Advanced Deposition Phase Diagrams for Guiding Si:H-Based Multijunction Solar Cells
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0989-A15-02
Advanced Deposition Phase Diagrams for Guiding Si:H-Based Multijunction Solar Cells Jason A. Stoke, Nikolas J. Podraza, Jian Li, Xinmin Cao, Xunming Deng, and Robert W. Collins Department of Physics and Astronomy, University of Toledo, Toledo, OH, 43606 ABSTRACT Phase diagrams have been established to describe very high frequency (vhf) plasmaenhanced chemical vapor deposition (PECVD) processes for intrinsic hydrogenated silicon (Si:H) and silicon-germanium alloy (Si1−xGex:H) thin films using crystalline Si substrates that have been over-deposited with n-type amorphous Si:H (a-Si:H). The Si:H and Si1−xGex:H processes are applied for the top and middle i-layers of triple-junction a-Si:H-based n-i-p solar cells fabricated at University of Toledo. Identical n/i cell structures were co-deposited on textured Ag/ZnO back-reflectors in order to correlate the phase diagram and the performance of single-junction solar cells, the latter completed through over-deposition of the p-layer and top contact. This study has reaffirmed that the highest efficiencies for a-Si:H and a-Si1−xGex:H solar cells are obtained when the i-layers are prepared under maximal H2 dilution conditions. INTRODUCTION State-of-the-art solar cells based on a-Si:H prepared by PECVD employ a triple-junction design [1,2]. Optimization of the a-Si:H i-layer of the top cell has been widely successful through the concept of maximal H2 dilution [3,4]. The benefits of H2 dilution include enhanced surface passivation and hence diffusion of film precursors in the PECVD process, as well as enhanced relaxation of sub-surface strained Si-Si bonds [4]. The resulting "protocrystalline" nature of the i-layer provides the highest device performance and stability. In this study, vhf PECVD phase diagrams have been developed for Si:H and Si1−xGex:H i-layers deposited from Si2H6 + GeH4 mixtures on a-Si:H n-layers under conditions used for high-efficiency triplejunction n-i-p cells. These diagrams have been correlated with single-junction cell performance in order to further explore and extend the applicability of the maximal H2 dilution concept. EXPERIMENTAL DETAILS The Si:H and Si1−xGex:H i-layers for phase diagram development by real time spectroscopic ellipsometry (RTSE) were deposited on c-Si/(native-oxide)/(n-type a-Si:H) substrate structures using multichamber vhf (70 MHz) PECVD. Such substrates ensure a specular surface to aid in the utilization of RTSE [5] for the first time in the characterization of PECVD a-Si1−xGex:H from S2H6 and GeH4. In order to apply deposition phase diagrams from RTSE for insights into n-i-p solar cell performance, additional samples ~2000 ≈ thick were co-deposited onto textured Ag/ZnO/(n-type a-Si:H) back-reflectors in the device configuration simultaneously with the specular c-Si/(native-oxide)/(n-type a-Si:H) substrates. For depositions performed versus the phase diagram variable R=[H2]/{[Si2H6]+[GeH4]}, all other parameters were selected as those used for the previously-optimized top and middle i-layers of a triple junction sola
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