An electrode design rule for organic photovoltaics elucidated using molecular nanolayers

Silane nanolayers deposited from the vapor phase onto indium-tin oxide (ITO) coated glass are shown to be an effective means of tuning the work function and stabilizing the surface of this complex ternary oxide. Using this approach a pair of model hole-extracting electrodes have been developed to investigate how the performance of bi-layer organic photovoltaics is impacted by built-in positive space charge in the critical region close to the holeextracting electrode. The magnitude and spatial distribution of positive space charge resulting from ground-state electron transfer from the donor layer to the ITO electrode upon contact formation, is derived from direct measurements of the interfacial energetics using the Kelvin probe technique. This judiciously designed experiment shows that it is unnecessary to engineer the work function of the hole-extracting electrode to match the ionization potential of the donor layer, rather only to ensure that the former exceeds the latter, thus simplifying an important aspect of device design. In addition, it is shown that silane nanolayers at the ITO electrode surface are remarkably effective at retarding device degradation under continuous illumination.