Nicolas Cavassilas

Modeling of nanoscale solar cells: The Green's function formalism

Solar cells incorporating nano-structures represent a promising solution to overtake the Schockley-Queisser limit. On the other hand, the non-equilibrium Greens function formalism provides a sound conceptual basis for the development of quantum simulators that are needed for nanoscale devices. While this approach has already been applied to solar cells, it remains unfamiliar to most photovoltaic physicists. In this paper we show the main concepts of this formalism and illustrate it with a simple 1D model of solar cell. This model is applied to a thin film GaAs solar cell. Our investigations permit to show and analyze current flowing in the solar cell at the nanometer scale.

Flexible Photodiodes Based on Nitride Core/Shell p–n Junction Nanowires

A flexible nitride p-n photodiode is demonstrated. The device consists of a composite nanowire/polymer membrane transferred onto a flexible substrate. The active element for light sensing is a vertical array of core/shell p–n junction nanowires containing InGaN/GaN quantum wells grown by MOVPE. Electron/hole generation and transport in core/shell nanowires are modeled within nonequilibrium Green function formalism showing a good agreement with experimental results. Fully flexible transparent contacts based on a silver nanowire network are used for device fabrication, which allows bending the detector to a few millimeter curvature radius without damage. The detector shows a photoresponse at wavelengths shorter than 430 nm with a peak responsivity of 0.096 A/W at 370 nm under zero bias. The operation speed for a 0.3 × 0.3 cm2 detector patch was tested between 4 Hz and 2 kHz. The −3 dB cutoff was found to be ∼35 Hz, which is faster than the operation speed for typical photoconductive detectors and which is compatible with UV monitoring applications.

Modeling of phonon scattering in n-type nanowire transistors using one-shot analytic continuation technique

We present a one-shot current-conserving approach to model the influence of electron-phonon scattering in nano-transistors using the non-equilibrium Greens function formalism. The approach is based on the lowest order approximation (LOA) to the current and its simplest analytic continuation (LOA+AC). By means of a scaling argument, we show how both LOA and LOA+AC can be easily obtained from the first iteration of the usual self-consistent Born approximation (SCBA) algorithm. Both LOA and LOA+AC are then applied to model n-type silicon nanowire field-effect-transistors and are compared to SCBA current characteristics. In this system, the LOA fails to describe electron-phonon scattering, mainly because of the interactions with acoustic phonons at the band edges. In contrast, the LOA+AC still well approximates the SCBA current characteristics, thus demonstrating the power of analytic continuation techniques. The limits of validity of LOA+AC are also discussed, and more sophisticated and general analytic con...

Theoretical comparison of multiple quantum wells and thick-layer designs in InGaN/GaN solar cells

This theoretical work analyzes the photovoltaic effect in non-polar InGaN/GaN solar cells. Our electronic transport model considers quantum behaviors related to confinement, tunneling, electron-phonon, and electron-photon scatterings. Based on this model, we compare a multiple quantum wells cell with its thick-layer counterpart. We show that the structure of multiple quantum wells is a promising design providing better compromise between photon-absorption and electronic transport. This balance is necessary since these two phenomena are shown to be antagonist in nanostructure based solar cells. In these devices, we also show that phonon absorption increases the short-circuit current, while phonon emission reduces the open-circuit voltage.

One-shot current conserving quantum transport modeling of phonon scattering in n-type double-gate field-effect-transistors

We apply a recently developed one-shot current conserving lowest order approximation (LOA) to the modeling of inelastic transport in silicon double-gate transistors using the non-equilibrium Greens function formalism. The transport properties are compared to those given by the commonly adopted selfconsistent Born approximation (SCBA). We find that LOA reproduces well the current reduction due to phonon scattering, as given by the SCBA. This good agreement is further improved by adopting a conserving analytical-continuation approach. In ultimate thin-film devices, the combination of LOA and analytical-continuation techniques offers the same accuracy as the SCBA but at a much reduced computational cost.

