Preparation Of CH₃NH₃PbI₃ Thin Films And Photovoltaic Characteristics Of Perovskite Solar Cells

Perovskite solar cells (PSCs), derived from traditional dye sensitized solar cells (DSSC), are a new kind of solar cell, whose name is derived from the important light absorption materials, metal halide CH3NH3PbX3 (X=I、Br、C1), which has perovskite crystal structure. For CH3NH3PbI3, it has unique features, such as narrower bandgap width, high light absorption coefficient, relatively low photo-generation exciton binding energy, higher carrier mobility and long carrier diffusion lengths, ambipolar charge transport etc. which make it quickly became one of the hot research materials for new type of solar cells. However, in the early development of the PSCs, the film fabrication of light absorption materials is the key to inproving performance. So preparation of the perovskite film with high cry stall inity, crystal grain density and grain boundary perpendicular to the substrate has become an important work and the challenge in this research field. We follow closely the forefront of international development in this paper, discussed the influence of reaction driving force on the nucleation, growth and crystallization process in perovskite film from the view of reaction driving force. Finally, Power conversion efficiency (PCE) of PSCs was improved significantly through the microstructure modulation, and lays the foundation for the further improving of PCE and the mechanism research in PSCs. In this paper, main conclusions are as follows:1. For preparation of perovskite films via sole step spin-coating, the demanding of environmental humidity conditions was extremely strict in the nucleation process. In order to surppress the influence of environmental humidity, two steps sequential deposition method has been successfully introduced and effectively improve the film coverage, and makes the PCE of PSCs improve significantly. However, there were some problems in the preparation of perovskite film via Sequential deposition methods, such as high surface roughness, a large number of pin-hole in grain boundary. These disadvantages will lead to part short circuit in device, and reduces the open circuit voltage of the PSCs. We learn from solid-liquid reaction model, consider in terms of reaction driving force, discussed the role of dipping pre-wetting processing in the reaction process, and established the model of concentration gradient diffusion. Meanwhile, we analyzed the reason of rapid and thorough reaction between CH3NH3I and PbI2 after dipping pre-wetting process. Furthermore, from reaction concentration, discussed the influence of CH3NH3I reaction concentration in isopropyl alcohol solution, reaction time and temperature on the microstructure of the perovskite thin films. The results found that optimal reaction condition:the reaction concentration, reaction temperature and reaction time is 10 mg/mL,50℃ and 1 min after dipping pre-wetting process, respectively. The optimal PCE reached 12.1%.2. Though Sequential deposition overcome the disadvantage of lower coverage because of crystal island growth in sole step spin-coating process, the method itself has some obvious flaws, such as large surface roughness (～100 nm), vast pin-hole, small crystal size and so on. These problems induce the leakage and radiative recombination increasing. According to the principle of solid-liquid growth as well as the concentration gradient diffusion model, we control the reaction process between CH3NH3I isopropyl alcohol and PbI2 film and successfully achieved the perovskite film with surface and without pin-hole on the crystal boundary. The roughness of perovskite film is about 16 nm, which reduced at least 5 times compare with the film fabrication via subsequence deposition. In addition, a comprehensive survey for the influence of the dipping, spin-coating and spray pre-wetting methods on the morphology of PbI2 film, and the results of SEM images suggested that the driving force from air pressure accelerates the reaction process between PbI2 and CH3NH3I isopropyl alcohol solution, and result in some pin-holes which provide a basis for rapid PbI2 films complete reaction. On this basis, we have realized the large crystal size and grain boundary perpendicular to substrate via controlling the amount of pre-wetting solvent as well as the annealing temperature and time.The PCE analysis results shows that the optimal PCE of spray-assisted in mesoporous and flat structure solar cells were 14.2% and 14.5% respectively.3. Laser irradiation technology has been widely used in the crystallization of inorganic semiconductor, metal materials and its compound oxide, selenium, sulphur and superconducting materials. To explore the implementation of laser irradiation crystallization for metal organic halide perovskite materials, this technique not only can further broaden its application scope, but also can be helpful to understand the crystallization process of metal organic perovskite materials, and realize the high quality film preparation. In view of this, laser irradiation as a rapid crystallization approach was successfully introduced to prepare homogeneous, dense-grained CH3NH3PbI3 films. The regularly change of crystallization quality and light absorption intensity in perovskite film can be clearly observed via regulation laser power density. Finally, planar-heterojunction solar cells employing these high-quality films showed the optimal efficiency of 17.8% with a remarkably high open-circuit voltage of 1.146 V. Furthermore, it is worth noting that the average open-circuit voltage (Voc) of the devices with laser irradiation introduced increased by 32 mV compared with the thermal ones. X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy analysis found that a proper amount of PbI2 appeared on the surface due to the thermal gradient distribution of laser irradiation, which effectively passivates the CH3NH3PbI3 film and reduces the surface state. The results suggest that suppressing the non-radiative recombination contributes to high performance, which is more critical to improve the Voc in PSCs. In our work, the iodide based PSCs gained a high Voc of 1.146 V, which has a gap of about 0.19 V from its theoretical maximum.