Polymer solar cells have been fabricated from a recently synthesized low band‐gap alternating polyfluorene copolymer, APFO‐Green2, combined with [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) from organic solutions. External quantum efficiencies (EQEs) of the solar cells show an onset at 850 nm and a peak of > 10 % located at 650 nm, which corresponds to the extended absorption spectrum of the polymer. Photocurrent of 3.0 mA cm–2, photovoltage of 0.78 V, and power conversion efficiency of 0.9 % have been achieved in solar cells based on this new low‐bandgap polymer under the illumination of air mass 1.5 (AM 1.5) (1000 W m–2) from a solar simulator.

Plastic solar cells were fabricated using a low-band-gap alternating copolymer of fluorene and a donor–acceptor–donor moiety (APFO-Green1), blended with [6,6]-phenyl-C61-butyric acid methylester or 3′-(3,5-Bis-trifluoromethylphenyl)-1′-(4-nitrophenyl)pyrazolino[60]fullerene as electron acceptors. The polymer shows optical absorption in two wavelength ranges from 300<λ<500nm and 650<λ<1000nm. Devices based on APFO-Green1 blended with the later fullerene exhibit an outstanding photovoltaic behavior at the infrared range, where the external quantum efficiency is as high as 8.4% at 840nm and 7% at 900nm, while the onset of photogeneration is found at 1μm. A photocurrent density of 1.76mA∕cm2, open-circuit voltage of 0.54V, and power conversion efficiency of 0.3% are achieved under the illumination of AM1.5 (1000W∕m2) from a solar simulator.

A new conjugated polymer, LBPP‐1, with an unusually low band‐gap (ca. 1.0 eV) is presented. Light absorption and photovoltaic response up to 1200 nm in composites with a fullerene is demonstrated. Solar cell performance is presented and the polymer's suitability for photodetection in the infrared region is discussed.

Solar cells based on organic molecules or conjugated polymers attract great attention due to their unique advantages, such as low cost, and their use in flexible devices, but are still limited by their low power conversion efficiency (PCE). To improve the PCEs of polymer solar cells, more efforts have been made to increase short-circuit current (J sc ) or open-circuit voltage (V oc ). However, the trade-off between J sc and V oc in bulk heterojunctions solar cells makes it tricky to find a polymer with a low band gap to efficiently absorb photons in the visible and near infrared region of the solar spectrum, while maintaining a high V oc in solar cells. Therefore, it is crucial to design and synthesize polymers with energy levels aligning with the LUMO (lowest unoccupied molecular orbital) of an electron acceptor to minimize the LUMO level difference between donor and acceptor to keep enough driving force for charge generation, thereby maximizing photovoltage in solar cells. Here a novel copolymer APFO-Green 9 was synthesized. Polymer solar cells based on APFO-Green 9 blended with a derivative of fullerene demonstrate high photovoltage by fine tuning the HOMO and LUMO level of APFO-Green 9. Solar cells based on APFO-Green 9 and [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM) present a photoresponse extended to 900 nm with J sc of 6.5 mA cm À2 , V oc of 0.81 V and PCE of 2.3% under illumination of AM1.5 with light intensity of 100 mW cm À2 . As a low band gap polymer with a V oc bigger than 0.8 V, APFO-Green 9 is a promising candidate for efficient tandem solar cells.

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