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Novel strategies for polymer based light sensors
Thin Solid Films 417 (2002) 75–77
Novel strategies for polymer based light sensors K.S. Narayan*, A.G. Manoj, Th.B. Singh, A.A. Alagiriswamy Chemistry and Physics of Materials Unit, Jawaharlal Nehru Center for Advanced Scientific Research, Jakkur, Bangalore 560 064, India Received 7 December 2001; accepted 22 February 2002
Abstract We present results of conjugated polymer based devices for optical detection and image sensors. These devices have efficiencies comparable to conventional inorganic semiconductor based devices, coupled with the inherent advantage of having polymers as the active component in the device especially in terms of large area, cost effectiveness, chemical tunability and flexibility. We have adopted different strategies and device-architecture resulting in novel photodetectors; (i) p-typeyn-type polymer heterostructures, with high performance in terms of photosensitivity and conversion efficiency, (ii) resonant cavity enhanced photodiodes, (iii) light responsive field effect transistors. We discuss the results and possible applications of these light detecting device structures. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Photodetectors; Conjugated polymers
1. Introduction Polymer based photodetectors complements the polymer-light-emitting diodes and can constitute the basic design element for an all-organic integrated opto-electronic circuit. An organicypolymer Schottky photodiodes can be fabricated with the active polymer layer sandwiched between the two electrodes with a suitable difference in work functions w1,2x. In the last decade efforts to increase the quantum efficiency of charge generation and separation efficiencies in polymers such as derivatives of polyphenylenevinylene (PPV) and polythiophene (PTH) were quite successful w3x. The photosensitivity of these polymers significantly increased upon dispersing electron acceptors such as derivatised C60 w3x. Along with this strategy of having the active layers in form of blends with electron acceptor w3,4x multilayers of donor–acceptor w5x systems for increasing the photocurrent in devices have also been reported with responsivity approaching traditional semiconductor photodiodes. We present here novel design options provided by active polymer photodetectors. The three examples mentioned in this report represents a set *Corresponding author. Tel.: q91-80-8462750-7x251; fax: q9180-8462766. E-mail address: [email protected] (K.S. Narayan).
of conceptually distinct methods for generating photocurrent. The essential commonality in all these type of detectors is the eventual generation of charge carriers upon photo-exciting the semiconducting polymer. A generally accepted view point is that the electron–hole separation resulting in free carriers is a secondary process. The photocurrent observed in polymers such as PPV and PTH are largely contributed by the transport of holes across the medium, while the electrons get severely trapped at the defect sites (mn-mp). Another class of heteroaromatic polymers, such as the ladder type polymer benzamidazobenzophenanthroline (BBL), however has electrons as the primary contribution for the photocurrent (mn)mp) w6x. The bilayer structure of this ptype and n-type polymer reveals interesting properties w7,8x. 2. Results and discussion Bilayer devices was obtained by spin coating the solution of poly(3-octylthiophene-2,5-diyl) (P3OT) in p-xylene on indium tin oxide (ITO) coated transparent glass plate. BBL in Lewis acid (1 N aluminium chloride in nitrobenzene) was spin coated on top of P3OT and washed thoroughly to remove the aluminium chloride.
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 5 7 4 - 6
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K.S. Narayan et al. / Thin Solid Films 417 (2002) 75–77
Fig. 1. The device structures of (a) bilayer p-type polymeryn-type polymer photodetector (b) resonant cavity device; with illumination from semi-transparent gold (Au) layer and chemical structure of the polymer P3OT. ITO—indium tin oxide, DBR—distributed Bragg reflector of 16 pairs of AlAsyAlGaAs. (c) Schematic of the light responding polymer FET.
