ITO Interface Modifiers Can Improve VOC in Polymer Solar Cells and Suppress Surface Recombination

We use dipolar phosphonic acid self-assembled monolayers (PA SAMs) to modify the work function of the hole-extracting contact in polymer/fullerene bulk heterojunction solar cells. We observe a linear dependence of the open-circuit voltage (V-OC) of these organic photovoltaic devices on the modified indium tin oxide (ITO) work function when using a donor polymer with a deep-lying ionization energy. With specific SAMs, we can obtain V-OC values exceeding those obtained with the common poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) hole-extraction layer. We measure charge-carrier lifetimes and densities using transient photovoltage and charge extraction in a series of devices with SAM-modified contacts. As expected, these measurements show systematically longer carrier lifetimes in devices with higher V-OC values; however, the trends provide useful distinctions between different hypotheses of how transient photovoltage decays might be controlled by surface chemistry. We interpret our results as being consistent with changes in the band bending at the ITO/bulk heterojunction interface that have the net result of altering the internal electric field to help prevent electrons in fullerene domains from undergoing surface recombination at the hole-extracting electrode.