Scientists solve the genetic puzzle of perovskite battery life

The team of Pan Xu and Tian Xingyou, researchers from the Institute of Solid State Physics, Hefei Institute of Physical Sciences, Chinese Academy of Sciences, and the Key Laboratory of Photovoltaic and Energy-saving Materials, Chinese Academy of Sciences, cooperated with Professor Nam-Gyu Park of Sungkyunkwan University in South Korea and Professor Dai Songyuan of North China Electric Power University to make new breakthroughs in the research of trans-perovskite solar cells. In this study, the non-uniform out-of-plane distribution of perovskite cations was found to be the main reason affecting the performance of the battery for the first time, and a photoelectric conversion efficiency (PCE) of 26.1% was obtained by designing 1-(benzenesulfonyl)pyrrole (PSP) as an additive to homogenize the phase distribution of perovskite films. On November 2, the research results were published online in Nature. 

Perovskite solar cells are solar cells that use perovskite-type organometallic halide semiconductors as light-absorbing materials, which are a new concept of solar cells. After years of development, the traditional interface passivation and crystallization control methods have promoted the improvement of cell efficiency to a large extent, but in recent years, the speed of cell efficiency improvement has slowed down significantly, and a “bottleneck” has been encountered. Researchers have found that phase separation is often unavoidable within perovskite films. The team’s previous work has shown that effective management of halogen phase separation can help improve device performance. High-efficiency perovskite materials are often obtained by using cation-doped components in pure iodine systems, especially FA1-xCsxPbI3 systems, and the distribution of different cation components in the out-of-phase direction of the perovskite body is very important for the phase carrier diffusion and interfacial extraction of the perovskite body. The study of the out-of-plane distribution of cations is helpful to explore the phase carrier dynamics of perovskite bodies, and is expected to further improve the efficiency of perovskite solar cells. However, the distribution of different cationic components of the perovskite body phase and the reasons that affect the stability and efficiency loss of the cell are not clear.  

Based on this, the team started from the FA1-xCsxPbI3 system, explored the longitudinal distribution of formamidine (FA) and cesium (Cs) cations through elemental quantitative analysis, combined with time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS), and found that inorganic Cs cations tend to be deposited at the bottom of the film, and organic FA cations are enriched at the interface on the film. Based on this, the crystalline phase distribution of perovskite thin films was explored, and the grazing incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM) analysis of the cross-section of the film proved that there was a crystalline phase with a small plane spacing at the bottom of the film, and the characteristic signal related to Cs-rich perovskite was shown at the bottom of the film. The above experiments illustrate the uneven distribution of cation gradient in the out-of-plane direction, which is the first visual verification of the non-uniform distribution of cationic components in perovskite films. 

The research team further analyzed the cause of the uneven distribution of the gradient using in-situ experiments, and found that the large rate difference between different cations during crystallization and phase transition was the main reason for the heterogeneity of the components. Furthermore, the team designed PSP molecules to compensate for the difference in crystallization and phase conversion rates between different cations, and prepared homogenized perovskite films. This perovskite film with uniformly distributed cationic components effectively inhibits the quasi-I-type energy level arrangement brought by the Cs-rich phase at the bottom, improves the carrier lifetime and diffusion length, and strengthens the carrier interface extraction. 

In this study, the trans-perovskite solar cells prepared using the PSP strategy obtained the highest efficiency of 26.1% and the certified efficiency of 25.8%. In addition, after 2500 hours of maximum power electrical tracking (MPPT), the unpackaged device retains 92% of the robust operational stability of the initial PCE. This study shows that excellent cell performance can be obtained by homogenizing the out-of-plane distribution of perovskite components, which opens up a new way to improve the stability of cell devices, is expected to break the efficiency bottleneck of perovskite solar cells, and proposes a clear direction for further improving the efficiency and stability of perovskite solar cells, which is of great significance for promoting the commercial development of PSCs. 

The research work has been supported by the National Key R&D Program of China, the National Natural Science Foundation of China, the Natural Science Foundation of Anhui Province, and the President Fund of Hefei Institute of Materials. Researchers from Southern University of Science and Technology participated in the research. (Source: Hefei Institute of Physical Sciences, Chinese Academy of Sciences)

Related Paper Information:

Figure 1. Spatially perpendicular FA-Cs phase separation of perovskite: (a) inhomogeneous phase distribution caused by out-of-plane FA-Cs segregation; (b) Electrostatic potential (ESP) image and molecular structure of PSP; (c) Time-of-flight secondary ion mass spectrometry (ToF-SIMS) of control samples and PSP devices for cation distribution; (d) Atomic percentage distribution based on deep XPS; HAADF transmission electron microscope images of control sample (e) and PSP-treated sample (f) (scale bar reference: 200 nm); GIXRD spectra collected from the bottom of control samples (g) and PSP (h) treated perovskite films. 

Figure 2. Origins of FA-Cs phase separation: (a) in-situ synchrotron radiation grazing-incidence wide-angle X-ray scattering (in-situ GIWAXS) spectra revealing crystallization and phase transition processes; Schematic diagram of the calculated results of free energy evolution during crystallization and phase transition of control sample (b) and PSP sample (c); (d-e) Fourier transform R-space results measured by X-ray absorption fine structure spectroscopy (EXAFS); (f) Pb-O coordination ratio plot calculated from EXAFS measurements; (g) FTIR spectra of PSP and PSP (PbI2) complexes. 

Figure 3. Device performance: (a) J-V curves of small-area perovskite solar cells; (b) IPCE curves of perovskite solar cells; (c) EQE curves of small-area perovskite solar cells operating in LED mode; (d) J-V curves of 1 cm2 perovskite solar cells; Operational stability of perovskite solar cells under (e) ISOS L-1I and (f) ISOS D-3 standards. 

Researchers test the performance of battery devices

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