| Perovskite solar cells(PSCs),as one of the third-generation solar cells,have attracted wide attention owing to their advantages of high power conversion efficiency(PCE),low production cost,high defect tolerance,and compatibility with flexible substrates.After more than ten years of rapid development,the efficiency of singlejunction PSCs has been boosted from 3.8%to 25.8%,which has great potential for commercial application.The commonly used planar heterojunction PSCs are mainly composed of electrode layer,charge transport layers and perovskite layer.However,a huge number of trap states has been inevitably enriched at the interlayer interface because of the deposition of these functional layers of PSCs by layer-by-layer,which seriously affects the efficiency and stability of PSCs.Therefore,it is valuable to passivate the defects at the interface,regulate the charge transfer performance at the interface,and improve the efficiency and stability of devices for promoting the commercialization of PSCs.Organic small molecule has been considered as ideal defect passivator owing to its advantages of simple structure,good solubility,and rich functional groups.Hence,we concentrated on improving the efficiency and stability of PSCs through interface engineering strategies,which employing different types of organic small molecules to modify the interface of electron transport layer(ETL),perovskite layer,and hole transport layer(HTL),respectively,and mainly carried out the following researches:(1)Tungsten oxide(WO3)is considered as a promising electron transport material due to its wide band gap,stable chemical properties,and relatively high electron mobility.However,WO3-based formal-structure planar PSCs exhibit serious light soaking effect,which a continuous light illumination can gradually increase the PCE of PSCs.This is adverse for operation stability of devices and the accurate assessment of the efficiency.In order to solve this problem,a fullerene derivative,Fullereno-C60pyrrolidine-2,5-di(carboxylic acid)-1-(acetic acid)(CPTA),was introduced to coat atop the WO3 film and modify the interface between WO3 and perovskite.The influence of CPTA on the films of both WO3 and perovskite was studied by a series of characterization.CPTA can not only form coordination bonds with WO3,which passivate the defects of its surface,but also diffuse into the perovskite layer to coordinate with Pb2+,resulting in passivating the defects of perovskite layer.Therefore,the double defect passivation effects of CPTA significantly improve the PCE of PSCs.The WO3/CPTA-based PSC achieves a PCE of 20.48%,which is significantly higher than the WO3-based device(17.42%).In addition,the introduction of CPTA can also regulate the energy band levels of WO3 and perovskite,leading to increase the built-in electric field between them,promote the electrons extraction from perovskite to ITO,suppress their accumulation at the interface,thus eliminate the light soaking effect.At the same time,the stabilities including dark storage,light,thermal,and humidity of unencapsulated devices are enhanced significantly.(2)The perovskite polycrystalline film which prepared by solution processing inevitably exhibits a large number of defects at its grain boundary and interface.To decrease these defects,we adopt a multifunctional small molecule,1-methylsulfonyl piperazine(MP),to coat atop the WO3 film and modify the interface between perovskite and HTL in formal-structure planar devices.Combined with the experimental and theoretical simulation results,we found that MP molecules can not only form Pb-N and Pb-O coordination bonds with Pb2+of perovskite,which effectively passivate the defect caused by excessive residual PbI2 in the film,but also form hydrogen bonds with organic A-site cations(methylammonium and/or formamidinium ions)of perovskite,which conductive to further strengthen the strength of its interaction with perovskite.In addition,the valence band of perovskite move up after MP modification,closer to the highest occupied molecular orbital of the hole transport layer(HTL),thus promoting the transport of hole carriers from perovskite layer to HTL.The multiple bonding effects between MP and perovskite enhance the quality of perovskite film,reduce the density of defect,improve the carriers transfer performance.Consequently,the MPbased PSC achieves a PCE of 23.41%,much better than that of the pristine device(21.51%).At the same time,the stabilities of both dark storage and thermal are enhanced significantly.(3)In comparison with the formal-structure planar devices,the inverted-structure planar devices similarly gain support by researchers owing to their advantages of negligible hysteresis effect and excellent environmental stability.Nickel oxide(NiOx)is a commonly used HTL in inverted-structure planar devices.While there are some problems of NiOx,such as poor charge transfer performance and complex surface chemical reactions which promote the perovskite degradation process.In order to solve these problems,self-assembled monolayer(SAM)was formed via organic small molecules spin-coating atop NiOx film,which acts as a spacer layer inserting between NiOx and the perovskite layer.The results show that the phosphoric acid groups in SAM molecules can form coordination bonds with NiOx,leading to passivate the defect on its surface and therefore suppress the trap-assisted non-radiation recombination.Moreover,SAM molecules can regulate the energy band structure of NiOx,which its value band moves downward and much more closer to the value band of perovskite,resulting in facilitating the transport of hole carries and reducing the open-circuit voltage loss.The synergistic effect of above results improves the efficiency of the devices from 18.87%to 20.19%.In addition,the intercalation of SAM blocks the direct contact between NiOx and perovskite,inhibiting the possible redox reaction between them and thus improving the stabilities including light,dark storage and thermal of PSCs. |
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