First Principles Investigation of Perovskite/Semiconductor Interfaces in Perovskite Solar Cells

Edoardo Mosconi^a, Filippo De Angelis^a

^aComputational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), CNR-ISTM, Via Elce di Sotto 8, I-06123, Perugia, Italy

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Oral, Edoardo Mosconi, presentation 117
Publication date: 5th February 2015

The field of solid-state solution-processable photovoltaics has experienced in the recent years a deep transformation thanks to the exploitation of the emergent class of selfassembling lead halide hybrid perovskites. Their revolutionary role in pushing the solar conversion efficiency up to 19.3% has to be ascribed to the unique combination of advantages that they present, including the intense and extended light harvesting and the superior, ambipolar charge-transport properties. Their crystalline structure allows an easy interchange of the components, enabling a fine-tuning of the material properties. For these characteristics, solar cells have been assembled in a variety of architectures, either in mesostructures or planar thin film devices, involving different preparation routes and possible morphology of the final compounds.^2-5 We investigate the prototypical interface between organohalide perovskites and TiO₂, by first principles electronic structure calculations.⁶^,⁷In 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.^6-9We 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.⁸Moreover, 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.⁹Furthermore, 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.⁹ Finally, we move to study the nature of the hetero-interface between perovskite and a series of substrates (ZnO, ITO, Al₂O₃, SiO₂), investigating the morphology, the stability and the electronic properties. Our findings represent a step forward to the rationalization of the peculiar properties of mixed halide perovskite, allowing one to further address material and device design issues.
Isodensity plot of the integrated DOS as a function of the distance from the TiO2 surface for MAPbI3 and interfacial chloride MAPbI3−xClx perovskite interfaces with TiO2. A blue to red color variation indicates an increase of the DOS value. The red circles highlight in both cases the energy range where occupied chloride states are found in MAPbI3−xClx (and are lacking in MAPbI3). The arrow in the lower panel indicates the sizable band bending observed in the MAPbI3−xClx case.
1. Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-b.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface engineering of highly efficient perovskite solar cells. Science 2014, 345, 542-546. 2. Laban, W. A.; Etgar, L. Depleted hole conductor-free lead halide iodide heterojunction solar cells. Energy Environ. Sci. 2013, 6, 3249-3253. 3. Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M. Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 2013, 499, 316-319. 4. Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature 2013, 501, 395-398. 5. Chung, I.; Lee, B.; He, J.; Chang, R. P. H.; Kanatzidis, M. G. All-solid-state dye-sensitized solar cells with high efficiency. Nature 2012, 485, 486-489. 6. Roiati, V.; Mosconi, E.; Listorti, A.; Colella, S.; Gigli, G.; De Angelis, F. Stark Effect in Perovskite/TiO2 Solar Cells: Evidence of Local Interfacial Order. Nano Lett. 2014, 14, 2168-2174. 7. De Angelis, F. Modeling Materials and Processes in Hybrid/Organic Photovoltaics: From Dye-Sensitized to Perovskite Solar Cells. Acc. Chem. Res. 2014, 47, 3349–3360. 8. Mosconi, E.; Ronca, E.; De Angelis, F. First-Principles Investigation of the TiO2/Organohalide Perovskites Interface: The Role of Interfacial Chlorine. J. Phys. Chem. Lett. 2014, 5, 2619-2625. 9. Colella, S.; Mosconi, E.; Pellegrino, G.; Alberti, A.; Guerra, V. L. P.; Masi, S.; Listorti, A.; Rizzo, A.; Condorelli, G. G.; De Angelis, F., et al. Elusive Presence of Chloride in Mixed Halide Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5, 3532-3538.

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