Effect of Polyelectrolyte Electron Collection Layer Counteranion on the Properties of Polymer Solar Cells Thu Trang Do, † Hee Seob Hong, † Ye Eun Ha, † Juyun Park, ‡ Yong-Cheol Kang, ‡ and Joo Hyun Kim* ,† † Department of Polymer Engineering, Pukyong National University, Busan 608-739, Korea ‡ Department of Chemistry, Pukyong National University, Busan 608-737, Korea * S Supporting Information ABSTRACT: Polyviologen (PV) derivatives are known materials used for adjusting the work function (WF) of cathodes by reducing the electron injection/collection barrier at the cathode interface. To tune and improve device performance, we introduce different types of counteranions (CAs), such as bromide, tetrafluoroborate, and tetraphenylborate, to a PV derivative. The effective WF of the Al cathode is shown to depend on the size of the CA, indicating that a Schottky barrier can be modulated by the size of the CA. Through the increased size of the CA from bromide to tetraphenylborate, the effective WF of the Al cathode is gradually decreased, indicating a decreased Schottky barrier at the cathode interface. In addition, the change of the power conversion efficiency (PCE) and the short circuit current (J sc ) value show good correlation with the change of the WF of the cathode, signifying the typical transition from a Schottky to an Ohmic contact. The turn-on electric field of the electron-only device without PV was 0.21 MV/cm, which is dramatically higher than those of devices with PV-X (0.07 MV/cm for PV-Br, 0.06 MV/cm for PV- BF 4 , and 0.05 MV/cm for PV-BPh 4 ) This is also coincident with a decrease in the Schottky barrier at the cathode interface. The device ITO/PEDOT/P3HT:PCBM/PV/Al, with a thin layer of PV derivative and tetraphenylborate CA as the cathode buffer layer, has the highest PCE of 4.02%, an open circuit voltage of 0.64 V, a J sc of 11.6 mA/cm 2 , and a fill factor of 53.0%. Our results show that it is possible to improve the performance of polymer solar cells by choosing different types of CAs in PV derivatives without complicated synthesis and to refine the electron injection/collection barrier height at the cathode interface. KEYWORDS: polyelectrolyte, anion exchange, polymer solar cell, buffer layer ■ INTRODUCTION Recently, polymer solar cells (PSCs) have attracted attention because of their possible application in energy harvesting devices due to their flexibility and low fabrication cost. 1-3 Photoinduced charge separation, transportation, and collection properties are very important factors for the construction of efficient PSCs. Among them, charge carrier injecting/collecting properties are crucial for improving the performance of the devices, which are strongly related to the interfacial properties between the organic semiconducting layer and the cathode (or anode). The interfacial property at the cathode interface is improved simply by the insertion of solution processable conjugated polymer electrolytes (CPEs), 4-11 alcohol soluble neutral conjugated polymers, 12,13 polyviologen (PV) deriva- tives, 14 nonconjugated polymer electrolytes, 15,16 and non- conjugated polymers with polar groups, such as poly- (vinylpyrrolidone), 17 poly(ethylene oxide), 18 and poly(vinyl alcohol). 19 These materials enable the fabrication of a multilayer device without destroying a precoated organic semiconducting layer because they are soluble in polar solvents (e.g., water, alcohol, etc.). Through the insertion of a thin layer of these materials at the cathode interface, the performance of PSCs is dramatically improved relative to that of devices lacking these materials as an interfacial layer. Park et al. 20 reported that spontaneous organization of CPEs occurs during the spin coating process. Ionic components of the CPE accumulate at the top of the CPE surface because the hydrophobic semiconducting polymer layer rejects the ionic groups. Thus, the ionic or polar groups of interfacial materials can help the electric fields redistribute within a device and allow them to show permanent dipoles via their spontaneous orientation on top of either a hydrophobic organic active layer or a hydrophilic metal electrode (i.e., cathode). Therefore, it is possible to refine the energy barrier for the electron injection/collection at the cathode interface through the formation of favorable interface dipoles. If these ionic or polar materials are placed at the cathode interface, the work function (WF) of the cathode can be modified, and the energy barrier between the organic semiconducting layer and the cathode can be reduced. The presence of counteranions (CAs) in CPE structures provides another simple way of fine-tuning Received: November 24, 2014 Accepted: January 22, 2015 Research Article www.acsami.org © XXXX American Chemical Society A DOI: 10.1021/am5082606 ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX