Kinetics of Light-Induced Changes in P-I-N Cells with Protocrystalline Si:H
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ABSTRACT
Studies have been carried out on the thickness dependent transition between the amorphous and microcrystalline phases in intrinsic Si:H materials (i-layers) and its effect on p-i-n solar cell performance [I]. P(a-SiC:H)-i(a-Si:H)-n(ýtcSi:H) cell structures were deposited with the intrinsic Si:H layer thickness and the flow ratio of hydrogen to silane, R=[H 2]/[SiH 4], guided by an evolutionary phase diagram obtained from real-time spectroscopic ellipsometry. The thickness range over which the fill factors are controlled by the bulk was established and their characteristics investigated with different protocrystalline i-layer materials (i.e., materials prepared near the amorphous to microcrystalline boundary but on the amorphous side). Insights into the properties of these materials and the effects of the transition to the microcrystalline phase were obtained from the systematic changes in the initial fill factors, their light-induced changes, and their degraded steady states for cells with i-layers of different thickness and H2 dilution. INTRODUCTION
Significant progress has been made in improving the performance and stability of a-Si:H based p-i-n and n-i-p solar cells using intrinsic layers prepared with hydrogen dilution of silane [2-5]. It is found that both materials and solar cells prepared with H2 dilution of SiH.j exhibit degradation kinetics distinctly different from their undiluted counterparts. They not only exhibit less degradation under AM1.5 illumination (100mW/cm 2 ), but they also reach a degraded steady state within approximately 100 hours [5]. These are distinguishing characteristics of what are called protocrystalline materials-a term used to describe a-Si:H grown close to the microcrystalline phase boundary [1,6,7]. However, there are many unanswered questions not only about the growth of these materials but also about the nature and properties of the amorphous phase, and in particular about the material in the optically deduced transition region between the amorphous and microcrystalline regimes. The optical properties and microstructural evolution of the transition region can be characterized using real-time spectroscopic ellipsometry (RTSE) which can guide systematic studies on further evaluating the basic material properties as well as their effects on the stabilized performance of solar cells [8]. However, it is very difficult to characterize the basic electronic properties of such a transition region even from quite sophisticated thin film measurements due to the limitations of the experimental techniques with respect to depth profiling [9]. Since the protocrystalline materials by their very nature evolve with thickness, the operation of the coplanar structures used in such thin film measurements is such that the transition material is probed in parallel with the amorphous phase rather than in series. Since in p-i-n (n-i-p) solar cells two such constituent materials are effectively in series, the solar cell can be used to obtain insights not only about the "sharpness" of these t
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