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Highly light-responsive ink-jet printed 6,13-bis(triisopropylsilylethynyl) pentacene phototransistors with suspended top-contact structure
Organic Electronics 11 (2010) 1529–1533
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Highly light-responsive ink-jet printed 6,13-bis(triisopropylsilylethynyl) pentacene phototransistors with suspended top-contact structure Yong-Hoon Kim a,1, Jeong-In Han b, Min-Koo Han a, John E. Anthony c, Jacky Park d, Sung Kyu Park d,* a
Department of Electrical Engineering, Seoul National University, Seoul 151-744, Republic of Korea Department of Chemical and Biochemical Engineering, Dongguk University-Seoul, Seoul 100-715, Republic of Korea c Department of Chemistry, University of Kentucky, Lexington, KY 40506-0055, USA d Convergence Materials and Devices Laboratory, Department of Textile Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 18 April 2010 Received in revised form 15 June 2010 Accepted 21 June 2010 Available online 30 June 2010 Keywords: Organic phototransistor TIPS-pentacene Suspended top-contact Ink-jet printing Dynamic range
a b s t r a c t Highly light-responsive ink-jet printed 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene thin films were investigated as a photo-sensing active layer by employing suspended top-contact phototransistors. Under a steady-state illumination, the suspended top-contact TIPS-pentacene organic phototransistors have shown a current modulation of 106–107, resulting in dynamic range of 120 dB even in the low-molecular ordered ink-jet printed films. These results suggest that the suspended top-contact phototransistors based on ink-jet printed TIPS-pentacene can be a new promising candidate for low-cost and highperformance photo-sensing element for digital imaging applications. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Organic thin-film transistors (OTFTs) based on polymers and small molecules have attracted considerable attention for flexible electronics such as displays, photo sensors, pressure sensors and disposable radio-frequency identification tags (RF-IDs) due to their potential advantages of low-cost manufacturing, simple device architecture and compatibility with plastic substrates [1–5]. Most of the current research on OTFTs is focused on realization of high-performance electrical switching devices to integrate into flexible display panels [2,6–8]. Recently several papers were reported concerning the light-responsive characteristics of organic semiconductor materials utilizing them as an active layer for photo-detecting sensors [9–12]. Similar
* Corresponding author. E-mail address: [email protected] (S.K. Park). 1 Present address: Flexible Display Research Center, Korea Electronics Technology Institute, Seongnam 463-816, South Korea. 1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.06.016
to hydrogenated amorphous silicon (a-Si:H) films, photogenerated excitons are formed when the organic semiconductor materials are exposed to light, and subsequently these photogenerated excitons diffuse and dissociate into free charge carriers thereby increasing the drain current of the transistor [13]. When this transistor is used as an electric switching device, the number of photogenerated excitons should be minimized because these excitons can increase the current level in the off-state and degrade the current modulation of the device. However, as a photosensing device, the number of photogenerated excitons or charge carriers should be maximized in order to increase the light sensitivity and dynamic range of the sensor. The focus of this work is to investigate the effect of a steady-state light illumination on the electrical properties of ink-jet printed 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene films and their applications for low-cost and high-performance phototransistors. With TIPS-pentacene as a photo-sensing layer, an organic phototransistor with a current modulation up to seven orders of magnitude was demonstrated. Additionally, to increase the sensitivity
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of light response and current modulation of the ink-jet printed low-molecular ordered TIPS-pentacene transistors, a suspended top-contact (STC) structure was employed [14].
