Revelation of Interfacial Energetics in Organic Multiheterojunctions

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.

The Impact of Local Morphology on Organic Donor/Acceptor Charge Transfer States

“…In the case of a ternary blend of polymer:fullerene solar cells studied by Kästner et al, a controlled ratio of semicrystalline and amorphous polymers was used to vary the D–A interface from disordered–disordered, over disordered–ordered, up to ordered–ordered ( Figure 3 a). By including these intercalated phases (for example, amorphous PCBM and amorphous polymer molecularly intermixed with PCBM) and probing the interfaces via electroluminescence and photoluminescence, the authors concluded that six unique CT transitions were present in this system, with the highest energy CT state forming between the disordered–disordered interface and the lowest energy CT state occurring at the ordered–ordered interface …”

“…a) Schematics of several morphological configurations of donor/acceptor interfaces with varying degrees of order and their associated energy‐level diagrams of resulting CT states reflecting the expected changes in CT state energy. Reproduced with permission . Copyright 2016, WILEY‐VCH Verlag GmbH & Co. KGaA.…”

“…The authors explained that the Gaussian width within the Voigt profile could be a measure of disorder‐induced inhomogeneous broadening while the Lorentzian width accounts for lifetime‐dependent homogeneous broadening. And since the peaks were non‐Gaussian, the typical technique of identifying E CT as the intersection between the CT state EL and EQE spectra had to be discarded, taking the energies of the center of the emission peaks to identify E CT instead . As the realm of systems being investigated with CT states increases, these works suggest that scientists may need to entertain modifications to Marcus theory, or to consider altogether new models of charge transfer.…”

Controlling donor crystallinity and phase separation in bulk heterojunction solar cells by the introduction of orthogonal solvent additives

“…This can be, connected to an improved vertical alignment of the materials within the photoactive layer, as already indicated by the increased parallel resistance, leading to a more efficient injection and transport of holes and electrons from the anode and cathode, respectively, towards the materials interface at the heterojunction. It is furthermore interesting to note that the peak energy of the CTtransition seems to be reduced (from ~920 to ~ 945 nm) for the methanol modified blend, which confirms a higher order in the transport states [65], respectively the access to them during charge injection -and vice versa, charge extraction. Having pointed out a possible increase in charge carrier mobility, specifically for the holes as these are transported within the potentially better-ordered domains of the aggregated polymer, hole-only space-charge-limited-current (SCLC) devices were to be investigated for validation.…”

“…The EL spectra are depicted in Figure 6 right. Here, the ratio of the single transitions, especially the polymer emission (peaking at ~ 630 nm) and the charge transfer emission (peaking at 920-950 nm), is relevant [65]. A dramatic increase of the charge transfer (CT) emission (around 800 nm to 1100 nm) is observed, compared to the polymer emission (around 560 nm to 730 nm).…”