Artur Turala

Antireflection Coating Design for Triple-Junction III–V/Ge High-Efficiency Solar Cells Using Low Absorption PECVD Silicon Nitride

The design of antireflection coating (ARC) for multijunction solar cells is challenging due to the broadband absorption and the need for current matching of each subcell. Silicon nitride, which is deposited by plasma-enhanced chemical vapor deposition (PECVD) using standard conditions, is widely used in the silicon wafer solar cell industry but typically suffers from absorption in the short-wavelength range. We propose the use of silicon nitride deposited by low-frequency PECVD (LFSiN) optimized for high refractive index and low optical absorption as a part of the ARC design for III–V/Ge triple-junction solar cells. This material can also act as a passivation/encapsulation coating. Simulations show that the SiO

AlGaAs tunnel junction for high efficiency multi-junction solar cells: Simulation and measurement of temperature-dependent operation

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CURRENT–VOLTAGE MEASUREMENTS WITHIN THE NEGATIVE DIFFERENTIAL RESISTANCE REGION OF AlGaAs/AlGaAs TUNNEL JUNCTIONS FOR HIGH CONCENTRATION PHOTOVOLTAICS

/LFSiN double-layer ARC can be very effective in reducing the reflection losses over the wavelength range of the limiting subcell for top subcell-limited, as well as middle subcell-limited, triple-junction solar cells. We also demonstrate that the structure’s performance is stable over expected variations in the layer parameters (thickness and refractive index) in the vicinity of the optimal values.

Time-dependent analysis of AlGaAs/AlGaAs tunnel junctions for high efficiency multi-junction solar cells

AlGaAs tunnel junctions are shown to be well-suited to concentrated photovoltaics where temperatures and current densities can be dramatically higher than for 1-sun flat-panel systems. Detailed comparisons of AlGaAs/AlGaAs tunnel junction experimental measurements over a range of temperatures expected during device operation in concentrator systems are presented. Experimental and simulation results are compared in an effort to decouple the tunnel junction from the overall multi-junction solar cell. The tunnel junction resistance is experimentally studied as a function of the temperature to determine its contribution to overall efficiency of the solar cell. The current-voltage behavior of the isolated TJ shows that as the temperature is increased from 25°C to 85°C, the resistance decreases from ~4.7×10-4 ¿·cm2 to ~0.3×10-4 ¿·cm2 for the operational range of a multi-junction solar cell under concentration.

Thermal behaviors and ageing of GaAs and InGaP solar cells for thermal-CPV hybrid energy systems

The current–voltage characteristics of AlGaAs/AlGaAs tunnel junctions for use in multi-junction solar cells are studied experimentally, where tunneling current peaks of 1100 A/cm2 and specific contact resistivities of 0.3 × 10-4Ω⋅cm2 at 7 A/cm2 (typical concentrated photovoltaic operating current) are measured. This represents an ideal tunnel junction design, with a very low resistance and one of the highest tunneling peak currents reported for solar cells. Normally, solar cell current–voltage characteristics are measured using time-averaged methods, which, in this study, reveal a tunneling peak current density of ~950 A/cm2. Due to nonlinear oscillations within the measurement circuit, the precise locations and magnitudes of the tunneling peak and valley current densities are obscured when using time-average measurement methods. Here we present an alternative method to determine the tunneling peak current density, in which the nonlinear oscillations in the current and voltage are recorded over time and a current density–voltage curve is reconstructed. This time-dependent method results in a measured tunneling peak current density of ~ 1100 A/cm2. The nonlinear oscillations of the experimental circuit are reproduced by modeling an equivalent circuit, resulting in qualitative agreement with the observed oscillations. This model predicts the capacitance and inductance of the equivalent circuit to be approximately 3 nF and 3.5 μH, respectively. This numerical model can be used to determine the inductance and the capacitance of any circuit having a negative differential resistance region.

Chemical beam epitaxy growth of AlGaAs/GaAs tunnel junctions using trimethyl aluminium for multijunction solar cells

The current density-voltage characteristic of an AlGaAs/AlGaAs tunnel junction is determined by taking a time-averaged measurement across the device. A tunnelling peak of ~950A/cm2 is recorded by this method. Measurements of the tunnelling peak and valley currents by the time averaging method are obscured due to the unstable nature of the negative differential resistance region of the current density-voltage characteristic. This AlGaAs/AlGaAs tunnel junction is then biased inside the negative differential resistance region of the current density-voltage characteristic, causing the current and the voltage to oscillate between the peak and the valley. The current and voltage oscillations are measured over time and then currents and voltages corresponding to the same time stamps are plotted against each other to form a timedependent curve from which a tunnelling peak of a value larger than 1100A/cm2 is determined. The peak determined by this method is 11-20% larger than previously determined using the time averaged measurement. An AlGaAs/InGaP tunnel junction having no negative differential resistance region is also presented.

GaAs, AlGaAs and InGaP Tunnel Junctions for Multi‐Junction Solar Cells Under Concentration: Resistance Study

The main parameters of InGaP and GaAs thin solar cells (SCs) at average light concentration ratios (X) of ~54 and ~93 suns were investigated in the temperature range 25-250 °C. The main parameters of the two devices showed quasi-linear behaviors with increasing the operating temperature. The conversion efficiencies were found to drop ~27 and ~40 % in InGaP and GaAs, respectively independently of the two concentration ratios. Furthermore, the two devices showed a decrease in output power averagely ranging from 25 to 27 %, which yields to a difference less than 2%. In term of thermal reliability, the two devices did not show significant degradation after approximately 4 months of heat dumping. Hence, these results imply that GaAs still deliver more output power at 250 °C.

Isolation of III-V/Ge Multijunction Solar Cells by Wet Etching

AlGaAs/GaAs tunnel junctions for use in high concentration multijunction solar cells were designed and grown by chemical beam epitaxy (CBE) using trimethyl aluminium (TMA) as the p-dopant source for the AlGaAs active layer. Controlled hole concentration up to 4⋅1020 cm−3 was achieved through variation in growth parameters. Fabricated tunnel junctions have a peak tunneling current up to 6140 A/cm2. These are suitable for high concentration use and outperform GaAs/GaAs tunnel junctions.

Advances in Cell Carriers for CPV Applications

The following four TJ designs, AlGaAs/AlGaAs, GaAs/GaAs, AlGaAs/InGaP and AlGaAs/GaAs are studied to determine minimum doping concentration to achieve a resistance of 5×10−4 ωcm2.

Performance comparison of AlGaAs, GaAs and InGaP tunnel junctions for concentrated multijunction solar cells

Microfabrication cycles of III-V multijunction solar cells include several technological steps and end with a wafer dicing step to separate individual cells. This step introduces damage at lateral facets of the junctions that act as charge trapping centers, potentially causing performance and reliability issues, which become even more important with today’s trend of cell size reduction. In this paper we propose a process of wet etching of microtrenches that allows electrical isolation of individual solar cells with no damage to the sidewalls. Etching with bromine-methanol, the solution that is typically used for nonselective etching of III-V compounds, results in the formation of unwanted holes on the semiconductor surfaces. We investigate the origin of holes formation and discuss methods to overcome this effect. We present an implementation of the isolation step into a solar cell fabrication process flow. This improved fabrication process opens the way for improved die strength, yield, and reliability.