Critical design criteria for silicon nanocrystals downshifting layers enhancement in CIGS solar cells

Conventional Cu ( In , Ga ) Se2 (CIGS) solar cells suffer poor response in the short-wavelength region (280 to 520 nm) due to the parasitic absorption in the ZnO transparent conductive oxide and inefficient collection of generated electron–hole pairs in the CdS layer. The short-wavelength response can be enhanced using a downshifting (DS) layer mounted on the top surface of the solar cell. The performance effects of coupling silicon nanocrystals DS layer to a CIGS solar cell are analyzed using numerical simulations. Measured photoluminescence of fabricated Si-nC using plasma-enhanced chemical vapor deposition is used to calibrate the model. A 13.9% relative enhancement of the conversion efficiency is observed when a DS layer with a photoluminescence quantum yield (PLQY) of 20% is added. The relative enhancement increases to 17.8% when the DS layer has a PLQY of 100%. The results are further analyzed to decouple and quantify the surface reflectance effect and the DS effect using a passive cell. The surface reflectance effect is dominant for DS layers with PLQY of 20% and accounts for 13.6% relative enhancement. Whereas the DS effect dominates when the PLQY is >70 % and reaches 17.1% relative enhancement for a DS layer with PLQY of 100%. Overall, cells with poor UV spectral responses are found to benefit the most from the coupling of a DS layer, albeit only for a very high PLQY.