ABSTRACT:The ultrafast charge carrier dynamics in GaAs/ conjugated polymer type II heterojunctions are investigated using time-resolved photoluminescence spectroscopy at 10 K. By probing the photoluminescence at the band edge of GaAs, we observe strong carrier lifetime enhancement for nanowires blended with semiconducting polymers. The enhancement is found to depend crucially on the ionization potential of the polymers with respect to the Fermi energy level at the surface of the GaAs nanowires. We attribute these effects to electron doping by the polymer which reduces the unsaturated surfacestate density in GaAs. We find that when the surface of nanowires is terminated by native oxide, the electron injection across the interface is greatly reduced and such surface doping is absent. Our results suggest that surface engineering via π-conjugated polymers can substantially improve the carrier lifetime in nanowire hybrid heterojunctions with applications in photovoltaics and nanoscale photodetectors.

Hybrid semiconductor‐polymer nanostructured solar cells hold the promise of photovoltaic energy conversion based on abundant and nontoxic materials and scalable manufacturing processes. After a decade of intense research activity, hybrid solar cells still exhibit low short‐circuit currents and moderate open‐circuit voltages. These bottlenecks call for a detailed understanding of the physics underlying the device operation at the nanoscale. Using first‐principles calculations the ideal energy‐level alignment of hybrid solar cell interfaces based on the wide bandgap semiconductor ZnO and the polymer poly(3‐hexylthiophene) (P3HT) is investigated. The interfacial charge transfer is quantified and it is shown that this effect increases the ideal open‐circuit voltage with respect to the electron‐affinity rule by as much as 0.5 V. The results of this work suggests that there is significant room for optimizing this class of excitonic solar cells by tailoring the semiconductor/polymer interface at the nanoscale.

Two-dimensional (2D) materials are increasingly being used as active components in nanoscale devices. Many interesting properties of 2D materials stem from the reduced and highly non-local electronic screening in two dimensions. While electronic screening within 2D materials has been studied extensively, the question still remains of how 2D substrates screen charge perturbations or electronic excitations adjacent to them. Thickness-dependent dielectric screening properties have recently been studied using electrostatic force microscopy (EFM) experiments. However, it was suggested that some of the thickness-dependent trends were due to extrinsic effects. Similarly, Kelvin probe measurements (KPM) indicate that charge fluctuations are reduced when BN slabs are placed on SiO2, but it is unclear if this effect is due to intrinsic screening from BN. In this work, we use first principles calculations to study the fully non-local dielectric screening properties of 2D material substrates. Our simulations give results in good qualitative agreement with those from EFM experiments, for hexagonal boron nitride (BN), graphene and MoS2, indicating that the experimentally observed thickness-dependent screening effects are intrinsic to the 2D materials.We further investigate explicitly the role of BN in lowering charge potential fluctuations arising from charge impurities on an underlying SiO2 substrate, as observed in the KPM experiments. 2D material substrates can also dramatically change the HOMO-LUMO gaps of adsorbates, especially for small molecules, such as benzene. We propose a reliable and very quick method to predict the HOMO-LUMO gap of small physisorbed molecules on 2D and 3D substrates, using only the band gap of the substrate and the gas phase gap of the molecule.

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