To keep up with the increasing demand for solar energy, higher efficiencies are necessary while keeping cost at a minimum. The easiest theoretical way to achieve that is using silicon-based multi-junction solar cells. However, there are major challenges in effectively implementing such a system. Much work has been done recently to integrate III-V with Si for multi-junction solar cell purposes. The focus of this paper is to explore GaP-based dilute nitrides as a possible top cell candidate for Si-based multi-junctions. The direct growth of dilute nitrides in a lattice-matched configuration epitaxially in literature is reviewed. The problems associated with such growths are outlined and pathways to mitigate these problems are presented. The need for a GaP buffer layer between the dilute nitride film and Si is established. Defects in GaP/Si system are explored in detail and a study on pit formation during such growth is performed. Effective suppression of pits in GaP surface grown on Si is achieved. Issues facing GaP-based dilute nitrides in terms of material properties are outlined. Review of these challenges is done and some possible future areas of interest to improve material quality are established. Finally, the growth process of dilute nitrides using Molecular Beam Epitaxy tool is explained. Results for GaNP grown on Si pre and post growth treatments are detailed.

Unlike conventional solar cells, modern high efficiency passivated contacts solar cells like silicon heterojunction (SHJ) cells have excellent surface passivation and use high bulk lifetime wafers which increase the operating injection level of these devices. These solar cell architectures can benefit from having lower doped substrates, with undoped solar cells becoming an attractive option. There has been very limited literature on high bulk resistivity substrates (>>10 Ωcm). This thesis work provides a comprehensive assessment of the potential of high resistivity/undoped substrates for high performance and more reliable silicon solar cells by demonstrating the results from modeling as well as characterization of SHJ solar cells fabricated with high resistivity/undoped substrates under real-world illumination and temperature conditions that the cells/modules experience in the field. In this work, the results from the analytical model demonstrated the effects of various defects, variation in doping and temperature on the performance of silicon solar cells. Experimentally, SHJ cells with bulk resistivities in the range of 1 Ωcm to >15k Ωcm were fabricated, and cell efficiencies over 20% were measured at standard testing conditions (STC) across the entire range of bulk resistivities. The illumination response (0.1-1 sun) and temperature coefficients (25-90 °C) were shown to be independent of the bulk resistivity. No light induced degradation was observed in the n-type SHJ cells of all resistivity ranges whereas high resistivity p-type SHJ cells showed less degradation compared to that of commercial resistivity range (<10 Ωcm). Very high reverse breakdown voltages (over 1 kV) were demonstrated for SHJ cells fabricated with high resistivity wafers. Using simulation, the importance of having cells in the modules with breakdown voltage higher than the series string voltage for safe and reliable operation of the photovoltaic (PV) system was highlighted. The ingot yield can be improved by moving towards high resistivity ranges to manufacture high efficiency reliable solar cells by utilizing the entire ingot and eliminating the need to adhere to narrow resistivity range. Thus, the novel findings from this work can have profound impact on ingot and module manufacturing resulting in significant cost savings as well as improvement in the system reliability.

The molecular beam epitaxy growth of the III-V semiconductor alloy indium arsenide antimonide bismide (InAsSbBi) is investigated over a range of growth temperatures and V/III flux ratios. Bulk and quantum well structures grown on gallium antimonide (GaSb) substrates are examined. The relationships between Bi incorporation, surface morphology, growth temperature, and group-V flux are explored. A growth model is developed based on the kinetics of atomic desorption, incorporation, surface accumulation, and droplet formation. The model is applied to InAsSbBi, where the various process are fit to the Bi, Sb, and As mole fractions. The model predicts a Bi incorporation limit for lattice matched InAsSbBi grown on GaSb.The optical performance and bandgap energy of InAsSbBi is examined using photoluminescence spectroscopy. Emission is observed from low to room temperature with peaks ranging from 3.7 to 4.6 μm. The bandgap as function of temperature is determined from the first derivative maxima of the spectra fit to an Einstein single oscillator model. The photoluminescence spectra is observed to significantly broaden with Bi content as a result of lateral composition variations and the highly mismatched nature of Bi atoms, pairs, and clusters in the group-V sublattice.
A bowing model is developed for the bandgap and band offsets of the quinary alloy GaInAsSbBi and its quaternary constituents InAsSbBi and GaAsSbBi. The band anticrossing interaction due to the highly mismatched Bi atoms is incorporated into the relevant bowing terms. An algorithm is developed for the design of mid infrared GaInAsSbBi
quantum wells, with three degrees freedom to independently tune transition energy, in plane strain, and band edge offsets for desired electron and hole confinement.
The physical characteristics of the fundamental absorption edge of the relevant III-V binaries GaAs, GaSb, InAs, and InSb are examined using spectroscopic ellipsometry. A five parameter model is developed that describes the key physical characteristics of the absorption edge, including the bandgap energy, the Urbach tail, and the absorption coefficient at the bandgap.
The quantum efficiency and recombination lifetimes of bulk InAs0.911Sb0.089 grown by molecular beam epitaxy is investigated using excitation and temperature dependent steady state photoluminescence. The Shockley-Read-Hall, radiative, and Auger recombination lifetimes are determined.

