Urs Aeberhard

Theory and simulation of quantum photovoltaic devices based on the non-equilibrium Green's function formalism

This article reviews the application of the non-equilibrium Green’s function formalism to the simulation of novel photovoltaic devices utilizing quantum confinement effects in low dimensional absorber structures. It covers well-known aspects of the fundamental NEGF theory for a system of interacting electrons, photons and phonons with relevance for the simulation of optoelectronic devices and introduces at the same time new approaches to the theoretical description of the elementary processes of photovoltaic device operation, such as photogeneration via coherent excitonic absorption, phonon-mediated indirect optical transitions or non-radiative recombination via defect states. While the description of the theoretical framework is kept as general as possible, two specific prototypical quantum photovoltaic devices, a single quantum well photodiode and a silicon-oxide based superlattice absorber, are used to illustrated the kind of unique insight that numerical simulations based on the theory are able to provide.

Optimized amorphous silicon oxide buffer layers for silicon heterojunction solar cells with microcrystalline silicon oxide contact layers

We report on the systematic optimization of the intrinsic amorphous silicon oxide buffer layer in interplay with doped microcrystalline silicon oxide contact layers for silicon heterojunction solar cells using all silicon oxide based functional layers on flat p-type float-zone wafers. While the surface passivation quality is comparably good within a wide range of low oxygen contents, the optical band gap increases and the dark conductivity decreases with increasing oxygen content, giving rise to an inevitable trade-off between optical transparency and electrical conductivity. On the cell level, fill factor FF and short circuit current density Jsc losses compete with the open circuit voltage Voc gains resulting from a thickness increase of the front buffer layers, whereas Jsc and Voc gains compete with FF losses resulting from increasing thickness of the rear buffer layers. We obtained the highest active area efficiency of ηact = 18.5% with Voc = 664 mV, Jsc = 35.7 mA/cm2, and FF = 78.0% using 4 nm front a...

Wide Gap Microcrystalline Silicon Oxide Emitter for a-SiOx:H/c-Si Heterojunction Solar Cells

This paper reports on the development of phosphorous doped microcrystalline silicon oxide (µc-SiOx:H) films as an emitter window layer in flat p-type silicon heterojunction (SHJ) solar cells featuring intrinsic a-SiOx:H buffer layers. We investigated the material properties of n-type µc-SiOx:H films grown at various input gas ratios and correlated the results of SHJ solar cells utilizing varying oxygen content and thickness of the emitter layer to the corresponding film properties. A maximum efficiency of 19.0% was achieved. The excellent short circuit current of 35.8 mA/cm2 for flat cells was attributed to the low optical losses in the emitter window.

Fluorescence of colloidal PbSe/PbS QDs in NIR luminescent solar concentrators

For applications in luminescent solar concentrators harvesting subgap photons, either via direct irradiation of solar cells with optimized band gap or via sensitization of an up-conversion process, exact knowledge and tunability of both the spectral shape and the intensity of the emission are of paramount importance. In this work, we investigate theoretically the photoluminescence spectra of colloidal core-shell PbSe/PbS QDs with type II alignments in the valence band. The method builds on a steady-state formulation of the non-equilibrium Greens function theory for a microscopic system of coupled electrons, photons and phonons interfaced with electronic structure calculations based on a k·p model for PbSe/PbS core-shell QDs. The resulting output spectral density of photons in a realistic QD ensemble is obtained via the renormalization of the incident spectrum according to the polarization of the system.

Theory and simulation of photogeneration and transport in Si-SiOx superlattice absorbers

Si-SiOx superlattices are among the candidates that have been proposed as high band gap absorber material in all-Si tandem solar cell devices. Owing to the large potential barriers for photoexited charge carriers, transport in these devices is restricted to quantum-confined superlattice states. As a consequence of the finite number of wells and large built-in fields, the electronic spectrum can deviate considerably from the minibands of a regular superlattice. In this article, a quantum-kinetic theory based on the non-equilibrium Greens function formalism for an effective mass Hamiltonian is used for investigating photogeneration and transport in such devices for arbitrary geometry and operating conditions. By including the coupling of electrons to both photons and phonons, the theory is able to provide a microscopic picture of indirect generation, carrier relaxation, and inter-well transport mechanisms beyond the ballistic regime.

