Fabrication Of High Quality Formamidine Perovskite Films And The Effect On Device Performance

In recent years,organic-inorganic halide perovskite is one of the most attractive materials in the photovoltaic field.The efficiency of perovskite solar cells(PSCs)using perovskite materials as light-absorbing layers has sharply increased from 3.8%to more than 25%,showing great research potential and application prospects.There are electron transport layer,perovskite absorption layer and hole transport layers in PSCs The efficiency and stability of devices can be affected by many factors.In order to improve the performance and stability of perovskite solar cells,we focused on perovskite absorption layer materials.Though pure formamidine lead iodide(FAPbI3)perovskite material has many advantages,such as more suitable band gap,higher short-circuit current density and better thermal stability,there are still some problems in the preparation of FAPbI3 perovskite films.It is difficult to fabricate compact and high crystallinity FAPbI3 perovskite films.Here,we have carried out a series of studies on FAPbI3 perovskite materials:(1)FAPbI3 perovskite films usually have some voids and low crystallinity.Also using anti-solvent increases the complexity of device fabrication and causes environment pollution.We introduced HI-MACl assisted 2D-3D conversion mechanism(HI-MACl ACM)strategy by mixing HI and MAC1(methylamine hydrochloride)into the precursor solution to fabricate high quality formamidine perovskite films.It was found that high quality perovskite films with high uniformity,density and large particle size can be successfully prepared by using the 2D-3D phase transition process involved in the HI-MACl ACM strategy.This strategy abandons the anti-solvent used in traditional methods,and reveals the mechanism of slow crystallization of films.The perovskite films prepared by HI-MACl ACM mechanism have the best morphology and the highest crystallinity.As a comparison,three kinds of film samples were prepared with different ammonium halide,and the devices were prepared based on three different film samples.The device prepared by HI-MACl ACM strategy showed the best photovoltaic performance,and the best efficiency was 1 8.45%.(2)Usually,FAPbI3 exists as a non-perovskite hexagonal yellow phase at room temperature(δ-FAPbI3)whereas the black phase(α-FAPbI3)is formed at a higher temperature.We provide an effective strategy to control the phase transition and suppress the phase decay of FAPbI3 perovskite films.We proposed a facile bi-additive(BA)strategy to fabricate black phase FAPbI3 with no non-perovskite yellow phase.A certain amount of HI and Pb(SCN)2 were used as BAs in the precursor solution of the perovskite.A new FAI-Pb(SCN)2-HI-DMF intermediate(derived from the precursor solution)was formed and was essential to the phase transition of the yellow phase to the black phase.Based on the investigation of the annealing time and temperature,we deduced that the intermediate could induce the phase transition and enhance the stability of α-FAPbI3.The δ-FAPbI3 was completely eliminated and high quality α-FAPbI3 perovskite film was obtained.As a result,the PCE of the device based on BA film increased from 14.57%to 20.05%compared with the control film.Based on the remarkable α-FAPbI3 phase stability,the devices prepared with the BA film showed improved stability,maintaining over 80%of their initial PCE after aging for over 400 h in air with a relative humidity of 35%.(3)A series of studies have already been published on improving the quality of perovskite films and enlarging the grain size.To obtain high quality large-size perovskite films,researchers usually proposed the preparation methods assisted by N2 atmosphere and high temperature.However,the modulation of precursor solution and the regulation of the nucleation process to increase the perovskite particle size are essential parts to be addressed.Therefore,we use a strategy to modify the precursor solution,which can effectively increase the size of the film and reduce the defect density of the film by suppressing the heterogeneous nucleation.When we modified the viscosity properties of the precursor solution,the wettability of the modified precursor solution on the substrate is also reduced,and the contact angle on the substrate is increased,which is beneficial to promote the homogeneous nucleation process of perovskite,inhibiting grain boundary migration and suppressing heterogeneous nucleation.Finally,the particle size of the modified perovskite film reached the micron level,and the device efficiency based on the film was improved to 20.15%.In addition to improving the device efficiency,the modified film has been proved to have a good resistance to moisture in the air,and the humidity stability of the film has been improved.This work provides a facile solution process for obtaining pure α-FAPbI3 perovskite films with high quality,low defect density and large grains.In this paper,we systematically investigated the film morphology and crystallinity,phase transformation mechanism,nucleation and crystallization process of FAPbI3 perovskite films,a series of effective strategies for preparing high quality FAPbI3 perovskite films were proposed.Finally,high quality FAPbI3 and efficient perovskite solar cells were obtained.These works in this paper provided the possibility for obtaining devices with high efficiency and commercial application.