Wide-bandgap (WBG) perovskites, with energy bandgaps ranging from 1.5 to 2.3 eV, have been extensively investigated as the photoactive layer in the top cell of tandem photovoltaic devices. These perovskites have gained prominence in photovoltaic applications due to their high absorption coefficient, tunable bandgap, and ease of fabrication. The WBG perovskites are realized by the substitution of bromide ions in place of iodide ions in the traditional APbI₃ perovskites (where A represents a monovalent organic or inorganic cation), allowing precise control over the bandgap within the aforementioned range [1]. However, these WBG perovskites suffer from intrinsic phase instability and halide segregation under light exposure, particularly when a critical bromine content (~ 20%) is surpassed [2]. A high concentration of bromine accelerates the perovskite crystallization due to the lower solubility of bromide salts. This results in perovskite films with higher defect density and reduced crystallinity. This then leads to high V_oc losses and devices with reduced stability [3]. In this study, we investigate the impact of a chloride additive in inhibiting the formation of intermediate phases (2H/4H polytypes) in the 1.72 eV FACsPbIBr wide-bandgap perovskite films. The chloride ions forced the crystallization kinetics to improve the crystallinity of the target perovskite films, as revealed by the X-ray diffraction and scanning electron microscopy measurements. In situ photoluminescence measurements conducted during the spin-coating of the films revealed the initial nucleation stage through bromide ions in control films (PL peak: 701-719 nm). This is subsequently replaced by nucleation through chloride ions (PL peak: 690-695 nm), resulting in halide homogenization in the target films. Temperature-dependent XRD measurement also confirms the elimination of the 2H/4H intermediate phases in the target films. This additive engineering strategy improved the photostability of the films by reducing the defect density in the perovskite bulk, thus significantly improving the photovoltaic performance of the WBG perovskite devices. The PCE of the devices increased from 15% for the control samples to 18% for the target samples with a high V_oc of 1.224 V.

References:

1. Oliver, R.D., et al., Understanding and suppressing non-radiative losses in methylammonium-free wide-bandgap perovskite solar cells. Energy & Environmental Science, 2022. 15(2): p. 714-726.
2. Hoke, E.T., et al., Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics. Chemical Science, 2015. 6(1): p. 613-617.
3. Hang, P., et al., Highly efficient and stable wide‐bandgap perovskite solar cells via strain management. Advanced Functional Materials, 2023. 33(11): p. 2214381.