Formation of the Donor–Acceptor Charge-Transfer Exciton and Its Contribution to Charge Photogeneration and Recombination in Small-Molecule Bulk Heterojunctions

Abstract

We report on the formation of charge-transfer (CT) excitons and their effect on photocurrent dynamics in composites with a fluorinated anthradithiophene (ADT-TES-F) donor (D) and acceptors (A) with (i) various LUMO energy offsets with respect to that of the donor (ΔLUMO) and (ii) several different side groups that modify spatial D/A separation at the D/A interface. Exciplexes and nonemissive CT excitons were formed in composites with ΔLUMO <0.6 and >0.6 eV, respectively. A competition between fast charge carrier photogeneration and CT exciton formation was observed, with outcomes depending on the applied electric field (E) and on the D/A separation. At low E fields, CT formation was dominant, and up to a factor of 2 increase in charge photogeneration efficiency due to CT exciton dissociation was observed in composites with large spatial D/A separation compared with that in pristine D films. At high E fields, fast charge carrier photogeneration was dominant in all composites, and no improvement in charge photogeneration efficiency with respect to that in pristine D films was observed. Dramatic changes in charge recombination dynamics were observed depending on the spatial D/A separation. These contributed to a factor of 5−10 improvement in continuous-wave photocurrents in composites with large spatial D/A separation as compared with those in pristine D films.

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.…”

Beyond Pentacenes: Synthesis and Properties of Higher Acenes

“…Very recently Anthony and co-workers reported the synthesis of the 1,2,3,4-tetrafluoro as well as the 1,2,3,4,9,10,11,12-octafluoro derivatives of 23e and the characterization of their charge transport properties [39]. The octafluoro compound was also investigated as an electron acceptor in organic bulk heterojunctions [40].…”

Unusually short excited state lifetimes of indenofluorene and fluorenofluorene derivatives result from a conical intersection

“…Numerous small-molecule systems have been developed toward this goal that employ extended pi-conjugated centers [5][6][7][8][9]. One such molecular class of interest is the p-quinodimethane-containing indenofluorenes and related molecules (e.g., 1-3, Figure 1), which have recently been realized through synthesis [10][11][12][13][14][15][16][17]. Intriguingly, these compounds have been found to be non-emissive and thus we wished to examine their photophysics.…”