Our results provide a simple relationship involving the driving force and binding energy of CT state to maximize charge generation in non-fullerene organic solar cells.

No abstract

The dynamics of photoinduced charge generation is studied for donor−acceptor (D−A) organic interfaces, with focus on the interplay of quantum dynamics, decoherence effects, and recombination. A coarse-grained molecular envelope function model is developed to enable the investigation of large scale D−A heterojunctions, taking into account morphology and molecular orientation as well as the underlying quantum nature of the system. Simulations show that, upon photoexcitation, Frenkel excitons delocalize over several molecules in <300 fs. At the interface, they dissociate without dwelling in intermediate charge transfer states, evincing that exciton motion and dissociation cannot be describe by point particle models. Moreover, as decoherence suppresses the excitonic quantum coherence length, it also decreases the geminate recombination rate. Although ultrafast coherent charge separation is more efficient at early times and, particularly, for excitons created at the interface, diffusion becomes important for excitons created far away from the D−A interface. In this case, decoherence provides a slower but steadier diffusion migration that protects the exciton from geminate recombination. We discuss the balance between charge dissociation and transport in OPV devices and photosynthesis.

Intermolecular electron‐transfer reactions are key processes in physics, chemistry, and biology. The electron‐transfer rates depend primarily on the system reorganization energy, that is, the energetic cost to rearrange each reactant and its surrounding environment when a charge is transferred. Despite the evident impact of electron‐transfer reactions on charge‐carrier hopping, well‐controlled electronic transport measurements using monolithically integrated electrochemical devices have not successfully measured the reorganization energies to this date. Here, it is shown that self‐rolling nanomembrane devices with strain‐engineered mechanical properties, on‐a‐chip monolithic integration, and multi‐environment operation features can overcome this challenge. The ongoing advances in nanomembrane‐origami technology allow to manufacture the nCap, a nanocapacitor platform, to perform molecular‐level charge transport characterization. Thereby, employing nCap, the copper‐phthalocyanine (CuPc) reorganization energy is probed, ≈0.93 eV, from temperature‐dependent measurements of CuPc nanometer‐thick films. Supporting the experimental findings, density functional theory calculations provide the atomistic picture of the measured CuPc charge‐transfer reaction. The experimental strategy demonstrated here is a consistent route towards determining the reorganization energy of a system formed by molecules monolithically integrated into electrochemical nanodevices.

One strategy to improve the photovoltaic properties of nonfullerene acceptors (NFAs), employed in state-of-art organic solar cells, is the rational fluorination or chlorination of these molecules. Although this modification improves important acceptor properties, little is known about the effects on the triplet states. Here, we combine the polarizable continuum model with an optimally tuned range-separated hybrid functional to investigate this issue. We find that fluorination or chlorination of NFAs decreases the degree of the highest occupied molecular orbital− lowest unoccupied molecular orbital (HOMO−LUMO) overlap along these molecules. Consequently, the energy gap between T 1 and S 1 states, ΔE ST = E S1 − E T1 , also decreases. This effect reduces the binding energy of triplet excitons, which favors their dissociation into free charges. Furthermore, the reduction of ΔE ST can contribute to mitigating the losses produced by the nonradiative deactivation of the T 1 excitons. Interestingly, although Cl has a lower electronegativity than F, chlorination is more effective to reduce ΔE ST . Since the chlorination of NFAs is easier than fluorination, Cl substitution can be a useful approach to enhance solar energy harvesting using triplet excitons.