The Control Of Perovskite/hole Transport Layer Interface In Perovskite Solar Cells

In the last 11 years,the perovskite solar cell（PSC）has experienced significant development and its efficiency has been boosted to 25.5%from the initial report of 3.8%.Owing to the excellent light absorption capacity,long charge carrier diffusion distance,low cost,high defect tolerance factor of perovskite and the high-power conversion efficiency（PCE）of PSCs,the PSCs are considered as one of the most promising next-generation photovoltaic technologies.Generally,there are vast defects presenting in the bulk and on the surface of the perovskite films produced by the solution process,mainly including crystal structure disorder defects,surface defects and grain boundary defects.It was reported that the number of surface defects is 1～2 order magnitude higher than bulk defects.The surface defects,mainly including lattice defects(such as I vacancies,under-coordinated Pb²⁺ions,etc.)and impurity defects（such as residual reactants of Pb I₂and organic salts）,will result in significant non-radiative recombination and thus lead to a large V_oc deficit.Therefore,suppressing the interfacial trap density via interfacial engineering is one of the key approaches to improve device performance.This dissertation focuses on the interfacial engineering of PSCs,and the research works together with the main outcomes are listed as follows.1.Direct surface modification of the perovskite films by 4-fluorophenethylammonium iodide（F-PEAI）molecules is conducted to significantly reduce the number of I vacancies on the perovskite surface and then suppress the non-radiative recombination loss.At the optimized condition,the PCE of PSCs raised from 19.5%to 21%,which results from the improvement in V_oc and FF of the device,especially the V_oc which increases from 1.07 V to 1.13 V.In addition,the humidity stability of the device has also been improved by F-PEAI modification.After 720h of storage in the air ambient with～40%relative humidity,the modified device remains 90%of its initial PCE while this value is 77%for the control device.2.The trap density at the perovskite/HTL interface has dramatically dropped by inserting a 2D perovskite layer converting from the phenylbutylammonium iodine（PBAI）via post-annealing.The crystal structure and fluorescence spectra measurements reveal that the 2D perovskite layer consists of mixed 2D perovskite phases with different n values,mainly composing of n=2 phase.Under the synergistic effect of 2D perovskite phases with different n values,the interfacial nonradiative recombination has been largely suppressed,which results in a great V_oc increase from 1.077 V to 1.180 V and thus the PCE enhancement from 20.42%to22.06%.Moreover,the 2D/3D perovskite heterojunction induced by the PBAI boosts the device stability,including the intrinsic stability,humidity stability and thermal stability.Especially for the humidity stability,the modified device remains 92.1%of initial PCE after 720 h of storage at～40%relative humidity.3.The ultrasonic cleaning approach is exploited to wipe off the defective layer on the surface of perovskite film and thus reduce the interfacial nonradiative recombination.The perfluorohexane（TFH）is the best ultrasonic medium among the selected solvents of isopropanol,chlorobenzene and TFH to peel off the defect-rich surface layer,under an optimized ultrasonic power of 100 W.The resulting device demonstrates a PCE increment from19.16%to 21.00%,resulting mainly from the improved V_oc（from 1.075 V to 1.132 V）due to the elimination of interface defects.Additionally,the humidity stability of PSCs based on the TFH-cleaning perovskite film has also been greatly improved because of the existence of the hydrophobic fluorine groups of TFH molecules attached to the perovskite surface.