Effective microscopic theory of quantum dot superlattice solar cells

We introduce a quantum dot orbital tight-binding non-equilibrium Green’s function approach for the simulation of novel solar cell devices where both absorption and conduction are mediated by quantum dot states. By the use of basis states localized on the quantum dots, the computational real space mesh of the Green’s function is coarse-grained from atomic resolution to the quantum dot spacing, which enables the simulation of extended devices consisting of many quantum dot layers.

Simulation of nanostructure-based and ultra-thin film solar cell devices beyond the classical picture

In this paper, an optoelectronic device simulation framework valid for arbitrary spatial variation of electronic potentials and optical modes, and for transport regimes ranging from ballistic to diffusive, is used to study non-local photon absorption, photocurrent generation and carrier extraction in ultra-thin film and nanostructure-based solar cell devices at the radiative limit. Among the effects that are revealed by the microscopic approach and which are inaccessible to macroscopic models is the impact of structure, doping or bias induced nanoscale potential variations on the local photogeneration rate and the photocarrier transport regime.

Simulation of Ultrathin Solar Cells Beyond the Limits of the Semiclassical Bulk Picture

The photovoltaic characteristics of an ultrathin GaAs solar cell with a gold back reflector are simulated using the standard semiclassical drift-diffusion-Poisson model and an advanced microscopic quantum-kinetic approach based on the nonequilibrium Greens function (NEGF) formalism. For the standard assumption of flat-band bulk absorption coefficient used in the semiclassical model, substantial qualitative and quantitative discrepancies are identified between the results of the two approaches. The agreement is improved by consideration of field-dependent absorption and emission coefficients in the semiclassical model, revealing the strong impact of the large built-in potential gradients in ultrathin device architectures based on high-quality crystalline materials. The full quantum-kinetic simulation results for the device characteristics can be reproduced by using the NEGF generation and recombination rates in the semiclassical model, pointing at an essentially bulk-like transport mechanism.

Microscopic theory and numerical simulation of quantum well solar cells

Quantum well solar cells have been introduced as high efficiency photovoltaic energy conversion devices nearly twenty years ago. Since then, their capability to outperform bulk devices under concentrated illumination was established and efficiencies close to the single gap Shockley-Queisser limit for the corresponding bulk materials were reached. In order to further increase the efficiency, the mechanisms behind the extraordinary performance need to be analyzed and understood. To this end, novel theoretical approaches to the simulation of quantum optoelectronic devices are required, since the device behavior depends on complex processes between localized and extended states, such as carrier capture and escape into and from quantum wells, which cannot be consistently described by the combination of macroscopic semiconductor transport equations with detailed balance rate models conventionally used in photovoltaics. This paper presents a suitable theoretical framework based on the nonequilibrium Greens function formalism that allows the unification of quantum optics and dissipative quantum transport on the quantum kinetic level and is thus able to provide insight into the microscopic mechanisms of quantum photovoltaic device operation.

Spectral properties of photogenerated carriers in quantum well solar cells

The use of low-dimensional structures such as quantum wells, wires or dots in the absorbing regions of solar cells strongly affects the spectral response of the latter, the spectral properties being drastically modified by quantum confinement effects. Due to the microscopic nature of these effects, a microscopic theory of absorption and transport is required for their quantification. Such a theory can be developed in the framework of the non-equilibrium Greens function approach to semiconductor quantum transport and quantum optics. In this paper, the theory is used to numerically investigate the density of states, non-equilibrium occupation and corresponding excess concentration of both electrons and holes in single quantum well structures embedded in the intrinsic region of a p-i-n semiconductor diode, under illumination with monochromatic light of different energies. Escape of carriers from the quantum well is considered via the inspection of the spectral photocurrent at a given excitation energy. The investigation shows that thermal escape from deep levels may be inefficient even at room temperature.