2008-2009 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Chan, HLW

Mok, SM

Yan, F

Applied Physics Letters

PolyU Institutional Repository

Organic phototransistor based on poly„3-hexylthiophene…/TiO2 nanoparticlecompositeSheung Man Mok, Feng Yan,a and Helen L. W. ChanDepartment of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, ChinaReceived 20 March 2008; accepted 24 June 2008; published online 18 July 2008Light sensitive phototransistor based on the composite of poly3-hexylthiophene and TiO2nanoparticles has been developed. The device shows a quick change in channel current under lightexposure, which can be attributed to a positive shift of the threshold voltage, while no change in thefield effect mobility and off current can be observed. The shift of the threshold voltage is inducedby accumulated electrons trapped by the TiO2 nanoparticles in the channel. The photosensitivity ofthe device has been found to be dependent on the concentration of TiO2 nanoparticles, the incidentwavelength and the voltage between the source and drain. © 2008 American Institute of Physics.DOI: 10.1063/1.2957981The study on organic materials including small mol-ecules and polymers for their applications in the electronicsand semiconductor industry has been of great scientific andtechnological interest for several decades. Although elec-tronic materials research has been dominated by the impres-sive performance of inorganic materials for many years, theattraction of using organic or plastic devices is driven bymany virtues of organic materials, including flexibility, lowcost, light weight, solution processibility, biocompatibility,and environmental friendliness.1–3Phototransistors are a type of optical transducer in whichlight detection and signal amplification are combined in asingle device without the noise increment associated withavalanche photodiodes.4 Organic phototransistors OPTs areconsidered to be one of the feasible application of organicthin film transistor OTFT because of their large adsorptionproperties in ultraviolet UV and visible light and the excel-lent photocurrent generation efficiency of organicsemiconductors.5–15 More importantly, some OPTs can befabricated by solution process,13–15 such as printing or spincoating, at room temperature and therefore can be easily in-tegrated in smart clothes, packages, and biological systemsas light sensors, biosensors, and multifunctional sensors.OPTs fabricated by thermal evaporation of someoligomers including pentacene,5–8 2,5-bis-biphenyl-4-yl-thieno3,2-bthiophene,9 rubrene,10 N-7-N ,N-di-phenylamino-9 ,9-spirobifluoren-2-yl- N-2,5-di-tert-but-ylphenyl-3,4:9,10-perylenetetracarboxylicdiimide,11 and2,7-bis-N ,N-diphenylamino-2,7-bisbiphenyl-4-yl-9,9-spirobifluorene,12 have been found to be very photosensitive.Normally, a positive shift of the threshold voltage of a OPTcan be observed under illumination, which can be attributedto trapping of electrons generated by photons at the insulator/semiconductor interface of the device. However, as ex-plained by Debucquoy et al.,5 the photosensitivity of penta-cene organic TFTs shares the same mechanism of biasstressing effect. Therefore the photosensitive behavior of thistype of phototransistor is normally associated with the unsta-bility of the device performance, which will be a shortcom-ing in its real application.Solution processible OPTs based on conjugated poly-mers, such as poly9,9-dioctylfluorene-co-bithiophene,12poly3-octylthiophene-2,5-diyl,13 and blends of poly2-methoxy-5-3,7-dimethyloctyloxy-1,4-phenylenevinyleneMDMO-PPV and methanof 6,6-phenyl C61-butyric acidmethyl ester14 PCBM have been reported. Light illumina-tion can induce pronounced changes both in threshold volt-age and off current of the devices, which can be attributed totrapping of electrons at the semiconductor/insulator interfaceand light induced photocurrent, respectively. It is worth not-ing that all of the OPT devices based on conjugated polymeror oligomers that have been reported show relatively longresponse time 1 s to light illumination, which may limittheir applications. Although most papers are focusing on themagnitude of current change under illumination, we considerstability and fast response under light illumination