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Cite this: DOI: 10.1039/c5cc00711a Received 25th January 2015, Accepted 20th February 2015
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Photoswitchable organic field-effect transistors and memory elements comprising an interfacial photochromic layer† Lyubov A. Frolova, Pavel A. Troshin,* Diana K. Susarova, Alexander V. Kulikov, Nataliya A. Sanina and Sergey M. Aldoshin
DOI: 10.1039/c5cc00711a www.rsc.org/chemcomm
Optical memory elements based on photoswitchable organic fieldeffect transistors have been designed by using an interfacial layer of photochromic spirooxazine molecules sandwiched between semiconductor and dielectric layers. Optical and electrical programming of the designed devices leads to multiple discrete states demonstrating drastically different electrical characteristics (VTH, IDS) and advanced stability.
Various photochromic molecules undergoing facile photoswitching between two stable isomeric forms attract considerable attention as a promising family of materials for organic electronics.1 In particular, they are applied in the design of memory devices.2 A number of reports describe diode-type memory cell configurations comprising photochromic materials in the active layer sandwiched between two electrodes.3 Light-induced isomerization of photochromic component results in the formation of bipolar traps affecting transport of charges through the device.4 Alternatively, photochromic molecules arranged at the interface between the semiconductor layer and one of the electrodes allow one to control charge injection in the diode device. This approach has been used for elegant demonstration of large-area OLEDs with optical memory capabilities.5 In spite of considerable progress in the field of diode-type memory devices, the achieved photoswitching effects were not very high thus challenging their practical implementation. This might be one of the reasons why transistor-type memory elements comprising photochromic materials have been intensively developed within the last few years.2 According to one of the proposed concepts, a photochromic compound is introduced as the so called dopant into the semiconductor layer of organic field-effect transistors (OFETs).6 However, such modification results in charge trapping in the semiconductor layer which affects the electrical characteristics of OFETs to some extent. At the same time, the best devices of this
IPCP RAS, Semenov Prospect 1, Chernogolovka, 141432, Russia. E-mail: [email protected]; Fax: +7 496515 5420; Tel: +7 496522 1418 † Electronic supplementary information (ESI) available: Experimental procedures and transfer characteristics of photoswitchable OFETs. See DOI: 10.1039/c5cc00711a
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type showed drain current modulation of ca. 80–95% of the nominal (dark) IDS current value.7 The estimated switching coefficients (ksw = IDS(state 1)/IDS(state 2)) of 5–18 are comparable to the characteristics achieved in diode-type memory devices.3 Alternatively, thin layers (or even monolayers) of photochromic molecules are placed under source and/or drain electrodes in order to control charge injection in OFETs.8,9 The best achieved switching coefficients of 2–3 were inferior compared to the results obtained using the previously described approach.9 It is known that the highest density of the charge carriers in operating OFET flows in a few molecular layers of the semiconductor adjacent to the dielectric.10 Therefore, the most promising approach to design photoswitchable OFETs is to place the photochromic compound at the interface between the dielectric and semiconductor. Isomerization of the photochromic molecules changes significantly the dielectric permittivity of the hybrid dielectric11 and capacitance of the device12 thus governing the charge transport in the channel of the OFET. This approach has been pursued by several research groups, however, the achieved switching coefficients were also not high (ksw = 0.3–10).11–13 The reported devices operated typically at high voltages (30–100 V) and required UV light for current modulation. Here we report low-voltage photoswitchable OFETs demonstrating two or many stable discrete states characterized by switching coefficients ksw 4 1000. The architecture of these devices is shown schematically in Fig. 1. To construct such devices, the aluminum gate electrodes are initially subjected to the electrochemical passivation in order to grow thin AlOx dielectric layers. Afterwards, the photoactive spirooxazine layer is deposited by spin-coating from a solution in toluene. Fullerene C60 applied as an n-type semiconductor in this work was deposited by sublimation in vacuum. Finally, silver source and drain electrodes were evaporated forming a transistor channel with W = 1 mm and l = 60 mm (see the Experimental procedure in ESI†). It is known that spirooxazine (SpOx) undergoes photoisomerization to the open zwitterionic form under illumination in solution with UV or violet light. Back isomerization occurs when the system
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Fig. 1 Architecture of the designed photoswitchable OFETs. Spirooxazine undergoes reversible isomerization under simultaneous action of light and electric field (programming voltage Vp).