2. Experimental The ink-jet printed STC–TIPS-pentacene phototransistors were fabricated by the following sequence. On Corning 1737 glass substrates, 100-nm-thick indium–tin-oxide (ITO) gate electrode was deposited by sputtering and then patterned. On the top of the gate electrode, an organic gate insulator, poly-4-vinylphenol (PVP) was spin-coated and thermally cured. The PVP solution was prepared with 10 wt.% of PVP powder mixed with 5 wt.% of poly (melamine-co-formaldehyde) methylated as a cross-linking agent in propylene glycol monomethyl ether acetate. The curing temperature and time were 200 °C and 30 min, respectively. For source/drain electrode, a STC electrode structure has been built. The STC electrode structure was fabricated with 50 nm-thick Cr and 50 nm-thick Au bi-layers. With over-etching of the Cr adhesion layer, suspended Au source/drain electrodes can be formed, resulting in somewhat similar to Au top-contact structure on TIPS-pentacene films. The channel width and length of were 125 lm and 10 lm, respectively. The organic semiconductor ink, TIPS-pentacene was prepared from 1 wt.% anisole solution. The deposition of TIPS-pentacene was then performed using a piezoelectric ink-jet printing system (UniJet UJ2100) on the STC electrodes. The piezoelectric ink-jet nozzle had a diameter of 50 lm (orifice size
of 50 lm). The frequency of the jetting was 150 Hz and the diameter of the ink drop was approximately 30–50 lm. The electrical measurements were performed using a Keithley 4200-SCS in a dark and air ambient at room temperature. The steady-state white light response characteristics were carried out using a halogen lamp as a light source. For UV irradiation tests, low-pressure mercury lamp (Hamamatsu Photonics, L937-02) was used as a light source. All the light sources were placed on the top of the sample. The X-ray diffraction patterns of TIPS-pentacene films were obtained by using X-ray diffractometer (Philips, X’pert MPD, k = 1.5406 Å).
3. Results and discussion The STC-structured device has suspended source/drain electrode over the gate insulator. There is a very narrow gap in between (typically 50 nm), and TIPS-pentacene solution fills this gap forming a channel layer. Therefore, the device is more likely to behave as a top-contact device (Fig. 1). Typical transfer characteristics of the ink-jet printed STC–TIPS-pentacene and conventional bottomcontact TIPS-pentacene phototransistors measured in the dark are shown in Fig. 2. As shown in this figure, the STC–TIPS-pentacene phototransistors typically show much improved mobility, on-current, subthreshold slope, and reduced off-current, possibly resulting from contact improvement [14]. Such a low off-current and subthreshold slope may enable the high photosensitivity and high dynamic range of the phototransistor. For the application of the STC–TIPS-pentacene transistors as photosensitive
Fig. 1. Fabrication procedure of ink-jet printed TIPS-pentacene phototransistor with suspended top-contact (STC) structure.
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Fig. 2. The comparison of transfer characteristics of ink-jet printed conventional and STC–TIPS-pentacene phototransistor measured under dark. The channel width and length of the phototransistor were 125 lm and 10 lm, respectively.
devices, we characterized the devices in the linear regime. The linear field-effect mobility was 0.02 cm2/V-s and the on/off ratio was 106–107. The threshold voltage (Vth) and subthreshold slope were determined as 4 V and 2.5 V/decade, respectively. The STC–TIPS-pentacene phototransistor showed extremely low off-current level (1014– 1015 A) at a low source-to-drain bias (Vds = 1 V) (Fig. 3). The gate bias dependent current modulation (Ilight/Idark) and dynamic range (20 log (Ilight/Idark)) of a phototransistor are shown in Fig. 4. The illuminated light power density was controlled as 9, 11 and 13 mW/cm2. When the light is illuminated on the phototransistor, the drain current of the phototransistor is increased due to the photogenerated excitons and localized heating by absorption and rapid thermalization of light [15]. With the light illumination the drain current (Ilight) increased up to 108 A in the offstate showing a current modulation of 106–107. In strong accumulation regime (Vgs < 10 V), the current modulation is nearly unity since the number of the photogenerated charge carriers is comparable to the number of carriers accumulated by the field-effect. On the other hand, as the gate bias moves toward positive, the current modulation increases and reaches a maximum point. Further increase of the gate bias towards positive direction decreases the current modulation due to increased Idark. In a-Si:H TFTs, the dependence of photocurrent (Iph; Iph = Ilight Idark) on illumination flux and gate bias can be explained by using the photofield effect model [16]. According to this model, the photocurrent shows a power law dependency on the illumination flux, as in Eq. (1),
Iph / F cðV gs Þ
ð1Þ
where, F is the illumination flux and c is power law exponent which is a gate bias dependent factor and describes the dependency of photocurrent on light intensity. When a gate bias is applied, conduction and valence bands are bent and electrons or holes are accumulated near the insulator/semiconductor interface depending on the polarity of
Fig. 3. The transfer characteristics of TIPS-pentacene phototransistors with a steady-state light illumination (at Vds = 1 V). The light power density was varied as 9, 11 and 13 mW/cm2; (a) in logarithm scale, and (b) in linear scale of the drain current.
