Bulk heterojunction (BHJ) organic photovoltaic (OPV) is one of the most promising techniques to generate electricity with advantages of flexibility, solution processing and capability for large area device fabrication. Although the power conversion efficiency (PCE) of BHJ solar cells has already achieved over 13%, there are still problems remain to be solved. This thesis presents the binary and ternary organic BHJ devices with alternative donor:acceptor (D:A) ratios, and the charge transport properties and electronic interactions in their BHJ films. In a high performance BHJ solar cell, the commonly optimized D:A weight ratio is about 1:x, where x is commonly in excess of 1.5, when PC71BM is used as the acceptor. We demonstrated how to achieve high PCEs of BHJ solar cells by enriching the D:A weight ratios. The PCEs of the re-optimized cells were improved for the PTB7:PC71BM, PCDTBT:PC71BM, PDTSTPD:PC71BM devices. Current-voltage (JV) and admittance spectroscopy (AS) measurements indicate enhanced hole mobilities for the polymer-rich BHJs based on PTB7, PCDTBT, and PDTSTPD. At the same time, although the relative weight ratio of PC71BM is reduced, the electron mobilities are maintained due to the dispersion of fullerene domains by increased DIO concentrations. The active layer thickness of most optimized BHJ solar cells is about 100nm. The thin active layer is unfavorable for optical absorption and film coating. We employed a ternary strategy to address this problem, and the thick-film BHJ devices can retain 90% PCEs of their optimized thin-film devices. Three model systems were studied, involving PTB7:PC71BM, PTB7-Th:PC71BM and P3HT:PCBM BHJs. Into these BHJs, a ternary component, p-DTS(fbtth2)2 (DTS) is introduced. With DTS, the corresponding thick film devices have significantly improved PCEs. The ternary component DTS improves hole mobility and reduces sub-bandgap trap states. Both observations are well correlated with improved FFs of the ternary BHJ cells. Photothermal deflection spectroscopy (PDS) and 1H nuclear magnetic resonance (1H NMR) results indicate that DTS behaves as conducting bridges in between two neighboring polymer segments. Most lab-based BHJ solar cells are optimized by their power conversion efficiencies (PCEs). We challenge this conventional view by showing that BHJ cells using fullerene acceptors should be optimized by their fill-factors (FFs). With the optimized-FF approach, BHJ cells tend to have higher fullerene content when compared to the BHJ cells that are optimized by PCEs. The FF-optimized BHJ cells have slightly reduced PCEs (due to smaller Jscs) compared to the PCE-optimized cells. Yet, FF-optimized cells enjoy a much better thermal stability. We demonstrate that these FF-optimized BHJs possess better-balanced electron-to-hole mobility ratios due to weakly field-dependent electron mobilities. The improved mobility ratio suppresses carrier recombination. Our results suggest that BHJ cells optimized by their PCEs should be meta-stable, and other D:A ratios should be considered for practical BHJ cell development.

abstract: The goal of any solar photovoltaic (PV) system is to generate maximum energy throughout its lifetime. The parameters that can affect PV module power output include: solar irradiance, temperature, soil accumulation, shading, encapsulant browning, encapsulant delamination, series resistance increase due to solder bond degradation and corrosion and shunt resistance decrease due to potential induced degradation, etc. Several PV modules together in series makes up a string, and in a power plant there are a number of these strings in parallel which can be referred to as an array. Ideally, PV modules in a string should be identically matched to attain maximum power output from the entire string. Any underperforming module or mismatch among modules within a string can reduce the power output. The goal of this project is to quickly identify and quantitatively determine the underperforming module(s) in an operating string without the use of an I-V curve tracer, irradiance sensor or temperature sensor. This goal was achieved by utilizing Radiovoltmeters (RVM). In this project, it is demonstrated that the voltages at maximum power point (Vmax) of all the individual modules in a string can be simultaneously and quantitatively obtained using RVMs at a single irradiance, single module operating temperature, single spectrum and single angle of incidence. By combining these individual module voltages (Vmax) with the string current (Imax) using a Hall sensor, the power output of individual modules can be obtained, quickly and quantitatively. / Dissertation/Thesis / Masters Thesis Engineering 2019

