We investigate the prototypical interface between organohalide perovskites and TiO₂, by first principles electronic structure calculations.[1-2] In particular, Stark spectroscopy analysis combined with DFT simulation results proves the existence of oriented permanent dipoles, consistent with the hypothesis of an ordered perovskite layer, close to the oxide surface. The existence of a structural order, promoted by specific local interactions, could be one of the decisive reasons for highly efficient carriers transport within perovskite films.[1-5] We find that the MAPbI₃ and MAPbI_3-xCl_x perovskites tend to grow in (110)-oriented films on TiO₂, due to the better structural matching between rows of adjacent perovskite surface halides and TiO₂ undercoordinated titanium atoms. Interfacial chlorine atoms further stabilize the (110) surface, due to an enhanced binding energy. We find that the stronger interaction of MAPbI_3-xCl_x with TiO₂ modifies the interface electronic structure, leading to a stronger interfacial coupling and to a slight TiO₂ conduction band energy up-shift.[3] Moreover, AR-XPS and DFT calculations indicate the preferential location of chloride at the TiO₂ interface compared to the bulk perovskite due to an increased chloride−TiO₂ surface affinity.[4] Furthermore, our calculations clearly demonstrate an interfacial chloride-induced band bending, creating a directional “electron funnel” that may improve the charge collection efficiency of the device and possibly affecting also recombination pathways.[4] We report a combined experimental and computational investigation to elucidate the effect of PbI₂ on the electronic properties of the TiO₂/methylammonium lead-iodide (MAPbI₃) perovskite interface. We observe that excess PbI₂ in the precursor solution is predominantly located at the interface with TiO₂, enhancing the interfacial electronic coupling and favorably modifying the electronic energy levels landscape at the interface. This is in turn found to facilitate electron transfer from the MAPbI₃ perovskite to the TiO₂ electron transport layer as revealed by a fast quenching in the photoluminescence dynamics, confirming the beneficial role of PbI₂ at the interface.[5]

References:

[1] Roiati, V.; Mosconi, E.; Listorti, A.; Colella, S.; Gigli, G.; De Angelis, F. Nano Lett. 2014, 14, 2168-2174.

[2] De Angelis, F. Acc. Chem. Res. 2014, 47, 3349–3360.

[3] Mosconi, E.; Ronca, E.; De Angelis, F. J. Phys. Chem. Lett. 2014, 5, 2619-2625.

[4] Colella, S.; Mosconi, E.; Pellegrino, G.; Alberti, A.; Guerra, V. L. P.; Masi, S.; Listorti, A.; Rizzo, A.; Condorelli, G. G.; De Angelis, F.; Gigli, G. J. Phys. Chem. Lett. 2014, 5, 3532-3538.

[5] Mosconi, E.; Grancini, G.; Roldan-Carmona, C.; Gratia, P.; Zimmermann, I.; Nazeeruddin, M. K.; De Angelis, F. Chem. Mater. 2016, Submitted.