Spectroscopy of the optical absorption edge of solar cell materials

Abstract

The focus of this thesis is relatively narrowly defined as spectroscopy of the optical absorption edge of the actual or potential photovoltaic materials. This includes the spectroscopy methods and interpretation of results, the photovoltaic materials and their technological parameters and also the potential of certain photovoltaic technologies as a renewable source of energy. The materials presented here are either absorbers: amorphous silicon (a-Si:H), microcrystalline silicon, organo-metalhalide perovskites (CH3NH3PbI3) or materials for transparent electrodes: graphene, amorphous InZnO and amorphous ZnSnO. The physical effects studied are related to material preparation methods, ageing, exposure to light, and annealing. The methods employed here for the absorption spectroscopy are photothermal deflection spectroscopy and photocurrent spectroscopy. The two methods are briefly explained and compared. Two main issues related to correct interpretation of these measurements are discussed: effect of surface states and the effects of light scattering. The absorption spectrum in sub-bandgap region is measured to obtain mainly the slope of the absorption edge parametrized by Urbach energy. It is shown here, how theoretically and practically this parameter correlates with the maximum attainable efficiency of solar cell – if the material is used as absorber. If the material is used as transparent electrode, correlation with conductivity is found. The effect of cost evolution in the photovoltaic industry is discussed with the consequence on the efficiency as a most critical parameter dictating the cost of energy. The competitiveness of the solar energy is reviewed for different levels of autonomy.