This thesis discusses two different photovoltaic systems, organic solar cells, and photonic power converters. The open-source software package Solcore was used to simulate and analyze optoelectronic properties of both systems. It is widely accepted that the transition from a fossil-fuel driven economy is necessary in the coming future. Organic solar cells are an alternative energy generation method with potential for fast energetic and economic payback periods. Bulk heterojunction organic solar cells are a common design, as they have particularly low manufacturing costs due to a simple device architecture. In this work, two bulk heterojunction blends are experimentally assessed using the acceptor molecule silicon phthalocyanine (bis(tri-n-butyl silyl oxide) silicon phthalocyanine ((3BS)2-SiPc) as a potential low-cost non-fullerene alternative to the typical acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM). These acceptors are compared within blends with the typical donor compound poly(3-hexylthiophene) (P3HT), and also poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo [1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1′,3′-di-2- thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c’]dithiophene-4,8-dione)] (PBDB-T). Device performance was assessed under standard conditions, increased angles of incidence, and reduced light intensities. Devices with the P3HT:(3BS)2-SiPc blend achieved a power conversion efficiency (PCE) of 3.6%, which outperformed P3HT:PC₆₁BM devices with a PCE of 3.0% due to a higher open-circuit voltage (VOC) of 0.76 V as opposed to 0.53 V. The PBDB-T:(3BS)2-SiPc achieved a high VOC of 1.09 V, but had a lower PCE of 3.4% in relation to the PBDB-T:PC₆₁BM device with a PCE of 6.4% and a VOC of 0.78 V. Photonic power converters are devices in optical networks that allow for optical power transmission rather than the conventional method of electrical power transmission. This provides benefits such as electrical isolation and resistance to electromagnetic interference, along with the ability to propagate along the same cable as data. These power converters are used to convert optical power to electrical power, and operate similarly to a solar cell with a narrow bandwidth. Multijunction designs are often used for increased operating voltage and efficiency. In such designs employing a vertical architecture, the bottom-most junction has the largest thickness along with the lowest efficiency due to increased recombination losses. To improve this lower efficiency, light trapping techniques can be employed to decrease the junction thickness while retaining the optical thickness. In this work, a current-matched 5- junction GaAs photonic power converter was simulated with both metallic and distributed Bragg reflectors at the rear of the device. These reflectors allowed for the thinning of the bottommost junction, which resulted in an increase in efficiency and overall power output of the power converter.

Multi-junction photovoltaic devices based on III-V semiconductors have applications in space power systems and terrestrial concentrating photovoltaics, as well as in power-over-fibre and optical power conversion systems. These devices have between two and twenty junctions arranged in tandem, connected in series with optically transparent tunnel diodes. In some cases, they may include as many as eight different materials, including ternary and quaternary alloys, and >100 epitaxial layers in total. A general method for simulating performance of these devices using drift-diffusion based device simulation tools is reviewed. This includes discussion of the geometry, discretization, and physical equations to be solved. A set of material parameters for some important materials is listed, and solutions are shown for an example of a lattice-matched four-junction GaInP / (In)AlGaAs / InGaAsN(Sb) / Ge solar cell including a dilute nitride based p-i-n junction with ∼ 0.9 eV band gap. A sample of this dilute nitride junction with a 650 nm absorber layer was grown by molecular beam epitaxy and was shown to have short-circuit current density of 15.1 mA/cm2, sufficient for use in the 4-junction structure, while transmitting sufficient light through to the bottom (germanium) junction. Open-circuit voltage was up to 0.186 V at 1-sun, increasing to 0.436 V under 1500 suns concentration. The device simulation methodology was extended to include effects of luminescent coupling and photon recycling. These effects are included by adding a term to the electron and hole continuity equations, and the resulting coupled system of equations is solved. No external iterative loop is required, as has been the case in other efforts to model these effects. A five-junction photonic power converter (PPC) is simulated and it is shown that the quantum efficiency of the device is significantly broadened through luminescent coupling. There is a 350 mV reduction in simulated open-circuit voltage (70 mV per junction) if luminescent coupling is neglected. This work was later extended to a 12-junction PPC device, where the simulation predicts a wavelength sensitivity of -1.1%/nm in the absence of luminescent coupling; this is reduced to -0.4%/nm when luminescent coupling is included in the calculation. The latter result, and the overall shape of the simulated quantum efficiency curve agree closely with experimental measurements. Finally, two specific applications of PPCs are demonstrated. The first is in a step-up DC-to-DC converter, where a linear regulator combined with a laser/PPC pair can convert a 3.3 V input (commonly available from a single lithium polymer battery cell) into 12 V. Unlike conventional switching boost converters, this ‘photonic boost converter’ is not a source of ripple. In testing, a >80 dB reduction in ripple was measured compared with an equivalent switching boost converter, limited only by input noise of the instrument.The second application is in a 60 kW, 650 V switching circuit such as might be found in a hybrid or electric vehicle drivetrain. These circuits need several isolated power supplies to power gate drivers for the IGBT or SiC MOSFET switching components. This isolation is commonly provided by a small transformer, which inherently has a parasitic capacitance between primary and secondary windings and creates a path for EMI currents to flow from the high-power components to the power supply and control circuitry. By using a laser/PPC pair to provide the needed isolation, this parasitic capacitance can be largely eliminated; a 20 dB reduction in EMI current reaching the control FPGA is demonstrated.

