Abstract
Efficient charge generation via exciton dissociation in organic bulk heterojunctions necessitates donor–acceptor interfaces, e.g., between a conjugated polymer and a fullerene derivative. Furthermore, aggregation and corresponding structural order of polymer and fullerene domains result in energetic relaxations of molecular energy levels toward smaller energy gaps as compared to the situation for amorphous phases existing in homogeneously intermixed polymer:fullerene blends. Here it is shown that these molecular energy level shifts are reflected in interfacial charge transfer (CT) transitions and depending on the existence of disordered or ordered interfacial domains. It can be done so by systematically controlling the order at the donor–acceptor interface via ternary blending of semicrystalline and amorphous model polymers with a fullerene acceptor. These variations in interfacial domain order are probed with luminescence spectroscopy, yielding various transition energies due to activation of different recombination channels at the interface. Finally, it is shown that via this analysis the energy landscape at the organic heterojunction interface can be obtained.
Highlights
As the low dielectric constant limits charge generation in organic semiconductors, organic solar cells require an additional process for exciton dissociation into free charge carriers, which is conveniently realized at an interface between
The semiconductor exhibiting higher molecular energy levels— the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO)—is called the donor, whereas the other is called the acceptor. This socalled type II heterojunction enables efficient electron transfer from the donor to the acceptor, which may be accompanied by the formation of charge transfer (CT) states, where the electron is located on the acceptor LUMO and the hole on the donor HOMO
We have demonstrated the existence of several CT transitions within one single organic semiconductor bulk heterojunction
Summary
By applying one more Voigt profile, i.e., allowing an additional emission peak, to the fitting procedure, good fits could be simultaneously obtained for all blend ratios between the semicrystalline and the amorphous polymer. The from electroluminescence obtained CT peak positions consistent for all blend compositions are around CTAEL ≈ 1.51 eV, CTBEL ≈ 1.39 eV, CTCEL ≈ 1.31 eV, and CTDEL ≈ 1.23 eV. Since the CT2EL transition is only probable under presence of semicrystalline polymer,[33] the lower in energy CTCEL peak, significantly present in blend ratios containing less than 50% of the amorphous polymer (AnE-PVba), corresponds to it.
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