Gold (Au) or aluminium (Al) was coated on top of the BBL as cathode, Fig. 1a. The interface facilitates hole transfer from BBL to P3OT and electron transfer from P3OT to BBL. The interface acts as a barrier for electron from BBL to P3OT and for hole from P3OT to BBL. The salient features of this device are (i) the photocurrent conversion efficiency is two–three orders of magnitude higher than the single layered devices, (ii) the broadening of spectral range covering the entire visible range as shown in Fig. 2, (iii) the open circuit voltage is controlled by the estimated Fermi level difference of
the polymers rather than the electrode work function, (iv) the energy conversion efficiency is f0.3% w9x. The concept of ‘resonance cavity enhanced’ (RCE) ¨ ¨ photodiodes for inorganic devices demonstrated by Unlu and Strite w10x is adapted for the polymer based devices. The procedure entails the introduction of a thin polymer active layer in the micro-cavity, which is essentially a resonator with an effective cavity length of half the wavelength of light, (Fig. 1b). We observe, upon implementing the RCE structure, that the gain of the photodiode is not compromised while decreasing the transit time. Thin polymer films cast from solution shows clear evidence of spectral broadening and band tailing with characteristic energies of a few tenths of an eV. We utilize this finite absorption at wavelengths lc-hco yEg, which is present in these polymers. In this approach we also demonstrate the tunability of the photodiode to a wavelength lc-hco yEg, approaching the near-infrared region. The essential features of this photodiode are (i) significant red-shifted photocurrent at the non-absorbing region of the polymer; (ii) A local maximum, lmax, in this red-shifted photocurrent controlled by the cavity parameters; (iii) Iph (l) variation with distributed Bragg reflector (DBR) mirror substrates tuned to different wavelength. In absence of the DBR substrate the Iph (l) (not shown here) essentially follows the absorption response; (iv) The photocurrent corresponding to lmax, has a stronger dependence to the incident angle compared to the photocurrent corresponding to high absorption wavelength; (v) the switching speed of photoresponse is in the GHz range. The micro-cavity enhances the photodetection property in terms of speed and gain at a desired detection wavelength, specifically in the low-absorption region much below the band-edge. We achieve a photoresponsivity f10 mAyW, along with a speed )1 GHz in these micro-cavity enhanced polymer based photodetectors, at zero bias in the near-IR region w11x. Field effect transistors (FETs) based on organic materials have recently been demonstrated to be a route for
Fig. 2. The spectral response of a bilayer polymer device along with single layer response of P3OT and BBL.
K.S. Narayan et al. / Thin Solid Films 417 (2002) 75–77
fabricating novel and efficient devices w12x. We observe a transistor action that gets considerably modified with photoexcitation with large changes in the drain–source current Id, and the saturation value of drain–source current Ids depending on both the intensity of light and the gate voltage. The large photo-induced drain current, Ilight is a consequence of an internal amplification prod cess resulting from the photogenerated carriers which is possible only in this transistor configuration. The response of an organic FET to visible light also opens up the utility of these devices as image sensors. We demonstrate that in an organic FET as shown in Fig. 1c, the incident light intensity can act as an added control parameter in the transistor operation. Dielectric layers such as silica and polyvinyl alcohol were chosen as the insulating medium with aluminium electrode forming the gate electrode. Regioregular P3OT with 98.5% head to tail regiospecific conformation was dissolved in chloroform and spin coated on the insulator to yield a 100-nm thick film. Gold electrodes, 3-mm wide with an inter-electrode spacing of 70 mm, forming the channel length, was deposited on P3OT to form the source and drain electrodes. The gain factor in the drain current Id; Ilight yId, due to light with photon flux rate as d low as f1 mW, in this case, is f100. It was also observed that the gain (Ilight yId) can be further increased d to as high as 103 for certain devices, with higher flux rates and a thicker P3OT (150 nm) layer. The dependence of Ilight on the gate bias Vg is significant only at d low light intensity, with a weak linear dependence on Vg at higher incident intensity. The large Ilight was d observed both for the enhanced and depleted mode of the FET. Responsivity as large as 1 AyW in these FET is observed at low light fluxes w13x. The substantial increase in Id upon illumination needs to be understood in the context of the following features: (i) the unipolar (hole) transport in the channel; (ii) the increase in Id at moderate-high light intensity regime, both in the enhancement and depletion mode, with a higher value (10%) of near-saturated light induced Ilight in the d
