Hybrid organic-inorganic perovskite semiconducting materials developed for field-effect transistors have received a significant research interest owing to their superior optical absorption, tolerances to defects, and charge-carrier diffusion. The power conversion efficiency of these perovskite solar cells have been recorded to rise over the past few years to about 20%. Notwithstanding, fabrication based on the planer architecture has become popular owing to its low fabrication temperature and compatibility with a wide range of substrates.
Unfortunately, incomplete and inhomogeneous coverage of the perovskite films have been identified as the main adversaries limiting device performance. Therefore, there has been extensive research aiming at improving the morphology of the perovskite films by modifying interface layers, precursor solution concentration, and by optimizing annealing time and temperature. Developing new methods for perovskite film deposition such as vacuum deposition and solution deposition have also been explored.
Vacuum processes for thermal co-deposition and sequential deposition of lead chloride and CH3NH3I have been focused most, as they are considered efficient for preparing films with excellent uniformity as well as high surface coverage. However, the diffusion of CH3NH3I into the vacuum chamber and poor uniformity of the solar cells have limited the number of successful reports implementing vacuum deposition methods.
Researchers led by Professor Shengzhong (Frank) Liu and Dr. Dong Yang from Dalian Institute of Chemical Physics, Chinese Academy of Sciences, achieved a uniform layer-by-layer vacuum deposition by alternating lead chloride and CH3NH3I precursor layers. The method allowed the authors to relax the stringent deposition control and monitoring measures, and realized excellent uniformity in film morphology, smoothness, and surface coverage. They recorded a solar cell efficiency of 16.03%, the highest efficiency reported at the time of its publication in peer-reviewed journals, Journal of Materials Chemistry A and Advanced Materials. It is worthwhile to point out that the group has increased the efficiency to as high as 18.34%, remaining their lead in this category.
The authors prepared samples of lead chloride film with different thickness. This was in a bid to investigate the thickness effect of lead chloride layer. They then ensured sufficient amount of CH3NH3I and removed the excess amount through post-annealing.
They achieved uniform and full coverage perovskite film when the lead chloride film thickness was smaller than 100nm. When it was more than 100nm, voids appeared in the films because of unreacted lead chloride, and the density and size of the voids increased with the lead chloride film thickness. Therefore, they take turns to deposite 100 nm of lead chloride and then suitable thickness of CH3NH3I precursor layers to obtain high quality perovskite films. This proposed method offered very high solar cell efficiency with suppressed performance variation. The power conversion efficiency of these devices implementing the present development reached approximately 16.03%, and the power conversion efficiency of the large active area device reaches 13.87%.
The method proposed in this study comes with a number of advantages. First, it relaxes the complicated operations concerning deposition rates control and monitoring. It provides high quality perovskite films with smaller roughness, uniform morphology, full surface coverage, and crystalline phases of higher purity. The controlled deposition environment makes possible the manufacture of dense and pure perovskite films, which results in efficient moisture protection yielding excellent device stability.
The proposed method in the study provided an efficient approach for the fabrication of large area perovskite solar cells. It appears promising for application in the manufacture of large area perovskite solar cells.
Dong Yang, Zhou Yang, Wei Qin, Yuliang Zhang, Shengzhong (Frank) Liu and Can Li. Alternating precursor layer deposition for highly stable perovskite films towards efficient solar cells using vacuum deposition. J. Mater. Chem. A, 2015, 3, 9401–9405.
Yang; R. Yang; X. Ren; X. Zhu; Z. Yang; C. Li; S. F. Liu, Hysteresis-Suppressed High-Efficiency Flexible Perovskite Solar Cells Using Solid-State Ionic-Liquids for Effective Electron Transport. Adv Mater 2016, 28 (26), 5206-13.Go To Advanced Materials