Effects of pressure on nano- and micro-scale morphological changes in conjugated polymer photovoltaic cells

This paper presents the results of an experimental study of the effects of pressure on polymer chain alignments in poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) blends that are used in bulk heterojunction organic photovoltaic cells (OPVs). The P3HT:PCBM blends on glass were subjected to pressure and annealing at 140 °C. The surface morphologies, nano-/micro-structures and the chain alignments were analyzed using atomic force microscopy techniques and grazing incidence x-ray scattering. The current-voltage characteristics of the resulting devices are also shown to change significantly with changes in the nano-/micro-structures. The polymer chains were aligned in the direction of the applied pressure (edge-on), which reduced the lamellae spacing between the polymer units and increased the degree of crystallinity. The increased crystallinity plays significant role in the current-voltage enhancements. The implications of the study are discussed for the design and control of the nano/microstructures of bulk heterojunction organic solar cells.