. Yokoyama, Chem. Rev. 2000, 100, 1717. [144] Y. Kishimoto, J. Abe, J. Am. Chem. Soc. 2009, 131, 4227. [145] D. A. Parthenopoulos, P. M. Rentzepis, Science 1989, 245, 843. [146] S. Kawata, Y. Kawata, Chem. Rev. 2000, 100, 1777. [147] M. Irie, Jpn. J. Appl. Phys., Part 1 1989, 28, 215. [148] A. J. Myles, N. R. Branda, Adv. Funct. Mater. 2002, 12, 167. [149] B. L. Yao, Y. L. Wang, N. Menke, M. Lei, Y. Zheng, L. Y. Ren, G. F. Chen, Y. Chen, M. G. Fan, Mol. Cryst. Liq. Cryst. 2005, 430, 211. [150] Y. Y. Huang, W. Liang, J. K. S. Poon, Y. Xu, R. K. Lee, A. Yariv, Appl. Phys. Lett. 2006, 88. [151] F. M. Raymo, Adv. Mater. 2002, 14, 401. [152] T. Tsujioka, M. Irie, Appl. Optics 1999, 38, 5066. [153] T. Tsujioka, M. Irie, Appl. Opticss 1998, 37, 4419. [154] S. J. Lim, J. Seo, S. Y. Park, J. Am. Chem. Soc. 2006, 128, 14542. [155] K. Uchida, M. Saito, A. Murakami, S. Nakamura, M. Irie, Adv. Mater. 2003, 15, 121. [156] D. Dulic, S. J. van der Molen, T. Kudernac, H. T. Jonkman, J. J. D. de Jong, T. N. Bowden, J. van Esch, B. L. Feringa, B. J. van Wees, Phys. Rev. Lett. 2003, 91. [157] S. J. van der Molen, J. H. Liao, T. Kudernac, J. S. Agustsson, L. Bernard, M. Calame, B. J. van Wees, B. L. Feringa, C. Schonenberger, Nano Lett 2009, 9, 76. [158] J. Li, G. Speyer, O. F. Sankey, Phys. Rev. Lett. 2004, 93. [159] G. Speyer, J. Li, O. F. Sankey, Phys. Status Solidi B 2004, 241, 2326. [160] A. J. Kronemeijer, H. B. Akkerman, T. Kudernac, B. J. van Wees, B. L. Feringa, P. W. M. Blom, B. de Boer, Adv. Mater. 2008, 20, 1467. [161] A. C. Whalley, M. L. Steigerwald, X. Guo, C. Nuckolls, J. Am. Chem. Soc. 2007, 129, 12590. [162] A. Yassar, F. Garnier, H. Jaafari, N. Rebiere-Galy, M. Frigoli, C. Moustrou, A. Samat, R. Guglielmetti, Appl. Phys. Lett. 2002, 80, 4297. [163] R. Hayakawa, K. Higashiguchi, K. Matsuda, T. Chikyow, Y. Wakayama, ACS Appl. Mater. Interfaces 2013, 5, 3625. [164] X. F. Guo, D. Zhang, Y. Gui, M. X. Wax, J. C. Li, Y. Q. Liu, D. B. Zhu, Adv. Mater. 2004, 16, 636. [165] C. P. Harvey, J. D. Tovar, Polym Chem-Uk 2011, 2, 2699. [166] M. J. Marsella, Z. Q. Wang, R. H. Mitchell, Org. Lett. 2000, 2, 2979. [167] W. J. Tan, X. Li, J. J. Zhang, H. Tian, Dyes Pigm. 2011, 89, 260. [168] K. Wagner, R. Byrne, M. Zanoni, S. Gambhir, L. Dennany, R. Breukers, M. Higgins, P. Wagner, D. Diamond, G. G. Wallace, D. L. Officer, J. Am. Chem. Soc. 2011, 133, 5453. [169] H. Logtenberg, J. H. M. van der Velde, P. de Mendoza, J. Areephong, J. Hjelm, B. L. Feringa, W. R. Browne, J. Phys. Chem. C 2012, 116, 24136. [170] R. Klajn, Chem. Soc. Rev. 2013, DOI: 10.1039/c3cs60181a. [171] M. Pars, C. C. Hofmann, K. Willinger, P. Bauer, M. Thelakkat, J. Kohler, Angew. Chem. Int. Ed. 2011, 50, 11405. [172] H. Choi, H. Lee, Y. J. Kang, E. Kim, S. O. Kang, J. Ko, J. Org. Chem. 2005, 70, 8291. [173] T. Kawai, Y. Nakashima, M. Irie, Adv. Mater. 2005, 17, 309. [174] J. H. Yang, M. K. Ng, Synthesis-Stuttgart 2006, 3075. [175] H. Tachibana, H. Komizu, T. Nakamura, M. Matsumoto, M. Tanaka, E. Manda, Y. Kawabata, T. Kato, Chem . Lett. 1989, 841.

