Enhanced performances in inverted small molecule solar cells by Ag nanoparticles Fangming Jin,1,2 Bei Chu,1 Wenlian Li,1,* Zisheng Su,1,4 Haifeng Zhao,1 and C. S. Lee3 1

State Key Laboratory of Luminescence and applications, Changchun Institute of Optics, Fine Mechanics, and Physics, Chinese Academy of Sciences, Changchun 130033, China 2 University of Chinese Academy of Sciences, Beijing 100039, China 3 Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Hong Kong, China 4 [email protected] *[email protected]

Abstract: We demonstrate a highly efficient inverted small molecular solar cell with integration of Ag nanoparticles (NPs) into the devices. The optimized device based on thermal evaporated Ag NPs provides a power conversion efficiency (PCE) of 4.87%, which offers 33% improvement than that of the reference device without Ag NPs. Such a high efficiency is mainly attributed to the improved electrical properties by virtue of the modification of the surface of ITO with Ag NPs and the enhanced light harvesting due to localized surface plasmon resonance (LSPR). The more detail enhanced mechanism of the PCE by introduction of Ag NPs is also discussed. ©2014 Optical Society of America OCIS codes: (040.5350) Photovoltaic; (250.5403) Plasmonics.

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#222359 - $15.00 USD Received 2 Sep 2014; revised 2 Oct 2014; accepted 9 Oct 2014; published 23 Oct 2014 (C) 2014 OSA 15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1669 | OPTICS EXPRESS A1669

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1. Introduction Organic solar cells (OSCs) are under intense investigation due to their potential to offer a low cost and low weight power generation. Continuous progress in designing new organic semiconductors, optimizing device architectures, and controlling active layer morphology has allowed OSC efficiencies dramatically improved. Although the perceived progresses of OSCs are attractive, their performance is limited by insufficient light absorption and low charge carrier mobility within the thin organic films. However, simply enhancing the absorbance with a thicker active layer typically leads to a rapid decrease in the internal quantum efficiency (IQE) due to low carrier mobility and short exciton diffusion lengths of the organic materials. Therefore, an essential aspect of developing high-efficiency OSCs lies in increasing the light absorption of an organic solar cell at a limited active film thickness. Recent years, metallic nanoparticles (NPs) were introduced into OSCs for highly improved light harvesting by utilizing the localized surface plasmonic resonances (LSPR) of metallic NPs [1–5]. NPs were included either between interfaces or inside the buffer or the active layers of OSC devices in order to promote absorption, thereby increasing the optical thickness of OSC materials for light harvesting. Due to LSPR excitation, NPs behave as subwavelength antennas for small NPs and the plasmonic near-field is coupled to the photoactive layer, increasing its effective absorption cross-section and thus exciton dissociation [6]. Many methods have been employed for the NPs fabrication such as thermal evaporation [7], chemical synthesis [8], electrodeposited [9], laser ablation in liquids [10]. Among these, thermal evaporation is a simple and cost-effective physical method for the preparation of metal NPs with dispersed sizes and has drawn much attention in recent years. While the efficiency of OSC has reached 10% [11], limited device stability remains one of the most significant roadblocks toward the wide success and commercialization of OSCs.

#222359 - $15.00 USD Received 2 Sep 2014; revised 2 Oct 2014; accepted 9 Oct 2014; published 23 Oct 2014 (C) 2014 OSA 15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1669 | OPTICS EXPRESS A1670