is alsovery important for the application of OPTs.Semiconducting polymer poly3-hexylthiopheneP3HT Ref. 1 is solution processible and feasible for rollto roll process. It has a field effect mobility up to0.1 cm2 /V s,16 which is near the value for amorphous sili-con, thus it is a promising material for applications in OTFTsand solar cells. However, OPT devices based on P3HT haverarely been reported. We find that OTFTs based onP3HT /TiO2 nanoparticle composite film are photosensitiveand show promising performance in terms of response timeand stability, which will be reported in this paper.The phototransistor is fabricated on a SiO2 /n-type Sisubstrate. The resistivity of the n-type Si is 0.001  cm. Thethickness of the SiO2 is 300 nm. Au source/drain electrodesare deposited through a shadow mask on top of the SiO2 filmby thermal evaporation with channel width/length of2000 /200 m. Here, the n-type Si layer acts as the gateelectrode and the SiO2 layer the gate insulator. RegioregularP3HT with head-to-tail linkages of 98% was purchased fromSigma-Aldrich, and used as received without additional pu-rification. TiO2 nanopowder with the size of 50–100 nm anda mixture phase of anatase and rutile was purchased alsofrom Sigma-Aldrich. P3HT and TiO2 powders are dissolvedin chloroform and spin coated on the substrate in air. Thenthe device is annealed at 120 °C for 2 h in a glovebox filledwith high purity N2. All of the devices are measured in theglovebox with a semiconductor parameter analyzer Agilent4156 C. Devices with different ratio between P3HT andTiO2 have been fabricated to find the most photosensitivecomposition.aElectronic mail: apafyan@polyu.edu.hk.APPLIED PHYSICS LETTERS 93, 023310 20080003-6951/2008/932/023310/3/$23.00 © 2008 American Institute of Physics93, 023310-1Downloaded 28 Sep 2008 to 158.132.12.80. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspAs shown in Fig. 1, transfer characteristics of OTFTswith different TiO2 concentration in P3HT have been mea-sured. The effective field effect mobility decreases with theincrease in TiO2 concentration, which can be attributed to theTiO2 nanoparticles that impede the transport of holes in thechannel.The performance of all of the OTFTs based onP3HT /TiO2 composite film is photosensitive. As shown inFig. 2, the channel current of a device weight ration ofP3HT:TiO2=1:0.75 changes sharply under light illumina-tion. The photosensitive channel currents have been mea-sured at different VDS 10, 5, 2, and 1 V and the same VGS0 V. The inset of Fig. 2 shows the transfer characteristicsVDS=−10 V of the device measured in dark and underwhite light illumination of 200 W /cm2, respectively. Thetransfer curve exhibits a parallel shift to positive gate voltageunder light illumination and no evident change can be foundin the field effect mobility, off current, and subthresholdslope. Therefore light illumination only induces a change inthe threshold voltage of the OTFT and this change can bedirectly calculated from the variation of channel current. Itcan be found that the channel current changes for two ordersof magnitude in the subthreshold region due to the light ex-posure.P3HT /TiO2 composite has been used in solar cell. Pho-tons absorbed by P3HT generate excitons in the film andcharge separation of the excitons may occur at theTiO2 /P3HT interface since the level of the conduction band4.3 eV of TiO2 is lower than that of the lowest unoccupiedmolecular orbit 3.2 eV of P3HT17 and thus some electronswill be trapped in the TiO2 nanoparticles. For the applicationof P3HT /TiO2 composite in an organic solar cell, high con-ductivity and continuous paths for electron transport amongTiO2 particles and the anode of the device are critical issuesfor the device efficiency. We consider that the photosensitiveof the OTFT based on P3HT /TiO2 is also due to the chargeseparation of excitons at the interface of P3HT and TiO2.Electrons trapped in TiO2 nanopartilces will increase the po-tential of the channel which can be regarded as a floatingbody effect of the transistor.18,19 Therefore the more elec-trons trapped in the channel, the bigger shift of the thresholdvoltage can be induced.Charge separation of excitons can be enhanced by apply-ing electric field. We find higher