abstract: To date, the most popular and dominant material for commercial solar cells is crystalline silicon (or wafer-Si). It has the highest cell efficiency and cell lifetime out of all commercial solar cells. Although the potential of crystalline-Si solar cells in supplying energy demands is enormous, their future growth will likely be constrained by two major bottlenecks. The first is the high electricity input to produce crystalline-Si solar cells and modules, and the second is the limited supply of silver (Ag) reserves. These bottlenecks prevent crystalline-Si solar cells from reaching terawatt-scale deployment, which means the electricity produced by crystalline-Si solar cells would never fulfill a noticeable portion of our energy demands in the future. In order to solve the issue of Ag limitation for the front metal grid, aluminum (Al) electroplating has been developed as an alternative metallization technique in the fabrication of crystalline-Si solar cells. The plating is carried out in a near-room-temperature ionic liquid by means of galvanostatic electrolysis. It has been found that dense, adherent Al deposits with resistivity in the high 10^–6 ohm-cm range can be reproducibly obtained directly on Si substrates and nickel seed layers. An all-Al Si solar cell, with an electroplated Al front electrode and a screen-printed Al back electrode, has been successfully demonstrated based on commercial p-type monocrystalline-Si solar cells, and its efficiency is approaching 15%. Further optimization of the cell fabrication process, in particular a suitable patterning technique for the front silicon nitride layer, is expected to increase the efficiency of the cell to ~18%. This shows the potential of Al electroplating in cell metallization is promising and replacing Ag with Al as the front finger electrode is feasible. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2016

<p>A new type of flexible silicon solar cell has been fabricated by interconnecting solar cell die on a flexible substrate. The fabrication process is described in this thesis. The solar cell die were diced using two methods. One method was to dice a solar cell completely through. The other method was to dice a solar cell from its back partially and then cleave through. To study the effects of different dicing methods on the performance of solar cell die, storage delay time measurement was employed to determine the lifetime of excess electrons in the p region of the two types of solar cell die. A laser beam induced current (LBIC) scanning technique was employed to study the electrical performance of the two types of solar cell die. The carrier diffusion lengths of two types of solar cell die were also determined by traveling light spot diffusion length measurement.</p><p>The theoretical response of a solar cell was compared to experimental results at various incident light angles. OpticLab software was used to model the incident light angle and lens spacing dependence of solar cell performance.</p> / Thesis / Master of Applied Science (MASc)

In this thesis, silicon oxide thin films fabricated on silicon substrates by reactive radio frequency (rf) magnetron sputtering and supercritical CO2 (SCCO2) treatment at room temperature were investigated. The electrical properties including I-V and C-V of the films prepared at different processing conditions were discussed. Using the Transmission Electron Microscope (TEM), the thickness of silicon oxide thin films were measured. The results suggested that the film quality can be significantly improved by the SCCO2 treatment after reactive sputtering. The leakage current of the films at an electrical field of 1 MV/cm is 1¡Ñ10-8A/cm2 with a hysteresis voltage of 0.01V. The silicon oxide thin films can be used as a barrier layer for Al/SiO2/Si silicon solar cells. The energy conversion efficiency of a single crystal silicon solae cell is 10.2% under AM1.5 (965W/m2) radiation. After rapid thermal annealing(RTA) at 500¢J, the measured short-circuit current, open- circuit voltage, fill factor are 53mA, 0.54V and 0.53, respectively.

The experiment divided into two parts. One is silicon solar cell process. The other is InGaN solar cell process. Borosilicafilm solution spin onto the n-type silicon (111) substrate and spread through the high-temperature furnace tube to form a p-n junction silicon solar cell. Then, evaporate top and rear contact by electron beam evaporation system. InGaN p-i-n structure solar cell grows on sapphire substrate by plasma-assisted molecular beam epitaxy system (PA-MBE) and its process is by repeated photolithography, inductive coupled plasma etching and wet etching. In the device fabrication process, the first is defining the sample size(mesa). Second, etched to the n-type GaN layer, and then coated metal as electrode. Finally, we get the device. In the measurement, the measurement of I-V curve of samples in the light by solar simulator of AM1.5 G light source observe open circuit voltage, short circuit current, fill factor, and efficiency. In addition, we measure the external quantum efficiency of the samples by IPCE and observe the photoelectric conversion efficiency of samples at different wavelength. Observed the sample quality and the indium composition of InGaN layer by XRD. We observe the InGaN band gap shift by variable-temperature photoluminescence spectra.

