D.B. Bushnell

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.

Strain-balanced quantum well solar cells

Abstract The exceptional power conversion efficiency of monolithic multi-junction solar cells requires careful current matching in the individual junctions. It is shown that the bulk GaInP/GaAs tandem configuration is not optimal for solar conversion. A means for achieving optimum tandem power conversion is demonstrated through the incorporation of thin strain-balanced layers, resulting in a multi-quantum-well (MQW) structure. Experimental results are presented and an efficiency of 27% AM0 projected for a GaInP/MQW tandem cell.

Recent results on quantum well solar cells

The Quantum Photovoltaic group at Imperial College has pioneered the use of quantum wells in photovoltaic applications, using material supplied by our collaborators in the EPSRC III-V Facility, University of Sheffield, University of Nottingham and the Center for Electronic Materials and Devices. We have shown, in a number of lattice-matched multi-quantum well systems, that quantum wells enhance the output current and efficiency compared to homogeneous cells made from the barrier material and output voltage compared with conventional cells made from the material in the well. This paper discusses recent results in three areas of our work: (1) fundamental studies relevant to the question of whether similar efficiency enhancements will be possible in an ideal cell where radiative recombination dominates, (2) the materials problems we have had to solve in order to enhance the highest efficiency GaAs solar cells with InGaAs wells and (3) the application of quantum well cells in the area of thermophotovoltaics.

Strain-balanced materials for high-efficiency solar cells

Traditionally, monolithic multi-junction solar cells have required lattice matched material combinations for efficient operation. However, strain-balanced structures allow lattice mismatched materials to be grown pseudomorphically, with low defect densities, and therefore offer interesting band-gap configurations for attaining optimal multi-junction solar cell structures. Several material combinations are identified and their suitability as highly efficient photovoltaic materials discussed; in particular In/sub x/Ga/sub 1-x/As, GaAs/sub 1-x/P/sub x/, GaInP and GaInNAs. Estimates for the limiting efficiency of strain-balanced multi-junction cells are presented, together with an outline of the technological requirements to achieve such cells.

Electron-beam-induced current and cathodoluminescence characterization of InGaAs strain-balanced multiquantum well photovoltaic cells

InxGa1−xAs/InyGa1−yAs strain-balanced quantum well cells (QWCs) have been shown to be beneficial for photovoltaic applications in particular to extend the light absorption edge of a single-junction cell toward the near infrared with a lower reduction of the open-circuit voltage compared to a single band-gap cell. The strain-balancing condition ensures that the multi-quantum well as a whole does not relax. However, if the mismatch between wells and barriers exceeds a critical limit, the structure becomes vulnerable to morphological or compositional fluctuations, which can lead to a local structural breakdown with the generation of extended defects of a completely different nature from misfit dislocations. In this work, we investigated a series of strain-balanced InGaAs QWCs grown on InP for thermophotovoltaic applications by means of electron-beam-induced current (EBIC) and cathodoluminescence (CL) measurements. Despite being electrically active, these defects appear to have a minor impact on the dark curr...

Light-trapping structures for multi-quantum well solar cells

Multi-quantum well (MQW) cells extend the absorption of bulk gallium arsenide cells to longer wavelengths, increasing short-circuit current and have been shown to exhibit a similar dark current to their p-i-n control cells. To complement the recent addition of rear surface mirrors, similar techniques to those employed in silicon cells may be used. These include the texturing of the front surface and use of diffraction gratings. Both are considered here and their effect upon the short-circuit current (J/sub sc/) calculated. Such techniques could allow the number of quantum wells to be reduced, improving the dark current of the cell.

Secondary Electron Emission Contrast of Quantum Wells in GaAs p-i-n Junctions

The secondary electron (SE) signal over a cleaved surface of GaAs p-i-n solar cells containing stacks of quantum wells (QWs) is analyzed by high-resolution scanning electron microscopy. The InGaAs QWs appear darker than the GaAsP barriers, which is attributed to the differences in electron affinity. This method is shown to be a powerful tool for profiling the conduction band minimum across junctions and interfaces with nanometer resolution. The intrinsic region is shown to be pinned to the Fermi level. Additional SE contrast mechanisms are discussed in relation to the dopant regions themselves as well as the AlGaAs window at the p-region. A novel method of in situ observation of the SE profile changes resulting from reverse biasing these structures shows that the built-in potential may be deduced. The obtained value of 0.7 eV is lower than the conventional bulk value due to surface effects.

The electric field and dopant distribution in p-i-n structures observed by ionisation potential (dopant contrast) microscopy in the HRSEM

The method of ionisation potential (dopant contrast) microscopy in the HRSEM is applied to the study of the electric field distribution in p-i-n structures used as quantum well solar cells. Our results show a secondary electron signal which varies between the different layers, being greatest in the p-type and smallest in the n-type regions respectively. The stacks of 8 nm wide quantum wells and their corresponding barriers are clearly distinguished in the intrinsic region of the devices. In-situ observation of reverse biased structures has been performed to determine the effect of bias on the potential distribution within the devices.

Dark-current suppression due to photon recycling in distributed Bragg reflector strain-balanced quantum well solar cells

Self-absorption or photon recycling (PR) has been observed experimentally as a reduction in the ideality n=1 reverse saturation current of SB-QWSCs. We believe this is the first example of PR effects on solar cell dark-currents. PR is observed at high bias when the primary recombination mechanism in SB-QWSCs is radiative recombination in the quantum wells corresponding to ideality n=1 dark-currents. Results are presented here on the effect of PR on the dark-current and electroluminescence (EL) spectrum; PR has resulted in DBR SB-QWSC dark-current and EL suppression of up to 35% both experimentally and theoretically. Comparison is made to control cells without a DBR. Modelling results will allow us to make predictions for expected efficiency enhancement in optimised SB-QWSC devices at high concentration levels. These results convincingly reveal how PR can increase the efficiency of SB-QWSCs.