Improved open-circuit voltage of benzodithiophene based polymer solar cells using bulky terthiophene side group

• Terthiophene side group can improve Voc of the polymer solar cells.
• Bulky side group help polymer (Eg~1.3 eV) produce ~0.78 V high Voc.
• Low photon energy loss (Eg−eVoc)~0.51 eV is achieved for PSCs donor.

Terthiophene, including one α–α and one branching α–β connection of the thiophene units, is introduced as benzodithiophene (BDT) side chain to build a novel two-dimensional (2D) conjugated BDT block. By copolymerizing this BDT block with three electron acceptors (DTTz (bis(thiophene-2-yl)-tetrazine), DPP (diketopyrrolopyrrole), DTffBT (4,7-bis(4-hexylthienyl)-5,6-difluoro-2,1,3-benzothiadiazole)) and one electron donor (TTT (2,5-Di(2-thienyl)thiophene)), four terthiophene side-chained benzodithiophene based copolymers were synthesized. Due to the difference in electron affinity among DTTz, DPP, DTffBT and TTT, these four polymers show different UV–vis absorption spectra and optical band gaps (1.3–2.0 eV), while fortunately they all remain deep highest occupied molecular orbital (HOMO) energy levels (−5.3 to 5.6 eV) which is very favorable to high open-circuit voltage (Voc) polymer solar cells (PSCs). By comparing the photovoltaic properties with polymers which have same backbone but do not have the bulky 2D side group in the literatures, our polymer solar cells devices show higher Voc. Especially for PQ3 (a copolymer of benzodithiophene and diketopyrrolopyrrole), the donor photon energy loss (Eg–eVoc) is 0.51 eV which is almost the lowest value achieved by the researchers. It can be concluded that: the bulky terthiophene side group helps to improve Voc of the PSCs devices. The overall performance of solar cells devices is correlated with the molecule conformation, polymer hole mobility and polymer/PCBM blend film morphology.

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