Intensity dependence of quantum efficiency and photo‐gating effects in thin film silicon solar cells

Abstract

AbstractSteady‐state photoconductivity measurements have been carried out on thin‐film silicon pin structures of i‐layer thickness typically 4 μm, where crystalline composition has been varied by adjustment of the silane concentration in the process gas. In amorphous and low‐crystallinity cells, strongly‐absorbed light incident from the p‐side at photon fluxes in excess of 1014 cm‐2 s‐1 produces strongly sub‐linear intensity dependence, ‘S’ shaped reverse current‐voltage curves and amplification of a second weakly‐absorbed beam, termed photogating. These effects are linked to the formation of space charge and attendant low‐field region close to the p‐i interface, as confirmed by computer simulation. More crystalline devices exhibit little or no such behaviour. At lower intensities of strongly‐absorbed light there is a markedly steeper increase in reverse current vs. voltage in low‐crystalline when compared to amorphous cells, particularly with light incident from the n‐side. This suggests the mobility‐lifetime product for holes is much larger in the former case, consistent with the higher hole mobilities reported in time of flight studies. Thus the prospect of composition‐dependent changes in mobility as well as defect density should be borne in mind when developing materials for application in microcrystalline silicon solar cells. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)