Direct Observation of Entropy-Driven Electron-Hole Pair Separation at an Organic Semiconductor Interface

Abstract

How an electron-hole pair escapes the Coulomb potential at a donor/acceptor interface has been a key issue in organic photovoltaic research. Recent evidence suggests that long-distance charge separation can occur on ultrafast timescales, yet the underlying mechanism remains unclear. Here we use charge transfer excitons (CTEs) across an organic semiconductor/vacuum interface as a model and show that nascent hot CTEs can spontaneously climb up the Coulomb potential within 100 fs. This process is driven by entropic gain due to the rapid rise in density of states with increasing electron-hole separation. In contrast, the lowest CTE cannot delocalize, but undergoes self-trapping and recombination.Charge generation in organic photovoltaic (OPV) devices is contingent upon the dissociation of excitons into charge-separated states across donor/acceptor (D/A) interfaces, a process that can occur on femtosecond timescales with near unity quantum efficiency [1][2][3]. However, such rapid formation of charge-separated states appears contrary to the excitonic nature of the materials that comprise organic solar cells [4]. Given their low dielectric constants, it is not immediately obvious how the electron-hole pair is able to escape the poorly screened Coulomb potential that can give rise to charge transfer excitons (CTEs) with binding energies an order of magnitude higher than thermal energy at room temperature [5][6][7][8][9]. Recent experimental [10][11][12][13][14] and theoretical [15][16][17] studies suggest that electronic delocalization enables the electron-hole pair to escape the CTE trap and promotes long-range charge separation, in agreement with the Onsager model for ionization in solution [18]. Excess energy from the offset in energy levels at the donor/acceptor interface [10][11][12][13][14][15][16][17] or from the initial excitation photon [19,20] is believed to assist long-range charge separation. The electron-hole pair in the CTE trap can also dissociate with the help of an additional photoexcitation step [ 21 ]. Despite this progress, the exact mechanism of long-range electron-hole pair formation at donor/acceptor interfaces remains Phys. Rev. Lett. submitted. 2 poorly defined and there are also seemingly contradicting findings of photo-carrier generation from low energy CTEs that are supposedly trapped [22][23][24]. In the latter case, there is likely a potential energy gradient that counters the Coulomb potential [25][26][27][28]. One universally present driving force for photo-carrier generation, which may be partially responsible for the initial longrange charge separation or the subsequent escape from the CTE trap, may be the entropic gain with increasing electron-hole separation [1,29 ]. However, there has been little experiment evidence for this proposal.Here we provide the first direct time domain view of entropy-driven charge separation using the model system of CTEs at a molecular semiconductor/vacuum interface. In this system, a molecular semiconductor is the donor and the free-electron...

The role of structural fluctuations and environmental noise in the electron/hole separation kinetics at organic polymer bulk-heterojunction interfaces

“…The results presented here corroborate recent ultrafast experimental evidence suggesting that free polarons can form on ultrafast timescales (sub 100fs) and underscore the dynamical nature of the bulk-heterojunction interface. 4,7,8,17,22,24,29,38,39 …”

“…This boost in efficiency indicates that mobile charge carriers can be generated efficiently in well-optimized organic heterostructures; however, the underlying mechanism for converting highly-bound Frenkel excitons into mobile and asymptotically free photocarriers remains elusive in spite of vigorous, multidisciplinary research activity. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Much of the difficulty in developing a comprehensive picture stems from our lack of detailed understanding of the electronic energy states at the interface between donor and acceptor materials and how these states are influenced by small-but significant-fluctuations in the molecular structure.…”

Charge Generation and Transport Phenomena in Disordered Organic Semiconductors and Photovoltaic Diodes

“…Indeed, there are several processes that could lower the Coulomb barrier for dissociation, for example entropic effects [68,98,99,100]. However, the observed differences in the geminate recombination losses upon a constant reduction of the electric field (~3.5x), which also depend on the slower carrier mobility of the system, render this argument incomplete or questionable.…”

“…Nevertheless, a quantitative model to describe Ω( ) for BHJs with disordered interfaces, varying domain sizes and composition ratios remains elusive. Moreover, direct experimental evidence of the entropy gain during separation is challenging, although first attempts were undertaken [99]. The effect of the entropy in devices with varying blend ratio compositions is further discussed in Chapter V.…”

“…Recent studies suggest that entropy can contribute to charge dissociation due to a vast expansion of the number of available states as the carriers diffuse away from the interface [68,98,99,100]. The entropy is a part of the Gibbs free energy ( ) that defines the free energy barrier (Δ B ) and the dissociation rate constant via the Arrhenius equation.…”