Revealing the Competing Contributions of Charge Carriers, Excitons, and Defects to the Non-Equilibrium Optical Properties of ZnO

Due to its wide band gap and high carrier mobility, ZnO is an attractive material for light-harvesting and optoelectronic applications. Its functional efficiency, however, is strongly affected by defect-related in-gap states that open up extrinsic decay channels and modify relaxation timescales. As a consequence, almost every ZnO sample behaves differently, leading to irreproducible or even contradicting observations. Here, a complementary set of time-resolved spectroscopies is applied to two ZnO samples of different defect density to disentangle the competing contributions of charge carriers, excitons, and defects to the non-equilibrium dynamics after photoexcitation: Time-resolved photoluminescence, excited state transmission, and electronic sum frequency generation. Remarkably, defects affect the transient optical properties of ZnO across more than eight orders of magnitude in time, starting with photodepletion of normally occupied defect states on femtosecond timescales, followed by the competition of free exciton emission and exciton trapping at defect sites within picoseconds, photoluminescence of defect-bound and free excitons on nanosecond timescales, and deeply trapped holes with microsecond lifetimes. These findings do not only provide the first comprehensive picture of charge and exciton relaxation pathways in ZnO, but also uncover the microscopic origin of previous conflicting observations in this challenging material and thereby offer means of overcoming its difficulties

Evidence for Hybrid Inorganic–Organic Transitions at the WS2/Terrylene Interface

The realization of the potential of hybrid inorganic organic systems requires an understanding of the coupling between the constituents: its nature and its strength. The observation of hybrid optical transitions in the monolayer WS2/terrylene hybrid is reported. The first-principle calculations, linear optical, and transient absorption spectroscopy are employed to investigate the optical spectrum of the hybrid, which exhibits a new transition that does not appear in the constituents’ spectra. The calculations indicate type II level alignment, with the highest occupied level of terrylene in the gap of WS2. Exploiting state-resolved transient absorption, the response of the hybrid interface to optical excitation is selectively probed. The dynamics reveal rapid hole transfer from WS2 to the terrylene layer, with a decay time of 88 ps. This hole transfer induces a bleach of the hybrid transition, which indicates that terrylene contributes to its initial state. Based on this, the hybrid resonance energy, and on our calculations, we assign the hybrid feature to a transition from the highest occupied molecular orbital of terrylene to the conduction band of WS2 close to the Γ point. The results indicate that the conditions for strong electronic coupling are met in this hybrid system.Peer Reviewe

Sulfur- and boron-containing porous donor-acceptor polymers for photocatalytic hydrogen evolution

Photocatalytic water-splitting provides a way to store solar energy as hydrogen gas, and hence, is an attractive alternative to energy-intensive electrolysis of water. Microporous polymer networks are an interesting class of heterogeneous photocatalysts due to the chemical modularity of their optically active backbone and their guest-accessible pore-structure. Photocatalytic action depends on efficient separation of photoexcited electron-hole pairs, and recently, it was discovered that this separation can be improved by incorporation of donor-acceptor motifs into the polymer backbones. While there are many examples of electron donors, there is little variety in electron acceptor motifs. Here, we present a series of microporous donor-acceptor networks that contain electron-deficient boron moieties (triarylborane) as the electron acceptors and sulfur moieties (thiophene) as the donors. Under sacrificial conditions, these sulfur- and boron-containing polymers (SBPs) show rates of hydrogen evolution up to 113.9 mmol h-1 g-1. Conventionally used platinum co-catalyst does not contribute meaningfully to the photocatalytic action. Instead, palladium that is incorporated during the stage of polymer synthesis acts as the co-catalyst