Special Issue: “Advanced Thin Film Materials for Photovoltaic Applications”

Photovoltaic (PV) technology is rapidly entering the energy market, providing clean energy for sustainable development in society, reducing air pollution. In order to accelerate the use of PV solar energy, both an improvement in conversion efficiency and reduction in manufacturing cost should be carried out continuously in the future. This can be achieved by the use of advanced thin film materials produced by low-cost growth techniques in novel device architectures. This effort intends to provide the latest research results on thin film photovoltaic solar energy materials in one place. This Special Issue presents the growth and characterisation of several PV solar energy materials using low-cost techniques to utilise in new device structures after optimisation. This will therefore provide specialists in the field with useful references and new insights into the subject. It is hoped that this common platform will serve as a stepping-stone for further development of this highly important field

Solar Energy Strategy for Sri Lanka: The Solar Village Solution for Sustainable Development and Poverty Reduction

This paper will first summarise few relevant projects for energy production using solar energy for Sri Lanka. Amongst these projects, the "Solar Village" will be the main focus of this paper. This will include a brief history of the development of this project through an HELink programme in the 1990s. The pilot project started in 2008 in Kurunegala District and its tremendous impacts on social development will be discussed. Solar village concepts used to empower village communities, combat climate change issues and find solutions to social problems like kidney diseases due to non-availability of clean and drinkable water will be presented. There are 13 out of 17 SDGs (sustainable development goals as highlighted by the United Nations) embedded in this particular project by channelling new Science and Technologies towards Social development. With the help of two charity organisations {APSL-UK (Association of Professional Sri Lankans in the UK) and Helasarana}, another few Solar Villages are commencing soon as a replica of the pilot project

Optimisation of pH of the CdCl2+Ga2(SO4)3 activation step of CdS/CdTe based thin-film solar cells

In order to produce high efficiency solar cells based on CdTe, CdCl2 post-growth treatment is an essential processing step. This treatment can be further improved by adding elements such as Fluorine and Gallium into the CdCl2 solution. Through systematic experimentation, it has been found that the pH value of the treatment solution also affect the conversion efficiency of the solar cells. This work therefore focuses on the effect of pH value of CdCl2+Ga2(SO4)3 aqueous solution on the device efficiencies. The graded bandgap device structure, glass/FTO/n-ZnS/n-CdS/n-CdTe/Au was used in this work. The pH values of 1.00, 2.00 and 3.00 for CdCl2+Ga2(SO4)3 solutions were utilised for the activation of glass/FTO/n-ZnS/n-CdS/n-CdTe layers and its effects were explored for both the CdTe material and device properties. It has been found that both CdTe material properties and solar cell device properties are superior when the pH value of 2.00 is used for post-growth treatment. The best conversion efficiency observed in this work for the above graded bandgap device is 12.2%

Electrical properties of cadmium telluride and associated devices

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Factors affecting electroplated semiconductor material properties: The case study of deposition temperature on cadmium telluride

Electrodeposition of cadmium telluride (CdTe) on fluorine doped tin oxide (FTO) using two electrode configurations was successfully achieved with the main focus on the growth temperature. The electroplating temperatures explored ranged between 55 and 85 °C for aqueous electrolytes containing 1.5 M cadmium nitrate tetrahydrate (Cd(NO3)2·4H2O) and 0.002 M tellurium oxide (TeO2). The ensuing CdTe thin-films were characterized using X-ray diffraction (XRD), UV-Vis spectrophotometry, scanning electron microscopy (SEM), energy dispersive X-ray (EDX), and photoelectrochemical (PEC) cell measurements. The electroplated CdTe thin-films exhibit a dominant (111) CdTe cubic structure, while the crystallite size increases with the increase in the electroplating temperature. The dislocation density and the number of crystallites per unit area decrease with increasing growth temperature. The optical characterization depicts that the CdTe samples show comparable absorbance and a resulting bandgap of 1.51 ± 0.03 eV for as-deposited CdTe layers. A marginal increase in the bandgap and reduction in the absorption edge slope towards lower deposition temperatures were also revealed. The annealed CdTe thin-films showed improvement in the energy bandgap as it tends towards 1.45 eV while retaining the aforementioned absorption edge slope trend. Scanning electron microscopy shows that the underlying FTO layers are well covered with increasing grain size observable relative to the increase in the deposition temperature. The energy dispersive X-ray analyses show an alteration in the Te/Cd relative to the deposition temperature. Higher Te ratio with respect to Cd was revealed at deposition temperature lower than 85 °C. The photoelectrochemical cell study shows that both p- and n-type CdTe can be electroplated and that deposition temperatures below 85 °C at 1400 mV results in p-type CdTe layers

