J. Barnes

Strained and strain-balanced quantum well devices for high-efficiency tandem solar cells

Abstract The state of GaAs/InGaAs quantum well solar cell research is reviewed. The effect of strain upon the GaAs/InGaAs cells is discussed and the limits to a strained GaAs/InGaAs cell established. The strain-balance approach is suggested as a means of overcoming the limits inherent to the strained approach and the principle is demonstrated in two differing device configurations. The strain-balance devices show enhanced efficiencies over their strained counterparts and in one case, comparable efficiency to a good GaAs control cell. The application of these cells to tandem structures is discussed, indicating the potential for a substantial efficiency enhancement.

Quantum well solar cells

In this paper we discuss the potential advantages of quantum wells (QWs) for enhancing solar cell efficiency. We present recent experimental results which show that the open-circuit voltage (Voc) of the quantum well solar cell (QWSC) is enhanced over that of comparable conventional cells formed from the well material, by more than the change in the absorption edge. We also report on theoretical and experimental studies which seek to determine the quasi-Fermi level separation in quantum wells inside a p-i-n system in order to understand the voltage behaviour of QWSCs and to be able to estimate the efficiency enhancements which may be achieved in the radiative limit. We discuss QWSCs in the InPInGaAs lattice matched system and present results which show that QWSCs in this deep well system have a better variation of efficiency with temperature than conventional cells made from either the well or barrier material. This is important for applications involving concentrated sunlight. We also consider the advantages of quantum well cells in the area of thermophotovoltaics (TPV).

Observation of suppressed radiative recombination in single quantum well p-i-n photodiodes

We have measured electroluminescence (EL) spectra of GaAs/InGaAs and AlGaAs/GaAs single quantum well (QW) p-i-n photodiodes at temperatures between 200 and 300 K and forward biases close to the open circuit voltage. Integrated EL spectra vary like eqV/nkT with an ideality factor n=1.05±0.05 over five decades, indicating purely radiative processes. The spectra are calibrated into absolute units enabling comparison to be made with the predictions of a theoretical model. For each temperature and bias we calculate the EL spectrum and radiative current expected in the detailed balance limit, integrating the theoretical emission spectrum over the surface of the device, in order to establish the quasi-Fermi potential separation, Δφf, in the QW and, where possible, in the host material. For the GaAs/InGaAs cell we are able to model emission from the QW and the host material simultaneously. We find that, in all cases, the QW emission is overestimated by theory if it is assumed that Δφf=V. QW emission corresponds i...

EFFECT OF STRAIN RELAXATION ON FORWARD BIAS DARK CURRENTS IN GAAS/INGAAS MULTIQUANTUM WELL P-I-N DIODES

The effect of the dislocation line density produced by the relaxation of strain in GaAs/InxGa1−xAs multiquantum wells where x=0.155–0.23 has been studied. There is a strong correlation between the dark line density, observed by cathodoluminescence, before processing of the wafers into photodiode devices, and the subsequent low forward bias (<1.5 V) dark current densities of the devices. A comparison is made of the correlation between the reverse bias current density and dark line density and it is found that, in this range of strain, the forward bias current density varies more. Two growth methods, molecular beam epitaxy and metal organic vapor phase epitaxy, have been used to produce the wafers and no difference between the growth methods has been found in dark line or current density variations with strain.

CHARACTERIZATION OF GAAS/INGAAS QUANTUM WELLS USING PHOTOCURRENT SPECTROSCOPY

We report on characterization studies of high quality metal‐organic vapor phase epitaxy and molecular beam epitaxy grown GaAs/InGaAs quantum wells, set within p‐i‐n diodes, to determine the well widths, indium mole fractions, and conduction band offset. We present photocurrent spectra containing a larger number of transitions than revealed in photoluminescence or photoluminescence excitation experiments. The energies of these transitions have been modeled using a theoretical characterization tool known as ‘‘contouring,’’ which is used in this strained system for the first time. This has enabled determination of the conduction band offset in GaAs/InGaAs quantum wells, to a value between 0.62 and 0.64, for a range of indium fractions between 0.155 and 0.23. As a final, additional check on our results, we compare the field dependence of the e1‐hh1 exciton transition energy with our theoretical calculations and find good agreement.

