We examined sub-nanosecond timescale charge carrier dynamics in crystalline films of a functionalized anthradithiophene (ADT) donor (D) with three different acceptor (A) molecules. A four-fold enhancement in ultrafast charge carrier separation efficiency was observed in D/A blends with a fullerene acceptor added at 7-10 wt. % concentrations, whereas a gradual decrease in peak photocurrent amplitude with acceptor concentration was observed with functionalized pentacene and indenofluorene acceptors. The results were directly correlated with the ADT-tri(ethylsilyl)ethynyl-F donor crystallinity. In the bestperforming blends, the presence of crystalline acceptor domains was also established.
We report on the dependence of time-resolved
photoluminescence
(PL) and photocurrent in small-molecule bulk heterojunctions on the
donor–acceptor (D/A) LUMO offset, D/A separation, and acceptor
domain structure. We chose a high-performance functionalized fluorinated
anthradithiophene (ADT) derivative, ADT-TES-F, as the donor and two
other fluorinated ADT derivatives, ADT-R-F (where R is a variable
side group), as well as two functionalized fluorinated pentacene (Pn)
derivatives, Pn-R-F8, as acceptors. The choice of ADT and Pn acceptors
enabled us to separate the effects of the D/A LUMO offset, which was
approximately zero in the case of ADT acceptors and ∼0.55 eV
in the case of Pn acceptors, from those of molecular packing on the
optoelectronic properties. The acceptor side groups R were chosen
based on (i) packing motifs in the solid state and (ii) size, to achieve
different D/A separations at the D/A interface. Addition of an ADT-R-F
acceptor to the ADT-TES-F donor introduced disorder, which resulted
in increased PL emission, depending on the acceptor’s packing
motif, and in reduced photocurrents. In ADT-TES-F/Pn-R-F8 films, charge
transfer from ADT-TES-F to Pn-R-F8 was observed with an acceptor packing-dependent
formation of an exciplex, which dissociated under applied electric
field, contributing to charge carrier photogeneration. However, this
contribution was not sufficient to compensate for a photocurrent reduction
due to an increased disorder at Pn-R-F8 concentrations of 7 wt % and
above, regardless of the acceptor’s R-groups and packing motifs.
We present a comparative study of optical absorption, photoluminescence (PL), and photoconductivity in bulk heterojunctions comprising a high performance functionalized anthradithiophene (ADT) derivative or the benchmark polymer P3HT as donor and functionalized pentacene (Pn) derivative or PCBM as acceptor. Of all D/A blends studied, the ADT/PCBM blend exhibited the highest charge photogeneration efficiencies under 532 nm excitation, leading to the highest amplitudes of time-resolved and continuous wave (cw) photocurrents. At nanosecond time scales after photoexcitation, both ADT-TES-F-based blends and the P3HT/Pn-TIPS-F8 blend exhibited photocurrents which were higher by a factor of 2-10, depending on the blend, than that in the P3HT/PCBM blend. However, cw photocurrents showed a different trend, with the ADT-TES-F/PCBM blend exhibiting only a factor of ~2.5 higher photoresponse than that in the P3HT/PCBM blends, and the ADT-TES-F-and P3HT-based blends with Pn-TIPS-F8 showing a factor of ~1.5-2.5 lower photoresponse than that in the P3HT/PCBM blend, due to other contributions, such as that of charge trap-limited transport, to cw photoresponse.
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