Effective thickness method for modeling absorption enhancement of forward-scattering nanoparticles in photovoltaic devices

Research in the field of solar cells is generally based on balancing the trade-off between high absorption in thicker and efficient carrier extraction in thinner samples. Nanoparticle-assisted forward-scattering of the sunlight results in an artificial thickness increase for the absorption while keeping the carrier extraction thickness at its initial optimal value. According to the Beer–Lambert law, where thickness and absorption coefficients are placed alongside, thickness variations can be assumed as absorption shifts. In this study, by proposing a model based on the calculation of the effective thicknesses, we arrive at a relation for the modified absorption coefficient. Scattering efficiency and angular distribution for metallic nanoparticles with different geometrical shapes are calculated. It is shown that pyramidal, cubical and cylindrical nanoparticles offer higher Mie scattering efficiency and forward scattering ratio than those of spherical and triangular shapes. As an application, non-fullerene absorber of PTQ:IDIC with lower absorption below 500 nm is chosen to evaluate the capability of forward-scattering nanoparticles for efficiency enhancements. Our studies are conducted through fitting experimental results of this absorber by the Drift-Diffusion equations. Other than optimizing the shape, cross-section, aspect and covering ratio of the introduced nanoparticles, we demonstrate that the back contact reflectivity plays an essential role after scattering the sunlight into the absorber layer. Finally, it is demonstrated that metallic particles located in the buffer layer reduce the optimal thickness of the sample, enabling fabrication of thinner solar cells.