A recent article published in EcoMat introduced a novel method called reverse-cool annealing (RCA) to fabricate stable and highly efficient two-dimensional (2D) Ruddlesden-Popper (RP) perovskite films. The proposed method involved sequential annealing at high and low temperatures for wet perovskite films.
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Background
Perovskite solar cells (PSCs) have received significant attention owing to their large charge carrier mobility, high absorption coefficient, and excellent photovoltaic performance. Specifically, 2D RP PSCs exhibit long-term environmental stability, but their photovoltaic performance is still inferior to their three-dimensional counterparts.
The quality of 2D RP perovskite films influences the photovoltaic performance of PSCs. Additionally, the solvent evaporation rate, nucleation, and grain growth rate highly depend on the annealing processes. Therefore, the annealing strategy of 2D RP perovskite films is crucial to the ordered distribution of multiple phases in the out-of-plane direction.
The flash crystallization approach with high nucleation and growth rate, obtained via the flash-annealing approach, yields high-quality perovskite films. Therefore, this study proposed a novel flash-annealing process, the RCA method, to fabricate 2D RP perovskite (4FPEA)2(MA0.9FA0.1)3Pb4I13 films.
Methods
Indium tin oxide (ITO) glasses were used as the substrates to prepare NiOx by spin-coating the corresponding precursor solution. Subsequently, polybis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) was spin-coated on NiOx to obtain NiOx/PTAA films. The 2D RP perovskite (4FPEA)2(MA0.9FA0.1)3Pb4I13 precursor solution was then spin-coated on these NiOx/PTAA films.
The wet 2D RP perovskite film was sequentially annealed at 120 °C for 8 seconds and 80 °C for 5 minutes. The resulting film was designated as RCA. Simultaneously, 2D RP perovskite films were fabricated by annealing at 120 °C for 5 min (high-temperature annealing (HTA)) or 80 °C for 5 min (low-temperature annealing (LTA)).
Subsequently, 6,6-phenyl C61 butyric acid methyl ester and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline solutions were spin-coated on 2D RP perovskite film. Finally, an Ag electrode was thermally evaporated on the composite perovskite film.
The scanning electron microscope (SEM) and atom force microscope (AFM) were used to characterize the morphology and roughness of 2D RP perovskite films, respectively. Their structure was determined using X-ray diffraction and grazing-incidence wide-angle X-ray scattering. Optical characteristics were studied using a photoluminescence (PL) system and ultraviolet-visible spectrophotometer.
An electrochemical workstation was used to characterize the dark current-voltage curves, electrochemical impedance spectra, and Mott-Schottky curves of 2D RP PSCs. Their external quantum efficiency (EQE) and electroluminescence (EL) spectra were also recorded.
Results and Discussion
The RCA method effectively improved the crystal quality of the perovskite films. While the first step, annealing at 120 °C for 8 s (HTA-8), promoted crystal growth in the out-of-plane direction, the subsequent low-temperature annealing at 80 °C for 5 min further facilitated the formation of smooth and uniform perovskite films.
The PL intensity of the RCA-derived perovskite film was higher than that of the film exposed only to HTA-8, indicating enhanced perovskite crystallization quality achieved through low-temperature annealing. Additionally, the photovoltaic performance of the RCA-derived PSC surpassed that of the PSC incorporating the HTA-8 film.
SEM images of RCA-derived perovskite film exhibited a considerably smooth and compact surface. Continuous large grains with nearly no grain boundary were evident in the case of RCA-based perovskite film. The smooth, strong surface and vertical orientation of the RCA-based perovskite film helped improve the charge transport and extraction ability, thereby significantly enhancing short-circuit current density.
The HTA-based perovskite film exhibited noticeable surface damage, while the LTA-based perovskite film showed clear pinholes, grain boundaries, and a few randomly oriented small crystalline grains. Despite having similar thicknesses, the roughness of the perovskite films derived from RCA, HTA, and LTA measured 23.1 nm, 47.2 nm, and 32.8 nm, respectively. These results indicate that the RCA approach effectively produces high-quality perovskite films.
The integrated current densities of PSCs based on RCA, HTA, and LTA were 18.36, 13.77, and 14.41 mA/cm2, respectively. Additionally, the steady-state output for photocurrent density of PSCs based on RCA, HTA, and LTA were 17.87, 9.56, and 13.61 mA/cm, respectively.
During the long-term storage of PSCs without encapsulation in a nitrogen-filled glove, the HTA-based PSC remained at 72.1 % of its original PCE after 195 hours; the PCE of the LTA-based PSC remained at 79.7 % of its original PCE after 1000 hours. Alternatively, the RCA-based PSC had excellent long-term stability and remained at 97.4 % of the initial PCE after 1000 hours.
Conclusion
The study demonstrated the effectiveness of the RCA strategy in fabricating high-quality 2D RP perovskite films with preferred out-of-plane growth. The RCA-derived films showed a compact, pinhole-free morphology, reducing defects and improving charge transport properties.
The resulting 2D RP PSCs exhibited high charge carrier extraction efficiency and minimal non-radiative open-circuit voltage loss. The best-optimized device achieved a power conversion efficiency (PCE) of 17.8 % and maintained stability in a nitrogen environment for over 1000 hours without significant degradation. This RCA strategy holds significant potential for advancing the practical application of 2D PSCs.
Journal Reference
Xie, Z., et al. (2024). A universal reverse‐cool annealing strategy makes two‐dimensional Ruddlesden‐popper perovskite solar cells stable and highly efficient with Voc exceeding 1.2 V. EcoMat. DOI: 10.1002/eom2.12501, https://onlinelibrary.wiley.com/doi/full/10.1002/eom2.12501
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