Selectively-transparent and conducting photonic crystals (STCPCs) made of alternating layers of sputtered indium-tin oxide (ITO) and spin-coated silica (SiO2) nanoparticle films have potential applications in micromorph solar cells and building integrated photovoltaics (BIPVs). In this work, theoretical calculations have been performed to show performance enhancement of the micromorph solar cell upon integration of the STCPC an intermediate reflector. Thin semi-transparent hydrogenated amorphous silicon (a-Si:H) solar cells with STCPC rear contacts are demonstrated in proof-of-concept devices. A 10% efficiency increase in a 135nm thick a-Si:H cell on an STCPC reflector with Bragg peak at 620nm was observed, while the transmitted solar irradiance and illuminance are determined to be 295W/m2 and 3480 lux, respectively. The STCPC with proper Bragg peak positioning can boost the a-Si:H cell performance while transmitting photons that can be used as heat and lighting sources in building integrated photovoltaic applications.

In this thesis, Terahertz Time-Domain spectroscopy (THz-TDS) was used in order to measure the electrical properties of silicon solar cells. The advantage of THz-TDS is that it allows us to measure the electrical properties without electrical contacts. In order to perform these measurements, a reflection based system was constructed and the changes in the peak amplitude in the time-domain under a, 450mW 808 nm continuous wave laser source were measured. The solar cell that was used in this thesis was manufactured in Middle East Technical University Microelectromechanical Systems (METU-MEMS) research laboratories located in Ankara, Turkey. The solar cell that we used in the measurements had a thickness of 0.45 mm and was produced on a single silicon crystal in &lt / 100&gt / direction. It is made up of a p-type base and n-type emitter to create p-n junction. Also, it has a Si4N3 AR coating and Al back contacts on it. To compare the THz measurements to that of electrical measurements, some electrical contact measurements were performed on the solar cell under laser illumination. By using these measurements, the energy conversion efficiency and the quantum efficiency of the solar cell were calculated and measured as 3.44 % and 7%, respectively under the 450mW, 808nm illumination on a specific area of the cell. The results that were obtained form the electrical measurements were compared with the THz results. We found that in order to understand the efficiency of the solar cell using THz-TDRS, a more comprehensive study needs to be done where the changes in the reflection of the THz radiation under different excitation powers and different configurations of the system need to be studied.

Selectively-transparent and conducting photonic crystals (STCPCs) made of alternating layers of sputtered indium-tin oxide (ITO) and spin-coated silica (SiO2) nanoparticle films have potential applications in micromorph solar cells and building integrated photovoltaics (BIPVs). In this work, theoretical calculations have been performed to show performance enhancement of the micromorph solar cell upon integration of the STCPC an intermediate reflector. Thin semi-transparent hydrogenated amorphous silicon (a-Si:H) solar cells with STCPC rear contacts are demonstrated in proof-of-concept devices. A 10% efficiency increase in a 135nm thick a-Si:H cell on an STCPC reflector with Bragg peak at 620nm was observed, while the transmitted solar irradiance and illuminance are determined to be 295W/m2 and 3480 lux, respectively. The STCPC with proper Bragg peak positioning can boost the a-Si:H cell performance while transmitting photons that can be used as heat and lighting sources in building integrated photovoltaic applications.

This master’s thesis deals with light induced degradation problems. In theoretical section it describes essentials of PN junction function and next light induced degradation mechanisms of solar cells and its symptoms at solar cell operation. In practical section it deals with set of measurements of solar cells since production of the silicon wafer to the complete solar cell. Selected cells are submitted to light induced degradation, measured dependencies are then evaluated. Degraded samples are subsequently recovered by two ways at high temperature treatment. The issues are evaluated.

