Organic Solar Cells: Understanding the Role of Förster Resonance Energy Transfer

Abstract

Organic solar cells have the potential to become a low-cost sustainable energy source. Understanding the photoconversion mechanism is key to the design of efficient organic solar cells. In this review, we discuss the processes involved in the photo-electron conversion mechanism, which may be subdivided into exciton harvesting, exciton transport, exciton dissociation, charge transport and extraction stages. In particular, we focus on the role of energy transfer as described by Förster resonance energy transfer (FRET) theory in the photoconversion mechanism. FRET plays a major role in exciton transport, harvesting and dissociation. The spectral absorption range of organic solar cells may be extended using sensitizers that efficiently transfer absorbed energy to the photoactive materials. The limitations of Förster theory to accurately calculate energy transfer rates are discussed. Energy transfer is the first step of an efficient two-step exciton dissociation process and may also be used to preferentially transport excitons to the heterointerface, where efficient exciton dissociation may occur. However, FRET also competes with charge transfer at the heterointerface turning it in a potential loss mechanism. An energy cascade comprising both energy transfer and charge transfer may aid in separating charges and is briefly discussed. Considering the extent to which the photo-electron conversion efficiency is governed by energy transfer, optimisation of this process offers the prospect of improved organic photovoltaic performance and thus aids in realising the potential of organic solar cells.

Organic Photovoltaics: Elucidating the Ultra‐Fast Exciton Dissociation Mechanism in Disordered Materials

“…After the first exciton hop, at time step > 1 ps, the exciton is localized (Figure 1 A) on one donor site (d = 1 nm) and then diffuses to neighboring donor sites by Fçrster energy transfer. [33] The localized (Frenkel) exciton proceeds to diffuse via Fçrster energy transfer until it either reaches ). [15-17, 21, 22] Fraction of excitons dissociated is represented by the rise of cation signal as measured experimentally in the donor materials.…”

Diffusion Length and Langevin Recombination of Singlet and Triplet Excitons in Organic Heterojunction Solar Cells

“…By using the diffusion lengths calculated from Equations (6) and (7), we have plotted the fraction of excitons that reach the D-A interface from Equation (16) as a function of d in Figure 2 for some organic semiconductors listed in Table 1. According to Figure 2, the comparable diffusion lengths allow a fairly good fraction between 0.99 to 0.64 of both singlet and triplet excitons to reach the D-A interface for d values of between 10 and 15 nm.…”

“…In Dexter's approach, the exciton transfer rate decays exponentially with distance. [6] The Fçrster and Dexter transfer rates are given, respectively, as Equation (3): [2] …”

Exciton delocalization incorporated drift-diffusion model for bulk-heterojunction organic solar cells

“…As described in the processes step II in Figure 1(a), after being photogenerated, the excitons will diffuse from the created locations to the donor/acceptor interfaces. 14,15 After overcoming the Coulomb attraction at the interfaces, the bounded electron-hole pairs will be dissociated, and the free carriers (electrons in the acceptor phase and holes in the donor phase) will be collected by electrodes. Regarding the excitons in the diffusion and dissociation processes, only singlet excitons are considered, because triplet excitons have no contribution in the carrier generation process due to the difficulty in their dissociation.…”