Surface Modification of Indium-Tin-Oxide Electrode for Improved Power Conversion Efficiency in Polymer Photovoltaic Device

摘要

Bulk heterojunction (BHJ) polymer photovoltaic devices based on a blend of an electron-donating π-conjugated polymer and an electron-accepting fullerene derivative are an attractive research target, because of their ease of fabrication, mechanical flexibility, and low cost. Among the numerous photoactive donor-acceptor composites, blends of poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PC_(61)BM) or [6,6]-phenyl C71-butyric acid methyl ester (PC_(71)BM) have been intensively studied in recent years, because polymer photovoltaic devices based on such composites have been shown to have power conversion efficiencies (PCEs) as high as 5%. An important factor in determining the performance of polymer photovoltaic devices, so as to achieve a high PCE, is the charge transport in the active layer. Indium-tin-oxide (ITO) is commonly used as the anode, due to its good transparency and low resistivity. However, the work function of untreated ITO is generally in the range of 4.5-4.6 eV, which does not provide for efficient hole extraction from the highest occupied molecular orbital (HOMO) of the common buffer layer such as poly(3,4-eth-ylenedioxythiophene) :poly(styrenesulfonate) (PEDOT:PSS) (HOMO, 5.0 eV). In order to reduce the high energy barrier for efficient hole extraction between ITO and PEDOT:PSS, different chemical and physical treatments have been proposed to increase the work function of ITO, including plasma treatment or UV-ozone treatment. More recently, organic surface modifiers based on silanes, as well as other organic molecules possessing anchoring groups such as COC1, SO2C12, and phosphonic acid, etc., have been utilized to try to control and modulate the work function of ITO in organic electronic devices. The bonding of these molecules to the ITO surface can result in an effective dipole moment at the interface that modifies the work function of ITO and, thus, improves the hole injection or extraction in the active layer. The effects of phosphonic acid modified ITO in organic light-emitting diodes (OLEDs) have also been evaluated. After the surface modification of ITO, its work function was increased to 4.9±0.5 eV. Previous studies have mainly focused on the effects of such surface modifiers on the performance of OLEDs. In this paper, we report on the modification of ITO using pentafluorobenzyl phosphonic acid as a surface modifier and its role in the improvement of the photovoltaic performance.