Correlating the nanoparticle size dependent refractive index of ZnO optical spacer layer and the efficiency of hybrid solar cell through optical modelling

The optical characteristics of nanoparticles will vary according to particle size. With decrease in the size of the nanoparticle (NP), the bandgap of the material increases, thus providing a blue-shift in the refractive index of the material. In this study, we analyzed the effect of different sized zinc oxide (ZnO) nanoparticle optical spacer on the ideal short-circuit current density (J_sc,ideal) of an hybrid photovoltaic cell. ZnO was used as an optical spacer in the solar cell structure to improve the light absorbed in the active layer. Refractive index and extinction coefficient of different sized ZnO nanoparticles were calculated using tight binding model. We implemented these results in two different morphological models: nanoparticle model, and thin-film model. In the nanoparticle model, the ZnO NPs were considered as nanospheres which scatter the incoming light. Comparison of these models helped us to evaluate the improvement in the J_sc,ideal due to scattering effects from nanoparticles. Finally, the optimized structure was obtained for different active layer thicknesses by varying the thickness of the ZnO layer.