Preparation Of FAPbI₃ Thin Films In Air And Study On Carbon-based Perovskite Solar Cells

Organic-inorganic hybrid perovskite materials have attracted wide attention because of their outstanding photoelectric properties and simple thin film preparation process.At present,the power conversion efficiency（PCE）of perovskite solar cells has been increased to 25.8%,showing great application potential.Pure formamidine lead iodide（FAPb I₃）perovskite has suitable band gap,excellent photoelectric properties and good thermal stability,and is widely used to prepare high-performance perovskite solar cells.However,it is difficult to control the phase structure of FAPb I₃perovskite,and it is a great challenge to prepare high-quality pureα-FAPb I₃ perovskite.Due to the high phase transition temperature ofα-FAPb I₃ perovskite,methylamine hydrochloride（MACl）is usually added to promote the phase transition process,but the residual MACl will lead to the decrease of phase purity and the widening of optical band gap of FAPb I₃.Annealing in air environment can accelerate the removal of MACl and improve the phase purity of FAPb I₃ perovskite.However,in air environment,αphase is easily transformed intoδphase.Therefore,how to promote the transformation ofδphase perovskite toαphase and inhibit the formation ofδphase perovskite in air environment is very important for the preparation of efficient and stable FAPb I₃ perovskite solar cells.In view of these problems,this paper has carried out the following research work:（1）The effect of air annealing on the quality of FAPb I₃ perovskite thin films was studied.In the preparation of FAPb I₃ perovskite thin films,a certain proportion of MACl is often added to reduce the formation temperature of FAPb I₃ perovskite.However,MACl remains in perovskite films,which leads to the impurity of perovskite phase,thus reducing the quality and stability of the films.By studying the annealing process in air environment,we found that annealing in air environment with relative humidity of 30-40%is helpful to prepare high-quality perovskite films.Humidity can hydrate organic cations,promote the reaction with lead iodide（Pb I₂）,accelerate the removal of MACl,improve the phase purity of FAPb I₃ perovskite,and make perovskite films have larger grains and higher crystallinity.The PCE of carbon-based perovskite solar cells annealed in air increased to17.22%,which was higher than that annealed in nitrogen（16.21%）.（2）The addition of all-inorganic perovskite quantum dots（Cs Pb I₃ QDs）into antisolvent to induce FAPb I₃ perovskite to formαphase directly in air was studied.During the crystallization of perovskite solution,the intermediate phase of solution complex such as FAI·Pb I₂·DMSO will inevitably be formed,which leads to the formation ofδ-FAPb I₃ impurity phase and a large number of defects.We have introduced Cs Pb I₃ QDs into bulk perovskite controlled crystallization through anti-solvent.The results show that Cs Pb I₃ QDs can be used as nucleation sites to promote the crystallization of bulk perovskite and the formation ofα-phase,and the ligand molecules of quantum dots can be transferred to the surface of perovskite thin films to passivate the surface defects of perovskite thin films and improve the hydrophobicity of the films.Finally,the PCE of carbon-based perovskite solar cells modified by Cs Pb I₃ QDs was 19.05%,while the PCE of unmodified devices prepared under the same conditions was only 17.27%.In addition,the stability of the modified films and devices has been improved.The FAPb I₃ perovskite films show no phase change after being exposed to air for 6 days,and the devices can still maintain 94%of the original efficiency（64%of unmodified devices）after being exposed to air for 300 days,which greatly improves the long-term stability of the devices.（3）The preparation of FAPb I₃ thin films in air by vacuum-flash method and the suppression of theδphase transition of FAPb I₃ in air by combining with the interfacial layer of Cs Pb I₃ QDs were studied.In FAPb I₃ films prepared by vacuum-flash method in air environment,complex solution intermediate phase andδ-FAPb I₃ impurity phase exist,which will lead to poor quality ofα-FAPb I₃films.We found that the interface layer of Cs Pb I₃ QDs can promote the formation ofαphase in the intermediate film and stabilize the intermediate film,thus obtaining high-quality and stable blackα-FAPb I₃ film.In addition,Cs Pb I₃ QDs promotes the surface reconstruction of FAPb I₃ thin films and forms a protective layer on the surface of perovskite thin films,hence reducing the surface defects of perovskite thin films,significantly improving the hydrophobicity of perovskite thin films and effectively improving the phase stability of perovskite thin films and devices.Finally,the perovskite films modified by Cs Pb I₃ QDs interface did not show phase change after being exposed to air for 10days.Moreover,the open circuit voltage(V_OC)of carbon-based perovskite solar cells modified by Cs Pb I₃ QDs interface is increased by 60 m V,and the PCE of devices is increased from 15.97%to18.05%.Finally,the interface modification of Cs Pb I₃ QDs improves the humidity stability of the device,and it can still maintain 80%of the original efficiency after 60 days of storage in an environment with relative humidity of 40-60%,while the unmodified device can only maintain 70%of the original efficiency.