Effects of surface modification and GXP annealing on PCBM dispersion in organic photovoltaic solar cells

Controlling and characterizing the morphology of the active layer in organic photovoltaics (OPVs) is essential for improving sunlight-to-energy conversion. While the most widely used active layer is based on a blend of the donor-type polymer, poly(3-hexylthiophene) (P3HT) and the fullerene [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), various thermal or solvent annealing protocols are required to achieve modest power conversions efficiencies of ~5%. Here the use of gas expanded polymer (GXP) annealing, which relies on varying pressure conditions by introducing and removing CO2 gas at a constant flow rate, is investigated as a process to improve the dispersion of PCBM within a P3HT film. GXP annealing causes the thin film to swell and collapse which leads to a change in crystallinity of the polymer P3HT, as observed by X-ray diffraction, UV-Vis spectroscopy and contact angle measurements. Along with this method of annealing both additive incorporation and substrate functionalization were used to alter PCBM dispersion in OPV active layers. In one set of experiments P3HT brushes were grafted to functionalize the substrate. This functionalization induced ordering of P3HT at the substrate and the air interface. In another set of experiments and computations porphyrins were incorporated as additives to interact with fullerenes. The chemical potential of this interaction was theorized as a means to drive mass transport in the photoactive layer as well as to utilize the porphyrin as an alternative electron donor. The exact morphology of the active layer is difficult to characterize directly, due to limited nanoscale visualization techniques. Neutron reflectivity provides a way in which the dispersion of PCBM within the P3HT polymer film can be described. This thesis sets out to describe the morphological changes incurred upon GXP annealing, substrate functionalization, and additive incorporation with the ultimate goal of providing scalable techniques by which OPVs can be manufactured.