Morphological studies of bulk heterojunction films made of polymers showing stable photovoltaic properties

In this project, the morphology of bulk heterojunction photovoltaic cells employing a 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-b0 ] dithiophene) (PTB) backbone with tetrahydropyranyl (THP) terminated side chains that are cleaved upon thermal treatment. Photovoltaic properties of the devices have been previously shown to demonstrate a stable performance during an accelerated aging process. I have used optical microscopy, transmission electron microscopy, and grazing incidence small angle X-ray scattering (GISAXS) to investigate the morphology of films made from blends of the polymer with PCBM before (PTB(THP):PCBM) and after (dPTB:PCBM) cleaving at micro and nanoscale. I have further studied the self-organization of these polymers and their blend solution with PCBM using grazing incidence wide angle X-ray scattering (GIWAXS). 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). Morphological investigations 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. Consequently, I have concluded that the removal of THP on the side chains can lead to stable morphology which results in stable performance of photovoltaic cells. GIWAXS studies show that PTB(THP) polymer show considerably stronger ability to self-organize compared with PTB4. Although deprotection process could deteriorate this self-organization but dPTB polymer still show better self-organization along ????????- stack. While this study has been focused on comparison to PTB4 blends, the results should be transferable to other polymers in the PTB series.