Reversible electron-hole separation in a hot carrier solar cell

Hot-carrier solar cells are envisioned to utilize energy filtering to extract power from photogenerated electron-hole pairs before they thermalize with the lattice, and thus potentially offer higher power conversion efficiency compared to conventional, single absorber solar cells. The efficiency of hot-carrier solar cells can be expected to strongly depend on the details of the energy filtering process, a relationship which to date has not been satisfactorily explored. Here, we establish the conditions under which electron-hole separation in hot-carrier solar cells can occur reversibly, that is, at maximum energy conversion efficiency. We thus focus our analysis on the internal operation of the hot-carrier solar cell itself, and in this work do not consider the photon-mediated coupling to the sun. After deriving an expression for the voltage of a hot-carrier solar cell valid under conditions of both reversible and irreversible electrical operation, we identify separate contributions to the voltage from the thermoelectric effect and the photovoltaic effect. We find that, under specific conditions, the energy conversion efficiency of a hot-carrier solar cell can exceed the Carnot limit set by the intra-device temperature gradient alone, due to the additional contribution of the quasi-Fermi level splitting in the absorber. We also establish that the open-circuit voltage of a hot-carrier solar cell is not limited by the band gap of the absorber, due to the additional thermoelectric contribution to the voltage. Additionally, we find that a hot-carrier solar cell can be operated in reverse as a thermally driven solid-state light emitter. Our results help explore the fundamental limitations of hot-carrier solar cells, and provide a first step towards providing experimentalists with a guide to the optimal configuration of devices.Comment: 31 pages, 5 figure

Implementing an Insect Brain Computational Circuit Using III–V Nanowire Components in a Single Shared Waveguide Optical Network

Recent developments in photonics include efficient nanoscale optoelectronic components and novel methods for sub-wavelength light manipulation. Here, we explore the potential offered by such devices as a substrate for neuromorphic computing. We propose an artificial neural network in which the weighted connectivity between nodes is achieved by emitting and receiving overlapping light signals inside a shared quasi 2D waveguide. This decreases the circuit footprint by at least an order of magnitude compared to existing optical solutions. The reception, evaluation and emission of the optical signals are performed by a neuron-like node constructed from known, highly efficient III-V nanowire optoelectronics. This minimizes power consumption of the network. To demonstrate the concept, we build a computational model based on an anatomically correct, functioning model of the central-complex navigation circuit of the insect brain. We simulate in detail the optical and electronic parts required to reproduce the connectivity of the central part of this network, using experimentally derived parameters. The results are used as input in the full model and we demonstrate that the functionality is preserved. Our approach points to a general method for drastically reducing the footprint and improving power efficiency of optoelectronic neural networks, leveraging the superior speed and energy efficiency of light as a carrier of information.Comment: 28 pages, 6 figures; supplementary information 15 pages, 8 figure

Single-nanowire, low-bandgap hot carrier solar cells with tunable open-circuit voltage

Compared to traditional pn-junction photovoltaics, hot carrier solar cells offer potentially higher efficiency by extracting work from the kinetic energy of photogenerated "hot carriers" before they cool to the lattice temperature. Hot carrier solar cells have been demonstrated in high-bandgap ferroelectric insulators and GaAs/AlGaAs heterostructures, but so far not in low-bandgap materials, where the potential efficiency gain is highest. Recently, a high open-circuit voltage was demonstrated in an illuminated wurtzite InAs nanowire with a low bandgap of 0.39 eV, and was interpreted in terms of a photothermoelectric effect. Here, we point out that this device is a hot carrier solar cell and discuss its performance in those terms. In the demonstrated devices, InP heterostructures are used as energy filters in order to thermoelectrically harvest the energy of hot electrons photogenerated in InAs absorber segments. The obtained photovoltage depends on the heterostructure design of the energy filter and is therefore tunable. By using a high-resistance, thermionic barrier an open-circuit voltage is obtained that is in excess of the Shockley-Queisser limit. These results provide generalizable insight into how to realize high voltage hot carrier solar cells in low-bandgap materials, and therefore are a step towards the demonstration of higher efficiency hot carrier solar cells

