Vincent Aimez

Ultrahigh efficiencies in vertical epitaxial heterostructure architectures

Optical to electrical power converting semiconductor devices were achieved with breakthrough performance by designing a Vertical Epitaxial Heterostructure Architecture. The devices are featuring modeled and measured conversion efficiencies greater than 65%. The ultrahigh conversion efficiencies were obtained by monolithically integrating several thin GaAs photovoltaic junctions tailored with submicron absorption thicknesses and grown in a single crystal by epitaxy. The heterostructures that were engineered with a number N of such ultrathin junctions yielded an optimal external quantum efficiencies approaching 100%/N. The heterostructures are capable of output voltages that are multiple times larger than the corresponding photovoltage of the input light. The individual nanoscale junctions are each generating up to ∼1.2 V of output voltage when illuminated in the infrared. We compare the optoelectronic properties of phototransducers prepared with designs having 5 to 12 junctions and that are exhibiting volt...

Advances with vertical epitaxial heterostructure architecture (VEHSA) phototransducers for optical to electrical power conversion efficiencies exceeding 50 percent

A monolithic compound semiconductor phototransducer optimized for narrow-band light sources was designed for and has achieved conversion efficiencies exceeding 50%. The III-V heterostructure was grown by MOCVD, based on the vertical stacking of a number of partially absorbing GaAs n/p junctions connected in series with tunnel junctions. The thicknesses of the p-type base layers of the diodes were engineered for optimal absorption and current matching for an optical input with wavelengths centered in the 830 nm to 850 nm range. The device architecture allows for improved open-circuit voltage in the individual base segments due to efficient carrier extraction while simultaneously maintaining a complete absorption of the input photons with no need for complicated fabrication processes or reflecting layers. Progress for device outputs achieving in excess of 12 V is reviewed in this study.

Electrothermal Mapping of AlGaN/GaN HEMTs Using Microresistance Thermometer Detectors

Self-heating effects in AlGaN/GaN high-electron mobility transistors (HEMTs) can notably reduce electron mobility and produce reliability concerns. Electrothermal characterization and appropriate thermal management are required to address this situation. This letter presents the measurement of channel temperature (T_ch) of GaN HEMTs in multiple bias conditions with a good accuracy. The measurements are executed using the integrated microresistance thermometer detector (μRTD) technique in AlGaN/GaN HEMTs on SiC and sapphire substrates. The integrated Ti/Pt μRTD sensor with linear resistance-temperature characteristic is used to obtain an I_ds-V_ds-T_ch map for each device. Thermal resistances are compared for similar operation conditions, obtaining RTH = 34.7 °C · W^-1 for the HEMT on SiC and RTH = 157.2 °C · W^-1 for the HEMT on sapphire.

Effect of Dot-Height Truncation on the Device Performance of Multilayer InAs/GaAs Quantum Dot Solar Cells

The effect of dot-height truncation on the device performance of multilayer InAs/GaAs quantum dot solar cells is investigated. The different structures were grown by chemical beam epitaxy, and an indium-flush process is used to control the dot height. A series of ten-layer samples with dots truncated at a height of 5 and 2.5 nm, respectively, are studied. Luminescence, atomic force microscopy, and high-resolution scanning transmission electron microscopy results indicate that the quantum dot properties are preserved up to the tenth layer for both structures. Under 1-sun illumination, the truncation of the dot height to 2.5 nm increased the short-circuit current density by 0.7 mA/cm2 and the open-circuit voltage by 31 mV. From the external quantum efficiency curves, limited to wavelengths> 500 nm, a 1.46-mA/cm2 current density enhancement is found over a GaAs reference cell. At least 45% of this enhancement has been attributed uniquely to the presence of quantum dots in the structure.

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

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.

