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 F¨orster 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 F¨orster 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.

Figure 1

Overview of the photoconversion mechanism in organic solar cells. Processes that involve FRET are indicated in green and recombination pathways in red.

Figure 2

Exciton hopping in the intrinsic DOS which is approximated by a Gaussian distribution.

Figure 3

(a) L as a function of R₀ for σ = 0 eV (squares), σ = 0.05 eV (diamonds) and σ = 0.09 eV (triangles). Reproduced with permission from [24]. Copyright 2012 by The American Institute of Physics; (b) Diffusion length (L_D) as a function of σ. Reprinted with permission from [62]. Copyright 2009 by The American Physical Society.

Figure 4

A typical sample for (a) single material PL measurements and (b) OPV devices.

Figure 5

Left: exciton transport through charge transfer. Right: exciton transport through energy transfer.

Figure 6

Exciton transport within a single semiconductor (ET1, ET2, ET3) followed by inter-species donor-acceptor energy transfer (ET4).

Figure 7

Energy transfer efficiency as a function of spacer thickness for a PFOBT–P3HT system. Reprinted with permission from [101]. Copyright 2008 by The American Physical Society.

Figure 8

The average energy of an exciton at dissociation in a p-type molecule when (left) exciton transport is modelled using a simple random walk and (right) energy relaxation is taken into account. The energy levels of an n-type molecule are shown in green. The LUMO offsets are indicated for both cases.

Figure 9

(a) Potential energy parabolas of two harmonic oscillators separated by a distance r; (b) normalised Marcus transfer rate as a function of the free energy difference.

Figure 10

Energy levels associated with donor and acceptor materials.

Figure 11

Cascade of energy transfer (k_EN) and electron transfer (k_ET ) leading to an unusually large CT state lifetime of 0.38 s. Reprinted with permission from [32]. Copyright 2001 by The American Chemical Society.