Wide-bandgap CsPbI1.5 Br1.5 Perovskite p–i–n Solar Cells
Olivera Vukovic a, Martijn Wienk a, Rene Janssen a
a Eindhoven University of Technology (TU/e), PO Box 513, Eindhoven, 5600, Netherlands
Materials for Sustainable Development Conference (MATSUS)
Proceedings of MATSUS23 & Sustainable Technology Forum València (STECH23) (MATSUS23)
#PerFut - Metal Halide Perovskites Fundamental Approaches and Technological Challenges
València, Spain, 2023 March 6th - 10th
Organizers: Wang Feng, Giulia Grancini and Pablo P. Boix
Poster, Olivera Vukovic, 370
Publication date: 22nd December 2022

By eluding thermalization and transmission losses, all-perovskite triple-junction solar cells are a potential low-cost strategy to achieve photovoltaic power conversion efficiencies up to 37% [1]. For a successful triple-junction solar cell, three complementary photovoltaic materials must be combined to achieve optimized current matching and minimal open-circuit voltage (VOC) losses in each sub-cell. Presently, the biggest limitation to an efficient triple-junction stack comes from limitations of the wide-bandgap sub-cell which is typically composed of a mixed-halide lead perovskite. Wide-bandgap semiconductors are much in need for future use in triple-junction, where a p–i–n configuration seems the most promising configuration.

            This work describes the development of a stable and improved wide-bandgap p–i–n single-junction solar cell based on a CsPbI1.5Br1.5 perovskite. To slow down crystallization kinetics and improve perovskite film morphology, different concentrations of methylammonium chloride (MACl) were introduced as an additive to the perovskite precursor solution to slow down the crystallization rate. X-ray diffraction (XRD) and Nuclear magnetic resonance (NMR) confirmed the presence of methylammonium in the crystal lattice, while XPS indicated an undetectable level of Cl. Hence the composition of the film is MA1−xCsxPbI1.5Br1.5. The inclusion of MA+ caused a narrowing of the bandgap from 2.05 to 1.98 eV. The use of MACl as additive increased the JSC and FF, reduced hysteresis, and removed the s-shape that was present in the J–V characteristics. While the film quality and device performance reached an optimum at 10 wt% MACl, no major improvement was achieved for VOC. Quasi-Fermi level splitting (QFLS) experiments indicated that the major contributors to the VOC loss is non-radiative recombination in the bulk of perovskite layer and at the interface between the perovskite and the C60 electron transport layer (ETL). An electron transport bilayer, consisting of [6,6]-phenyl-C61-butyric acid methyl ester ([60]PCBM) with C60 on top, improves the photovoltaic performance compared to single electron transport layers of C60 or [60]PCBM, especially regarding the VOC. Optimized devices have good stability and reproducibility. The champion device has an efficiency of 9.9%, with a VOC of 1.23 V. [60]PCBM successfully minimized losses caused by defect states. The Urbach energy for devices with bilayer ETL decreased from 20.8 meV for CsPbI1.5Br1.5 to 17.0 meV for MA1−xCsxPbI1.5Br1.5, implying a lower energetic disorder at the band edge for devices processed with MACl. As the bandgap narrowed, the defect state in the bandgap became relatively shallower, which possibly lowered non-radiative recombination.

By developing MA1−xCsxPbI1.5Br1.5 thin film absorbers via additive engineering and using a [60]PCBM/C60 bilayer ETL we have significantly reduced the non-radiative recombination losses that currently limit wide-bandgap mixed-halide perovskite p–i–n configuration solar cells and achieved stable performance under continuous illumination for 2 h. These results contribute to solving the main challenges of the wide-bandgap perovskite, i.e., high VOC and high stability, needed to make these materials suitable for triple-junction all-perovskite solar cells.

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