Modification Of Self-assembly Process In Perovskite Films And Its Impact On Perovskite Solar Cells

Perovskite solar cells have gained tremendous attention due to their easy fabrication,low cost and high efficiency.An unpredictable progress of PSCs has been made since the initial work;PSCs now show great potential for that the highest certified efficiency has reached22.7%.Methods for depositing perovskite films that is compatible with large area fabrication technology,such as roll-to-roll,are in urgent need when it comes to the transition from laboratory to industrial scale.Despite the rapid increasing of efficiency,stability is still a key challenge for the commercialization of PSCs.Moisture,oxygen and even light can promote the degradation of perovskite,so,for a more stable device,it is rather important to enhance the stability of perovskite absorber itself.Another issue that concerns is the toxicity of lead;it makes sense to replace lead with low toxicity metals though the lead content in PSCs is not high.This research work is pioneered by revealing the mechalism in the reaction between CH₃NH₂ and CH₃NH₃PbI₃,based on the self-assembly of liquid perovskite intermediate,a film deposition method that is compatible with industrial production process was innovated.Then,perovskite films with grains coated with PMMA monolayer were fabricated and the stability were greatly enhanced without scarifying the efficiency.Based on the intercalation of CH₃NH₂,ethanol was introduced as solvent to deposit MA₃Bi₂I₉ lead-free perovskite film and its impact on the performance of solar cell was examed.The main conclusions are summarized as follows:（1）When exposed to CH₃NH₂ gas,CH₃NH₃PbI₃ reacted with CH₃NH₂ quickly and liquid perovskite intermediate was formed.However,when the reaction time was prolonged,a white solid ctystal was obtained and analysed.CH₃NH₂ can intercalate and slice CH₃NH₃PbI₃crystals into low dimension perovskite intermediate and the intermediate can form CH₃NH₃PbI₃ again through a self-assembly process.（2）Utilizing the self-assembly process and introducing acetonitrile as solvent,a new method to deposit high quality perovskite films was developed.This method is compitable with large-scale fabricating technicals and the pervsokite film deposited showed enhanced stability comparing with film deposited by conventional method.In 60%humid air,the perovskite film was stable for 7 days without any degradation and its PSC device showed a high power conversion efficiency of 16.4%.（3）By introducing PMMA into perovskite intermediate solution,perovskite films with PMMA monolayer coated grains were developed.Through the self-assembly process,PMMA monolayer mainly distributed along grain boundary and the stability of perovskite film was enhanced with water contact angle increased from 39.1°to 65.1°.PMMA can compensate vacancies and reduce the trap density through passivation,resulting in the increasing of PL intensity.Through this procedure,staility of the device was greatly enhanced,it retained more than 90%of its initial PCE after stored in an environment with 50⁷0%hudity for 1200 h.（4）With CH₃NH₂ intercalation,the reaction betweent BiI₃ and MAI was promoted and the solubility of BiI₃ in ethanol was also increased.A deposition process with methylamine ethanol solution as solvent was developed to fabricate pinhole free MA₃Bi₂I₉ film,with this pinhole free film,photovoltaic performance of the lead free PSC device was enhanced and high V_oc of 0.84 V was obtained.