Controlling vertical morphology within the active layer of organic photovoltaics using poly(3-hexylthiophene) nanowires and phenyl-C61-butyric acid methyl ester.

In this study, we demonstrate how the vertical morphology of bulk heterojunction solar cells, with an active layer consisting of self-assembled poly(3-hexylthiophene) (P3HT) nanowires and phenyl-C(61)-butyric acid methyl ester (PCBM), can be beneficially influenced. Most device fabrication routes using similar materials employ an annealing step to influence active layer morphology, but this process can create an unfavorable phase migration where P3HT is driven toward the top of the active layer. In contrast, we demonstrate devices that exhibit an increase in relative fullerene concentration at the top of the active layer by introducing the donor phase as a solid nanowire in the active layer solution and altering the pre-spin drying time. X-ray photoelectron spectroscopy and conductive and photoconductive atomic force microscopy provide detailed images of how the surface of the active layer can be influenced; this is done by tracking the concentration and alignment of P3HT and PCBM domains. Using this new procedure, devices are made with power conversion efficiencies surpassing 2%. Additionally, we show that nanowires grown in the presence of the fullerene perform differently than those that are grown and mixed separately; exposure to the nanowire during self-assembly may allow the fullerene to coat nanowire surfaces and influence the photocurrent within the device.