Defects such as surface imperfections and grain boundaries within microcrystalline thin films have been the cause of performance killer in perovskite-based devices. Large single-crystalline materials such as methylammonium iodide have been used since they are free from the defects and possess superior characteristics such as ultralow trap state density. Research advancements have shown that the formamidinium lead iodide performs even better as an optoelectronic material with solar cell efficiency greater than 22%. One of the impediments to the advancement of the formamidinium lead iodide is that only smaller single crystalline formamidinium perovskite have been produced and therefore larger wafers and crystals urgently need to be developed.
Shengzhong (Frank) Liu and coworkers applied an inverse temperature reactive crystallization strategy to develop large single crystalline formamidinium perovskite. Their work is reported in Advanced Optical Materials, 2016, 4 (11), 1829-1837;. Advanced Materials, 2016, 28, 9204-9209; Sci. China Chem. 2017, DOI:10.1007/s11426-017-9081-3.
The authors observed that most harvested crystals exhibited rhombic hexagonal dodecahedra shapes with black glossy surfaces. They also noted that a higher relative humidity accelerates color change of the crystal from black to yellow. Thin wafers were prepared using a slicing machine with the thinnest wafer obtained having a 100 µm thickness.
The x-ray diffraction test showed the single crystalline formamidinium perovskite wafer adopts a perovskite structure and that no residue of the reactant precursors is present. Further, from rocking curve measurements it was evident that the wafer exhibits excellent crystalline quality.
From the absorption spectrum of the formamidinium lead iodide there was a cut-off feature indicating that it is a direct bandgap semiconductor, with the absorption edge located at approximately 870 nm, and an optical bandgap of approximately 1.49 electron-volts. The thermogravimetric analysis of the perovskite shows that it is highly stable without any sign of thermal decomposition up to 300 °C as compared with the methylammonium lead iodide perovskite.
The authors established that the trap state density of the as-prepared perovskite was lower than many of the known inorganic semiconductors, and that it has a low carrier concentration with a greater carrier mobility. This affirms that the as-prepared perovskite is of high quality.
The research team also compared photodetectors made of the as-prepared perovskite wafer and a thin film microcrystalline perovskite. They observed that the photocurrent of as-prepared perovskite was about 90 times higher than that of the thin film perovskite, and that it exhibits a significant response in the near-infrared region as compared with the latter, which confirms that the former has a broader optical absorption. The external quantum efficiency, photoresponsivity and response time of the as-prepared perovskite wafer were found to be even greater in the as-prepared perovskite wafer.
Yucheng Liu, Jiankun Shun, Zhou Yang, Dong Yang, Xiaodong Ren, Hua Xu, Zupei Yang, Shengzhong(Frank) Liu. 20-mm-Large Single-Crystalline Formamidinium-Perovskite Wafer for Mass Production of Integrated Photodetectors. Advanced Optical Materials, 2016, 4, 1829-1837.Go To Advanced Optical Materials