Polymer photovoltaic detectors: progress and recent developments

Thin Solid Films 451 – 452 (2004) 105–108

Polymer photovoltaic detectors: progress and recent developments Pavel Schilinskya,b, Christoph Waldaufa,b, Jens Haucha, Christoph J. Brabeca,* b

a SIEMENS AG, CT MM1, Innovative Polymers, Paul-Gossen-Str. 100, D-91052 Erlangen, Germany Energy and Semiconductor Research Laboratory, Institute of Physics, University of Oldenburg, D-26111 Oldenburg, Germany

Abstract Polymer photovoltaic detectors develop rather fast to a mature technological level, significantly faster than the technological closely related solar cells. The lack of near IR absorbing polymers, currently limiting the polymer solar cell efficiency, is not relevant for the performance of UVyV is detectors. Recent progress improved the polymer photovoltaic detectors to a level sufficient for many applications. External quantum efficiencies exceeding 80% in the visible range, linearity over several decades, low dark currents, a capacitance limited noise and a fast transient behaviour are demonstrated for prototype thin film detector with an active layer thickness of approximately 100 nm. Uncritical processing via printing methods together with a favourable cost structure suggest these detectors for various applications like chemicalymedical sensing and analysis as well as line or matrix arrays for full colour picture recognition and safety applications. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Polymer photodetectors; Bulk heterojunction; Organic semiconductors

1. Introduction The increasing need for new electronic devices in the wide area of information technology enforced the research for alternative processing methods for semiconductors. Among the alternatives to inorganic semiconductors, organic semiconductors gain more and more interest, because they can be processed from solution, meet the requirements for flexible substrates and consequently also for flexible devices and finally, have a huge potential for monolithic integration of different devices onto one substrate within one production step. Light emitting diodes are certainly in the main focus of research, but, additional to the polymer displays, further polymer optoelectronic devices have gained interest. Among them are the polymer photovoltaic devices and polymer photodetectors. The classical inorganic photovoltaics or photodetectors have a difficult cost structure for automated large area processing. The essential cost driving factor for production of photovoltaic cells or photodetectors is the expensive invest into costly semiconductor processing technologies. The disadvantage can be dominantly attributed to processing parameters *Corresponding author. Tel.: q49-9131-7-422-45; fax: q49-91317-324-69. E-mail address: [email protected] (C.J. Brabec).

demanding elevated temperatures, vacuum, lithographic structuring and discrete processing due to glass carriers. It is therefore only consequent to think of photovoltaic or photodetector elements based on thin plastic carriers, manufactured by printing and coating techniques from reel to reel and packaged by lamination techniques. Polymeric semiconductors are such a class of semiconductors that could fulfil these requirements, since their flexible chemical tailoring allows the design of semiconductors with desired electronically, optically and processing properties. Polymer photovoltaics and polymer photodetectors are based on the same fundamental principle, conversion of light into electrical power, and of course, all the arguments in favour of organic semiconductors can be fully transferred. Due to the excitonic nature of most polymeric semiconductors, photovoltaic cells made from single organic semiconductors achieve low power conversion efficiencies and incident-photon–to–current or external quantum efficiencies (EQE). A high EQE alone does not guarantee sufficient photodetector performance, but it is a prerequisite. A solution was found in 1995, when several groups independently showed that the EQE could be enhanced by several orders of magnitude upon blending two organic semiconductors, a p-type and an

0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.11.062

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Table 1 Typical parameter table of a polymer photodetector Parameter

Symbol

Value

Conditions

Peak photosensitivity Ext. quantum efficiency Spectral range Dark current Open circuit voltage Short circuit current Complete refresh Rise and fall time Capacitance Deviation of linearity

Smax h l IR Vo ISC TRA TR,TF Co Da

0.34 AyW 78% 300–630 nm -50 pAymm2 0.55–0.95 V )100 mAymm2 ;5 ms 10 ns , 200 ns - 50 pFymm2 - 0.1 %

550 nm, 0 V 550 nm, 0 V S)10% of Smax VRs10 mV 1 mWymm2, white light 1 mWymm2, white light 4 decades, white light, 50 Hz 0 V, 10 mm2 VRs0 V, fs1 kHz–10 kHz over 4 decades

