Photovoltaics at the mesoscale: insights from quantum-kinetic simulation

Abstract

This Topical Review discusses insights into the physical mechanisms of nanostructure solar cell operation as provided by numerical device simulation using a state-of-the-art quantum-kinetic framework based on the non-equilibrium Green's function (NEGF) formalism. After a brief introduction to the field of nanostructure photovoltaics and an overview of the existing literature on theoretical description and experimental implementation of such devices, the quantum-kinetic formulation of photovoltaic processes is discussed in detail, together with more conventional modeling approaches such as global detailed balance theory and the semi-classical drift-diffusion-Poisson-Maxwell picture. Application examples provided subsequently include III-V semiconductor nanostructures ranging from ultra-thin absorbers to quantum well and quantum dot solar cell devices. The focus is on common features encountered in photovoltaic nanostructure architectures such as the impact of configurational parameters and operating conditions on device characteristics, and the pronounced deviations from the semiclassical bulk picture. Ultra-thin absorbers are investigated with focus on the effect of built-in fields and contact configuration on radiative rates and currents. For the case of single and multi-quantum-well p-in devices, generation, recombination, and escape of carriers are discussed, and quantum well superlattice solar cells are considered with regard to charge carrier transport regimes ranging from band-like transport in miniband states to sequential tunneling between neighboring periods. Double quantum well structures are further studied in the context of tunnel junctions for multijunction solar cell. The investigation of quantum dots covers the fluorescence of colloidal nanoparticles for luminescent solar concentrators as well as the impact of configurational parameters on the photovoltaic properties of regimented quantum dot arrays, both in single-junction and intermediate-band configurations.

Hot-carrier optoelectronic devices based on semiconductor nanowires

“…The simulations are based on a NEGF modelling framework for nanostructure-based photovoltaic devices, which includes the effects of interactions of charge carriers with photons and phonons in a picture of quantum transport ranging from the ballistic to the diffusive regime. 163,164 Simulations were carried out using a simple effective mass picture for the electronic struc-Fig. 11.…”

“…The simulations are based on a NEGF modelling framework for nanostructure-based photovoltaic devices, which includes the effects of interactions of charge carriers with photons and phonons in a picture of quantum transport ranging from the ballistic to the diffusive regime. 163,164 Simulations were carried out using a simple effective mass picture for the electronic struc- (e) Short circuit current, Isc, as a function of irradiance at a variety of excitation energies, the exponential/linear regimes are interpreted as thermionic emission/internal photoemission being the main source of the generated current. Reprinted with permission from Nano Lett.…”

Challenges in the NEGF Simulation of Quantum‐Well Photovoltaics Posed by Non‐Locality and Localization

“…It was realized early in the development of NEGF transport methods that resolution of bound states lying outside the bandwidth of the electrodes is not possible in the ballistic situation, but requires in general an inelastic scattering process that couples the carrier population on the bound states to that in the extended states that are connected to the contacts . In the case of carriers that are photogenerated in the confined states of a QW, photocurrent flow is enabled by inelastic scattering with phonons (slow) and tunneling (fast), where the latter depends strongly on the built‐in field …”

“…The non‐equilibrium Green's function formalism (NEGF) is widely regarded as the state‐of‐the‐art in simulation of nano‐electronic devices, most notably for cases where transport beyond the ballistic regime needs to be considered. While in many realistic devices operating at room temperature, inelastic electron‐phonon interaction is the main scattering mechanism to be included, the additional treatment of electron–photon interaction is mandatory for the simulation of quantum‐opto‐electronic devices …”

Light-matter interactions in van der Waals photodiodes from first principles

“…Such models are ideal to validate methodological developments or to investigate quantum well structures made of conventional semiconductors, e.g. GaAs or Si [41][42][43][44] and carbon nanotubes 39,40 . However, they are not suitable to examine hetero-bilayers of 2D materials that approach the ultimate atomic thickness and exhibit complex bandstructure, inter-layer coupling, and electron-photon coupling matrix elements, whose features are unknown.…”