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enhancement mode; (iii) Presence of the enhanced Ilight d in absence of Vg at higher light intensity; (iv) A finite potential drop across the P3OT layer under a gate bias. The additional factors present for Ilight besides the lateral d field is more obvious upon comparison with the planar, surface configured, 2-terminal photodetector device. 3. Conclusions The polymer-based devices offer attractive structures for the development of novel photodetectors. The devices can be tailored to meet specific requirements such as wavelength region, sensitivity, response time, and substrate flexibility. Acknowledgments K.S.N. acknowledges the Department of Information Technology, Government of India for partially funding the project. References w1x G. Yu, J. Wang, J. McElvain, A.J. Heeger, Adv. Mater. 10 (1998) 1431. w2x G. Yu, J. Gao, J.C. Hummelen, F. Wudi, A.J. Heeger, Science 258 (1992) 1474. w3x N.S. Sariciftci, L. Smilowitz, A.J. Heeger, F. Wudl, Science 258 (1992) 1474. w4x K.S. Narayan, T.B. Singh, Appl. Phys. Lett. 74 (1999) 3456. w5x J.J.M. Halls, C.A. Walsh, N.C. Greenham, F.A. Marseglia, R.H. Friend, S.C. Moratti, A.B. Holmes, Nature 376 (1995) 498. w6x X.L. Chen, Z. Bao, J.H. Schon, A.J. Lovinger, Y.Y. Lin, B. Crone, A. Dodabalapur, B. Batlogg, Appl. Phys. Lett. 78 (2001) 228. w7x C.W. Tang, Appl. Phys. Lett. 48 (1986) 183. w8x S.A. Jenekhe, S. Yi, Appl. Phys. Lett. 77 (2000) 2635. w9x A.G. Manoj, A.A. Alagiriswamy, K.S. Narayan, Opt. Mater., in press. ¨ ¨ S. Strite, J. Appl. Phys. 78 (1995) 607. w10x M.S. Unlu, w11x Th.B. Singh, U.V. Waghmare, K.S. Narayan, Appl. Phys. Lett. 80 (2002) 1213. w12x J.H. Schon, ¨ Ch. Kloc, A. Dodabalapur, B. Batlogg, Science 289 (2000) 599. w13x K.S. Narayan, N. Kumar, Appl. Phys. Lett. 79 (2001) 1891.
Novel strategies for polymer based light sensors K.S. Narayan*, A.G. Manoj, Th.B. Singh, A.A. Alagiriswamy Chemistry and Physics of Materials Unit, Jawaharlal Nehru Center for Advanced Scientific Research, Jakkur, Bangalore 560 064, India Received 7 December 2001; accepted 22 February 2002
Abstract We present results of conjugated polymer based devices for optical detection and image sensors. These devices have efficiencies comparable to conventional inorganic semiconductor based devices, coupled with the inherent advantage of having polymers as the active component in the device especially in terms of large area, cost effectiveness, chemical tunability and flexibility. We have adopted different strategies and device-architecture resulting in novel photodetectors; (i) p-typeyn-type polymer heterostructures, with high performance in terms of photosensitivity and conversion efficiency, (ii) resonant cavity enhanced photodiodes, (iii) light responsive field effect transistors. We discuss the results and possible applications of these light detecting device structures. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Photodetectors; Conjugated polymers
1. Introduction Polymer based photodetectors complements the polymer-light-emitting diodes and can constitute the basic design element for an all-organic integrated opto-electronic circuit. An organicypolymer Schottky photodiodes can be fabricated with the active polymer layer sandwiched between the two electrodes with a suitable difference in work functions w1,2x. In the last decade efforts to increase the quantum efficiency of charge generation and separation efficiencies in polymers such as derivatives of polyphenylenevinylene (PPV) and polythiophene (PTH) were quite successful w3x. The photosensitivity of these polymers significantly increased upon dispersing electron acceptors such as derivatised C60 w3x. Along with this strategy of having the active layers in form of blends with electron acceptor w3,4x multilayers of donor–acceptor w5x systems for increasing the photocurrent in devices have also been reported with responsivity approaching traditional semiconductor photodiodes. We present here novel design options provided by active polymer photodetectors. The three examples mentioned in this report represents a set *Corresponding author. Tel.: q91-80-8462750-7x251; fax: q9180-8462766. E-mail address: [email protected] (K.S. Narayan).
