A simple molecular additive rewires how wide-bandgap perovskites crystallize - unlocking record-setting single-junction and tandem solar cell efficiencies while sharply boosting device durability.
Study: Boron-halide interactions for crystallization regulation of a 1.68 eV wide-bandgap perovskite prepared via a two-step method. Image Credit: LuYago/Shutterstock.com
In a recent article published in the journal Energy & Environmental Science, researchers introduced an all-solution two-step sequential deposition process for solar cells with additive tris(pentafluorophenyl)borane (BCF).
The study's primary goal was to regulate crystallization behavior, enhance film morphology, and improve photostability of wide-bandgap perovskites, to ultimately result in high-performance solar cells suitable for industrial-scale fabrication and enabling advanced tandem architectures.
The pursuit of efficient, stable, and scalable wide-bandgap perovskite solar cells (PSCs) is key to advancing photovoltaic technology, particularly for tandem solar cell configurations with silicon.
Wide-bandgap perovskites, with band gaps of approximately 1.7 eV, are particularly desirable as they can be used as the top cell in tandem architectures, enabling a broader solar spectrum and surpassing the efficiencies of single-junction devices.
However, fabricating such films is challenging. To produce them scalably and reproducibly is currently a significant bottleneck in their manufacturing: The core material issues revolve around controlling crystallization kinetics, minimizing defects, and achieving pinhole-free, stress-relieved films with uniform composition.
Developing the Films
The research primarily focuses on a two-step sequential deposition process tailored to produce high-quality 1.68 eV wide-bandgap perovskite films.
First, an inorganic lead halide layer composed of a mixed solution of PbI2 and PbBr2 at a molar ratio of 3:1 is spin-coated onto a substrate, followed by annealing.
To modulate the morphology and chemical uniformity of this inorganic layer, BCF is added directly to the inorganic precursor solution at an optimized concentration of 1 mg/mL.
The second step involves dripping a mixed organic salt solution of methylammonium bromide (MABr) and formamidinium iodide (FAI) onto the inorganic layer, where the solution is converted into the perovskite structure.
During this conversion, the BCF molecules influence the crystallization dynamics by delaying the reaction between organic salts and inorganic precursors, reducing phase segregation, and promoting the growth of larger grains.
The BCF molecules simultaneously form boron-halide bonds and hydrogen bonds with organic cations. This dual interaction mechanism plays a central role in regulating nucleation, halide distribution, and grain-boundary chemistry.
Advanced characterization techniques confirmed the material properties at each stage. Fourier-transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), and density functional theory (DFT) calculations reveal the interactions between BCF molecules and precursor ions.
FTIR detected characteristic shifts indicating halide-boron bonding and hydrogen bonds with organic cations. NMR supported the existence of such interactions, while DFT calculations modeled the stabilization of energy and the geometry of these molecular interactions.
The films were morphologically assessed using scanning electron microscopy (SEM) for grain size and distribution, and X-ray diffraction (XRD) to determine crystallinity and phase purity.
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BCF Molecules: A Profound Effect
The researchers found that integrating BCF into the inorganic precursor yielded profound effects on the material properties and device performance.
Morphologically, BCF-treated perovskite films resulted in significantly larger grain sizes, often exceeding several micrometers, compared to control films.
The larger grains correlated with reduced grain boundary density, which minimizes the number of charge trapping sites and non-radiative recombination pathways that impair electronic properties.
SEM images revealed a more uniform and pinhole-free surface. XRD measurements revealed enhanced crystallinity, characterized by sharper peaks, indicating a well-ordered perovskite phase.
Phase purity was maintained across different batches, demonstrating the reproducibility, and possible scalability, of the process.
Hydrogen bonding between fluorine substituents on BCF and organic cations also contributed to defect passivation at grain boundaries, reducing trap states.
The authors showed in physical tests that BCF modulates residual film stress, shifting it from tensile to compressive, which contributes to improved structural stability during operation.
Optoelectronically, the BCF-treated films exhibited increased photoluminescence intensity and prolonged carrier lifetimes in time-resolved PL, indicative of fewer non-radiative pathways.
Charge extraction efficiency also improved, as evidenced by transient photocurrent measurements. These materials properties translate to device-level improvements, with champion PSCs achieving efficiencies exceeding 23.5 %.
The PSC devices exhibited high open-circuit voltages (~1.29 V) and low voltage loss (~0.39 V).
Long-term stability testing under continuous illumination showed that BCF-treated devices retained over 90 % of their initial efficiency after hundreds of hours (up to 750 hours of MPP tracking), contrasting with control devices that degraded more rapidly.
Notably, the optimized wide-bandgap films were also integrated into monolithic perovskite/silicon tandem solar cells, achieving a 31.12 % efficiency - currently the highest efficiency reported for tandem devices produced via an all-solution two-step method.
Conclusion
This research represents a significant advancement in the materials science of wide-bandgap perovskite solar cells, demonstrating meticulous control over crystallization through the strategic addition of BCF in an all-solution, two-step process.
The findings emphasize the impact of minor molecular additives in materials engineering, underlining how subtle chemical interactions can significantly influence the structural and electronic quality of perovskite layers.
Journal Reference
Luo S. et al. (2025). Enhanced Wide-Bandgap Perovskite Solar Cells via All-Solution Two-Step Method with BCF Modification. Energy & Environmental Science. DOI: 10.1039/d5ee03984c