Morphological characterization of a new low-bandgap thermocleavable polymer showing stable photovoltaic properties
The stability of the morphology of bulk heterojunction photovoltaic cells employing a novel low-bandgap polymer as donor is studied. The polymer is based on a poly(thieno[3,4-b]thiophene-benzo[1,2-b:4,5-b′]dithiophene) (PTB) backbone with tetrahydropyranyl (THP) terminated side chains that are cleaved upon thermal treatment, which leads to stable performance of the photovoltaic properties during an accelerated aging process achieved by thermal annealing. The morphology of films made from blends of the polymer with PCBM is investigated before (PTB(THP):PCBM) and after (dPTB:PCBM) cleaving at micro and nanoscale using optical microscopy, transmission electron microscopy, and grazing incidence small angle X-ray scattering. Results are compared to films made from blends of poly(3-hexylthiophene) (P3HT:PCBM) and the version of the PTB series whose structure is closest to that of PTB(THP), PTB4 (PTB4:PCBM). All three techniques demonstrate that phase separation is suppressed at micro and nanoscale in the dPTB:PCBM films, while large micron-sized PCBM aggregates develop during thermal annealing in P3HT:PCBM, PTB4:PCBM and PTB(THP):PCBM films. Our studies show that the removal of THP on the side chains can lead to stable morphology, and result in stable performance of photovoltaic cells. While we have focused on comparison to PTB4 blends, our results should be transferable to other polymers in the PTB series.