n-type semiconductor w1x. The difference in electron affinity creates a driving force at the interface between the two materials that is strong enough to split photogenerated excitons. By blending materials on a nanostructured scale (approx. some 10 nanometers, i.e. the exciton diffusion length), the interface is distributed throughout the device. The concept of blending p-type semiconductors with n-type semiconductors has become popular under the name bulk heterojunction composites and significant development of the device performance has been reported during the last years w2–8x. The EQE represents that fraction of photoexcitations that survive both, charge separation and transport toward the electrodes. For bulk heterojunction devices the EQE has been increased from 29% w1x to 50% w9x and recently an EQE of 80% was reported w10x. These impressively high values certainly qualify bulk heterojunction composites for detector applications, because the performance is beginning to match their inorganic counterparts. Besides performance, lifetime and reliability are decisive. Accelerated temperature lifetime analysis suggests that these devices are of long term stability, meeting the

conditions for first applications and for further product development. 2. Experimental Devices were fabricated on glassyITO or PETyITO substrates. After cleaning of the ITO, a conducting polymer poly(ethylene dioxythiophene) doped with polystyrene sulfonic acid (PEDOT:PSS, Bayer AG) was spin-coated to a layer thickness of 60 nm. The active layer of the photodetectors consists in all cases of a polymeryfullerene composite. High quantum efficiencies are reported for blends of P3HT (poly(3-hexylthiophene)) with a methanofullerene PCBM (w6,6x-phenyl C61 butyric acid methyl ester). The metal electrode (Al or CayAg) are thermally deposited with a thickness exceeding 100 nm. Characterization of the dark and illuminated devices was performed between 0 and 85 8C. The spectral photocurrent and the detector linearity were measured by lock in technique, transient measurement were taken in a pumpyprobe geometry or by analysing the device performance upon periodically modulated excitation. Lifetime was determined by temperature accelerated degradation cycles, continually monitoring the short circuit current under illumination at 85 8C. 3. Results and discussion The decisive performance parameters for photodetectors are the spectral response, the photosensitivity or EQE, the dark current and the noise equivalent detection power, linearity, the transient behaviour or response speed and finally the lifetime. This set of essential parameters will be discussed at the hand of a prototype bulk heterojunction composite and summarized in a parameter table (Table 1). 3.1. Spectral response and performance

Fig. 1. EQE of a bulk heterojunction device with a layer thickness of 350 nm. The photocurrent has been converted to the EQE according to: EQE w%xs1240=Isc wmAycm2xyl wnmx I wWym2x.

The spectral response of polymer detectors based on P3HTyPCBM bulk heterojunction composites covers the

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3.2. Linearity

Fig. 2. Typical IyV characteristics of a polymer photodetector, in the dark (j) and under illumination (s) with 1 sun (solar simulator with 100 mWycm2).

UV and visible range. A typical spectrally resolved EQE is plotted in Fig. 1. All data were taken under zero bias. The peak EQE is observed approximately at 550 nm with ;75%. Highest values observed are over 80%, corresponding to a peak photosensitivity exceeding 0.3 AyW in the visible range. A detailed performance analysis of the detector revealed, that reflection and absorption losses of the electrodes and of the substrate are responsible that the EQE is limited approximately to 80%. Further increase can be expected by using antireflection coatings and low absorbing electrodes. One of the advantages operating a photodetector under zero bias is the low dark current. Especially for the semiconducting polymers discussed here, the rather wide bandgap of typical 2 eV meets the expectation of a low noise level and a large dynamic range if operated under zero applied bias. Fig. 2 shows a typical current–voltage (IyV) characteristics of a photodetector in the dark and illuminated under 1 sun. The dark current density under small (10 mV) or zero bias is below the nAycm2 range, caused by a small ohmic shunt in the order of )5Ø106 V cm2. Comparing the dark and illuminated IyV curves, the dynamic range of the detector is exceeding six orders in magnitude if operated at small voltages. Impedance analysis was applied to determine the capacity of the photodetectors. As expected, capacity is found to be governed by the geometrical capacity, depending directly on the detector thickness or on the detector area. Under reverse voltage, little or even no bias dependence of the capacity is observed, indicating, that the detector either does not have a space charge region or that the space charge region extends over the whole active layer thickness. Both explanations are in accordance to the picture for the working principle of thin film bulk heterojunction devices: an intrinsic layer sandwiched between two quasi-ohmic, selective contacts.

