This paper talks about the performance improvement of a wireless sensing system when it is powered by a low power source under very low operating temperature and extreme weather conditions. Two approaches have been used to increase the system reliability: mesh network improvement and optimal energy management. The system has been operated at its full performance for two consecutive production years under freezing operation temperature using Alkaline IEC-LR20 battery as a power source.

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.

In order to optimize the design of various optoelectronic devices utilizing quantum dots (QD), we must better understand the properties of carrier capture and escape times in these systems. Some of the properties of cylindrical quantum dots were studied. The wavefunctions and eigenenergies of a cylindrical quantum dot were approximated by a product of solutions of an infinite cylinder and a quantum well. It was assumed that the majority of transitions are caused by absorption/emission of polar optical phonons, and Fermi's golden rule was applied to calculate the transition rates to and from the QD. We have restricted our calculations to one-phonon (first order) processes only. We determined the carrier capture and escape times as a function of the size of the QD and carrier density. We have shown that the smallest capture times are achieved if the QD has only one quantum level. A short capture time is also achieved for a low carrier density. Similarly, the capture time has the smallest value when the QD has only one quantum level. The dependence of the carrier escape time for a fixed dot dimension shows a minimum as a function of carrier density. For small QDs, the capture and escape times are approximately independent of carrier density.

The N incorporation is studied in AlGaNAs with low Al content grown by chemical beam epitaxy at low temperature using dimethylhydrazine as the N precursor. The incorporation efficiency is significantly enhanced by introducing a relatively low Al concentration. The relation between the N incorporation and N/(N+As) flow ratio for Al concentrations of 0–15% is presented. The highest N incorporation and the best AlGaNAs crystal quality are obtained between 400°C and 440°C, where the growth mode starts to change from 2D to 3D. The activation energies for N incorporation in both the 2D and 3D growth mode regions are extracted.

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.

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.

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.

Quantum dot (QD) enhanced GaInP/InGaAs/Ge solar cells are presented and characterized under flash and continuous solar simulators. InAs QD within the middle sub‐cell increase the carrier generation due to absorption in the range 900–940 nm. These QD‐enhanced solar cells routinely achieve production efficiencies of ∼40%, and this set of research samples obtain a peak efficiency of >38% under flash solar simulators. Continuous solar simulator testing is performed to test the QD‐enhanced solar cells under thermal loads similar to concentrated photovoltaic systems, in which cells demonstrate excellent reliability. Numerical simulations of the QD‐enhanced solar cells are performed using an effective medium to model the additional absorption due to the QD layers. Temperature dependence of the QD‐enhanced solar cells are modeled, in which temperature‐dependent bandgap narrowing changes the dark current and the semiconductor absorption profiles. Comparison between the experimental results and numerical model show good agreement under flash testing. Further work is needed to improve the match between simulation and experiment under continuous solar simulators.

AbstractFour tunnel junction (TJ) designs for multijunction (MJ) solar cells under high concentration are studied to determine the peak tunnelling current and resistance change as a function of the doping concentration. These four TJ designs are: AlGaAs/AlGaAs, GaAs/GaAs, AlGaAs/InGaP and AlGaAs/GaAs. Time‐dependent and time‐average methods are used to experimentally characterize the entire current–voltage profile of TJ mesa structures. Experimentally calibrated numerical models are used to determine the minimum doping concentration required for each TJ design to operate within a MJ solar cell up to 2000‐suns concentration. The AlGaAs/GaAs TJ design is found to require the least doping concentration to reach a resistance of &lt;10−4 Ω cm2 followed by the GaAs/GaAs TJ and finally the AlGaAs/AlGaAs TJ. The AlGaAs/InGaP TJ is only able to obtain resistances of ≥5 × 10−4 Ω cm2 within the range of doping concentrations studied. Copyright © 2010 John Wiley &amp; Sons, Ltd.