Investigating the Impact of Ultrathin Interlayers Inserted Between Perovskite Films and Charge-Transport Layers Using Ultrafast Spectroscopy

Edward Butler-Caddle^a, Lauren Tidmarsh^b, W Hashini K Perera^b, Anjana Wijesekara^a, K. D. G. Imalka Jayawardena^a, James Lloyd-Hughes^a, Rebecca L. Milot^a

^aDepartment of Physics, University of Warwick, Coventry, United Kingdom

^bAdvanced Technology Institute (ATI), University of Surrey, UK, Guilford, United Kingdom

Proceedings of MATSUS Fall 2025 Conference (MATSUSFall25)

E4 (Ultrafast) Spectroscopy for Energy Materials - #SpEM

València, Spain, 2025 October 20th - 24th

Organizers: Jaco Geuchies and Freddy Rabouw

Oral, Edward Butler-Caddle, presentation 288
Publication date: 21st July 2025

For perovskite solar cells, the fullerene molecule (C₆₀) is one of the best performing electron transport layers (ETLs) that selectively extracts electrons from the perovskite layer, due to its suitable band alignment, good surface contact and high mobility. However, studies have shown that the perovskite-fullerene interface suffers from unwanted non-radiative recombination, which limits the device performance [1,2]. Ultrathin interlayers inserted between the perovskite surface and fullerene have been shown to improve the device performance [3], suggesting they prevent the unwanted interface recombination. In this work, we used ultrafast and nanosecond spectroscopy to investigate how interlayers affect the charge-carrier recombination and transfer at the interface.

The samples were formed by depositing C₆₀ layers on top of triple-cation ((FA_0.79MA_0.16Cs_0.05)Pb(I_0.83Br_0.17)₃) perovskite layers, with or without interlayers deposited in between. They were studied using two different versions of optical-pump terahertz-probe (OPTP) spectroscopy, which measured the carrier density in the perovskite layer as a function of time after injection. One version was the standard OPTP setup using a femtosecond laser to pump the sample and a mechanical stage to vary the pump-probe delay over a range of 3ns with sub-picosecond resolution. The other version was an electronically delayed OPTP (E-OPTP) setup [4] that used a separate electronically-triggered laser to pump the sample, giving an unlimited delay range with nanosecond resolution. Together these techniques allowed the carrier population to be studied from sub-picosecond to microsecond timescales.

The carrier dynamics were interpreted by comparison to a numerical model of the spatially and temporally varying carrier density, crucially including the Poisson equation to account for Coulombic effects of charge separation across the interface. A significant separation of charge should leave a fingerprint in the decay dynamics, and its absence implies recombination across the interface. The experimental results indicate that whilst the perovskite-C₆₀ interface suffers from rapid cross-interface recombination, the interlayers slow the extraction of electrons into the C₆₀.

References:

[1] E. Butler-Caddle, K. D. G. I. Jayawardena, A. Wijesekara, R. L. Milot, and J. Lloyd-Hughes, ‘Distinguishing carrier transport and interfacial recombination at perovskite/transport-layer interfaces using ultrafast spectroscopy and numerical simulation’, Phys Rev Appl, vol. 22, no. 2, p. 024013, Aug. 2024

[2] C. M. Wolff, P. Caprioglio, M. Stolterfoht, and D. Neher, ‘Nonradiative Recombination in Perovskite Solar Cells: The Role of Interfaces’, Advanced Materials, vol. 31, no. 52, p. 1902762, Dec. 2019

[3] J. Liu et al., ‘Efficient and stable perovskite-silicon tandem solar cells through contact displacement by MgFx’, Science (1979), vol. 377, no. 6603, pp. 302–306, Jul. 2022

[4] E. Butler-Caddle, N. E. Grant, S. L. Pain, J. D. Murphy, K. D. G. I. Jayawardena, and J. Lloyd-Hughes, ‘Terahertz photoconductance dynamics of semiconductors from sub-nanosecond to millisecond timescales’, Appl Phys Lett, vol. 122, no. 1, p. 012101, Jan. 2023

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