Influence of substrate on crystallization in polythiophene/fullerene blends

The nanoscale morphology of the active layer in organic, bulk heterojunction (BHJ) solar cells is crucial to device performance. Often a combination of casting conditions and post deposition thermal treatment is used to optimize the morphology. In general, the development of microscopic crystals is deleterious, as the exciton diffusion length is ≈10 nm. We find that the microscopic crystallization behavior in polythiophene/fullerene blends is strongly influenced by the substrate on which the BHJ is cast. With a silicon oxide substrate, the crystal nucleation density is high and significant crystallization occurs at a temperature of 140 °C. On more hydrophobic substrates, significantly higher temperatures are required for observable crystallization. This difference is attributed to the interfacial segregation of the PCBM, controlled by the substrate surface energy. The substrate dependence of crystallization has significant implications on the fullerene crystal growth mechanisms and practical implications for device studies.

Graphical AbstractFigure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights► PCBM crystallization behavior in bulk heterojuction films depends strongly on the nature of the substrate. ► On clean, hydrophilic substrates PCBM crystallization is readily observed at 140 °C. ► On more hydrophobic substrates, such as PEDOT and OTS treated oxides, observable PCBM crystallization requires higher temperatures. ► PCBM crystallization can be correlated to PCBM segregation at the substrate interface.