Protocrystalline Si:H p-type Layers for Maximization of the Open Circuit Voltage in a-Si:H n-i-p Solar Cells
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PROTOCRYSTALLINE Si:H p-TYPE LAYERS FOR MAXIMIZATION OF THE OPEN CIRCUIT VOLTAGE IN a-Si:H n-i-p SOLAR CELLS R. J. Koval, Chi Chen, G. M. Ferreira, A. S. Ferlauto, J. M. Pearce, P. I. Rovira, C. R. Wronski, and R. W. Collins Materials Research Institute, Center for Thin Film Devices, and Department of Physics, The Pennsylvania State University, University Park, PA 16802. ABSTRACT We have revisited the issue of p-layer optimization for amorphous silicon (a-Si:H) solar cells, correlating spectroscopic ellipsometry (SE) measurements of the p-layer in the device configuration with light current-voltage (J-V) characteristics of the completed solar cell. Working with p-layer gas mixtures of H2/SiH4/BF3 in rf plasma-enhanced chemical vapor deposition (PECVD), we have found that the maximum open circuit voltage (Voc) for n-i-p solar cells is obtained using p-layers prepared with the maximum possible hydrogen-dilution gas-flow ratio R=[H2]/[SiH4], but without crossing the thickness-dependent transition from the a-Si:H growth regime into the mixed-phase amorphous + microcrystalline [(a+µc)-Si:H] regime for the ~200 Å p-layers. As a result, optimum single-step p-layers are obtained under conditions similar to those applied for optimum i-layers, i.e., by operating in the so-called “protocrystalline” Si:H film growth regime. The remarkable dependence of the p-layer phase (amorphous vs. microcrystalline) and n-i-p solar cell Voc on the nature of the underlying i-layer surface also supports this conclusion. INTRODUCTION In previous investigations, novel p-layers deposited with high hydrogen-dilution gas flow ratios R=[H2]/[SiH4] were incorporated into amorphous silicon (a-Si:H) n-i-p solar cells, resulting in high open circuit voltages (Voc) and record high efficiencies [1-3]. It was proposed that (i) these optimum p-layers are, in fact, high-conductivity doped microcrystalline Si:H (µc-Si:H) and that (ii) such µc-Si:H p-layers in general lead to a significant increase in the builtin potential of the junction, as well as a reduced series resistance and reduced optical losses. In the earliest work, however, the conductivity measurements were actually performed on thick players (>1000 Å), whereas the p-layers incorporated into devices were very thin (~100 Å) [1]. In addition, the follow up reports showed that the high Voc values actually increase as the p-layer deposition temperature is decreased. Increased microcrystallinity in the lower temperature p-layers was hypothesized [2,3], in spite of a previous report showing an opposite trend with deposition temperature [4]. Reflection high energy electron diffraction (RHEED) patterns were also presented as evidence that the optimum p-layers in n-i-p solar cells were µc-Si:H. The grazing incidence angle of RHEED, however, makes this a highly surface sensitive technique that does not probe the p-layer phase closest to the i/p interface. Thus, in view of the evidence presented to date, an explanation of the results of Refs. [1-3] in terms of a predominantly microcrystalline p-layer may sti
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