Morphological, thermal and electrical characterization of organic photovoltaic blends

Abstract

The major objective of this thesis was to correlate morphology to the electrical properties of bulk heterojunction solar cells. Since at the beginning of this work P3HT:PCBM solar cells gave the best efficiencies, the focus was on this type of blend. First, an extensive morphology study was needed since it is generally known that morphology influences the properties of bulk heterojunction solar cells. Secondly, the relationship between morphology and electrical properties was still inefficiently known and therefore a few options were explored how to correlate them. First, the P3HT:PCBM blend was studied morphologically with a wide variety of techniques, i.e. transmission electron microscopy (TEM), X-Ray diffraction (XRD), atomic force microscopy (AFM), confocal fluorescence microscopy and optical microscopy. Different blend ratios and various annealing conditions were studied. Using XRD, the non annealed pure P3HT was found to be semicrystalline. With addition of PCBM the crystallization is suppressed and can subsequently be regained by annealing treatment. With annealing treatment, a twofold crystallization was observed using TEM and optical microscopy. Upon short (i.e. 5 min) annealing times and lower annealing temperatures (i.e. 75°C- 100°C) an enhanced crystallization of P3HT appeared. Increasing further annealing temperature and time, revealed the formation of novel needle-like crystalline PCBM-structures, identified by SAED as triclinic PCBM, of a few μm up to 100 μm in size. At specific preparation conditions these crystals can grow out to a two-dimensional network of PCBM-needles and in some cases to PCBMpallets. It has been demonstrated that key-parameters to “tune” the dimensions and spatial distribution of the PCBM needles are blend ratio and annealing conditions. It was found that the crystallization mechanism of P3HT was not responsible for this specific shape of crystals, since annealing of regiorandom P3HT:PCBM blends rendered the same needle like PCBM crystals, but rather the low Tg of P3HT in combination with the different start morphology compared to MDMO-PPV:PCBM blends. Secondly, the PCBM needles were studied more in detail. Around the PCBM needles a thinner area of almost pure and highly crystalline P3HT was found. This PCBM depleted region originates from diffusion of PCBM out of the matrix towards a growing PCBM crystal upon annealing. Confocal microscopy with fluorescence confirmed that regions surrounding the needles are made of almost pure P3HT. A stable morphology was obtained when all the PCBM has diffused out of the matrix. This was achieved for the 1:2 blend when annealed between 8-16 hours at 100°C. Thus, longer annealing times result in an irreversible morphology consisting of a pure matrix of highly crystalline P3HT embedding PCBM crystals. Moreover, these large PCBM crystals were also encountered in solar cell devices, hampering the perfect operation of the solar cell....