Lab to large scale transition for non-vacuum thin film CIGS solar cells: Phase I annual technical report, 1 August 2002-31 July 2003

OCLC Number: 55129170 Excerpt: ... 1.2.1 Current Problems in Gallium Distribution It is well known that the addition of gallium to the CIS structure opens up the bandgap and increases the V of the resulting cell. At ISET, past OC efficiency and spectral response results had indicated that in spite of adding Ga to the absorber layer, we were not getting the full benefit of it in increasing the open circuit voltage. SIMS measurements on our samples had indicated that during the conversion process in which the Cu-In-Ga alloy was converted to CIGS, the gallium is accumulating at the rear of the absorber layer nearing the Mo interface ( Figure 1 ) due to the kinetics of the selenization step. Consequently, controlled stoichiometries, which introduce gallium nearly molecularly in our ink based deposition appears to be ineffectual at opening up the bandgap of the semiconductor. We are countering the problem of gallium depth-distribution by exploring modification routes. Gallium surface treatments will help us achieve higher cell efficiencies by opening up the material bandgap from otherwise CIS-type laboratory cells during the selenization of nanoparticle metal alloy precursors. Figure 1. SIMS Depth profile of a CIGS Absorber layer using ISET's ink-based processing scheme involving selenization of nanoparticle precursors. 1.2.2 Surface Modification - Method Development The general strategy is to form a gallium containing interlayer over the base solar cell material. Through film annealing and gas-solid exchange, we take one of two routes for absorber modification. From the first strategy outlined in Figure 2, we modify an existing selenized film with a very thin coating of a gallium compound and subsequently drive Ga into the lattice at high temperature ( as limited by the glass substrate ). The second strategy is to modify a film of the reduced metal alloy with a thin layer of a gallium compound prior to carrying out selenization; both 3