Optical Coupling Efficiency, Photon Loss, and Efficiency Estimates for c-Si PERC Modules Enhanced with Downconverting Films and Nanocomposites

Downconversion is an attractive process to enhance the performance of photovoltaic devices, enhancing conversion efficiency by both increasing photon flux into the active layers and improving the spectral response. The achievable efficiency enhancement, however, depends on the coupling efficiency between the downconverting material and the solar cell and on the placement and method of incorporation into the photovoltaic module. This study uses a combination of wave and geometrical optics to model the coupling efficiency and resulting photovoltaic performance for three configurations of Si photovoltaic modules containing downconverters. We find that the optical coupling is greater than 80% when the downconverting film is incorporated at the textured SiN_x/Si interface or when the downconverters are dispersed within the encapsulant to form a composite. However, the optical coupling does not exceed 30% when the downconverting film is placed between the encapsulant and the cover glass because of increased internal reflection and trapped modes. The efficiency estimates show that placing the downconverting film on the SiN_x/Si pyramids yields the greatest increase in power conversion efficiency compared to the reference device, approximately 2% absolute, and the efficiency enhancement is primarily due to photon multiplication rather than spectral shifting.