Cyrium Technologies (Canada)

The effect of Al on the surface morphology of chemical beam epitaxy grown AlGaNAs alloys is studied. Pits attributed to N clustering appearing on the dilute nitride surface become smaller, denser, and more uniformly distributed in the presence of Al. This reveals that the introduction of Al results in more homogenous N atoms spatial distribution. A growth temperature study reveals the formation of 3D structures at high temperature due to phase separation. The density of these structures decreases, while their diameter and height increase when the temperature is raised from 380 °C to 565 °C. At growth temperatures in the 380–420 °C range, the phase separation is suppressed and the growth mode is 2D. At 420 °C, the N incorporation is also maximized, making it the optimum temperature. The absorption coefficient and the bandgap of AlGaNAs alloys are extracted from transmittance measurement. A good agreement is obtained between the experimentally measured bandgap and the theoretical values calculated using the band anticrossing model. A bandgap as low as 1.22 eV was reached using Al and N concentrations of ∼15% and ∼3.4%, respectively.

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 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{{\rm 2}}$</tex> </formula> /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.

This presentation reviews how multi-junction III-V cells are optimized for performance and cost, using semiconductor bandgap engineering. Conversion efficiencies around 40% are obtained when used in CPV applications at concentrations between 100 and 1000 suns.

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.

InAs quantum dots in a GaAs matrix are studied. Those quantum dots are used in applications to enhance the overall efficiency of multi-junction solar cells beyond 40%. Photoluminescence measurements at 77 K using a 532 nm laser have been performed on an epitaxially grown structure of self-assembled InAs quantum dots in a GaAs matrix upon a Ge substrate, where three energy levels are determined at E_n=0=1.01 eV, E_n=1=1.07 eV and E_n=2=1.13 eV. Theoretical calculations of the energy levels determine the quantum dots to be 7 nm high and have a 37 nm base diameter, which is close to atomic force microscopy measurements performed on the samples. Intensity dependant photoluminescence measurements reveal the saturation of the first excited energy level at 5&times;10⁶ W/m². A general model for the saturation of the first quantum dot excited energy level is then developed. This saturation model is applied to the AM1.5D solar spectrum at 297 K to determine the concentration of solar energy needed to saturate the first excited energy level within a multi-junction solar cell. Saturation was determined to be at ~1.56&times;10⁴ suns (where 1 sun = 1000 W/m²). Since current solar concentrations are between 500-1000 suns concentration, the saturation of such quantum dots will not occur.

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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> ¿·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> to ~0.3×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> ¿·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for the operational range of a multi-junction solar cell under concentration.