Photovoltaic performances of TiO2/Se heterojunction devices with different crystallographic structures of sputter-deposited TiO2 thin films

This study investigates the impact of sputter-deposited TiO2 n-type window layers with varied crystallographic structures on the power conversion of TiO2/Se photovoltaic devices, one of the most promising indoor photovoltaic energy harvesters and wide-gap top cells in tandem solar cells. Since incident light illuminates from TiO2 n-type window layer of TiO2/Se photovoltaic devices, most of photons carriers is absorbed in the Se layer. It is essential for highly efficient photovoltaic conversion of TiO2/Se devices to improve carrier collection by adequate formation of built-in potential in the Se layer and to reduce recombination by optimizing energy band alignment at TiO2/Se interface. In this study, TiO2 window layers with different contents of anatase and rutile crystallographic structures are applied to TiO2/Se heterojunction devices. Impacts of TiO2 n-type window layers with varied crystallographic structures on the power conversion of TiO2/Se photovoltaic devices are investigated in conjunction with different photoconductivity and energy band structures between anatase and rutile TiO2 thin films. In TiO2/Se photovoltaic devices with anatase/rutile mixture window layer, the power conversion efficiency was increased from 3.04% to 4.48% by combining the high open circuit voltage and high short circuit current, owing to the smaller cliff-type conduction band offset at -0.63 eV and smaller series resistance resulting from fitting the current density-voltage curves.