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Chain Length Dependence of the Photovoltaic Properties of Monodisperse Donor–Acceptor Oligomers as Model Compounds of Polydisperse Low Band Gap Polymers
Author(s) -
Zhou Cheng,
Liang Yamin,
Liu Feng,
Sun Chen,
Huang Xuelong,
Xie Zengqi,
Huang Fei,
Roncali Jean,
Russell Thomas P.,
Cao Yong
Publication year - 2014
Publication title -
advanced functional materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 6.069
H-Index - 322
eISSN - 1616-3028
pISSN - 1616-301X
DOI - 10.1002/adfm.201401945
Subject(s) - materials science , dispersity , stacking , polymer , homo/lumo , photoluminescence , conjugated system , polymer solar cell , absorption (acoustics) , band gap , chain (unit) , side chain , absorption edge , energy conversion efficiency , chemical physics , polymer chemistry , optoelectronics , organic chemistry , molecule , composite material , chemistry , physics , astronomy
Well‐defined conjugated oligomers ( Sn ) containing from 1 to 8 units of a tricyclic building block involving a dioctyloxybenzothiadiazole unit with two thienyl side rings ( S1 ) are synthesized by a bottom‐up approach. UV–Vis absorption data of solutions show that chain extension produces a narrowing of the HOMO–LUMO gap (Δ E ) to values slightly smaller than that of the parent polymer ( P1 ). Plots of Δ E and of the band gap of films ( E g ) versus the reciprocal chain length show that Δ E and E g converge towards a limit corresponding to an effective conjugation length (ECL) of 7–8 S1 units. UV–Vis absorption and photoluminescence data of solutions and solid films show that chain extension enhances the propensity to inter‐chain aggregation. This conclusion is confirmed by GIXD analyses which reveal that the edge‐on orientation of short‐chain systems evolves toward a face‐on orientation as chain length increases while the π‐stacking distance decreases beyond 7 units. The results obtained on solution‐processed BHJ solar cells show a progressive improvement of power conversion efficiency (PCE) with chain extension; however, the convergence limit of PCE remains inferior to that obtained with the polymer. These results are discussed with regard to the role of mono/polydispersity and chain aggregation.