Massimo Mazzer

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.

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 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.

Efficiency enhancement of ideal photovoltaic solar cells by photonic excitations in multi-intermediate band structures

We present an efficiency analysis of ideal photovoltaic solar cells based on multi-intermediate band structures. It is shown that the difference between the thermodynamic limit of photovoltaic conversion and the limit of efficiency of traditional bulk semiconductor solar cells can be gradually bridged if an optimum energy band structure is achieved. Efficiency enhancement originates from photonic excitations among multiple energy bands. Several possible ways to design the optimum energy band structures are proposed.

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.

Influence of surface morphology on ordered GaInP structures

Ga0.51In0.49P layers, grown by organometallic chemical vapor deposition on differently misoriented (001) GaAs substrates exhibit CuPtB‐type domains. The increase in structural homogeneity of the ordered domains, with increasing substrate misorientation, results in a very sharp distribution of the degree of ordering. The correlation between cathodoluminescence emission from ordered regions and changes in surface step distribution direction confirm the influence of the surface morphology on the distribution of ordered regions and their antiphase boundaries.

Nanoscale potential distribution across multiquantum well structures: Kelvin probe force microscopy and secondary electron imaging

Ultrahigh vacuum cross-sectional Kelvin probe force microscopy has been used to characterize In0.17GaAs∕GaAsP0.06 multiquantum well structures, together with secondary electron microscopy. Individual 8nm quantum wells were well resolved in both methods, and were found to be in a good agreement with numerical simulations of the work function profile. It is shown that the surface potential contrast in the Kelvin probe force microscopy measurements is greatly enhanced using deconvolution algorithms, and the reasons for the different contrast in the electron microscopy images are discussed.

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.