Key Takeaways & Executive Findings
- •• • Green solvent GVL replaces toxic GBL, achieving ambient-stable α-FAPbI3 single crystals with markedly improved phase stability, addressing environmental and scalability bottlenecks in perovskite manufacturing. • • Additive modulation with oxalic acid (OA) suppresses δ-phase formation via H+ and reductive groups, yielding FA0.9MA0.1PbI3 single crystal thin films with a low defect density of 8.3 × 10^11 cm−3, critical for high-performance optoelectronics. • • Photodetectors fabricated from these crystals exhibit a responsivity of 14.5 mA/W, detectivity of 3.75 × 10^10 Jones, and response speed of 149/65 μs under zero bias and 780 nm illumination, demonstrating competitive performance for low-power sensing. • • Unencapsulated devices retain 83% of initial performance after 30 days in ambient conditions, indicating exceptional operational stability for practical deployment without costly encapsulation.
Abstract
Formamidine lead-based perovskites (FAPbI3) exhibit significant potential in optoelectronic applications. Nevertheless, they encounter challenges related to δ-phase instability and reliance on toxic solvents. In this study, we present a green solvent-additive synergy strategy that employs γ-valerolactone (GVL) in conjunction with reductive acids like oxalic acid (OA), to stabilize α-FAPbI3 single crystals (SCs). GVL enhances the stability of the precursors through the formation of FA+-GVL hydrogen bonds and high-valence [PbIx]2−x clusters, resulting in ambient-stable α-phase SCs, yielding a marked improvement in stability compared to SCs prepared with the toxic solvent γ-butyrolactone (GBL). Additive modulation demonstrates that H+ and reductive groups play a critical role in regulating crystallization, suppressing the δ-phase by promoting FA+ dissociation and inhibiting MA+ deprotonation. A solvent-involved intermediate, δ-FAPbI3-GVL, has been identified; this intermediate evolves into α-FAPbI3 at a low temperature of 60 °C, thereby reducing the energy barriers associated with the α to δ phase transition. In contrast, non-reductive acids and reductive ionic liquids do not inhibit δ-phase formation, with the latter even promoting the crystallization of pure δ-FAPbI3. By utilizing low-volatility OA as an additive, optimized FA0.9MA0.1PbI3 single crystal thin films exhibit a low defect density of 8.3 × 10^11 cm−3. Subsequently, a photodetector was fabricated. Under zero bias voltage and 780 nm illumination, the device exhibited a responsivity of 14.5 mA/W, a detectivity of 3.75 × 10^10 Jones, and a response speed of 149/65 μs. Moreover, without any encapsulation, the device’s performance diminished by only 17% after 30 days of storage in ambient conditions, indicating remarkable stability.
1. Introduction
Metal halide perovskites (MHPs) have emerged as promising candidates for photovoltaic and optoelectronic devices due to their exceptional light absorption coefficients, long carrier diffusion lengths, tunable band gaps, and low defect densities. While most MHP-based photodetectors (PDs) currently utilize polycrystalline thin films, their performance and stability are significantly hindered by the presence of amorphous or low-crystallinity grain boundaries. In contrast, MHP single-crystal (SC) films exhibit superior optoelectronic characteristics owing to their grain-boundary-free structure. Formamidinium lead triiodide (FAPbI3) perovskites, which possess enhanced optoelectronic properties and greater thermal stability compared to MAPbI3, are regarded as ideal candidates for high-efficiency optoelectronics. Nonetheless, the spontaneous α-to-δ phase transition at ambient conditions, induced by the lattice distortion resulting from the large ionic radius of FA+ (2.79 Å), poses a significant challenge for their practical application.
Recent advances in compositional engineering, such as MA+/Cs+ doping, and additive modulation using MAAc or formic acid, have partially mitigated phase instability. Progress has also been achieved in improving the overall stability of perovskite optoelectronic devices through strategies like interface engineering and defect passivation. Nevertheless, these methods still depend on the use of toxic or flammable solvents like γ-butyrolactone (GBL) or 2-methoxyethanol (2-ME), which hinder scalability and environmental friendliness. GBL is known for its toxicity and its tendency to hasten phase degradation. This study introduces a green solvent-additive synergy strategy employing γ-valerolactone (GVL) and reductive acids like oxalic acid (OA) to stabilize α-FAPbI3 single crystals, directly addressing the dual bottlenecks of phase instability and solvent toxicity.
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GUO Meitong, WANG Ying, ZHOU Binyi, LI Siyv, ZHANG Bingbing, HUANG Hongbo, WANG Zhenguang, LI Leipeng, WANG Shufang, GUO Linjuan, PAN Caofeng, YANG Zheng (2026). Green Solvent Engineering and Additive Modulation in Stabilized Black FAPbI3 Perovskite Single Crystals for High-Performance Photodetectors. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-4000-1
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Frequently Asked Questions
What is the mechanism by which GVL stabilizes the α-phase of FAPbI3 compared to GBL?
GVL forms hydrogen bonds with FA+ and promotes the formation of high-valence [PbIx]2−x clusters, which stabilize the precursor solution and facilitate the crystallization of α-phase single crystals. This results in ambient-stable α-phase SCs, whereas GBL does not provide such stabilization, leading to δ-phase formation.
How does oxalic acid (OA) as an additive suppress δ-phase formation and what is its impact on defect density?
OA provides H+ and reductive groups that promote FA+ dissociation and inhibit MA+ deprotonation, thereby suppressing δ-phase nucleation. This additive modulation yields FA0.9MA0.1PbI3 single crystal thin films with a low defect density of 8.3 × 10^11 cm−3, which is critical for high-performance photodetectors.
What are the key performance metrics of the photodetector fabricated from the optimized single crystals?
Under zero bias voltage and 780 nm illumination, the photodetector exhibits a responsivity of 14.5 mA/W, a detectivity of 3.75 × 10^10 Jones, and a response speed of 149/65 μs (rise/fall). These metrics indicate competitive performance for low-power, high-speed detection applications.
How does the stability of the unencapsulated device compare to conventional perovskite photodetectors?
The unencapsulated device retains 83% of its initial performance after 30 days of storage in ambient conditions, demonstrating remarkable stability. This is a significant improvement over typical perovskite devices that degrade rapidly without encapsulation, highlighting the effectiveness of the green solvent and additive strategy.
What is the significance of the solvent-involved intermediate δ-FAPbI3-GVL and its low-temperature conversion?
The identification of δ-FAPbI3-GVL intermediate that converts to α-FAPbI3 at 60 °C reduces the energy barrier for phase transition, enabling low-temperature processing. This is industrially beneficial as it lowers thermal budget and simplifies manufacturing, while maintaining phase purity.
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