Renewable and clean energy is urgently needed to cope with the climate crisis. Photovoltaics (PV) has been the fastest growing technology in the clean energy market due to its low cost, and the abundance of solar energy. The capacity of silicon-based PV is rapidly expanding with evolving technologies. Passivating the solar cell’s electrical contacts is a widely accepted strategy for the PV industry to improve device power conversion efficiency (PCE). Polycrystalline silicon (Poly-Si) passivating contacts are one of the promising concepts in the emerging class of passivating contacts. In this dissertation, the passivation mechanism of Poly-Si passivating contacts is investigated. Moreover, the influence of dopant diffusion on the passivation quality is revealed. To address the side-effects of dopant diffusion, a thin buffer layer is inserted between the Poly-Si(p) layer and the $SiO_x$ layer. With such a buffer layer, the passivation of the Poly-Si passivating contact is improved, which in turn, enhances the device PCE. In addition to passivating contacts, this dissertation also explores carrier-selective contact of crystalline silicon (c-Si) and low work function metal – Li. Li is a very reactive metal which makes the fabrication process a challenge. To overcome such a challenge, the c-Si/ Li contact is fabricated by thermally decomposing stable $Li_3N$ powder instead of metal evaporation. The c-Si/Li contact shows an excellent electron-selective transport performance with a 0.39 eV energy barrier. Full-area Si/Li rear contact devices are fabricated, and >19% PCE and >80% fill factor are achieved. To accelerate the device optimization, a physical model embedded machine-learning approach is applied to transparent conductive oxide (TCO) materials optimization. In this work, empirical correlations between sputtering parameters and the deposited TCOs’ electrical properties are established. Then a Bayesian Parameter Estimation (BPE) algorithm is applied to learn the empirical model. With this BPE network, the TCOs’ electrical properties are successfully predicted with limited material characterizations. Thanks to the combination of BPE and a physical model network, the material optimization process is significantly accelerated. In summary, this dissertation explores different aspects to develop novel passivating and carrier-selective contacts for c-Si solar cells, and introduces an approach to accelerate the development processes.

In this thesis study, fabrication and doping of silicon thin films prepared by electron beam evaporation equipped with effusion cells for solar cell applications have been investigated. Thin film amorphous Si (a-Si) layers have been fabricated by the electron beam evaporator and simultaneously doped with boron (B) and phosphorous (P) using effusion cells. Samples were prepared on glass substrates for the future solar cell operations. Following the deposition of a-Si thin film, crystallization of the films has been carried out. Solid Phase Crystallization (SPC) and Metal Induced Crystallization (MIC) have been employed to obtain thin film crystalline Si. Crystallization was performed in a conventional tube furnaces and Rapid Thermal annealing systems (RTA) as a function of process parameters such as annealing temperature and duration. Produced films have been characterized using chemical and structural characterization techniques such as Raman Spectroscopy, X-Ray Diffractometer and Secondary Ion Mass Spectrometer (SIMS). The electrical properties of the films have been studied using Hall Effect and I-V measurements as a function of doping. We have demonstrated successful crystallization of a-Si by SPC at temperatures above 600 &deg / C. The crystallization occurred at lower temperatures in the case of MIC. For doping, P was evaporated from the effusion cell at a temperature between 600 &deg / C and 800 &deg / C. For B, the evaporation temperature was 1700 &deg / C and 1900 &deg / C. The thickness and the band gap of the Si films were determined by ellipsometry method and the results were compared for different evaporation temperatures. The effect of doping was monitored by the I-V and Hall Effect measurements. We have seen that the doping was accomplished in most of the cases. For the samples annealed at relatively high temperatures, the measured doping type was inconsistent with the expected results. This was attributed to the contamination from the glass substrate. To understand the origin of this contamination, we analyzed the chemical structure of the film and glass by X-ray Fluorescence (XRF) and seen that the glass is the main source of contamination. In order to prevent this contamination we have suggested covering the glass substrate with Si3N4 (Silicon Nitride) which act as a good diffusion barrier for impurities.