Bandgap and Molecular Energy Level Control of Conjugated Polymer Photovoltaic Materials Based on Benzo[1,2-b:4,5-b′]dithiophene

Abstract

Bandgap and molecular energy level control are of great importance in improving photovoltaic properties of conjugated polymers. A common approach to tuning these parameters is to modify the structure of conjugated polymers by copolymerizing with different units. In this paper, research work focuses on the synthesis of benzo[1,2-b:4,5-b′]dithiophene (BDT) with different conjugated units and their photovoltaic performance. Eight new BDT-based polymers with commonly used conjugated units, including thiophene, benzo[c][1,2,5]thiadiazole (BT), thieno [3,4-b]pyrazine (TPZ), etc., were synthesized. The bandgaps of the polymers were tuned in the range of 1.0-2.0 eV, and their HOMO and LUMO energy levels could also be tuned effectively. The absorption spectra as well as electrochemical and photovoltaic properties of these polymers were investigated systematically. Some units exhibiting the same effect of bandgap lowering exhibited different effects on molecular energy levels of the polymers. For example, the TPZ unit can reduce the bandgap by lowering the LUMO energy level and elevating the HOMO level of the polymer, but the BT unit can lower the bandgap only by depressing the LUMO level. Since open-circuit voltage (V oc ) of the heterojunction polymer solar cell is believed to be inversely proportional to the HOMO level of electron donor material, V oc of the devices based on H9, the copolymer of BDT and TPZ, was ca. 0.5 V lower than that of the device based on H7, the copolymer of BDT and BT. The effects of seven commonly used units on bandgap, molecular energy level, and photovoltaic properties of the BDT based polymers are studied and discussed in this paper, which can provide a guideline not only for design of photovoltaic materials but also for materials of various other electronic devices. In addition, the PCE of the device based on PCBM and H6, one of the BDT-based polymers, reached 1.6%, and V oc , I sc , and FF of the device were 0.75 V, 3.8 mA/cm 2 , and 56%, respectively, which indicates that BDT is a promising common unit for photovoltaic conjugated polymers. Since we have developed the synthetic method of the 4,8-bisalkoxy-BDT monomer, the BDT unit will play an important role in future research on conjugated polymer design.

Effects of branching position of alkyl side chains on ordering structure and charge transport property in thienothiophenedione- and quinacridone-based semiconducting polymers

“…3 In the development of semiconducting polymers, the design of the conjugated backbone is of primary importance because it substantially affects the molecular orbital energies, [4][5][6] bandgaps, 7,8 and ordering structure 9,10 that determine the device performance of the polymers. To date, enormous effort has been focused on the exploration of backbone structures (that is, new conjugated building units and their combinations).…”

Synthesis of π-conjugated poly(thienylenearylene)s with nickel-catalyzed C–H functionalization polycondensation

“…Higher catalytic activity was achieved with 5.0 mol% of PdCl 2 dppf (dppf: 1,1'-bis(diphenylphosphino)ferrocene), which afforded 1b with a 61% yield. Benzo[1,2-b:4,5-b']dithiophene-4,8-dione was synthesized using Hou and Yang's procedure, 31 with a 63% overall yield from thiophene-3-carboxylic acid. Quinone 9 was reduced by zinc powder in an aqueous sodium hydroxide solution, and then treated with alkylbromide and a catalytic amount of tetrabutylammonium bromide.…”

Donor–acceptor semiconducting polymers for organic solar cells

“…To address this bandgap challenge, the thiophene analogs of the fused dibenzene units were developed. 78 The fused thiophene analogs can produce D-A copolymers with higher planarity because the repulsion caused by the protruding hydrogen atoms of the phenyl groups with their neighboring units is relieved by replacing the phenyl with the thiophene units as demonstrated by DFT calculation (Fig. 8).…”