Impact of Initial Bulk-Heterojunction Morphology on Operational Stability of Polymer:Fullerene Photovoltaic Cells

Controlling initial bulk-heterojunction (BHJ) morphology of photoactive layer is critical for device efficiency of organic photovoltaic (OPV) cells. However, its impact on performance, specifically long-term operational stability is still poorly understood. This is mainly due to limitations in direct measurements enabling in situ monitoring of devices at a molecular level. Here, a thermal annealing preconditioning step is utilized to tune initial morphology of model polymer:fullerene BHJ OPV devices and molecular resonant vibrational spectroscopy to identify in situ degradation pathways. Direct spectroscopic evidence is reported for molecular-scale phase-segregation temperature (T-PS) which critically determines a boundary in high efficiency and long operational stability. Under operation, initially well-mixed blend morphology (no annealing) shows interface instability related to the hole-extracting layer via de-doping. Likewise, initially phase-segregated morphology at a molecular level (annealed above T-PS) shows instability in the photoactive layer via continuous phase-segregation between polymer and fullerenes in macroscales, coupled with further fullerene photodegradation. The results confirm that a thermal annealing preconditioning step is essential to stabilize the BHJ morphology; in particular annealing below T-PS is critical for improved operational stability while maintaining high efficiency.