1843

REVIEW

www.advmat.de

1844

[208] G. Pace, V. Ferri, C. Grave, M. Elbing, C. von Hanisch, M. Zharnikov, M. Mayor, M. A. Rampi, P. Samorì, Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 9937. [209] S. Kobayashi, T. Nishikawa, T. Takenobu, S. Mori, T. Shimoda, T. Mitani, H. Shimotani, N. Yoshimoto, S. Ogawa, Y. Iwasa, Nat. Mater. 2004, 3, 317. [210] C. R. Kagan, A. Afzali, R. Martel, L. M. Gignac, P. M. Solomon, A. G. Schrott, B. Ek, Nano Lett 2003, 3, 119. [211] H. Zhang, X. Guo, J. Hui, S. Hu, W. Xu, D. Zhu, Nano Lett 2011, 11, 4939. [212] J. J. Zhang, Q. Zou, H. Tian, Adv. Mater. 2013, 25, 378. [213] A. Virkar, S. Mannsfeld, J. H. Oh, M. F. Toney, Y. H. Tan, G. Y. Liu, J. C. Scott, R. Miller, Z. Bao, Adv. Funct. Mater. 2009, 19, 1962. [214] H. S. Lee, D. H. Kim, J. H. Cho, M. Hwang, Y. Jang, K. Cho, J. Am. Chem. Soc. 2008, 130, 10556.


[215] C.-W. Tseng, D.-C. Huang, Y.-T. Tao, ACS Appl. Mater. Interfaces 2012, 4, 5483. [216] M. Yoshida, K. Suemori, S. Uemura, S. Hoshino, N. Takada, T. Kodzasa, T. Kamata, Jpn. J. Appl. Phys. 2010, 49. [217] P. Lutsyk, K. Janus, J. Sworakowski, G. Generali, R. Capelli, M. Muccini, J. Phys. Chem. C 2011, 115, 3106. [218] S. Nespurek, J. Sworakowski, Thin Solid Films 2001, 393, 168. [219] P. Toman, W. Bartkowiak, S. Nespurek, J. Sworakowski, R. Zalesny, Chem. Phys. 2005, 316, 267. [220] M. Vala, M. Weiter, O. Zmeskal, S. Nespurek, P. Toman, Macromol Symp 2008, 268, 125. [221] Q. A. Shen, L. J. Wang, S. Liu, Y. Cao, L. Gan, X. F. Guo, M. L. Steigerwald, Z. G. Shuai, Z. F. Liu, C. Nuckolls, Adv. Mater. 2010, 22, 3282.


Adv. Mater. 2014, 26, 1827–1845

Editorial Advisory Board Christoph Brabec (Chair) Manfred Waidhas (Chair) Zhenan Bao Peter Bruce Jaephil Cho Bruce Dunn Dirk Guldi Alan J. Heeger Wenping Hu John T. S. Irvine Réne A. J. Janssen Hagen Klauk Frederik C. Krebs Pooi See Lee Karl Leo Max Lu Paul Meredith David B. Mitzi Peter H.L. Notten John A. Rogers Debra Rolison Gregory D. Scholes Henning Sirringhaus Takao Someya Michael Strano Zhong Lin Wang Martin Winter Dongyuan Zhao

