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We studied the crystal quality and optical properties of ITO produced with optical thin film coatings to replace conventional metal electrodes as electrodes for III-V solar cells. ITO films were grown on glass, GaAs and silicon substrate. ITO is a polycrystalline with predominant (222) crystalline plane. A smooth surface with protruding nano-crystallite was observed. The light transmittance of ITO has a strong relationship with thickness and annealing temperature. The transmittance was obviously enhanced after being annealed at 500 °C. With the thickness increase, the transmittance decreases in total. The band gap of the ITO film was estimated 3.8 eV by extrapolating the linear part of the Tauc plot curves to intercept the energy axis. The refractive index n of ITO film is was measured. The reflectivity is greatly influenced by the thickness of ITO.

The carrier recombination dynamics of InGaAsP material with a bandgap energy of 1 eV for quadruple-junction solar cells grown by solid-source molecular beam epitaxy have been investigated by the employment of time-resolved photoluminescence (PL) measurement. For the nominally undoped material, the PL decay time increases with increasing temperature, which indicates that radiative recombination dominates the recombination process. The radiative and the nonradiative recombination time constants were calculated on the basis of the temperature-dependent PL decay time and the integrated PL intensity. With the incorporation of Be (as the p-type dopant) into the material, the PL decay time decreases with increasing temperature, and a double-exponential PL decay curve is observed in the case of the material with a higher doping density. An InGaAsP-based single-junction photovoltaic device with a bandgap of 1 eV was fabricated, and an efficiency of 16.4% was obtained under the AM1.5G solar spectra.

There are many difficulties limiting the further development of monolithic multi-junction solar cells, such as the growth of lattice-mismatched material and the current matching constraint. As an alternative approach, the light-splitting photovoltaic system is investigated intensively in different aspects, including the energy loss mechanism and the choice of energy bandgaps of solar cells. Based on the investigation, a two-dual junction system has been implemented employing lattice-matched GaInP/GaAs and InGaAsP/InGaAs cells grown epitaxially on GaAs and InP substrates, respectively.

To investigate factors limiting the performance of a GaAs solar cell, genetic algorithm is employed to fit the experimentally measured internal quantum efficiency (IQE) in the full spectra range. The device parameters such as diffusion lengths and surface recombination velocities are extracted. Electron beam induced current (EBIC) is performed in the base region of the cell with obtained diffusion length agreeing with the fit result. The advantage of genetic algorithm is illustrated.

Zn1-xMnxSe thin films with different Mn compositions are grown by metal-organic chemical vapor deposition on GaAs substrate. Good crystallinity of sample is evidenced by X-ray diffraction and rocking-curve measurements. Photoluminescence (PL) properties were carefully studied. A dominant PL peak close to the band edge is observed at low temperature for samples with higher Mn concentration. The temperature-dependent PL and time-resolved photoluminescence show that this emission peak is associated with the recombination of exciton bound to Mn-induced impurity bound states. It is found that rapid thermal annealing can induce reorganization of Mn composition in alloys and significantly reduce the density of impurity induced by Mn incorporation and improve the intrinsic interband transition. (C) 2002 Elsevier Science B.V. All rights reserved.

We report the specific contact resistance for ITO with both metal and a semiconductor. Good quality ITO was deposited by electron beam evaporation with the resistivity of 2.32 × 10-4 Ω ·cm and an averaged transmittance of 92.8% in the visible light region. The circular transmission line model (c-TLM) method was used to evaluate and compare the properties of the ITO/metal and ITO/semiconductor ohmic contacts. The lowest specific contact resistance of the ITO/Ni is 2.81 × 10-6 Ω ·cm2, while that of ITO/n-GaAs is 7 × 10-5 Ω ·cm2. This is the best ohmic contact between ITO and n-GaAs ever reported. These results suggest that good quality ITO has strong potential to be used to realize highly efficient solar cells.

We report the recent result of GaAs/GaInP dual-junction solar cells grown by all solid-state molecular-beam-epitaxy (MBE). The device structure consists of a GaIn0.48P homojunction grown epitaxially upon a GaAs homojunction, with an interconnected GaAs tunnel junction. A photovoltaic conversion efficiency of 27% under the AM1.5 globe light intensity is realized for a GaAs/GaInP dual-junction solar cell, while the efficiencies of 26% and 16.6% are reached for a GaAs bottom cell and a GaInP top cell, respectively. The energy loss mechanism of our GaAs/GaInP tandem dual-junction solar cells is discussed. It is demonstrated that the MBE-grown phosphide-containing III—V compound semiconductor solar cell is very promising for achieving high energy conversion efficiency.

Single-junction, lattice-mismatched In0.69Ga0.31As thermophotovoltaic (TPV) devices each with abandgap of 0.6 eV are grown on InP substrate by metal—organic chemical vapour deposition (MOCVD). Compositionally undulating step-graded InAsyP1−y buffer layers with a lattice mismatch of ~1.2% are used to mitigate the effect of lattice mismatch between the device layers and the InP substrate. With an optimized buffer thickness, the In0.69Ga0.31As active layers grown on the buffer display a high crystal quality with no measurable tetragonal distortion. High-performance single-junction devices are demonstrated, with an open-circuit voltage of 0.215 V and a photovoltaic conversion efficiency of 6.9% at a short-circuit current density of 47.6 mA/cm2, which are measured underthe standard solar simulator of air mass 1.5-global (AM 1.5 G).

Structural and optical properties of Si-doped as well as nominally undoped GaInP epilayers grown on Germanium substrates by metal-organic chemical vapor deposition have been investigated by high resolution transmission electron microscope and photoluminescence (PL). Si incorporation results in an increased inner band PL transition and a blue shift of PL energy with increasing temperature, which arises from the trapping states around Ge-GaInP interface due to Ge diffusion to GaInP epilayer as well as Si doping. For the inter band PL transition, a competition between the emission processes near the band edge and in the ordered GaInP domains is responsible for the inverted S shape temperature dependence of PL peaks. By analyzing the time-resolved PL results, we attribute this emission near the ordered states to the localized states due to the potential fluctuation, which is induced by the compositional inhomogeneity of Ga and In in the partially ordered GaInP.

Magnetophotoluminescence properties of Zn0.88Mn0.12Se thin films grown by metal-organic chemical vapor deposition on GaAs substrates are investigated in fields up to 10 T. The linewidth of the excitonic luminescence peaks decreases with the increasing magnetic field (< 1 T), but the peak energy is almost unchanged. There is a crossover of the photoluminescence intensities between interband and bound excitonic transitions as the magnetic field is increased to about 1 T. These behaviors are interpreted by the strong tuning of the local alloy disorder potential by the applied magnetic field. In addition, the magnetic field-induced suppression of the energy transfers from excitons to Mn2+ ions is also observed.