the gate bias. However, when light is illuminated on the semiconductor, a redistribution of space charge in localized donor- and acceptor-like states takes place and band bending is relaxed [16]. Also, under illumination the semiconductor is at a non-equilibrium steady-state and quasi-Fermi levels (QFLs) for trapped electrons and holes are created [17]. The QFLs are separated from the equilibrium Fermi level (Fermi level under dark condition) by the factor DEf, which depends on the intensity of the illumination. That is, DEf increases as the illumination intensity increases and for a very high illumination level the QFLs can reach their respective band edges [17]. In the case of a-Si:H TFTs, the c exponent gradually decreases as the gate bias is swept from negative to positive. Similar behavior of gate bias dependent c is observed in the case of STC– TIPS-pentacene phototransistors, although the polarity of the gate bias is opposite. However, these models may not be directly applicable to organic semiconductor based TFTs since the charge transport mechanism is different and also the conventional band theory may be not appropriate to
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Fig. 4. The dependence of current modulation (Ilight/Idark) and dynamic range on the illuminated light power density and the gate bias. The dynamic range was calculated from the following equation. (dynamic range = 20 log (Ilight/Idark)).
explain the conducting behavior of organic semiconductors. In Fig. 5, the field-effect mobility, Vth and subthreshold slope of the phototransistors are shown, as a function of illuminated light power density. The field-effect mobility decreases from 0.021 cm2/V-s to 0.005 cm2/V-s when the device was illuminated and the Vth shifts to positive as the power density increases. Also, the subthreshold slope degrades from 2.5 V/decade to 11 V/decade. It is likely that the dependency of Vth on light power density is due to increasing photogenerated excitons and dissociated charged carriers. When the light is illuminated, the photogenerated carriers are formed in the bulk region of the STC–TIPS-pentacene and increase the off-current of the phototransistor resulting in the Vth shift. Concerning the continuous decrease of mobility, we notice from Fig. 3b that the on-state drain current decreases as the illuminated light power density increases. Especially at a light power density of 13 mW/cm2, the drain current is even lower than that measured under dark conditions. From this observation, it is clearly understood that the white-light illumination may be one of the reasons for decreased drain current and mobility. Previously it has been reported that UV irradiation of vacuum deposited pentacene TFTs destroys the crystalline structure of pentacene film by dissociation of pentacene molecules [18]. Since most of the light sources emit at least partly into the UV spectrum, the TIPS-pentacene active layer may be damaged by irradiation by the white-light source. To clarify the effect of UV irradiation on the electrical and physical characteristics of STC–TIPS-pentacene phototransistor, UV light was irradiated on the device and the results are shown in Fig. 6a. The intensity of irradiated UV light at a distance of 2 cm was 12.5 mW/cm2. During the UV exposure, photogenerated excitons are formed as in the case of light illumination and increase the drain current in the off-state. However, dissimilar to white-light
Fig. 5. The variation of field-effect mobility, threshold voltage and subthreshold slope of STC–TIPS-pentacene phototransistor as a function of incident light power density.
