Stephen P. Bremner

Energy conversion approaches and materials for high-efficiency photovoltaics

The past five years have seen significant cost reductions in photovoltaics and a correspondingly strong increase in uptake, with photovoltaics now positioned to provide one of the lowest-cost options for future electricity generation. What is becoming clear as the industry develops is that area-related costs, such as costs of encapsulation and field-installation, are increasingly important components of the total costs of photovoltaic electricity generation, with this trend expected to continue. Improved energy-conversion efficiency directly reduces such costs, with increased manufacturing volume likely to drive down the additional costs associated with implementing higher efficiencies. This suggests the industry will evolve beyond the standard single-junction solar cells that currently dominate commercial production, where energy-conversion efficiencies are fundamentally constrained by Shockley-Queisser limits to practical values below 30%. This Review assesses the overall prospects for a range of approaches that can potentially exceed these limits, based on ultimate efficiency prospects, material requirements and developmental outlook.

Limiting efficiency of an intermediate band solar cell under a terrestrial spectrum

The limiting efficiency of an intermediate band (IB) solar cell under the terrestrial AM1.5 spectrum was calculated by detailed balance for various concentration levels. The results show four energy gap combinations giving similar limiting efficiencies. This is in contrast to the more studied case of an IB solar cell under a blackbody spectrum where a single optimum combination is found. A design with a subenergy gap of ∼0.57eV is found to be viable, leading to the conclusion that the design space for an IB solar cell is larger when under the AM1.5 spectrum than when under a Blackbody spectrum.

Detailed balance efficiency limits with quasi-Fermi level variations [QW solar cell]

A central assumption in detailed balance efficiency limit calculations has been that the light generated carriers are collected by drift transport processes and have an infinite mobility, giving rise to constant quasi-Fermi levels (RFLs) across the solar cell. However, recent experimental and theoretical results for quantum well (QW) devices indicate that the QFLs need not be constant across the device. It is shown in this paper that transport mechanisms which cause a variation in the difference between the electron and hole QFLs give an increase in the limiting efficiency compared to previous detailed balance calculations. Further, QW solar cells which employ hot carrier transport across a well will have an efficiency limit in excess of a tandem solar cell while using the same number of semiconductor materials.

Site-Control of InAs Quantum Dots using Ex-Situ Electron-Beam Lithographic Patterning of GaAs Substrates

Conventional e-beam lithography followed by either dry or wet etching of small holes in GaAs substrates has been used to control the position of InAs self-assembled quantum dots. The dependence of hole occupancy on both hole area and hole depth has been investigated. We show a range of hole sizes where greater than 30% of sites contain a single dot with up to 60% single dot occupancy seen for dry-etched holes ~60 nm wide, ~35 nm deep and for wet-etched holes ~90 nm wide, ~20 nm deep. Single dot luminescence from these placed dots is demonstrated despite only a 10 nm GaAs buffer between dots and regrowth interface.

Use of a GaAsSb buffer layer for the formation of small, uniform, and dense InAs quantum dots

InAs quantum dots grown on GaAsSb buffer layers with varying Sb content have been studied. Atomic force microscopy results show that the dot size is reduced as the Sb content increases with a concomitant increase in number density. Analysis of the size distribution indicates that the spread of dot sizes narrows with increasing Sb content. This is confirmed by photoluminescence measurements showing a significant narrowing of the dot emission peak for a GaAs0.77Sb0.23 buffer compared to a GaAs buffer. The results are attributed to the strained buffer reducing interactions between dots and the Sb acting as a surfactant.

Impact of stress relaxation in GaAsSb cladding layers on quantum dot creation in InAs/GaAsSb structures grown on GaAs (001)

We describe InAs quantum dot creation in InAs/GaAsSb barrier structures grown on GaAs (001) wafers by molecular beam epitaxy. The structures consist of 20-nm-thick GaAsSb barrier layers with Sb content of 8%, 13%, 15%, 16%, and 37% enclosing 2 monolayers of self-assembled InAs quantum dots. Transmission electron microscopy and X-ray diffraction results indicate the onset of relaxation of the GaAsSb layers at around 15% Sb content with intersected 60° dislocation semi-loops, and edge segments created within the volume of the epitaxial structures. 38% relaxation of initial elastic stress is seen for 37% Sb content, accompanied by the creation of a dense net of dislocations. The degradation of In surface migration by these dislocation trenches is so severe that quantum dot formation is completely suppressed. The results highlight the importance of understanding defect formation during stress relaxation for quantum dot structures particularly those with larger numbers of InAs quantum-dot layers, such as those proposed for realizing an intermediate band material.

Surface-acoustic-wave-driven luminescence from a lateral p-n junction

The authors report surface-acoustic-wave-driven luminescence from a lateral p-n junction formed by molecular beam epitaxy regrowth of a modulation doped GaAs∕AlGaAs quantum well on a patterned GaAs substrate. Surface-acoustic-wave-driven transport is demonstrated by peaks in the electrical current and light emission from the GaAs quantum well at the resonant frequency of the transducer. This type of junction offers high carrier mobility and scalability. The demonstration of surface-acoustic-wave luminescence is a significant step towards single-photon applications in quantum computation and quantum cryptography.

Design trade-offs and rules for multiple energy level solar cells

Abstract Solar cell efficiency limit are receiving renewed examination due to the realisation that many types of solar cell structures can theoretically achieve similar efficiencies to those of multiple p–n homojunctions, without the need for a large number of different semiconductors. This paper shows that the proposed high efficiency device structures fit into one of three general classes and therefore only three ideal efficiency limit calculations are required. However, many of the suggested structures violate assumptions in the ideal efficiency limit calculations. Hence, these calculations should be modified to include additional transport, generation and recombination effects.

Supercharging Silicon Solar Cell Performance by Means of Multijunction Concept

This study aims to comprehensively calculate the conversion efficiency limits of multijunction solar cells having crystalline silicon (c-Si) not only as a growth substrate but as the lowermost active subcell as well. The first set of efficiency limits is calculated based on the detailed balance principle, assuming a stepwise absorption profile. Practical limits are then calculated using Si absorption data and Auger recombination parameters from the literature. The Si wafer thickness is also considered in the two-stack tandem design. Performances of the tandems are compared when various Si solar cell technologies are used as the bottom subcells. The impact of external radiative efficiency and external quantum efficiency of the top subcell on the tandem performance is studied. Efficiency limits using state-of-the-art devices, and the effect of varying cell thickness on tandem efficiencies is also reported.

Controllability of the subband occupation of InAs quantum dots on a delta-doped GaAsSb barrier

Optical properties of InAs quantum dots (QDs) embedded in GaAsSb barriers with delta-doping levels equivalent to 0, 2, 4, and 6 electrons per dot (e/dot) are studied using time-integrated photoluminescence (PL). When the PL excitation power is increased the full width at half maximum (FWHM) of the 4 and 6 e/dot samples is found to increase at a much greater rate than the FWHMs for the 0 and 2 e/dot samples. PL spectra of the 4 e/dot sample show a high energy peak attributed to emission from the first excited states of the QDs, a result deduced to be due to preoccupation of states by electrons supplied by the delta-doping plane. When temperature dependent PL results are fitted using an Arrhenius function, the thermal activation energies for the 4 and 6 e/dot samples are similar and greater than the thermal activation energies for the 0 and 2 e/dot samples (which are similar to each other). This increased thermal activation energy is attributed to the enhanced Coulombic interaction in the InAs QD area by th...