Highly reflective back surfaces are critical for reaching the detailed balance efficiency limits of photovoltaics. In addition to being highly reflective, the back surface and contact of the cell must have low resistance. A traditional approach to balance reflectance and contact resistance has been to use point contact geometries, which are process intensive. This work considers using a transparent conductive oxide and metal mirror, which, due to being two planar layers, can be fabricated much more easily. To study the tradeoff between resistance and absorptance for this contact, the oxide doping concentration is varied. Test structures to measure the doping concentration, contact resistance, and parasitic absorptance were fabricated. Using measured parameters, the performance of high-quality GaAs photonic power converters is modeled. Measurements show that although the contact resistance is comparatively high, it can be controlled through doping in the oxide and semiconductor composition. Furthermore, modeling shows the contact resistance is not prohibitively high for one-sun or lower illumination level devices. The hemispheric reflectance of the experimental oxide/metal back contact is modeled to be 96.7%, which is quite high considering that it is a conductive back contact. Although the oxide/metal contact structure does not perform electrically or optically as well as more complex point contact structures, this work indicates the advantages of the planar transparent conductive oxide/metal contact structure near one-sun equivalent current densities for solar cells and photonic power converters, where it is desirable to avoid the device fabrication costs of back contact patterning.

In this work, experimental photonic power converter (PPC) design, fabrication and characterization has been used, along with electrical and optical modeling, to study theoretical efficiency limits of monochromatic photovoltaic (PV) energy conversion due to photon recycling. The back-surface reflectance of a photovoltaic (PV) cell is known to strongly influence external radiative efficiency, a photon recycling metric (ERE), especially when reflectance is close to 100 %. Considering a perfect back reflector, an upper PV cell efficiency limit of 70.9 % and 85 % is calculated for 870.7 nm illumination at an intensity that would generate 32 mA/cm2 (1-sun) and 100 A/cm2 (3125-sun eq) photocurrent, respectively. However, when realistic non-idealities are introduced, ideal efficiency can drop by 21 % for both cases as long as the series resistivity for cells under high intensity illumination is limited to 1 mΩ cm^2. This presents a challenge for photonic energy conversion technology where high intensity lasers are typically used to deliver power to equipment from remote locations. This work discusses ways to provide reflectance enhancement while allowing sufficient current flow at the back surface. One way to do this is to use a planar transparent conductive oxide and reflective metal at the back surface. This work measures and compares the back-surface reflectance of IZO/Ag to standard reflective/conductive materials such as Au and Ag. A comparison between cells with the highest V_OC for cells processed with Au and IZO/Ag as reflective back contacts show that the V_OC for the IZO/Ag cell outperforms that of the Au cell by 6.6 mV measuring V_OC=1.071 V with a cell efficiency of 51.0 % at 780 nm LED illumination. Efficiency calculations extrapolated to other monochromatic light sources identified 841 nm as the optimal wavelength for the IZO/Ag cells with a projected efficiency of η_cell=55.5 % for incident intensity corresponding to 1-sun photocurrent. With the fill factors comparable between the cell types, at least at intensities near 1-sun equivalent photocurrent, the IZO/Ag reflective back contact design demonstrates benefits from photon recycling while not sacrificing voltage drop due to series resistance compared to cells with a standard Au back contact.