An Object-Oriented Multiscale Verification Scheme

Object-oriented verification methodology is becoming more and more common in the evaluation of model performance on high-resolution grids. The research herein describes an advanced version of an object-oriented approach that involves a combination of object identification on multiple scales with Procrustes shape analysis techniques. The multiscale object identification technique relies heavily on a novel Fourier transform approach to associate the signals within convection to different spatial scales. Other features of this new verification scheme include using a weighted cost function that can be user defined for object matching using different criteria, delineating objects that are more linear in character from those that are more cellular, and tagging object matches as hits, misses, or false alarms. Although the scheme contains a multiscale approach for identifying convective objects, standard minimum intensity and minimum size thresholds can be set when desirable. The method was tested as part of a spatial verification intercomparison experiment utilizing a combination of synthetic data and real cases from the Storm Prediction Center (SPC)/NSSL Weather Research and Forecasting (WRF) model Spring Program 2005. The resulting metrics, including error measures from differences in matched objects due to displacement, dilation, rotation, and intensity, from these cases run through this new, robust verification scheme are shown

Heterostructure semiconductor nanowires for hot carrier photovoltaics and photothermoelectric detectors: theory and experiment

This thesis seeks to addresses a question raised in the photovoltaic literature: “How can the kinetic energy of hot photogenerated carriers in a semiconductor be utilized to obtain larger open-circuit voltages than possible in a conventional pn-junction photovoltaic composed of the same material?”This thesis shows by theory and experiment that by using light to drive a photothermoelectric heat engine, larger open-circuit voltages than possible in a conventional pn-junction photovoltaic composed of the same material can be achieved. Specifically, it is demonstrated that hot photogenerated carriers in a single InAs nanowire can be used to obtain an open-circuit voltage in excess of the InAs detailed balance limit. This is accomplished by employing a highly resistive InP thermionic barrier to block the transit of holes and lattice-temperature electrons. The InP thermionic barrier, which is located between two identical, degenerately n-type InAs regions, allows high-energy electron photocurrents to pass, while simultaneously ensuring that bias-induced drift currents of lattice-temperature carriers are blocked. This enables achievement of high open-circuit voltages in excess of the InAs detailed balance limit.In the course of this work, two other findings are made. Firstly, it is shown theoretically that a thermally driven light emitting diode is thermodynamically allowed and that it is the reverse mode of operation of a hot carrier photovoltaic. Secondly, it is found that the wavelength dependent location of strong photon absorption in a metal-contacted, heterostructure single-nanowire can be exploited to obtain a photothermoelectric device in which the polarity of the photovoltage and photocurrent depend upon the wavelength of illumination. Methods for numerically modelling this optoelectronic phenomenon are detailed and possible uses are suggested

Absorption in and scattering from single horizontal Au-contacted InAs/InP heterostructure nanowires

Finite element modelling (FEM) is used to show that the addition of Au contacts to a single horizontal InAs/InP heterostructure nanowire substantially alters the nanowire's optical properties in comparison to the uncontacted case. It is found that the addition of contacts can increase absorption efficiency, decrease scattering efficiency and shift the location of absorption within the nanowire. Localized surface plasmon resonances are found to develop at nanowire/contact interfaces at infrared wavelengths and contribute to the alteration of the optical response of the nanowire

Long-wavelength intersubband quantum disc-in-nanowire photodetectors with normal incidence photoresponse

Semiconductor nanowire (NW) technology has emerged as a key facilitator of novel optoelectronics e.g. solar cells, photodetectors and LEDs. The functional wavelength range of current NW-based photodetectors is typically limited to the visible/near-infrared region. In this work, we present the first ever reported electrical and optical characteristics of longwavelength IR photodetectors based on largesquare millimeter ensembles of vertically grown and processed InAsP/InP heterostructure NWs grown on InP substrates1. More specifically, the MOVPE-grownNWs comprise single or multiple InAsP quantum discs (QDiscs) axially embedded in an n+-i-n+ geometry. The NWs are contacted together in a vertical geometryby uniformly depositing a thin insulating SiO2 layer, selective etching of the oxide from the tip of the NWs followed by sputtering of ITO as a common topcontact to all NWs. Using Fourier transform photocurrent spectroscopy, we demonstrate a photoresponse extending from the visible to far infrared1,2.In particular, the infrared response from 3-20 μm is enabled by intersubband transitions in the lowbandgap InAsP quantum discs synthesized axiallywithin the InP NWs. The detector elements exhibit an unexpected sensitivity to normal incident radiation, apparently in contradiction to well-known selection rulesfor intersubband transitions in quantum wells. From in-depth 2D and 3D optical simulations we attribute this result to an excitation of the longitudinal component ofoptical modes in the photonic crystal formed by the nanostructured portion of the detectors. Key advantages with the proposed design include a large degree offreedom in choice of material compositions, enhanced optical resonance effects due to periodically ordered NW arrays and the compatibility with silicon substrates.We believe that our novel detector design offers a route towards monolithic integration of compact and sensitive broadband III-V NW detectors with main-streamsilicon technology which could seriously challenge existing commercially available photodetectors