I. M. Dharmadasa

Strengths and Advantages of Electrodeposition as a Semiconductor Growth Technique for Applications in Macroelectronic Devices

This paper reviews the strengths and advantages of electrodeposition as a low-cost and large-area semiconductor growth technique for applications in macroelectronic devices such as solar panels and large-area display devices. To highlight its strengths, experimental evidence obtained from X-ray diffracton and photoluminescence is presented. It has been shown that when materials are grown with the right conditions, electrodeposition is capable of producing high-quality materials for electronic device applications. The paper also summarizes the new science and new solar cell designs emanating from the research on electrodeposited electronic materials and solar cell devices. The new designs have been tested with well-researched III-V materials, GaAs and AlxGa((1-x))As, and the device performances show a drastic improvement in device parameters (V-oc approximate to 1110 mV and FF approximate to 0.83). These new designs have also been tested using electrodeposited CuInGaSe2 materials, with encouraging preliminary results which are also presented and discussed. (c) 2005 The Electrochemical Society.

Recent developments and progress on electrical contacts to CdTe, CdS and ZnSe with special reference to barrier contacts to CdTe

A summary of experimental work on electrical contacts to CdTe, CdS and ZnSe is presented and recent progress of research on electrical contacts to these materials is reviewed in this paper. The surface preparation and surface characterisation prior to metallisation, interactions at the interface during contact fabrication, Schottky barrier characterisation and subsequent aging effects are considered. XPS, AES, SIMS and PL are used for surface characterisation; XPS and soft-XPS are used for interface interaction studies; I-V, C-V, DLTS and BEEM are used for Schottky barrier characterisation; and AES, GDOES and EDX profiling are used to study aging effects. The surfaces of all three materials behave in a similar way when etched in wet chemical etchants. The semiconductor cation is preferentially etched by acidic solutions and the semiconductor anion is preferentially removed by alkaline solutions. It has also been shown that the surface stoichiometry affects the Fermi level pinning position at metal/semiconductor interfaces. Furthermore, the observed Schottky barrier heights with all three materials demonstrate a multi-level pinning behaviour producing different barrier heights depending on the history of the materials used and the fabrication procedure followed. Barrier heights observed are independent of the metal work function, and their stability depends mainly on interactions occurring at the metal/semiconductor interface and are strongly related to the native defect levels within the bulk material.

Investigation of electronic quality of chemical bath deposited cadmium sulphide layers used in thin film photovoltaic solar cells

The investigation of electronic quality of chemical bath deposited cadmium sulphide (CdS) layers was the main objective of this work. For completeness, the US layers were characterised using X-ray diffraction, atomic force microscopy, optical absorption, photoelectrochemical cell, DC electrical conductivity measurements, current-voltage and capacitance-voltage measurements using Gold/CdS Schottky contacts. It has been found that the US layers grown are hexagonal with (002) preferential orientation. The n-type CdS materials show 1-2 mum clusters consisting of 0.3-0.4 mum size crystallites. The optical band gap is 2.42 eV, which shows a red-shift to 2.25 eV upon heat treatment. Gold Schottky contacts produce large Schottky barriers of 1.02 eV with ideality factors of 1.50, indicating excellent electronic qualities. Schottky-Mott plots indicate a moderate doping concentration of 1.2 X 10(17) cm(-3), suitable for electronic device fabrication. However, the DC electrical conductivity measurements carried out at room temperature indicate a very low electrical conductivity in the range (4-11) X 10(-5) (Omega cm)(-1). This indicates a very low mobility value of (2-5) X 10(-3) cm(2) V-1 s(-1), which are five orders of magnitude below that of single crystal CdS. The way forward for further improvement of the electrical conductivity is discussed

Investigation of n-type Cu2O layers prepared by a low cost chemical method for use in photo-voltaic thin film solar cells

A low cost and simple chemical method of boiling copper plates in CuSO4 solution is used to prepare Cu2O layers. X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), glow discharge optical emission spectroscopy (GDOES) and optical absorption have been used to characterise these layers. It has been found that the layers consist of Cu2O phase with a thickness of about 1.4 μm for 60 minutes boiling in CuSO4 solution. The largest grain sizes are in the order of 1 μm and the layers contain cubic Cu2O phases. The layers are n-type in electrical conduction and the optical band gap observed is 2.2 eV.