illumination, the on-current level is reduced by 2–3 orders of magnitude compared to a fresh device and no gate field modulation is observed. After turning off the UV source the gate field modulation is again observed. However, the oncurrent level is much lower than for the fresh sample. As suggested in the literature [18], the TIPS-pentacene film can be damaged seriously by the UV exposure losing its molecular structure, its unique crystal packing structure and its ordering. Fig. 6b shows the X-ray diffraction patterns of spin-coated TIPS-pentacene films before and after UV irradiation. Single crystal TIPS-pentacene has a triclinic structure with unit cell parameters a = 7.5650 Å, b = 7.7500 Å, c = 16.835 Å, a = 89.15°, b = 92.713° and c = 83.63° [19]. Therefore, the peaks observed at 5.4° correspond to intermolecular spacing of 16.8 Å indicating preferential orientation of the (0 0 1)-axis normal to the surface. While the as-coated TIPS-pentacene film has a polycrystalline structure, the UV-irradiated film typically exhibits amorphous features with no diffraction peaks, which may show that the intense UV irradiation disrupts the molecular order in the film, likely by decomposition of the TIPS-pentacene molecules. Since the carrier mobility in organic semiconductors is closely related to the molecular ordering of the semiconductors [20], the degradation of field-effect mobility and reduced drain current are possibly due to the decomposition of TIPS-pentacene molecules caused by UV irradiation. These results suggest that in order to employ TIPS-pentacene phototransistors as photo-sensing elements, a proper UV-protective
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Fig. 6. (a) Effect of UV irradiation on the transfer characteristics of TIPS-pentacene phototransistors. The transfer characteristics were measured before, after and during the UV irradiation. (b) X-ray diffraction patterns (h/2h mode) of TIPS-pentacene films before and after UV irradiation.
passivation layer should be integrated into the phototransistor to enhance their reliability. 4. Conclusion We demonstrated highly light-responsive organic phototransistors employing STC electrode structure and ink-jet printed TIPS-pentacene as a photo-sensing active layer. Under a steady-state illumination, the STC–TIPS-pentacene phototransistor showed high current modulation and dynamic range which suggests that the organic phototransistors based on ink-jet printed TIPS-pentacene can be a new promising candidate for the low-cost and high-performance photosensors for digital imaging applications. References [1] S.E. Burns, W. Reeves, B.H. Pui, K. Jacobs, S. Siddique, K. Reynolds, M. Banach, D. Barclay, K. Chalmers, N. Cousins, P. Cain, L. Dassas, M. Etchells, C. Hayton, S. Markham, A. Menon, P. Too, C. Ramsdale, J. Herod, K. Saynor, J. Watts, T. von Werne, J. Mills, C.J. Curling, H. Sirringhaus, M.D. McCreary, SID Int. Symp. Digest Tech. Papers 37 (2006) 74. [2] S.K. Park, J.E. Anthony, D.A. Mourey, T.N. Jackson, Appl. Phys. Lett. 91 (2007) 0563514. [3] J.B. Chang, V. Liu, V. Subramanian, K. Sivula, C. Luscombe, A. Murphy, J. Liu, J.M.J. Fréchet, J. Appl. Phys. 100 (2006) 014506.
[4] T. Someya, T. Sekitani, S. Iba, Y. Kato, H. Kawaguchi, T. Sakurai, Proc. Natl. Acad. Sci. U.S.A. 101 (2004) 9966. [5] V. Subramanian, J.M.J. Fréchet, P.C. Chang, D.C. Huang, J.B. Lee, S.E. Molesa, A.R. Murphy, D.R. Redinger, S.K. Volkman, Proc. IEEE 93 (2005) 1330. [6] M. Mizukami, N. Hirohata, T. Iseki, K. Ohtawara, T. Tada, S. Yagyu, T. Abe, T. Suzuki, Y. Fujisaki, Y. Inoue, S. Tokito, T. Kurita, IEEE Elec. Dev. Lett. 27 (2006) 249. [7] L. Zhou, S. Park, B. Bai, J. Sun, S.-C. Wu, T.N. Jackson, S. Nelson, D. Freeman, Y. Hong, IEEE Elec. Dev. Lett. 26 (2005) 640. [8] 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. USA 98 (2001) 4835. [9] K.S. Narayana, N. Kumar, Appl. Phys. Lett. 79 (2001) 1891. [10] G. Yu, G. Srdanov, J. Wang, H. Wang, Y. Cao, A.J. Heeger, Synth. Met. 111–112 (2000) 133. [11] G. Yu, Y. Cao, J. Wang, J. McElvain, A.J. Heeger, Synth. Met. 102 (1999) 904. [12] S.R. Forrest, IEEE J. Sel. Top. Quant. 6 (2000) 1072. [13] M.C. Hamilton, J. Kanicki, IEEE J. Sel. Top. Quant. 10 (2004) 840. [14] Y.H. Kim, S.M. Han, W. Lee, M.K. Han, Y.U. Lee, J.I. Han, Appl. Phys. Lett. 91 (2007) 042113. [15] T. Tokumoto, J.S. Brooks, R. Clinite, X. Wei, J.E. Anthony, D.L. Eaton, S.R. Parkin, J. Appl. Phys. 92 (2002) 5208. [16] R.E.I. Schropp, T. Franke, J.F. Verwey, J. Non-Cryst. Solids 90 (1987) 199. [17] J.G. Simmons, G.W. Taylor, Phys. Rev. B 4 (1971) 502. [18] W.J. Kim, W.H. Koo, S.J. Jo, C.S. Kim, H.K. Baik, D.K. Hwang, K. Lee, J.H. Kim, S. Im, Electrochem. Solid-State Lett. 9 (2006) G251. [19] J.E. Anthony, J.S. Brooks, D.L. Eaton, S.R. Parkin, J. Am. Chem. Soc. 123 (2001) 9482. [20] C.R. Newman, R.J. Chesterfield, M.J. Panzer, C.D. Frisbie, J. Appl. Phys. 98 (2005) 084506.