Submitted to the Faculty of Science, School of Chemistry at University of the Witwatersrand, in partial fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 05 June 2017. / The colloidal method has extensively been used to synthesize ternary and quaternary copper sulfides and selenides. Although tellurides form part of the chalcogenides, little has been reported on them particularly the synthesis of these nanostructures. Achieving high-quality nanocrystals through colloidal synthesis requires thorough monitoring of parameters such as time, solvent, precursor as they affect nucleation and growth of the nanocrystals. Herein, we report on the colloidal synthesis of ternary CuInTe2 and quaternary CuIn1-xGaxTe2 nanostructured semiconductor materials. A typical synthesis of CuInTe2 entailed varying reaction temperature. At temperatures below 250 °C, no formation of CuInTe2 was seen. At 250 °C formation of CuInTe2 could be observed with the formation of binary impurities. A change in the sequence in which precursors were added at 250 °C yielded pure CuInTe2. Applying different surfactants aided in achieving differently structured morphologies of CuInTe2 nanocrystals. Morphology varied from rods, cubes, nanosheets etc. Different morphologies resulted in different optical properties with the high optical band gap of 1.22 eV measured for 1D rods. Different precursors were employed in the synthesis of quaternary CuIn1-xGaxTe2. Precursor 2 (entailed the use of Cu (acac)2, In (acac)3 and Ga(acac)3) yielded pure CuIn1-xGaxTe2 phase with no formation of impurities. Variation in reaction time influenced the optical properties of the quaternary CuIn1-xGaxTe2 with high band gap obtained at low reaction time (30 min). A change in Ga and In concentration resulted in reduced lattice parameters a and c with lowest values obtained with the highest Ga concentration. However, achieving the intended concentration proved challenging due to the loss of the material during synthesis. Increasing the Ga concentration resulted in a high optical band gap. Conducting the reaction with Hexadecylamine (HDA) resulted in a relatively high optical band though the formation of impurities was evident. The obtained band gap can be attributed to small sized particles as evident from TEM results. Heterojunction ZnO/CIT and ZnO/CIGT solar cell devices were fabricated through a simple solution approach. The performance of ZnO/CIGT device was superior to that of ZnO/CIT in which efficiency increased from 0.26-0.78%. In the ZnO/CIT device, high Voc of 880 mV was recorded while 573.66 mV was measured for ZnO/CIGT device. Chemical and thermal treatments were performed on the ZnO/CIGT devices. The efficiency increased from 0.78 1.25% when the device was chemically treated with a short-chain EDT ligand. A high conversion efficiency of 2.14% was recorded for devices annealed at 300 °C. High annealing temperatures resulted in poor device performance with the lowest efficiency of 0.089% obtained at annealing temperatures of 500 °C attributed to the leaching out of In and Ga into the ZnO layer. / LG2017

As Organic solar cells (OSCs) become a promising complementary to traditional inorganic solar cells, studying the optical properties of OSCs plays a critical role to understand and improve the performance of organic solar cells. Studying optical properties is essential because it can help to understand how light interacts with the materials used in organic solar cells, which can help to improve the efficiency of organic solar cells. In this work, the optical properties of the organic photovoltaic system PM6:Y6 prepared from two different solvents, namely, chloroform and o-xylene, were investigated. The optical constants, specifically the refractive index and absorption coefficient of thin films of these materials, and the effects of thermal annealing on the optical properties were studied. The optical properties of isotropic and anisotropic organic materials were also compared, and the obtained optical constants were used to simulate the optical properties of the devices using the transfer matrix approach. The results suggest that more accurate measurements and analysis of the optical constants help to achieve more accurate simulations. This, in turn, provides more information about how the molecular orientation affects the optical properties of OSCs. However, it is important to note that the optical properties of PM6:Y6 blends that were studied are limited to those obtained under the conditions used to prepare the films. In fact, changes in the thickness or concentrations of solutions will need to be considered as well. Lastly, the glass transition temperature was determined using the change in the ellipsometric data (Ψ). This helps to select and test different thermal annealing temperatures for the material system, which could improve the efficiency of the respective solar cells.

The materials Fe���(Si,Ge)(S,Se)���, Cu���PS[subscript 4-x]Se[subscript x] (0 ��� x ��� 4), and Cu���PxAs[subscript 1-x]S��� (0 ��� x ��� 1) have been synthesized and studied as new earth-abundant absorbers for single and multijunction photovoltaic cells as well as solar fuel generation. The synthesis, single-crystal growth, and optical and electrical properties of these materials are described and discussed in Chapter 2, 3, and 4. Inspired by the photovoltaic absorber Cu���ZnSnS���, the new compound CuZnPS��� has been discovered; its structure and properties are discussed in Chapter 5, including a comparative analysis to Cu���PS���. The compound YZrF��� (Chapter 6) has been synthesized and evaluated for the first time as a new optical host for green up-conversion. / Graduation date: 2013

