Defect State Regulation Of Perovskite Polycrystalline Thin Films And Their Effects On Optoelectronic Properties

Lead halide-based perovskite materials have been attracting ever-increasing attention on account of intriguing optoelectronic properties,such as large absorption coefficient,wide and tunable bandgap,high ambipolar carrier mobility and long carrier diffusion length.In just a few years,they are recognized as icon materials not only in solar cells,with the greatest improvement in power conversion efficiency from 3.8%in year 2009 to 25.5%rapidly,but also showing huge potential in photodetectors,photodiodes,lasers and other applications.However,the polycrystalline perovskite films have high trap state density,which has a vital impact on carrier recombination and ion migration in the optoelectronic device,limiting the optoelectronic performance of the device severely.Therefore,in order to convert the attractive optoelectronic properties of halide perovskite materials into the best device performance,and move forward to achieve their theoretical performance limit,scientists must face the challenges to understand and control the structural defects in perovskite materials,and to study how the defects’ would affect the performance of optoelectronic devices,so as to truly realize their full potential in commercial applications.The halide perovskite polycrystalline films are usually simply processed from solution at low temperature and has a very lowcost advantage,While the preparation method will lead to the generation of a large amount of crystal defects.The defects often act as non-radiative recombination center of charge carriers,affecting the crystal quality and optoelectronic properties of the polycrystalline films.Thereby,it is a crucial research direction to reduce or eliminate the number of defects and minimize the impact of defects on the performance of optoelectronic devices by various passivation strategies.Based on the above description,this dissertation is structured in four sections,with the following details:(1)Triple-cation mixed-halide perovskite of composition Csx(FAyMA1-y)1-xPb(IzBr1-z)3 is a promising light absorber owing to their attractive photovoltaic properties.In this study,FA0.9MA0.05Cs0.05PbI2.7Br0.3(denoted as FAMACs)polycrystalline films are prepared by spin coating method,and high-quality,low-defect density perovskite polycrystalline film doped with Cd2+are obtained by adding CdCl2 to the precursor solution.The effect of CdCl2 on the carrier dynamics of FAMACs polycrystalline film was investigated by analyzing steady-state photoluminescent and time-resolved photoluminescent,and we found the bulk carrier lifetime is prolonged to～20μs,similar to that in single crystals,and the intrinsic surface defect density is reduced to as low as 6.72±0.01×1013 cm-3 by incorporating CdCl3 into the FAMACs polycrystalline films.The reduction of defect density in FAMACs polycrystalline film is essential to improve its photovoltaic performance,leading to an increase of power conversion efficiency(PCE)of FAMACs solar cell from 20.4%to 22.1%,which is attributed to a larger VOC due to the removal of defects by Cd ions doping and Cl-ion-assisted crystallization.However,further improvement of PCE is likely limited by the impurities on the surface of FAMACs film produced by the Cd doping.(2)Organic-inorganic hybrid halide perovskite materials have recently been recognized as a promising optical gain medium,and the development of optically pumped amplified spontaneous emission of perovskite materials with low threshold and high optical gain will be on the only way to go the realization of its commercial perovskite laser.In this part of study,we obtain enhanced performance ASE with lower threshold and higher optical gain in the CdCl2 passivated FAMACs polycrystalline films coated PMMA layer.The ASE threshold of FAMACs polycrystalline films with CdCl2 is significantly reduced from 18.7 μJ·cm-2 to 7.9μJ·cm-2,with an enhanced ASE output intensity and stability.The improved ASE performance can be attributed to the synergistic effect of Cd cation doping and Cl anion assisted crystallization in FAMACs polycrystalline films,exhibiting remarkably enhanced PL,a prolonged decay lifetime and a low defect density,and thus leading to easily achieve the population inversion due to the reduction of non-radiative recombination of carriers in the optical gain medium.(3)The development of CsPbBr3 film with low defect density and high photoluminescent quantum yield is a potential direction for achieving low threshold,high gain and high stable ASE performance.In this study,the long-chain organic cation amphetamine PPA doped-CsPbBr3 films are successfully prepared by incorporating PPABr into precursor solution,and the crystalline structure,surface morphology and photophysical properties of CsPbBr3 films with PPABr was investigated in detail,and we found the doping of PPABr enable denser and smoother morphology,a prolonged carrier lifetime,increased exciton binding energy and photoluminescent Quantum yield,thereby reducing the optical scattering loss and nonradiative recombination loss during the realization of ASE.Finally,amplification spontaneous emission with a low gain threshold of 22.7 μJ·cm-2 and an optical gain coefficient of 109.8±2.4 cm-1 was achieved in 0.1mol/mL PPABr doped-CsPbBr3 thin film at room temperature.