Advances in thin-film solar cells

Preface List of Symbols and Abbreviations Used in the Book Photovoltaic Solar Energy Conversion Introduction Photovoltaic Effect Solar Energy Materials Electronic Devices Used for Solar Energy Conversion Characteristics of a Solar Cell Next-Generation Solar Cells Summary Status Report on Solar Energy Technologies Introduction Si Solar Cell Technology PV-Manufacturing Cost Based on Si Technology PV Technology Based on III-V Compounds New Technology for PV and Nano-Divices Emerging Low-Cost Thin-Film Technologies Summary Electrochemical Deposition of Solar Energy Materials Introduction Electrodeposition of Semiconductors Strengths and Advantages of Electrodeposition Experimental Evidence Issues in Electrodeposition of Semiconductors Current Work and Future Prospects Summary Background of the CdTe Solar Cell and the New Device Concept Introduction The Conventional Model for a Glass/Conducting Glass/CdS/CdTe/Metal Solar Cell Key Observations That Led to the Formulation of a NewModel New Concept for CdS/CdTe Solar Cell Description of Experimental Results Using the Two Models Predictions for Further Development of CdS/CdTe Solar Cells and Latest Observations Summary Extension of the New Model to CIGS Thin-Film Solar Cells Introduction Summary of Accumulated Knowledge on CIGS-Based Materials Summary of Accumulated Knowledge on CIGS-Based Solar Cells Current Views of the Physics Behind CIGS Solar Cells Reported Device Performance RecentWork on Metal/p-CIGS Interfaces Deeper Understanding of Mo/CIGS/CdS/i-ZnO/ n-ZnO:Al/Metal-Grid Solar Cells Discussion on Further Improvements of CIGS Solar Cells Conclusions Summary Effective Harvesting of Photons Introduction Tandem Solar Cells Comparison of the Two Connecting Methods Conclusions Summary Multi-Layer Graded Bandgap Solar Cells Introduction Summary of Growth and Process Details of the Device Structure Experimental Results of Fully Processed Devices Discussions Summary Solar Cells Active in Complete Darkness Introduction Summary of Experimental Results Search for Experimental Evidence of the Impurity PV Effect Responsivity Measurements I-V Measurements Under Dark Conditions Discussion Conclusions Summary Effects of Defects on Photovoltaic Solar Cell Characteristics Introduction Variations of I-V Characteristics of Metal/n-CdTe Interfaces Effects on the Performance of CdS/CdTe Solar Cells Variations in GaAs/AlGaAs Solar Cells Variations in CIGS Solar Cells Summary A Future Dominated by Solar Energy Introduction Early Applications with Low Power Requirements Early Applications with Moderate Power Requirements Applications in Solar Home Systems (~50WRange) Applications in Drip Irrigation Systems (~100WRange) Applications in Powering Computers (500-1,000WRange) Applications in Large-ScaleWater Pumping (~1,000Wand Above) Solar Power Applications on Roads Solar Power Applications on Buildings (~3 kW and Above) Energy from Solar Farms and Deserts (MW Range) Recommendations for Developing Countries Recommendations for Developed Countries Summary Is Fermi-Level Pinning Affecting GaAs-Based Solar Cells? Introduction Observation of Discrete Sets of I-V Characteristics Observation of Discrete Sets of Voc Values Discussion of New Observations Thoughts on Future Directions of Thin-Film Solar Cell Research and Development Introduction Areas for Research and Development Efforts Conclusions Index

Electrodeposition and characterisation of CuInSe2 for applications in thin film solar cells

Copper indium diselenide (CuInSe2) layers have been grown at room temperature by electrochemical deposition technique in an aqueous medium. Resulting thin films have been characterised using XRD, XRF, XPS, GDOES and SEM for structural, stoichiometric and morphological properties. A considerable influence of the deposition potential on the atomic composition of In and Se present in the film was observed. Cu composition remains the same within the deposition potentials used in this investigation. The deposited layers are polycrystalline and annealing at 350 degreesC for 30 min improves the crystallinity. The film quality deteriorates due to dissociation when annealed at temperatures above 350 degreesC. Excessive annealing results in a surface which is depleted in Cu and rich in In and Se

