Research Achievements and Highlights

All-perovskite tandem solar cells are considered a key technology for breaking through the efficiency limits of single-junction cells. However, their long-term stability is constrained by thermal degradation issues associated with the volatile methylammonium (MA) component in narrow-bandgap (NBG) lead-tin (Pb-Sn) perovskites. While replacing MA with formamidinium (FA) and cesium (Cs) can enhance thermal stability, the low solubility of Cs salts in solvents introduces new crystallization kinetics challenges.

Led by Professor Hairen Tan and Professor Renxing Lin’s team from Nanjing University, this research published in Nature Communications addresses these critical challenges. The study reveals that in traditional anti-solvent (AS) processes, low-solubility Cs tends to preferentially crystallize at the gas-liquid interface, forming a dense “Cs-rich surface layer.” This dense film acts as a barrier, impeding anti-solvent penetration for internal solvent (DMF/DMSO) extraction, resulting in residual solvents within the film. This ultimately causes void formation during annealing and leads to non-uniform vertical compositional distribution, severely compromising device efficiency and fill factor.

To overcome this bottleneck, the research team proposed a “Sustained Solvent-Extraction Channels (SSC)” strategy. By introducing L-glutamic acid hydrochloride (Glu), a multi-Lewis base regulator with dual carboxyl groups (-COOH), they leveraged its strong coordination with Pb²⁺/Sn²⁺ and modulation of precursor solution surface tension to effectively delay surface nucleation rates. This method successfully suppressed premature formation of the dense surface layer, maintaining open solvent extraction channels during anti-solvent treatment, allowing complete solvent removal, eliminating void defects at the film bottom, and achieving uniform vertical distribution of Cs elements.

Research Team

This groundbreaking research was conducted by the team led by Professor Hairen Tan and Professor Renxing Lin from Nanjing University. The study was published in the prestigious international journal Nature Communications.

Research Background and Challenges

Methylammonium-free perovskites face significant crystallization kinetics challenges that have hindered the development of stable, high-efficiency all-perovskite tandem solar cells.

Main challenges in MA-free Pb-Sn perovskite subcells:

• Thermal stability limitations: The volatile MA component in narrow-bandgap Pb-Sn perovskites undergoes thermal degradation, limiting long-term device stability.
• Crystallization dynamics issues: While FA and Cs substitution improves thermal stability, the low solubility of Cs salts in solvents creates new crystallization challenges.
• Cs-rich surface layer formation: In conventional anti-solvent processes, low-solubility Cs preferentially crystallizes at the gas-liquid interface, forming a dense barrier layer.
• Solvent extraction impedance: The dense Cs-rich surface layer blocks anti-solvent penetration, preventing efficient extraction of internal solvents (DMF/DMSO), leading to residual solvents within the film.
• Film quality degradation: Residual solvents cause void formation during annealing and result in non-uniform vertical compositional distribution, severely compromising device efficiency and fill factor.

Solution Approach

To address the severe crystallization dynamics challenges in methylammonium-free Pb-Sn perovskites, the research team proposed an innovative “Sustained Solvent-Extraction Channels (SSC)” strategy. They introduced L-glutamic acid hydrochloride (Glu), a multi-Lewis base regulator featuring dual carboxyl groups (-COOH), to fundamentally alter the crystallization process.

Mechanism of SSC Strategy

The SSC approach exploits Glu’s unique properties to control crystallization dynamics:

• Strong coordination interactions: The dual carboxyl groups in Glu form strong coordination bonds with Pb²⁺ and Sn²⁺ ions, modulating the nucleation and growth kinetics.
• Surface tension regulation: Glu modulates the surface tension of the precursor solution, effectively delaying surface nucleation rates and preventing premature Cs-rich layer formation.
• Sustained extraction channels: By suppressing early-stage dense surface layer formation, the SSC strategy maintains open channels throughout the anti-solvent treatment, enabling thorough solvent extraction from the film interior.
• Defect elimination: Complete solvent removal eliminates void defects at the film bottom that would otherwise form during annealing.
• Compositional uniformity: The sustained extraction channels facilitate uniform vertical distribution of Cs elements throughout the film thickness.

QFLS Characterization: Analyzing Carrier Dynamics and Non-Radiative Recombination Losses

This research utilized Quasi-Fermi Level Splitting (QFLS) as a key indicator for quantitatively evaluating perovskite film quality and device voltage losses. QFLS represents the energy difference between electron and hole Fermi levels under illumination, directly corresponding to the upper limit of achievable open-circuit voltage (V_oc) in devices. The research team performed precise photoluminescence quantum yield (PLQY) measurements to correlate optical properties with electrical losses, confirming the effectiveness of the SSC strategy in suppressing non-radiative recombination.

