Single Second Laser Annealed CuInSe2 Semiconductors from Electrodeposited Precursors as Absorber Layers for Solar Cells

Abstract

Cu(In,Ga)Se2 (CIGSe) is a polycrystalline absorber layer in thin film solar cells with solar conversion efficiencies exceeding 20%. High temperature annealing for periods of minutes to hours is currently required to convert amorphous or nanocrystalline precursor material into high quality Cu(In,Ga)Se2 absorber layers. In this work, we perform the critical annealing step, using a 1064 nm laser, on electrodeposited precursor layers containing Cu, In, and Se, for times of 0.3–60 s thus synthesizing CuInSe2 absorber layers. An annealing time of 1 s is found to be sufficient to remove elemental concentration gradients in the bulk of the layer and to increase the average implied crystallite size (crystal coherence length, as determined by X-ray diffraction, XRD). Therefore the rate-determining step in producing higher quality layers with short annealing times is the rate of grain growth and not atomic diffusion. Optoelectronic analysis of the absorber layers revealed p-type doping with improved radiative recombination compared to the precursors. Laser annealed CuInSe2 layers did not produce working photovoltaic devices. This is first attributed to a loss of Se that occurs during laser annealing, resulting in detrimental substoichiometric quantities of Se in the absorber. Second, the likely presence of a thick surface layer of the CuIn3Se5 phase is expected to detrimentally impact device performance. These findings must be addressed if annealing times of the CuInSe2 absorber layer are to be reduced to seconds.