An Effective Two-Orbital Quantum Chemical Model for Organic Photovoltaic Materials

The role of entropy gains in the exciton separation in organic solar cells

In organic solar cell (OSC), the lower dielectric constant of organic semiconductor material induces a strong Coulomb attraction between electron-hole pairs, which leads to a low exciton separation efficiency, especially the charge transfer (CT) state. The CT state formed at the electron-donor (D) and electron-acceptor (A) interface is regarded as an unfavorable property of organic photovoltaic devices. Since the OSC works in a nonzero temperature condition, the entropy effect would be one of the main reasons to overcome the Coulomb energy barrier and must be taken into account. In this review, we review the present understanding of the entropy-driven charge separation and describe how factors such as the dimensionality of the organic semiconductor, energy disorder effect, the morphology of the active layer, and the nonequilibrium effect affect the entropy contribution in compensating the Coulomb dissociation barrier for CT exciton separation and charge generation process. We focus on the investigation of the entropy effect on exciton dissociation mechanism from both theoretical and experimental aspects, which provides pathways for understanding the underlying mechanisms of exciton separation and further enhancing the efficiency of OSCs. This article is protected by copyright. All rights reserved.

The impact of chemical structure and molecular packing on the electronic polarisation of fullerene arrays

Electronic polarisation contributes to the electronic landscape as seen by separating charges in organic materials. The nature of electronic polarisation depends on the polarisability, density, and arrangement of polarisable molecules. In this paper, we introduce a microscopic, coarse-grained model in which we treat each molecule as a polarisable site, and use an array of such polarisable dipoles to calculate the electric field and associated energy of any arrangement of charges in the medium. The model incorporates chemical structure via the molecular polarisability and molecular packing patterns via the structure of the array. We use this model to calculate energies of charge pairs undergoing separation in finite fullerene lattices of different chemical and crystal structures. The effective dielectric constants that we estimate from this approach are in good quantitative agreement with those measured experimentally in C₆₀ and phenyl-C₆₁-butyric acid methyl ester (PCBM) films, but we find significant differences in dielectric constant depending on packing and on direction of separation, which we rationalise in terms of density of polarisable fullerene cages in regions of high field. In general, we find lattices containing molecules of more isotropic polarisability tensors exhibit higher dielectric constants. By exploring several model systems we conclude that differences in molecular polarisability (and therefore, chemical structure) appear to be less important than differences in molecular packing and separation direction in determining the energetic landscape for charge separation. We note that the results are relevant for finite lattices, but not necessarily for infinite systems. We propose that the model could be used to design molecular systems for effective electronic screening.

Ultrafast decoherence dynamics govern photocarrier generation efficiencies in polymer solar cells

All-organic-based photovoltaic solar cells have attracted considerable attention because of their low-cost processing and short energy payback time. In such systems the primary dissociation of an optical excitation into a pair of photocarriers has been recently shown to be extremely rapid and efficient, but the physical reason for this remains unclear. Here, two-dimensional photocurrent excitation spectroscopy, a novel non-linear optical spectroscopy, is used to probe the ultrafast coherent decay of photoexcitations into charge-producing states in a polymer:fullerene based solar cell. The two-dimensional photocurrent spectra are interpreted by introducing a theoretical model for the description of the coupling of the electronic states of the system to an external environment and to the applied laser fields. The experimental data show no cross-peaks in the twodimensional photocurrent spectra, as predicted by the model for coherence times between the exciton and the photocurrent producing states of 20 fs or less.

Charge Separation and Recombination at Polymer-Fullerene Heterojunctions: Delocalization and Hybridization Effects

We address charge separation and recombination in polymer/fullerene solar cells with a multiscale modeling built from accurate atomistic inputs and accounting for disorder, interface electrostatics and genuine quantum effects on equal footings. Our results show that bound localized charge transfer states at the interface coexist with a large majority of thermally accessible delocalized space-separated states that can be also reached by direct photoexcitation, thanks to their strong hybridization with singlet polymer excitons. These findings reconcile the recent experimental reports of ultrafast exciton separation ("hot" process) with the evidence that high quantum yields do not require excess electronic or vibrational energy ("cold" process), and show that delocalization, by shifting the density of charge transfer states toward larger effective electron-hole radii, may reduce energy losses through charge recombination.

The effect of donor content on the efficiency of P3HT:PCBM bilayers: optical and photocurrent spectral data analyses

A numerical analysis of optical absorption and photocurrent data reveals extensive interdiffusion in P3HT:PCBM bilayer devices.

Models of charge pair generation in organic solar cells

A critical perspective on modelling of charge generation in organic photovoltaics, focussing on interfacial electronic states, electrostatics, and dynamic processes.