Mixed-halide wide-bandgap perovskites are a cornerstone for tandem solar cells—but they have a stubborn habit: they segregate into I-rich and Br-rich domains, sometimes even from the very first moments of film formation. That asymmetric nucleation breaks the intended bandgap uniformity, undermining both efficiency and long-term stability. Conventional “homogenization” approaches try to modify how Pb²⁺ coordinates, yet they act globally across all Pb sites and don’t specifically prevent the faster-forming PbBrₓ-rich nuclei.
A new study proposes a more selective way to steer crystallization. Instead of adjusting Pb²⁺ everywhere, the researchers tune the local Lewis-base “hardness” at the donor atom using a molecular dipole. By constraining the polarizability of the O-donor’s outer electrons, the strategy changes which Pb²⁺ species is most strongly stabilized during early growth.
The core idea is a hardness–selectivity match: PbBrₓ requires coordination with “harder” Pb²⁺ character, while PbIₓ nucleation proceeds differently. The harder O-donor preferentially coordinates the harder Pb²⁺ associated with Br-rich chemistry, selectively retarding PbBrₓ nucleation. In effect, it synchronizes Br-rich and I-rich growth rather than letting Br-rich regions run ahead.
The result is compositionally homogeneous wide-bandgap films—an outcome that directly targets the earliest stage of segregation, not just its later evolution. Optically and structurally, the films show bandgaps at 1.62 eV, 1.68 eV, and 1.88 eV, aligning with the requirements of high-performance tandem architectures. Device testing shows both higher power conversion efficiency and improved stability.
In solar cells built from these films, the authors report durability over 1500 hours and thermal endurance with ≥T₉₀ beyond 65 °C under 1 sun. These metrics are especially compelling because mixed-halide instability is often a thermal and light-driven process.
To demonstrate tandem relevance, the team integrated the wide-bandgap perovskites into perovskite/organic stacks with an infrared-active organic cell. The tandem reached certified 27.0% efficiency (steady-state 26.4%) and an ISOS-L2 T₉₁ of 1000 hours at 65 °C.
The work points to a broader design rule for perovskite film control: use molecular dipoles to impose selective donor hardness, so crystallization pathways align across mixed anions. For “viral” science news, the headline is simple—phase segregation can be suppressed at birth, enabling wide-bandgap perovskites that keep their bandgap longer in real devices.
Subject of Research: Phase-homogeneous mixed-halide perovskites for stable wide-bandgap tandem photovoltaics.
Article Title: Phase-homogeneous mixed halide perovskites for stable tandem photovoltaics.
Article References: Shi, P., Zhuang, J., Zhang, D. et al. Nature (2026). https://doi.org/10.1038/s41586-026-10929-2
Image Credits: AI Generated
DOI: 10.1038/s41586-026-10929-2
Keywords: mixed-halide perovskites; phase segregation; wide-bandgap; Lewis-base hardness; molecular dipole; tandem photovoltaics; stability; PbBrₓ nucleation; O-donor coordination
