Understanding recombination processes in perovskite solar cells using frequency and time domain measurements to deconvolve electronic and ionic interactions
Adam Pockett a, Matthew Carnie a
a SPECIFIC, Swansea University, Baglan Bay Innovation and Knowledge Centre, Baglan, SA12 7AX, United Kingdom
Proceedings of Perovskite Thin Film Photovoltaics (ABXPV17)
València, Spain, 2017 March 1st - 2nd
Organizers: Henk Bolink and David Cahen
Oral, Adam Pockett, presentation 032
Publication date: 18th December 2016

The existence of slow dynamic processes in perovskite solar cells is well-known. This is most commonly observed as hysteresis in the current-voltage curve during device efficiency measurements. Slow processes have also been observed in a range of frequency and time domain measurements. Whilst there is considerable evidence linking the origins of these observed processes to the migration of ions within the perovskite, the exact nature of their interaction with the electronic structure of the device is still unclear.

In this work a range of complimentary frequency and time domain characterization techniques have been utilized to help understand the wide range of effects this electronic-ionic coupling has upon recombination in perovskite solar cells. By combining the use of transient photovoltage (TPV) and intensity modulated voltage spectroscopies (IMVS), over a wide range of temperatures from 77‑323 K, relaxations with time constants on microsecond, millisecond and second timescales have been observed. Measurements are complimented further with the use of impedance spectroscopy, large amplitude open-circuit photovoltage decay and current-voltage measurements.

Excellent agreement is shown between the time and frequency domain results, with the combination of techniques being used to aid understanding. A range of architectures have been studied including planar and mesoporous based TiO2 and organic contact cells. Similar processes are observed in all types highlighting that the underlying physical process is intrinsic to the perovskite. The recombination rate is found to be frequency/time dependent, relating to the particular ionic environment and its impact on electronic band structure. The high frequency response is representative of recombination in a fixed ionic system. On longer timescales the relaxation of the electronic and ionic species to re-establish electrochemical equilibrium results in a reduction in recombination rate.

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