Thieno[3,2‑b]thiophene-Substituted Benzo[1,2‑b:4,5‑b′]dithiophene as a Promising Building Block for Low Bandgap Semiconducting Polymers for High-Performance Single and Tandem Organic Photovoltaic Cells

• Published in: Chemistry of materials Vol. 26; no. 2; pp. 1234 - 1242
• Main Authors: Kim, Ji-Hoon, Song, Chang Eun, Kim, BongSoo, Kang, In-Nam, Shin, Won Suk, Hwang, Do-Hoon
• Format: Journal Article
• Language: English
• Published: American Chemical Society 28.01.2014
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/cm4035903

We designed and synthetized a new poly{4,8-bis((2-ethyl­hexyl)thieno[3,2-b]thiophene)-benzo[1,2-b:4,5-b′]dithio­phene-alt-2-ethyl­hexyl-4,6-dibromo-3-fluoro­thieno[3,4-b]thio­phene-2-carboxylate} (PTTBDT-FTT) comprising bis(2-ethyl­hexyl­thieno[3,2-b]thio­phenyl­benzo[1,2-b:4,5-b′]dithio­phene (TTBDT) and 2-ethylhexyl 3-fluorothieno[3,4-b]thiophene-2-carboxylate (FTT). The optical bandgap of PTTBDT-FTT was 1.55 eV. The energy levels of the highest occupied and lowest unoccupied molecular orbitals of PTTBDT-FTT were −5.31 and −3.73 eV, respectively. Two-dimensional grazing-incidence X-ray scattering measurements showed that the film’s PTTBDT-FTT chains are predominantly arranged with a face-on orientation with respect to the substrate, with strong π–π stacking. An organic thin-film transistor fabricated using PTTBDT-FTT as the active semiconductor showed high hole mobility of 2.1 × 10–2 cm2/(V·s). Single-junction bulk heterojunction photovoltaic cells with the configuration ITO/PEDOT:PSS/PTTBDT-FTT:PC71BM/Ca/Al were fabricated, which showed a maximum power conversion efficiency (PCE) of 7.44%. Inverted photovoltaic cells with the structure ITO/PEIE/PTTBDT-FTT:PC71BM/MoO3/Ag were also fabricated, with a maximum PCE of 7.71%. A tandem photovoltaic device comprising the inverted PTTBDT-FTT:PC71BM cell and a P3HT:ICBA-based cell as the top and bottom cell components, respectively, showed a maximum PCE of 8.66%. This work demonstrated that the newly developed PTTBDT-FTT polymer was very promising for applications in both single and tandem solar cells. Furthermore, this work highlighted the fact that an extended π-system in the electron-donor moiety in low bandgap polymers is crucial for improving polymer solar cells.