Luminescence Imaging for Studying Ion Movements in Perovskite Solar Cells

With power conversion efficiencies reaching 26.7% and improving stability against moisture, heat, and light, perovskite solar cells (PSCs) are approaching commercialization, offering a promising, low-cost photovoltaic technology.^[1] However, fabrication of large-area PSCs comes with its own challenges such as significantly lower PCE (~ 20% for 64 cm² cell) and stability compared to small devices^[2], mostly due to spatial inhomogeneity in film thickness, interfacial voids, and delamination, etc. that degrades charge collection efficiencies and overall device performance. These defects are difficult to detect using standard characterization techniques, especially in a full device stack. Luminescence-based techniques, such as camera-based photoluminescence (PL) and electroluminescence (EL) imaging, are widely used to study spatial imperfections in solar cells.^[3] When coupled with potentiostatic and galvanostatic measurements, PL/EL imaging reveals fabrication defects, non-radiative recombination centers that contribute to device performance degradation. ^[3,4] This fast, contactless, and nondesctructive characterization technique works on a full device as well. However, analyzing PL/EL image data can be challenging due to the complexity of solar cell structures.

In this work, we have used PL/EL imaging techniques to identify performance loss and device degradation mechanisms in PSCs. PL and EL imaging was conducted at different electrical biases on a formamidinium lead iodide based PSC with 20.62% PCE. According to Rau’s reciprocity relation, PL and EL should exhibit analogous behavior.^[5] However, we have observed significant dissimilarities in the PL and EL images of PSCs, particularly in the spatial distribution of charge extraction and injection efficiencies calculated from the PL and EL images, respectively. This discrepancy redirects to lateral inhomogeneity in the series and shunt resistance and formation of rectifying/blocking interfaces at the perovskite-carrier selective layer interfaces. High ionic activity in these perovskites often induces potential barriers at the ETL-perovskite interface due to steep conduction band bending.^[6] This effect hinders the extraction of photogenerated carriers into the ETL and HTL, leading to increased carrier concentration in the bulk perovskite that results in stronger PL emission. Considering this, we further investigated the PSCs with selective light soaking (LS) i.e. exposing part of the PSC to white light for a long time before capturing the PL and EL images under different electric biasing conditions. The PL images revealed stronger emission from the LS region compared to the dark region, indicating charge carrier confinement in the LS region due to the potential barrier formed by interfacial ion accumulation. This is further supported by significantly lower EL emission from the LS region, which is attributed to hindered charge injection during EL operation by the same potential barrier. The strong PL and weak EL from the LS region gradually disperse outward into the dark areas over tens of seconds, aligning with the time scale of ion movements. Thus, selective light-soaking PL/EL imaging effectively “visualizes” ion dynamics in PSCs, providing new insights into the ionic activity of halide perovskites. Further analysis quantifies ionic mobility in PSCs with various perovskite and ETL/HTL combinations, offering insights into lateral ion kinetics and identifying degradation mechanisms. The comparative results guide the selection of better active and carrier-selective layers for improved PSC performance and stability.

References
[1] https://www.nrel.gov/pv/cell-efficiency.html
[2] Science (80-. ). 2022, 375, 302.
[3] Adv. Energy Mater. 2018, 8, 1.
[4] Matter 2022, 5, 2352.
[5] Matter Mater. Phys. 2007, 76, 1.
[6] Energy Environ. Sci. 2024, DOI 10.1039/D4EE02669A.