This thesis addresses barriers to the widespread adoption of high-efficiency photovoltaic devices through the use of innovative semiconductor materials and device design. The feasibility of various strategies is explored through experimental characterization and modeling of semiconductor materials and devices. High-efficiency photovoltaic devices are made from epitaxially grown III-V semiconductor materials. Epitaxial devices are highly sensitive to lattice mismatch between the epi-layers and the substrate, requiring sophisticated substrate engineering or growth strategies to access materials outside of the lattice-matched regime. One promising strategy involves the electrochemical porosification of germanium on a lattice-mismatched silicon substrate to create a compliant interface for high-quality epitaxial growth of Ge, GaAs, and other equivalent-bandgap III-V semiconductors on silicon. This results in a threading dislocation density of ~10^4 cm^-2, a reduction of 4 to 6 orders of magnitude compared to direct epitaxy of germanium on silicon. This technology could enable the development of highly efficient III-V multi-junction photovoltaic devices on cost-effective silicon substrates that benefit from well-established commercial supply chains. In the first part, I present characterization of the electrical properties of porous germanium. Experimental measurements revealed conductivities ranging from 0.6 to 33 (x10^-3) Ohm^-1 cm^-1, depending on the morphology. The relationship between the electrical properties and the morphology is described using an electrostatic model that can be generalized to other porous semiconductors including silicon. For a compliant interface designed to integrate a standard triple-junction solar cell onto a silicon substrate, the porous Ge/Si layers are predicted to introduce < 0.01 Ohm cm^2 of series resistance to the device, which is sufficiently low for concentrated photovoltaic applications. Optoelectronic device modelling of the triple-junction solar cell on silicon demonstrates that III-V triple-junction solar cells fabricated on silicon using this compliant Ge/Si porous interface could achieve 93% of the efficiency of a comparable defect-free device. The remainder of this thesis is concerned with the design and characterization of photovoltaic devices optimized for monochromatic illumination, known as photonic power converters. Most commercially available photonic power converters are based on GaAs and are suitable for short-range photonic power transmission through optical fiber (< 1 km). Extended reach power-over-fiber systems require the use of photonic power converters that are compatible with longer-wavelength light, which travels further in optical fiber. One candidate material for this application is the semiconductor quaternary alloy InAlGaAs lattice-matched to InP for photonic power converter operation in the telecommunications O-band, near 1310 nm. I describe the design and characterization of multi-junction InAlGaAs/InP photonic power converters grown by molecular beam epitaxy, including the analysis of material properties and characterization of single- and dual-junction devices under 1319-nm laser illumination. Optically thick devices are found to be diffusion-limited and device simulations suggest that non-radiative recombination is significant. The performance of InAlGaAs tunnel diodes, which act as interconnections for the absorbing junctions within a multi-junction device, is demonstrated to be highly dependent on the growth temperature, with peak tunneling current densities exceeding 1200 A/cm^2 in the best measured devices. In addition to molecular beam epitaxy-grown InAlGaAs/InP devices, I also characterize single-junction O-band photonic power converters grown by metal-organic vapour phase epitaxy with two alternative absorber materials. A lattice-matched InGaAsP/InP device is compared to a more cost-effective lattice-mismatched GaInAs device grown on GaAs using a metamorphic buffer layer. Both devices are measured under 1319-nm laser illumination with a variety of beam sizes and peak efficiencies of 52.9% and 48.8% were measured for the InGaAsP/InP and the metamorphic-GaInAs/GaAs devices respectively. At illumination powers exceeding 100 mW, the performance begins to degrade with increasingly non-uniform illumination, indicating that illumination profiles should be as uniform as possible to maximize device performance.

Photonic power converters (PPCs) are one of the main components of optical power transmission systems, converting optical power injected by a monochromatic optical source (laser or LED) to electrical power via the photovoltaic effect. This thesis focuses on designing and optimizing ultra-thin single junction InAlGaAs PPC with integrated back reflectors (BR) for operation at the telecommunications wavelength of 1310 nm and numerically studies the light trapping capability of three BR types: planar, cubic nanotextured, and pyramidal nanotextured. Optical simulations were performed by coupling finite difference time-domain (FDTD) calculations with a particle swarm optimization, while electrical simulations were carried out by the finite element drift-diffusion method. With 90% absorptance, optoelectrical simulations revealed that ultra-thin PPCs with 5.6- to 8.4-fold thinner absorber layers can have open circuit voltages (Voc) that are 9-12% larger and power conversion efficiencies that are 9-10% (relative) larger than conventional thick PPCs. Of the studied BR designs, pyramidal BRs exhibit the highest performance for ultra-thin designs, reaching an efficiency of 43.2% with 90% absorptance, demonstrating the superior light trapping capability relative to planar and cubic nanotextured BRs. The sensitivity of optical absorptance to variations in device thickness and incident light wavelength is also investigated numerically in thin PPCs with planar and pyramidal nanotextured BRs. Optical simulation results revealed that BR-induced resonances shift from constructive to destructive interference with thickness variations of ~100 nm and ~70 nm in planar and pyramidal nanotextured BRs, respectively. Also in PPCs with pyramidal BR, a 50 nm variation of the nanotextures’ geometry (base width and height of pyramids) drops the absorptance by more than 25% (absolute).