Haimei Wu

Fluorine substituted thiophene–quinoxaline copolymer to reduce the HOMO level and increase the dielectric constant for high open-circuit voltage organic solar cells

This study reported a novel fluorinated copolymer (FTQ) and shown it to exhibit a significantly higher open circuit voltage (VOC) in bulk heterojunction solar cells with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) compared to the low band-gap polymer Thiophene–Quinoxaline (TQ). Fluorination lowers the polymer HOMO level effectively which pushes down the highest occupied molecular orbital (HOMO) level of the TQ from −5.36 eV to −5.51 eV and increases the relative dielectric constant from 4.2 to 5.5, resulting in a high VOC. The highest VOC of 950 mV was achieved in the FTQ/PCBM solar cell device. For these optimized blends, the device made of FTQ:PC71BM with a 1 : 1 weight ratio yielded a high power conversion efficiency of 5.3% after a very short time thermal annealing process. These findings will be of importance for achieving high-performance of polymer solar cells by functional group substitution in low band gap polymers.

Side chain modification: an effective approach to modulate the energy level of benzodithiophene based polymers for high-performance solar cells

Over the past few years, it has been proven that deepening the highest occupied molecular orbital (HOMO) levels of conjugated polymers is one of the most successful strategies to develop novel materials for high performance bulk heterojunction polymer solar cells. Here we report an effective approach of side chain modification in a donor moiety to modulate the highest occupied molecular orbital (HOMO) energy level of benzo[1,2-b:4,5-b′]dithiophene (BDT) based two-dimensional conjugated polymers and hence obtain improved power conversion efficiency. Through the introduction of 2,3-dioctylthienyl groups instead of conventional 2-ethylhexylthienyl groups as side chains in the 4,8-positions of the BDT moiety, four mono-fluorinated quinoxaline based alternating polymers: poly{4,8-di(2-ethylhexylthiophene-5-yl)-2,6-benzo[1,2-b:4,5-b′]dithiophene-alt-5,5-[5′,8′-di-2-thienyl-(6′-fluoro-2′,3′-bis(3′′-octyloxyphenyl)quinoxaline)]} (PBDTTFTQ-EH), poly{4,8-di(2-ethylhexythiophene-5-yl)-2,6-benzo[1,2-b:4,5-b′]dithiophene-alt-5,5-[5′,8′-di-2-thienyl-(6′-fluoro-2′,3′-bis(5′′-octylthiophen-2′′-yl)quinoxaline)]} (PBDTTFTTQ-EH), poly{4,8-di(2,3-dioctylthiophene-5-yl)-2,6-benzo[1,2-b:4,5-b′]dithiophene-alt-5,5-[5′,8′-di-2-thienyl-(6′-fluoro-2′,3′-bis-(3′′-octyloxyphenyl)-quinoxaline)]} (PBDTTFTQ-DO) and poly{4,8-di(2,3-dioctylthiophene-5-yl)-2,6-benzo[1,2-b:4,5-b′]dithiophene-alt-5,5-[5′,8′-di-2-thienyl-(6′-fluoro-2′,3′-bis(5′′-octylthiophen-2′′-yl)quinoxaline)]} (PBDTTFTTQ-DO) were synthesized, in which the former two EH-based polymers contain the 2-ethylhexylthienyl side chain and the latter two DO-based polymers contain the 2,3-dioctylthienyl side chain. The results clearly indicated that the variation in the side chain of the BDT unit from the 2-ethylhexylthienyl group to the 2,3-dioctylthienyl group can cause a deep HOMO level and slightly enlarged bandgap of the polymer. Consequently, polymer solar cells from PBDTTFTQ-DO and PBDTTFTTQ-DO showed high Vocs of 0.88 V and 0.85 V, while those of 0.78 V and 0.72 V were observed in PBDTTFTQ-EH and PBDTTFTTQ-EH based devices, respectively. Both polymers deliver high power conversion efficiency (PCE) exceeding 6.8%, with an outstanding efficiency of 7.61% and an excellent fill factor (FF) of 75.9% for PBDTTFTQ-DO after tetrahydrofuran solvent vapour annealing for 30 s (while a PCE of 7.29% for PBDTTFTQ-EH). Similar results were investigated in PBDTTFTTQ-EH (a PCE of 6.82%) and PBDTTFTTQ-DO (a PCE of 7.25% with high FF of 75.7%) based solar cells. This finding should provide valuable guidelines for the design and synthesis of novel polymer donor materials for highly efficient polymer solar cells via fine tuning of the molecular energy levels and absorption spectra through a modulation of the side chain onto the donor moiety.

An alternating polymer with fluorinated quinoxaline and 2,7-carbazole segments for photovoltaic devices

A novel alternating polymer, poly{[N-9′-heptadecyl-2,7-carbazole]-alt-5,5-[5′,8′-di-2-thienyl-(6′-fluoro-2′,3′-bis-(3′′-octyloxyphenyl)-quinoxaline)]} (PCzFTQx), based on mono-fluorinated quinoxaline derivative and 2,7-carbazole was synthesized and applied as electron donor material in polymer solar cells. Compared to the corresponding counterpart polymer without fluorine substituent (PCzTQx), PCzFTQx possesses similar absorption properties and optical bandgap (∼2.0 eV). However, the highest occupied molecular orbital (HOMO) energy level of PCzFTQx was lowered to −5.31 eV, about 0.09 eV deeper than that of PCzTQx. Benefit from the low-lying HOMO energy level caused by the strong electron deficient fluorine atom on the quinoxaline unit, the optimized photovoltaic device based on PCzFTQx and phenyl-C71-butyric acid methyl ester (PC71BM) exhibited an enhanced power conversion efficiency (PCE) of 5.19% with corresponding high open-circuit voltage (Voc) of 0.94 V, relatively to those of 4.72% and 0.82 V for PCzTQx-based device. The experimental data indicated that fluorinated quinoxaline based polymer PCzFTQx should be a promising donor for polymer solar cells.