Improving solar cell efficiency is an enormously powerful driver of the cost reduction of solar power. While the silicon solar cell efficiency approaches theoretical limits, many thin-film solar cell technologies fall behind. In particular, cadmium telluride (CdTe) solar cells have only reached a maximum efficiency of 22.1%. One of the challenges associated with the development of CdTe solar cells is due its high electron affinity and the difficulty of achieving heavy p-type doping. This challenge results in the formation of a Schottky barrier at the hole contact, which reduces solar cell efficiency, primarily through the reduction of open circuit voltage (Voc) and fill factor (FF). The Schottky barrier makes the characterization of the actual solar cell p-n junction through current voltage (I-V), capacitance voltage (C-V), and thermal admittance spectroscopy (TAS) more difficult and not straightforward. However, interpreted through accurate physical models and under the correct experimental conditions, these techniques can then also be used to extract the impact of the contact on device performance, chiefly through analysis of the barrier height. Additionally, characterization of the open circuit voltage as a function of the illumination intensity (Suns-Voc) and the open circuit voltage as a function of temperature [Voc(T)] offer insight into the potential impact of the contact barrier. A comprehensive review of characterization of the barrier through the above techniques is given, primarily through a two-diode model. Further, a discussion of the utility of electrochemical capacitance-voltage (ECV) profiling to recover carrier concentrations in device regions otherwise difficult to access through traditional C-V measurements is provided along with modeling to support this conclusion. A discussion of and justification for the experimental extraction of barrier height from TAS measurements are also provided. Experimentally measured Voc(T), C-V, and Suns-Voc characteristics are presented and compared for a CdTe and a gallium arsenide (GaAs) solar cell. Experimental results indicate that the contact barriers and other possible non-idealities strongly affect the performance of the CdTe solar cell. Modeling results demonstrate the use of ECV to characterize solar cell absorbers can offer information unavailable via conventional C-V measurements.

Realization of efficient, high-bandgap photovoltaic cells produced using economically viable methods is a technological advance that could change the way we generate and use energy, and thereby accelerate the development of human civilization. There is a need to engineer a semiconductor material for solar cells, particularly multijunction cells, that has high (1.6-2.0 eV) bandgap, has relatively inactive defects, is thermodynamically stable under normal operating conditions with the potential for cost-effective thin-film growth in mass production.This work focuses on a material system made of gallium, indium, and phosphorus – the ternary semiconductor GaInP. GaInP based photovoltaic cells in single-crystal form have demonstrated excellent power conversion efficiency, however, growth of single-crystal GaInP is prohibitively expensive. While growth of polycrystalline GaInP is expected to lower production costs, polycrystalline GaInP is also expected to have a high density of electronically active defects, about which little is reported in scientific literature. This work presents the first study of synthesis, and structural and optoelectronic characterization of polycrystalline GaInP thin films. In addition, this work models the best performance of polycrystalline solar cells achievable with a given grain size with grain-boundary/surface recombination velocity as a variable parameter. The effects of defect characteristics at the surface and layer properties such as doping and thickness on interface recombination velocity are also modeled. Recombination velocities at the free surface of single-crystal GaInP and after deposition of various dielectric layers on GaInP are determined experimentally using time-resolved photoluminescence decay measurements. In addition, experimental values of bulk lifetime and surface recombination velocity in well-passivated single crystal AlInP-GaInP based double heterostructures are also measured for comparison to polycrystalline material systems. A novel passivation method – aluminum-assisted post-deposition treatment or Al-PDT – was developed which shows promise as a general passivation and material improvement technique for polycrystalline thin films. In the GaInP system, this aluminum post-deposition treatment has demonstrated improvement in the minority carrier lifetime to 44 ns at 80 K. During development of the passivation process, aluminum diffusivity in GaInP was measured using TEM-EDS line scans. Introduction, development, and refinement of this novel passivation mechanism in polycrystalline GaInP could initiate the development of a new family of passivation treatments, potentially improving the optoelectronic response of other polycrystalline compound semiconductors as well.