is illuminated with visible light. Very similar processes might occur also in the photochromic thin films incorporated in the structure of the OFET. Indeed, illumination of the C60–SpOx bilayer films with the violet light (l = 405 nm) promotes the formation of the zwitterionic form as it can be concluded from the increased intensity of the 520–660 nm absorption band. At the same time, illumination of the films with green light (532 nm) leads to decrease in the intensity of this band thus suggesting that zwitterionic form is converted back to the neutral isomer under these conditions (Fig. 2a). It is notable that illumination of the OFET under zero bias with violet or green light leads to a very minor photoswitching effect (Fig. S1, ESI†). It is very likely that spirooxazine does not undergo photoisomerization at the interface with the semiconductor ([60]fullerene) which is responsible for the charge transport in the channel of the transistor. The situation becomes drastically different when violet light is applied simultaneously with the electrical bias between source and gate electrodes of the device (programming voltage Vp, Fig. 1). Fig. 2b illustrates a strong photoswitching effect which was fully reproducible for ca. 50 investigated OFETs. It is rather remarkable that we can switch the transistor between multiple discrete states using different programming voltages. The switching time can be set between 10 and 500 ms depending on the desired switching coefficient (longer time results in a higher ksw values, Fig. S2, ESI†). Other striking finding was that applying violet (405 nm), blue (460 nm), green (532 nm) or red (650 nm) light together with VP induced very similar programming effects (Fig. 2 and Fig. S3, ESI†). Fig. 2c shows that OFET threshold voltage (VTH) depends almost linearly on the bias voltage VP regardless the wavelength of the light which was applied at the programming step. However, the observed switching of the transistor has a clear photoinduced nature since applying the electric field (bias VP) in the absence of light shows a very minor influence on the threshold voltage (Fig. 2c). It is rather clear that the observed programming behavior of photoswitchable OFETs cannot be achieved via classical forward and backward photoisomerization of spirooxazine. It is possible that photoinduced charge separation between the spirooxazine (electron donor) and [60]fullerene (electron acceptor) facilitates the isomerization of SpOx and changes the electrical characteristics of the transistor. However, realization of this pathway was not supported by light-induced electron spin resonance (LESR)
Chem. Commun.
Fig. 2 Evolution of the absorption spectra of C60–SpOx under illumination with violet and green light (a). Transfer characteristics of the photoswitchable OFET obtained under different programming conditions (b). Threshold voltage as a function of the programming voltage (c).
spectroscopy (T = 100 K) which did not reveal any charge separated states in the SpOx–C60 composites. Alternatively, isomerization of SpOx molecules in the ground or excited state might be induced by injection of charges from the source electrode of the transistor. A similar behavior was reported previously for photochromic diarylethenes.14 To verify this hypothesis, we fabricated diode devices comprising bare spirooxazine (ITO–SpOx–Ag) and its stack with C60 (ITO–SpOx–C60–Ag). The behavior of these systems was studied by applying electrical bias and/or light pulses to the diodes and then measuring their reflectance spectra. Indeed, it was shown that positive electrical bias applied together with the light pulse to the ITO–SpOx–Ag diode increases the concentration of the zwitterionic form of SpOx in the films. On the contrary, applying negative bias shifts the equilibrium towards neutral SpOx isomer (Fig. S4a, ESI†). Note that the reference ITO–C60–Ag diodes showed no changes in spectral characteristics under these conditions (Fig. S5, ESI†). Applying the positive electrical bias (+2 V) and light (405 nm) to the bilayer ITO–SpOx–C60–Ag diodes resulted mainly in the bleaching of the 630–860 nm band. This band reappeared again with the same intensity when negative bias and light were
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applied (Fig. S4b, ESI†). The observed optical switching of the system was fully reversible and could be repeated many times. The origin of the 630–860 nm band is particularly interesting. Most probably, it is a signature of the charge transfer (CT) states formed at the interface between the SpOx and C60 layers. This broad band appeared also in the absorption spectra of the SpOx–C60 bilayer films (Fig. S5, ESI†). It should be noted that other known fullerene-based CT complexes show similar low intense broad bands in absorption spectra.15 The obtained results proved that simultaneous application of the electrical bias and light can govern not only the isomerization of SpOx but also the formation of CT states at the SpOx/C60 interface. The latter process seems to give the strongest contribution to the photoswitching of the OFETs reported in this work. This pathway also explains why using violet, blue, green or red light induces the same device programming effects. Considering the potential multibit memory applications of the designed devices, special attention has to be paid to their stability and reliability as well as to the achievable current ratios for ‘‘on’’ and ‘‘off’’ states. Fig. 3a shows that the devices can be switched many times between any two arbitrary selected states with a high accuracy. No noticeable degradation was observed in the device characteristics when they were periodically measured inside a nitrogen box for ca. 300 times within 1 year. Fig. 3b shows ten manually recorded ‘‘write–read–erase’’ cycles which demonstrate appreciable reproducibility and cycling stability of the devices. Retention characteristics presented in Fig. 3c also prove the
Fig. 3 Transfer characteristics of the OFET switched between two arbitrary selected states (a); ten manually recorded write–read–erase cycles for one of the devices (b); retention characteristics of the device in ‘‘on’’ and ‘‘off’’ states (c).
