Research of growth and devices base on perovskite single-crystalline thin films [PhD thesis]
Authors: Yu-Hao Deng
Date: 2020-07-30 00:00:00

Monocrystalline materials are the cornerstone of the modern high-performance semiconductor industry. In recent years, the new perovskite (ABX3) materials with organic-inorganic hybrid structure have attracted extensive attention in the scientific research field due to their excellent and unique photoelectric properties. Photovoltaic devices based on perovskite materials have made sufficient breakthroughs in efficiency in just a few years, rapidly increasing from only 3.9% in 2009 to 25.2% today. In addition to the photovoltaic field, perovskite materials also have great application prospects in photodetectors, light-emitting diodes, and lasers. However, almost all of the absorption layers of photovoltaic devices prepared in the past are polycrystalline thin films. However, the grain boundaries and defects in polycrystalline thin films increase carrier scattering and recombination, which limits the further improvement of photovoltaic device performance. The single-crystal bulk of the perovskite material has been proven to have advantages of lower defect states density, higher carrier mobility and longer carrier recombination life time than polycrystalline films. However, due to the thicker thickness of the monocrystalline bulk material, it is not suitable for the direct preparation of perovskite optoelectronic devices, so the preparation of perovskite single-crystal thin films has become a new hot spot in the field.
In this paper, we begin from the field of materials growth for perovskite single crystal thin films and work on the growth of high quality, ultra-thin, large scale and controllable thickness of thin films. We also have carried out research based on photoelectric detectors fabricated with perovskites single crystal thin films. This paper mainly includes the following three parts:
1. Optimization of single-crystal thin film growth method. 
Based on the previously reported monocrystalline bulk growth method, we have optimized the growth method to grow thinner and larger-scale monocrystalline thin films. The specific optimizations are as follows: first, we provide an internal gap between two superimposed substrates for nucleation and growing of the perovskite monocrystalline films, breaking the isotropic growth conditions of perovskite materials. Second, by applying pressure to the substrate to regulate the thickness of the gap between two substrates, the thickness of the single crystal film can be adjusted from a few micrometers to 100 nm. Third, the growth substrate was hydrophobically treated. Experiments have demonstrated that hydrophobic substrates can reduce the nucleation density and greatly increase the crystal growth rate. The growth rate of the perovskite in hundreds of nanometers gap under hydrophobic substrates is even higher than that of single crystal growth in free solution space, and we also give a theoretical explanation of the mechanism of hydrophobic effect.
2. Preparation and characterization of ultra-thin perovskite single crystal thin film and their optoelectronic devices. 
Based on the above growth methods, we have successfully prepared large-scale ultra-thin perovskite single crystal thin films with controlled thickness. By changing the thickness of the single-crystal films, we systematically investigated the relationship between the thickness with the absorption of light and carrier collection, obtained a proper thickness of the perovskite film. Then, by controling the pressure, ultra-thin perovskite single crystal films with a side length of several hundred micrometers and a thickness of only a few hundred nanometers, corresponding to an aspect ratio of 1900, are successfully grown. The surface roughness (RMS) of the films is only 0.54 nm, the full width at half maximum (FWHM) of X-Ray diffrection peak is as low as 0.035o, and the defect density is at 1011 cm-3 level. At the same time, the photodetector based on the corresponding material was fabricated. The detector has a gain of 50 million and a gain-bandwidth product of 70 GHz, which is 5000 times and two orders of magnitude higher than that of all previously reported perovskite photoconductive detectors. The detection limit of the detector is as low as 100 photons, which is more than 50 times lower than that of all previously reported perovskite photodetectors, also the highest performance among all reported perovskite photodetectors.
3. Preparation and characterization of large-sscale perovskite single crystal thin films and their optoelectronic devices.  
At the same time, the optimized growth method was used to prepare single crystal films as large as possible, together keeping the thickness of the film at the carrier diffusion length level. Finally, we successfully prepared a perovskite single crystal thin film with a thickness of about 10 µm with a side length of 1.12 cm, corresponding to an aspect ratio of 1100, this is the largest single crystal perovskite film in micron scale thickness. The defect density of the perovskite single crystal film is on the order of 1011 cm-3, and the mobility reaches to 62 cm2 / V∙s, which is close to the single crystal bulk. Photodetector devices based on the film is also prepared, and the rise time and the fall time of the photodetector are measured to be 170 µs and 320 µs, respectively. The gain of the photodetector is close to 10000, and the 3dB bandwidth of the device is 750 Hz.