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 (V_OC) 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 CsPbI_1.5Br_1.5 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 MA_1−xCs_xPbI_1.5Br_1.5. 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 J_SC 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 V_OC. Quasi-Fermi level splitting (QFLS) experiments indicated that the major contributors to the V_OC 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 V_OC. Optimized devices have good stability and reproducibility. The champion device has an efficiency of 9.9%, with a V_OC 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 CsPbI_1.5Br_1.5 to 17.0 meV for MA_1−xCs_xPbI_1.5Br_1.5, 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 MA_1−xCs_xPbI_1.5Br_1.5 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 V_OC and high stability, needed to make these materials suitable for triple-junction all-perovskite solar cells.