Process development and optimisation for efficient and cost-effective Cu(In,Ga)Se2 thin film solar cells

Chalcopyrite copper indium gallium diselenide (Cu(In,Ga)Se2 or CIGS) solar cells have achieved the highest laboratory power conversion efficiency among thin film PV technologies, and have the potential to be manufactured cost-effectively at large scale. In this thesis, detailed studies on the growth of CIGS solar cells by means of a pilot scale inline co-evaporation system have been performed to explore the beneficial deposition techniques to successfully transfer the research achievements into cost-effective industrial production. The work involved the development and optimisation of several new processes along with the design of the inline pilot scale system. Effects of the thin absorber layers (with thickness of ~1 μm) on material quality and the device performance have been investigated using (i) a two-step process with room temperature evaporated metal precursors, (ii) a three-stage process at constant substrate temperature and (iii) a three-stage process at varied substrate temperatures. Changing the copper and gallium compositions of the CIGS layers was observed to have a strong impact on the structural and electronic properties and hence on the performance of the solar cells. In one study, simple mechanical compression was employed on a batch of low quality porous CIGS films to significantly improve the surface morphological and the optoelectronic properties. Photoluminescence measurements revealed the band-to-tail defect-related recombination that was detrimental to the quality of as-grown and compressed films. In another study, rapid thermal processing (RTP) was applied to the low temperature Cu-In-Ga-Se precursor layers in an attempt to reduce the cost and optimise the reaction mechanism of the chalcopyrite material. After fine tuning of the process conditions, the CIGS layers with larger grain size exhibited power conversion efficiencies up to 10.8%. While these results are promising, the device performance was mainly limited by the low open circuit voltage and the low fill factor. It has been found that the Cu-In-Ga-Se precursors doped with low concentrations of sodium were beneficial for re-crystallisation of the CIGS films due to the reduced grain size and crystallinity of the precursors. An in-situ X-ray diffraction technique was used to investigate the phase evaluation in both Na-free and Na-doped Cu-In-Ga-Se precursor layers as a function of temperature. The results confirmed that the formation of both CIGS as well as secondary phases in the layer with the Na-doped precursors started at higher temperatures compared to Na-free precursors. It is therefore expected that further improvements in the solar cell efficiency might be achieved following this RTP and low-temperature precursor process

Temperature-light-dependent JV and TPV analysis of pure sulfide based Cu<inf>2</inf> ZnSnS<inf>4</inf> solar cells

In this work, we exploit temperature-light-dependent current-density-voltage (T-JV) and transient photovoltage measurements (T-TPV) to investigate charge dynamics, especially at the back contact, in solution-processed Cu 2 ZnSnS 4 solar cells. A Si x N y hole barrier was grown on top of Mo to help to investigate carrier dynamics. By using T-JV techniques, we are able to observe the dominant recombination mechanism occurring at the back contact interface that could lead to significant open-circuit voltage (V oc ) loss. In combination with T-TPV, TPV decay time mapping across temperature in a range of 213-313 K and light intensity range of 0.01-1 suns was used to explore interface related recombination and charge transport for CZTS solar cell devices

Electronic Structure in Gapped Graphene with Coulomb Potential

In this paper, we numerically study the bound electron states induced by long range Coulomb impurity in gapped graphene and the quasi-bound states in supercritical region based on the lattice model. We present a detailed comparison between our numerical simulations and the prediction of the continuum model which is described by the Dirac equation in (2+1)-dimensional Quantum Electrodynamics (QED). We also use the Fano's formalism to investigate the quasi-bound state development and design an accessible experiments to test the decay of the supercritical vacuum in the gapped graphene.Comment: 5 page, 4 figure

A Comparison of Different Textured and Non-Textured Anti-Reflective Coatings for Planar Monolithic Silicon-Perovskite Tandem Solar Cells

Multijunction solar cells offer a route to exceed the Shockley–Queisser limit for single-junction devices. In a few short years, silicon-perovskite tandems have significantly passed the efficiency of the best silicon single-junction cells. For scalable solution processing of silicon-perovskite tandem devices, with the avoidance of vacuum processing steps, a flat silicon sub-cell is normally required. This results in a flat top surface that can lead to higher optical reflection losses than conformal deposition on textured silicon bottom cells. To overcome this, textured anti-reflective coatings (ARCs) can be used on top of the finished cell, with textured polydimethylsiloxane (PDMS), a promising candidate. In this work, we vary the texture geometry and film thickness of PDMS anti-reflective foils to understand the effect of these parameters on reflectance of the foil. The best film is selected, and anti-reflective performance is compared with two common planar ARCs─lithium fluoride (LiF) and magnesium fluoride (MgF2) showing considerable reduction in reflectance for a non-textured silicon-perovskite tandem cell. The application of a PDMS film is shown to give a 3–5% increase in integrated JSC in each sub-cell of a silicon-perovskite tandem structure

Successes and Challenges Associated with Solution Processing of Kesterite Cu2ZnSnS4 Solar Cells on Titanium Substrates

