Small-Molecule Bulk Heterojunctions: Distinguishing Between Effects of Energy Offsets and Molecular Packing on Optoelectronic Properties

Abstract

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.

Photophysics of organic semiconductors: from ensemble to the single-molecule level

“…20 Similar trends were observed in ADT-TES-F/Pn-R-F8 blends, in which the emission efficiency and lifetime of exciplex significantly depended on the size of the acceptor's R-group, so that larger R groups led to a less emissive and shorter lived exciplex. 2,5 If the ADT-TES-F donor and Pn-R-F8 acceptor molecules are separated by a neutral spacer such as PMMA, FRET, with the FRET radius of 4.2-4.5 nm, depending on the side group R, dominates over the exciplex formation. FRET between ADT-TES-F donor and Pn-NODIPS-F8 acceptor molecules is illustrated in Fig.5 which shows fluorescence spectra from ADT-TES-F donor-only, Pn-NODIPS-F8 acceptor-only, and ADT-TES-F/Pn-NODIPS-F8 D/A pair in PMMA under a 490 nm excitation.…”

Charge carrier dynamics in small-molecule- and polymer-based donor-acceptor blends

“…In the case of the ADT-TES-F/Pn-TIPS-F8 blend, strong PL emission due to the ADT-TES-F/Pn-TIPS-F8 CT state (exciplex) was observed, with a peak at ~ 710 nm (1.75 eV) (Fig. 2(b)) which closely matched the energy difference between the LUMO of Pn-TIPS-F8 and HOMO of ADT-TES-F. 9,10,12 No emissive CT states were observed in any other D/A blend studied. The fast decay component has been previously attributed to charge trapping and various recombination processes such as bimolecular and trap-assisted recombination; 15 for example, a faster decay in ADT-TES-F/PCBM blends as compared to pristine ADT-TES-F films was determined to be due to enhanced bimolecular recombination in these blends.…”

“…8 Addition of the Pn-TIPS-F8 acceptor to the ADT-TES-F donor has been shown to result in the formation of an emissive charge transfer state (exciplex), which was detrimental for charge photogeneration. 9 Photoinduced interactions between P3HT and Pn-TIPS-F8 and their effect on photoconductivity are not known; also not known is how charge carrier dynamics in P3HT-based D/A blends compare to those in ADT-TES-F-based D/A blends utilizing same acceptors, which is investigated here.…”

“…10 The D/A film samples were prepared by spin-casting 10 µL of the solution at 3000 rpm for 30 seconds on glass substrates with 10 pairs of Cr/Au (5 nm/50 nm thick) interdigitated gold electrodes with a 25 μm gap. Just before spin-casting, each substrate was treated with pentafluorobenzenethiol (PFBT), which was found to form a selfassembled monolayer on the substrate and facilitate crystallization in fluorinated SMs such as ADT-TES-F. 9 No annealing procedures were applied to any of the samples. Such film preparation method resulted in polycrystalline ADT-TES-F films and blends of ADT-TES-F with PCBM or Pn-TIPS-F8, as confirmed by the x-ray diffraction (XRD) ( Fig.…”

“…Time resolution was ~ 1.0 ns, limited by the laser pulse width and the DSO response time. 8,9,12,13 All the measurements were carried out in air at room temperature.…”