Theoretical design and performance of InxGa1-xN two-junction solar cells

The efficiencies of InxGa1-xN two-junction solar cells are calculated with various bandgap combinations of subcells under AM1.5 global, AM1.5 direct and AM0 spectra. The influence of top-cell thickness on efficiency has been studied and the performance of InxGa1-xN cells for the maximum light concentration of various spectra has been evaluated. Under one-sun irradiance, the optimum efficiency is 35.1% for the AM1.5 global spectrum, with a bandgap combination of top/bottom cells as 1.74 eV/1.15 eV. And the limiting efficiency is 40.9% for the highest light concentration of the AM1.5 global spectrum, with the top/bottom cell bandgap as 1.72 eV/1.12 eV.

Electroluminescence from nano-crystalline Si/SiO2 structures embedded in pn junctions

Phosphorous-doped and boron-doped amorphous Si thin films as well as amorphous SiO2/Si/SiO2 sandwiched structures were prepared in a plasma enhanced chemical vapor deposition system. Then, the p-i-n structures containing nano-crystalline Si/SiO2 sandwiched structures as the intrinsic layer were prepared in situ followed by thermal annealing. Electroluminescence spectra were measured at room temperature under forward bias, and it is found that the electroluminescence intensity is strongly influenced by the types of substrate. The turn-on voltages can be reduced to 3 V for samples prepared on heavily doped p-type Si (p(+)-Si) substrates and the corresponding electroluminescence intensity is more than two orders of magnitude stronger than that on lightly doped p-type Si (p-Si) and ITO glass substrates. The improvements of light emission can be ascribed to enhanced hole injection and the consequent recombination of electron-hole pairs in the luminescent nanocrystalline Si/SiO2 system. (C) 2008 Elsevier Ltd. All rights reserved.

Simulation of In0.65Ga0.35N single-junction solar cell

The performances of In0.65Ga0.35N single-junction solar cells with different structures, including various doping densities and thicknesses of each layer, have been simulated. It is found that the optimum efficiency of a In0.65Ga0.35N solar cell is 20.284% with 5 x 10(17) cm(-3) carrier concentration of the front and basic regions, a 130 nm thick p-layer and a 270 nm thick n-layer.

Quantum efficiency and temperature coefficients of GaInP/GaAs dual-junction solar cell

GaInP/GaAs dual-junction solar cell with a conversion efficiency of 25.2% has been fabricated using metalorganic chemical vapor deposition (MOCVD) technique. Quantum efficiencies of the solar cell were measured within a temperature range from 25 to 160A degrees C. The results indicate that the quantum efficiencies of the subcells increase slightly with the increasing temperature. And red-shift phenomena of absorption limit for all subcells are observed by increasing the cell's work temperature, which are consistent with the viewpoint of energy gap narrowing effect. The short-circuit current density temperature coefficients dJ (sc)/dT of GaInP subcell and GaAs subcell are determined to be 8.9 and 7.4 mu A/cm(2)/A degrees C from the quantum efficiency data, respectively. And the open-circuit cell voltage temperature coefficients dV (oc)/dT calculated based on a theoretical equation are -2.4 mV/A degrees C and -2.1 mV/A degrees C for GaInP subcell and GaAs subcell.

The junction temperature and forward voltage relationship of GaN-based laser diode

In GaAs-based light-emitting diode (LED) or laser diode (LD), the forward voltage (V) will decrease linearly with the increasing junction temperature (T). This can be used as a convenient method to measure the junction temperature. In GaN-based LED, the relationship is linear too. But in GaN-based LD, the acceptor M (g) in p-GaN material can not ionize completely at-room temperature, and the carrier density will change with temperature. But we find finally that, this change won't lead to a nonlinear relationship of V-T. Our experiments show that it is Linear too.

Frequency dependence of junction capacitance of GaN p-i-n UV detectors

In this paper frequency dependence of small-signal capacitance of p-i-n UV detectors, which were fabricated on GaN grown on sapphire substrate by metalorganic chemical vapor deposition, has been studied. The Schibli-Milnes model was used to analyze the capacitance-frequency characteristics. According to high frequency C-V measurements, the deep level mean concentration is about 2.98 x 10(20) cm(-3). The deep level is caused by the un-ionised Mg dopant. The calculated Mg activation energy is 260 meV and the hole thermal capture cross section of the deep level is about 2.73 x 10(-22) cm(2). The applicability of the Schibli-Milnes model is also discussed when the concentration of deep levels exceeds that of the heavily doped n-side. It is concluded that the analytic expression of the Schibli-Milnes model can still be used to describe the capacitance-frequency characteristics of GaN p-i-n UV detectors in good agreement with experiment. (c) 2005 Elsevier Ltd. All rights reserved.

