Highly Mobile Excitons in Single Crystal Methylammonium Lead Tribromide Perovskite Microribbons.

Excitons are often given negative connotation in solar energy harvesting in part due to their presumed short diffusion lengths. We investigate exciton transport in single-crystal methylammonium lead tribromide (MAPbBr3) microribbons via spectrally, spatially, and temporally resolved photocurrent and photoluminescence measurements. Distinct peaks in the photocurrent spectra unambiguously confirm exciton formation and allow for accurate extraction of the low temperature exciton binding energy (39 meV). Photocurrent decays within a few µm at room temperature, while a gate-tunable long-range photocurrent component appears at lower temperatures (about 100 µm below 140 K). Carrier lifetimes of 1.2 µs or shorter exclude the possibility of the long decay length arising from slow trapped-carrier hopping. Free carrier diffusion is also an unlikely source of the highly nonlocal photocurrent, due to their small fraction at low temperatures. We attribute the long-distance transport to high-mobility excitons, which may open up new opportunities for novel exciton-based photovoltaic applications.

Transport Modeling of Locally Photogenerated Excitons in Halide Perovskites.

Excitons have fundamental impacts on optoelectronic properties of semiconductors. Halide perovskites, with long carrier lifetimes and ionic crystal structures, may support highly mobile excitons because the dipolar nature of excitons suppresses phonon scattering. Inspired by recent experimental progress, we perform device modeling to rigorously analyze exciton formation and transport in methylammonium lead triiodide under local photoexcitation by using a finite element method. Mobile excitons, coexisting with free carriers, can dominate photocurrent generation at low temperatures. The simulation results are in excellent agreement with the experimentally observed strong temperature and gate dependence of carrier diffusion. This work signifies that efficient exciton transport can substantially influence charge transport in the family of perovskite materials.

Nitrogenated, phosphorated and arsenicated monolayer holey graphenes.

Motivated by a recent experiment that reported the synthesis of a new 2D material nitrogenated holey graphene (C2N) [Mahmood et al., Nat. Commun., 2015, 6, 6486], the electronic, magnetic, and mechanical properties of nitrogenated (C2N), phosphorated (C2P) and arsenicated (C2As) monolayer holey graphene structures are investigated using first-principles calculations. Our total energy calculations indicate that, similar to the C2N monolayer, the formation of the other two holey structures are also energetically feasible. Calculated cohesive energies for each monolayer show a decreasing trend going from the C2N to C2As structure. Remarkably, all the holey monolayers considered are direct band gap semiconductors. Regarding the mechanical properties (in-plane stiffness and Poisson ratio), we find that C2N has the highest in-plane stiffness and the largest Poisson ratio among the three monolayers. In addition, our calculations reveal that for the C2N, C2P and C2As monolayers, creation of N and P defects changes the semiconducting behavior to a metallic ground state while the inclusion of double H impurities in all holey structures results in magnetic ground states. As an alternative to the experimentally synthesized C2N, C2P and C2As are mechanically stable and flexible semiconductors which are important for potential applications in optoelectronics.