Hot Electron Injection from Graphene Quantum Dots to TiO2

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Abstract
The Shockley–Queisser limit is the maximum power conversion efficiency of a conventional solar cell based on a single semiconductor junction. One approach to exceed this limit is to harvest hot electrons/holes that have achieved quasi-equilibrium in the light absorbing material with electronic temperatures higher than the phonon temperature. We argue that graphene based materials are viable candidates for hot carrier chromophores. Here we probe hot electron injection and charge recombination dynamics for graphene quantum dots (QDs, each containing 48 fused benzene rings) anchored to the TiO2(110) surface via carboxyl linkers. We find ultrafast electron injection from photoexcited graphene QDs to the TiO2 conduction band with time constant τi < 15 fs and charge recombination dynamics characterized by a fast channel (τr1 = 80–130 fs) and a slow one (τr2 = 0.5–2 ps). The fast decay channel is attributed to the prompt recombination of the bound electron–hole pair across the interface. The slow channel depends strongly on excitation photon energy or sample temperature and can be explained by a “boomerang” mechanism, in which hot electrons are injected into bulk TiO2, cooled down due to electron–phonon scattering, drifted back to the interface under the transient electric field, and recombine with the hole on graphene QDs. We discuss feasibilities of implementing the hot carrier solar cell using graphene nanomaterials.

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