Current leading thin-film solar cell technologies, i.e., cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS), employ elements which are either toxic (Cd), or rare and/or expensive (In, Te, Ga, and Cd). The aim of this thesis is to investigate new, abundant, non-toxic p-type semiconductors for potential solar absorber application. Two ternary chalcogenides, Cu���PSe��� and CuTaS���, were selected for their attractive calculated optical absorption properties. Thin films of both materials were synthesized using physical vapor deposition (PVD) techniques in conjunction with post-deposition annealing. Cu���PSe��� appears promising for solar absorber applications, with a measured optical bandgap of 1.2 eV, an absorption coefficient (��) reaching 10��� cm�����, Hall mobilities of 19.8���30.3 cm��/V���s, and carrier concentrations of 3.3���4.9 10����� cm�����. Optical characterization of CuTaS��� thin-films showed a rapid turn-on of absorption, with �� exceeding 10��� cm����� within 0.5 eV of the bandgap. To date, reproducible synthesis of CuTaS��� thin films has been problematic. Moreover, these films are insulating and thus not yet appropriate for thin-film solar cell absorber applications. / Graduation date: 2013

Bulk heterojunction organic solar cells (BHJ OSCs) have been recognized as one of the most promising next generation green technology alternatives to inorganic solar cells because of the low-cost, lightweight, flexibility. Specifically, the use of small molecules instead of polymers as donors in BHJ OSC have been developed very fast recently because small molecules can be facilely synthesized and easily purified, and have a determined molecular structure without batch-to-batch variations. To date, those among the most efficient small molecules were constructed as acceptor-donor-acceptor (A-D-A) structural configuration from electron-rich units such as benzodithiophene (BDT), dithienosilole (DTS), oligothiophene units, and electron-deficient units such as benzothiadiazole (BT), diketopyrrolopyrrole (DPP), isoindigo (IID) and perylenediimide (PDI). Surprisingly, porphyrins were rarely studied either in polymers or π-conjugated small molecules as donor materials, though they have unique chemistry together with excellent photochemical and electrochemical properties, such as facile functionalization of the periphery and the variation of the central atom (metal ions), strong UV-visible absorption, ultrafast photoinduced charge separation in porphyrin-fullerene systems. In this research work, we design, synthesize and characterize new porphyrin-based small molecules with acceptor-donor-acceptor (A-D-A) configuration for bulk heterojunction organic solar cells, and investigate their structure-property relationships, specifically the effect of peripheral and backbone alkyl side-chains, π-conjugated linkers as well as electron-deficient ending units on the charge mobility, film morphology and solar cell performances. In Chapter 1, a general review on the historic and recent development of BHJ OSCs was given first, including the major components and working principle of OSC, the versatile organic semiconductors and their performances in OSCs. In chapter 2, six A-D-A structural porphyrin small molecules were designed and synthesized, in which different peripheral alkyl substitutions are attached to the meso-position of porphyrin core (CS-I, CS-II, CS-III, CS-4, CS-5 and CS-6), and 3-ethylrhodanine is used as terminal group. Their UV-visible absorption in solid, energy level, blend film morphology, charge mobility and cell performance are dependent on the different peripheral substitutions. The active layer consists of these six small molecules as donor materials and PC71BM as the acceptor material with an optimized film thickness. Although all six molecules show similar optical spectrum in solutions, the introduction of linear alkyl side chains can promote thin-film nanostructural order, especially shown to shorten π-π stacking distances between backbones and increase the correlation lengths of both π-π stacking and lamellar spacing, leading to higher efficiency in this serial. Among them, the highest power conversion efficiency of 9.09% has been