Synthesis Of Novel Fullerene Derivatives And Defects Passivation In Inverted Perovskite Solar Cells

Since the first reported in 2009,perovskite solar cells(PSCs)have been developed rapidly in the past decade and the laboratory-reported power conversion efficiency(PCE)of single-junction solar cell has been risen from 3.9%to a certified PCE of 25.7%.Particularly,inverted PSCs have received extensive attention due to high PCE,negligible hysteresis and low cost.However,the charge transport barriers at interfaces and defects at grain boundaries significantly hinder the improvement of device performance.Compositional engineering,interface engineering and additive engineering have been widely used to improve the performance of PSCs,which can adjust light absorption,optimize energy level alignment between different layers,and passivate various defects generated form the rapid crystal growth,respectively.Incorporating of fullerene materials into PSCs has been proved to be an effective strategy to address the above mentioned problems.As a good electron acceptor,fullerene materials possess excellent electron transport property and it is easy to regulate the solubility,energy level and band gap by introducing functional groups through chemical functionalization,making fullerene and fullerene derivatives widely utilized as interface layer materials or additives in PSCs.Although several fullerene derivatives have been used to passivate defects in PSCs,they only interact with perovskite via a single site,which limits the passivation effect.In addition,the existence of charge transfer barrier at interface hinders the charge extraction and sacrifices the device performance in further.Therefore,it is of great significance to develop novel multifunctional fullerene derivatives to achieve better defects passivation of perovskite films or interfacial charge transfer through optimizing design schemes.This dissertation mainly focuses on designing and synthesizing novel fullerene derivatives to optimize interface contact or achieve multi-site defects passivation to improve the efficiency and stability of inverted PSCs and the main research contents include the following three aspects:(1)A novel bis-dimethylamino-functionalized fullerene derivative(PCBDMAM)was synthesized through a facile esterification reaction,and was employed as a cathode buffer layer(CBL)atop of phenyl-C61-butyric acid methyl ester(PCBM)via spincoating process to construct inverted PSCs with PCBM/PCBDMAM double fullerene CBLs.An interfacial dipole layer between PCBM and silver electrode was formed after incorporating PCBDMAM CBL.which can reduce the work function of Ag electrode.facilitate the charge transfer and minimize the energy offset.As a result,the device with PCBM/PCBDMAM double fullerene CBLs achieved an improved PCE of 18.11%,which is higher than the device with PCBM single-layer(14.21%).In addition,the excellent hydrophobicity of PCBDMAM also endows the device improved moisture stability.(2)In order to achieve the versatility of fullerene derivatives,we introduced Lewis base functional groups(pyridine group)with strong coordination interactions and fluoroalkyl groups with excellent hydrophobicity into fullerene cage simultaniously to construct a pyridine and perfluoroalkyl cofunctionalized fullerene derivative(C60PyF15).C60-PyF15 was added into CH3NH3PbI3 precursor to construct inverted bulk heterojunction(BHJ)PSCs.To further elucidate the role of bifunctional fullerene derivatives in device performance enhancement,another fullerene derivative functionalized with a single pyridine group was also synthesized for comparison.It was found that the pyridine moiety within C60-PyF 15 can passivate the defects of naked Pb2+through coordination interaction,and fluorine atoms within the perfluoroalkyl group can form hydrogen bonding with CH3NH3+cations to order the orientation,thus to regulate the crystallinity of perovskite film and suppress ion migration via double site defects passivation.The device based on C60-PyF 15 co-functionalized additive achieved a double-site defect passivation and an improved PCE of 20.10%,which is among the highest efficiency of inverted BHJ-PSCs with fullerene additives at the time of publication.Meanwhile,the hydrophobicity of perfluoroalkyl chain also significantly improves the ambient and thermal stabilities of the C60-PyF 15 incorporated device.(3)Considering that dye groups can effectively regulate the crystallization of perovskite film and can be easily introduced into fullerene cage.We developed a novel rhodanine-functionalized fullerene derivative(C60-R-S)and another rhodaninefunctionalized fullerene derivative bearing double binding cyano terminals with the same backbone(C60-R-CN)via molecular clipping strategy by Prato reaction,thus to assemble the excellent electron transport property of fullerene and unique chemical properties of dye groups.Both of them were introduced into CH3NH3PbI3 active layer by anti-solvent method to prepare inverted PSCs.The top-down gradient distribution of fullerene materials in the perovskite film can facilitate charge transport and extraction,and the existence of rhodanine terminal goups within C60-R-S/C60-R-CN can effectively improve the perovskite crystallinity.In contrast,the two-CN groups in C60R-CN exhibit stronger coordinate interactions with neighboring Pb than that of the S terminal in C60-R-S.Meanwhile,the higher dielectric constant of C60-R-CN material significantly reduces the bimolecular recombination rate of the device,and promotes charge transport/extraction at the interface of fullerene-perovskite bulk heterojunction.The device based on C60-R-CN additive achieved an improved PCE of 20.81%with a impressive fill factor nearly 83%.Furthermore,due to the enhanced hydrophobicity and larger grain size of the fullerene incorporated perovskite films,both of the devices demonstrated improved environmental and thermal stability.