Fabrication And Characterization Of Perovskite Solar Cell In Air

Organic-inorganic metal halide perovskite compounds have attracted a significant amount of attention, due to their strong light absorption, long charge carrier lifetime and diffusion length. Over the last six years, the power conversion efficiency（PCE） of organic-inorganic perovskite solar cells had increased from 3.8% to 22.1%. This dramatical improvement in performance is mainly attributed to the efforts devoted towards the precise control of their fabrication processes and conditions. In particular, the mitigation of the influence of moisture has been intensively investigated for various perovskite compound fabrication methods, including vacuum vapor-deposition, solution processing in glove box. However, these fabrication methods are still cost-ineffective for the mass-production of large-scale perovskite solar cells. Can we obtain efficiency perovskite solar cell in air? Here we attempted to prepare perovskite solar cell in air using one-step spin-coating method. The main contents are as follows:1. We prepared two kinds of perovskite films(CH3NH3Pb I3, CH₃NH₃PbI_3-xCl_x) in air using different methods. The performance of perovskite solar cells using CH₃NH₃PbI_3-xCl_x were better than that using CH3NH3 Pb I3, due to the higher coverage of CH₃NH₃PbI_3-xCl_x film. Here we fabricated mesoporous perovskite solar cell and planar perovskite solar cell using one-step spin-coating method in ambient air. For mesoporous solar cell, by annealing perovskite films with 70℃ for 30 min and then 100℃ for 15 min, a best efficiency of 7.22% has been obtained. For planar solar cell(ITO/PEDOT:PSS/CH₃NH₃PbI_3-xCl_x/PCBM/C60/Al), a best efficiency of 4.66% has been achieved.2. One-step spin coating is a simple method which has been widely used in fabricating perovskite solar cells. However, this method was vastly demonstrated in glove box wherein the influence of moisture is negligible. Thus the use of one-step spin-coating in ambient air has not been comprehensively investigated. In this work, we employ one-step spin-coating method to coat perovskite films in ambient air（with humidity above 50%）, and then the perovskite films were annealed in vaccum or air. Experimental results show that by using vacuum annealing, a power conversion efficiency of 12.98% is attained, and this is 45% higher than that attained by air annealing method. This improvement is mainly attributed to the fast solvent evaporation process in vacuum during annealing, which induces high supersaturation that leads to higher coverage of perovskite films. All the perovskite solar cells, either annealed in air or in vacuum, the open circuit voltage and short circuit current of the devices are very stable in glove box over 1000 hours. The degradation of the PCE is mainly due to the decreasing of fill factor.3 The SEM and XRD characterization showed that different annealing process had great influence on the crystallinity and surface morphology of perovskite films. Here we prepared perovskite films with four different methods（annealing in vacuum, vacuum treated in room temperature for 10 min and then thermal annealing in vacuum, vacuum treated in room temperature for 10 min and then thermal annealing in air, annealing in air）. The efficiency of devices prepared with different methods were 13.44%, 11.44%, 8.13% and 7.96% respectively. A higher performance perovskite solar cell can be obtained by annealing in vacuum quickly after spin-coated in ambient air.4. After vacuum treatment in room temperature for 10 min, the color of the films spin-coated in air turn from pale yellow to dark brown quickly and then become grey after exposing in air for 2 min. One hour later, a white film formed finally. XRD and SEM measurements showed that the crystal structure and surface morphology of the films were changed during exposing in air.