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Highly light-responsive ink-jet printed 6,13-bis(triisopropylsilylethynyl) pentacene phototransistors with suspended top-contact structure Yong-Hoon Kim a,1, Jeong-In Han b, Min-Koo Han a, John E. Anthony c, Jacky Park d, Sung Kyu Park d,* a
Department of Electrical Engineering, Seoul National University, Seoul 151-744, Republic of Korea Department of Chemical and Biochemical Engineering, Dongguk University-Seoul, Seoul 100-715, Republic of Korea c Department of Chemistry, University of Kentucky, Lexington, KY 40506-0055, USA d Convergence Materials and Devices Laboratory, Department of Textile Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 18 April 2010 Received in revised form 15 June 2010 Accepted 21 June 2010 Available online 30 June 2010 Keywords: Organic phototransistor TIPS-pentacene Suspended top-contact Ink-jet printing Dynamic range
a b s t r a c t Highly light-responsive ink-jet printed 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene thin films were investigated as a photo-sensing active layer by employing suspended top-contact phototransistors. Under a steady-state illumination, the suspended top-contact TIPS-pentacene organic phototransistors have shown a current modulation of 106–107, resulting in dynamic range of 120 dB even in the low-molecular ordered ink-jet printed films. These results suggest that the suspended top-contact phototransistors based on ink-jet printed TIPS-pentacene can be a new promising candidate for low-cost and highperformance photo-sensing element for digital imaging applications. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Organic thin-film transistors (OTFTs) based on polymers and small molecules have attracted considerable attention for flexible electronics such as displays, photo sensors, pressure sensors and disposable radio-frequency identification tags (RF-IDs) due to their potential advantages of low-cost manufacturing, simple device architecture and compatibility with plastic substrates [1–5]. Most of the current research on OTFTs is focused on realization of high-performance electrical switching devices to integrate into flexible display panels [2,6–8]. Recently several papers were reported concerning the light-responsive characteristics of organic semiconductor materials utilizing them as an active layer for photo-detecting sensors [9–12]. Similar
* Corresponding author. E-mail address: [email protected] (S.K. Park). 1 Present address: Flexible Display Research Center, Korea Electronics Technology Institute, Seongnam 463-816, South Korea. 1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.06.016
to hydrogenated amorphous silicon (a-Si:H) films, photogenerated excitons are formed when the organic semiconductor materials are exposed to light, and subsequently these photogenerated excitons diffuse and dissociate into free charge carriers thereby increasing the drain current of the transistor [13]. When this transistor is used as an electric switching device, the number of photogenerated excitons should be minimized because these excitons can increase the current level in the off-state and degrade the current modulation of the device. However, as a photosensing device, the number of photogenerated excitons or charge carriers should be maximized in order to increase the light sensitivity and dynamic range of the sensor. The focus of this work is to investigate the effect of a steady-state light illumination on the electrical properties of ink-jet printed 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene films and their applications for low-cost and high-performance phototransistors. With TIPS-pentacene as a photo-sensing layer, an organic phototransistor with a current modulation up to seven orders of magnitude was demonstrated. Additionally, to increase the sensitivity
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of light response and current modulation of the ink-jet printed low-molecular ordered TIPS-pentacene transistors, a suspended top-contact (STC) structure was employed [14].
