Chang Zoo Kim

Te doping in the GaAs tunnel junction for GaInP/GaAs tandem solar cells

Heavily tellurium (Te)-doped GaAs layers with diethyltellurium (DETe) grown by metalorganic chemical vapor deposition (MOCVD) were investigated. The existence of Te-rich microprecipitates might degrade both the electrical and optical properties. Compared to Si doping, the tunnel junction diode doped with Te doping revealed lower tunneling resistance. A comparative study using both Si and Te doping in the GaAs tunnel junction of GaInP/GaAs tandem solar cells showed a higher efficiency for Te doping. Therefore, the GaAs tunnel junction with Te doping can be considered to improve the device performance of GaAs-based multi-junction solar cells.

Properties of Si-Doped

Nonpolar Si-doped a-plane GaN layers were grown using metal organic chemical vapor deposition (MOCVD) with different silane (SiH4) flow rates. The on-axis full width at half maximum (FWHM) of X-ray rocking curves (XRCs) along the c- and m-axis directions showed that Si doping barely affected the anisotropy of a-plane GaN. A decrease in the edge threading dislocations (TDs) with increasing Si doping was confirmed by the decreased off-axis FWHM values of the XRCs. With increasing SiH4 flow rate, both the carrier concentration and mobility increased through a reduction in the edge dislocation density. The photoluminescence (PL) spectra revealed that the PL peak positions were first slightly redshifted and then blueshifted with increasing carrier concentration.

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The properties of nonpolar a-plane GaN layers grown with different Mg doping levels were investigated. With increasing the Mg flow rate, the hole concentration initially increased and then decreased, indicating the formation of compensation centers. The dominant photoluminescence (PL) emission at relatively low Mg doping levels is the blue luminescence (BL) band due to the donor-acceptor pair (DAP) transition with MgGaVN (deep donor) and MgGa (acceptor). In addition to the weak BL band, both the ultraviolet luminescence (UVL) and yellow luminescence (YL) bands are observed at higher Mg doping level. The UVL band, especially dominant at 10 K, can be related to bound excitonic emissions involving Mg-induced extended defects, whereas one or more mechanisms may contribute to the YL band.

-Plane GaN Grown with Different SiH4 Flow Rates

The performance of GaInP/GaAs tandem solar cells with AlInP growth temperatures of 680 ℃ and 700 ℃ on n-type GaAs (100) substrate with 2° and 6° tilt angles has been investigated. The series resistance and open circuit voltage of the fabricated tandem solar cells are affected by the substrate tilt angles and the growth temperatures of the window layer when zinc is doped in the tunnel diode. With carbon doping as a p-type doping source in the tunnel diode and the effort of current matching between top and bottom cells, GaInP/GaAs tandem solar cell has been exhibited 25.58% efficiency.

Mg Doping Effect in Nonpolar a-Plane GaN

Using GaInP as a nucleation layer, the Ge bottom cell was fabricated with a proper junction depth in the p-n junction and this Ge bottom cell was used to grow GaInP/GaAs/Ge triple junction solar cells. Then, the growth temperature of p-type GaAs base layer in the middle cell was investigated as a means of improving the solar cell efficiency. The solar cell grown at 550°C produced the improved efficiency compared to that grown at 680°C. The result suggests that the growth temperature of GaAs base layer is an important factor to improve the cell efficiency, related to the thermal stability of previously formed bottom cells.

The Effects of Growth Temperature and Substrate Tilt Angle on GaInP/GaAs Tandem Solar Cells

The feasibility of the GaInP/AlInP window layer, proposed to prevent an unintentional thin‐film degrading the performance of anti‐reflection coating, on a multi‐junction photovoltaic cell was investigated by using two dimensional numerical simulation. Simulated and calibrated GaInP/GaAs/InGaAs triple‐junction solar cell under AM 1.5 global illumination showed exactly the same open circuit voltage, and 0.4% efficiency discrepancy compared with measured data. Based on the simulation results, a 5 nm‐thick GaInP‐oxidation stop layer with a 25 nm‐thick AlInP window layer on the GaInP/GaAs/InGaAs triple‐junction solar cell provided both 0.35%‐efficiency and 30%‐concentration ratio increment.

Improving GaInP/GaAs/Ge triple junction solar cell efficiency with reduced growth temperature of GaAs base layer

Flexible thin film (In)GaAs solar cells are grown by metalorganic chemical vapor deposition on GaAs substrates and transferred to 30 μm thick Au foil by internal stress-assisted epitaxial lift-off processes. The internal stress is induced by replacing the solar cell epi-layers from GaAs to In0.015Ga0.985As, which has a slightly larger lattice constant. The compressive strained layer thickness was varied from 0 to 4.5 μm to investigate the influence of the internal stress on the epitaxial lift-off time. The etching time in the epitaxial lift-off process was reduced from 36 to 4 h by employing a GaAs/In0.015Ga0.985As heterojunction structure that has a compressive film stress of −59.0 MPa. We found that the partially strained epi-structure contributed to the much faster lateral etching rate with spontaneous bending. Although an efficiency degradation problem occurred in the strained solar cell, it was solved by optimizing the epitaxial growth conditions.

Numerical Simulation of GaInP/AlInP Window Layer For High Concentration Photovoltaic Cells

Tellurium (Te) doping of InGaP with diethyl Te (DETe) was investigated to apply for tunnel junctions (TJs) in multijunction solar cells. When the DETe flow rate was 2 sccm, the electron concentration was found to increase with decreasing growth temperature. The obtained electron concentration reached up to ~1 × 1019 cm-3. The photovoltaic (PV) properties of InGaP/InGaAs/Ge triple-junction solar cells with a Te-doped InGaP layer in TJ were measured under concentrated light condition. Here, Te-doped InGaP layers in TJ were prepared using different growth temperatures ranging from 540 °C to 660 °C. Compared with other samples, open-circuit voltage (VOC), fill factor, and conversion efficiency were higher for the sample with an InGaP grown at 570 °C. For this sample, external quantum efficiency was also found to be higher than the other samples, associated with the improved crystalline quality.

Internal stress-assisted epitaxial lift-off process for flexible thin film (In)GaAs solar cells on metal foil

We investigated the optical and electrical properties of Si-doped polar (c-plane) and nonpolar (a-plane) GaN layers grown by metal-organic vapor phase epitaxy (MOVPE) using photoluminescence (PL) and Hall-effect measurements. For c-plane GaN, the dominant emission bands were related to band-acceptor and donor-acceptor transitions, and mobility was dominated by optical phonon and ionized impurity scattering at high and low temperature, respectively. For a-plane GaN, the dominant emission bands were related to extended defects such as basal stacking faults (BSFs) and prismatic stacking faults (PSFs) and mobility was dominated by the scattering mechanism due to dislocations.