Constructing High-performance Organic Photovoltaic Devices Via Interfacial Modification Of ZnO

With high electron mobility,low cost,good environmental stability,high transparency,and excellent hole blocking characteristics,inorganic zinc oxide（ZnO）is often used in the construction of cathode interface layers（CILs）of inverted organic photovoltaic devices（OPVs）.Sol-gel method is one of the commonly used preparation methods of ZnO CIL.However,the ZnO layer derived from the sol-gel method is not prefect:a high density of defect states leads to more serious carrier recombination.Besides,incompatible chemical interfaces between inorganic ZnO and the photosensitive organic materials layer,are not conducive to electron extraction.In view of the above two problems,new interfacial structures and modified ZnO layers have been designed to promote the construction of high-performance OPVs in this thesis.Details are as follows:（1）A simple one-step solution spin coating method is used to prepare a new type of organic/inorganic gradient diffusion structure.This newly designed organic/inorganic gradient diffusion interface layer（G-ZnO/ITIC）is composed of n-type organic semiconductor 3,9-bis（2-methylene-（3-（1,1-dicyanide Methylene）-indanone））-5,5,11,11-tetrakis（4-hexylphenyl）dithio[2,3-d:2,3-d]-s-indanone[1,2-b:5,6-b]-dithiophene（ITIC）and inorganic ZnO.Various characterizations indicate that the constructed G-ZnO/ITIC structure can optimize the contact interface and adjust the energy levels via regulating the distribution of ITIC molecules in the cathode interfacial layers.Therefore,G-ZnO/ITIC-based PTB7-Th:PC₇₁BM OPVs exhibit power conversion efficiency（PCE）up to 8.73%,which is remarkably larger than these of conventional ZnO-based devices（7.88%for pure ZnO CIL device）.This gradient diffusion structure is also effective in the PTB7-Th:ITIC-based nonfullerene OPVs,showing improved PCE values from 6.63%to 7.29%.（2）The organic amine additives diethylenetriamine（DTA）and tetraethylenepentamine（TPA）with amino groups are used to adjust the ZnO interfacial layer.After DTA molecules are introduced into ZnO,the regulation of surface work function（WF）can be achieved by the interaction of the amino group with the inorganic ZnO.At the same time,organic amine additives can optimize the interfacial contact between the inorganic ZnO and the organic photosensitive layer.The regulation effect of DTA molecules on the ZnO film reduces carrier recombination at the interface,and promotes electron extraction and transportation.For PTB7-Th:PC₇₁BM-based OPV devices,its fill factor（FF）significantly increases from 59.3%of the original ZnO device to 64.5%of the DTA-modified ZnO device.Correspondingly,the final PCE value of the device is significantly increased by 16%compared to that of the original device.However,when using TPA to modify the ZnO interface,the PCE of the device is even lower than that of pure ZnO,which indicates that the selection of organic amines to optimize OPV devices performance is rather crucial.（3）A novel organic-inorganic composite film made up of zinc oxide（ZnO）and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane（F4TCNQ）is designed to investigate the effect of F4TCNQ doping on the performance of photovoltaic devices.Thus constructed ZnO:F4TCNQ composite film exhibits enhanced surface hydrophobicity and adjustable energy levels.Moreover,the trap density in the ZnO CILs is efficiently reduced by the coordination of C≡N group with ZnO.Under the optimal doping ratio,the FF of ZnO:F4TCNQ CIL-based PTB7-Th:PC₇₁BM OPVs improves from 62.2%to 67.1%significantly,and the FF of ZnO:F4TCNQ CIL-based PTB7-Th:ITIC OPVs improved from 52.8%to 58.3%.The above results show that F4TCNQ as a defect filler for ZnO can effectively improve the carrier transport properties of ZnO and play a beneficial role in a variety of OPVs,which also provides new research ideas for the design of the interfacial layer structure for high-performance OPV devices.