Organic cation steered interfacial electron transfer within organic-inorganic perovskite solar cells

Abstract

Methylammonium lead-iodide (MAPbI(3), MA: CH3-NH3) interfaced with rutile TiO2 is widely used in photovoltaic devices. These devices utilize the electron transfer from MAPbI(3) to TiO2, which may not be explained solely by the band structures of the two bulk materials. To elucidate the interface dynamics and its impact on the electron transfer process, we have studied the interfacial features of a TiO2/MAPbI(3) system. First principles calculations and ab initio molecular dynamics simulations show that the rotational freedom of MA present within the bulk is considerably suppressed due to interaction of MA with the TiO2 substrate, highlighting orientationally ordered MA at the interface. The optimized interface structure shows the C-N axis of MA titled towards the TiO2 surface so as to maximize the interaction between N-attached H and underlying O. The very short O center dot center dot center dot H center dot center dot center dot N distance with very large hydrogen bonding energy identifies short strong hydrogen bonding (SSHB) as the origin of structural re-organization at the interface. As for the electronic structure, this proton sharing between MA and TiO2 has a critical impact on the energy level alignment at the interface, thus driving the electron transfer process from MA to TiO2. Indeed, significant reduction in the electron transfer barrier is observed for the energetically optimal interface configuration which promotes the electron transfer across the interface and photovoltaic properties.