Electrical properties and electroluminescence of 4H-SiC p-n junction diodes

Homoepitaxial growth of 4H-SiC on off-oriented Si-face(0001) substrates was performed by using the step-controlled epitaxy technique in a newly developed low-pressure hot-wall CVD (LP-HWCVD) system with a horizontal air-cooled quartz tube at around 1500 degreesC and 1.33 x 10(4) Pa by employing SiH4 + C2H4 + H-2. In-situ doping during growth was carried out by adding NH3 gas into the precursor gases. It was shown that the maximum Hall mobility of the undoped 4H-SiC epilayers at room temperature is about 430 cm(2) (.) V-1 (.) s(-1) with a carrier concentration of similar to 10(16) cm(-3) and the highest carrier concentration of the N-doped 4H-SiC epilayer obtained at NH3 flow rate of 3 sccm is about 2.7 x 10(21) cm(-3) with a mobility of 0.75 cm(2) (.) V-1 (.) s(-1). SiC p-n junctions were obtained by epitaxially growing N-doped 4H-SiC epilayers on Al-doped 4H-SiC substrates. The C - V characteristics of the diodes were linear in the 1/C-3 - V coordinates indicating that the obtained p-n junctions were graded with a built-in voltage of 2.7 eV. The room temperature electroluminescence spectra of 4H-SiC p-n junctions are studied as a function of forward current. The D-A pair recombination due to nitrogen donors and the unintentional, deep boron center is dominant at low forward bias, while the D-A pair recombination due to nitrogen donors and aluminum acceptors are dominant at higher forward biases. The p-n junctions could operate at temperature of up to 400 degreesC, which provides a potential for high-temperature applications.

Synthesis and photoluminescence properties of vertically aligned ZnO nanorod-nanowall junction arrays on a ZnO-coated silicon substrate

Arrays of vertically well-aligned ZnO nanorod-nanowall junctions have been synthesized on an undoped ZnO-coated silicon substrate by a carbothermal reduction and vapour phase transport method. X-ray diffraction (XRD) and scanning electron microscopy (SEM) show that the nanostructures are well-oriented with the c-axis perpendicular to the substrate. The room temperature photoluminescence (PL) spectrum of the as-prepared ZnO nanostructure reveals a dominant near-band-edge (NBE) emission peak and a weak deep level (DL) emission, which demonstrates its good optical properties. Temperature-dependent PL spectra show that both the intensity of NBE and DL emissions increased with decreasing temperature. The NBE emission at 3.27 eV is identified to originate from the radiative free exciton recombination. The possible growth mechanism of ZnO nanorod-nanowall junctions is also proposed.

Novel coupled multi-active region high power semiconductor lasers cascaded via tunnel junction

A novel semiconductor laser structure is put forward to resolve the major difficulties of high power laser diodes. In this structure, several active regions are cascaded by tunnel junctions to form a large optical cavity and to achieve super high efficiency. This structure can solve the problems of catastrophic optical damage of facet, thermal damage and poor light beam quality effectively. Low-pressure metalorganic chemical vapor deposition method is adopted to grow the novel semiconductor laser structures, which are composed of Si:GaAs/C:GaAs tunnel junctions, GaAs/InGaAs strain quantum well active regions. External differential quantum efficiency as high as 2.2 and light power output of 2.5 W per facet (under 2A drive current) are achieved from an uncoated novel laser device with three active regions.

Monolithic Integration of Light Emitting Diodes, Photodetector and Receiver Circuit in Standard CMOS Technology

A monolithic silicon CMOS optoelectronic integrated circuit (OEIC) is designed and fabricated with standard 0.35 mu m CMOS technology. This OEIC circuit consists of light emitting diodes (LED), silicon dioxide waveguide, photodiodes and receiver circuit. The silicon LED operates in reverse breakdown mode and can be turned on at 8.5V 10mA. The silicon dioxide waveguide is composed of multiple layers of silicon dioxide between different metals layers. A two PN-junctions photodetector composed of n-well/p-substrate junction and p(+) active implantation/n-well junction maximizes the depletion region width. The readout circuitry in pixels is exploited to handle as small as 0.1nA photocurrent. Simulation and testing results show that the optical emissions powers are about two orders higher than the low frequency detectivity of silicon CMOS photodetcctor and receiver circuit.

Electron injection assisted phase transition in a nano-Au-VO2 junction

The semiconductor-metal transition of vanadium dioxide (VO2) thin films epitaxially grown on C-plane sapphire is studied by depositing Au nanoparticles onto the thermochromic films forming a metal-semiconductor contact, namely, a nano-Au-VO2 junction. It reveals that Au nanoparticles have a marked effect on the reduction in the phase transition temperature of VO2. A process of electron injection in which electrons flow from Au to VO2 due to the lower work function of the metal is believed to be the mechanism. The result may support the Mott-Hubbard phase transition model for VO2.

METASTABLE DEFECT IN AMORPHOUS-SILICON SOLAR-CELLS INVESTIGATED BY JUNCTION RECOVERY TECHNIQUE

The effect of metastable defects caused by light soaking and carrier injection on the transport of carriers in undoped a-Si:H has been investigated by a junction recovery technique. The experiments show that after light soaking or carrier injection the product of mu-p-tau-p decreases, but no detectable change in the distribution of shallow valence band tail states was found.