Comparison of the Effect of Light Soaking in Porous Silicon and a-Si:H
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A9.8.1
Comparison of the Effect of Light Soaking in Porous Silicon and a-Si:H
N. P. Mandal and S. C. Agarwal Department of Physics, Indian Institute of Technology, Kanpur, 208016, India.
ABSTRACT In undoped a-Si:H thin films, light soaking (LS) decreases dark current and photocurrent and increases the electron spin resonance signal monotonically with increasing exposure time, (Staebler-Wronski Effect, SWE). On the other hand, in porous silicon (PS) the light-soaking effect is non-monotonic. For short durations, LS increases dark current, photo current and photoluminescence and decreases the ESR signal. Long exposures of PS, however, have the opposite effect, which is similar to the SWE observed in a-Si:H. All metastabilities in PS as well as in a-Si:H can be removed by annealing, but not by exposure to infrared light. We find that the combined effect of NH3 vapor exposure and LS on PS yields a final state that depends upon the order in which the two steps are performed. This is in contrast to the corresponding observation in a-Si:H. We could arrest the light-induced degradation of PS over long times by coating with a thin polystyrene, which resulted in constant PL and ESR intensities. However, in the case of a-Si:H the polymer coating decreases the SWE, but does not eliminate it. The results can be understood, if we were to propose that LS affects the surface in PS, but affects mainly the bulk in a-Si:H.
INTRODUCTION Light induced degradation of a-Si:H limits the use of a-Si:H in solar photovoltaic applications [1]. This effect, popularly known as the Staebler Wronski effect [2], is believed to be the result of the photostructural changes caused by the movement of hydrogen [1]. Porous silicon (PS) consists of nanometer size crystallites of silicon and shows photoluminescence (PL) in the visible at room temperature. PS has attracted quite a lot of attention because of the possibility of its use in optoelectronic devices and sensors [3]. PS also degrades upon exposure to light and is sensitive to the ambient conditions [3, 4]. These instabilities are a hurdle in fabricating stable optoelectronic devices based on PS. The degradation caused by light soaking in PS and a-Si:H are carefully studied and compared. It is found that although PS and a-Si:H, both show photostructural changes upon light soaking, they seem to have different origins. We find that LS in PS affects mainly the surface, whereas in a-Si:H it is widely believed to affect the bulk [5]. This conclusion is arrived at by carefully noting the changes observed in dark current (DC) and photocurrent (PC) as a function of light exposure and under different ambient conditions in the two materials. Upon exposing the samples to cycles of LS and NH3, it is found that in PS the final metastable state depends on the order in which these steps are performed. However, in the case of a-Si:H it is reported [5] that the order of the operations is not important. Moreover, we report that a coating of polymer arrests the photo-induced degradation of PS. Although a coating
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