J.P. Connolly

Strain-balanced GaAsP/InGaAs quantum well solar cells

A strain-balance multiquantum well (MQW) approach to enhance the GaAs solar cell efficiency is reported. Using a p-i-n diode structure, the strain-balanced GaAsP/InGaAs MQW is grown on a GaAs substrate and equals a good GaAs cell in terms of power conversion efficiency. The cell design is presented together with measurements of the forward bias dark current density, quantum efficiency, and 3000 K light-IV response. Cell efficiencies under standard air mass (AM) 1.5 and AM 0 illumination are projected from experimental data and the suitability of this cell for enhancing GaInP/GaAs tandem cell efficiencies is discussed.

Modeling the spectral response of the quantum well solar cell

The quantum well solar cell is an alternative to more conventional multiband gap approaches to higher cell efficiency. Preliminary studies have shown that the insertion of a series of quantum wells into the depletion region of a GaAs/AlxGa1−xAs p‐i‐n solar cell can significantly enhance the cell’s short‐circuit current. We present here a model for the spectral response of GaAs and AlxGa1−xAs p‐n and p‐i‐n solar cells, with and without quantum wells, based on a standard solution of the minority‐carrier equations. Particular emphasis is placed on modeling the absorption coefficient of the AlxGa1−xAs and of the quantum wells. We find that our model can accurately predict the spectral response of a wide variety of cells: both conventional p‐n junctions in GaAs and AlxGa1−xAs, and various geometries of quantum well solar cell in AlxGa1−xAs/GaAs (x∼0.3). We discuss the strengths and weaknesses of the model and its underlying assumptions, and conclude by using the model to design p‐i‐n quantum well solar cells w...

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).

Voltage enhancement in quantum well solar cells

It is known that quantum well solar cells (QWSCs) can enhance short circuit current and power conversion efficiency in comparison with similar, conventional solar cells made from the quantum well (QW) barrier material alone. In this article we report measurements of the dark‐current and open‐circuit voltage (Voc) of a number of quantum well cells in three different lattice‐matched material systems, namely, Al0.35Ga0.65As/GaAs, GaInP/GaAs, and InP/InGaAs. We also present the results obtained from comparable control cells without wells formed either from the material of the barriers or the well material alone. Our results clearly demonstrate in all three cases that, at fixed voltage, QWSC dark currents are systematically lower than would be expected from control cells with the same effective absorption edge. Measurements of Voc in a white‐light source show that the open‐circuit voltages of the QWSCs are higher than those of control cells formed from the well material. Furthermore, this enhancement is more t...

Observation of photon recycling in strain-balanced quantum well solar cells

Photon recycling in strain-balanced quantum well solar cells grown on distributed Bragg reflectors has been observed as a suppression of the dark current and a change in electroluminescence spectra. Comparing devices grown with and without distributed Bragg reflectors we have demonstrated up to a 33% reduction in the ideality n=1 reverse saturation current. Furthermore, to validate the observations we demonstrate how both the measured dark currents and electroluminescence spectra fit very well to a photon recycling model. Verifying our observations with the model then allows us to calculate optimized device designs.

Effect of well number on the performance of quantum-well solar cells

The effect of increasing the number of quantum wells in a strain-compensated, multiquantum-well solar cell is investigated. It is found that as the well number is increased, dark current level close to the operating point rises linearly. Short-circuit current in the AM0 spectrum also rises linearly with the inclusion of more quantum wells. This allows the cell to maintain a constant open-circuit voltage irrespective of the number of wells grown. This is anticipated to have advantages when the cell is used as a replacement for the GaAs junction in the existing generation of tandem and triple-junction cells since current levels can be matched to the upper junction without detriment to the voltage performance. This result allows us to predict a tandem cell AM0 efficiency of 23.8% based on the 50-well cell.

Effect of quantum well location on single quantum well p-i-n photodiode dark currents

The photocurrent available from a p-i-n solar cell can be increased by the addition of quantum wells (QWs) to the undoped region. At the same time the QWs reduce the open-circuit voltage by introducing areas of lower band gap where recombination is enhanced. This increase in recombination should be as small as possible for the most favorable effect on the photovoltaic efficiency of the device. Theoretical considerations indicate that nonradiative recombination, which is the dominant loss mechanism in AlxGa1−xAs/GaAs QW structures, may be reduced by positioning the QWs away from the point where the electron-hole product is a maximum. For p-i-n diodes, where recombination is greatest at or near the center of the space charge region, this means locating the QWs closer to the doped regions. Spectral response should not be affected so long as the QWs are still located within the field bearing region. Thus, improved photovoltaic performance may be expected through strategic location of the QWs. We report on mea...

Conversion efficiency enhancement of AlGaAs quantum well solar cells

The quantum efficiency and photocurrent for AlGaAs quantum well solar cell is calculated and compared with experimental results obtaining good agreement. The conversion efficiency as a function of Al composition in barriers and wells is presented showing that there is a wide range of Al composition barrier and Al composition well where the QWSC efficiency is always higher than corresponding homogeneous p-i-n cell without quantum wells. We also show that for up to 15 wells in the intrinsic region an efficiency enhancement for the QWSC over the baseline cell is obtained.

Demonstration of Photon Coupling in Dual Multiple-Quantum-Well Solar Cells

Multiple-quantum-well (MQW) top cells can enhance the performance of multi-junction solar cells since the absorption edge of top and middle subcells can be tuned with the MQWs to maximize the efficiency. The radiative dominance of MQW top cells can enhance photon coupling, which can potentially reduce the spectral sensitivity of the device and, thus, raise the energy harvest. We present experimental results on photon coupling in dual-junction cells with GaInP top cells containing GaInAsP quantum wells along with theoretical calculation based on a detailed balance model. It is observed that at high concentration, approximately 50% of the dark current of an MQW top cell is transferred to the photocurrent of the cell in the bottom, which is much higher than any previously reported values.