achieved by CS-4 based devices. In chapter 3, another two new A-D-A porphyrin small molecules (PTTR and PTTCNR) have been developed, which are similar in structure to CS-I, II and III, except that the linker is phenylethynyl in CS-I, II and III, whereas it is terthiophenylethynyl in PTTR and PTTCNR. The highest power conversion efficiency of 8.21% is achieved by PTTCNR, corresponding to a JSC of 14.30 mA cm−2, VOC of 0.82 V, and FF of 70.01%. The excellent device performances can be ascribed to the conjugated structure of porphyrin with 3,3''-dihexyl-terthiophene and the aliphatic 2-octylundecyl peripheral substitutions, which not only effectively increase the solar flux coverage between the conventional Soret and Q bands of porphyrin unit, but also optimize molecular packing through polymorphism associated with side-chain and the π-conjugated backbones, and form the blend films with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) characteristics of bi-continuous, interpenetrating networks required for efficient charge separation and transportation.;In chapter 4, we designed and synthesized a new dimeric porphyrin donor molecule (CS-DP) containing A-π2-D-π1-D-π2-A architecture by coupling of two zinc porphyrin cores through ethynyl linker. Interestingly, it can harvests the photons up to deep near-infrared (NIR) region in the absorption spectrum. From the past decades, it has been found that developing donor molecules with the absorption spectral in NIR region is a challenging key factor to get the high performance BHJ OSCs. Solar cell devices employing CS-DP as a donor exhibit a highest power conversion efficiency of 8.23%, corresponding to JSC = 15.14 mA cm-2, VOC = 0.781 mV and FF = 69.8% under AM 1.5G solar radiation. The high efficiency of this molecule is attributed to a panchromatic IPCE action spectrum from 300 nm to 1000 nm. Also, this performance is best for the reported deep NIR organic solar cells based on single small molecule and PC71BM system so far. We envision that this new small bandgap dimeric porphyrin is very promising to use in ternary and multi-junction applications as well as NIR photodetectors. In chapter 5, a series of new A-D-A structural porphyrin small molecules (CS-10, CS-11 and CS-12) have been prepared, that contain the same meso-thienyl-thioalkyl substituted porphyrin core and 3-ethylrhodanine ending unit, but varies with different numbers of phenylethynyl linker. Using them as donors for solution-processed organic solar cells, the device based on CS-10 featuring single phenyl ethynyl π-linker exhibits high power conversion efficiency (PCE) of 7.0%. The results indicate that meso-thienyl-thioalkyl substitution and controlled π-linker length is beneficial to tune the optoelectronic properties, film morphology and consequently performance of porphyrin-based BHJ OSCs. In chapter 6, two symmetrical tetra-meso-substituted porphyrin molecules (ZnP and CuP) have been prepared in gram-scale through the direct condensation of pyrrole and 4-[bis(4-methoxyphenyl)amino]benzaldehyde. Its Zn(II) and Cu(II) complexes exhibit excellent thermal and electrochemical stability, specifically, high hole mobility and very favorable energetics for hole extraction that render them attractive for implementation as new hole transporting materials in organometallic halide perovskite solar cells (PSCs). As expected, the use of ZnP as HTM in PSCs affords a competitive PCE of 17.78%, which is comparable to the most powerful HTM of Spiro-OMeTAD (18.59%) under the same working conditions. Meanwhile, the metal centers affect somewhat the photovoltaic performances that CuP as HTM produces a relative lower PCE of 15.36%. Notably, the perovskite solar cells employing ZnP show longer stability than that of Spiro-OMeTAD. Moreover, the two porphyrin-based HTMs can be prepared from relatively cheap raw materials with a facile synthetic route. The results demonstrate that ZnP and CuP can be a new class of HTMs for efficient and stable perovskite solar cells. To the best of our knowledge, this is the highest performance for porphyrin-based perovskite solar cells with PCE > 17%. The dissertation was completed with conclusions and outlooks in chapter 7.