The absorption of the sun's radiation and its efficient conversion to useful work by a photovoltaic solar cell is of interest to the community at large. Scientists and engineers are particularly interested in approaches that exceed the Shockley-Queisser limit of photovoltaic solar-energy conversion. The abstract notion of increasing the efficiency of photovoltaic solar cells by constructing a three-transition solar cell via an absorber with intermediate levels is well-established. Until now, proposed approaches to realize the three-transition solar cell do not render the efficiency gains that are theorized; therefore, researchers are experimenting to ascertain where the faults lie. In my opinion, it is unclear if the abstract efficiency gains are obtainable. Furthermore, it is difficult to determine whether three-transition absorbers are even incorporated in the existing three-transition solar cell prototypes. I assert that there are material systems derived from the technologically important compound semiconductors and their ternary alloys that more clearly determine the suitability of employing nanostructured absorbers to realize a three-transition solar cell. The author reports on a nanostructured absorber composed of InAs quantum dots completely enveloped in a GaAsSb matrix that is grown by molecular beam epitaxy. The material system, InAs/GaAs$_{0.88}$Sb$_{0.12}$, is identified as an absorber for a three transition solar cell. This material system will more easily determine the suitability of employing nanostructured absorbers because its quantum-dot heterojunctions have negligible valence-band discontinuities, which abate the difficulty of interpreting optical experimental results. A key tool used to identify the GaAs$_{1-x}$Sb$_{x}$ ($xapprox 0.12$) is a maximum-power iso-efficiency contour plot. This contour plot is only obtainable by first having analyzed the impact of both finite intermediate-band width and spectral selectivity on the optimized detailed-balance conversion efficiencies of the three-transition solar cell. Obtaining the contour plot is facilitated by employing a rapid and precise method to calculate particle flux (Appendix~ ef{ch:Rapid-Precise}). The author largely determines the electronic structure of the InAs/GaAs$_{1-x}$Sb$_{x}$ ($xapprox 0.12$) absorber that is grown by molecular beam epitaxy from optical experimental methods and in particular, from photoluminescent spectroscopy. The interpretation of the experimental photoluminescent spectrum is facilitated by having first studied the theoretical photoluminescent spectra of idealized three-transition absorbers.

The aim of the diploma work is to introduce analytical methods concerning solar panels and systems. Firstly, the attention is drawn to a principle of photovoltaic conversion including an explanation of photoelectric phenomenon as well as the issue of P-N junction considered to be the fundamental building block of a solar cell. With respect to the fact that the diploma work is to be focused on testing of solar panels and the identification of defects in solar cells, the work describes individual solar cells, solar panels as well as individual methods of defect identification. Secondly, the work includes sections dealing with the assessment of the measurement performed by using electroluminescence method as well as the measurement method of volt-ampere parameters. The last section of the work includes the assessment of solar panel aging after one-year performance using the electroluminescence measurement method.

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The semiconducting sulphosalt Wittichenite has been identified as a possible absorber material for thin film photovoltaic devices. It has the chemical formula Cu3BiS3 and its component elements are those of low toxicity and high abundance making it a very attractive prospect for photovoltaic devices. The copper bismuth sulphur material system is not very well understood and information on it limited to a few small regions. To aid understanding of this system a pseudo-binary phase diagram along the CuS-Bi join of the Cu-Bi-S ternary phase diagram was constructed by making bulk samples of various compositions along the join and analysing them using X-ray diffraction and differential scanning calorimetry. This join was chosen because is crosses the point at which Cu3BiS3 would be expected to occur due to its stoichiometry. The CuS-Bi phase diagram shows Cu3BiS3 forms across a wide compositional range but is mixed with either bismuth metal or copper sulphides depending on composition. Films of Cu3BiS3 were made using sputtered copper and bismuth films annealed in a sulphur atmosphere and thermal co- evaporation of copper sulphide and bismuth.

This research applies a recently developed dynamic state-estimation based protection scheme, the settingless protection, to the photovoltaic (PV) industry for the first time. At this time, the proposed protection algorithm has been implemented on traditional protection zones for individual power system devices, but this research extends this protection to a microgrid, specifically, a system of PV network composed of several PV modules. Several illustrative examples on various anomalies such as high impedance faults and shorted-out PV modules have been provided to demonstrate the effectiveness of this protection scheme. The detection of these anomalies has been demonstrated in the presence of changing atmospheric conditions, and with the operation of maximum power point tracking (MPPT) equipped dc-dc converters. This protection scheme requires an accurate model of the PV module, therefore, a two-diode PV model has been developed using quadratic integration modeling. In this PV model development, a scaling factor is applied to the Taylor series expansion of the exponential terms of the model of the PV module. Then the higher order terms of the Taylor series expansion are reduced to at most second order terms using the quadratization technique. Furthermore, a novel approach for extracting the PV parameters, namely, the ideality constants, leakage currents, PV module internal current, shunt and series resistances, has been presented. A comparison was performed between numerically generated data using the determined PV module parameters and data measurements from a physical PV module. It was shown that the maximum error from this comparison was below 0.12A, and less than 0.05A around the maximum power point region of the PV modules used for this research. The residual data from the PV array protection scheme has been used to develop a method for identifying the location of faulted PV modules. Also, condition-based monitoring of the PV array system has also been presented with examples. From the PV array system monitoring, the shading and underperformance of a PV module have been identified. From the contributions of this research, an accurate module of the PV array has been developed in a form that can be integrated with other power system devices. This accurate module can be used for state estimation of the PV array, load flow analysis, short circuit analysis, and other power system analytical studies. Also, by determining the location of the faulted PV module, the time to identify this faulted PV module in a large PV installation is drastically reduced. Lastly, by identifying shading conditions and underperforming PV modules, the PV system operator can quickly bring the underperforming module or modules to optimal performance, thereby, maximizing the power yield of the PV array, and maximizing the revenue of the PV system owner.