Energy Cascades in Mixed-Phase Perovskite Thin Films: Charge-Carrier Dynamics and Mobilities
Silvia Motti a, Timothy Crothers a, Rong Yang b, Jianpu Wang b, Laura Herz a
a Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom
b Nanjing Tech Unversity, 5 Xinmofan Road, Nanjing, 210009, China
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Poster, Silvia Motti, 012
Publication date: 11th February 2019

Perovskite semiconductors have recently emerged as promising materials for optoelectronic applications, with photovoltaic efficiencies that have now reached over 23%. However, the poor stability of the material is still a major challenge for commercial application. One possible strategy for improving environmental stability is the incorporation of large organic hydrophobic cations in the perovskite structure, which results in a layered two-dimensional (2D) material where the organic molecules form a dielectric barrier between the semiconductor layers. 2D perovskites show enhanced stability in contrast to the conventional 3D compositions,[1] however the presence of dielectric confinement results in higher exciton binding energies, wider bandgaps and limited charge carrier diffusion lengths.[2]  Mixed phase compositions can be a viable approach for obtaining films that combine the enhanced environmental stability of 2D perovskite and good charge transport properties of the 3D semiconductor for device application. We have investigated the transport properties of mixed-phase 2D-3D perovskite thin films deposited from solution using time resolved Photoluminescence (PL) and Optical-Pump Terahertz-Probe (OPTP) spectroscopy. We demonstrate the preferential formation of 2D domains close to the substrate and predominance of 3D crystallites on the front surface, consequently creating an energy cascade with a preferential direction across the semiconductor film.[3,4] Evidence for the charge and energy transfer through the cascade could be observed in unusual OPTP transients. Based on the experimental observations we proposed a model that reproduces the recombination and transfer dynamics, combining the effects of charge diffusion and photon reabsorption. This allows us to identify the most relevant transfer mechanisms in play in such self-assembled energy cascades. Our findings show that the presence of 2D domains does not hinder the charge transport properties of the semiconductor film with respect to the pure 3D structures, demonstrating the suitability of the mixed phase 2D-3D perovskite films for optoelectronic applications.

Samples have been provided by the group of Professor Jianpu Wang and fabricated at Nanjing Tech, China. This project has received funding from the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Skłodowska-Curie grant agreement No 675867.

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