Abstract
Benzotrithiophene (BTT), where three thiophenes are fused to a central benzene, was polymerized with three kinds of electron accepting units, thiadiazolopyridine (TP), difluorobenzothiadiazole (FT), or naphthobisthiadiazole (NTz). These low bandgap polymers exhibited deep HOMO level (-5.4 ~ -5.6 eV) which can lead to increasing open circuit voltage (VOC) of bulk heterojunction organic photovoltaics (OPV). One of the BTT polymers and methano[60]fullerene (PCBM) blend showed a high VOC of 0.99 V, owing to the deep HOMO level and low energy loss. The best power conversion efficiency (PCE) of 2.7% was obtained for the BTT-NTz polymer blended with methano[70]fullerene. We have intensively characterized optoelectronic and morphological features of the new polymers by atomic force microscopy, 2-dimensional X-ray diffraction, and time-resolved microwave conductivity. This work highlights the potential of BTT-based polymers for an increase of PCE, because there are extra rooms for further improvement.
