We present photophysical properties of functionalized anthradithiophene (ADT) and pentacene (Pn) derivatives, as well as charge and energy transfer properties of donor-acceptor (D/A) pairs of these derivatives. All molecules studied were fluorescent and photostable enough to be imaged on the single-molecule level in a variety of polymeric and in a functionalized benzothiophene (BTBTB) crystalline host using room-temperature wide- field epifluorescence microscopy. Flexibility of functionalization of both guest (ADT, Pn) and host (BTBTB or polymer) molecules can be used for systematic studies of nanoscale morphology and photophysics of D/A organic semiconductor bulk heterojunctions, as well as in applications relying on FRET, using single-molecule fluorescence microscopy.
We explored relationships between photophysical processes and solar cell characteristics in solution-processable bulk heterojunctions (BHJs), in particular: (1) polymer donor:fullerene acceptor:small-molecule (SM) nonfullerene acceptor, (2) polymer donor:SM donor:SM nonfullerene acceptor, and (3) SM donor:SM nonfullerene or fullerene acceptor. Addition of a nonfullerene SM acceptor to “efficient” polymer:fullerene BHJs led to a reduction in power conversion efficiency (PCE), mostly due to decreased charge photogeneration efficiency and increased disorder. By contrast, addition of an SM donor to “inefficient” polymer:SM nonfullerene acceptor BHJs led to a factor of two to three improvement in the PCE, due to improved charge photogeneration efficiency and transport. In most blends, exciplex formation was observed and correlated with a reduced short-circuit current (Jsc) without negatively impacting the open-circuit voltage (Voc). A factor of ∼5 higher PCE was observed in SM donor:fullerene acceptor BHJs as compared to SMBHJs with the same SM donor but nonfullerene acceptor, due to enhanced charge carrier photogeneration in the blend with fullerene. Our study revealed that the HOMO and LUMO energies of molecules comprising a blend are not reliable parameters for predicting Voc of the blend, and an understanding of the photophysics is necessary for interpreting solar cell characteristics and improving the molecular design of BHJs.
We performed numerical simulations of transient photocurrents in organic thin films, in conjunction with experiments. This enabled us to quantify the contribution of multiple charge generation pathways to charge carrier photogeneration, as well as extract parameters that characterize charge transport, in functionalized anthradithiophene (ADT-TES-F) films prepared using two different deposition methods: drop casting on an untreated substrate and spin casting on a pentauorobenzenethiol (PFBT)-treated substrate. These deposition methods yielded polycrystalline lms with considerably larger grain sizes in the case of the spin cast lm. In both drop cast and spin cast films, simulations revealed two competing charge photogeneration pathways: fast charge generation on a picosecond (ps) or sub-ps time scale with efficiencies below 10%, and slow charge generation, on the time scale of tens of nanoseconds, with efficiencies of 11-12% in drop cast and 50-60% in spin cast films, depending on the applied electric field. The total charge photogeneration efficiency in the spin cast sample was 59-67% compared to 14-20% in the drop cast sample, whereas the remaining 33-41% and 80-86%, respectively, of the absorbed photon density did not contribute to charge carrier generation on these time scales. The spin cast film also exhibited higher hole mobilities, lower trap densities, shallower traps, and lower charge carrier recombination, as compared to the drop cast lm. As a result, the spin cast lm exhibited higher photocurrents despite a considerably lower lm thickness (and thus reduced optical absorption and cross section of the current flow).
We present a study of optical, photoluminescent (PL), and photoconductive properties of small-molecule D/A bulk heterojunctions of functionalized fluorinated anthradithiophene (ADT-R-F) and pentacene (Pn-R-F8) derivatives. We chose one of the ADT derivatives, ADT-TES-F, which exhibits a 2D “brick-work” .π-stacking, as the donor, and ADT-TIPS-F (2D “brick-work”), ADT-TSBS-F (1D “sandwich-herringbone”), Pn-TIPS-F8 (2D “brick-work”), or Pn-TCHS-F8 (1D “sandwich-herringbone”), as acceptors. We measured PL and photoconductivity at time scales from sub-nanoseconds to many seconds after photoexcitation, at various acceptor concentrations, under various experimental conditions. The choice of acceptors enabled us to distinguish between effects of the LUMO energy offsets between the donor and acceptor molecules and those of the molecular packing in the acceptor domains on exciton and charge carrier dynamics.