Performance Optimization And Mechanism Study Of Perovskite Photovoltaic Devices With High Efficiency And Stability

Perovskite has become a promising photoelectric conversion material due to its excellent photoelectric properties such as suitable optical band gap,long carrier diffusion length,weak exciton binding energy and high absorption coefficient in the visible light range.In the past decade,the power conversion efficiency（PCE）of perovskite solar cells（PSCs）has increased from 3.8%to 25.5%,which brings great prospects for the subsequent photovoltaic industrialization.Good charge transfer channel and excellent active layer films are the key to the fabrication of high efficiency devices.In this paper,the modification of transport layer and passivation of perovskite interface defects are studied.Then a method of preparing large perovskite grain is found,and the mechanism of grain secondary growth is proposed.Finally,high stability and high efficiency perovskite photovoltaic devices were obtained by preparing 2D/3D mixed dimension perovskite.The research results of this paper are as follows:（1）We demonstrate the continuous decrease of ionic interface charge（IIC）density,weakening of current-voltage hysteresis,decrease of leakage current as well as constant increase of PCE from～6.0%to～18.8%were achieved through step-by-step modification of the HTL and ETL of the PSCs.A new device model is presented to understand the correlation between the IIC density and the photovoltaic performance.The model suggests that the PSCs has two junction regions,the p-perovskite and perovskite-n regions,which are separated by field-free perovskite bulk.The measured IIC is a reflection of junction capacitance and junction widths.From the model,the continuous decrease of IIC density and the constant improvement of the performance are the result of constant increase of junction widths through step-by-step modification of the HTL and ETL.The work shows that although the IIC is originated from mobile ions in perovskite layer,the IIC density is determined by the properties of the charge-transport layer.These conclusions and the proposed device model have important implication for future study in pursuing efficient and stable PSCs.（2）Based on the improvement and optimization of the transport layer that has been completed above.In this work,we try to optimize the perovskite interface to improve the device efficiency.Therefore,we try a small amount of fullerene(C₆₀)is added to toluene in the antisolvent dripping process.The resulting C₆₀-containing perovskite solar cells show better stability,PCE,and reproducibility than the device fabricated by using only toluene as an antisolvent.C₆₀ adheres to the pinhole and grain boundaries of perovskite film,improves perovskite crystallinity,and enhances absorption without changing the thickness of the film.It also plays a passivation role on the surface of perovskite film and prevents the migration of I and Pb elements.The change of the morphology and the passivation of the interface increase the FF and J_sc of C₆₀-containing device.The best PCE of 19.8%is obtained.The light stability of C₆₀-containing perovskite films was also enhanced.The PCE of encapsulated PSCs still maintains 70%of its initial value after1000 h continuous full sun illumination.（3）The grain boundaries of perovskite polycrystalline are regarded as a defect region that not only provides carrier recombination sites but also introduces device degradation pathways.In addition to the passivation treatment on the perovskite surface,efforts to enlarging the grain size of perovskite film and reducing its grain boundary are crucial for highly efficient and stable PSCs.Some effective methods that facilitate grain growth are post-deposition thermal annealing and solvent vapor annealing.However,a detailed understanding of grain growth mechanisms in perovskite films is lacking.In this study,perovskite films were prepared by adding ethylamine hydrochloride（EACl）to the precursor solution.This additive strategy promotes a new grain growth mode,secondary grain growth in perovskite films.Secondary grain growth leads to much larger grains with high crystallographic orientation.These excellent properties lead to reduced grain boundaries and the densities of boundary defects.The improved film quality results in a prolonged charge–carrier lifetime and a significantly enhanced PCE.Compared with the PCE 18.42%of the control device,the PCE of the device with EACl additives reaches21.07%.Therefore,EACl additive has been proved to be one of the simple methods to obtain high efficiency PSCs with large grains.（4）Based on the optimized preparation of perovskite with high efficiency,we found that the light stability of the device is still an urgent problem.In this study,2D/3D heterojunction perovskite was formed in situ by introducing long chain alkyl PBAI passivation layer.It is found that not only the grain boundary of perovskite is reduced,but also the photoluminescence（PL）and time-resolved photoluminescence（TRPL）spectra also show the decrease of non-radiative recombination of perovskite.Moreover,the n-type doping of perovskite in the heterojunction optimizes the charge transfer channel.Finally,the PBAI passivation strategy can significantly improve the V_oc and FF of the devices without sacrificing current,and the PCE can be improved from 19.22%to21.76%.PBAI passivation strategy also improves the humidity stability of the films and the efficiency stability of the devices under continuous illumination.Finally,the efficiency of the champion heterojunction perovskite device remains 87%after 1000hours of illumination.The results show that heterojunction engineering plays an important role in the preparation of efficient and stable perovskite solar cells.