This journal is © The Royal Society of Chemistry 2015
Communication
capability of applying the designed photoswitchable OFETs as memory elements. It should also be noted that the ratio between the device drain currents in ‘‘on’’ and ‘‘off’’ states (switching coefficient) stays typically between 103 and 104 which is about two or three orders of magnitude higher compared to the previously reported values for photoswitchable OFETs comprising photochromic materials. The devices presented in this work also demonstrate faster switching rates (Table S1, ESI†). In conclusion, we have shown that introduction of photosensitive spirooxazine film at the interface between the semiconductor and dielectric layers in OFET represents a highly promising approach for construction of multibit optical memory elements with advanced electrical characteristics, good stability and reliability. Further exploration of this concept and using different combinations of photochromic and semiconductor materials might lead to development of industrially interesting technologies of novel types of memory devices and light sensors based on organic materials. This work was supported by the Russian Foundation for Basic Research (grants 13-03-01170 and 15-03-06175) and Russian President Science Foundation (MK-5260.2014.3).
Notes and references 1 T. Tsujioka and M. Irie, J. Photochem. Photobiol., C, 2010, 11, 1; R. C. Shallcross, P. Zacharias, A. Koehnen, P. O. Koerner, E. Maibach and K. Meerholz, Adv. Mater., 2013, 25, 469; J. J. Zhang, Q. Zou and H. Tian, Adv. Mater., 2013, 25, 378. 2 E. Orgiu and P. Samorı`, Adv. Mater., 2014, 26, 1827. 3 T. Tsujioka and H. Kondo, Appl. Phys. Lett., 2003, 83, 937; T. Tsujioka and K. Masuda, Appl. Phys. Lett., 2003, 83, 4978; T. Tsujioka, M. Shimizu and E. Ishihara, Appl. Phys. Lett., 2005, 87, 213506; P. Andersson, N. D. Robinson and M. Berggren, Synth. Met., 2005, 150, 217; M. Weiter, M. Vala, O. Zmeskal, S. Nespurek and P. Toman, Macromol. Symp., 2007, 247, 318. 4 S. Nespurek, J. Sworakowski, C. Combellas, G. Wang and M. Weiter, Appl. Surf. Sci., 2004, 234, 395; P. Andersson, N. D. Robinson and M. Berggren, Adv. Mater., 2005, 17, 1798. 5 P. Zacharias, M. C. Gather, A. Koehnen, N. Rehmann and K. Meerholz, Angew. Chem., Int. Ed., 2009, 48, 4038. 6 Y. R. Li, H. T. Zhang, C. M. Qi and X. F. Guo, J. Mater. Chem., 2012, 22, 4261; C. Raimondo, N. Crivillers, F. Reinders, F. Sander, M. Mayor and P. Samorı`, Proc. Natl. Acad. Sci. U. S. A., 2012, 109, 12375; Y. Ishiguro, R. Hayakawa, T. Chikyow and Y. Wakayama, J. Mater. Chem. C, 2013, 1, 3012. 7 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 and P. Samorı`, Nat. Chem., 2012, 4, 675. 8 Q. Shen, Y. Cao, S. Liu, M. L. Steigerwald and X. Guo, J. Phys. Chem. C, 2009, 113, 10807. 9 N. Crivillers, E. Orgiu, F. Reinders, M. Mayor and P. Samorı`, Adv. Mater., 2011, 23, 1447. 10 R. Ruiz, A. Papadimitratos, A. C. Mayer and G. G. Malliaras, Adv. Mater., 2005, 17, 1795. 11 P. Lutsyk, K. Janus, J. Sworakowski, G. Generali, R. Capelli and M. Muccini, J. Phys. Chem. C, 2011, 115, 3106. 12 Q. A. Shen, L. J. Wang, S. Liu, Y. Cao, L. Gan, X. F. Guo, M. L. Steigerwald, Z. G. Shuai, Z. F. Liu and C. Nuckolls, Adv. Mater., 2010, 22, 3282. 13 H. Zhang, X. Guo, J. Hui, S. Hu, W. Xu and D. Zhu, Nano Lett., 2011, 11, 4939; C.-W. Tseng, D.-C. Huang and Y.-T. Tao, ACS Appl. Mater. Interfaces, 2012, 4, 5483; M. Yoshida, K. Suemori, S. Uemura, S. Hoshino, N. Takada, T. Kodzasa and T. Kamata, Jpn. J. Appl. Phys., 2010, 49. 14 T. Tsujioka, Y. Hamada, K. Shibata, A. Taniguchi and T. Fuyuki, ¨rner, Appl. Phys. Lett., 2001, 78, 2282; R. C. Shallcross, P. O. Ko ¨hnen and K. Meerholz, Adv. Mater., 2013, 25, 4807. E. Maibach, A. Ko 15 D. V. Konarev, R. N. Lyubovskaya, N. V. Drichko, V. N. Semkin and A. Graja, Chem. Phys. Lett., 1999, 314, 570.
Chem. Commun.
Published on 20 February 2015. Downloaded by University of Pittsburgh on 12/03/2015 20:31:29.
COMMUNICATION
