Lead Author: Jaewang Park, Jongbeom Kim
Corresponding Authors: Min Gyu Kim, Tae Joo Shin, Sang Il Seok
Corresponding unit: Ulsan National Institute of Science and Technology (UNIST), South Korea Pohang University of Science and Technology (POSTECH)
Research highlights:
1. This paper reports the controllable crystallization of perovskite thin films by the combination of alkylammonium chloride (RACl) added to FAPbI3.
2. The δ-to α-phase transition of FAPbI3 and the crystallization process and surface morphology of RACl-coated perovskite films under different conditions were investigated by in situ grazing incidence wide-angle X-ray diffraction and scanning electron microscopy.
3. The RACl added to the precursor solution is considered to be easily volatilized during coating and annealing.
4. The obtained perovskite thin layer facilitates the preparation of PSCs with a power conversion efficiency of 26.08% (certified as 25.73%) under standard illumination.
Research Status of Alkylammonium Chlorides on Perovskite Thin Films
I. Introduction
There have been numerous studies on the use of shorter alkylammonium chlorides (RACl) such as methylammonium chloride (MACl) to deposit perovskite thin films, resulting in low-temperature formation, grain growth, and preferred orientation of α-formamidinium lead iodide (FAPbI3). However, there are few reports on the use of longer-chain RACls, such as propylammonium chloride (PACl) and butylammonium chloride (BACl), as reagents to improve perovskite crystallization and surface morphology. PACl and BACl undergo slower deprotonation to produce propylamine (PA0) and butylamine (BA0) compared to MACl, as their boiling points are considerably higher (47.8°C for PA0 and 78°C for BA0) than methylamine (MA0, -6.3°C). Therefore, adding RACls with different physical properties is expected to have a significant impact on the crystallization behavior during the perovskite thin film formation. However, little research has been conducted to investigate whether the benefits of adding RACl arise from Cl- ions, RA+ ions, or the synergistic effect of both Cl- and RA+ ions.
II. Summary of Achievements
Controlling the crystallinity and surface morphology of perovskite layers through solvent engineering and the addition of methylammonium chloride additives is an effective strategy for achieving efficient perovskite solar cells (PSCs). In particular, depositing α-formamidinium lead iodide (FAPbI3) perovskite thin films with fewer defects is crucial as it exhibits excellent crystallinity and large grain size. Here, we report the influence of alkylammonium chlorides (RACl) combined with FAPbI3 on the control of perovskite thin film crystallization. The δ-to-α phase transition of FAPbI3 and the crystallization process and surface morphology of RACl-coated perovskite thin films under different conditions were studied using in-situ grazing incidence wide-angle X-ray diffraction and scanning electron microscopy. The RACl added to the precursor solution is believed to easily evaporate during the coating and annealing process due to the association of RA·····H+-Cl- with PbI2 in FAPbI3, resulting in the deprotonation of RA+ to form RA0 and HCl. Hence, the type and amount of RACl determine the rate of δ-to-α phase transition, crystallinity, preferred orientation, and surface morphology of the final α-FAPbI3. The obtained perovskite thin film improved the performance of PSCs, achieving a power conversion efficiency (PCE) of 26.08% (certified as 25.73%) under standard sunlight.
III. Results and Discussion
Key Point 1: Preparation of α-FAPbI3 Thin Films and the δ-α Phase Transition of FAPbI3
These results indicate that the increase in RACl, which can associate with PbI2 in FAPbI3, reduces the nucleation rate of intermediate phases induced by drop-casting anti-solvent. Furthermore, depending on the type of RA+, it leads to the formation of uniform thin films with large grain size. To prepare efficient PSCs based on α-FAPbI3, it is crucial to maintain the uniformity of the intermediate film even after high-temperature annealing. Additionally, the deposited films were annealed at 120°C for 40 minutes and compared regarding their surface morphology. It was observed that adding 10mol% RACl to the FAPbI3 precursor containing 35mol% MACl reduced the surface roughness of the film, but the smoothest surface was obtained with PACl. Therefore, PACl added to the 35mol% MACl plays a decisive role in improving the surface morphology, resulting in enhanced efficiency.
Figure 1c shows the structural evolution monitored by in-situ GI-WAXD during heating the deposited film from room temperature to 120°C. The initial δ-FAPbI3 thin film transforms to α-FAPbI3 under heating conditions around 60-80°C. The film coated with a precursor containing 15mol% PACl exhibits a mixture of α- and δ-FAPbI3 at room temperature. Typically, α-FAPbI3 requires heating temperatures higher than 150°C to obtain a corner-sharing PbI6 octahedral structure. It is speculated that the synergistic effect of PbI6 octahedral rotation and coexistence of PACl and MACl at low temperatures (60-80°C), and even at room temperature, may lead to the observed behavior. Thus, it can be reasonably concluded that the content of Cl- ions added to the perovskite precursor and the control of Cl- ion release rate through RA+ binding to Cl- ions are important factors determining the δ-to-α phase transition.
Figure 1d compares the 2D GI-WAXD of perovskite thin films obtained after annealing at 120°C for 30 minutes. The intermediate phases with a small amount of MACl and PACl, as well as solvent and the δ-phase, disappear after thermal treatment, transitioning to nearly pure α-FAPbI3 perovskite structure. This result indicates that PACl enhances the crystallinity and improves the grain orientation in the (100) α-plane along the Debye-Scherrer ring at q~10 nm^-1. This suggests an improvement in crystal orientation perpendicular to the substrate. The influence of these controlled grain, crystal orientation, and surface morphology on the device performance was studied based on the amount of PACl added to the precursor solution.
