Photocarrier Transport and Recombination in Amorphous Silicon
- PDF / 536,014 Bytes
- 12 Pages / 414.72 x 648 pts Page_size
- 118 Downloads / 242 Views
PHOTOCARRIER TRANSPORT AND RECOMBINATION IN AMORPHOUS SILICON C.R. WRONSKI, R.M. DAWSON, M. GUNES, Y.M. LI, AND R.W. COLLINS Electronic Materials Processing Research Laboratory, The Pennsylvania State University, University Park, PA 16802 ABSTRACT The effect of microstructure in undoped a-Si:H films on carrier transport, recombination, densities of midgap states and solar cell characteristics has been investigated. Extended state mobilities of electrons were obtained from photo and dark conductivity measurements between 400 C and 1900 C and the gap states characterized using Dual Beam Photoconductivity. In these films the estimated room temperature electron mobilities increase from about 1 to 30 cm2/V sec as the dihydride concentrations and void volume fractions decrease. It is found that the carrier mobility-lifetime products are not solely determined by the dangling bond states. The effects of changes in the mobilities and midgap states on p-i-n homojunction solar cell characteristics are presented and discussed. INTRODUCTION Significant progress has been made in improving the stabilized efficiencies of a-Si-H based solar cells and panels despite the lack of quantitative correlations between material properties, as evaluated in thin films, and solar cells fabricated from these materials. '12 The approach taken, which has led to these improvements, has been primarily based on the use of actual solar cells to characterize the effects of different materials rather than relying on results obtained from the studies on thin films. This approach however has made many of the studies that have been carried out on thin films not only somewhat irrelevant, but also has sidestepped the possibility that this lack of correlation could in part, be due to elements that are missing from our fundamental understanding of carrier transport and recombination in a-Si:H based materials. Recently considerable interest has been shown in a-Si:H materials deposited at high temperatures and in their potential for improving solar cell performance0 4 However the extent to which the structural changes in these materials affect the carrier transport and Mat. Res. Soc. Symp. Proc. Vol. 297. @1993 Materials Research Society
444
recombination as well as solar cell performance has not been quantified. This paper presents and discusses results on carrier transport, recombination and gap states in undoped a-Si:H materials with different hydrogen contents and microstructure. The effect of the changes in these properties on solar cell performance is also explored by incorporating them into p-i-n homojunction cells. EXPERIMENTAL DETAILS The undoped a-Si:H films (0.8-1.0 prm thick) were prepared by RF PECVD using pure silane at a Ptal pressure of 0.5 Torr, a flow rate of 15 sccm, substrate temperature T, between 200 and 380°C, and RF power density of 200 mW/cm 2 . The device quality films were codeposited on crystalline silicon, for FTIR measurements of hydrogen and or spectroscopic ellipsometric measurements of void fractions5 , as well as corning 705
Data Loading...