Using inverted configuration instead of conventional structure is regarded as valid method to improve devices stability [12]. Such improvement is realized by nonuse of strong acidity poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), or/and by avoiding the low work function metal top cathode oxidized in ambient conditions, which produces an insulating barrier that reduces the conductivity of the electrode and effectively increases the serial resistance of the device. Besides, it is also expected that the inverted configuration has the advantage over the normal configuration because of the vertical phase separation [13]. For an inverted planar heterojunction (PHJ) based small molecular OSC, what is more, unstable C60 or C70 layer is located at the inner side of the devices and can be shielded from ambient conditions, which is in favor of device stability. For realizing efficient and stable inverted configuration OSCs, cathode buffer layer plays a pivotal role. Several successful n-type buffer layers such as Ca [14], TiO2 [15], ZnO [16], and Cs2CO3 [17] have been adopted to alter the carrier selectivity of the ITO electrode and convert it to a cathode contact. An excited result was reported by Cao group, where they used conjugated polymer poly [(9,9-bis(3′-(N, N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)] (PFN) as a cathode buffer layer and highly efficient OSCs with PCE of 9.2% were extracted [18]. Although polymer inverted solar cells are widely studied, there are only a few reports about inverted small molecule solar cells [19] and plasmonic-enhanced OSCs with inverted structure are still very rare. In this work, vapor-deposited Ag NPs were incorporated into inverted small molecular OSCs. As a result, incorporated Ag NPs not only enhanced light harvesting but also worked as a cathode buffer layer combined with bathocuproine (BCP). A peak PCE of 4.87% was determined, which increased by 33% as compared to the reference cell without the NPs. The mechanism for rising PCE was also argued in more details. 2. Experimental methods The organic materials for fabrication were procured commercially and were used without further sublimation. Devices were fabricated on patterned ITO-coated glass substrates with a sheet resistance of 15 Ω/sq. Prior to deposition, the ITO surface was cleaned in a series of solvents and then treated by ultraviolet-ozone in a chamber for 15 min. All the layers, including Ag, were deposited onto the substrates in sequence via thermal evaporation in the vacuum chamber at a pressure of 5 × 10−4 Pa without a vacuum break. Deposition rates were monitored with a quartz oscillating crystal and controlled to be 1 Å/s for Ag, MoO3 and the organic layers, and 5 Å/s for the Al cathode. Current density -voltage (J − V) characteristics of the devices were measured with a Keithley 2400 source meter both in dark and illuminated with a Xe lamp with an AM 1.5 G filter, and the irradiation intensity was certified to be 100 mW/cm2. The incident photon to current conversion efficiency (IPCE) spectra was performed with a Stanford SR803 lock-in amplifier under monochromatic illumination at a chopping frequency of 130 Hz by a Stanford SR540 chopper. Scanning electron microscopy (SEM) images were measured on a Hitachi S4800. Absorption spectra were recorded with a Shimadzu UV-3101PC spectrophotometer. The surface topographies were imaged with a Bruker MultiMode 8 atomic force microscope (AFM) in tapping mode. All the measurements were carried out at room temperature under ambient conditions.

#222359 - $15.00 USD Received 2 Sep 2014; revised 2 Oct 2014; accepted 9 Oct 2014; published 23 Oct 2014 (C) 2014 OSA 15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1669 | OPTICS EXPRESS A1671

3. Results and discussion

Fig. 1. Normalized absorption spectra of the Ag films with different thickness on an ITO substrate.

The absorption spectra of the Ag films with various thicknesses heat-deposited on ITO substrate are presented in Fig. 1. The absorption peak at 440 nm in the absorption spectrum of 0.5 nm Ag NPs corresponds to the typical surface plasmon resonance of Ag, which clearly indicates the formation of Ag NPs [20]. With increasing the thickness of Ag film, absorption spectra band is broaden, and the absorption peak appears shifted towards the longer wavelength side, i.e., as the Ag film thickness reaches to 4 nm an absorption peak of 550 nm was observed. Such variation of the bsorption spectra can be understood by considering that the resonant wavelength region of metallic nanomaterials is typically determined by the nanomaterials size and shape, and the local dielectric environment [21]. From the top-view SEM images of Ag NPs with different thicknesses presented in Fig. 2, one can see that density of Ag NPs gradually increased with their distance decreased, rather than the shape or size changes (the average diameter of the Ag NPs is all about 4−5nm for different Ag thickness), indicating the distribution density of Ag NPs is mainly responsible for the absorption spectra variation with increasing the nominal thickness.