Cite this: DOI: 10.1039/c5cc00711a Received 25th January 2015, Accepted 20th February 2015
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Photoswitchable organic field-effect transistors and memory elements comprising an interfacial photochromic layer† Lyubov A. Frolova, Pavel A. Troshin,* Diana K. Susarova, Alexander V. Kulikov, Nataliya A. Sanina and Sergey M. Aldoshin
DOI: 10.1039/c5cc00711a www.rsc.org/chemcomm
Optical memory elements based on photoswitchable organic fieldeffect transistors have been designed by using an interfacial layer of photochromic spirooxazine molecules sandwiched between semiconductor and dielectric layers. Optical and electrical programming of the designed devices leads to multiple discrete states demonstrating drastically different electrical characteristics (VTH, IDS) and advanced stability.
Various photochromic molecules undergoing facile photoswitching between two stable isomeric forms attract considerable attention as a promising family of materials for organic electronics.1 In particular, they are applied in the design of memory devices.2 A number of reports describe diode-type memory cell configurations comprising photochromic materials in the active layer sandwiched between two electrodes.3 Light-induced isomerization of photochromic component results in the formation of bipolar traps affecting transport of charges through the device.4 Alternatively, photochromic molecules arranged at the interface between the semiconductor layer and one of the electrodes allow one to control charge injection in the diode device. This approach has been used for elegant demonstration of large-area OLEDs with optical memory capabilities.5 In spite of considerable progress in the field of diode-type memory devices, the achieved photoswitching effects were not very high thus challenging their practical implementation. This might be one of the reasons why transistor-type memory elements comprising photochromic materials have been intensively developed within the last few years.2 According to one of the proposed concepts, a photochromic compound is introduced as the so called dopant into the semiconductor layer of organic field-effect transistors (OFETs).6 However, such modification results in charge trapping in the semiconductor layer which affects the electrical characteristics of OFETs to some extent. At the same time, the best devices of this
IPCP RAS, Semenov Prospect 1, Chernogolovka, 141432, Russia. E-mail: [email protected]; Fax: +7 496515 5420; Tel: +7 496522 1418 † Electronic supplementary information (ESI) available: Experimental procedures and transfer characteristics of photoswitchable OFETs. See DOI: 10.1039/c5cc00711a
This journal is © The Royal Society of Chemistry 2015
type showed drain current modulation of ca. 80–95% of the nominal (dark) IDS current value.7 The estimated switching coefficients (ksw = IDS(state 1)/IDS(state 2)) of 5–18 are comparable to the characteristics achieved in diode-type memory devices.3 Alternatively, thin layers (or even monolayers) of photochromic molecules are placed under source and/or drain electrodes in order to control charge injection in OFETs.8,9 The best achieved switching coefficients of 2–3 were inferior compared to the results obtained using the previously described approach.9 It is known that the highest density of the charge carriers in operating OFET flows in a few molecular layers of the semiconductor adjacent to the dielectric.10 Therefore, the most promising approach to design photoswitchable OFETs is to place the photochromic compound at the interface between the dielectric and semiconductor. Isomerization of the photochromic molecules changes significantly the dielectric permittivity of the hybrid dielectric11 and capacitance of the device12 thus governing the charge transport in the channel of the OFET. This approach has been pursued by several research groups, however, the achieved switching coefficients were also not high (ksw = 