Zhongwei An

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.

High performance alternating polymers based on two-dimensional conjugated benzo[1,2-b:4,5-b′]dithiophene and fluorinated dithienylbenzothiadiazole for solar cells

Three donor–acceptor polymers based on two-dimensional conjugated alkylthienyl substituted benzo[1,2-b:4,5-b′]dithiophene (BDTT) and fluorinated 4,7-dialkylthienyl-2,1,3-benzothiadiazole (DTBT) derivatives were synthesized and applied as donor materials for bulk heterojunction solar cells. The effects of fluorine substitution on the absorption properties, energy levels, dielectric constants and photovoltaic properties of the polymers were investigated. We found that the incorporation of fluorine atoms on the DTBT unit can effectively deepen the highest occupied molecular orbital (HOMO) energy level and increase the dielectric constant of the polymers. As a result of these improved properties, a high open-circuit voltage (Voc) of 0.85 V was obtained for the optimal conventional polymer solar cells (PSCs) based on a two-fluorine-substituted analogue P3 blended with [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) as the active layer (1:1.5), giving rise to a high power conversion efficiency (PCE) of 7.48%. The influences of the molecular weight (MW) of P3 on the device performances were further studied and enhanced photovoltaic parameters were achieved for the medium molecular weight P3 based device, with a corresponding open circuit voltage (Voc) of 0.80 V, short-circuit current density (Jsc) of 14.7 mA cm−2, high fill factor (FF) of 71.9% and the resulting best PCE of 8.47%, verifying that P3 with an appropriate MW should be a promising polymer for highly efficient photovoltaic application.