Reflective Barrier Optimization in Ultrathin Single-Junction GaAs Solar Cell

This paper proposes a theoretical analysis of electronic transport in ultrathin (220 nm) single-junction GaAs solar cell. Using an in-house electronic quantum transport model, we shed light on two detrimental phenomena, namely the “backdiffusion” and the “contact-to-contact diffusion.” While the back-diffusion degrades both the short-circuit current and the fill factor, the contact-to-contact diffusion reduces the open-circuit voltage. The so-called window and back-surface-field barriers used to reflect minority carriers away from contacts reduce these two detrimental phenomena. In a second part, we then show a synthesis of performance optimization of window/GaAs/backsurface-field heterojunctions varying thicknesses, materials, and material composition profiles. Our results conclude that the Al0.4Ga0.6As(10 nm)/GaAs/In0.49Ga0.51P(10 nm) structure provides the best output power.

Three-dimensional k · p real-space quantum transport simulations of p-type nanowire transistors: Influence of ionized impurities

We present a three-dimensional quantum transport simulator for p-type nanowire transistors. This self-consistent model expresses a six-band k · p Hamiltonian within the non-equilibrium Green’s function formalism. Transport properties are analyzed with and without the presence of ionized impurities in the channel. We observe that inter-subband coupling generates a rich structure of peaks in the transmission coefficients even in the intrinsic situation. A single donor leads to a current decrease whereas its acceptor counterpart induces complicated resonant and anti-resonant features. Unlike n-type devices, our conclusions pinpoint that the p-type nanowire transistors exhibit intricate transmission variations that can potentially generate larger variability and whose modeling requires a multi-band based simulator.

Simulation of hole phonon-velocity in strained Si/SiGe metal-oxide-semiconductor transistor

Hole phonon velocity in a strained Si inversion layer grown on a relaxed SiGe substrate has been theoretically investigated. We used: (i) a 20 band k.p Hamiltonian method for the valence-band structure calculation, (ii) a self-consistent Schrodinger and Poisson equations solver for the confined hole subband determination, (iii) a direct matrix Boltzmann transport equation solver including hole-phonon interactions for the carrier velocity estimation in the subband structure. The present work particularly focuses on the influence of SiGe alloy composition and strained Si layer thickness on the hole dynamic in the inversion layer. Our results highlight the linear slope of the hole velocity enhancement factor with strain in the Si layer. But at the same time, large strain facilitates transfer in the parasitic channel at the Si/SiGe interface for which the carrier mobility is highly degraded. Consequently, in order to optimize a p-channel transistor with Si layer strain on SiGe virtual substrate, a compromise ...

Theoretical comparison of Si, Ge, and GaAs ultrathin p-type double-gate metal oxide semiconductor transistors

Based on a self-consistent multi-band quantum transport code including hole-phonon scattering, we compare current characteristics of Si, Ge, and GaAs p-type double-gate transistors. Electronic properties are analyzed as a function of (i) transport orientation, (ii) channel material, and (iii) gate length. We first show that ⟨100⟩-oriented devices offer better characteristics than their ⟨110⟩-counterparts independently of the material choice. Our results also point out that the weaker impact of scattering in Ge produces better electrical performances in long devices, while the moderate tunneling effect makes Si more advantageous in ultimately scaled transistors. Moreover, GaAs-based devices are less advantageous for shorter lengths and do not offer a high enough ON current for longer gate lengths. According to our simulations, the performance switching between Si and Ge occurs for a gate length of 12 nm. The conclusions of the study invite then to consider ⟨100⟩-oriented double-gate devices with Si for gat...

Material challenges for solar cells in the twenty-first century: directions in emerging technologies

Abstract Photovoltaic generation has stepped up within the last decade from outsider status to one of the important contributors of the ongoing energy transition, with about 1.7% of world electricity provided by solar cells. Progress in materials and production processes has played an important part in this development. Yet, there are many challenges before photovoltaics could provide clean, abundant, and cheap energy. Here, we review this research direction, with a focus on the results obtained within a Japan–French cooperation program, NextPV, working on promising solar cell technologies. The cooperation was focused on efficient photovoltaic devices, such as multijunction, ultrathin, intermediate band, and hot-carrier solar cells, and on printable solar cell materials such as colloidal quantum dots.