0.3–10).11–13 The reported devices operated typically at high voltages (30–100 V) and required UV light for current modulation. Here we report low-voltage photoswitchable OFETs demonstrating two or many stable discrete states characterized by switching coefficients ksw 4 1000. The architecture of these devices is shown schematically in Fig. 1. To construct such devices, the aluminum gate electrodes are initially subjected to the electrochemical passivation in order to grow thin AlOx dielectric layers. Afterwards, the photoactive spirooxazine layer is deposited by spin-coating from a solution in toluene. Fullerene C60 applied as an n-type semiconductor in this work was deposited by sublimation in vacuum. Finally, silver source and drain electrodes were evaporated forming a transistor channel with W = 1 mm and l = 60 mm (see the Experimental procedure in ESI†). It is known that spirooxazine (SpOx) undergoes photoisomerization to the open zwitterionic form under illumination in solution with UV or violet light. Back isomerization occurs when the system
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Fig. 1 Architecture of the designed photoswitchable OFETs. Spirooxazine undergoes reversible isomerization under simultaneous action of light and electric field (programming voltage Vp).
is illuminated with visible light. Very similar processes might occur also in the photochromic thin films incorporated in the structure of the OFET. Indeed, illumination of the C60–SpOx bilayer films with the violet light (l = 405 nm) promotes the formation of the zwitterionic form as it can be concluded from the increased intensity of the 520–660 nm absorption band. At the same time, illumination of the films with green light (532 nm) leads to decrease in the intensity of this band thus suggesting that zwitterionic form is converted back to the neutral isomer under these conditions (Fig. 2a). It is notable that illumination of the OFET under zero bias with violet or green light leads to a very minor photoswitching effect (Fig. S1, ESI†). It is very likely that spirooxazine does not undergo photoisomerization at the interface with the semiconductor ([60]fullerene) which is responsible for the charge transport in the channel of the transistor. The situation becomes drastically different when violet light is applied simultaneously with the electrical bias between source and gate electrodes of the device (programming voltage Vp, Fig. 1). Fig. 2b illustrates a strong photoswitching effect which was fully reproducible for ca. 50 investigated OFETs. It is rather remarkable that we can switch the transistor between multiple discrete states using different programming voltages. The switching time can be set between 10 and 500 ms depending on the desired switching coefficient (longer time results in a higher ksw values, Fig. S2, ESI†). Other striking finding was that applying violet (405 nm), blue (460 nm), green (532 nm) or red (650 nm) light together with VP induced very similar programming effects (Fig. 2 and Fig. S3, ESI†). Fig. 2c shows that OFET threshold voltage (VTH) depends almost linearly on the bias voltage VP regardless the wavelength of the light which was applied at the programming step. However, the observed switching of the transistor has a clear photoinduced nature since applying the electric field (bias VP) in the absence of light shows a very minor influence on the threshold voltage (Fig. 2c). It is rather clear that the observed programming behavior of photoswitchable OFETs cannot be achieved via classical forward and backward photoisomerization of spirooxazine. It is possible that photoinduced charge separation between the spirooxazine (electron donor) and [60]fullerene (electron acceptor) facilitates the isomerization of SpOx and changes the electrical characteristics of the transistor. However, realization of this pathway was not supported by light-induced electron spin resonance (LESR)
Chem. Commun.