Development of p + , p, i, n, and n + -Type CuInGaSe2 Layers for Applications in Graded Bandgap Multilayer Thin-Film Solar Cells

Copper indium gallium diselenide layers with p(+), p, i, n, and n(+)-type electrical conduction, as predetermined, have been electrodeposited from aqueous solutions in a single bath. The photoelectrochemical cell has been used as the key analytical tool to determine the electrical conduction type, and X-ray fluorescence has been used to determine the stoichiometry of the corresponding layers. Optical absorption, X-ray diffraction, and atomic force microscopy have been used to investigate the bandgap, bulk structure, and surface morphology of the material layers, respectively. It has been found that the bandgaps of these layers can be varied in the range 1.10-2.20 eV. A four-layer n-n-i-p solar cell structure was fabricated and a corresponding energy band diagram for the device constructed. Current-voltage and capacitance-voltage measurements were carried out to assess the devices, and these parameters (V-oc approximate to 570 mV, J(sc)approximate to 36 mA cm(-2), and FF approximate to 0.40) indicate encouraging characteristics enabling further development of multilayer thin-film solar cells based on CuInGaSe2. The addition of a p(+) layer to the structure improved device parameters as expected due to improvements at the metal contact to the p(+) surface of the n-n-i-p-p(+) structure. (c) 2007 The Electrochemical Society.

Growth of CdS Layers to Develop All-Electrodeposited CdS/CdTe Thin-Film Solar Cells

The electrodeposition (ED) of CdS (continuous process) was investigated in detail with the aim of replacing chemical bath deposited (CBD) CdS (batch process) from the solar cell production process. This avoids the daily production of toxic waste and, hence, drastically reduces manufacturing cost. Material studies were carried out using voltammetry, X-ray diffraction, X-ray fluorescence, optical absorption, photoelectrochemical studies, and scanning electron microscopy. The main difference between the two processes is in the structure of material layers: ED-CdS is hexagonal while CBD-CdS is cubic. All other properties, the bandgap (2.42 eV), the electrical conduction type (n-type), and the grain size (~20 nm), remain very similar. The comparison of initial solar cell devices shows that ED-CdS layers have similar or superior performance in thin-film CdS/CdTe solar cells.

Comparison of electrodeposited and sputtered intrinsic and aluminium-doped zinc oxide thin films

Intrinsic zinc oxide (i-ZnO) and aluminium-doped ZnO (ZnO:Al) are components of high-efficiency copper indium gallium diselenide solar cells. This paper examines both of these materials grown by two different techniques, namely radio frequency sputtering and electrodeposition (ED) for comparison and a better understanding. X-ray diffraction showed all materials to be polycrystalline and hexagonal (wurtzite) ZnO. Scanning electron microscopy indicated crystallites with different orientations for ED materials compared to agglomerated nanocrystallites of the sputtered layers. The band-gap energy was determined to be in the range 3.27-3.45 eV. The transmission was 85% for both ED materials and 95% for the sputtered layers. Glass/FTO/i-ZnO/Al structures were rectifying, and glass/FTO/ZnO:Al/Al contacts were ohmic for both ZnO:Al layers. Addition of Al decreases the bulk resistivity for both i-ZnO layers by 1-2 orders of magnitude. The photovoltage response to pulsed illumination showed a slow relaxation hysteresis, and all materials showed n-type electrical conduction.

Sulphidation of electrodeposited cuprous oxide thin films for photovoltaic applications

Abstract Electrodeposited cuprous oxide thin films on indium-doped tin oxide (ITO) substrates were sulphided by exposing them to a spray of aqueous solution of sodium sulphide or to a mixture of hydrogen sulphide and nitrogen gases. Both methods produced light darker and darker films having different photovoltaic characteristics in a solar cell structure. The photovoltages produced by the light darker films under AM 1.5 illumination was negative as compared to the positive photovoltages produced by the darker films. Spectral response measurements revealed that most of the light darker films produced positive photovoltages in the shorter wavelengths and negative photovoltages in the longer wavelengths. However, some of the light darker films produced only the negative photovoltage for the entire spectral range and their photovoltaic properties were comparatively better. Darker films resulted in only the positive photovoltages in the entire spectral range. As a result of the sulphidation, the bulk crystal structure of the cuprous oxide thin films was not changed, however, the interfacial characteristics of the solar cell structure were modified.