Optimization of hydrogenated amorphous silicon p-i-n solar cells with two-step i layers guided by real-time spectroscopic ellipsometry

Hydrogenated amorphous silicon iir-Si:I-Ij p-i-n solar cell performance has been optimized using a two-step i-layer growth process. This effort has been guided by real-time spectroscopic ellipsometry (RTSE) studies of the nucleation and growth of a-Si:H films by plasma-enhanced chemical vapor deposition at 200 degrees C using a variable H-2-dilution gas flow ratio R=[H-2]/[SiH4]. RTSE studies during film growth with R>15 reveal a transition from the amorphous to microcrystalline (a-->mu c) phase at a critical thickness that decreases with increasing R. From such results, the optimum two-step process was designed such that the initial stage of the i layer (similar to 200 Angstrom) is deposited at much higher R than the bulk to ensure that the film remains within the amorphous side of the a-->mu c phase boundary, yet as close as possible to this boundary at low i-layer thicknesses. (C) 1998 American Institute of Physics.