Steady state photocurrent and photoluminescence from single quantum wells as a function of temperature and bias

We have studied the variation with applied bias and temperature of steady state photoluminescence (DCPL) and photoconductivity (DCPC) from a series of GaAs/AlGaAs single quantum well, p-i-n structures with different well widths. We present the DCPC and DCPL results, which when combined, allow us to assess how significant nonradiative recombination is in the samples and hence the quality of the material. We discuss the qualitative features in the light of a new theoretical approach presented here for the first time. This includes contributions from escape (of both electrons and holes) and makes it possible to extract from the experimental data two parameters, each reflecting the competition between escape and one of the recombination processes (radiative or nonradiative) in the absence of the other. We further comment qualitatively on the bias and temperature dependence of these different processes.

Space charge effects in carrier escape from single quantum well structures

Recently published data on the variation with applied bias and temperature of steady-state photoluminescence and photoconductivity from a series of GaAs/AlGaAs single quantum well p-i-n structures are subjected to detailed theoretical analysis, using phenomenological variables introduced in connection with these results. The data are interpreted as revealing the presence in the well of a space charge, which causes band bending and hence indirectly modifies carrier escape lifetimes. It is shown that the thermionic escape of holes can affect the electron tunneling escape lifetime so that the latter displays a thermal activation energy which is quantitatively similar to the hole well depth.

A carrier escape study from InP/InGaAs single quantum well solar cells

Carrier escape from InP/AlGaAs single quantum well structures is studied by means of simultaneous steady state photocurrent and photoluminescence measurements. The activation energy for escape is measured for the first time in this system. The photoluminescence from the InGaAs wells indicates that a significant number of carriers do not escape at room temperature thus affecting the temperature dependence of the cell. An estimate of the nonradiative efficiency of the device studied is given as a function of bias and temperature. The relevance to new applications is discussed.

Study of misfit dislocations by EBIC, CL and HRTEM in GaAs/InGaAs lattice-strained multi-quantum well p-i-n solar cells

Abstract The location, density and nature of misfit dislocations (MDs) in lattice-strained multi-quantum well (MQW) structures were investigated by depth-resolved electron-beam-induced current (EBIC) and cathodoluminescence (CL) modes in a scanning electron microscope. A planar network of dark recombination lines due to MDs was observed at the lower and upper interfaces of the MQW stack. Their density was correlated with the MQW average strain before relaxation, giving-information on the equilibrium and catastrophic strain relaxation processes which take place at the two MQW stack interfaces. High-resolution transmission electron microscopy (HRTEM) showed the location and nature of the MDs at an atomic level; they are mostly close to the lower MQW stack interface, on a 111 plane constituting glissile-60 ° dislocations, composed of two partials including a stacking fault. Comparison of their density with the dark line density indicates that each dark line represents a group of about 9 MDs. Quantitative information on the electrical properties of solar cells was obtained by (i) determining the average MD contrast at the lower MWQ interface using EBIC gain measurements and (ii) establishing the existence of a strong correlation between the dark current in forward bias and the MD density.

Tailored carrier escape rates in asymmetric double quantum wells

An experimental study of carrier escape in a series of p - i - n asymmetric double quantum well (ADQW) structures using time-resolved photoluminescence is presented. By performing complementary continuous-wave photoluminescence and photocurrent measurements the carrier escape times were extracted as a function of temperature and perpendicular applied electric field and compared with a quantitative model which includes the mechanisms of tunnelling, thermally assisted tunnelling and thermionic emission. Holes were found to dominate the escape rate, and in the light of this it is shown that if narrow wells are grown next to wider wells and nearer the p region a significantly enhanced carrier escape rate can result. This is explained by resonant hole tunnelling and may have implications in the design of QW modulator and solar cell devices with improved performance.