Comparing Charge Carrier Removal Rates in Perovskite-Transport Layer Heterostructures Using Time-Resolved THz and Optical Spectroscopies
Edward Butler-Caddle a, Imalka Jayawardena b, James Lloyd-Hughes a
a Department of Physics, University of Warwick, Coventry, CV47AL, United Kingdom
b Advanced Technology Institute (ATI), University of Surrey, UK, Guilford, United Kingdom
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV22)
València, Spain, 2022 May 19th - 25th
Organizers: Pablo Docampo, Eva Unger and Elizabeth Gibson
Poster, Edward Butler-Caddle, 277
Publication date: 20th April 2022

As the fabrication of high quality thin films of lead-halide perovskites has improved rapidly over recent years, the performance of the adjacent charge transport layers (CTLs) and their interfaces with the perovskite layer have become increasingly important[1]. In this work we have studied the performance of some of the most commonly used charge transport layers deposited on a three-dimensional perovskite using a range of time resolved spectroscopies.

In a perovskite solar cell, the charge transport layers on either side of the light absorbing perovskite layer provide the asymmetry necessary to extract electrons and holes though opposite sides of the device. The extraction currents on the two opposite sides are influenced by the energy bands and fermi levels of the CTLs, the recombination rates at the CTL interface and in the CTL bulk, and the conductivity of the CTL.

Here we gain insight into the rate of carrier removal at individual CTL interfaces by studying bilayers comprising a commonly used transport layer (Spiro-OMeTAD, PCBM, C60) deposited on top of a high performance perovskite[2] layer (FA0.79MA0.16Cs0.05)Pb(I0.83Br0.17)3. Terahertz (THz) photoconductivity and transient absorption (TA) spectroscopies, track the perovskite carrier population from femtosecond to nanosecond timescales, whilst time resolved photoluminescence (TRPL) follows the population decay on nanosecond to microsecond timescales. The techniques are complimentary as the TRPL signal is proportional to the product of the electron and hole populations, whilst the TA signal is proportional to their sum and the THz signal is proportional to their sum weighted by their mobilities. These optical techniques avoid the use of additional contacts which will complicate the interpretation of the results.

In order to generate different initial carrier distributions within the perovskite layer, different excitation wavelengths are used and different sides of the bilayer are excited. Overall, we find that PCBM and C60 layers accelerate the population decay more than the Spiro-OMeTAD, and the thickness of the PCBM and C60 layers also has some influence on the carrier removal. This has important implications for device design as CTL thickness also affects the series resistance of the device and light absorption in the perovskite layer.

EPSRC (Engineering and Physical Sciences Research Council)

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