#222359 - $15.00 USD Received 2 Sep 2014; revised 2 Oct 2014; accepted 9 Oct 2014; published 23 Oct 2014 (C) 2014 OSA 15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1669 | OPTICS EXPRESS A1672

Fig. 2. Top-view SEM images of (a) 0.5, (b) 1, (c) 2, and (d) 4 nm Ag.

The device structure for a typical inverted planar heterojunction OSC device fabricated in this study is shown in Fig. 3(a). Here, tetraphenyldibenzoperiflanthene (DBP) was selected as a donor material. It was recently reported that DBP-based BHJ OSC achieves a high PCE of 6.4% and 8.1% by vacuum-deposited single junction manifesting that DBP is a good donor material [22, 23]. Inset of Fig. 3(b) shows the molecular structure of active layer DBP and C60. The amorphous DBP has a symmetrical molecular structure and is horizontally oriented, enabling the active layers in the OSC to be very thin while maintaining high absorption. In particular, DBP has shown high absorption in the range 550–600 nm wavebands (Fig. 3(b)), where the sun’s spectrum strength is strong. The large energy offset between EHOMO of DBP and ELUMO of acceptor C60 enable a large Voc. All merits mentioned above are in favor of achieving high PCE. Besides, since the LSPR peak (430: 530 nm) of the Ag NPs just located at the absorption wavelength region of C60 film, we speculate that a rising photovoltaic response would be obtained derived from the contribution of the advantage of LSPR.

#222359 - $15.00 USD Received 2 Sep 2014; revised 2 Oct 2014; accepted 9 Oct 2014; published 23 Oct 2014 (C) 2014 OSA 15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1669 | OPTICS EXPRESS A1673

Fig. 3. (a) Schematic of the inverted OSC device structure. (b) Absorption spectra of C60 and DBP films on ITO substrate. The inset is the molecular structure of DBP and C60.

The current density–voltage (J–V) characteristics of the optimized devices with or without Ag NPs are shown in Fig. 4, with their performances summarized in Table 1. Our best reference cell with no Ag NPs offers a short-circuit current (JSC) = 7.37 mA/cm2, open-circuit voltage (VOC) = 0.92 V, fill factor (FF) = 0.54, and PCE = 3.66%. It is shown that the inclusion of 0.5 and 1 nm Ag NPs sandwiched between ITO and BCP induces a significant improvement of both the device JSC, by 6% and 9%, and the FF, by 18.5% and 22%, whereas the VOC remains constant. As a result, a 24% and 33% increase in the device efficiencies are obtained, reaching to 4.57% and 4.87%, which is among the highest efficiencies reported for inverted OSCs based on small molecule planar heterojunction. Table 1. Comparison of the PCE, Jsc, FF, and VOC of the optimized OSC devices with different thickness of Ag. Thickness (nm) 0 0.5 1 2 4

Jsc (mA/cm2) 7.37 7.85 8.03 6.87 5.00

FF 0.54 0.64 0.66 0.66 0.67

Voc (V) 0.92 0.91 0.92 0.92 0.91

PCE (%) 3.66 4.57 4.87 4.17 3.05

#222359 - $15.00 USD Received 2 Sep 2014; revised 2 Oct 2014; accepted 9 Oct 2014; published 23 Oct 2014 (C) 2014 OSA 15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1669 | OPTICS EXPRESS A1674

Fig. 4. J–V characteristics under 1 sun, AM 1.5G illumination for optimized devices with different thickness of Ag.

A plot of FF and Jsc as a function of the BCP thickness with or without Ag NPs is indicated in Fig. 5(a) and 5(b). We can see that device performance strongly depends on the thickness of BCP. It shows that for the control device 2 nm BCP can improve FF and JSC enormously, but decrease severely with further increase the thickness of BCP. The improvement effect of 2 nm BCP can be easily understood by considering the exciton and hole block effect, optimized contact between C60 and ITO and the suppression effect of exciton quenching by Ag NPs. The decreased performance for thick BCP may come from two reasons: on the one hand, energy band bending occurs at the C60/BCP interface when BCP layer is thick, generating a considerable barrier for electrons transport and will induce charge accumulation at the C60/BCP interface in OSCs [24]; On the other hand, defect states created when metal cathode is deposited are largely absent for inverted organic-on-metal interfaces [25].

