title: Effect of PTB7 Properties on the Performance of PTB7:PC₇₁BM Solar Cells.
abstract: The effect of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexy)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) properties on the optical properties, charge transport and photovoltaic performance of PTB7:[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) blend films is investigated. We found that the variations in the molecular weight (Mw) and polydispersity index (PDI) of PTB7 mainly affect the phase separation and charge transport (hole mobility) in the blend films. The optical properties are also affected, but the increase in the extinction coefficient does not necessarily imply increased power conversion efficiency. The obtained power conversion efficiency for optimized thickness varied from 4.8% to 7.8% depending on the properties of PTB7. The wide range of obtained power conversion efficiencies illustrates the importance of optimizing the Mw and PDI to optimize bulk heterojunction morphology and achieve high performance solar cells.
authors.affiliation: †Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong Island, Hong Kong.
authors.forename: Chap Hang
authors.initials: CH
authors.lastname: To
authors.affiliation: ‡Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong.
authors.forename: Annie
authors.initials: A
authors.lastname: Ng
authors.affiliation: †Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong Island, Hong Kong.
authors.forename: Qi
authors.initials: Q
authors.lastname: Dong
authors.affiliation: †Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong Island, Hong Kong.
authors.forename: Aleksandra B
authors.initials: AB
authors.lastname: Djurišić
authors.affiliation: §Center of Super Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong.
authors.forename: Juan Antonio
authors.initials: JA
authors.lastname: Zapien
authors.affiliation: ⊥Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong Island, Hong Kong.
authors.forename: Wai Kin
authors.initials: WK
authors.lastname: Chan
authors.affiliation: ‡Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong.
authors.forename: Charles
authors.initials: C
authors.lastname: Surya
keywords: polymer solar cells; spectroscopic ellipsometry