Conjugated Liquid-crystalline Small Molecular Additives Boosting The Efficiency And Stability Of Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells(PSCs)have attracted the attention of many researchers due to their excellent photoelectric performance and uncomplicated fabrication process.The power conversion efficiency(PCE)of PSCs has dramatically increased from 3.8%to 25.2%in ten years,which is comparable to the efficiency of commercialized crystalline silicon solar cells and Cu(In,Ga)Se2(CIGS)thin film solar cells.However,the poor stability of PSCs severely hampers their large-scale industrial application.At present,most of perovskite films used in PSCs are fabricated using low-temperature solution preparation methods.However,in these methods,the nucleation and crystallization rate of the perovskite film cannot be effectively controlled during the preparation process,resulting in the polycrystalline films with many grain boundaries and defects.A large number of defects and grain boundaries in the perovskite films will lead to nonradiative recombination and ion migration,which is detrimental to the efficiency and stability of the devices.Among the numerous methods to optimize the quality of perovskite films,the additive engineering is the simplest and the most efficient.However,the present additives are difficult to control the orderness of microstructure of perovskite films on the molecular level.Conjugated liquid-crystalline small molecules combine the the ordered self-assembly of liquid crystals with superior optoelectric properties.Application of conjugated liquid-crystalline small molecular additive in perovskite films will effectively passivate the defects,improving the crystalline quality.At the same time,the orderness of perovskite microstructures can be controlled via the self-assembly ability of liquid crystals.These attractive features of conjugated liquid-crystalline small molecules make them show promising potential applications in PSCs.However,the employment of such functionalized additive in PSCs has not been investigated.In this paper,two kinds of conjugated liquid-crystalline small molecular additives have been selected to be added into perovskite precursor solution to improve the crystal quality of the perovskite film,and further enhance the PCE and stability of the devices.The main research contents are as follows:Firstly,A functionalized ?-conjugated ionic liquid crystal(ILC),4'-(N,N,N-trimethyl ammonium bromide hexyloxy)-4-cyanobiphenyl(6CNBP-N),is developed as a novel chemical additive to obtain CH3NH3PbI3(MAPbI3)PSCs with high-efficiency and excellent moisture stability.Such 6CNBP-N ILC possesses characteristics of ionic liquids and liquid crystals.The inclusion of 6CNBP-N ILC can effectively improve the quality and stability of perovskite films,as well as reduce the trap-state densities and promote the carrier transport induced by the cyano group(C?N),rod-like ?-conjugated biphenyl mesogenic unit and quaternary alkyl ammonium cations(R4N+)in 6CNBP-N.Through this functionalized ILC engineering strategy,the PCE of PSC is greatly increased from 16.26%for the control PSC to 19.76%for the PSC with 6CNBP-N,along with the depressed hysteresis effect and enhanced moisture stability of PSCs.Secondly,The 4,4'-biphenyldicarboxylic acid(BPDA),a liquid crystal molecule containing the biphenyl mesogenic unit and two carboxyl groups,has been applied as additives to prepare MAPbI3 perovskite solar cells.The impact of conjugated biphenyl mesogenic units on the properties of perovskite films and devices is investigated.The suberic acid(SA)and p-phthalic acid(PTA),which also contain two carboxyl groups,are also been selected as references.The results show that the carboxyl groups in the three additives can effectively passivate defects and improve the crystal quality of perovskite films.Compared with the insulating SA additive,the additives containing the conjugated benzene rings can enhance the carrier transporting,which is benefiting from the delocalized ? electrons in the benzene rings.Specially,the conjugated biphenyl mesogenic units in BPDA additives are apt to form ordered ?-? stacking,which can regulate the orientation of perovskite crystals,further boosting the carrier transport.As a result,the perovskite films and devices prepared by introducing BPDA additives exhibit the best performance and the PCE of BPDA-containing device can be improved to 19.46%.