Linear behaviour of the photodetector with light dependence is a prerequisite to guarantee exploitation of the full dynamic range. Especially for imaging application linearity over the full dynamic range is essential. The linearity of the polymer photodetectors was measured in two different ways: First, detectors were illuminated with light of different intensities while the absolute illumination level was monitored by a reference diode. The disadvantage of this method is that linearity cannot be determined more precisely than the linearity of the reference diode allows. Second, linearity was measured by evaluating the illumination intensity modulated photocurrent by a lock-in amplifier while varying the intensity of the background illumination over several decades. This method has the advantage that the precision of a reference diode, monitoring the background illumination, is not limiting. For the polymer detectors investigated, the deviation from linearity was found to be better than 0.1% over several decades (Table 1), which is directly comparable to the performance of Si detectors. 3.3. Transient behaviour The recognition or reconstruction of pictures is of course one of the main applications for photodetectors or photodetector arrays, as it is required for scanners, cameras or copiers. Independent from the specific application requirements, picture recognition demands the periodic read out of a detector signal. The frame rate for picture generation determines the requirements on the transient behaviour of the photodetector. Recent progress reported a time constant of ;0.8 ms w11x for polymer photodetectors with an active area of 0.16 cm2. We investigated photodetectors with an active area of 0.05 cm2, and response times as fast as 10 ns can be observed (Table 1). Equally important as the turn on kinetics is the time required to completely refresh the detector, i.e. to sweep out all photogenerated carriers. Fig. 3 shows the slow transient behaviour of a typical polymer photodetector over 4 decades in magnitude. A signal decay of 4 orders in magnitude is observed within the first 5 ms after switching off illumination. Such a fast decay of the photocurrent is a sign for a rather pure semiconductor with a low density of traps. Please note that the noise level of the measurement is caused by the resolution limit of the measurement setup, which uses a 16 bit Dy A card. 3.4. Stability Favourable shelf life times of polymer photodetectors exceeding one year have been reported recently w11x.

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Lifetimes exceeding 1000 h under intense illumination and at 85 8C are observed. Assuming a similar temperature dependence of the acceleration factor like for OLEDs w12x, operational lifetimes exceeding 10 000 h are expected under indoor conditions. 4. Conclusion

Fig. 3. Slow transient behaviour of a polymer photodetector. The detector was operated in the photovoltaic mode under zero bias, a pulsed white LED was used for the excitation. Photocurrent was converted by a currentyvoltage amplifier and fed into the voltage port of a 16 bit DyA card.

Additional to the shelf life time we have monitored the operational life time under different elevated temperatures. Fig. 4 shows a typical ageing curve for an organic photodetector under operation. Small changes in the low percentage range are monitored within the first 250 h.

Fig. 4. Accelerated ageing of a polymer photodetector. The detector is illuminated by halogen lamps with an intensity of app. 0.6 suns and operated in the photovoltaic mode at an elevated temperature of 85 8C. No corrections were taken to compensate temperature or illumination fluctuations.

The performance of polymer photodetectors has been significantly improved during the last years. High efficiencies, a wide dynamic range and long lifetimes are reported, meeting all specifications for practical applications. Together with the advantages of solution processing, enabling the low cost production of large area detectors or detector arrays, this technology becomes mature for applications including biomedical and medical sensing and analysis, controlling automation, full colour picture recognition, security applications, environmental UV sensing and in various fields of information technology. References w1x (a) G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995) 1789 ¨ (b) M. Granstrom, K. Petritsch, A.C. Arias, A. Lux, M.R. Andersson, R.H. Friend, Nature 395 (1998) 257. w2x J. Nelson, Mater. Today 5 (2002) 5. w3x C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Func. Mater. 2001. w4x S.E. Shaheen, C.J. Brabec, N.S. Sariciftci, F. Padinger, T. Fromherz, J.C. Hummelen, Appl. Phys. Lett. 78 (2001) 841. w5x C.J. Brabec, S.E. Shaheen, N.S. Sariciftci, P. Denk, Appl. Phys. Lett., 2002. w6x C.J. Brabec, G. Zerza, G. Cerullo, S. de Silvestri, S. Luzzati, J.C. Hummelen, N.S. Sariciftci, Chem. Phys. Lett. 340 (2001) 232. w7x G. Yu, Y. Cao, G. Srdanow, A.J. Heeger, MRS Spring Meeting, San Francisco, April 13–17 (1999). w8x G. Yu, J. Wang, J. McElvain, A.J. Heeger, Adv. Mater. 10 (1998) 1431. w9x S.E. Shaheen, C.J. Brabec, N.S. Sariciftci, F. Padinger, T. Fromherz, J.C. Hummelen, Appl. Phys. Lett. 78 (2001) 841. w10x P. Schilinsky, C. Waldauf, C. Brabec, Appl. Phys. Lett. 81 (2002) 3885–3887. w11x G. Yu, G. Srdanov, H. Wang, Y. Cao, A.J. Heeger, Proc. SPIE 4108 (2001) 48. w12x S. Schuller, P. Schilinsky, C.J. Brabec, in preparation.