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ABSTRACT The photocurrent delivered by microcrystalline hydrogenated silicon p-i-n devices and the spectral response are analysed under different applied bias voltage. The spectral response is extended far beyond 900 nm. Under reverse bias the spectral response is high and essentially unchanged while under increasing forward bias it decreases continuously. For forward bias higher than the open circuit voltage, the usual reversal of the spectral response is not observed and the photocurrent remains negative and almost independent of the bias voltage.

A heterojunction model based on the presence of additional local electric fields near the grain boundaries is presented to explain this behaviour. Those fields lead to a strong increase of the recombination at the grain boundaries decreasing the contribution from the photogenerated carriers for the secondary photocurrent. Reverse bias restores the electric field at the interfaces minimizing the influence of the local barriers. INTRODUCTION Microcrystalline hydrogenated silicon materials (kc-Si:H) have become promising candidates for low-cost applications in the field of large-area optoelectronic sensors due to their efficient absorption of visible/infrared light and their suitability in device realization on large area substrates since they combine some advantages of amorphous and crystalline silicon. When compared with the amorphous counterpart they have also the advantage of a faster response, high current capability and excellent stability under illumination [1, 2, 3]. The microcrystalline silicon films are growing in columnar fashion and consist of silicon grains embedded in an amorphous matrix. Due to their mixed structure the electronic transport under illumination and the exact operation of these devices has not been completely clarified [4]. The correlation between the experimental current-voltage dependence, the spectral response and a detailed simulation analysis can provide important information on the transport mechanism inside the device. In this study we used the closed-chamber chemical vapor deposition method (CC-CVD) [5] to produce thin entirely ic-Si:H p-i-n structures resulting in wide spectral range photodiodes with an enhanced sensitivity to the near infrared region and a positive spectral response under forward bias. A heterojunction model, based on the band discontinuities near the grain boundaries, is presented and supported by numerical simulation. EXPERIMENTAL DETAILS AND DEVICE CHARACTERIZATION All work was carried out on devices having a glass/ZnO/p/i/n/metal configuration in which the light is incident through the glass. The entirely microcrystalline p/i/n-Si:H structures were deposited on ZnO-coated glass substrates by the new CC-CVD method using 193 Mat. Res. Soc. Symp. Proc. Vol. 507 01998 Materials Research Society

an alternating process of silane or hydrogen plasma processes [6]. For the characterization of the films, optoelectronic measurements have been applied as described elsewhere [7]. The current-voltage (I-V) characteristics in