Researchers from Uppsala University and Linköping University in Sweden, along with collaborators from other institutions, have conducted a study to better understand the energy level alignment at the interface between lead halide perovskites and electron transport layers, which is crucial for improving the efficiency of perovskite solar cells.
The team, led by Alberto García-Fernández and Håkan Rensmo from Uppsala University, used in-situ photoelectron spectroscopy to investigate the interface between methylammonium lead iodide (MAPbI3) single crystals and C60 (a type of fullerene). Their goal was to resolve inconsistencies reported in earlier studies and gain a deeper understanding of the charge extraction process in perovskite solar cells.
The researchers found that both MAPbI3 and C60 remain chemically stable when they come into contact with each other, unlike the strong reactions typically seen when metals contact perovskites. By analyzing the core level positions of lead (Pb 4f) and carbon (C 1s), they determined that C60 consistently exhibits a downward energy shift toward MAPbI3. This shift works against efficient charge extraction, but its magnitude is highly sensitive to the surface composition of the perovskite. Small variations in surface composition can lead to significant differences in the energy level alignment and, consequently, in the performance of the solar cells.
At higher coverages of C60 (more than 5 monolayers), the researchers obtained a constant highest occupied molecular orbital (HOMO)-valence band offset of 0.52 eV. Assuming a 1.86 eV HOMO-lowest unoccupied molecular orbital (LUMO) gap for C60, they found that the LUMO level of C60 is 0.25 eV below the conduction band of MAPbI3. This energy level alignment is favorable for charge extraction, but the observed energetic alignment explains why further interface modification by charge-blocking layers or surface passivation is often needed to achieve optimized device efficiencies.
The findings of this study, published in the journal Advanced Energy Materials, underscore the decisive role of surface chemistry on interfacial energetics and explain performance variability in perovskite devices. The researchers emphasize the need to control and accurately measure surface properties to improve the efficiency of perovskite solar cells. This work provides valuable insights for the energy industry, particularly for companies and researchers developing perovskite-based solar cells and other optoelectronic devices. By understanding and controlling the energy level alignment at interfaces, it may be possible to minimize recombination losses and enhance the overall performance of these devices.
Source: García-Fernández, A., Radetzky, K., Riva, S., Kammlander, B., Rydgren, B., Johannesson, E., Varma, R. M., Rensmo, H., & Cappel, U. B. (2021). Resolving the energy alignment between methylammonium lead iodide and C60: an in-situ photoelectron spectroscopy study. Advanced Energy Materials, 11(26), 2100506.
This article is based on research available at arXiv.