Chemical Surface Grafting And Zwitter-ionic Polymer For Interfacial Layer: Towards High Performance Polymer Solar Cells

Polymer solar cells（PSCs）have attracted extensive attention,owing to their advantages,such as light-weight,low-cost,and mechanical flexibility.Recently,significant progresses have been achieved in the s ingle-junction PSCs with the power conversion efficiencies（PCEs）over 14%,primarily owing to the exploration of high performance materials,the optimization of device architecture,and the development of interfacial materials.1.Chemical modification of n-type-material naphthalene diimide on ITO for efficient and stable inverted polymer solar cells.In order to provide orthogonal and solvent-processible interfacial layer and improve the device stability of bulk-heterojunction polymer solar cells（PSCs）,n-type semiconducting material naphthalene diimide（NDI）was chemically introduced on ITO surface as cathode interlayer（CIL）using3-bromopropyltrimethoxysilane（BrTMS）as coupling agent.After modification,the work function of ITO decreased from 4.70 to 4.23 eV.The modified ITO cathode was applied in inverted PSCs based on PTB7-Th:PC₇₁BM.With the CIL modification,a champion power conversion efficiency（PCE）of 5.87%was achieved,showing a dramatic improvement compared with that of the devices without CIL（PCE=3.58%）.More importantly,with this chemical bonded interlayers,the stability of inverted PSCs was greatly enhanced.The improved PCE and stability can be attributed to the increased open-circuit vo ltage and the formation of chemical bond within NDI-TMS films,respectively.This study demonstrated that chemical modification of ITO with n-type semiconducting materials provides an avenue for not only solving the solvent orthogonal problem,but also improving the device performance in terms of the PCE and the stability.In this article we investigated the influence of chemical modification of n-type-material and spin-coated the zwitterionic polymer as cathode interfacial layer on PSCs.2.A nonconjugated zwitterionic polymer:cathode interfacial layer comparable with PFN for narrow-bandgap polymer solar cells.A non-conjugated alcohol soluble zwitter-ionic polymer poly（sulfobetaine methacrylate）（denoted by PSBMA）was employed as cathode interfacial layer（CIL）in polymer solar cells（PSCs）based on PTB7-Th:PC₇₁BM.Compared with the control device without CIL,PSCs with PSBMA CILs showed significant enhancement on the resulting performance,and the highest power conversion efficiency（PCE）of 8.27%was achieved.Under parallel conditions,PSCs with PSBMA as CIL showed comparable performance than thosewithwidely-usedPFN（poly[（9,9-bis（30-（N,Ndimethylamino）propyl）-2,7-fluorene）-alt-2,7-（9,9-iocty lfluorene）]）as CIL.The polar groups of PSBMA can not only provide a good solubility in polar solvent which is orthogonal to the solvent used for preparation of the photoactive layers,but also lead to interfacial dipole to the electrode,which promises a better energy level alignment.In addition,PSBMA-based devices show better abilities of hole blocking.These results indicate that the zwitter-ionic polymer PSBMA should be a promising CIL in PSCs based narrow-bandgap polymers.