The emergence of perovskite and practical efficiency limit to silicon solar cells has opened door for perovskite and silicon based tandems with the possibility to achieve >30% efficiency. However, there are material and optical challenges that have to be overcome for the success of these tandems. In this work the aim is to understand and improve the light management issues in silicon and perovskite based tandems through comprehensive optical modeling and simulation of current state of the art tandems and by characterizing the optical properties of new top and bottom cell materials. Moreover, to propose practical solutions to mitigate some of the optical losses.

Highest efficiency single-junction silicon and bottom silicon sub-cell in silicon based tandems employ monocrystalline silicon wafer textured with random pyramids. Therefore, the light trapping performance of random pyramids in silicon solar cells is established. An accurate three-dimensional height map of random pyramids is captured and ray-traced to record the angular distribution of light inside the wafer which shows random pyramids trap light as well as Lambertian scatterer.

Second, the problem of front-surface reflectance common to all modules, planar solar cells and to silicon and perovskite based tandems is dealt. A nano-imprint lithography procedure is developed to fabricate polydimethylsiloxane (PDMS) scattering layer carrying random pyramids that effectively reduces the reflectance. Results show it increased the efficiency of planar semi-transparent perovskite solar cell by 10.6% relative.

Next a detailed assessment of light-management in practical two-terminal perovskite/silicon and perovskite/perovskite tandems is performed to quantify reflectance, parasitic and light-trapping losses. For this first a methodology based on spectroscopic ellipsometry is developed to characterize new absorber materials employed in tandems. Characterized materials include wide-bandgap (CH3NH3I3, CsyFA1-yPb(BrxI1-x)3) and low-bandgap (Cs0.05FA0.5MA0.45(Pb0.5Sn0.5)I3) perovskites and wide-bandgap CdTe alloys (CdZnSeTe). Using this information rigorous optical modeling of two-terminal perovskite/silicon and perovskite/perovskite tandems with varying light management schemes is performed. Thus providing a guideline for further development.

A Fundamental study of bulk, layered, and monolayers bromide lead perovskites structural, optical, and electrical properties have been studied as thickness changes. X-Ray Diffraction (XRD) and Raman spectroscopy measures the structural parameter showing how the difference in the thicknesses changes the crystal structures through observing changes in average lattice constant, atomic spacing, and lattice vibrations.

Optical and electrical properties have also been studied mainly focusing on the thickness effect on different properties where the Photoluminescence (PL) and exciton binding energies show energy shift as thickness of the material changes. Temperature dependent PL has shown different characteristics when comparing methylammonium lead bromide (MAPbBr3) to butylammonium lead bromide (BA2PbBr4) and comparing the two layered n=1 materials butylammonium lead bromide (BA2PbBr4) to butylammonium lead iodide (BA2PbI4). Time-resolved spectroscopy displays different lifetimes as thickness of bromide-based perovskite changes. Finally, thickness dependence (starting from monolayers) Kelvin Probe Force Microscopy (KPFM) of the layered materials BA2PbBr4, Butylammonium(methylammonium)lead bromide (BA2MAPb2Br7), and molybdenum sulfide (MoS2) were studied showing an exponential relation between the thickness of the materials and their surface potentials.

This comprehensive library of photovoltaic functions (PVSimLib) is an attempt to help the photovoltaics community to solve one of its long-lasting problems, the lack of a simple, flexible and comprehensive tool that can be used for photovoltaic calculations. The library contains a collection of useful functions and detailed examples that will show the user how to take advantage of the resources present in this library. The results will show how in combination with other Python libraries (Matplotlib), this library becomes a powerful tool for anyone involved in solar power.

It has been a long-standing goal to epitaxially integrate III-V alloys with Si substrates which can enable low-cost microelectronic and optoelectronic systems. Among the III-V alloys, gallium phosphide (GaP) is a strong candidate, especially for solar cells applications. Gallium phosphide with small lattice mismatch (~0.4%) to Si enables coherent/pseudomorphic epitaxial growth with little crystalline defect creation. The band offset between Si and GaP suggests that GaP can function as an electron-selective contact, and it has been theoretically shown that GaP/Si integrated solar cells have the potential to overcome the limitations of common a-Si based heterojunction (SHJ) solar cells.