#222359 - $15.00 USD Received 2 Sep 2014; revised 2 Oct 2014; accepted 9 Oct 2014; published 23 Oct 2014 (C) 2014 OSA 15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1669 | OPTICS EXPRESS A1675

Fig. 5. (a) FF, (b) Jsc under 1 sun, AM 1.5G illumination for devices with or without Ag NPs as a function of BCP thickness.

From Fig. 5(b), we can see that introduce Ag NPs can significantly increase devices FF, especially for thick BCP devices. For example, including 1 nm Ag NPs to the 10 nm BCP devices improves FF from 0.447 to 0.586, and the champion FF with Ag NPs reaches to 0.68, which is closed to the FF of the conventional devices reported in [26]. It is pointed out that here Ag NPs work as a cathode buffer layer, like Ca, TiO2, ZnO, and Cs2CO3 mentioned above. We speculate that significant improvement of FF is attributed to a better charge transfer or transport resulting from a decreased series resistance and an increased electrical conductivity. The improvement of the electrical conductivity and the effect of cathode buffer by including Ag NPs were further affirmed by the dark J –V characteristics of inverted OSCs and the J-V curves of the electron-only devices. Figure 6(a) plots the dark J–V characteristics of inverted OSCs with or without Ag NPs inserted between ITO and 10 nm BCP. The increased current density in the J–V curves in the dark strongly indicates that the total series resistance of the diode devices is decreased with incorporation of interfacial Ag NPs. The same conclusion can be drawn from the J-V curves of the electron-only devices with the structure of ITO/Ag (0 or 2 nm)/BCP (20 nm)/C60 (100 nm)/BCP (20 nm)/Al. As show in Fig. 6(b), a great rise in current density values of the electron only devices when inserted 2 nm Ag on ITO has been observed. These characteristics are clear signs of improvement in electron extraction and transport to ITO cathode via the Ag/BCP interlayer.

#222359 - $15.00 USD Received 2 Sep 2014; revised 2 Oct 2014; accepted 9 Oct 2014; published 23 Oct 2014 (C) 2014 OSA 15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1669 | OPTICS EXPRESS A1676

Fig. 6. (a) Dark current of the 10 nm BCP devices with or without Ag NPs. (b) J-V curves of the electron-only devices of ITO/BCP (20 nm)/C60 (100 nm)/BCP (20 nm)/AL with or without 2 nm Ag on ITO substrate.

Fig. 7. 3D AFM images of (a) 1 nm Ag NPs and (b) 1nm Ag NPs covered by 5 nm BCP.

To illuminate the electrical conductivity improvement,we took AFM images of 1 nm Ag NPs covered with and without 5 nm BCP on ITO substrate (Fig. 7). It is clearly observed islands like Ag NPs dispersedly distributed on ITO substrate with a height about 5 nm. While the neat 1 nm Ag film has a root-mean-square (rms) roughness of 1.15 nm, rms roughness decreases to 0.821 nm for the surface covered by 5 nm BCP, and islands like Ag NPs cannot be recognized, which suggest that 5 nm BCP can cover Ag NPs uniformly and Ag NPs are embedded in BCP completely. The deposition of Ag and BCP layer by layer is more like Ag NPs doped in BCP, which is believed to enhance the conductivity of BCP enormously. Such electrical conductivity and series resistance improvement may attribute to the introduction of dopant states in the layer of BCP. Similar mental doping for increase the conductivity of organic materials has been reported in other reference [27].What is more, charge distribution at BCP/ electrode may change as inclusion mental Ag film with low workfunction, leading to energy shift in the gap states and eventually directly affected the electronic conductivity of BCP layers [28]. #222359 - $15.00 USD Received 2 Sep 2014; revised 2 Oct 2014; accepted 9 Oct 2014; published 23 Oct 2014 (C) 2014 OSA 15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1669 | OPTICS EXPRESS A1677

Fig. 8. Percentage enhancement in the IPCE of the devices with Ag NPs over the reference.

