With the continuing depletion of nonrenewable energy sources, the demand for solar energy is rising. Perovskite oxide thin films are a promising material system that could advance technological progress in solar cell design, as little is known about their photovoltaic properties. They have a band gap in the visible range and are composed of non-toxic and chemically stable elements. These perovskite films are grown via molecular beam epitaxy by Prof. Steve May’s group (Drexel MSE). The carrier lifetimes and recombination mechanisms were studied by using ultrafast optical probe-pump spectroscopy. This method uses a single wavelength pump pulse which is fired into a material to excite charge carriers (electrons) across the band gap. A probe pulse is fired after a time delay to study the relaxation of photoexcited carriers. The data is obtained in terms of energy-resolved change in the reflectivity of the sample after excitation and then fitted to a recombination-diffusion transport model. In the model, parameters were set to study surface, trap-assisted and Auger recombination that occur in the material. Surface recombination limited carrier lifetimes in the thinnest films (~5 nm), but lifetimes of nanoseconds could be achieved in films with thicknesses >40 nm . Despite surface recombination being the limiting factor, increased research into perovskite oxide thin films could lead to possible solutions on how to increase carrier lifetimes and improve solar cell performance.