Fig. 2 Evolution of the absorption spectra of C60–SpOx under illumination with violet and green light (a). Transfer characteristics of the photoswitchable OFET obtained under different programming conditions (b). Threshold voltage as a function of the programming voltage (c).
spectroscopy (T = 100 K) which did not reveal any charge separated states in the SpOx–C60 composites. Alternatively, isomerization of SpOx molecules in the ground or excited state might be induced by injection of charges from the source electrode of the transistor. A similar behavior was reported previously for photochromic diarylethenes.14 To verify this hypothesis, we fabricated diode devices comprising bare spirooxazine (ITO–SpOx–Ag) and its stack with C60 (ITO–SpOx–C60–Ag). The behavior of these systems was studied by applying electrical bias and/or light pulses to the diodes and then measuring their reflectance spectra. Indeed, it was shown that positive electrical bias applied together with the light pulse to the ITO–SpOx–Ag diode increases the concentration of the zwitterionic form of SpOx in the films. On the contrary, applying negative bias shifts the equilibrium towards neutral SpOx isomer (Fig. S4a, ESI†). Note that the reference ITO–C60–Ag diodes showed no changes in spectral characteristics under these conditions (Fig. S5, ESI†). Applying the positive electrical bias (+2 V) and light (405 nm) to the bilayer ITO–SpOx–C60–Ag diodes resulted mainly in the bleaching of the 630–860 nm band. This band reappeared again with the same intensity when negative bias and light were
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View Article Online
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applied (Fig. S4b, ESI†). The observed optical switching of the system was fully reversible and could be repeated many times. The origin of the 630–860 nm band is particularly interesting. Most probably, it is a signature of the charge transfer (CT) states formed at the interface between the SpOx and C60 layers. This broad band appeared also in the absorption spectra of the SpOx–C60 bilayer films (Fig. S5, ESI†). It should be noted that other known fullerene-based CT complexes show similar low intense broad bands in absorption spectra.15 The obtained results proved that simultaneous application of the electrical bias and light can govern not only the isomerization of SpOx but also the formation of CT states at the SpOx/C60 interface. The latter process seems to give the strongest contribution to the photoswitching of the OFETs reported in this work. This pathway also explains why using violet, blue, green or red light induces the same device programming effects. Considering the potential multibit memory applications of the designed devices, special attention has to be paid to their stability and reliability as well as to the achievable current ratios for ‘‘on’’ and ‘‘off’’ states. Fig. 3a shows that the devices can be switched many times between any two arbitrary selected states with a high accuracy. No noticeable degradation was observed in the device characteristics when they were periodically measured inside a nitrogen box for ca. 300 times within 1 year. Fig. 3b shows ten manually recorded ‘‘write–read–erase’’ cycles which demonstrate appreciable reproducibility and cycling stability of the devices. Retention characteristics presented in Fig. 3c also prove the
Fig. 3 Transfer characteristics of the OFET switched between two arbitrary selected states (a); ten manually recorded write–read–erase cycles for one of the devices (b); retention characteristics of the device in ‘‘on’’ and ‘‘off’’ states (c).
This journal is © The Royal Society of Chemistry 2015
Communication
capability of applying the designed photoswitchable OFETs as memory elements. It should also be noted that the ratio between the device drain currents in ‘‘on’’ and ‘‘off’’ states (switching coefficient) stays typically between 103 and 104 which is about two or three orders of magnitude higher compared to the previously reported values for photoswitchable OFETs comprising photochromic materials. The devices presented in this work also demonstrate faster switching rates (Table S1, ESI†). In conclusion, we have shown that introduction of photosensitive spirooxazine film at the interface between the semiconductor and dielectric layers in OFET represents a highly promising approach for construction of multibit optical memory elements with advanced electrical characteristics, good stability and reliability. Further exploration of this concept and using different combinations of photochromic and semiconductor materials might lead to development of industrially interesting technologies of novel types of memory devices and light sensors based on organic materials. This work was supported by the Russian Foundation for Basic Research (grants 13-03-01170 and 15-03-06175) and Russian President Science Foundation (MK-5260.2014.3).
