Development and analyses of innovative thin films for photovoltaic applications

In solar cell current research, innovative solutions and materials are continuously requested for efficiency improvements. Si-based technology rules over 95% of the market, with silicon heterojunction (SHJ) solar cell reaching 26.7% record efficiency. Nonetheless, hydrogenated amorphous silicon (a-Si:H) layers employed in the structure still have challenges, resolvable with oxygen/nitrogen inclusion. In parallel, new technologies based on different materials still lack in the market due to stability issues or low efficiencies. However, a preliminary study of their properties creates a deeper knowledge exploitable in photovoltaic application. In this perspective, we investigated both innovative Si-based materials (nanocrystalline and amorphous silicon oxy-nitride and oxide thin films, nc-SiOxNy, a-SiOxNy and a-SiOx, respectively) and innovative materials (perovskite lanthanum-vanadium oxide LaVO3 thin films, indium gallium nitride InxGa1-xN and aluminium indium gallium nitride AlxInyGa1-x-yN layers) for solar cell concepts. Different deposition conditions have been employed to extract their influence on compositional, optical, and electrical properties. The study on nc-SiOxNy layers by conductive atomic force microscopy (c-AFM) and surface photovoltage (SPV) has allowed to clarify O, N, and B content, and annealing treatment role on microscopic transport properties. On a-SiOx and a-SiOxNy layers, by spectral ellipsometry, Fourier transform infrared spectroscopy, photoconductance decay and SPV, we can conclude that moderate insertions of O/N in a-Si:H lead to a decrease of optical parasitic absorption, preserving the passivation quality of the layers. The measurements by AFM and Kelvin probe force microscopy on LaVO3 have clearly shown that it is a poor charge-transport medium, thus not suitable for photovoltaic applications. The analysis on InGaN and AlGaInN by SPV measurements has shown how low In content, Si doping and no misfit dislocations in InGaN/GaN structure cause less recombination processes at the interface, whereas, the strain relaxation (tensile and compressive) with the formation of pinholes produces better interfaces in the AlGaInN/GaN samples.