This dissertation details a study of wide-bandgap molecular beam epitaxy (MBE)-grown single-crystal MgxCd1-xTe. The motivation for this study is to open a pathway to reduced $/W solar power generation through the development of a high-efficiency 1.7-eV II-VI top cell current-matched to low-cost 1.1-eV silicon. This paper reports the demonstration of monocrystalline 1.7-eV MgxCd1-xTe/MgyCd1-yTe (y>x) double heterostructures (DHs) with a record carrier lifetime of 560 nanoseconds, along with a 1.7-eV MgxCd1-xTe/MgyCd1-yTe (y>x) single-junction solar cell with a record active-area efficiency of 15.2% and a record open-circuit voltage (VOC) of 1.176 V. A study of indium-doped n-type 1.7-eV MgxCd1-xTe with a carrier activation of up to 5 × 1017 cm-3 is presented with promise to increase device VOC. Finally, this paper reports an epitaxial lift-off (ELO) technology using water-soluble MgTe for the creation of free-standing MBE-grown II-VI single-crystal CdTe and 1.7-eV MgxCd1-xTe solar cells freed from lattice-matched InSb(001) substrates. Photoluminescence (PL) spectroscopy measurements comparing intact and free-standing films reveal the survival of optical quality in CdTe DHs after ELO. This technology opens up several possibilities to drastically increase cell conversion efficiency through improved light management and transferability into monolithic multijunction devices. Lastly, this report will present considerations for future work in each of the study areas mentioned above.

This work explores the application and optimization of laser patterning of dielectrics on textured crystalline silicon for improving the performance of industrial silicon solar cells. Current direct laser patterning processes introduce defects to the surface of the solar cell as a result of the film transparency and the intensity variation of the laser induced by the textured surface. As a means of overcoming these challenges, a co-deposited protective masking film was developed that is directly patterned with laser light at greatly depreciated light intensities that allows for selective chemical etching of the underlying dielectric films without incurring substantial defects to the surface of the device. Initial defects produced by the process are carefully evaluated with electron microscopy techniques and their mechanism for generation is identified and compensated. Further, an analysis of the opening fraction within the laser spot is evaluated –the area of removed film within the laser spot divided by the area of the laser spot– and residue produced by the laser process within the contact opening is studied. Once identified, this non-damaging laser process is a promising alternative to the standard screen print and fire process currently used by industry for metallization of silicon solar cells. Smaller contacts may be made with the laser process that are as of yet unattainable with screen printing, allowing for a decrease in shading losses. Additionally, the use of patterning allows for silver-free metallization and improved conductivity in the contacts, thereby decreasing parasitic losses in the device.

Silicon photovoltaics (PV) is approaching its theoretical efficiency limit as a single-junction technology. To break this limit and further lower the PV-generated levelized cost of electricity, it is necessary to engineer a silicon-based “tandem” technology in which a solar cell of another material is stacked on top of silicon to make more efficient use of the full solar spectrum.

This dissertation understands and develops four aspects of silicon-based tandem PV technology. First, a new “spectral efficiency” concept is proposed to understand how tandem cells should be designed and to identify the best tandem partners for silicon cells. Using spectral efficiency, a top-cell-design guide is constructed for silicon-based tandems that sets efficiency targets for top cells with various bandgaps to achieve targeted tandem efficiencies.

Second, silicon heterojunction solar cells are tuned to the near-infrared spectrum to enable world-record perovskite/silicon tandems both in two- and four-terminal configurations. In particular, for the 23.6%-efficient two-terminal tandem, a single-side textured silicon bottom cell is fabricated with a low-refractive-index silicon nanoparticle layer as a rear reflector. This design boosts the current density to 18.5 mA/cm2; this value exceeds that of any other silicon bottom cell and matches that of the top cell.

Third, “PVMirrors” are proposed as a novel tandem architecture to integrate silicon cells with various top cells. A strength of the design is that the PVMirror collects diffuse light as a concentrating technology. With this concept, a gallium-arsenide/silicon PVMirror tandem is demonstrated with an outdoor efficiency of 29.6%, with respect to the global irradiance.

Finally, a simple and versatile analytical model is constructed to evaluate the cost competitiveness of an arbitrary tandem against its sub-cell alternatives. It indicates that tandems will become increasingly attractive in the market, as the ratio of sub-cell module cost to area-related balance-of-system cost—the key metric that will determine the market success or failure of tandems—is decreasing.

As an evolution of silicon technology, silicon-based tandems are the future of PV. They will allow more people to have access to clean energy at ultra-low cost. This thesis defines both the technological and economic landscape of silicon-based tandems, and makes important contributions to this tandem future.