Donor-acceptor conjugated polymers PBDT-DTBT and PBDT-DTNT, based on 2,1,3-benzothiadiazole (BT) and naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole (NT), have been designed and synthesized for polymer solar cells. NT contains two fused 1,2,5-thiadiazole rings that lower the band gap, enhance the interchain packing, and improve the charge mobility of the resulting polymer. Consequently, the NT-based polymer PBDT-DTNT exhibited considerably better photovoltaic performance with a power conversion efficiency (PCE) of 6.00% when compared with the BT-based polymer PBDT-DTBT, which gave a PCE of 2.11% under identical device configurations.

All-polymer solar cells based on a pair of crystalline low-bandgap polymers (NT and N2200) are demonstrated to achieve a high short-circuit current density of 11.5 mA cm-2 and a power conversion efficiency of up to 5.0% under the standard AM1.5G spectrum with one sun intensity. The high performance of these NT:N2200-based cells can be attributed to the low optical bandgaps of the polymers and the reasonably high and balanced electron and hole mobilities of the NT:N2200 blends due to the crystalline nature of the two polymers.

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A series of donor-acceptor type of π-conjugated copolymers based on tert-butoxycarbonyl (t-Boc) substituted indigo, isoindigo or diketopyrrolopyrrole as the acceptor unit and a benzodithiophene derivative as the donor unit was designed and synthesized. These copolymers can be readily dissolved in organic solvents and can produce uniform films by solution deposition. Thermal treatment of copolymer films at 200 °C for 10 min resulted in elimination of t-Boc side groups in nearly quantitative yield as suggested by thermogravimetric analysis and Fourier transform infrared spectroscopy. The elimination of the bulky t-Boc side groups resulted in the emergence of N-H···O═C hydrogen bonding interactions by virtue of the lactam structures of the indigo, isoindigo and diketopyrrolopyrrole units. Of particular interests is the distinctly increased field-effect mobility of these copolymers after thermal treatment, which may arise from the enhanced coplanarity and intermolecular ordering of the indigo, isoindigo or diketopyrrolopyrrole units after elimination of the bulky t-Boc side groups. These results demonstrate that the incorporation of latent side groups provides a viable strategy to construct conjugated polymers that can attain more ordered intermolecular stacking by simple thermal treatments. On the other hand, despite the thermal cleavage of t-Boc groups can also lead to increased ordering of polymer chains when blending with [6,6]-phenyl C71 butyric acid methyl ester, the photovoltaic performances of the resulting bulk heterojunction solar cells did not obviously increase due to the serious phase separation and coarsening of the film morphology.

Three donor–acceptor type of narrow band gap conjugated polymers with enlarged coplanar skeleton were synthesized via Stille copolymerization of indacenodithiophene (IDT) and naphtho[1,2-c:5,6-c]bis(1,2,5-thiadiazole) (NT) based monomers. The energy levels, absorption spectra and band gaps of the resulting polymers were well tuned by utilizing different thiophene derivatives as spacer between IDT and NT units, and polymer PIDT-C12NT which employed bithiophene attached with dodecyl side chain as spacer exhibited superior properties compared to the other two copolymers. All polymers exhibited deep highest occupied molecular orbital energy levels and subsequently lead to high open circuit voltages of fabricated solar cell devices. Best performance of the bulk-heterojunction solar cells with a power conversion efficiency of 5.05% was achieved with PIDT-C12NT as donor and (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) as acceptor, which can be attributed to its higher charge carrier mobility, the optimized interpenetrating network as well as enhanced absorption coefficient of photoactive layer with respect to the other two polymers. The results demonstrated that the combination of IDT and NT with appropriate spacers was a promising molecular design strategy for the application of solar cells.