Polaron absorption for photovoltaic energy conversion in a manganite-titanate pn heterojunction

The relation among structure, electric transport, and photovoltaic effect is investigated for a pn heterojunction with strong correlation interactions. A perovskite interface is chosen as a model system consisting of the p-doped strongly correlated manganite Pr0.64Ca0.36MnO3 (PCMO) and the n-doped titanate SrTi1-yNbyO3 (y = 0.002 and 0.01). High-resolution electron microscopy and spectroscopy reveal a nearly dislocation-free, epitaxial interface and give insight into the local atomic and electronic structure. The presence of a photovoltaic effect under visible light at room temperature suggests the existence of mobile excited polarons within the band-gap-free PCMO absorber. The temperature-dependent rectifying current-voltage characteristics prove to be mainly determined by the presence of an interfacial energy spike in the conduction band and are affected by the colossal electroresistance effect. From the comparison of photocurrents and spatiotemporal distributions of photogenerated carriers (deduced from optical absorption spectroscopy), we discuss the range of the excited polaron diffusion length.