(News) Nature: SKKU Nam-Gyu Park & CAS & NCEPU Research Teams Proposed Perovskite Solar Cells that Achieves Nearly 26% Efficiency and Stability Through Cation Homogenization
Highlights
The research article was published by Nam-Gyu Park from SKKU, Xu Pan & Jiajiu Ye form Chinese Academy of Sciences (CAS*)* and Songyuan Dai from NCEPU et.al.
- Perovskite solar cells with cesium cation additions (FA1-xCsxPbI3) are promising but have issues with compositional inhomogeneity.
- The researchers visualized vertical variations in cation composition in perovskite films.
- This was caused by cesium cation segregation between grain boundaries and grains.
- They used 1-(Phenylsulfonyl)pyrrole (PSP) to homogenize the cation distribution in the films.
- Solar cells made with the cation-homogenized films showed enhanced 25.2% (champion PCE of 26.1%) efficiency and stability.
Background
Perovskite solar cells have garnered much attention due to their high efficiency, stability and scalability. However, research has found that the segregation of A-site Cs cations leads to compositional inhomogeneity in perovskite films, impacting cell performance. The issues with Cs cation segregation include: 1) Non-uniform distribution, with Cs cations tending to aggregate at the bottom of the film; 2) Compositional inhomogeneity, resulting in a gradient distribution from Cs-poor to Cs-rich; 3) Lattice mismatch and distortion due to size differences with other cations; 4) Reduced stability. These problems negatively affect cell performance and reliability.
Recent research visualized the compositional non-uniformity in the vertical direction of perovskite films, identifying the causes and effects on devices. The researchers used 1-(Phenylsulfonyl)pyrrole to homogenize the cation distribution within the films. This strategy overcame the composition variations caused by cation segregation, achieving higher efficiency and stability. This study provides new insights to optimize perovskite films and facilitates the development of perovskite solar cells.
Results
The research discovered vertical phase separation of formamidinium and cesium cations in perovskite films, which significantly impacted cell performance. Researchers observed cesium cations tending to aggregate at the bottom of films, forming cesium-rich phases and causing lattice strain and mismatch. By using 1-(phenylsulfonyl)pyrrole (PSP) to inhibit cation segregation and improve lattice arrangement, PSP-treated films exhibited better out-of-plane alignment and reduced lattice stress. Thus, the origin of cation inhomogeneity may be related to structural changes. Resolving out-of-plane cation inhomogeneity is very important for improving device performance.
Introducing the PSP organic molecule can inhibit FA-Cs phase separation, making the cation distribution uniform in the perovskite. ToF-SIMS and XPS results support the existence of out-of-plane cation inhomogeneity, with PSP molecules clustered at the bottom of the film. PSP can mitigate cation inhomogeneity in the perovskite.
Evaluation results showed PSP treatment enhanced PL intensity, prolonged carrier lifetime, and improved carrier recombination kinetics. Calculations indicated PSP treatment can reduce defect formation energy and enhance film quality. Analysis also showed PSP generated a more favorable band structure. In summary, PSP improved the optical and electronic properties of the perovskite.
Perovskite solar cells treated with PSP exhibited enhanced performance. The champion device achieved a PCE of 26.1% (certified reverse PCE of 25.8%, certified steady-state PCE of 25.2%), displaying enhanced fill factor and carrier extraction. After 2500 hours of testing, unpackaged PSP-treated devices retained 92% of initial PCE, while reference devices dropped to around 80%. Humidity reliability testing also demonstrated the superior performance of PSP-treated devices.
Therefore, PSP can effectively inhibit cation segregation and improve perovskite solar cell performance. This study systematically revealed the issue of cation distribution non-uniformity in perovskite films, and successfully controlled out-of-plane cation distribution using the PSP organic molecule, significantly improving device performance and reliability. This strategy provides new insights to optimize perovskite solar cell performance.
Methods
- Fabricated and visualized vertical inhomogeneity in FA1-xCsxPbI3 perovskite films
- Added 1-(Phenylsulfonyl)pyrrole (PSP) as an additive
- Characterized films with PL, TRPL, ToF-SIMS, XPS, UV-vis
- Performed DFT calculations on defect and band structure
- Fabricated and tested p-i-n and n-i-p solar cells
- Certified champion device power conversion efficiency
Conclusions
Perovskite solar cells with FA1-xCsxPbI3 formula offer efficiency and stability but suffer A-site cation segregation causing detrimental inhomogeneity. Visualizing vertical composition variations in films revealed reasons and impacts. Adding PSP homogenized cations, enabling 25.2% certified steady-state PCE in stable p-i-n devices. Out-of-plane cation homogenization with PSP presents a viable strategy to overcome composition issues and realize efficient, stable perovskite photovoltaics.
Keywords: solar cell, PCE, PCE
