Abstract
A long standing question in organic electronics concerns the effects of molecular orientation at donor/acceptor heterojunctions. Given a well-controlled donor/acceptor bilayer system, we uncover the genuine effects of molecular orientation on charge generation and recombination. These effects are studied through the point of view of photovoltaics—however, the results have important implications on the operation of all optoelectronic devices with donor/acceptor interfaces, such as light emitting diodes and photodetectors. Our findings can be summarized by two points. First, devices with donor molecules face-on to the acceptor interface have a higher charge transfer state energy and less non-radiative recombination, resulting in larger open-circuit voltages and higher radiative efficiencies. Second, devices with donor molecules edge-on to the acceptor interface are more efficient at charge generation, attributed to smaller electronic coupling between the charge transfer states and the ground state, and lower activation energy for charge generation.
Highlights
A long standing question in organic electronics concerns the effects of molecular orientation at donor/acceptor heterojunctions
The two orientations have distinctly different structures seen by high-resolution transmission electron microscope (HR-TEM) images
Using a combination of Monte Carlo (MC) and molecular dynamics (MD) simulations, we find that p-SIDT(FBTTh2)[2] has a unique unit cell, shown in Supplementary Fig. 9, true for both molecular orientations
Summary
A long standing question in organic electronics concerns the effects of molecular orientation at donor/acceptor heterojunctions. Most studies have found that within the same material system, face-on solar cells have a superior power conversion efficiency (PCE) when compared to the edge-on orientation[7, 15] This has been attributed primarily to changes in the donor ionization potential (IP) (or, to a first approximation, highest occupied molecular orbital energy level), which directly affects the open circuit voltage (VOC)[7,8,9, 11, 15,16,17], but has been explained by differences in recombination rates[1, 7, 12, 15]. The driving force for charge generation has remained a disputed topic in the literature, with researchers quoting the need for energetic offsets[33, 34], hot charges[35, 36], delocalization[37, 38], low reorganization energies[39], electric fields[40, 41], energetic cascades and disorder[42, 43], and entropy[29, 44], to achieve efficient charge generation
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