A CMOS Buried Quad p-n Junction Photodetector Model

A buried quad junction (BQJ) photodetector has been designed and fabricated with a high-voltage CMOS process. It implements four vertically stacked p-n junctions with four different spectral responses. This feature allows high spectral discriminating ability, greater than both conventional buried double junction and buried triple junction detectors. In this paper, we propose a SPICE-like model, based on the physical properties of the device structure. The proposed model has been integrated in EDA software. It could be used for rapid and reliable design of system on chip, integrating the BQJ sensor, and its signal processing. The analytical expressions of the four BQJ photocurrents, as well as dark currents, have been developed. The spectral characteristics of the photodetector, computed with the proposed model, have been compared with those from TCAD simulations and experimental measurements. The analytical is close to the measurement with an average error on spectral responses in the range of 3%-17%, depending on the considered junction.

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

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.

Trough-Lens-Cone optics with microcell arrays: High efficiency at low cost

Tandem cells give HCPV high efficiency, but HCPV has been burdened by high optics, receiver, and balance-of-module costs. Trough/Lens/Cone optics can provide arrays of very-high-concentration microcell-sized foci at low cost, multi-microcell receivers can match the arrays of foci at low receiver cost, and the trough’s initial concentration can reduce sealed-module size and balance-of-module costs. Efficiency and cost analyses indicate that Trough-Lens-Cone modules should provide higher efficiency than Fresnel/box CPV, at a cost that is competitive with silicon PV.

Permanent bonding process for III-V/Ge multijunction solar cell integration

In this paper, we propose a process to permanently bond multi-junction solar cells (MJSCs) front side to a transparent superstrate using polydimethylsiloxane (PDMS). This process could allow for substrate handling during several fabrication schemes such as thin MJSCs, substrate lift-off /recycling or through cell via contacts (TCVC) solar cells. The developed process is compatible with standard microfabrication processes such as photolithography, etching or metallization. Permanent bonding of III-V/Ge triple junction solar cell on quartz superstrate has been used to handle the solar cell and to demonstrate a 20 µm thick TCVC device. No significant performance loss was observed on I-V characteristics for a bonded triple junction solar cell.In this paper, we propose a process to permanently bond multi-junction solar cells (MJSCs) front side to a transparent superstrate using polydimethylsiloxane (PDMS). This process could allow for substrate handling during several fabrication schemes such as thin MJSCs, substrate lift-off /recycling or through cell via contacts (TCVC) solar cells. The developed process is compatible with standard microfabrication processes such as photolithography, etching or metallization. Permanent bonding of III-V/Ge triple junction solar cell on quartz superstrate has been used to handle the solar cell and to demonstrate a 20 µm thick TCVC device. No significant performance loss was observed on I-V characteristics for a bonded triple junction solar cell.

Design of thin InGaAsN(Sb) n-i-p junctions for use in four-junction concentrating photovoltaic devices

Abstract. Four-junction solar cells for space and terrestrial applications require a junction with a band gap of ∼1  eV for optimal performance. InGaAsN or InGaAsN(Sb) dilute nitride junctions have been demonstrated for this purpose, but in achieving the 14  mA/cm2 short-circuit current needed to match typical GaInP and GaAs junctions, the open-circuit voltage (VOC) and fill factor of these junctions are compromised. In multijunction devices incorporating materials with short diffusion lengths, we study the use of thin junctions to minimize sensitivity to varying material quality and ensure adequate transmission into lower junctions. An n-i-p device with 0.65-μm absorber thickness has sufficient short-circuit current, however, it relies less heavily on field-aided collection than a device with a 1-μm absorber. Our standard cell fabrication process, which includes a rapid thermal anneal of the contacts, yields a significant improvement in diffusion length and device performance. By optimizing a four-junction cell around a smaller 1-sun short-circuit current of 12.5  mA/cm2, we produced an InGaAsN(Sb) junction with open-circuit voltage of 0.44 V at 1000 suns (1  sun=100  mW/cm2), diode ideality factor of 1.4, and sufficient light transmission to allow >12.5  mA/cm2 in all four subcells.