voltage between source anddrain VDS can induce higher shift of threshold voltage un-der the same light illumination, which means higher VDS in-duces higher density of electrons trapped in TiO2 nanopar-ticles. So this is a clear evidence for field enhanced chargeseparation of excitons20 at the interface of P3HT and TiO2.On the other hand, our experiments show that the influenceof gate voltage on charge separation in the channel is negli-gible. Normally a gate voltage only can induce electric fieldin the active layer within several nanometers from thesemiconductor/gate insulator interface21 and thus it has littleeffect on the charge separation of excitons in the channel.It is worth noting that the off currents of the phototransi-tors measured in dark and under light illumination show littledifference, as shown in the inset of Figs. 2 and 3. Thereforephotocurrent in the channel generated by photon absorption,in other words, a photovoltaic effect,15 is negligible inthe performance of the organic device, which is differentFIG. 1. Color online Transfer characteristics of OTFTs based on the com-posite of P3HT /TiO2 nanoparticles. VDS=−10 V. From bottom to top, theweight ratio between P3HT and TiO2 nanoparticles are 1:1, 1:0.75, 1:0. 5,and 1:0.25, respectively.FIG. 2. Color online a Channel current of an OPT weight ratio ofP3HT:TiO2 is 1:0.75 measured at different drain voltages VDS and VGS=0 V. The rising and falling edges of current correspond to switching onand off of a white light illumination of 200 W /cm2, respectively. Inset:transfer characteristics of the OPT VDS=−10 V measured in dark and un-der light illumination. b Shift of the threshold voltage of the OPT calcu-lated from the change of the channel current measured at different VDS.FIG. 3. Color online Shift of the threshold voltage of the OPT weightratio of P3HT:TiO2 is 1:0.75 illuminated with different light intensities anddifferent wavelengths. VDS=−10 V. Inset: from left to right, curves fortransfer characteristics of the OPT VDS=−10 V measured under light illu-mination wavelength: 530 nm with the intensity of 0, 0.43, 5.76, 26.07,60.82, and 102.09 W /cm2, respectively. Linear log plot is shown by leftright axis.023310-2 Mok, Yan, and Chan Appl. Phys. Lett. 93, 023310 2008Downloaded 28 Sep 2008 to 158.132.12.80. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspfrom the device based on the composite of MDMO-PPV/PCBM.15 In our sample, since TiO2 nanoparticles cannot form a continuous conduction path of electrons in thechannel, the contribution of electrons to the channel currentis very small. In the case of phototransistors based onMDMO-PPV/PCBM, PCBM may form a network for elec-tron conduction in the channel and thus the transfer curve ofthis type of devices shows pronounced changes in on/offcurrent and the subthreshold slope under light illumination.The photosensitivity of the OPTs is dependent on lightintensity and wavelength. A device with P3HT:TiO2=1:0.75 has been measured under three different wavelengths 530, 590, and 640 nm. As shown in Fig. 3a, theshift of the threshold voltage of the transistor shows a non-linear relationship with light intensity. This nonlinear behav-ior can be attributed to the following process. Most of theTiO2 nanoparticles in the channel are not connected to thesource/drain electrodes and the trapped electrons accumulatein the nanoparticles. Therefore the built-in electric field at theinterface between TiO2 and P3HT induced by the accumu-lated electrons will retard the charge transfer at the interfaceand induce a lower photosensitivity at higher light intensity.Another possible reason that may induce lower photosensi-tivity at higher light intensity is the occurrence of exciton-exciton annihilation within the P3HT layer, resulting effec-tively in a decrease in the exciton lifetime.17 More detailedtheoretical simulation will be carried out for the nonlinearphotosensitivity of the device.The photosensitivity of the device is obviously related tothe light absorbance of the composite film. As show in Fig.4a, pure P3HT and P3HT /TiO2 composite thin films bothexhibit a maximum light absorbance at 520 nm and a de-creasing value with the increasing wavelength. Therefore,the lower photosensitivity for longer