N-type Small Molecular Conjugated Electrolytes As Cathode Buffer Layer For High-performance Organic Solar Cells

Polymer solar cells(PSCs),which are low-cost,light-weight,mechanical flexibility and printable,have attracted much attention in the past few years.Recently,power conversion efficiencies(PCEs)of the single-junction PSCs have improved up to ~12% for small-area devices.However,it still lag behind the requirement for the commercial application.To improve efficiency of PSCs,the design of new active layer photovoltaic materials,device optimization and interfacial engineering have been demonstrated to be the successful strategies.Among of these,interfacial engineering is an efficient approach to obtain relatively high PCE.An excellent interlayer can effectively control the active layer/electrode interface to form compatible ohmic contact,which can alleviate the interfacial energy barrier and facilitate holes and electrons to transfer to the corresponding electrodes.In addition,the interlayer also impacts the vertical phase separation of the active layer,which is very crucial for the charge selective injection.Therefore,interfacial modification have significant influence on the device performance parameters,such as open-circuit voltage(Voc),short-circuit current density(Jsc),and fill factor(FF).Two novel amphipathic small molecular conjugated electrolytes(SMCEs)with self-doping effect were successfully synthesized by incorporating diketopyrrolopyrrole(DPP)as n-type backbone and amine-based groups as polar pendants.In favor of the n-type backbone and amine-based groups,an obvious n-type doping was obtained,resulting in the dramatically improved conductivity of the SMCEs.The formation of the polar group induced dipoles at the SMCEs/Al interface to provide a favorable energy for cathode.Subsequently,the two small molecule electrolytes as cathode interlayers have been successfully applied in the polymer solar cells(PSCs)with a wide thickness range from 7~33nm.A notable power conversion efficiency(PCE)of 10.2% for PSCs were achieved.And the two SMCEs also showed a good universality with substantial performance.These findings indicate that these small molecular conjugated electrolytes are promising candidates as cathode interlayers for highly efficient polymer solar cells.