Deidra Hodges

Feasibility study of thermal energy harvesting using lead free pyroelectrics

Energy harvesting has significant potential for applications in energizing wireless sensors and charging energy storage devices. To date, one of the most widely investigated materials for mechanical and thermal energy harvesting is lead zirconate titanate (PZT). However, lead has detrimental effects on the environment and on health. Hence, alternative materials are required for this purpose. In this paper, a lead free material, lithium niobate (LNB) is investigated as a potential material for pyroelectric energy harvesting. Although its theoretical pyroelectric properties are lower compared to PZT, it has better properties than other lead free alternatives such as ZnO. In addition, LNB has a high Curie temperature of about 1142 °C, which makes it applicable for high temperature energy harvesting, where other pyroelectric ceramics are not suitable. Herein, an energy harvesting and storage system composed of a single crystal LNB and a porous carbon-based super-capacitor was investigated. It is found that with controlled heating and cooling, a single wafer of LNB (75 mm diameter and 0.5 mm thickness) could generate 437.72 nW cm–3 of power and it could be used to charge a super-capacitor with a charging rate of 2.63 mV (h cm3)–1.

Arrays of Position-Sensitive Virtual Frisch-Grid CdZnTe Detectors: Results From a

Position-sensitive virtual Frisch-grid (VFG) CdZnTe (CZT) detectors offer a unique capability for correcting the response nonuniformities caused by crystal defects. This allowed us to achieve high energy resolution, while using typical-grade commercial CZT crystals with relaxed requirements to their quality, thus reducing the overall cost of detectors. Another advantage of the VFG detectors is that they can be integrated into arrays and used in small compact hand-held instruments or large-area gamma cameras that will enhance detection capability for many practical applications, including nonproliferation, medical imaging, and gamma-ray astronomy. Here, we present the results from testing small array prototypes coupled with front-end application-specified integrated circuit. Each detector in the array is furnished with 5-mm-wide charge-sensing pads placed near the anode. The pads signals are converted into XY coordinates, which combined with the cathode signals (for Z coordinates) provide 3-D position information of all interaction points. The basic array consists of a number of detectors grouped into

4\times 4

2\times 2

Array Prototype

subarrays, each having a common cathode made by connecting together the cathodes of the individual detectors. These features can significantly improve the performance of detectors while using typical-grade low-cost CZT crystals to reduce the overall cost of the proposed instrument.

Tellurides as back contacts for substrate CdTe thin film solar cells on flexible foil substrates

The formation of a low resistance ohmic contact on p-type CdTe is a significant challenge for the superstrate solar cell configuration, where surface preparation techniques including wet etches are often used in order to modify the surface of CdTe prior to applying the contact material(s). It is even a greater challenge for the less commonly used substrate configuration, since the p-contact/CdTe interface will be formed first and will be exposed to the remaining solar cell fabrication steps. This paper presents results on the properties of two tellurides and their performance as back contacts in substrate type CdTe solar cells fabricated on flexible foil substrates. Solar cell efficiency is limited by the properties of the back contact and is currently in the 6.0–6.5% range.

Development of back contacts for CdTe thin film solar cells deposited on flexible foil substrates

The formation of low resistance ohmic contacts on p-type CdTe is a major challenge for the superstrate solar cell the dominant device configuration for this technology. It is even a greater challenge for the substrate configuration, since the back contact material is deposited first and is therefore influenced by subsequent high temperature fabrication processing. In this paper, the structural and electrical properties of ZnTe and Sb2Te3 films deposited by the close-spaced sublimation and evaporation respectively are presented. Thin film CdTe solar cells of the substrate configuration have been fabricated on flexible foil substrates using ZnTe and Sb2Te3 as back contacts, and their performance has been evaluated.

Characterization and analysis of structural and optical properties of perovskite thin films

The development of perovskite thin films by spin-coating, deposition techniques have been investigated. The methyl ammonium lead iodide (CH3NH3PbI3) perovskite has a direct band gap of 1.5 eV and a large absorption coefficient of over 104 cm-1. The perovskite absorber was deposited by a non-vacuum liquid-based coating method with chlorobenzene and without chlorobenzene by using Laurell Technologies WS650 spin processor. Optical, structural and electronic characterization and analysis of the thin films were performed by using the Hitachi S-4800 Scanning Electron Microscope (SEM) and the Bruker D8 Discover X-ray diffractometer (XRD), the Thermo Scientific DXR SmartRaman spectrometer, and the Cary 5000 UV-Vis spectrophotometer. Results were used to determine the crystal structure, orientation and crystallite size, and energy band gap (Eg) of perovskite thin films.

Development of CZTS thin films by non-vacuum, liquid-based techniques for efficient, low-cost CZTS solar cells

The fundamental development of Cu2ZnSnS4 (CZTS) thin films by non-vacuum, liquid-based techniques have been investigated. CZTS has a suitable optical band gap of ~1.5 eV and a large optical absorption coefficient of over 104 cm-1. CZTS polycrystalline thin films were prepared by a non-vacuum liquid-based coating method. Characterization and analysis of the thin films were performed using the JEOL 7600F scanning electron microscope (SEM) and the Panalytical X-ray diffractometer (XRD) to determine the crystal structure, orientation and crystallite size. Results show the formation of kesterite CZTS.

Substrate based CdTe solar cells fabricated on metallic foils: Device, material, and processing issues

Substrate CdTe cells are uncommon due to the difficulty in forming effective contacts to p-type CdTe. The development of such solar cells could further increase the applications and enhance the product characteristics of the lowest cost thin film PV technology, by making lightweight and flexible products possible. This paper reviews the current status of the development of substrate CdTe solar cells on flexible metallic substrates. The influence of substrate impurities, the effect of impurity barrier layers, the effect of heat treatments, and the use of various compounds as back contacts are among the topics to be discussed. Presently the efficiency of foil/CdTe-type solar cells is in the 6.5% range. The performance is limited by low VOCs (<700 mV) and the fact that the p-type contact to CdTe is rectifying leading to poor FFs.

Structural properties of CdTe and ZnTe thin films deposited on flexible foil substrates

A common feature of the most efficient laboratory scale CdS/CdTe solar cells has been the use of close-spaced sublimation (CSS) for the deposition of CdTe [1]. In this study, the growth of CdTe and ZnTe thin films deposited on flexible foil substrates by CSS has been investigated in order to study their structural properties for solar cell applications. Thin films of CdTe were deposited by CSS onto substrates held at temperatures in the range of 400–550°C. The effect of the substrate-source temperature and the growth rate on the structure and surface morphology of CdTe films were analyzed. The structural and surface morphology of the CdTe films were determined by XRD and SEM. Similar studies have been done on the growth characteristics of ZnTe which is often used as a back contact interlayer.