2. Experimental The ink-jet printed STC–TIPS-pentacene phototransistors were fabricated by the following sequence. On Corning 1737 glass substrates, 100-nm-thick indium–tin-oxide (ITO) gate electrode was deposited by sputtering and then patterned. On the top of the gate electrode, an organic gate insulator, poly-4-vinylphenol (PVP) was spin-coated and thermally cured. The PVP solution was prepared with 10 wt.% of PVP powder mixed with 5 wt.% of poly (melamine-co-formaldehyde) methylated as a cross-linking agent in propylene glycol monomethyl ether acetate. The curing temperature and time were 200 °C and 30 min, respectively. For source/drain electrode, a STC electrode structure has been built. The STC electrode structure was fabricated with 50 nm-thick Cr and 50 nm-thick Au bi-layers. With over-etching of the Cr adhesion layer, suspended Au source/drain electrodes can be formed, resulting in somewhat similar to Au top-contact structure on TIPS-pentacene films. The channel width and length of were 125 lm and 10 lm, respectively. The organic semiconductor ink, TIPS-pentacene was prepared from 1 wt.% anisole solution. The deposition of TIPS-pentacene was then performed using a piezoelectric ink-jet printing system (UniJet UJ2100) on the STC electrodes. The piezoelectric ink-jet nozzle had a diameter of 50 lm (orifice size
of 50 lm). The frequency of the jetting was 150 Hz and the diameter of the ink drop was approximately 30–50 lm. The electrical measurements were performed using a Keithley 4200-SCS in a dark and air ambient at room temperature. The steady-state white light response characteristics were carried out using a halogen lamp as a light source. For UV irradiation tests, low-pressure mercury lamp (Hamamatsu Photonics, L937-02) was used as a light source. All the light sources were placed on the top of the sample. The X-ray diffraction patterns of TIPS-pentacene films were obtained by using X-ray diffractometer (Philips, X’pert MPD, k = 1.5406 Å).
3. Results and discussion The STC-structured device has suspended source/drain electrode over the gate insulator. There is a very narrow gap in between (typically 50 nm), and TIPS-pentacene solution fills this gap forming a channel layer. Therefore, the device is more likely to behave as a top-contact device (Fig. 1). Typical transfer characteristics of the ink-jet printed STC–TIPS-pentacene and conventional bottomcontact TIPS-pentacene phototransistors measured in the dark are shown in Fig. 2. As shown in this figure, the STC–TIPS-pentacene phototransistors typically show much improved mobility, on-current, subthreshold slope, and reduced off-current, possibly resulting from contact improvement [14]. Such a low off-current and subthreshold slope may enable the high photosensitivity and high dynamic range of the phototransistor. For the application of the STC–TIPS-pentacene transistors as photosensitive
Fig. 1. Fabrication procedure of ink-jet printed TIPS-pentacene phototransistor with suspended top-contact (STC) structure.
