Synthesis and photovoltaic properties of naphthobisthiadiazole-triphenylamine-based donor-acceptor π-conjugated polymer

- The synthesis of the amorphous D-A polymer, poly(NTD-TPA), is reported.
- Poly(NTD-TPA) had an absorption edge at longer wavelengths than poly(CR-TPA).
- BHJ OPVs based on poly(NTD-TPA) exhibited a PCE of 2.15%.

We report the synthesis and photovoltaic properties of a donor-acceptor polymer, poly(naphthobisthiadiazole-triphenylamine) [poly(NTD-TPA)], which consists of TPA as a donor unit and NTD as a stronger acceptor than benzothiadiazole. This polymer had an absorption edge at wavelengths up to 610 nm. The optimized bulk-heterojunction (BHJ) organic photovoltaic (OPV) cell using poly(NTD-TPA) and [6,6]-phenyl C71 butyric acid methyl ester (PC70BM) exhibited a short-circuit current of 6.87 mA cm−2, an open-circuit voltage Voc of 0.79 V, a fill factor of 0.40, and a power conversion efficiency of 2.15%. The validities of these photovoltaic properties are discussed using the incident-photon-to-current conversion efficiency, film morphology, field-effect hole mobility, and electroluminescence (EL) spectrum from the charge transfer states between the highest occupied molecular orbital of poly(NTD-TPA) and the lowest unoccupied molecular orbital of PC70BM. In particular, the Voc estimated from the current density-voltage curve agreed well with that estimated from the peak energy of the EL spectra.