Photocatalytic solar fuel generation is the most green, sustainable and viable approach to deal with both the ever-growing energy crisis and environmental issues, simultaneously. The work presented in this thesis is focused on the development of new organic carbonaceous semiconductors, typically, carbon quantum dots (CQDs) and graphitic carbon nitride (g-C3N4), and porphyrin small molecules and their hybrids with graphitic carbon nitride, meanwhile, their application in the field of photocatalytic solar fuel generation. In the chapter 1, a general review about background and mechanism of photocatalytic solar fuel generation are introduced first. Next, the features and developments of porphyrin and graphitic carbon nitride for the photocatalytic redox reaction are discussed. In chapter 2, the synthesis, characterization methods and photocatalytic experiment in details are described. In chapter 3, gram-scale CQDs are facilely synthesized by simple thermal treatment of citric acid monohydrate, and microporous 1D nanorods of g-C3N4 are prepared through template-free chemical approach. The photocatalytic properties of 1D protonated g-C3N4 (HCN) modified with different amount of CQDs were evaluated by the rate of H2- evolution under visible light irritation. The results demonstrate that g-C3N4/CQDs with the optimal CQDs amount of 0.25 wt.% afford the highest H2-production rate of 382 μmol h-1 g-1 with apparent quantum yield (AQY) of 1.9% which was about 3-fold of pure g- C3N4. The composite g-C3N4/CQDs show a remarkable stability against the photocorrosion within a continuous experiment period over 12h. The enhanced photocatalytic H2-production activity could be attribute to the intimate interface between CQDs and g-C3N4, which not only significantly improves the visible-light absorption, but also prolongs the lifetime of charge carriers and suppresses the recombination of photogenerated electron-hole pairs. This work showed that low-cost and metal-free CQDs could be an efficient photosensitizer to promote photocatalytic hydrogen generation. In chapter 4, we reported a new array of push-pull isomeric naphthalimide- porphyrins (ZnT(p-NI)PP) to investigate the effect of naphthalimide and molecular constitution on light driven hydrogen evolution from water splitting. These compounds were synthesized by integration of four naphthalimide moieties on meso-substituion of porphyrin macrocycle through para phenyl linker. Porphyrins were characterized by UV- Vis, Fluorescence and DFT calculations and compared with those of zinc tertapheylporphyrin (ZnTPP). When these porphyrins were employed as photocatalyst for the photocatalytic hydrogen production (PHP) with platinum co-catalyst, they delivered high hydrogen efficiency compared to that of ZnTPP. Particularly, ZnT(p-NI)PP exhibited 203 times higher hydrogen efficiency than the ZnTPP. This could be ascribed to the efficient exciton dissociation into holes and electrons at the photoexcited state of ZnT(p-NI)PP and then electrons were transferred to the proton through platinum. These results indicate that introduction of naphthalimide at meso-position of porphyrin through para phenyl linker is a versatile strategy to improve the photocatalytic hydrogen evolution of porphyrin based materials. In addition, the other two isomeric naphthalimide conjugated porphyrins through meta-and ortho-phenyl linker, ZnT(m-NI)PP and ZnT(o-NI)PP are also developed for photocatalytic H2 production. The para-linked isomer, ZnT(p-NI)PP delivered a much higher H2 production rate of 973 μmol h−1g -1 compared to that of ZnT(m-NI)PP (597 μmol h−1g −1) and ZnT(o-NI)PP (54 μmol h−1g −1), respectively. This could be attributed to the efficient intramolecular energy transfer from the naphthalimide to the porphyrin ring. In chapter 5, a series of NP/g-C3N4 hybrids of graphitic carbon nitride (g-C3N4) with naphthalimide-porphyrin (ZnT(p-NI)PP, labelled as NP) have been developed for photocatalytic hydrogen production. Planar naphthalimide-porphyrins are adsorbed onto flexible two-dimensional g-C3N4 through π-π stacking, which are characterized by scanning electronic microscopy and X-ray photoelectron spectroscopy. Except for its function as photosensitizer, NP in the hybrids could efficient inhibit the charge recombination by electron transfer for the lower lowest unoccupied molecular orbital of NP than g-C3N4, whereas facilitate energy transfer from g-C3N4 donor to NP acceptor for efficient overlap of emission spectrum of NP and absorption of g-C3N4. As a result, the hybrid containing weigh ratio of 2% NP (2%NP/g-C3N4) exhibits an enhanced photocatalytic hydrogen production rate (HPR) of 2297 μmol g−1 h −1, while pristine g- C3N4 shows a HPR of 698 μmol g−1 h −1. The 2%NP/g-C3N4 shows the best performance when compared with the reported hybrids of g-C3N4 with Zn(II) -tetrakis(4- carboxylphenyl) porphyrin (ZnTCPP/g-C3N4) and Zn(II)-tetrakis(4- hydroxyphenyl)porphyrin (ZnTHPP/g-C3N4) in photocatalytic hydrogen production under the same conditions. In the chapter 6, the future work on photocatalytic CO2 reduction, perspectives and conclusions are included

