The efficiency of organic light-emitting devices (OLEDs) is often limited by roll-off, where efficiency decreases with increasing bias. In most OLEDs, roll-off primarily occurs due to exciton quenching, which is commonly assumed to be active only above device turn-on. Below turn-on, exciton and charge carrier densities are often presumed to be too small to cause quenching. Using lock-in detection of photoluminescence, we find that this assumption is not generally valid; luminescence can be quenched by >20% at biases below turn-on. We show that this low-bias quenching is due to hole accumulation induced by intrinsic polarization of the electron transport layer (ETL). Further, we demonstrate that selection of nonpolar ETLs or heating during deposition minimizes these losses, leading to efficiency enhancements of >15%. These results reveal design rules to optimize efficiency, clarify how ultrastable glasses improve OLED performance, and demonstrate the importance of quantifying exciton quenching at low bias.

ABSTRACT:Electrodeposited thin films and nanoparticles of Ni 3 S 2 are highly active, poison and corrosion resistant catalysts for oxygen reduction to water at neutral pH. In pH 7 phosphate buffer, Ni 3 S 2 displays catalytic onset at 0.8 V vs the reversible hydrogen electrode, a Tafel slope of 109 mV/decade, and high Faradaic efficiency for four--electron reduction of O 2 to water. Under these conditions, the activity and stability of Ni 3 S 2 exceeds that of polycrystalline platinum and manganese, nickel, and cobalt oxides illustrating the catalytic potential of pairing labile first row transition metal active sites with a more covalent sulfide host lattice.The interconversion of water and O 2 is an essential chemistry underlying a future renewable energy economy. 1 Nature exe--cutes this kinetically demanding multi--proton, multi--electron interconversion with remarkable selectivity and efficiency. Oxygen evolution is carried out at the Mn 4 Ca co--factor of the oxygen evolving complex of photosystem II 2 whereas oxygen reduction is carried out at the heme/Cu ac--tive site of cytochrome C oxidase 3 and Cu 3 cluster active sites of multicopper oxidases. 4 While these catalysts operate effi--ciently and selectively under benign conditions of neutral pH and ambient temperature and pressure, precious and base metal containing heterogeneous catalysts typically require highly alkaline or acidic electrolytes ( Figure 1).The paucity of heterogeneous electrocatalysts capable of efficient oxygen reduction at neutral pH 5 arises from two seemingly divergent kinetic/materials requirements: 1) the catalyst must remain active in the presence of buffering elec--trolytes that are required to maintain neutral pH stability and deliver protons to drive the proton--coupled electron transfer (PCET) activation of O 2 6 and 2) the catalyst must resist protolytic corrosion under reducing conditions. Pre--cious metal catalysts such as Pt and Au meet the latter re--quirement but also strongly adsorb buffering electrolyte ions such as phosphate, degrading their catalytic efficiency. 7 In contrast, low valent mid to late first row transition metal ions are substitutionally labile, 8 allowing them to meet the first requirement, but this very property makes their corre--sponding oxides unstable with respect to corrosion in all but highly alkaline environments. 9Unlike metal oxides, bonding in transition metal sulfides is more covalent, inhibiting their corrosion under similar con--ditions. 10 Thus, we envisioned that both of the above re--quirements could be met if a labile first row transition metal active site ion can be exposed at the surface of a sulfide host lattice. Here, we illustrate the effectiveness of this design strategy by uncovering a novel earth abundant catalyst for oxygen reduction at neutral pH, the heazlewoodite phase of nickel sulfide, Ni 3 S 2 . Under phosphate buffered neutral pH conditions, Ni 3 S 2 outperforms state of the art ORR catalysts including MnO x and platinum owing to its unique combina--tion of labile ...

Polymer electrolyte membranes employed in contemporary fuel cells severely limit device design and restrict catalyst choice, but are essential for preventing short-circuiting reactions at unselective anode and cathode catalysts. Herein, we report that nickel sulfide Ni S is a highly selective catalyst for the oxygen reduction reaction in the presence of 1.0 m formate. We combine this selective cathode with a carbon-supported palladium (Pd/C) anode to establish a membrane-free, room-temperature formate fuel cell that operates under benign neutral pH conditions. Proof-of-concept cells display open circuit voltages of approximately 0.7 V and peak power values greater than 1 mW cm , significantly outperforming the identical device employing an unselective platinum (Pt) cathode. The work establishes the power of selective catalysis to enable versatile membrane-free fuel cells.

Polymer electrolyte membranes employed in contemporary fuel cells severely limit device design and restrict catalyst choice, but are essential for preventing short‐circuiting reactions at unselective anode and cathode catalysts. Herein, we report that nickel sulfide Ni3S2 is a highly selective catalyst for the oxygen reduction reaction in the presence of 1.0 m formate. We combine this selective cathode with a carbon‐supported palladium (Pd/C) anode to establish a membrane‐free, room‐temperature formate fuel cell that operates under benign neutral pH conditions. Proof‐of‐concept cells display open circuit voltages of approximately 0.7 V and peak power values greater than 1 mW cm−2, significantly outperforming the identical device employing an unselective platinum (Pt) cathode. The work establishes the power of selective catalysis to enable versatile membrane‐free fuel cells.

The authors produce plasmonic ZnO-TiN nanocomposite films by depositing plasma-synthesized ZnO nanocrystals onto a substrate and then by infilling the nanocrystal network's pores with TiN via remote plasma-enhanced atomic layer deposition (PEALD). This ZnO-TiN nanocomposite exhibits a plasmonic resonance that is blueshifted compared to planar titanium nitride thin films. The authors study the effects of PEALD conditions and the ZnO film thickness on the plasmonic response of these nanocomposites and exploit the optimized film in a device that generates photocurrent at zero bias.

Charge-transfer (CT) states formed at organic donor–acceptor (D–A) semiconductor heterojunctions play a critical role in optoelectronic devices. While mobile, their migration has not been extensively characterized. In addition, the factors impacting the CT state diffusion length (L D) have not been elucidated. Here, CT state L D is measured by using photoluminescence quenching for several D–A mixtures, with migration occurring along the bulk heterojunction. All D–A pairings considered yield a similar L D ∼ 5 nm in equal mixtures despite variations in the CT state energy and the constituent molecular structures. The CT state L D varies strongly with mixture composition and is well-correlated to the slowest charge carrier mobility, suggesting a direct method to tune CT state transport. These findings may be applied to elucidate the role of CT state migration in organic photovoltaic and light-emitting devices as well as to broadly explain the transport of interfacial excited states along inorganic and hybrid organic–inorganic heterojunctions.

A membrane‐free fuel cell relies on the selectivity of catalysts at both the anode and the cathode. In their Communication on page 7496 ff., Y. Surendranath et al. show that the sulfide N3S2 selectively catalyzes the oxygen reduction reaction (ORR) in the presence of a high concentration of formate. Paired with the known formate oxidation catalyst Pd/C, the selective ORR catalyst Ni3S2 enables the construction of a membrane‐free formate fuel cell that operates at neutral pH and outperforms the Pt–Pd device.

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