Extracting Electrons from Delocalized Excitons by Flattening the Energetic Pathway for Charge Separation

Abstract

At organic donor−acceptor (D−A) interfaces, electron and hole are bound together to form charge transfer (CT) excitons. The electron and hole wave functions in these CT excitons can spatially delocalize. The electron delocalization opens up possibilities of extracting free charges from bound excitons by manipulating the potential energy landscape on the nanoscale. Using a prototype trilayer structure that has a cascade band structure, we show that the yield of charge separation can be doubled as compared to the bilayer counterpart when the thickness of the intermediate layer is around 3 nm. This thickness coincides with the electron delocalization size of CT excitons typically found in these organic films. Tight-binding calculation for the CT states in the trilayer structure further demonstrates that electron delocalization, together with the energy level cascade, can effectively flatten the energetic pathway for charge separation. Hence, it is possible to add nanometer-thick layers between the donor and the acceptor to significantly enhance the charge separation yield.

Entropy-Driven Charge Separation: A Potential Explanation for the Low Energy Loss Found in OPVs with Non-fullerene Acceptors

“…To determine the effect of the crystal anisotropy and the orientation/shape of the delocalized electron and hole wave functions on the DOS function, we use a quantum mechanical tight-binding Hamiltonian that accounts for the anisotropy in the nearest neighbor electronic coupling. 62,65,70 Figure 4a,b shows the energies and the average electron− hole distances of all eigenstates (each dot represents an eigenstate) for F 8 ZnPc/ZnPc interfaces with the two different molecular orientations. Figure 4c shows the DOS function calculated from the distribution in Figure 4a,b.…”

“…The entropy change (Δ S ) for an enthalpy change of (Δ E ) can be related to the DOS by Δ S false( E false) = S false( E + normalΔ E false) − S false( E false) = k B ⁡ ln true( normalDOS ( E + Δ E ) normalDOS ( E ) true) where k B is the Boltzmann constant. To determine the effect of the crystal anisotropy and the orientation/shape of the delocalized electron and hole wave functions on the DOS function, we use a quantum mechanical tight-binding Hamiltonian that accounts for the anisotropy in the nearest neighbor electronic coupling. ,, …”

Recent Progress in 2D Inorganic/Organic Charge Transfer Heterojunction Photodetectors

“…For example, to explore the multilayer CT like the D-A-A, P-I-N organic-inorganic heterojunction and even the tandem active layer. [38,195] These initiatives flatten the energetic path and promote the charge separation efficiency.…”