Spontaneous Exciton Dissociation at Organic Semiconductor Interfaces Facilitated by the Orientation of the Delocalized Electron–Hole Wavefunction

Abstract

In organic semiconductors, optical excitation does not necessarily produce free carriers. Very often, electron and hole are bound together to form an exciton. Releasing free carriers from the exciton is essential for the functioning of photovoltaics and optoelectronic devices, but it is a bottleneck process because of the high exciton binding energy. Inefficient exciton dissociation can limit the efficiency of organic photovoltaics. Here, nanoscale features that can allow the free carrier generation to occur spontaneously despite being an energy uphill process are determined. Specifically, by comparing the dissociation dynamics of the charge transfer (CT) exciton at two donor–acceptor interfaces, it is found that the relative orientation of the electron and hole wavefunction within a CT exciton plays an important role in determining whether the CT exciton will decompose into the higher energy free electron–hole pair or relax to the lower energy tightly‐bound CT exciton. The concept of the entropic driving force is combined with the structural anisotropy of typical organic crystals to devise a framework that can describe how the orientation of the delocalized electronic wavefunction can be manipulated to favor the energy‐uphill spontaneous dissociation of CT excitons over the energy‐downhill CT exciton cooling.

Identifying the Electrostatic and Entropy‐Related Mechanisms for Charge‐Transfer Exciton Dissociation at Doped Organic Heterojunctions

“…where the first term represents for the change in Coulomb binding energy, and the second term represents for the entropy driving force for CT exciton dissociation. [27][28][29][30] Here, W represents for the electronic degeneracy, that is, the molecular states available to accommodate electrons and holes (DOS FL ); ε 0 and ε r are the vacuum permittivity and the relative dielectric constant respectively; and r is the initial electron-hole distance of the CT exciton. For doped heterojunction, we estimate the local carrier density to be ≈5 × 10 16 cm −3 using simple parallel capacitor model (with ΔE = 80 mV as indicated in Figure 1e,f).…”

“…Larger entropy means more states for carriers to stay, which reduces the free energy of CT-CS transition according to Gibbs equation. [27][28][29][30] Previous studies have shown that all abovementioned properties are closely related to the local morphology of crystallinity, mixing ratio, and molecular orientation at the D/A heterojunction. [31][32][33][34] The complexity in local morphology regulation and characterization hinders the rational optimization of heterojunction properties in OPVs.…”

Research Progress on Triarylmethyl Radical-Based High-Efficiency OLED

“…Finally, the electrons and holes combine to generate excitons in EML under the action of coulomb static electricity [ 63 , 64 ]. As the excitons are excited, they need to release energy in some way and return to the ground state [ 65 , 66 ]. There are two main ways to release energy: one is in the form of light radiation, and the other is in the form of non-radiation [ 67 ].…”

How the Size and Density of Charge-Transfer Excitons Depend on Heterojunction’s Architecture

“…The dissociation mechanisms of interfacial CT excitons, having binding energies ∼10 times higher than ambient thermal energies 1,15 , are also currently in discussion. Several physical effects can impact the exciton dissociation dynamics, such as entropy 6,[17][18][19] , vibronic couplings [19][20][21][22] , energy offset 5,[22][23][24][25][26][27][28][29][30][31] , disorder [32][33][34][35][36] , hybridization between CT and localized states 7,22,[37][38][39] , and also electronic delocalization and polarization 14,34,[40][41][42][43][44][45][46][47][48][49][50] , among others.…”

“…A number of recent studies have addressed the microscopic structures of the DA interfaces and their impact on free-carrier-generation [5][6][7]34,38,46,47,[51][52][53][54] . While atomistic simulations can provide invaluable mechanistic and structure-property insights, the importance of going beyond minimal (bimolecular) interface models has been emphasized 34,47,52,54 .…”