Design And Interfacial Modification Of Water/Alcohol-soluble Conjugated Polymer For High Efficient Polymer Solar Cells

Research on polymer solar cells (PSCs) has achieved tremendous progressduring the past decade, due to their advantages of scalable printing manufacturing andlow materials costs. Power conversion efficiency (PCE) of PSCs has successfullysurpassed10%，with sophisticated design of novel light-harvesting polymers, usageof efficient device structures, development of new fabrication techniques, andincorporation of interfacial materials for electrode modification. Interfacialmodification for PSC plays a crucial role in improving the performance of PSCs. Anappropriate interfacial material could efficiently hamper the excitons recombinationand lower the contact resistance at the light-harvesting layer/electrode interface.A variety of charge transporting layer materials for modifying electrodes inPSCs have been explored. Recently, conjugated polyelectrolyte (CPE) characterizedby the amphiphilic characteristics with a π-conjugated main backbone and pendantionic functionalities has emerged as a remarkable interfacial material to establish aconnection between hydrophilic electrode and the hydrophobic organic layer in PSCs.In this paper, we introduced different kinds of CPEs as cathode interlayer in PSCs togive the insight of the effect on the device performance and to develop strategies foroptimization this effect.Firstly, fluorene-based CPEs PFNBr and PFSO3Na with cationic and anionicside chains respectively, are synthesized and applied as cathode interlayer intraditional PSCs. The opposite charges exert great influence on the effective workfunction of cathode and interfacial interaction through the orientation of theinterfacial dipole at the active layer/metal electrode interface. Compared with thecationic PFNBr, PFSO3Na with anionic sulfonate groups can dramatically reduce thework function of Al by accumulation of the polar groups at the PFSO3Na/Al interfaceto induce more favorable the interfacial dipole, subsequently enhance the deviceperformance. Then, three CPEs based on polythiophene bearing anionic (PTSO-Na),neutral (PTNOH) and cationic (PTN-Br) groups, respectively, were combined withZnO to form a ZnO/CPE bilayer as electron transporting layer (ETL) in invertedPSCs. For the deposition of cationic PTN-Br with quaternary amine side chains on the ZnO layer, we observed a strong electrostatic interaction between cationic groupsof the CPE and anionic oxygen ions of the ZnO surfaces, which obtained a uniformformation of strong dipoles across the interfaces and an intimate interfacial contact.Inverted PSCs ITO/ZnO/CPE/P3HT:PCBM/PEDOT:PSS/Ag with PTN-Br as ETLnot only delivered the device with the best PCE (4.08%) among the three CPEs butalso yielded an exceptional device lifetime without encapsulation. These resultsindicate that the pendant groups of CPE play a crucial role in dipole formation andmutual contact at device interface.In order to optimize the effect of the CPE on PSC performance, a simpleapproach was demonstrated to manipulate dipole moment of interlayer. The ionicliquid crystals (ILCs) CbpNSO were blended with CPE PTNBr to afford a novelCPE ILC complex. The spontaneous orientation of liquid crystal (LC) favors moreordered structural arrangement in CPE ILC complexes. More importantly,LC-assistant assembly improves the orientation of dipole at cathode and significantlyreduces the work function of ITO. The PCE of P3HT:PC60BM-based inverted PSCswith the layer of PTNBr-CbpNSO is increased by37%with respect to that of thedevice with pure PTNBr. After that, we utilized another CPE-ILC PFN-CbpSO asETL and solution processed tungsten oxide (WO3) together with conjugated polymerPBDTT-TT-TEG as hole transporting layer (HTL) in high performance PBDTTT-C-T:PC71BM-based solar cells. The interfacial modification of these interlayers achievesenergy alignment at both electrodes. The PCE of the inverted PSC based onITO/PFN-CbpSO/PBDTTTC-T:PC71BM/PBDTT-TT-TEG/WO3/Ag with solutionprocessed interlayers reaches to7.8%. It is worthy to note that, except for theelectrodes, all layers of device are fabricated by solution process at room temperatureand without annealing. In the case of incorporating ZnO layer into this device, thedevice efficiency further increases to8.5%, which is the best value reported fromPBDTTT-C-T:PC71BM-based solar cells with solution processed interlayers at bothelectrodes so far. With the help of CPE-ILC ETL, we have successfully developed aPSC with high efficiency, solution processibility and long-term air stability, which areanticipated to be used for fabrication of printable and large scale PSCs.