Modulation of Perovskite Crystals for Optoelectronic Application

Abstract

Solution processed metal halide perovskite semiconductors have gained extensive momentum in field of solar cells and light-emitting diodes due to the advantages of strong light absorption, low exciton bonding energy, long electron and hole diffusion, high charge carrier mobility, high photoluminescence quantum efficiency, narrow emission linewidth, simple color tuning and low material cost, et al. The perovskite solar cells (PeSCs) exhibited dramatically improved powder conversion efficiency (PCE) from 9.7 % to 24.2 % within a few years, and the perovskite light-emitting diodes (PeLEDs) have also achieved significant breakthroughs in the electroluminescence (EL) efficiency. In particle, the perovskite crystal quality, including film coverage, surface roughness, grain size, grain gap and grain boundaries et al, is crucial for perovskite optoelectronic device (PeSCs and PeLEDs) performance. The purpose of this dissertation is to fabricate high quality perovskite film by using some novel techniques for achieving high-performance PeSCs and PeLEDs. The formation of perovskite film is very sensitive to the environment moisture and annealing process. The moisture can induce a recrystallization of perovskite film, and the annealing process is essential to remove the remaining solution for achieving complete perovskite film. Based on these facts, a moisture pre-treatment (MPT) and hot air annealing process (HAAP) were developed for improving the quality of CH3NH3PbI3-xClx film, further enhancing the PeSCs efficiency. In the solution-processed fabrication of inverted p-i-n planar PeSCs, both of perovskite and PC61BM film need to be heated for removing the remaining solvent, which is complicated and time-consuming. For simplifying the PeSCs fabrication, a novel merging annealing (MA) process was developed, which combined the separate annealing process of perovskite and PC61BM in one stop. In particular, the MA process also remarkably increase the crystallinity of CH3NH3PbI3, leading to a large single-crystal-like CH3NH3PbI3 grain, and the top PC61BM deeply penetrated into the CH3NH3PbI3 film through the grain boundaries, enhancing the interface engineering. That results in not only simple device fabrication but also efficient inverted planar PeSCs with high efficiency of 18.27 % and a fill factor of 81.34 %. For further controlling the quality of perovskite film, including film coverage, surface roughness, grain boundaries, grain gap and grain size, an innovative bulk heterojunction (BHJ)-assisted grain growth (BAGG) method was develop to accurately control the quality of perovskite film. In this method, the perovskite precursor film was shielded by a polymer-to-PC61BM blending film during annealing, by simply modulating the polymer-to-PC61BM blending ratio, the transition of complete perovskite film from perovskite precursor was accurately regulated, resulting in controllable grain growth and high-quality final perovskite film. Finally, the PeSCs containing an optimized CH3NH3PbI3 film presented a high efficiency of 18.38 % with excellent fill factor of 83.71 %, and the PeLEDs that contained uniform and full-covered CH3NH3PbBr3 film emitted bright green light with brightness value of 1600 cd/m2 and a luminous efficiency of 0.56 cd/A. The planar PeSCs and PeLEDs contains similar device architecture, and the low exciton binding energy and excellent charge transport properties of perovskite materials allow the conversion of photons into free electrons and holes and vice versa with minimal energy loss. These aspects guarantee the fabrication of dual-functional light-emitting perovskite solar cells (LEPeSCs) with both solar cells and light-emitting diodes functions in single device. A high quality perovskite active is crucial for efficient LEPeSCs. Here, a facile soft-covered annealing (SCA) method was developed for achieving high-quality CH3NH3PbBrI2 film with large grain size, high uniformity and minimized grain boundaries, which ensuring an efficient LEPeSCs with powder conversion efficiency of 14.02 % in solar cells mode and bright red-light emission of 1710 cd/cm2 in LED mode. This work focuses on fabrication of high quality of perovskite active layer by developing some new methods, and regulation between perovskite crystal quality and corresponding optoelectronic devices was deeply investigated. The continuation of this work will promote the development of efficient PeSCs and PeLEDs.