To elucidate the effect of the Ag NPs on improving the Jsc, IPCE was measured for the devices with or without Ag NPs, and the IPCE curves for the reference and plasmatic OSCs are displayed in Fig. 8, along with the increase in IPCE (ΔIPCE) and in device absorption (ΔAbsorption). We can see that the cell with DBP and C60 as the active layers offers a wide spectral response wavelength ranging from 300 to 700 nm. Compared to the pristine cell, IPCE increases remarkably upon the incorporation of Ag NPs, which complies with the enhanced Jsc observed. It is notable that the enhancement in the IPCE by Ag is more pronounced in the range of 300:500 nm, which is close to the plasmon resonance energies of Ag NPs and the wavelength where absorption increases. This suggests that device performance does be enhanced by LSPR effect. However, an impressive increase in external quantum efficiency at longer wavelengths (>500 nm) is also observed in silver samples. Such increasement in whole broad spectral range is ascribed to the improvement of the internal quantum efficiency as charge collection improves by introducing Ag NPs. Considering Jsc and FF enhancement contributed by the improved interface that the electron extraction efficiency enhances, absorption enhancement due to incorporation of Ag NPs is insignificant and provides only a minor contribution to PCE enhancement. Depositing 4 nm Ag on ITO lowers the device efficiency to 3.05% due to the severe reduction of Jsc, which may be attributed to the low light transmittance and a serious exciton quenching. In the Fig. 1 we can see that 4 nm Ag presents a strong absorption in a wide range from 300 nm 800 nm, much photons are absorbed by Ag film and cannot reach to organic materials to generate photocurrent. Besides, C60 has a weak absorption in the long wave (e.g.>500 nm), much longwave light is wasted due to the parasitic absorption of 4 nm Ag and cannot be used by LSPR. Thick Ag may also induce a much more serious exciton quenching. As presented in the Fig. 2, deposition 4 nm Ag results in a compact distribution of Ag nanoparticles on the ITO substrate. The following deposited thin BCP cannot cover the compact Ag nanoparticles completely and results in the direct contact between Ag and C60 leading to exciton quenching, hence decreased the JSC.

#222359 - $15.00 USD Received 2 Sep 2014; revised 2 Oct 2014; accepted 9 Oct 2014; published 23 Oct 2014 (C) 2014 OSA 15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1669 | OPTICS EXPRESS A1678

4. Conclusions In summary, we demonstrate a highly efficient inverted small molecule OSC with the integration of thermal evaporated Ag NPs on the ITO side in the devices. The plasmonic device exhibits a 33% increase in PCE from 3.66% to 4.87%. This efficiency gain is attributed to the improved electrical properties by virtue of modification of the ITO surface and the enhanced light harvesting due to localized surface plasmon resonance. Our findings provide a new way to utilize mental nanoparticles, where mental nanoparticles can be used as cathode buffer layer combined with BCP to modify ITO electrode in inverted configuration OSCs. The preparation of such buffer layer consisted of nanoparticles is easily and conveniently, and complex chemical synthesis or annealing process is avoided. Acknowledgments This work was supported by the National Natural Science Foundation of China (61376062, 61376022,61107082, and 11004187), and the Science and Technology Development Plan of Jilin Province (20140101094JC).

#222359 - $15.00 USD Received 2 Sep 2014; revised 2 Oct 2014; accepted 9 Oct 2014; published 23 Oct 2014 (C) 2014 OSA 15 December 2014 | Vol. 22, No. S7 | DOI:10.1364/OE.22.0A1669 | OPTICS EXPRESS A1679

Enhanced performances in inverted small molecule solar cells by Ag nanoparticles.

We demonstrate a highly efficient inverted small molecular solar cell with integration of Ag nanoparticles (NPs) into the devices. The optimized devic...
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