Stoichiometric dependence of TiOx as a cathode modifier on band alignment of polymer solar cells

• We use stoichiometric TiOx for cathode work function engineering of polymer solar cells.
• Different stoichiometric TiOx (x=1.56, 1.76, 1.93) are synthesized by controlling O/Ti ratio and electronic structure.
• Inserting the TiOx modification layer with low O/Ti ratio largely improves device performance.
• TiOx with low O/Ti ratio up-shifts the effective work-function of the cathode and improves the built-in potential and Voc.
• Electron extraction and hole blocking are more significant due to increased built-in potential and result in lower recombination rate.

In this study, we systematically investigated the stoichiometric dependence of titanium oxide (TiOx, x=1.56–1.93) as a cathode modifier on the device performance of polymer solar cells. Electronic structures of the synthesized TiOx modifier layers were controlled by tuning the compositions of various O/Ti ratios. The effective cathode work-functions and the corresponding device performances of polymer solar cells are systematically changed as a result of inserting the TiOx modification layers. Interfacial modification of the Al cathode with a low O/Ti ratio of TiOx layer yields the best performing photovoltaic device as a result of a largest built-in potential. The correlation of power conversion efficiencies and carrier dynamics of these devices by inserting various TiOx modification layer is further examined by using the Mott-Schottky analysis and the impedance spectroscopy technique. The consistent result shows an enhanced carrier collection efficiency and a reduced charge recombination rate of the device via adequate band alignment between the photoactive layer and the cathode using the TiOx modification layer with an optimized O/Ti ratio.