Optimization of Si:H multijunction n-i-p solar cells through the development of deposition phase diagrams

Phase diagrams have been developed to guide very high frequency (vhf) plasma-enhanced chemical vapor deposition (PECVD) of intrinsic hydrogenated amorphous silicon (a-Si:H), amorphous silicon-germanium alloys (a-Si1-xGex:H), and nanocrystalline silicon (nc-Si:H) for use as the top, middle, and bottom cell i-layer components, respectively, of triple junction n-i-p solar cells. These phase diagrams have been used in conjunction with cross-sectional transmission electron microscopy (XTEM) to identify nucleation and growth behavior in order to gain a better understanding of phase evolution. The substrates for phase diagram development by real time spectroscopic ellipsometry (RTSE) are crystalline Si wafers that have been over-deposited with either n-type a-Si:H for the top and middle cell amorphous i-layers, or n-type nc-Si:H for the bottom cell nanocrystalline i-layer. Identical n/i cell structures were co-deposited on textured (stainless steel)/Ag/ZnO, which serve as substrate/back-reflectors, in order to relate the RTSE-developed phase diagrams to the performance parameters of single-junction solar cells. 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 previously-described maximal H2 dilution conditions. The phase diagram for the bottom cell using a nc-Si:H n-layer reveals for the first time a bifurcation at a critical R value between mixed-toamorphous phase transitions [(a+nc) → a] at low R and mixed-to-single phase nanocrystalline transitions at high R [(a+nc) → nc]. The highest efficiency single-step nc-Si:H solar cell is obtained at minimal R while remaining on the nanocrystalline side of the identified bifurcation where suitable grain boundary passivation is assured.