Morphology of bulk heterojunction materials in polymer based solar cells

Solar cell technology based on conjugated polymer/fullerene composites continues to be of interest as a potential source of renewable energy. By applying rigorous fabrication conditions and postproduction annealing at 150°C, polymer solar cells with 5% power conversion efficiency are demonstrated. These devices exhibit remarkable thermal stability. We attribute the improved performance to morphology changes in the bulk heterojunction material induced by thermal annealing.; Poly(3-hexylthiophene), P3HT, as a semiconducting polymer with relatively high hole mobility, has been widely used in organic polymer photovoltaic (PV) cells. After investigation of the morphology and performance of Poly(3-hexylthiophene)/PCBM solar cell using P3HT with different molecular weight, we conclude that the optimum annealing temperature for solar cell devices is related to the MW of P3HT; the best performance can be obtained by using mixed P3HT with optimum ratio between high MW and low MW polymers. The corresponding "ideal morphology" would be comprised of highly ordered crystalline regions formed by low MW P3HT embedded and interconnected by a high MW P3HT matrix.; Extensive studies of the morphology revealed that polymer solar cell performance depends strongly on the nanoscale phase separation network of the P3HT and fullerene components. Spatial Fourier transforms and power spectral density were used to analyze digital images (obtained by Transmission Electron Microscopy, TEM) of such network in bulk heterojunction material. The power spectral density obtained from the spatial Fourier transform of the TEM images provides a detailed and quantitative description of the bulk heterojunction material and the kinetics of the phase separation. We also calculated the fractal dimension, the chemical distance, the fractal dimension of a random walk, and finally the effect of an applied electric field on the transport within the interpenetrating network of P3HT/fullerene bulk heterojunction materials. According to the simulations, charge carriers travel along a pathway ∼4 times longer than the regular Euclidean distance. In an applied electric field, the drift speed of charged carriers is reduced (lower by a factor 10∼20 than in a pure single component film) because of field-induced trapping on the tortuous network.