Organic photovoltaics(OPVs)have attracted a lot of research interest in the past decade as a promising alternative clean and renewable energy resource.[1] The power conversion efficiency(PCE)of the solar cells,their stability,and cost are key factors for widespread application.Much work has been carried out to optimize the methods of fabrication,and to improve the PCE; one approach being considered is interfacial engineering to enable better charge collection at the electrodes.A typical solar cells architecture contains light-absorbing bulk-heterojunction(BHJ)sandwiched between two electrodes; in a conventional solar cell structure,transparent indium tin oxide(ITO)is used as an anode for hole collection,and a metal cathode is used for electron collection.Upon absorption of a photon,electron-hole pairs are generated and separated in the BHJ,which are then transported to BHJ-electrode interfaces and collected by the electrodes.Generally,properties of interfacial layers play important roles in several aspects,such as making ohmic contact,align energy levels,adjusting surface energetics,and defining BHJ vertical phase segregation.[2] Traditionally,PEDOT:PSS is a popular material at the ITO interface,due to its highly transparency,and high conductivity.[3]With the development of low-band gap conjugated polymer,which has deep highest occupied molecular orbital(HOMO),the work function of PEDOT:PSS is not optimal to achieve full potential of the polymer.Herein,we describe the synthesis and characterization of a carboxylate-functionalized conjugated polymer,and its application as an interfacial layer,inserted between a BHJ and a PEDOT:PSS-modified ITO electrode.We will also describe the influence of this interfacial layer on the performance of deep,and shallow HOMO donor polymers in the BHJ,and demonstrate that pairing of the correct interfacial layer and donor polymer is necessary,and productive.