PLQY and QFLS Measurements

The team measured PLQY values for different layer configurations and calculated QFLS using thermodynamic relationships. Data shown in Fig. 2f and Table S2 reveal that SSC-treated perovskite films exhibit significantly enhanced PLQY, with calculated QFLS values reaching 0.94 eV, higher than the control group’s 0.93 eV. When integrated with electron transport layers and hole transport layers, SSC devices showed much smaller QFLS degradation compared to control devices, indicating substantially reduced non-radiative recombination losses at interfaces.

Electroluminescence quantum efficiency (ELQY) spectra in Fig. 2e further corroborate these findings. Under injection conditions equivalent to short-circuit current (J_sc), SSC devices achieved ELQY of 1.04%, superior to the control group’s 0.12%, explaining the higher open-circuit voltage exhibited by SSC devices.

In Situ Crystallization Dynamics Monitoring

Beyond static QFLS values, the research team employed in situ photoluminescence spectroscopy to monitor crystallization process dynamics (Fig. S19). Control group PL signals exhibited “appearance-disappearance-reappearance” characteristics, corresponding to the process where dense surface layer formation blocks solvent extraction, causing temporary film dissolution and recrystallization. In contrast, SSC-treated films showed continuous and smooth PL signal evolution, confirming sustained open solvent extraction channels and uniform crystallization mechanisms.

Steady-State and Time-Resolved Photoluminescence Analysis

Steady-state PL spectra (Fig. 2c) demonstrate that SSC films exhibit consistent emission intensity from both front and back surfaces with no peak shift, confirming vertical compositional homogeneity.

Time-resolved photoluminescence (TRPL) data (Fig. 2d) reveal that SSC films possess longer carrier lifetimes (increasing from 61 ns / 367 ns to 931 ns / 3811 ns), quantifying the reduction in defect density. These optical characterization data provide the microscopic physical basis for QFLS enhancement, confirming the dual role of Glu additive in defect passivation and crystallization control.

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Photovoltaic Performance Testing and Validation

Current-Voltage (J-V) Measurements

For current density-voltage (J-V) characterization, experiments employed an EnliTech AAA-grade solar simulator (SS-X) equipped with a Keithley 2450 source meter, performing tests under standard AM 1.5G spectrum (100 mW cm^-2) conditions.

Fig. 3b demonstrates that narrow-bandgap perovskite solar cells (NBG PSCs) optimized via the SSC strategy achieved a power conversion efficiency (PCE) of 22.7%, with open-circuit voltage improved to 0.871 V and fill factor reaching 81.9%. Precise light source control is crucial for all-perovskite tandem cells, as narrow-bandgap subcells are sensitive to infrared spectrum response. The AAA-grade simulator’s spectral matching ensures accuracy in J_sc measurements.

External Quantum Efficiency (EQE) Measurements

External quantum efficiency (EQE) measurements utilized EnliTech’s QE system (QE-R). Fig. 3e and Fig. S33 present EQE spectra for both tandem cells and single-junction cells. Integrated J_sc obtained from EQE spectral integration yielded 16.1 mA cm^-2, highly consistent with J-V measurement values, eliminating errors from spectral mismatch. These data generated by high-specification instruments confirm the SSC strategy’s contribution to actual device performance, supporting the research’s claim of a 29.2% tandem cell efficiency record.

Conclusion

The Nanjing University team, through the SSC strategy utilizing dual-carboxyl molecule Glu to modulate crystallization dynamics, successfully resolved issues of Cs surface enrichment and solvent retention in methylammonium-free Pb-Sn perovskites. This research achieved high-quality homogeneous thick film deposition through process optimization and elucidated the relationship between crystallization uniformity and non-radiative recombination losses via optical characterization methods including QFLS and PLQY, establishing the physical mechanism for enhancing open-circuit voltage.

Ultimately, the team fabricated all-perovskite tandem solar cells with third-party certified efficiency reaching 29.2%, setting a new record for methylammonium-free all-perovskite tandem cells. With IZO transparent electrode implementation, unencapsulated devices maintained over 80% of initial efficiency after 800 hours of aging at 85°C (Fig. 4d), demonstrating excellent thermal stability. These research achievements prove that high efficiency and high stability can be achieved simultaneously, providing a concrete technical pathway for commercialization of all-perovskite tandem solar cells.

Original Publication Information

Article Title: Homogeneous crystallization via sustained solvent-extraction channels for methylammonium-free all-perovskite tandem solar cells
Journal: Nature Communications
Publication Date: January 5, 2026
DOI: https://doi.org/10.1038/s41467-025-68213-2

This content is organized based on an early version of the paper for academic exchange and research sharing purposes. Related conclusions and data may be revised and refined before formal publication. For any discrepancies or inaccuracies, please refer to the final published version in the journal.