Analysis of the Electronic Properties of All-electroplated ZnS, CdS and CdTe Graded Bandgap Photovoltaic Device Configuration

All-electrodeposited ZnS, CdS and CdTe thin layers have been incorporated in a graded bandgap solar cell structure of glass/FTO/n-ZnS/n-CdS/n-CdTe/Au have been fabricated and an average conversion efficiency of 14.18% was achieved under AM1.5 illuminated condition. Based on former work in which 10% conversion efficiency was reported, optimisation has been made to the semiconductor layers, precursors, thicknesses and the post-growth treatment. These results demonstrate the advantages of multi-layer graded bandgap device configuration and the inclusion of gallium based post-growth treatment (CdCl2+Ga2(SO4)3) on the CdS/CdTe-based device structure. The fabricated devices were characterised using both current-voltage (I-V) and capacitance-voltage (C-V) techniques. Under dark I-V condition, a rectification factor (R.F.) of 104.8, ideality factor (n) of 1.60 and a barrier height (ϕb) >0.82 eV were observed. Under AM1.5 illuminated I-V condition, short-circuit current density (Jsc) of 34.08 mAcm-2, open-circuit voltage (Voc) of 730 mV, fill-factor (FF) of 0.57 and conversion efficiency of 14.18% were observed. Under dark C-V condition, doping density (ND) of 7.79×1014 cm-3 and a depletion width (W) of 1092 nm were achieved. In addition, the work demonstrates the capability of two-electrode system as a simplification to the conventional three-electrode system in the electrodeposition of semiconductors

Analysis of electrodeposited CdTe thin films grown using cadmium chloride precursor for applications in solar cells

Deposition of cadmium telluride (CdTe) from cadmium chloride (CdCl2) and tellurium oxide has been achieved by electroplating technique using two-electrode configuration. Cyclic voltammetry shows that near-stoichiometric CdTe is achievable between 1330 and 1400 mV deposition voltage range. The layers grown were characterised using X-ray diffraction (XRD), UV–Visible spectrophotometry, scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), photoelectrochemical (PEC) cell and DC conductivity measurements. The XRD shows that the electrodeposited CdTe layer is polycrystalline in nature. The UV–Visible spectrophotometry shows that the bandgap of both as-deposited and heat-treated CdTe films are in the range of (1.44–1.46) eV. The SEM shows grain growth after CdCl2 treatment, while, the EDX shows the effect of growth voltage on the atomic composition of CdTe layers. The PEC results show that both p- and n-type CdTe can be electrodeposited and the DC conductivity reveals that the high resistivity is at the inversion growth voltage (Vi) for the as-deposited and CdCl2 treated layers

Unravelling complex nature of CdS/CdTe based thin film solar cells

Thin film solar cells based on CdS/CdTe hetero- structure has shown a drastic improvement changing from 16.5 to 22.1% efficiency during a short period of time from ~2013 to ~2016. This has happened in the industrial environment and the open research in this field has stagnated over a period of two decades prior to ~2013. Most of the issues of this hetero-structure were not clear to the photovoltaic (PV) community and research efforts should be directed to unravel its complex nature. Issues related to materials, post-growth treatment, chemical etching prior to metallisation and associated device physics are the main areas needing deeper understanding in order to further develop this device. After a comprehensive research programme in both academia and in industry on these materials, surfaces and interfaces and fully fabricated devices over a period of over three decades by the main author, the current knowledge as understood today, on all above mentioned complex issues are presented in this paper. Full understanding of this structure will enable PV developers to further improve the conversion efficiency beyond 22.1% for CdS/CdTe based solar cells

