Fine tuning the HOMO energy levels of polythieno[3,4-b]thiophene derivatives by incorporation of thiophene-3,4-dicarboxylate moiety for photovoltaic applications

Xiao-Lian Hu, Li-Jian Zuo, Ya-Xiong Nan, Martin Helgesen, Ole Hagemann, Eva Bundgaard, Min-Min Shi, Frederik C Krebs, Hong-Zheng Chen

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Abstract

To lower the HOMO (highest occupied molecular orbital) energy level of polythieno[3,4-b]thiophene (∼−4.5eV), a series of ester-functionalized polythieno[3,4-b]thiophene derivatives (P1–P3) were designed and synthesized by Stille cross coupling reaction. The resulting copolymers exhibited broad and strong absorption bands from visible to near infrared region with low optical band gaps of 1.23–1.42eV. Through cyclic voltammetry measurements, it was found that the HOMO energy levels of the copolymers gradually decreased with increasing the content of the thiophene-3,4-dicarboxylate moiety, i.e. −4.91eV for P1, −5.00eV for P2, and −5.11eV for P3. Preliminary photovoltaic properties of the copolymers blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as electron acceptor were investigated. Among the three copolymers, P1 exhibited the best photovoltaic performance with an open circuit voltage (Voc) of 0.54V, a short circuit current density (Isc) of 3.3mA/cm2, a fill factor (FF) of 0.57, and a power conversion efficiency (PCE) of 1.02%. A high Voc up to 0.71V was achieved in the solar cell based on a P3:PCBM blend.
Original languageEnglish
JournalSynthetic Metals
Volume162
Issue number23
Pages (from-to)2005-2009
ISSN0379-6779
DOIs
Publication statusPublished - 2012

Keywords

  • Polythieno[3,4-b]thiophene
  • Ester group
  • HOMO
  • Polymer solar cells

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