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Fig. 2. The comparison of transfer characteristics of ink-jet printed conventional and STC–TIPS-pentacene phototransistor measured under dark. The channel width and length of the phototransistor were 125 lm and 10 lm, respectively.
devices, we characterized the devices in the linear regime. The linear field-effect mobility was 0.02 cm2/V-s and the on/off ratio was 106–107. The threshold voltage (Vth) and subthreshold slope were determined as 4 V and 2.5 V/decade, respectively. The STC–TIPS-pentacene phototransistor showed extremely low off-current level (1014– 1015 A) at a low source-to-drain bias (Vds = 1 V) (Fig. 3). The gate bias dependent current modulation (Ilight/Idark) and dynamic range (20 log (Ilight/Idark)) of a phototransistor are shown in Fig. 4. The illuminated light power density was controlled as 9, 11 and 13 mW/cm2. When the light is illuminated on the phototransistor, the drain current of the phototransistor is increased due to the photogenerated excitons and localized heating by absorption and rapid thermalization of light [15]. With the light illumination the drain current (Ilight) increased up to 108 A in the offstate showing a current modulation of 106–107. In strong accumulation regime (Vgs < 10 V), the current modulation is nearly unity since the number of the photogenerated charge carriers is comparable to the number of carriers accumulated by the field-effect. On the other hand, as the gate bias moves toward positive, the current modulation increases and reaches a maximum point. Further increase of the gate bias towards positive direction decreases the current modulation due to increased Idark. In a-Si:H TFTs, the dependence of photocurrent (Iph; Iph = Ilight Idark) on illumination flux and gate bias can be explained by using the photofield effect model [16]. According to this model, the photocurrent shows a power law dependency on the illumination flux, as in Eq. (1),
Iph / F cðV gs Þ
ð1Þ
where, F is the illumination flux and c is power law exponent which is a gate bias dependent factor and describes the dependency of photocurrent on light intensity. When a gate bias is applied, conduction and valence bands are bent and electrons or holes are accumulated near the insulator/semiconductor interface depending on the polarity of
Fig. 3. The transfer characteristics of TIPS-pentacene phototransistors with a steady-state light illumination (at Vds = 1 V). The light power density was varied as 9, 11 and 13 mW/cm2; (a) in logarithm scale, and (b) in linear scale of the drain current.
the gate bias. However, when light is illuminated on the semiconductor, a redistribution of space charge in localized donor- and acceptor-like states takes place and band bending is relaxed [16]. Also, under illumination the semiconductor is at a non-equilibrium steady-state and quasi-Fermi levels (QFLs) for trapped electrons and holes are created [17]. The QFLs are separated from the equilibrium Fermi level (Fermi level under dark condition) by the factor DEf, which depends on the intensity of the illumination. That is, DEf increases as the illumination intensity increases and for a very high illumination level the QFLs can reach their respective band edges [17]. In the case of a-Si:H TFTs, the c exponent gradually decreases as the gate bias is swept from negative to positive. Similar behavior of gate bias dependent c is observed in the case of STC– TIPS-pentacene phototransistors, although the polarity of the gate bias is opposite. However, these models may not be directly applicable to organic semiconductor based TFTs since the charge transport mechanism is different and also the conventional band theory may be not appropriate to
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Fig. 4. The dependence of current modulation (Ilight/Idark) and dynamic range on the illuminated light power density and the gate bias. The dynamic range was calculated from the following equation. (dynamic range = 20 log (Ilight/Idark)).
explain the conducting behavior of organic semiconductors. In Fig. 5, the field-effect mobility, Vth and subthreshold slope of the phototransistors are shown, as a function of illuminated light power density. The field-effect mobility decreases from 0.021 cm2/V-s to 0.005 cm2/V-s when the device was illuminated and the Vth shifts to positive as the power density increases. Also, the subthreshold slope degrades from 2.5 V/decade to 11 V/decade. It is likely that the dependency of Vth on light power density is due to increasing photogenerated excitons and dissociated charged carriers. When the light is illuminated, the photogenerated carriers are formed in the bulk region of the STC–TIPS-pentacene and increase the off-current of the phototransistor resulting in the Vth shift. Concerning the continuous decrease of mobility, we notice from Fig. 3b that the on-state drain current decreases as the illuminated light power density increases. Especially at a light power density of 13 mW/cm2, the drain current is even lower than that measured under dark conditions. From this observation, it is clearly understood that the white-light illumination may be one of the reasons for decreased drain current and mobility. Previously it has been reported that UV irradiation of vacuum deposited pentacene TFTs destroys the crystalline structure of pentacene film by dissociation of pentacene molecules [18]. Since most of the light sources emit at least partly into the UV spectrum, the TIPS-pentacene active layer may be damaged by irradiation by the white-light source. To clarify the effect of UV irradiation on the electrical and physical characteristics of STC–TIPS-pentacene phototransistor, UV light was irradiated on the device and the results are shown in Fig. 6a. The intensity of irradiated UV light at a distance of 2 cm was 12.5 mW/cm2. During the UV exposure, photogenerated excitons are formed as in the case of light illumination and increase the drain current in the off-state. However, dissimilar to white-light
Fig. 5. The variation of field-effect mobility, threshold voltage and subthreshold slope of STC–TIPS-pentacene phototransistor as a function of incident light power density.