Vol. 3 • No. 1 • January • 2013

First Impact Factor

www.advenergymat.de at.de

10.043 Advanced Energy Materials is an international, interdisciplinary, English-language journal of original peer-reviewed contributions on materials used in all forms of energy harvesting, conversion and storage. Volume 4, 12 Issues in 2014. Print ISSN: 1614-6832. Online ISSN: 1614-6840. AENM-3-1-2013-U1-Cover.indd 1

12/20/12 4:00:25 AM

Cover picture by Harald Ade et al. DOI: 10.1002/aenm.201200377

Highly Cited Papers on solar cells, batteries, supercapacitors, fuel cells, thermoelectrics: Interdiffusion of PCBM and P3HT Reveals Miscibility in a Photovoltaically Active Blend N. D. Treat, M. A. Brady, G. Smith, M. F. Toney, E. J. Kramer, C. J. Hawker, M. L. Chabinyc

http://onlinelibrary.wiley.com/doi/10.1002/aenm.201000023/full Graphene–Cellulose Paper Flexible Supercapacitors Zhe Weng, Yang Su, Da-Wei Wang, Feng Li, Jinhong Du, Hui-Ming Cheng*

All-Solid-State Lithium-Ion Microbatteries: A Review of Various Three-Dimensional Concepts J. F. M. Oudenhoven, L. Baggetto, P. H. L. Notten http://onlinelibrary.wiley.com/doi/10.1002/aenm.201000002/full Beyond 11% Efficiency: Characteristics of State-of-the-Art Cu2ZnSn(S,Se)4 Solar Cells T. K. Todorov, J. Tang, S. Bag, Oki Gunawan, Tayfun Gokmen, Yu Zhu, and David B. Mitzi*

http://onlinelibrary.wiley.com/doi/10.1002/aenm.201100312/full


Recent Progress in the Development of Anode Materials for Solid Oxide Fuel Cells P. I. Cowin, C. T. G. Petit, R. Lan, J. T. S. Irvine, S. Tao

Thermoelectric Property Studies on Cu-Doped n-type CuxBi2Te2.7Se0.3 Nanocomposites W.-S. Liu, Q. Zhang, Y. Lan, S. Chen, X. Yan, Q. Zhang, H. Wang, D. Wang, G. Chen, Z. Ren



25th anniversary article: progress in chemistry and applications of functional indigos for organic electronics.

25th anniversary article: supramolecular materials for regenerative medicine.

Flexible Organic Electronics in Biology: Materials and Devices.

25th anniversary article: carbon nanotube- and graphene-based transparent conductive films for optoelectronic devices.

25th anniversary article: Designer hydrogels for cell cultures: a materials selection guide.

25th anniversary article: metal oxide particles in materials science: addressing all length scales.

25th anniversary article: semiconductor nanowires--synthesis, characterization, and applications.

25th anniversary article: label-free electrical biodetection using carbon nanostructures.

25th anniversary article: hybrid nanostructures based on two-dimensional nanomaterials.

25th anniversary article: organic photovoltaic modules and biopolymer supercapacitors for supply of renewable electricity: a perspective from Africa.

25th anniversary article: Rational design and applications of hydrogels in regenerative medicine.

25th anniversary article: A soft future: from robots and sensor skin to energy harvesters.

25th anniversary article: double emulsion templated solid microcapsules: mechanics and controlled release.

Spiers memorial lecture. Organic electronics: an organic materials perspective.

25th anniversary article: ordered polymer structures for the engineering of photons and phonons.

25th anniversary article: Bulk heterojunction solar cells: understanding the mechanism of operation.

25th anniversary article: scalable multiscale patterned structures inspired by nature: the role of hierarchy.

25th anniversary article: high-mobility hole and electron transport conjugated polymers: how structure defines function.

25th anniversary article: 25 years of fullerene research in electron transfer chemistry.

Editorial. The 25th Anniversary of the BMS.

[On the 25th anniversary of the PAPPS ].

Organic Materials for Time-Temperature Integrator Devices.

Highlights of the SVM 25th Anniversary and Scientific Sessions.

Materials and optimized designs for human-machine interfaces via epidermal electronics.