To further understand the δ-to-α phase transition of FAPbI3 perovskite thin films with volatile RACls on a low-temperature and highly flat surface, we first monitored the surface and structural evolution of the deposited films at room temperature. It was concluded that, above 60°C, the activation barrier is lowered in the presence of RACl, making the phase transition thermodynamically favorable. At room temperature, the barrier kinetically suppresses the return of the δ-phase. The δ-to-α phase transition occurs at higher temperatures, such as 150°C, or even at room temperature, due to the complex behavior of metastable intermediates, dependent on the amount of Cl- ions and the type of RA+ ions present, as well as the removal of residual solvent. There have been many reports on the δ-to-α phase transition, and the addition of Cl- anions (such as HCl and MACl) favors preferred orientation at low temperatures. Moreover, the 2D phase generated by RA+ reduces the δ-α phase transition temperature and enhances the stability of the α-phase. The addition of RACl to the FAPbI3 precursor significantly lowers the transition temperature, even causing it to disappear during room temperature aging. This indicates that the δ-to-α phase transition in the coated films is influenced by the amount and type of RACl present.
In the chemical formation process, the transition of α-FAPbI3 perovskite from the precursor film was studied by measuring the Extended X-ray Absorption Fine Structure (EXAFS) on the Pb LIII edge. The results suggest an initial increase in coordination between RACl and PbI2, leading to the formation of PbI6 octahedra, which then gradually disappears. RA+ (soft acid) behaves more like a soft base (I-), forming stable products, while RA+ (soft acid) and Cl- (hard base) form unstable products. Therefore, RACls coordinating with PbI2 (or FAPbI3) are expected to transform into highly volatile substances (e.g., MA0, PA0, and HCl) through processes like RA+ deprotonation, while remaining bound as RA+ cations and Cl- anions with PbI2.
Key Point 2: Characterization of α-FAPbI3 Thin Films Coated with Volatile RACls
Characteristics of perovskite layers deposited with volatile RACls. (a) Steady-state photoluminescence and (b) time-resolved photoluminescence spectra of films deposited on glass substrates. The highest PL intensity and longest lifetime in the target film indicate the lowest non-radiative recombination, suggesting the formation of films with the lowest defect concentration using 10mol% PACl. (c) Spatial charge-limited current (SCLC) analysis of target and control films, where VTFL represents the trap-filled limit voltage. Clearly, the target film shows the lowest trap density, indicating a reduction in defects in the perovskite film coated with a precursor containing 10mol% added PACl. Although the target film and reference 2 are almost similar in terms of surface morphology and δ-to-α phase transition, the analysis shows that the lowest trap density in the target film is due to higher crystallinity than reference 2. This may be attributed to the relatively rapid crystallization rate during the formation of the perovskite film in reference 2. Urbach energy (Eu) was also compared, as the low defect concentration in the perovskite film leads to high performance due to the superior structural quality during perovskite crystallization. The Eu of the perovskite films was calculated from UV-visible absorption spectra using the formula α=α0 exp(hν/Eu), where α is the absorption coefficient and hν is the photon energy. The target film exhibited the lowest Eu value, indicating the highest structural quality (Figure 3d).
Key Point 3: Photovoltaic Performance and Stability of Devices
As expected from the analysis of defect concentration and carrier lifetime in the formed perovskite thin films under each condition, the average power conversion efficiency (PCE) of the target and reference 2 devices is the highest, with a slight decrease compared to the control. The target device exhibited the best performance in reverse and forward bias scans, with the J-V characteristics shown in Figure 4b. The calculated JSC, VOC, and FF values from the J-V curves under reverse bias were 25.69 mA/cm2, 1.178 V, and 86.15%, respectively, corresponding to a PCE of 26.08% under standard air environment conditions. Under forward bias, the JSC, VOC, FF, and PCE were 25.64 mA/cm2, 1.178 V, 85.06%, and 25.7%, respectively. The PCE of the device was further validated by an accredited laboratory (Newport, USA) using the quasi-steady-state method, and the measured final stable efficiency was 25.73% with JSC, VOC, and FF values of 25.80 mA/cm2, 1.179 V, and 84.60%, respectively, which is the highest efficiency ever recorded.
Figure 4c shows the external quantum efficiency (EQE) of the same devices as the J-V measurements. The JSC obtained by integrating the EQE over wavelength was 25.3 mA/cm2, which closely matches the JSC measured using the solar simulator in the J-V curves. When tested under full sunlight exposure without a UV cutoff filter, the encapsulated target device maintained about 88% (25.2%) of its initial efficiency after 600 hours (Figure 4d).
IV. Conclusion
In conclusion, the authors achieved controlled crystallization of perovskite thin films by adding a combination of alkylammonium chlorides (RACl) to FAPbI3. The δ-to-α phase transition of FAPbI3 and the crystallization process and surface morphology of perovskite thin films coated with RACl under different conditions were studied using in-situ grazing incidence wide-angle X-ray diffraction and scanning electron microscopy. The RACl added to the precursor solution was found to be easily volatile during the coating and annealing process due to RA⋅⋅H+-Cl- association with PbI2 in FAPbI3, leading to the deprotonation of RA+ and dissociation into RA0 and HCl. Consequently, the type and amount of RACl determined the rate of δ-to-α phase transition, crystallinity, preferred orientation, and surface morphology of the final α-FAPbI3. The obtained perovskite thin films contributed to the preparation of PSCs, achieving a power conversion efficiency of 26.08% (certified as 25.73%) under standard illumination.
V. References
Park, J. et al. Controlled growth of perovskite layers with volatile alkylammonium chlorides. Nature
Doi: 10.1038/s41586-023-05825-y
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