Researchers achieved 13.47% efficiency in an all-inorganic CsPbIBr₂ perovskite solar cell with a record fill factor of 82.3% using TPP-Zn surface engineering
The zinc porphyrin treatment reduced surface defects and grain-boundary recombination while improving crystallinity, carrier extraction, and hysteresis behavior
The treated devices retained around 82% of their initial efficiency after 10 days under ambient conditions with ~40% relative humidity
All-inorganic wide-bandgap perovskite solar cells are attracting growing interest due to their superior thermal stability compared to hybrid perovskites. However, their progress is still constrained by relatively low efficiencies, recombination losses, and defect-driven instability. Among the available compositions, CsPbIBr₂ has emerged as a promising candidate because its ~2.03 eV bandgap offers a balance between phase stability and light absorption. Yet, improving voltage retention and fill factor remains difficult.
A collaborative research team from Syracuse University, Ankara Yıldırım Beyazıt University, and King Fahd University of Petroleum and Minerals has demonstrated that tetraphenylporphyrin-zinc (TPP-Zn) surface treatment can improve charge transport and suppress recombination in CsPbIBr₂ perovskite solar cells.
The researchers deposited a thin TPP-Zn layer on top of the CsPbIBr₂ perovskite absorber. According to the study, the zinc porphyrin molecules interact with uncoordinated metal cations and surface defects in the perovskite film. This deposition passivated grain boundaries, reduced charge trapping, and improved charge transport across the device.
The surface-engineered CsPbIBr₂ solar cells reached 13.47% efficiency with an open-circuit voltage of 1.29 V and a fill factor of 82.3% in 0.16 cm² devices. Large-area cells also showed relatively limited performance loss, retaining 11.29% efficiency on a 1.02 cm² area.
A major improvement came from changes in the perovskite film morphology. XRD measurements showed that TPP-Zn treatment improved crystallinity without altering the crystal phase. The strongest enhancement was observed at a TPP-Zn concentration of 2.5 mg/mL, which produced sharper diffraction peaks and lower full-width half maximum values.
Microscopy measurements further revealed that untreated films contained rough surfaces and visible pinholes. After TPP-Zn treatment, the films became denser and more uniform. Average grain size increased from ~401 nm in pristine films to 654 nm in optimized passivated films. Surface roughness also dropped from 30.09 nm to 19.97 nm.
The study linked these morphological changes directly to improvements in charge carrier dynamics. Time-resolved photoluminescence measurements showed that carrier lifetime increased from 4.07 nanoseconds(ns) in untreated films to 6.68 ns after passivation. The treated films also demonstrated lower non-radiative recombination and improved carrier extraction.
Electrical characterization showed a reduction in trap-assisted recombination and enhanced electron mobility. The researchers also reported significantly lower hysteresis. The hysteresis index decreased from 21% in the untreated devices to 5% after TPP-Zn modification. According to the paper, the larger grains and reduced grain-boundary density likely suppressed ion migration and phase segregation in the bromide-rich perovskite absorber.
The group additionally used SCAPS-1D simulations to validate the experimental findings and analyze carrier transport behavior. The simulations showed good agreement with measured current-voltage characteristics, EQE response, and temperature-dependent performance trends.
Stability improvements were another important outcome. Under a nitrogen atmosphere at 85°C, the treated devices maintained around 75% of their original efficiency after 12 days, whereas untreated devices dropped to about 30%. Under ambient conditions with ~40% relative humidity, TPP-Zn-treated devices retained around 82% of their initial efficiency after 10 days.
The authors noted that the TPP-Zn layer also increased surface hydrophobicity, helping block moisture diffusion into the perovskite layer. Contact angle measurements increased from 43.4° for untreated films to over 74° for optimized passivated samples.
Despite remaining below the record efficiency of inorganic perovskite devices, the work shows that targeted surface engineering can significantly improve fill factor, recombination behavior, and stability in CsPbIBr₂ solar cells. The researchers also see potential for integrating these wide-bandgap absorbers into tandem photovoltaic structures.
The work is reported in the paper titled Surface engineered wide-bandgap all-inorganic perovskite solar cells achieve a fill factor exceeding 82%.