SURFACE MODIFICATIONS TO THE INDIUM TIN OXIDE (ITO) ANODES THROUGH PLASMA OXIDIZED SILVER FOR EFFICIENT P3HT:PCBM (1 :0.8) BULK HETEROJUNCTION PHOTOVOL TAlC DEVICES 1 • Woo-Jun Yoon 1 and Paul R. Berger 1 ,2 Department o! Electrical and Computer Engineering, The Ohio State University, Columbus, OH Department of Physics, The Ohio State University, Columbus, OH ABSTRACT Since reporting an improved power conversion efficiency via an increased short-circuit current density with a high fill factor of -63% for a poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) bulk heterojunction PV devices using surface modifications to the indium tin oxide (ITO) anodes through plasma oxidized silver [Appl. Phys. Lett. 92, 2008, p. 013306], the properties of this thin AgOx were investigated to shed light on the physical mechanism for the measured enhancement. Here, we present the physical, chemical and optical properties of this modified ITO anode using various surface characterization techniques. In particular, we elucidate the role of plasma oxidized thin islanded Ag in the interfacial energy level and its chemical bonding interfacial morphology using the Kelvin-prove technique and X-ray photoelectron spectroscopy. INTRODUCTION Among recent advances towards improving the power conversion efficiency (17eff) in organic photovoltaic (PV) devices, various interface and surface engineering techniques to the ITO anode have demonstrated improved efficiencies for organic PV devices by an optimization of the short-circuit current density (J sc) , the open-circuit voltage (Voe) and the fill factor (FF). Various interface and surface engineering techniques include, alternative transparent conducting oxide anodes [1,2], direct surface modification of the ITO anodes [3], of the hole transporting layer (HTL) [4], Insertion of exciton blocking layers [5] and modification of the cathodes [6). Although direct surface modifications to the ITO anode has been successfully implemented for organic light emitting diodes (OLEO) to improve their device performance by creating an interface energy step between the ITO and the HTL, thereby enhancing hole injection by effectively lowering the hole injection barrier, 978-1-4244-1641-7/08/$25.00 ©2008 IEEE this approach has not been fully explored for organic PV yet. For an OLEO, carriers are inserted over the energy step, whereas for PV devices, the carriers are extracted down this energy barrier, which means the carriers are not inhibited from extraction from the active layer by the injection barrier [7). However, it has been proposed that the reduction of an interface energy step at the electrode could improve the charge-transfer rate organic PV devices, leading to the increase of the charge collection efficiency by improving the J se [8). We further speculate that the transport through this staircase of energy levels could reduce thermal generation loss mechanisms, such as phonon creation. Khodabakhsh et al. [9] investigated the performance of an organic solar cell through surface modifications of ITO anode using self-assembled monolayers (SAM), which similarly enhanced J se without any significant change to Voc. By introducing the interfacial energy step, it was shown that the J se in the device with SAM-modified ITO anode was -4.6x greater than that of devices with bare ITO, while Voc remained nearly the same. Recently, we reported an improved 17eff for polymer fullerene bulk heterojunction PV devices, mainly due to the enhanced J sc, through plasma modification of Ag atop ITO anodes [10). Certification of these results is currently underway. The PV device performances are summarized in Table 1. Considering the enhanced hole injection from their current density-voltage (J- V) curves under darkness with oxygen plasma treated Ag on ITO anode, the measured increased J se without significant changes to Voc, FF, and the series resistance (Rs) suggest the improved charge-transfer rate at the modified anode occurs by creating an interfacial energy step between the ITO anode and the hole transporting layer. Here, we present the physical, chemical and optical properties of this modified ITO anode using various surface characterization techniques. This will include a detailed analysis of the ITO/AgO x interface via the Kelvin prove technique and x-ray photoelectron spectroscopy.