Thiophene spacers impart crystallinity and enhance the efficiency of benzotrithiophene-based conjugated polymers for bulk heterojunction photovoltaics

Shang Che Lan, Po An Yang, Meng Jie Zhu, Chia Min Yu, Jian Ming Jiang, Kung-Hwa Wei*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

35 Scopus citations

Abstract

In this study we synthesized the donor-acceptor conjugated copolymers PBTT4BT and PBTT4BO featuring benzotrithiophene (BTT) units as donors and benzothiadiazole (BT) and benzoxadiazole (BO) units, respectively, as acceptors, linked through 4-dodecylthiophene spacers. The presence of the spacer units enhanced not only the solubility of the synthesized polymers but also their molecular packing in the solid state; both of these polymers exhibited good crystallinity, as evidenced by a d-spacing of 23.8 Å in the (100) plane in their X-ray diffraction curves. When we used these synthesized polymers in bulk heterojunction photovoltaic device applications, the optimal device incorporating PBTT4BO/PC61BM as the active layer exhibited a low efficiency of 3.2%, due to the poor solubility of PBTT4BO, whereas the optimal device incorporating the more-soluble PBTT4BT and PC71BM displayed an efficiency of 4.4%, which is substantially 1.5% higher than that for the PBTTBT/PC71BM device, where PBTTBT was formed by copolymerizing BTT and BT units without any spacer. After thermal annealing, the efficiency of the PBTT4BT/PC71BM device improved further to 5.6%, with a VOC value of 0.72 V, a JSC value of 11.58 mA cm-2 and a fill factor of 67%. The annealed PBTT4BT/PC71BM active layer possessed a nanoscaled network-like morphology with rod-like PBTT4BT domains that were beneficial for charge separation and transport; accordingly, the power conversion efficiency of the annealed PBTT4BT/PC71BM photovoltaic device was enhanced greatly over that of the as-cast PBTT4BT/PC71BM device.

Original languageEnglish
Pages (from-to)1132-1140
Number of pages9
JournalPolymer Chemistry
Volume4
Issue number4
DOIs
StatePublished - 21 Feb 2013

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