Notes and references 1 T. Tsujioka and M. Irie, J. Photochem. Photobiol., C, 2010, 11, 1; R. C. Shallcross, P. Zacharias, A. Koehnen, P. O. Koerner, E. Maibach and K. Meerholz, Adv. Mater., 2013, 25, 469; J. J. Zhang, Q. Zou and H. Tian, Adv. Mater., 2013, 25, 378. 2 E. Orgiu and P. Samorı`, Adv. Mater., 2014, 26, 1827. 3 T. Tsujioka and H. Kondo, Appl. Phys. Lett., 2003, 83, 937; T. Tsujioka and K. Masuda, Appl. Phys. Lett., 2003, 83, 4978; T. Tsujioka, M. Shimizu and E. Ishihara, Appl. Phys. Lett., 2005, 87, 213506; P. Andersson, N. D. Robinson and M. Berggren, Synth. Met., 2005, 150, 217; M. Weiter, M. Vala, O. Zmeskal, S. Nespurek and P. Toman, Macromol. Symp., 2007, 247, 318. 4 S. Nespurek, J. Sworakowski, C. Combellas, G. Wang and M. Weiter, Appl. Surf. Sci., 2004, 234, 395; P. Andersson, N. D. Robinson and M. Berggren, Adv. Mater., 2005, 17, 1798. 5 P. Zacharias, M. C. Gather, A. Koehnen, N. Rehmann and K. Meerholz, Angew. Chem., Int. Ed., 2009, 48, 4038. 6 Y. R. Li, H. T. Zhang, C. M. Qi and X. F. Guo, J. Mater. Chem., 2012, 22, 4261; C. Raimondo, N. Crivillers, F. Reinders, F. Sander, M. Mayor and P. Samorı`, Proc. Natl. Acad. Sci. U. S. A., 2012, 109, 12375; Y. Ishiguro, R. Hayakawa, T. Chikyow and Y. Wakayama, J. Mater. Chem. C, 2013, 1, 3012. 7 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 and P. Samorı`, Nat. Chem., 2012, 4, 675. 8 Q. Shen, Y. Cao, S. Liu, M. L. Steigerwald and X. Guo, J. Phys. Chem. C, 2009, 113, 10807. 9 N. Crivillers, E. Orgiu, F. Reinders, M. Mayor and P. Samorı`, Adv. Mater., 2011, 23, 1447. 10 R. Ruiz, A. Papadimitratos, A. C. Mayer and G. G. Malliaras, Adv. Mater., 2005, 17, 1795. 11 P. Lutsyk, K. Janus, J. Sworakowski, G. Generali, R. Capelli and M. Muccini, J. Phys. Chem. C, 2011, 115, 3106. 12 Q. A. Shen, L. J. Wang, S. Liu, Y. Cao, L. Gan, X. F. Guo, M. L. Steigerwald, Z. G. Shuai, Z. F. Liu and C. Nuckolls, Adv. Mater., 2010, 22, 3282. 13 H. Zhang, X. Guo, J. Hui, S. Hu, W. Xu and D. Zhu, Nano Lett., 2011, 11, 4939; C.-W. Tseng, D.-C. Huang and Y.-T. Tao, ACS Appl. Mater. Interfaces, 2012, 4, 5483; M. Yoshida, K. Suemori, S. Uemura, S. Hoshino, N. Takada, T. Kodzasa and T. Kamata, Jpn. J. Appl. Phys., 2010, 49. 14 T. Tsujioka, Y. Hamada, K. Shibata, A. Taniguchi and T. Fuyuki, ¨rner, Appl. Phys. Lett., 2001, 78, 2282; R. C. Shallcross, P. O. Ko ¨hnen and K. Meerholz, Adv. Mater., 2013, 25, 4807. E. Maibach, A. Ko 15 D. V. Konarev, R. N. Lyubovskaya, N. V. Drichko, V. N. Semkin and A. Graja, Chem. Phys. Lett., 1999, 314, 570.
Chem. Commun.