Despite the promising potential of GaP/Si heterojunction solar cells, there are two main obstacles to realize high performance photovoltaic devices from this structure. First, the growth of the polar material (GaP) on the non-polar material (Si) is a challenge in how to suppress the formation of structural defects, such as anti-phase domains (APD). Further, it is widely observed that the minority-carrier lifetime of the Si substrates is significantly decreased during epitaxially growth of GaP on Si.

In this dissertation, two different GaP growth methods were compared and analyzed, including migration-enhanced epitaxy (MEE) and traditional molecular beam epitaxy (MBE). High quality GaP can be realized on precisely oriented (001) Si substrates by MBE growth, and the investigation of structural defect creation in the GaP/Si epitaxial structures was conducted using high resolution X-ray diffraction (HRXRD) and high resolution transmission electron microscopy (HRTEM).

The mechanisms responsible for lifetime degradation were further investigated, and it was found that external fast diffusors are the origin for the degradation. Two practical approaches including the use of both a SiNx diffusion barrier layer and P-diffused layers, to suppress the Si minority-carrier lifetime degradation during GaP epitaxial growth on Si by MBE were proposed. To achieve high performance of GaP/Si solar cells, different GaP/Si structures were designed, fabricated and compared, including GaP as a hetero-emitter, GaP as a heterojunction on the rear side, inserting passivation membrane layers at the GaP/Si interface, and GaP/wet-oxide functioning as a passivation contact. A designed of a-Si free carrier-selective contact MoOx/Si/GaP solar cells demonstrated 14.1% power conversion efficiency.

Cadmium Telluride (CdTe) possesses preferable optical properties for photovoltaic (PV) applications: a near optimum bandgap of 1.5 eV, and a high absorption coefficient of over 15,000 cm-1 at the band edge. The detailed-balance limiting efficiency is 32.1% with an open-circuit voltage (Voc) of 1.23 V under the AM1.5G spectrum. The record polycrystalline CdTe thin-film cell efficiency has reached 22.1%, with excellent short-circuit current densities (Jsc) and fill-factors (FF). However, the Voc (~900 mV) is still far below the theoretical value, due to the large non-radiative recombination in the polycrystalline CdTe absorber, and the low-level p-type doping.

Monocrystalline CdTe/MgCdTe double-heterostructures (DHs) grown on lattice-matched InSb substrates have demonstrated impressively long carrier lifetimes in both unintentionally doped and Indium-doped n-type CdTe samples. The non-radiative recombination inside of, and at the interfaces of the CdTe absorbers in CdTe/MgCdTe DH samples has been significantly reduced due to the use of lattice-matched InSb substrates, and the excellent passivation provided by the MgCdTe barrier layers. The external luminescent quantum efficiency (η_ext) of n-type CdTe/MgCdTe DHs is up to 3.1%, observed from a 1-µm-thick CdTe/MgCdTe DH doped at 1017 cm-3. The 3.1% η_ext corresponds to an internal luminescent quantum efficiency (η_int) of 91%. Such a high η_ext gives an implied Voc, or quasi-Fermi-level splitting, of 1.13 V.

To obtain actual Voc, the quasi-Fermi-level splitting should be extracted to outside the circuit using a hole-selective contact layer. However, CdTe is difficult to be doped p-type, making it challenging to make efficient PN junction CdTe solar cells. With the use of MgCdTe barrier layers, the hole-contact layer can be defective without affecting the voltage. P-type hydrogenated amorphous silicon is an effective hole-selective contact for CdTe solar cells, enabling monocrystalline CdTe/MgCdTe DH solar cells to achieve Voc over 1.1 V, and a maximum active area efficiency of 18.8% (Jsc = 23.3 mA/cm2, Voc = 1.114 V, and FF = 72.3%). The knowledge gained through making the record-efficiency monocrystalline CdTe cell, particularly the n-type doping and the double-heterostructure design, may be transferable to polycrystalline CdTe thin-film cells and improve their competitiveness in the PV industry.

A basic theory and terminology that comprehensively applies to all different types

of contacts in silicon solar cells has, thus far, been elusive. While the well established diode model has been applied to many of the complex contacts, the theory is not adequate to intuitively describe the characteristics of novel contacts. This thesis shows that the many desirable characteristics of contacts that are discussed in the literature—carrier selectivity, passivation, and low majority carrier conductance, key among them—originate from the resistance to electrons and holes in the contact. These principles are applied to describe a few popular contact technologies in order to pave the path to envisioning novel contacts. Metrics for contact performance is introduced to quantify each of the above characteristics using the two carrier resistances. The the validity of the proposed metrics is explored using extensive PC-1D simulations.