Study On Critical Interface And Device Performance Of Perovskite Solar Cells

The power conversion efficiency（PCE）of metal halide perovskite solar cells（PSCs）has exceeded 25%,which has been comparable to commercial crystalline silicon solar cells.However,to realize the commercial application of perovskite photovoltaic devices,there are still some difficulties such as lack of stability.Conventional PSCs are usually constructed with various functional layers such as perovskite active layer,charge transport layer and electrodes.The intrinsic defects in perovskite polycrystalline films and the instability of the heterogeneous interface between the various functional layers of the devices will not only hinder the transfer and transportation of charge,but also limit the efficiency of the devices.In addition,it will also induce material degradation and accelerate device performance degradation.This thesis mainly focuses on the basic scientific issues related to perovskite solar cells.Adopting vapor-assisted crystallization process and multifunctional material design strategy,which improve the efficiency and long-term stability of photovoltaic devices.The specific research contents are as follows:（1）The grain boundaries of perovskite polycrystalline films are optimized by adopting a vapor-phase assisted crystallization strategy,in which the secondary phases such as Pb I₂andδ-FAPb I₃ that derived from the crystallization process are eliminated.Consequently,this process can purify the crystals and improve the photothermal stability of perovskite polycrystalline films.Firstly,by comparing the effects of different vapor-phase materials on the phase transformation and crystallization of Pb I₂ andδ-FAPb I₃,methylammonium chloride（MACl）/formamidinium iodide（FAI）mixed vapor materials which can promote the secondary phase transformation and elimination at the same time is screened.In the multi-stage assisted crystallization process of MACl/FAI mixed vapor,the secondary phases such as Pb I₂ andδ-FAPb I₃ in perovskite films are eliminated through intermediate phase modulation process and ion exchange reaction process,which healing the defects at grain boundaries.Modified PSC with a structure of FTO/Sn O₂/Perovskite/Spiro-OMe TAD/Carbon electrode is fabricated and delivers a champion efficiency of 19.8%.After aging at 85℃for 500 h,the optimized device can still maintain 93%of the initial efficiency,and can still maintain 84%of the initial efficiency after aging for 500 h under continuous illumination.（2）The high quality and stable 3D/2D perovskite heterostructures are constructed by depositing a two-dimensional（2D）perovskite passivation layer with controllable thickness and pure phase on the interface of the three-dimensional（3D）perovskite absorption layer through the dimension engineering.The Pb I₂ inorganic skeleton that deposited on the surface of 3D perovskite via thermal evaporation process can act as a reaction template to control the thickness,morphology,crystallization quality and growth orientation of 2D perovskite,and prevent solvent erosion of the 3D perovskite underneath.Experimental results show that the dense and orderly constructed 3D/Pb I₂ bilayer film can significantly inhibit the formation of quasi-2D perovskite to obtain 2D perovskite capping layer with small n value（n=1）.High-quality 2D perovskite capping layer can effectively passivate surface defects of 3D perovskite and inhibit non-radiative recombination at interfaces.Finally,the efficiency of the modified device with a structure of FTO/Sn O₂/3D perovskite/2D perovskite/Spiro-OMe TAD/Carbon electrode does not decay after aging for about 1000 h under the condition of 30%relative humidity.In addition,the modified device can still maintain the 90%of the initial efficiency after aging for 670 h under continuous light conditions.（3）Based on molecular engineering,the ion diffusion mechanism in 3D/2D perovskite heterostructures is further studied,and a cross-linked polymer（CLP）interfacial material with excellent ion barrier properties and defect passivation function is selected to inhibit ion migration in 3D/2D perovskite heterostructures,which further improving the operational stability of the device.By comparing the barrier effects of small molecule,linear polymer and crosslinked polymer on cations,it was found that the highly crosslinked network polymer CLP can effectively inhibit the diffusion of FA⁺and 2-（4-fluorophenyl）ethyl ammonium cation（4F-PEA⁺）cations to Pb Br₂ films and prevent the formation of perovskite.The highly crosslinked network CLP can effectively inhibit the diffusion of cations in3D/2D perovskite heterostructures,thus improving the stability of heterostructures.In addition,CLP can improve the carrier transfer between 3D perovskite and hole transport layer.Finally,the device with a structure of ITO/Sn O₂/3D perovskite/CLP/2D perovskite/Spiro-OMe TAD/carbon electrode obtains a 21.2%efficiency.The device can still maintain 90%of the initial efficiency after aging for 1030 h under continuous illumination.