of conceptually distinct methods for generating photocurrent. The essential commonality in all these type of detectors is the eventual generation of charge carriers upon photo-exciting the semiconducting polymer. A generally accepted view point is that the electron–hole separation resulting in free carriers is a secondary process. The photocurrent observed in polymers such as PPV and PTH are largely contributed by the transport of holes across the medium, while the electrons get severely trapped at the defect sites (mn-mp). Another class of heteroaromatic polymers, such as the ladder type polymer benzamidazobenzophenanthroline (BBL), however has electrons as the primary contribution for the photocurrent (mn)mp) w6x. The bilayer structure of this ptype and n-type polymer reveals interesting properties w7,8x. 2. Results and discussion Bilayer devices was obtained by spin coating the solution of poly(3-octylthiophene-2,5-diyl) (P3OT) in p-xylene on indium tin oxide (ITO) coated transparent glass plate. BBL in Lewis acid (1 N aluminium chloride in nitrobenzene) was spin coated on top of P3OT and washed thoroughly to remove the aluminium chloride.
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 5 7 4 - 6
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K.S. Narayan et al. / Thin Solid Films 417 (2002) 75–77
Fig. 1. The device structures of (a) bilayer p-type polymeryn-type polymer photodetector (b) resonant cavity device; with illumination from semi-transparent gold (Au) layer and chemical structure of the polymer P3OT. ITO—indium tin oxide, DBR—distributed Bragg reflector of 16 pairs of AlAsyAlGaAs. (c) Schematic of the light responding polymer FET.
Gold (Au) or aluminium (Al) was coated on top of the BBL as cathode, Fig. 1a. The interface facilitates hole transfer from BBL to P3OT and electron transfer from P3OT to BBL. The interface acts as a barrier for electron from BBL to P3OT and for hole from P3OT to BBL. The salient features of this device are (i) the photocurrent conversion efficiency is two–three orders of magnitude higher than the single layered devices, (ii) the broadening of spectral range covering the entire visible range as shown in Fig. 2, (iii) the open circuit voltage is controlled by the estimated Fermi level difference of
the polymers rather than the electrode work function, (iv) the energy conversion efficiency is f0.3% w9x. The concept of ‘resonance cavity enhanced’ (RCE) ¨ ¨ photodiodes for inorganic devices demonstrated by Unlu and Strite w10x is adapted for the polymer based devices. The procedure entails the introduction of a thin polymer active layer in the micro-cavity, which is essentially a resonator with an effective cavity length of half the wavelength of light, (Fig. 1b). We observe, upon implementing the RCE structure, that the gain of the photodiode is not compromised while decreasing the transit time. Thin polymer films cast from solution shows clear evidence of spectral broadening and band tailing with characteristic energies of a few tenths of an eV. We utilize this finite absorption at wavelengths lc-hco yEg, which is present in these polymers. In this approach we also demonstrate the tunability of the photodiode to a wavelength lc-hco yEg, approaching the near-infrared region. The essential features of this photodiode are (i) significant red-shifted photocurrent at the non-absorbing region of the polymer; (ii) A local maximum, lmax, in this red-shifted photocurrent controlled by the cavity parameters; (iii) Iph (l) variation with distributed Bragg reflector (DBR) mirror substrates tuned to different wavelength. In absence of the DBR substrate the Iph (l) (not shown here) essentially follows the absorption response; (iv) The photocurrent corresponding to lmax, has a stronger dependence to the incident angle compared to the photocurrent corresponding to high absorption wavelength; (v) the switching speed of photoresponse is in the GHz range. The micro-cavity enhances the photodetection property in terms of speed and gain at a desired detection wavelength, specifically in the low-absorption region much below the band-edge. We achieve a photoresponsivity f10 mAyW, along with a speed )1 GHz in these micro-cavity enhanced polymer based photodetectors, at zero bias in the near-IR region w11x. Field effect transistors (FETs) based on organic materials have recently been demonstrated to be a route for
Fig. 2. The spectral response of a bilayer polymer device along with single layer response of P3OT and BBL.