An investigation of the influence of different transparent conducting oxide substrates/front contacts on the performance of CdS/CdTe thin-film solar cells

CdS/CdTe/Au thin film solar cells have been fabricated on different transparent conducting oxide (TCO) substrates/front contacts to study the influence of these different TCOs on the performance of the devices. The TCOs used were ZnO, ZnO:Al and SnO2:F. Under dark condition, all three device structures of the type glass/TCO/n-CdS/n-CdTe/Au n-n heterojunction+Schottky barrier, show interesting rectifying behaviors with rectification factors (RF) in the range (102.5 – 105.0), Schottky barrier heights (ΦB) greater than (0.69 – 0.81) eV, diode ideality factors (n) in the range (1.85 – 2.12), reverse saturation current densities (J0) in the range (3.18×10-6 – 3.18×10-8) Acm-2, series resistances (Rs) in the range (507 – 1114) Ω and shunt resistances (Rsh) in the range (0.84 – 271) MΩ. The device structures glass/SnO2:F/n-CdS/n-CdTe/Au and glass/FTO/ZnO:Al/n-CdS/n-CdTe/Au show the best performance with equal J0 of 3.18×10-8 Acm-2, equal ΦB > 0.81 eV, RF of 104.9 and 105.0, n value of 2.01 and 2.12, Rs of 615 Ω and 507 Ω and Rsh of 197 and 271 MΩ respectively. The device structure with ZnO shows the least performance. Under AM1.5 illumination, the device structure glass/SnO2:F/n-CdS/n-CdTe/Au shows the best solar cell performance with open-circuit voltage of 630 mV, short-circuit current density of 23.5 mAcm-2, fill factor of 0.44 and conversion efficiency of 6.5%, and is followed by the device structure with ZnO:Al showing a conversion efficiency of 6.0%. Suggested energy band diagrams of the devices as well as possible reasons for the observed trends in performance are presented and discussed

One-sided rectifying p-n junction diodes fabricated from n-CdS and p-ZnTe:Te semiconductors

The fabrication of a one-sided p-n hetero-junction (HJ) diodes have been successfully carried out using both p-type ZnTe and n-CdS semiconductors. Chemical bath deposition (CBD) and electrodeposition (ED) techniques have been used in the deposition of n-CdS and p-ZnTe layers respectively. Before the fabrication of the one-sided p-nHJ diodes, the electrical properties of glass/ FTO/p-ZnTe/Al and glass/FTO/n-CdS/Au rectifying structures were separately studied using capacitance-voltage (C-V ) technique so as to determine the doping density of each of the thin films. The results from C-V analyses showed that p-ZnTe is moderately doped with an acceptor density of 3.55×1015 cm3 while n-CdS is heavily doped with a donor density of 9.00×1019 cm3. The heavy doping of n-CdS and moderate doping of p-ZnTe will make the interface between n-CdS and p-ZnTe thin films a one-sided n+p diode. Therefore, to fabricate the CdS/ZnTe hetero-structure, it was ensured that approximately same thickness of CdS and ZnTe thin films being used in the initial experiment to study the electrical properties of glass/FTO/n-CdS/Au and glass/FTO/p-ZnTe/Al were also used in the development of the one-sided n+p junction diodes to obtain more accurate results. The electronic properties of the device structure were studied using both current-voltage (IV ) and C-V measurement techniques. The I-V results show that the one-sided n+pHJ diodes possess good rectifying quality with a series resistance (Rs) of35 and rectification factors exceeding 102.7 under dark condition. The results of theC-Vanalyses showed that the acceptor density of the onesided n+pHJ diode is of the order of 1015 cm3 while the donor density is of the order of 1018 cm3. The results obtained from this analysis still showed the moderate doping of p-ZnTe and the degenerate nature of n-CdS