Orientation nucleation mechanism of formamidine-based photovoltaic perovskite materials

On June 21, 2023, the journal Nature published a new paper titled “Oriented nucleation in formamidinium perovskite for photovoltaics” online. By monitoring the perovskite crystallization process in situ, the research team reported a key mechanism of oriented nucleation to avoid the generation of non-photoactive phases.

Shi Pengju, a doctoral student jointly trained by Wang Rui’s research group of Westlake University and Xue Jingjing’s research group of Zhejiang University, is the first author of the paper, and Ding Yong, Ding Bin and Xing Qiyu are the co-first authors. Xue Jingjing of Zhejiang University, Mohammad K. Nazeeruddin of Lausanne Federal Institute of Technology, Yang Yang of UCLA, and Wang Rui of Westlake University are co-corresponding authors, and Academician Yang Deren, academic leader of Zhejiang University, gave important guidance and support for this work.

Perovskite solar cells, as a new generation of semiconductor photovoltaic technology with great potential, have attracted widespread attention in the field of renewable energy. Formamidine-lead iodine-based perovskites (FAPbI3) are considered to be the most promising materials in the perovskite family to achieve high photoelectric conversion efficiency due to their ideal optical bandgap and thermal stability. However, the photoactive black phase FAPbI3 is often accompanied by the presence of other crystalline phases that are not optically active during crystallization due to the thermodynamic disadvantage of its crystal phase. The rapid crystallization kinetics (measured in seconds) of perovskites still make the key microscopic mechanism of its phase transition process unknown, which hinders the design and development of targeted crystal phase regulation strategies. Especially in different perovskite deposition scenarios, such as one-step and two-step deposition schemes, small-area and large-area devices, etc., due to the lack of understanding of their common key mechanisms, the phase regulation strategies applicable in one scenario are usually not applicable to other scenarios.

In order to clarify the key microscopic mechanism in the process of rapid phase transition of perovskite, the researchers discovered a universal orientation nucleation mechanism in the formation of black phase FAPbI3 through an in-situ multi-channel real-time monitoring method, which inhibits the formation of non-optically active crystal phases and enables the formation of pure black phase FAPbI3 at room temperature, which is applicable in perovskite deposition in various scenarios. Based on this, the research team achieved a photoelectric conversion efficiency of 25.4% for perovskite small-area solar cell devices, and amplified it to achieve a 21.4% aperture efficiency in large-area modules (27.83 cm2). The research results were published online in the top international journal Nature on June 21, 2023.

Figure 1: Phase change driving force at room temperature of formamidine-based perovskite.

In perovskite film processes, PbI2 and organohide precursors are pre-deposited, and non-photoactive mesophases are always observed at room temperature. XRD patterns of perovskite films deposited at room temperature show diffraction peaks corresponding to the black phase (100) plane, accompanied by diffraction peaks in the 2H and 6H phases. With the addition of pentamidine (PAD) to the precursor of organic cations, the non-optically active phase is eliminated. The research team used Fourier transform infrared spectroscopy, XRD, combined with density functional theory calculations to study the interaction between PAD and Pb-I framework, and found that the black phase FAPbI3(100) crystal plane energy decreased significantly. This thermodynamic driving force led to the preferential formation of the crystal plane orientation of the black phase perovskite (100), which ultimately determined the crystalline framework and promoted the formation of black phase perovskite crystals. In contrast, the energy distribution of the phase transition process is relatively uniform without PAD, resulting in the coexistence of multiple phases and orientations.

The research team used synchrotron radiation-based in situ detection technology to deeply study the whole process of perovskite from precursor solution to crystallization, and verified the nucleation mechanism of this orientation by systematically changing the crystal plane energy of the (100) surface. In situ incident X-ray diffraction measurements show that when the FAI solution is dropped onto the pre-deposited PbI2, the perovskites begin to nucleate (N0 step), and during spin coating, the peak intensity of the (100) plane gradually increases, describing it as the nucleation phase (ns step), in which the disappearance, formation, and growth of crystal nuclei occur simultaneously. The subsequent annealing step results in a rapid rise in (100) peak intensity, indicating a rapid crystal growth process (G-step). During the nucleation phase, the control perovskite film showed diffraction peaks of multiple crystal phases, while the film with PAD added showed a (100) diffraction pattern of clean black phase perovskite. By extracting the azimuth of the crystal plane of perovskite (100) in the nucleation stage, the research team found that the azimuth in the control group showed a wide distribution from about 60° to 120°, indicating that its crystallization orientation was relatively random. A sharp peak centered on a 90° azimuth with a half-peak width maintained at about 8° is observed after the addition of PAD, which reveals the orientation nucleation mechanism along the (100) crystal face. This mechanism is further elucidated by changing the length of alkyl chains linked to organamidine cations, thereby regulating the effect on crystal plane energy. In situ monitoring showed that the decrease of (100) crystal plane energy in different systems would induce the nucleation of the orientation, and with the intensification of the decrease of (100) crystal plane energy, orientation nucleation would be more significant. The slow nucleation dynamics were further verified by in situ photoluminescence measurements, and its kinetic trend was consistent with the trend of crystal plane orientation distribution, which further confirmed the important role of crystal plane in regulating crystallization. This orientation nucleation dominates subsequent crystal growth along the (100) crystal plane orientation. The resulting perovskite film exhibits higher crystallinity and conductivity.

Figure 2: In situ real-time monitoring and orientation nucleation mechanism of perovskite crystallization film formation.

Based on this, the research team optimized the perovskite thin film deposition strategy, and achieved photoelectric conversion efficiency of more than 24% and 25% in the two-step and one-step deposition processes, respectively. The research team further applied it to the preparation of solar modules, achieving a pore size efficiency of up to 21.4% on a pore size area of 27.83 cm2, which was certified by a third-party organization. The prepared solar cell devices also showed significantly improved working stability. When tracked at maximum power point under constant illumination at 30±3 °C, the device maintained 95% of the initial efficiency after more than 1000 hours, while the photoelectric conversion efficiency of the control device decreased by 30% under the same conditions.

Figure 3: Perovskite photovoltaic device and module performance.

By capturing the rapid crystallization process of perovskite, this study reveals the key role of orientation nucleation mechanism in achieving FAPbI3 crystal phase regulation, which provides an important theoretical basis and technical exploration for the development of targeted perovskite film quality improvement strategies and large-scale deposition schemes. (Source: Science Network)

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