The effect of post annealing in CdCl2 was investigated to elucidate the correlation between the evolution of the microstructure of CdTe thin films with the annealing time and the performance of CdS/CdTe thin‐film solar cells made by in situ pulsed laser deposition (PLD). It has been found that the crystallinity of the annealed film is improved considerably as the CdTe polycrystalline grains evolved into grain clusters with a dendrite structure. While longer annealing time allows larger grain size, the increased film surface roughness outweighs the benefit of the improved crystallinity, resulting in increased structural disorders, such as pinholes. In fact, the concentration of the pinholes was found to increases monotonically with annealing time when it exceeds the optimal value, resulting in degradation of device performance and yield especially when pinholes appeared near the CdS/CdTe interface. This result illustrates the importance of controlling the microstructure of CdTe film and its interface with CdS to high‐performance CdTe/CdS thin‐film solar cells.

Raman spectra specific to CdS and CdTe were obtained on the CdS/CdTe heterojunction interface by employing two excitation wavelengths of λ1 = 488 nm and λ2 = 633 nm, respectively, from the glass side of Glass/FTO/CdS/CdTe/HgTe:Cu:graphite/Ag solar cells fabricated using pulsed-laser deposition (PLD). This two-wavelength Raman spectroscopy approach, with one wavelength selected below the absorption edge of the window layer (λ2 in this case), allows nondestructive characterization of the CdS/CdTe heterojunction and therefore correlation of the interfacial properties with the solar cell performance. In this study, the evolution of the interfacial strain relaxation during cell fabrication process was found to be affected not only by the inter-diffusion of S and Te corresponding to the formation of CdSxTe1-x ternary alloy with a various x from ∼0.01 to ∼0.067, but also by the variation in misfit dislocations (MDs) at CdS/CdTe interface from Raman TO/LO ratio ∼2.85 for as-deposited sample to TO/LO ∼4.44 for the cells post treatment. This is consistent with the change of the Urbach energy from 0.03 eV to 0.09 eV, indicative of the deterioration of crystalline quality of CdTe at interface although improved CdTe crystalline quality was observed away from the interface after the CdCl2 annealing. This difference crucially impacted on the rectification characteristics of the CdS/CdTe heterojunction and therefore the solar cell performance.