Application Of Materials Based On Fused Benzothiadiazole And Benzotriazole In Perovskite Solar Cells

Perovskite solar cells（PSCs）are emerging as the most promising next-generation photovoltaic technology,owing to their high efficiency,low-cost and flexibility.In fact,the highest efficiency recorded for single-junction devices has already exceeded 25%,indicating the vast potential of PSCs in industrial applications.Critical to achieving optimal performance in PSCs is the charge extraction and transfer between the hole transport layer and the perovskite layer.However,the most commonly used hole transport material（HTM）,Spiro-OMe TAD,currently requires the addition of a dopant that absorbs water to improve its hole mobility and conductivity.Unfortunately,this practice is detrimental to the stability of perovskite films.Moreover,the lengthy synthesis process and cumbersome purification procedures result in increased costs,making it challenging to achieve low-cost,large-scale production of the material.Finally,the relative shallowed highest occupied molecular orbital（HOMO）energy level of this molecule cannot match the energy level at the top of the perovskite valence band,which also causes serious energy loss.Therefore,it is of great importance to develop undoped HTMs with excellent hole transport properties that are easy to synthesize.During the preparation of perovskite active layers,a large number of charge defect states inevitably exist at the interface.This leads to non-radiative recombination of charges which severely impedes carrier transport.Moreover,defects also serve as channels for water and oxygen penetration which accelerate the degradation of perovskite.As a result,interface engineering becomes a key strategy in improving the overall performance and stability of PSC devices.Although a variety of small molecule passivation strategies have been developed in current research to suppress non-radiative recombination at the interface,resulting in efficient and stable PSCs devices,a lack of in-depth research on the relationship between the characteristics of small molecule passivation materials,such as different structures,energy levels,hole mobility,and interactions with perovskite,significantly impacts the effective selection of passivation molecules.Therefore,this study explores the impact of different core acceptor A-units in small molecules through molecular engineering,designing and synthesizing four D-A-D（donor-acceptor-donor）passivation molecules,SBT,DBT,CBT-EH,and CBT-BO,to investigate their effects on the optoelectronic properties of perovskite devices.Research has shown that compared to other structural small molecules,CBT-BO with the most heteroatoms and the longest conjugated system exhibits an energy level that is well-matched with the valence band maximum of perovskite.Additionally,its good molecular coplanar structure and longer alkyl side chains promote charge transfer between molecules and solubility in solvents for processing.As a result,CBT-BO demonstrates suitable energy levels and excellent passivation ability by facilitating hole extraction at the interface.After modification with CBT-BO,the PCE of PSCs reached 22.16%,significantly higher than that of unmodified PSC（19.9%）,while exhibiting smaller hysteresis effects and superior stability.Even after aging for 1800 hours,the PSC maintained 95%of its initial efficiency.It has been proven in work（1）that the condensed benzothiadiazole unit has great potential for passivating interface defects and facilitating hole transportation direction,which inspired us to condensed 2,1,3-benzothiadiazole unit into the condensed benzothiadiazole unit with different thienylene conjugated bridges to design and synthesize two defect-passivating,highly planar and structurally simple D-A type polymers PCBTz-T and PCBTz-TF as non-doped HTMs for p-i-n perovskite solar cells.Results showed that both PCBTz-T and PCBTz-TF have energy levels well-matched with perovskite and achieved high hole mobilities(1.732×10^-4 and 1.282×10^-4 cm² V^-1s^-1)based on non-doped devices.The efficient hole extraction and transportation were caused by the superior hole transportation properties and defect-passivating function of the fused benzothiadiazole-based polymer HTMs.The power conversion efficiencies of PSCs without dopants based on PCBTz-T and PCBTz-TF HTMs reached 21.04%and 10.20%,respectively,suggesting the superiority of the fused benzothiadiazole-based polymer over Spiro-OMe TAD in terms of performance and cost.Moreover,PCBTz-T showed excellent hole-conducting ability even at a low concentration of 5mg/m L with lower molecular weight and degree of polymerization than Spiro-OMe TAD,which typically requires 70 mg/m L or more,indicating a great potential for future commercial production.In addition,the high density and hydrophobicity of the PCBTz-T polymer effectively blocked water vapor corrosion of the perovskite film,and the PSC device using PCBTz-T as a hole-transporting layer maintained over 95%initial PCE after storage at 25°C and 20-40%relative humidity for 1000 hours,demonstrating better environmental stability.