Charge Transfer from Methylammonium Lead Iodide Perovskite to Organic Transport Materials: Efficiencies, Transfer Rates and Interfacial Recombination
Tom Savenije a, Ruben Abellon a, Michiel Moes a, Jan-Jaap Hofman a, Eline Hutter a, Michiel Petrus b, Pablo Docampo b
a Delft University of Technology, The Netherlands, Julianalaan, 136, Delft, Netherlands
Proceedings of Perovskite Thin Film Photovoltaics (ABXPV17)
València, Spain, 2017 March 1st - 2nd
Organizers: Henk Bolink and David Cahen
Oral, Eline Hutter, presentation 090
Publication date: 18th December 2016

In spite of the substantial progress that has been made in improving power conversion efficiencies of perovskite-based solar cells and understanding the photo-physics of perovskites, there are only a handful of reports investigating the kinetics of charge transfer from perovskite to charge-specific transport materials (TMs). In order to rationally design efficient and stable perovskite-based solar cells, it is crucial to understand processes occurring at the perovskite/TM interfaces, such as charge transfer and interfacial recombination. Some groups have extracted transfer rates or yields from global analysis of spectroscopic data. However, a quantitative description accounting both for the dynamics of charges in the perovskite itself and the transfer to charge-specific electrodes is still lacking. In this work, Time-Resolved Microwave Conductivity measurements are performed to investigate interfacial processes for methylammonium lead iodide and various organic TMs. Both the frequently used hole transporting material Spiro-OMeTAD and the recently reported low-cost alternatives H101 and EDOT-OMeTPA are investigated. Similarly, state-of-the-art PCBM and C60 are compared to less commonly used electron transport materials such as ICBA and bis-PCBM. We introduce a global kinetic model to describe both the dynamics of excess charges in the perovskite layer and transfer to charge-specific TMs. Hence, we find the rates of charge transfer and interfacial recombination for the above mentioned organic TMs. Additionally, this model enables us to separate the electron and hole mobilities and to deduce the charge collection efficiency as a function of charge carrier density (i.e., illumination intensity). We conclude that for state-of-the-art materials, such as Spiro-OMeTAD and PCBM, charge extraction is not significantly affected by intra-band gap traps. That is, for trap densities under 1015 cm-3, the trapping rates (<107 s-1) are substantially lower than the transfer rates (typically ~ 108 s-1). Finally, our results show that transfer rates to C60, PCBM, EDOT-OMeTPA and Spiro-OMeTAD are sufficient to outcompete second order recombination under excitation densities representative for illumination by AM1.5. These results pave the way for rational design of perovskite-based solar cells with balanced extraction of charges, which is essential for avoiding accumulation of charges at one of the electrodes.

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