Ian Mathews

Theoretical performance of multi-junction solar cells combining III-V and Si materials.

A route to improving the overall efficiency of multi-junction solar cells employing conventional III-V and Si photovoltaic junctions is presented here. A simulation model was developed to consider the performance of several multi-junction solar cell structures in various multi-terminal configurations. For series connected, 2-terminal triple-junction solar cells, incorporating an AlGaAs top junction, a GaAs middle junction and either a Si or InGaAs bottom junction, it was found that the configuration with a Si bottom junction yielded a marginally higher one sun efficiency of 41.5% versus 41.3% for an InGaAs bottom junction. A significant efficiency gain of 1.8% over the two-terminal device can be achieved by providing an additional terminal to the Si bottom junction in a 3-junction mechanically stacked configuration. It is shown that the optimum performance can be achieved by employing a four-junction series-connected mechanically stacked device incorporating a Si subcell between top AlGaAs/GaAs and bottom In0.53Ga0.47As cells.

A Vision for Thermally Integrated Photonics Systems

Thermal management has traditionally been relegated to the last step in the design process. However, with the exponential growth in data traffic leading to ever-greater levels of component integration and ever-higher levels of energy consumption, thermal management is rapidly becoming one of the most critical areas of research within the ICT industry. Given the vast use of optics for efficient transmission of high-speed data, this paper focuses on a new thermal solution for cooling the components within pluggable optical modules. Thermally Integrated Photonics Systems (TIPS) represents a new vision for the thermal building blocks required to enable exponential traffic growth in the global telecommunications network. In the TIPS program, existing thermal solutions cannot scale to meet the needs of exponential growth in data traffic. The main barriers to enabling further growth were identified and a research roadmap was developed around a scalable and efficient integrated thermal solution. In particular, the effects of replacing inefficient materials and large macroTECs with better thermal spreaders and μTECs are investidated. In addition, new forms of μChannel cooling into the package to more efficiently remove the heat generated by the lasers and the TECs are being studied which can lead to future photonic devices that can be deployed in a vastly more dense and integrated manner to address the requirements of future telecommunication networks.

InAlAs solar cell on a GaAs substrate employing a graded InxGa1−xAs–InP metamorphic buffer layer

Single junction In0.52Al0.48As solar cells have been grown on a (100) GaAs substrate by employing a 1 μm thick compositionally graded InxGa1−xAs/InP metamorphic buffer layer to accommodate the 3.9% mismatch. Cells processed from the 0.8 μm thick InAlAs layers had photovoltaic conversion efficiency of 5% with an open circuit voltage of 0.72 V, short-circuit current density of 9.3 mA/cm2, and a fill factor of 74.5% under standard air mass 1.5 illumination. The threading dislocation density was estimated to be 3 × 108 cm−2.

GaAs solar cells for Indoor Light Harvesting

GaAs solar cells are the highest efficiency single-junction photovoltaic technology. Their wide bandgap lends them to efficient conversion of indoor light. In this paper GaAs solar cells are experimentally compared to Dye Sensitised solar cells, traditionally used for indoor light harvesting. The power density of GaAs solar cells was found to be over 3× greater than DSSC modules under indoor light levels. A credit card sized GaAs cell can provide 0.67 mW or 4 mW to a wireless sensor node in a dimly lit corridor (~200 lux) or well-lit office space (~1000 lux) respectively. A near linear relationship is measured between the open-circuit and maximum power voltages of GaAs solar cells at low light levels allowing their use with power conditioning circuits (MPPT tracking) based on the fractional-voltage method.

Reducing thermal crosstalk in ten-channel tunable slotted-laser arrays

Given the tight constraints on the wavelength stability of sources in optical networks, the thermal crosstalk between operating devices in a ten-channel thermally-tunable slotted laser array for DWDM applications has been investigated. It was found experimentally the current standard thermal solution with the laser array chip mounted on an AlN carrier does not allow for wavelength stability of ± 25 GHz ( ± 2 K) with a temperature rise of 5 K measured in a device with 100 mA (CW) applied to a neighbouring laser (device spacing = 360 µm). A combined experimental/numerical approach revealed solid state submounts comprising diamond or highly ordered pyrolytic graphite are inadequate to reduce crosstalk below an allowable level. Numerical simulations of advanced cooling technologies reveal a microfluidic enabled substrate would reduce thermal crosstalk between operational devices on the chip to acceptable levels. Critically our simulations show this reduced crosstalk is not at the expense of device tunability as the thermal resistance of individual lasers remains similar for the base and microfluidic cases.

Towards AlN optical cladding layers for thermal management in hybrid lasers

Aluminium Nitride (AlN) is proposed as a dual function optical cladding and thermal spreading layer for hybrid ridge lasers, replacing current benzocyclobutene (BCB) encapsulation. A high thermal conductivity material placed in intimate contact with the Multi-Quantum Well active region of the laser allows rapid heat removal at source but places a number of constraints on material selection. AlN is considered the most suitable due to its high thermal conductivity when deposited at low deposition temperatures, similar co-efficient of thermal expansion to InP, its suitable refractive index and its dielectric nature. We have previously simulated the possible reduction in the thermal resistance of a hybrid ridge laser by replacing the BCB cladding material with a material of higher thermal conductivity of up to 319 W/mK. Towards this goal, we demonstrate AlN thin-films deposited by reactive DC magnetron sputtering on InP.

Patterning Submicrometer Thick Inorganic Nanoparticle Films by Solution Process and Application for Light Trapping in Solar Cells

We present a low-cost fabrication process to deposit patterned inorganic nanoparticle films with submicrometer thickness and in turn to build higher dimensional structures through sequential multilayer deposition. Oxide nanoparticle films including semiconductors, dielectrics, and conductors have been patterned by moulding or imprinting from their solvent-suspension/paste using polydimethylsiloxane stamps. The easily controlled film thickness and good duplication fidelity with high resolution allows one to fabricate various layered structures, such as double layer and multilayer structures with minimized residual materials between them to finally define quasi-3D structures. Our experiment shows that colloidal suspension of materials can readily be patterned by stamping techniques with similar quality as compared to well-developed thermal or UV imprinting using solvent-free molecule-based materials. The usability of the fabricated structure is further demonstrated by integration of a 2-D grating on dye sensitized solar cell for improved power conversion efficiency.