Over the past 30 years, the world has witnessed the rapid development of optoelectronic devices based on III-V compound semiconductors. Past effort has mainly been directed to the theoretical understanding of, and the technology development for, these devices in applications in telecommunication networks and compact disk (CD) data storage.
On the basis of real-time simulations, we devise a method to extend the capability of scanning tunneling microscopy (STM) to track the electronic dynamics of molecules on a material’s surface with the ultrafast temporal resolution of laser pulses. The intrinsic mechanism of visualization of electronic dynamics by measuring tunneling charge is attributed to the interference between the electronic oscillations stimulated by pump and probe pulses. The charge-transfer rate from molecule to the surrounding environment can be estimated with the decay time of electronic dynamics, which can also be detected by measuring the tunneling charge across the STM junction. Moreover, it is found that the tunneling charge can be varied precisely by changing the carrier-envelope phase (CEP) of the pulses, and this phase-dependence of tunnelling diminishes as the duration of incident laser pulses increases. The proposed scheme provides an alternative means for visualization of electron dynamics of single molecules by measuring tunnelling charges.
Using an innovative quantum mechanical method for an open quantum system, we observe in real time and space the generation, migration, and dissociation of electron-hole pairs, transport of electrons and holes, and current emergence in an organic photovoltaic cell. Ehrenfest dynamics is used to study photoexcitation of thiophene:fullerene stacks coupled with a time-dependent density functional tight-binding method. Our results display the generation of an electron-hole pair in the donor and its subsequent migration to the donor-acceptor interface. At the interface, electrons transfer from the lowest unoccupied molecular orbitals (LUMOs) of thiophenes to the second LUMOs of fullerene. Further migration of electrons and holes leads to the emergence of current. These findings support previous experimental evidence of coherent couplings between electronic and vibrational degrees of freedom and are expected to stimulate further work toward exploring the interplay between electron-hole pair (exciton) binding and vibronic coupling for charge separation and transport.
Van der Waals heterostructures composed of two-dimensional materials offer an unprecedented control over their properties and have attracted tremendous research interest in various optoelectronic applications. Here, we study the photoinduced charge transfer in graphene/WS2 heterostructure by time-dependent density functional theory molecular dynamics. Our results show that holes transfer from graphene to WS2 two times faster than electrons, and the occurrence of interlayer charge transfer is found correlated with vibrational modes of graphene and WS2. It is further demonstrated that the carrier dynamics can be efficiently modulated by external electric fields. Detailed analysis confirms that the carrier transfer rate at heterointerface is governed by the coupling between donor and acceptor states, which is the result of the competition between interlayer and intralayer relaxation processes. Our study provides insights into the understanding of ultrafast interlayer charge transfer processes in heterostructures and broadens their future applications in photovoltaic devices.
Graphene nanoribbons (GNRs) with atomically precise heterojunction interfaces are exploited as nanoscale light emitting devices with modulable emission frequencies. By connecting GNRs with different widths and lengths, topological boundary states can be formed and manipulated. Using first-principles-based atomistic simulations, we studied the luminescence properties of a STM GNR junction and explored the applications of these topological states as nanoscale light sources. Taking advantage of the ultrahigh resolution of the STM tip, direct injection of high energy carriers at selected boundary states can be achieved. In this way, the emission color can be controlled by precisely changing the tip position. The GNR heterojunction can therefore represent a robust and controllable light-emitting device that takes a step forward towards the fabrication of nanoscale graphene-based optoelectronic devices.
Control of absorption and photocurrent conversion is of practical importance for the design of photoelectric devices. In this paper, using simulations, we demonstrate that the photoresponse of a bilayer graphene nanoribbon (GNR) device can be controlled by gate voltage modulation. A vertical gate field shifts the potential on the top and bottom layers in opposite directions, resulting in a continuous change of band gap with applied gate voltage. This field simultaneously facilitates separation of photoexcited electron–hole pairs and gives rise to a photocurrent in a selected photon energy range. The photoresponse of a bilayer GNR device can thus be tuned by adjusting the applied gate voltage. In addition, the light frequency range can be changed by using nanoribbons of different widths. These findings provide a basis for the design of adjustable optoelectronic devices using two-dimensional materials.
Nanometallic structures that support surface plasmons provide new ways to confine light at deep-subwavelength scales. The effect of light scattering in nanowire array solar cells is studied by a multiscale approach combining classical electromagnetic (EM) and quantum mechanical simulations. A photovoltaic device is constructed by integrating a silicon nanowire array with a plasmonic silver nanosphere. The light scatterings by plasmonic element and nanowire array are obtained via classical EM simulations, while current–voltage characteristics and optical properties of the nanowire cells are evaluated quantum mechanically. We found that the power conversion efficiency (PCE) of photovoltaic device is substantially improved due to the local field enhancement of the plasmonic effect and light trapping by the nanowire array. In addition, we showed that there exists an optimal nanowire number density in terms of optical confinement and solar cell PCE.
Understanding of the electroluminescence (EL) mechanism in optoelectronic devices is imperative for further optimization of their efficiency and effectiveness. Here, a quantum mechanical approach is formulated for modeling the EL processes in nanoscale light emitting diodes (LED). Based on non-equilibrium Green's function quantum transport equations, interactions with the electromagnetic vacuum environment are included to describe electrically driven light emission in the devices. The presented framework is illustrated by numerical simulations of a silicon nanowire LED device. EL spectra of the nanowire device under different bias voltages are obtained and, more importantly, the radiation pattern and polarization of optical emission can be determined using the current approach. This work is an important step forward towards atomistic quantum mechanical modeling of the electrically induced optical response in nanoscale systems.
Previous simulations of photovoltaic devices are based on classical models, which neglect the atomistic details and quantum-mechanical effects besides the dependence on many empirical parameters. Here, within the nonequilibrium Green’s function formalism, we present a quantum-mechanical study of the performance of inorganic nanowire-based photovoltaic devices. On the basis of density-functional tight-binding theory, the method allows simulation of current–voltage characteristics and optical properties of photovoltaic devices without relying on empirical parameters. Numerical studies of silicon nanowire-based devices of realistic sizes with 10 000 atoms are performed, and the results indicate that atomistic details and nonequilibrium conditions have a clear impact on the photoresponse of the devices.