Cu₂S/CdS solar cells have been studied extensively for the past two decades due to their potentially high efficiencies per unit cost. The operation and characteristics of Cu₂S/CdS solar cells are fairly well understood. However, the properties of the newer Cu₂S/ZnCdS cell type are not well understood. The main goals of this thesis were to compare Cu₂S/CdS and Cu₂S/ZnCdS cells using Cu₂S/CdS cells as a reference, and to understand the operation and properties of Cu₂S/ZnCdS cells in order to improve cell performance. Four different measurements were used in this research to achieve these goals. They were; electrical, spectral, capacitance and deep trap measurements. I-V measurements give important electrical parameters of the cells; cell efficiency, fill factor, short circuit current, open circuit voltage, shunt resistance and series resistance are reported. From a In(I_SC) versus V_OC measurement, the diode factor, A, was found to be about 1 for Cu₂S/CdS, Cu₂S/Zn_0.11Cd_0.89S, and about 1.2 for Cu₂S/Zn_0.25Cd_0.75S cells. The relation between In(J_oo) (current density) and ϕ (potential barrier height) is linear for both types of cells. The slope of this linear relationship increases as the content of Zn increases in Zn_xCd_1-xS. Under air mass 1 (100 mW/cm²) illumination, it was found that V_OC decays and capacitance increases for Cu₂S/ZnCdS cells. This is attributed to electron relaxation from deep traps near the junction. Spectral response with and without bias light were measured for both Cu₂S/CdS and Cu₂S/ZnCdS cells. White and blue bias light enhance the spectral response, while red bias light quenches the response. This is attributed to ionization and filling of deep traps near the junction. Capacitance measurements on both cell types show that 1/C² versus voltage is quite flat, which indicates the existence of an i-layer (insulation layer) in the CdS or ZnCdS near the junction. Three methods–photocapacitance, space-charge-limited current, and thermally stimulated. current techniques–were used for deep trap measurements. Photocapacitance measurements indicate one deep donor energy and two deep acceptor energy levels. These trap energies become larger as the content of Zn in ZnCdS increases. Space-charge-limited current measurements give a trap density of the order of 10¹⁶ cm³ for both cell types. The shallow energy trap is found to be 0.26 eV below the conduction band edge of CdS. The occurrence of a current-saturated region for Cu₂S/ZnCdS is attributed to the filling of the interface traps near the junction. Thermally stimulated current measurements give two energy levels below the conduction band of CdS; 0.05 eV and 0.26 eV. From the above results, several differences between the Cu₂S/CdS and the Cu₂S/ZnCdS cells can be seen. The Cu₂S/ZnCdS cells show stronger red quenching, smaller electron lifetime at the interface near the junction, and deeper traps than the Cu₂S/CdS cells. These differences can account for the decline of I_SC and the V_OC decay. The smaller I_SC for the Cu₂S/ZnCdS cells can also possibly result from smaller electron lifetime at the interface, larger interface recombination velocity, different deep trap levels, and enhanced Zn concentration near the junction. The V_OC decay for the Cu₂S/ZnCdS cells is mostly due to long decay of charge. Longer decay could be attributed to deeper traps. / Ph. D.

This thesis focuses on improving the adhesion of electroless metal layers plated to pit contacts in interdigitated, backside buried contact (IBBC) solar cells. In an electrolessly plated, pit contact IBBC cell, the contact grooves are replaced with lines of pits which are interconnected by the plated metal. It is shown, however, that electroless metal layers, plated by the standard IBBC plating sequence, are not adherent on pit contact IBBC solar cells. The cause of this adhesion problem is investigated by examining the adhesive properties of each of the metal layers in the electroless metallization sequence on planar test structures. This investigation reveals that Pd activation of heavily P diffused Si impedes Ni silicide growth and that, in the absence of a silicide at the Ni/Si interface, an electrolessly plated Cu layer will cause the underlying Ni layer to peel away from the substrate. It is also found that the Ni silicidation process itself intermittently causes the unreacted Ni to spontaneously peel away from the substrate. An electroless metallization sequence that results in thick, adhesive Cu deposits on planar &lt 100&gt surfaces is developed in this thesis. It is shown that this process leads to the formation of a Ni silicide on both n- and p- type, heavily diffused surfaces. Fully plated, pit contact IBBC solar cells were not able to be fabricated during the course of this work but it is reasonable to expect that the modified plating sequence developed in this work will result in the metal layers being adhesive on these cells.