illumination, the on-current level is reduced by 2–3 orders of magnitude compared to a fresh device and no gate field modulation is observed. After turning off the UV source the gate field modulation is again observed. However, the oncurrent level is much lower than for the fresh sample. As suggested in the literature [18], the TIPS-pentacene film can be damaged seriously by the UV exposure losing its molecular structure, its unique crystal packing structure and its ordering. Fig. 6b shows the X-ray diffraction patterns of spin-coated TIPS-pentacene films before and after UV irradiation. Single crystal TIPS-pentacene has a triclinic structure with unit cell parameters a = 7.5650 Å, b = 7.7500 Å, c = 16.835 Å, a = 89.15°, b = 92.713° and c = 83.63° [19]. Therefore, the peaks observed at 5.4° correspond to intermolecular spacing of 16.8 Å indicating preferential orientation of the (0 0 1)-axis normal to the surface. While the as-coated TIPS-pentacene film has a polycrystalline structure, the UV-irradiated film typically exhibits amorphous features with no diffraction peaks, which may show that the intense UV irradiation disrupts the molecular order in the film, likely by decomposition of the TIPS-pentacene molecules. Since the carrier mobility in organic semiconductors is closely related to the molecular ordering of the semiconductors [20], the degradation of field-effect mobility and reduced drain current are possibly due to the decomposition of TIPS-pentacene molecules caused by UV irradiation. These results suggest that in order to employ TIPS-pentacene phototransistors as photo-sensing elements, a proper UV-protective
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Fig. 6. (a) Effect of UV irradiation on the transfer characteristics of TIPS-pentacene phototransistors. The transfer characteristics were measured before, after and during the UV irradiation. (b) X-ray diffraction patterns (h/2h mode) of TIPS-pentacene films before and after UV irradiation.
passivation layer should be integrated into the phototransistor to enhance their reliability. 4. Conclusion We demonstrated highly light-responsive organic phototransistors employing STC electrode structure and ink-jet printed TIPS-pentacene as a photo-sensing active layer. Under a steady-state illumination, the STC–TIPS-pentacene phototransistor showed high current modulation and dynamic range which suggests that the organic phototransistors based on ink-jet printed TIPS-pentacene can be a new promising candidate for the low-cost and high-performance photosensors for digital imaging applications. References [1] S.E. Burns, W. Reeves, B.H. Pui, K. Jacobs, S. Siddique, K. Reynolds, M. Banach, D. Barclay, K. Chalmers, N. Cousins, P. Cain, L. Dassas, M. Etchells, C. Hayton, S. Markham, A. Menon, P. Too, C. Ramsdale, J. Herod, K. Saynor, J. Watts, T. von Werne, J. Mills, C.J. Curling, H. Sirringhaus, M.D. McCreary, SID Int. Symp. Digest Tech. Papers 37 (2006) 74. [2] S.K. Park, J.E. Anthony, D.A. Mourey, T.N. Jackson, Appl. Phys. Lett. 91 (2007) 0563514. [3] J.B. Chang, V. Liu, V. Subramanian, K. Sivula, C. Luscombe, A. Murphy, J. Liu, J.M.J. Fréchet, J. Appl. Phys. 100 (2006) 014506.
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