Towards Small-Molecule Organic Photovoltaics

Morphology plays a significant role in the research field of organic photovoltaics. In particular, the device performance and efficiency of bulk-heterojunction organic solar cells, relying on the concept of phase-separated donor and acceptor active layers, is strongly affected by morphology. The low dielectric numbers of organic functional materials lead to strong exciton binding energies and short diffusion path lengths therefore demanding an active layer with a bicontinuous interpenetrating donor-acceptor network and a pattern length scale of a few ten nanometers. In this work, a special class of spiro-linked small-molecule functional materials was used to develop a design approach to fulfill these criterions. As a first step, seven donor-acceptor combinations, which exhibit an uncommon liquid-liquid miscibility gap, were identified by thermomicroscopy. Combined with thermoanalysis and rigorous application of the Flory-Huggins solution theory, asymmetric binary phase diagrams were derived. For this, only a few measurements with a total material consumption of not more than a few milligrams were necessary. Two other donor-acceptor binaries showed complete miscibility in the liquid. Thus, for these systems the solid-liquid behavior was studied, and for both, a eutectic phase behavior was identified. While preparation of these blend systems was straight-forward and was achieved via mixing and subsequent melting, for the non-mixing systems, a solvent-assisted blend preparation method had to be developed. As studied by light microscopy and electron microscopy, such samples phase-separated by spinodal decomposition, which in its early stages produces a bicontinuous morphology. To gain some control about the pattern size of the evolving structures, a surfactant-like third compound, which resembles structural motifs of both blend compounds, was added in analogy to ternary A/B/AB polymer blends and oil/water/surfactant systems. It turned out that the addition of the third compound leads to a shift of the critical point contrary to the case of the analogous systems. It therefore rather behaves as an impurity than a surfactant. With increasing volume fraction of the third compound, a linear decrease in the critical temperature and a shift of the critical composition according to Timmermann’s rule was observed. The third compound does not migrate to the interface but is distributed in both phases with a partitioning distribution satisfying the Bancroft rule. By taking this shift into account, ternary systems were be prepared and characterized, which exhibit the desired morphology on the proper pattern length scale of a few ten nanometers. Thereby, the pattern lengths scale non-linearly with added impurity. In the course of this, several experimental methods suitable for tiny sample sizes had to be elaborated. The compound’s densities had to be known precisely for modelling the phase behavior. Knowledge about the vapor pressure curves gave valuable information about possible compositional changes upon thermal treatment and thermoanalysis.