Finding renewable sources of energy is becoming an increasingly important component of scientific research. Greater competition for existing sources of energy has strained the world’s supply and demand balance and has increased the prices of traditional sources of energy such as oil, coal, and natural gas. The experiment discussed in this paper is designed to identify and build an inexpensive and simple method for creating an effective organic solar cell.

Solar cells have extrinsic losses from a variety of sources which can be minimized by optimization of the design and fabrication processes. Reflection from the front surface is one such loss mechanism and has been managed in the past with the usage of planar antireflection coatings. While effective, these coatings are each limited to a single wavelength of light and do not account for varying incident angles of the incoming light source. Three-dimensional nanostructures have shown the ability to inhibit reflection for differing wavelengths and angles of incidence. Nanocones were modeled and show a broadband, multi-angled reflectance decrease due to an effective grading of the index. Finite element models were created to simulate incident light on a zinc oxide nanocone textured silicon substrate. Zinc oxide is advantageous for its ease of production, benign nature, and refractive index matching to the air source region and silicon substrate. Reflectance plots were computed as functions of incident angle and wavelength of light and compared with planar and quintic refractive index profile models. The quintic profile model exhibits nearly optimum reflection minimization and is thus used as a benchmark. Physical quantities, including height, width, density, and orientation were varied in order to minimize the reflectance. A quasi-random nanocone unit cell was modeled to better mimic laboratory results. The model was comprised of 10 nanocones with differing structure and simulated a larger substrate by usage of periodic boundary conditions. The simulated reflectance shows approximately a 50 percent decrease when compared with a planar model. When a seed layer is added, simulating a layer of non-textured zinc oxide, on which the nanocones are grown, the reflectance shows a fourfold decrease when compared with planar models. At angles of incidence higher than 75 degrees, the nanocone model outperformed the quintic model.

A research report submitted in partial fulfilment to the degree of Master of Science in the School of Physics, University of the Witwatersrand, 2018 / With the global energy consumption at an all-time high and the demand for energy estimated to triple by 2050, renewable energy sources such as solar are pivotal in an addressing this global energy demand. Solar power generation by photovoltaic cells enjoys several advantages compared to other forms of electricity generation such as a reduced fossil fuel dependence, modularity, easy and flexible installation, and scalability. The development of novel solar cells that offer increased efficiencies is an integral component of the process of addressing the global energy needs. Solar cell device simulations offer a cost-effective means to explore the impact of different material properties on the overall efficiency of the solar cell. The use of ab initio calculated material properties that serve as an import for the device simulations offers a means to easily study and estimate the typical solar cell efficiencies of different types of solar cells. The implementation of new light harnessing features, like frequency conversion layers or plasmonic nanoparticles, and the integration of these futures into existing device simulation codes serves as a useful tool that aids solar cell development. This work explores the theoretical and numerical background for the simulation of solar cell devices. A brief explanation of how ab initio calculated parameters can be used, together with the implementation of frequency conversion techniques in existing simulation codes is given. It is shown that the solar cell performance parameters can be well approximated using ab intio parameters. Also, the positive effect of frequency conversion techniques is demonstrated with examples of how this tool can be implemented in existing solar cell device simulation codes. The approaches discussed in this work can serve as a good framework for the modeling of novel solar cell devices / MT 2019

