Free charge photogeneration in a single component high photovoltaic efficiency organic semiconductor

Abstract

Organic photovoltaics (OPVs) promise cheap and flexible solar energy. Whereas light generates free charges in silicon photovoltaics, excitons are normally formed in organic semiconductors due to their low dielectric constants, and require molecular heterojunctions to split into charges. Recent record efficiency OPVs utilise the small molecule, Y6, and its analogues, which – unlike previous organic semiconductors – have low band-gaps and high dielectric constants. We show that, in Y6 films, these factors lead to intrinsic free charge generation without a heterojunction. Intensity-dependent spectroscopy reveals that 60–90% of excitons form free charges at AM1.5 light intensity. Bimolecular recombination, and hole traps constrain single component Y6 photovoltaics to low efficiencies, but recombination is reduced by small quantities of donor. Quantum-chemical calculations reveal strong coupling between exciton and CT states, and an intermolecular polarisation pattern that drives exciton dissociation. Our results challenge how current OPVs operate, and renew the possibility of efficient single-component OPVs.

Surfactant Effects on Hydrogen Evolution by Small Molecule Non-Fullerene Acceptor Nanoparticles

“…7,10,34 This low EQE is likely due to the single-component nature of the NPs, in which there is a lower driving force for charge separation when compared to a donor:acceptor heterojunction. In Y6 films, most excited states are lost within ∼100 ps of excitation,, 24,27,35 indicating that the formation and persistence of free charges may be limited by exciton-exciton and charge recombination in the NP bulk. Y6 NPs have similarly short excited-state lifetimes, which are further discussed later in this article.…”

“…To understand the reason for such a significant difference in H 2 evolution between Y6 NPs stabilized by TEBS and SDS, we considered how Y6 NPs are able to evolve H 2 . Studies of Y6, both on its own and in donor:Y6 blends, have suggested that the unique packing of the material allows Y6 excitons to partially 24 or fully 26,27 separate into free charges within Y6 domains, as well as at donor-Y6 interfacial sites. During photocatalysis, the generation of separated charges should allow the material to form surface deposits of Pt, through reduction of Pt cations by free electrons at the surface of the NP.…”

“…24,43 Finally, in a number of dimer configurations the binding energy of Y6 excitons has been calculated to be very low (≤ 0.15 eV 26 ), and free charge formation has been demonstrated in which in neat Y6 films. 26,27 There are few studies on whether these species are also formed in Y6 NPs, which are likely to have morphological and hence photophysical differences compared to Y6 films.…”

“…attributed the formation of triplets to non-geminate charge recombination. 27 Triplet formation by singlet fission or charge recombination is feasible at this power as the TA kinetics remain power-dependent (Figure S7), indicating that bi-molecular annihilation processes are present.…”

“…[21][22][23][24][25] Y6 also has an exceptionally low exciton binding energy (≤ 0.15 eV 26 ), and has been shown to generate free charges even in the absence of an electron donor. 26,27 Additionally, Y6 has a electron affinity of −4.10 eV relative to the vacuum potential, appropriate for the reduction of H + to H 2 (Figure S1). 28 Hence, Y6 NPs have significant potential for photocatalytic H 2 evolution.…”