Roll-to-roll (R2R) processing of solution-based Cu2ZnSn(S,Se)4 (CZT(S,Se)) solar cells on flexible metal foil is an attractive way to achieve cost-effective manufacturing of photovoltaics. In this work we report the first successful fabrication of solution-processed CZTS devices on a variety of titanium substrates with up to 2.88% power conversion efficiency (PCE) collected on flexible 75 μm Ti foil. A comparative study of device performance and properties is presented aiming to address key processing challenges. First, we show that a rapid transfer of heat through the titanium substrates is responsible for the accelerated crystallisation of kesterite films characterised with small grain size, a high density of grain boundaries and numerous pore sites near the Mo/CZTS interface which affect charge transport and enhance recombination in devices. Following this, we demonstrate the occurrence of metal ion diffusion induced by the high temperature treatment required for the sulfurization of the CZTS stack: Ti4+ ions are observed to migrate upwards to the Mo/CZTS interface whilst Cu1+ and Zn2+ ions diffuse through the Mo layer into the Ti substrate. Finally, residual stress data confirm the good adhesion of stacked materials throughout the sequential solution process. These findings are evidenced by combining electron imaging observations, elemental depth profiles generated by secondary ion mass spectrometry, and x-ray residual stress analysis of the Ti substrate

Perovskite photovoltaics for aerospace applications − life cycle assessment and cost analysis

In the past few years, we have witnessed a rapid evolution of perovskite solar cells. In this study, we employ life cycle assessment (LCA) to identify the potential environmental impacts of perovskite solar cells (PSC) optimised for aerospace applications but could be used in conventional terrestrial applications too. One PSC module is manufactured by spin coating equipped with ITO glass and gold cathode. The other PSC module is manufactured by slot-die coating with a PET layer and carbon cathode and gold cathode respectively. Life cycle assessment is employed to compare potential environmental impact of two manufacture methods by impact method of Recipe(H), as well as the fabrication cost of PSC module. The primary data of material and energy used for fabricating PSCs are collected from spin coating with lab scale and slot-die coating with pilot scale. The life cycle impact assessment of the PSC module in the pilot scale shows much lower in all the assessed 18 impact categories than in the lab scale thanks to the material use efficiency and reducing energy consumption. Gold as a conduct electrode has the highest impacts in both spin coating and slot-die coating modules. Calculating with a two-year lifetime (typical of aerospace applications), the impact of global warming potential from the PSC module with carbon electrode with pilot scale used in a terrestrial application is calculated to be 12 g/kWh

Spin Fluctuation Induced Linear Magnetoresistance in Ultrathin Superconducting FeSe Films

The discovery of high-temperature superconductivity in FeSe/STO has trigged great research interest to reveal a range of exotic physical phenomena in this novel material. Here we present a temperature dependent magnetotransport measurement for ultrathin FeSe/STO films with different thickness and protection layers. Remarkably, a surprising linear magnetoresistance (LMR) is observed around the superconducting transition temperatures but absent otherwise. The experimental LMR can be reproduced by magnetotransport calculations based on a model of magnetic field dependent disorder induced by spin fluctuation. Thus, the observed LMR in coexistence with superconductivity provides the first magnetotransport signature for spin fluctuation around the superconducting transition region in ultrathin FeSe/STO films

Design and optimisation of process parameters in an in-line CIGS evaporation pilot system

Substantial efforts have been made globally towards improving Cu(In,Ga)Se2 thin film solar cell efficiencies with several organisations successfully exceeding the 20% barrier on a research level using the three-stage CIGS process, but commercial mass production of the three-stage process has been limited due to the technological difficulties of scaling-up. An attempt has been made to identify these issues by designing and manufacturing an in-line pilot production deposition system for the three-stage CIGS process which is capable of processing 30 cm × 30 cm modules. The optimisation of the process parameters such as source and substrate temperature, deposition uniformity, flux of copper, indium, gallium and selenium and thickness control has been presented in this investigation. A simplistic thickness distribution model of the evaporated films was developed to predict and validate the designed deposition process, which delivers a comparable simulation compared with the experimental data. These experiments also focused on the optimisation of the temperature uniformity across 30 cm × 30 cm area using a specially designed graphite heating system, which is crucial to form the correct α-phase CIGS in the desired time period. A three-dimensional heat transfer model using COMSOL Multiphysics 4.2a software has been developed and validated with the help of experimental data

Radiation Hardness of Perovskite Solar Cells Based on Aluminum‐Doped Zinc Oxide Electrode Under Proton Irradiation

Due to their high specific power and potential to save both weight and stow volume, perovskite solar cells have gained increasing interest to be used for space applications. However, before they can be deployed into space, their resistance to ionizing radiations such as high‐energy protons must be demonstrated. In this report, we investigate the effect of 150 keV protons on the performance of perovskite solar cells based on aluminium‐doped zinc oxide (AZO) transparent conducting oxide (TCO). Record power conversion efficiency of 15% and 13.6% were obtained for cells based on AZO under AM1.5G and AM0 illumination, respectively. We demonstrate that perovskite solar cells can withstand proton irradiation up to 1013 protons.cm−2 without significant loss in efficiency. At this irradiation dose, Si or GaAs solar cells would be completely or severely degraded when exposed to 150 keV protons. From 1014 protons.cm−2, a decrease in short‐circuit current of the perovskite cells is observed, which is consistent with interfacial degradation due to deterioration of the Spiro‐OMeTAD HTL during proton irradiation. Using a combination of non‐destructive characterization techniques, results suggest that the structural and optical properties of perovskite remain intact up to high fluence levels. Although shallow trap states are induced by proton irradiation in perovskite bulk at low fluence levels, they can release charges efficiently and are not detrimental to the cell's performance. This work highlights the potential of perovskite solar cells based on AZO TCO to be used for space applications and give a deeper understanding of interfacial degradation due to proton irradiation