In this thesis, the structural, optical and electrical properties of crystalline silicon quantum dots (SiQDs) are examined for application to silicon based tandem cells. The approach has been to concentrate on all silicon devices by taking advantage of quantum confinement in low-dimensional Si. RF magnetron co-sputtering provided the capability of creating superlattice structures in conjunction with high temperature annealing, to form Si nanocrystals in an oxide matrix. Structural techniques, including Fourier transform infrared spectroscopy (FTIR), micro-Raman spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), and Secondary ion mass spectroscopy (SIM) were employed to gather structural information about the SiQD/SiO2 SLs. The result combine presents that the packing density of Si QDs, correlated to the oxygen content of the silicon rich oxide layer can be control independently. The effect of Si nanocrystallite density on Raman scattering is investigated. The preliminary results present that a decrease in the oxygen content (x) results in an increased sharpness of the Strokes-mode peak of nanocrystalline Si, attributed to an increase in the proportion of crystalline Si because of the increased number of SiQDs. However the influence of the surface region on the crystallite core intensity scattering becomes dominant, when SiQD size diameter is very small (less than 3 nm). The present work shows that a decrease in x-content leading to an increase of the SiQD concentration, initially results in the enhancement of the lateral conductivity in the SiQD superlattice material. In this work, the Al contacting scheme, using a prolonged heat treatment technique at elevated temperature less than the eutectic point of Al and Si (577C) has been successfully applied to making Ohmic contacts on both SiQD SLs in oxide and nitride matrices. Activation energy (Ea) of SiQDs, extracted from a linear Arrhenius plot is investigated in the present work in order to expand the understanding of engineering electrical injection in laterally active paths. It is found that a lower barrier height of dielectric matrix influences to the lateral electron transport of the SiQDs in such dielectric matrix. PL results confirm that the band gap of surface oxidized SiQDs widens due to quantum confinement. The present results reveal that the strong peak (Q-peak) due to quantum confinement is more effective in the emission with increasing SiQD concentration. The surface oxide is believed to play an important role in the reduction of SiQD luminescence due to a trapped exiciton. It is concluded that SiQDs surface oxide accompanied by a SiO2 matrix may not provide a good passivation in very small SiQD size. However the energy band gap and conductivity of the SiQDs are tunablity, in the optimum range of SiQD size and concentration. This observation may be important for future nanoelectronics applications.

In this thesis, the structural, optical and electrical properties of crystalline silicon quantum dots (SiQDs) are examined for application to silicon based tandem cells. The approach has been to concentrate on all silicon devices by taking advantage of quantum confinement in low-dimensional Si. RF magnetron co-sputtering provided the capability of creating superlattice structures in conjunction with high temperature annealing, to form Si nanocrystals in an oxide matrix. Structural techniques, including Fourier transform infrared spectroscopy (FTIR), micro-Raman spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), and Secondary ion mass spectroscopy (SIM) were employed to gather structural information about the SiQD/SiO2 SLs. The result combine presents that the packing density of Si QDs, correlated to the oxygen content of the silicon rich oxide layer can be control independently. The effect of Si nanocrystallite density on Raman scattering is investigated. The preliminary results present that a decrease in the oxygen content (x) results in an increased sharpness of the Strokes-mode peak of nanocrystalline Si, attributed to an increase in the proportion of crystalline Si because of the increased number of SiQDs. However the influence of the surface region on the crystallite core intensity scattering becomes dominant, when SiQD size diameter is very small (less than 3 nm). The present work shows that a decrease in x-content leading to an increase of the SiQD concentration, initially results in the enhancement of the lateral conductivity in the SiQD superlattice material. In this work, the Al contacting scheme, using a prolonged heat treatment technique at elevated temperature less than the eutectic point of Al and Si (577C) has been successfully applied to making Ohmic contacts on both SiQD SLs in oxide and nitride matrices. Activation energy (Ea) of SiQDs, extracted from a linear Arrhenius plot is investigated in the present work in order to expand the understanding of engineering electrical injection in laterally active paths. It is found that a lower barrier height of dielectric matrix influences to the lateral electron transport of the SiQDs in such dielectric matrix. PL results confirm that the band gap of surface oxidized SiQDs widens due to quantum confinement. The present results reveal that the strong peak (Q-peak) due to quantum confinement is more effective in the emission with increasing SiQD concentration. The surface oxide is believed to play an important role in the reduction of SiQD luminescence due to a trapped exiciton. It is concluded that SiQDs surface oxide accompanied by a SiO2 matrix may not provide a good passivation in very small SiQD size. However the energy band gap and conductivity of the SiQDs are tunablity, in the optimum range of SiQD size and concentration. This observation may be important for future nanoelectronics applications.