Mobile Ions as a Researcher's Ally—Identifying Performance Limiting Factors in Halide Perovskite Solar Cells

It is often assumed that mobile ions are detrimental to the function of halide perovskite solar cells. While their presence certainly introduces some challenges when optimizing performance and stability of devices, a complete picture of their role is far from established.¹ One such challenge is understanding the long-time scale response of perovskite solar cells, a feature that is evident in experimental measurements of their electrical and photophysical properties.
Here, we show that long-time scale reversible polarization effects due to mobile ions bear useful information when searching for performance bottlenecks in solar cells. We interpret the frequency domain response of solar cells with different architectures, by referring to the concept of ionic-to-electronic current amplification occurring in the bulk and at interfaces of the device.^2,3 Focusing on mesoscopic perovskite solar cells, we discuss methods to identify the region in the device with the dominant recombination contribution. In addition, DC voltage bias dependent impedance measurements and drift-diffusion simulations using Setfos⁴ are interpreted using simplified equivalent circuit models of the complete device. We present how not only the recombination properties of the device, but also electronic transport limitations influence the electrical response of mixed conducting solar cells. Our interpretation is supported by photoluminescence and electroluminescence imaging of the solar cells,⁵ which allows us to further evaluate the time scale of polarization processes occurring in the active layer.
The presented approach represents a self-consistent tool to the analysis of impedance spectra of solar cells under bias as well as of data obtained using other optoelectronic techniques. The power of simplified equivalent circuit models to derive a physically consistent picture of device behavior is emphasized.
References
1. L. J. F. Hart et al. Energy Environ. Sci., 2024, 17, 7107
2. D. Moia, arXiv:2405.10477 [physics.app-ph], 2024
3. D. Moia, et al., Energy Environ. Sci. 2019, 12, 1296–1308.
4. https://www.fluxim.com/
5. Comi, E., L., et al. Solar Energy Advances 2021, 1, 100003