Ultrafast charge separation and nongeminate electron–hole recombination in organic photovoltaics

Abstract

The mechanism of electron-hole separation in organic solar cells is currently hotly debated. Recent experimental work suggests that these charges can separate on extremely short timescales (<100 fs). This can be understood in terms of delocalised transport within fullerene aggregates, which is thought to emerge on short timescales before vibronic relaxation induces polaron formation. However, in the optimal heterojunction morphology, electrons and holes will often re-encounter each other before reaching the electrodes. If such charges trap and cannot separate, then device efficiency will suffer. Here we extend the theory of ultrafast charge separation to incorporate polaron formation, and find that the same delocalised transport used to explain ultrafast charge separation can account for the suppression of nongeminate recombination in the best devices.The best solution-processed organic photovoltaic cells (OPVs) now exhibit efficiencies exceeding 9%1 . Devices consist of a nanostructured "heterojunction" morphology of intermixed electron donor and acceptor semiconductors 2 . Usually fullerene-derivatives are used as the electron acceptor. Photons are absorbed within the device generating tightly bound excitons. These excitons diffuse to interfaces between donor and acceptor semiconductor, where electron and hole can separate into free charges. However in order to separate, charges must overcome their mutual Coulomb attraction, which is an order of magnitude greater than thermal energies at room temperature 3 . Experimentally, charge separation has been observed on ultrafast timescales (<100 fs) 4-7 . This observation is incompatible with conventional theories of charge transport in organic media, and new proposals have emerged [8][9][10][11] . It has been proposed that ultrafast charge transfer is sustained by spatially coherent delocalised states, which arise on short timescales before molecular vibrations can respond to the presence of charges 4,6,12 . It is assumed that more localised polarons will form on longer timescales, although Bakulin et al. found that delocalised states could be repopulated at late a Theory of Condensed Matter, Cavendish Laboratory, University of Cambridge, times by an infrared pulse 4 . Electrons and holes remaining in close proximity at long times are thought to trap into bound charge transfer (CT) states at the interface, while separated charges are free to generate a photocurrent.Electron hole pairs generate a dipolar electric field as they separate, this field can be observed as a Stark shift in the optical spectra of neighboring molecules. Using this signature, Gélinas et al. directly observed the separation of charges on femtosecond timescales 6 . They found that electron and hole separated by a few nanometres within just 40 fs, but that this was only observed in devices containing nanoscale aggregates of the fullerene-derivative electron acceptor PC 71 BM. Alongside this experiment, we presented a simple phenomenological model of ultrafast charge separation through delocalised...

Quantum dynamics of a vibronically coupled linear chain using a surrogate Hamiltonian approach

“…Such a system represents a good minimal model [90][91][92][93] for the process of charge generation/recombination in OPV cells. These states can be thought as those of an electron acceptor under the influence of a positive hole in the electron donor separated by a distance D from the nearest electron.…”

Charge Dynamics in Organic Photovoltaic Materials: Interplay between Quantum Diffusion and Quantum Relaxation

“…Permittivity of a dielectric medium is close to the reported experimental value 29,30 and τ is an intermediate value encountered in computational studies. 7,14,15,23,27,31,32 Charge is initially localized on the second nearest site to the interface (|ψ(t=0) = |2 ), which corresponds to the electron-hole separation of 2.6 nm.…”

“…[13][14][15][16][17][18][19][20][21][22] For example, Tamura and Burghardt 13 attributed fast CS to the two main factors, (i) electron delocalization within the fullerene condensate and (ii) the role of vibronically hot CT states, by carrying out a combined approach of electronic structure calculations and quantum dynamical analysis. Sun To obtain a complete description of charge dynamics it is therefore important to model the charge dynamics of both processes, "low-energy CT to CS states" and "CS to low-energy CT states", which would occur on two different timescales.…”

Impact of quantized vibrations on the efficiency of interfacial charge separation in photovoltaic devices

“…The electron dynamics considered here concerns the fast electron unbinding from the Coulomb well, occurring on the 10 to 100 femtosecond time scale [15,16]. We thus neglect the impact of weakly coupled low frequency phonon modes [10,11,13,28], as these will lead to final equilibration at longer relaxation times than relevant for the exciton dissociation process. Fourth, we take a pure Coulomb law for the electron-hole binding potential, but any other type of interaction should not drastically affect our general results (see Refs.…”