Additive and Interface Engineering for High-Efficiency Methylammonium-free Wide-bandgap Perovskite Solar Cells

Wide-bandgap perovskite solar cells are crucial for enhancing solar efficiency in tandem cells by capturing the broad spectrum of sunlight.¹ Their tunable bandgap, ranging from 1.68 eV to 2.3 eV, makes them ideal candidates for top cells of tandem architectures.² While the low-bandgap bottom cells are already well-established, wide-bandgap perovskite top cells have yet to reach the same power conversion efficiency as their bottom cell counterparts. Wide-bandgap perovskite solar cells suffer from several issues that negatively affect their performance. Lead thiocyanate (Pb(SCN)₂) is introduced as an additive to enhance the quality of the perovskite film, promoting better crystallization and enlarged grain sizes. This improvement is confirmed from scanning electron microscopy images and X-ray diffraction measurements. However, the excessive amount of lead iodide (PbI₂), formed from Pb(SCN)₂, deteriorates the device’s performance.³ A post-treatment with a solution containing a mixture of phenethylammonium iodide (PEAI) and methylammonium iodide (MAI) is applied. This treatment helps to convert residual PbI₂ into quasi-2D perovskites efficiently. This approach contrasts with using PEAI alone, which mainly leads to the formation of 2D perovskite. Treating the perovskite film with a mixed solution of PEAI and MAI proves to be more efficient in reducing residual PbI₂. The presence of quasi-2D perovskite helps minimize V_OC losses by reducing non-radiative recombination and enhances charge extraction at the perovskite/electron transport layer interface, thereby improving overall device performance. The quasi-2D perovskite layer effectively minimized defects, as confirmed by photoluminescence analysis, transient photovoltage measurement, and space charge-limited current measurement. These techniques provided clear evidence of the defect reduction in the perovskite film. As a result, the modified wide-bandgap perovskite solar cell exhibited a power conversion efficiency of 19.34% with an open circuit voltage of 1.23 V, short circuit current density of 19.36 mA/cm², and fill factor of 81%. The encapsulated passivated perovskite solar cells maintained above 90% of their initial efficiency after 200 hours under 1-sun illumination in ambient conditions.

References

1. He, R.; Ren, S.; Chen, C.; Yi, Z.; Luo, Y.; Lai, H.; Wang, W.; Zeng, G.; Hao, X.; Wang, Y.; Zhang, J.; Wang, C.; Wu, L.; Fu, F.; Zhao, D., Wide-bandgap organic–inorganic hybrid and all-inorganic perovskite solar cells and their application in all-perovskite tandem solar cells. Energy & Environmental Science 2021, 14 (11), 5723-5759.
2. Hörantner, M. T.; Leijtens, T.; Ziffer, M. E.; Eperon, G. E.; Christoforo, M. G.; McGehee, M. D.; Snaith, H. J., The Potential of Multijunction Perovskite Solar Cells. ACS Energy Letters 2017, 2 (10), 2506-2513.
3. Roose, B.; Dey, K.; Chiang, Y.-H.; Friend, R. H.; Stranks, S. D., Critical Assessment of the Use of Excess Lead Iodide in Lead Halide Perovskite Solar Cells. The Journal of Physical Chemistry Letters 2020, 11 (16), 6505-6512.