wavelength, as shownin Fig. 3, can be explained. In addition, it can be found thatTiO2 nanoparticles increase the light absorbance in the UVregion. The UV light sensitivity of the device is under inves-tigation.As shown in Fig. 4b, the photosensitivity of the OTFTbased on P3HT /TiO2 is dependent on the concentration ofTiO2 nanoparticles in the composite film. The shift of thethreshold voltage increases with the increase of TiO2 weightratio under the same intensity of white light illumination,which can be explained in terms of higher density of elec-trons trapped in the channel by higher density of TiO2 nano-particles. Since light absorbance of TiO2 nanoparticles towhite light is negligible, the photosensitive behavior of thedevice can be attributed to light absorption of P3HT part,which generates excitons in the composite film. Charge sepa-ration of excitons occurs at the interface of P3HT /TiO2. Forhigher density of TiO2 nanoparticles, bigger interface areaexists in the composite film and thus more electrons can betrapped in the TiO2 nanoparticles and induce higher photo-sensitivity of the device.In conclusion, photosensitive OTFTs based on the com-posite film of P3HT and TiO2 nanoparticles have been fab-ricated and show a stable performance and relatively fastresponse time under light illumination. Since the device canbe fabrication by solution process, it can be easily integratedin organic circuits and multifunctional sensor chips, whichimplies a broad potential application in the future.This work was financially supported by the ResearchGrant No. 1-BB9S of the Hong Kong Polytechnic University.1H. Sirringhaus, Adv. Mater. Weinheim, Ger. 17, 2411 2005.2Y. Hong, F. Yan, P. Migliorato, S. H. Han, and J. Jang, Thin Solid Films515, 4032 2007.3F. Yan, Y. Hong, and P. Migliorato, J. Appl. Phys. 101, 064501 2007.4M. A. Romeo, M. A. G. Martinez, and P. R. Herczfeld, IEEE Trans.Microwave Theory Tech. 44, 2279 1996.5M. Debucquoy, S. Verlaak, S. Steudel, K. Myny, J. Genoe, and P. Her-emans, Appl. Phys. Lett. 91, 103508 2007.6Y. T. Noh and D. Y. Kim, Solid-State Electron. 51, 1052 2007.7B. Park, P. Paoprasert, I. In, J. Zwickey, P. E. Colavita, R. J. Hamers, P.Gopalan, and P. Evans, Adv. Mater. Weinheim, Ger. 19, 43553 2007.8Y. Hu, G. Dong, C. Liu, L. Wang, and Y. Qiu, Appl. Phys. Lett. 89,072108 2006.9Y. Y. Noh, D. Y. Kim, Y. Yoshida, K. Yase, B. J. Jung, and E. Lim, Appl.Phys. Lett. 86, 043501 2005.10V. Podzorov and M. E. Gershenson, Phys. Rev. Lett. 95, 016602 2005.11T. P. I. Saragi, K. Onken, I. Suske, T. Fuhemann-Lieker, and J. Salbeck,Opt. Mater. Amsterdam, Neth. 29, 1332 2007.12T. P. I. Saragi, R. Pudzich, T. Fuhrmann, and J. Salbeck, Appl. Phys. Lett.84, 2334 2004.13M. C. Hamilton, S. Martin, and J. Kanicki, IEEE Trans. Electron Devices51, 877 2004.14K. S. Narayan and N. Kumar, Appl. Phys. Lett. 79, 1891 2001.15N. Marjanovic, T. B. Singh, G. Dennler, S. Gunes, H. Neugebauer, N. S.Sariciftci, R. Schwodiauer, and S. Bauer, Org. Electron. 7, 188 2007.16H. Sirringhaus, N. Tessler, and R. H. Friend, Science 280, 1741 1998.17J. E. Kroeze, T. J. Savenije, M. J. W. Vermeulen, and J. M. Warman, J.Phys. Chem. B 107, 7696 2003.18H. C. Shin, I. S. Lim, M. Racanelli, W. L. M. Huang, J. Foerster, and B. Y.Hwang, IEEE Trans. Electron Devices 43, 318 1996.19F. Yan, P. Migliorato, and R. Ishihara, Appl. Phys. Lett. 88, 1535072006.20P. Peumans and S. R. Forrest, Chem. Phys. Lett. 398, 27 2004.21S. M. Sze, Physics of Semiconductor Devices, 2nd ed. Wiley, New York,1981.FIG. 4. Color online a Absorbance of the P3HT /TiO2 composite films.Along the arrow, from bottom to top, the weight ratio of P3HT:TiO2 are1:0, 1:0.25, 1:0.5, 1:0.75, and 1:1, respectively. b Shift of the thresholdvoltage of the OTFTs with different weight ratios of P3HT:TiO2 measuredunder the white light illumination of 200 W /cm2 and VDS=−20 V. Frombottom to top, the weight ratios of P3HT:TiO2 are 1:0, 1:0.05, 1:0.10,1:0.25, 1:0.5, 1:0.75, and 1:1, respectively.023310-3 Mok, Yan, and Chan Appl. Phys. Lett. 93, 023310 2008Downloaded 28 Sep 2008 to 158.132.12.80. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Organic phototransistor based on poly(3-hexylthiophene)/TiO₂ nanoparticle composite

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