Guangdong University of Technology researchers have developed a solvent-engineering strategy to improve the crystallization and performance of carbon-based CsPbBr₃ perovskite solar cells, addressing a known challenge associated with precursor solubility mismatch.
All-inorganic CsPbBr₃ perovskites are widely recognized for their superior stability compared to hybrid counterparts, avoiding degradation pathways linked to organic cations. Despite a relatively wide bandgap of approximately 2.3 eV, limiting absorption to the 270–532 nm range, CsPbBr₃ exhibits strong intrinsic transport properties, including a carrier diffusion length of around 10 μm and a carrier mobility of up to 2000 cm² V⁻¹ s⁻¹. These characteristics make it a promising candidate for stable photovoltaic devices, provided high-quality films can be reliably fabricated.
A key bottleneck in conventional spin-coating approaches lies in the large solubility difference between PbBr₂ and CsBr. While PbBr₂ dissolves readily in common organic solvents, CsBr shows limited solubility in methanol, often resulting in incomplete conversion and the formation of secondary phases such as CsPb₂Br₅ or Cs₄PbBr₆. These impurities degrade film morphology and device performance.
To overcome this limitation, the researchers introduced a water-methanol (WMe) binary solvent system to precisely tune CsBr solubility. Water enhances CsBr dissolution, while methanol improves wettability and film coverage on the PbBr₂ layer. By carefully adjusting the water content, the team achieved a balanced stoichiometric reaction between CsBr and PbBr₂ during a multi-step spin-coating process.
The study identified an optimal solvent composition containing 30% water, which enables sufficient CsBr solubility while maintaining favorable film formation dynamics. This balance plays a critical role in governing nucleation and crystal growth: higher solubility promotes complete phase conversion, while controlled solvent properties suppress residual CsBr and improve grain formation. As a result, the process yields dense, full-coverage, and phase-pure CsPbBr₃ films.
The resulting films were incorporated into a device structure of FTO/c-TiO₂/m-TiO₂/CsPbBr₃/carbon. Devices fabricated under ambient conditions achieved a power conversion efficiency of 6.59%. Notably, the unencapsulated devices retained 87% of their initial efficiency after 30 days of air exposure, highlighting the inherent stability of the inorganic absorber and the improved film quality.
The work provides insight into the mechanism of solvent-controlled crystallization. The water fraction directly regulates CsBr solubility, which in turn influences the reaction kinetics with PbBr₂ and the evolution of crystal phases. This approach avoids the need for more complex treatments such as additives, post-processing, or multi-step annealing, offering a comparatively simple and scalable route to high-quality CsPbBr₃ films.
Overall, the study demonstrates that precise solvent engineering in a binary water-methanol system can simultaneously optimize precursor chemistry, crystallization behavior, and device stability, offering a practical pathway toward more robust and manufacturable all-inorganic perovskite solar cells.
