Microstructure Analysis of Cdte/cds Thin-film Solar Cells for Optimization of Devices Fabricated at Low Temperature with Pulsed Laser Deposition

Low temperature fabrication of CdTe (1.25 μm)/CdS (120 nm) thinfilm solar cells was explored using in situ pulsed laser deposition (PLD) in order to control the CdTe grain morphology, crystallinity, and CdS/CdTe interface, which have proven to be crucial in CdTe/CdS solar cell performance. A comparative study was carried out on solar cells fabricated using PLD with the CdTe layer either grown directly at 400°C or at200°C followed with in situ annealing at 400°C for 5-40 minutes in 20 Torr Ar. The post annealed CdTe films exhibit better crystalline characteristics confirmed by optical transmittance and atomic force microscopy as well as improved CdTe/CdS p-n junction, which results in increasingly higher open circuit voltage and fill factor with increasing anneal times up to 20 minutes. Quantum efficiency measurements revealed that the post-annealed cells have more efficient electron collection, particularly below the CdS band edge, and a correlation is drawn between PLD annealing time and CdCl2 annealing time to optimize device efficiency. The results suggest that the in situ post PLD annealing process developed in this work is advantageous because of much improved crystallinity of CdS and CdTe, a considerably lower deposition thermal budget for better control of the CdS and CdTe interface, plus flexibility for in situ materials mixing for energy band gap engineering.