Steady State Photocarrier Charge Collection in a-Si:H for Electrons and Holes
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STEADY STATE PHOTOCARRIER CHARGE COLLECTION IN a-Si:H FOR ELECTRONS AND HOLES G.H. Bauer* Abel*, H.R. Paes**, Elektronik, C.-D. *Institut fur Physikalische Universit~t Stuttgart, Pfaffenwaldring 47, D-7000 Stuttgart-80, F.R.Germany **COPPE / PEMM Universidade Federal, Rio de Janeiro, Brasil Abstract Carrier transport in amorphous silicon pin-diodes has been analyzed by steady state photocarrier charge collection applying strongly absorbed light. For low intensities at room temperature electron charge collection is limited by recombination in the generation region. For increasing intensity 4)and/or decreasing temperature charge collection becomes nonlinear in 4) and shows S-like characteristics versus voltage. We present a model for this behaviour, including space charge limitation which e.g. for holes in a-Si:H limits charge collection even at room temperature due to low extended state mobility of holes and dark Fermi level position above midgap. Introduction Steady state photocarrier charge collection measured on pin-structures gives an insight on transport and recombination. A local probing of tsr-products in pin-diodes has been achieved by variation of photon energy for charge generation recently discussed in detail [1]. The drop of tsr-products in the intrinsic layer close to the pi-interface at least to a considerable degree results from the shift of Fermi level towards midgap which in a-Si:H pin-structures has to be accepted as an intrinsic effect. For higher intensities and/or decreasing temperature the voltage dependence of charge collection departs from traditional Hecht-curves to a S-like function. Further the photocurrent depends sublinearly on 4)except in the saturation regime where is still proportional to 4). Shift of quasi Fermi levels with illumination intensity well known from steady state secondary photocurrents [2],[3] and thus with voltage has been ruled out by simulation. Furthermore diffusion would increase the charge collection efficiency in contradiction to the experimental result. Photoinduced space charge in the bulk due to single carrier transport can explain both facts as will be shown in a theoretical approximation. The S-like functional dependence can neither be explained by the usual models [4]-[6] nor by our proposed one [1] since they all presuppose a photocurrent always proportional to the illumination intensity. There are several indications for space charge: normal space charge limited currents are measured on nin- or pip-samples. The injection current iSCLC as a function of applied voltage V shows iSCLC~-V with -y> 1 [7]-[9], similar to the charge collection efficiency in the low voltage regime. For crystalline semiconductors [10] one expects a square law dependence, whereas for amorphous semiconductors -ycan differ from 2 shown in a simple model for exponential gap state density by Rose ([11], -y=Eoc/kT+ 1). Further space charge effect should not only appear for high intensity but also for low temperature verified in the experiment. For holes space charge effects are already ex
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