K.S. Narayan et al. / Thin Solid Films 417 (2002) 75–77
fabricating novel and efficient devices w12x. We observe a transistor action that gets considerably modified with photoexcitation with large changes in the drain–source current Id, and the saturation value of drain–source current Ids depending on both the intensity of light and the gate voltage. The large photo-induced drain current, Ilight is a consequence of an internal amplification prod cess resulting from the photogenerated carriers which is possible only in this transistor configuration. The response of an organic FET to visible light also opens up the utility of these devices as image sensors. We demonstrate that in an organic FET as shown in Fig. 1c, the incident light intensity can act as an added control parameter in the transistor operation. Dielectric layers such as silica and polyvinyl alcohol were chosen as the insulating medium with aluminium electrode forming the gate electrode. Regioregular P3OT with 98.5% head to tail regiospecific conformation was dissolved in chloroform and spin coated on the insulator to yield a 100-nm thick film. Gold electrodes, 3-mm wide with an inter-electrode spacing of 70 mm, forming the channel length, was deposited on P3OT to form the source and drain electrodes. The gain factor in the drain current Id; Ilight yId, due to light with photon flux rate as d low as f1 mW, in this case, is f100. It was also observed that the gain (Ilight yId) can be further increased d to as high as 103 for certain devices, with higher flux rates and a thicker P3OT (150 nm) layer. The dependence of Ilight on the gate bias Vg is significant only at d low light intensity, with a weak linear dependence on Vg at higher incident intensity. The large Ilight was d observed both for the enhanced and depleted mode of the FET. Responsivity as large as 1 AyW in these FET is observed at low light fluxes w13x. The substantial increase in Id upon illumination needs to be understood in the context of the following features: (i) the unipolar (hole) transport in the channel; (ii) the increase in Id at moderate-high light intensity regime, both in the enhancement and depletion mode, with a higher value (10%) of near-saturated light induced Ilight in the d
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enhancement mode; (iii) Presence of the enhanced Ilight d in absence of Vg at higher light intensity; (iv) A finite potential drop across the P3OT layer under a gate bias. The additional factors present for Ilight besides the lateral d field is more obvious upon comparison with the planar, surface configured, 2-terminal photodetector device. 3. Conclusions The polymer-based devices offer attractive structures for the development of novel photodetectors. The devices can be tailored to meet specific requirements such as wavelength region, sensitivity, response time, and substrate flexibility. Acknowledgments K.S.N. acknowledges the Department of Information Technology, Government of India for partially funding the project. References w1x G. Yu, J. Wang, J. McElvain, A.J. Heeger, Adv. Mater. 10 (1998) 1431. w2x G. Yu, J. Gao, J.C. Hummelen, F. Wudi, A.J. Heeger, Science 258 (1992) 1474. w3x N.S. Sariciftci, L. Smilowitz, A.J. Heeger, F. Wudl, Science 258 (1992) 1474. w4x K.S. Narayan, T.B. Singh, Appl. Phys. Lett. 74 (1999) 3456. w5x J.J.M. Halls, C.A. Walsh, N.C. Greenham, F.A. Marseglia, R.H. Friend, S.C. Moratti, A.B. Holmes, Nature 376 (1995) 498. w6x X.L. Chen, Z. Bao, J.H. Schon, A.J. Lovinger, Y.Y. Lin, B. Crone, A. Dodabalapur, B. Batlogg, Appl. Phys. Lett. 78 (2001) 228. w7x C.W. Tang, Appl. Phys. Lett. 48 (1986) 183. w8x S.A. Jenekhe, S. Yi, Appl. Phys. Lett. 77 (2000) 2635. w9x A.G. Manoj, A.A. Alagiriswamy, K.S. Narayan, Opt. Mater., in press. ¨ ¨ S. Strite, J. Appl. Phys. 78 (1995) 607. w10x M.S. Unlu, w11x Th.B. Singh, U.V. Waghmare, K.S. Narayan, Appl. Phys. Lett. 80 (2002) 1213. w12x J.H. Schon, ¨ Ch. Kloc, A. Dodabalapur, B. Batlogg, Science 289 (2000) 599. w13x K.S. Narayan, N. Kumar, Appl. Phys. Lett. 79 (2001) 1891.