Effect of mechanical compression on Cu(In,Ga)Se films : micro-structural and photoluminescence analysis

Cu(In,Ga)Se (CIGS) thin films were deposited by a two-step process on Mo-coated soda-lime glass substrates. The CuInGa (CIG) precursors were prepared in an in-line evaporation system at room temperature, and then selenised at 500 °C. The two-step processed CIGS films were mechanically compressed at 25 MPa to improve their optoelectronic properties, which were verified by photoluminescence (PL). The surface and structural properties were compared before and after compression. The mechanical compression has brought changes in the surface morphology and porosity without changing the structural properties of the material. The PL technique has been used to reveal changes in the electronic properties of the films. PL spectra at different excitation laser powers and temperatures were measured for as-grown as well as compressed samples. The PL spectra of the as-grown films revealed three broad and intense bands shifting at a significant rate towards higher energies (j-shift) with the increase in excitation power suggesting that the material is highly doped and compensated. At increasing temperature, the bands shift towards lower energies, which is a characteristic of the band tails generated by spatial potential fluctuation. The compression increases the intensity of energy bands by an order of magnitude and reduces the j-shift, demonstrating an improvement of the electronic properties

Reliability investigation for a built ultrahigh concentrator prototype

This is the final version of the article. Available from AIP Publishing via the DOI in this record.Ultrahigh concentrator photovoltaics hold a great potential in both reducing the cost of photovoltaic energy and to higher conversion efficiencies. The challenges in their design and manufacturing however have not yet permitted a reliable ultrahigh ( > 2000X) system. Here we propose an ultrahigh concentrator photovoltaic design of 5800X geometrical concentration ratio based on multiple primary Fresnel lenses focusing to one central solar cell. The final stage optic is of a novel design to accept light from four different directions and focus the light towards the solar cell. The extremely high geometrical concentration of 5800X was chosen in anticipation of the losses accompanied with ultrahigh concentration due to alignment difficulties. The system was designed with manufacturability as one of the priorities and resulted in easily achieving > 2000X concentration for a first prototype with non-achromatic Fresnel lenses and in house secondarys. Higher concentrations are anticipated for future prototypes but investigation into the cell performance is required. An acceptance angle of 0.4°was achieved for this design which is considered good for such an ultrahigh concentration level and what's more, even at higher misalignment angles (such as 0.8 or 1 degree) ultrahigh concentration ratios are still achieved in simulations. Such a design could be the breakthrough in concentrator photovoltaic research for reaching higher concentration ratios. The use of flat optics to ease manufacturing and alignment is a simple but effective method to achieve a reliable system that will achieve ultrahigh concentration even at 36% optical efficiency. Such a design will be of use in investigations of concentration, concentrator solar cell development, temperature effects and more; achieving ultrahigh concentration levels not yet tested.The authors acknowledge the funding bodies of the SUNTRAP project and as part of the encouragement of open access data, any part of the presented investigation and results can be given upon email request to the authors

Conjugate Refractive-Reflective Homogeniser in a 500x Cassegrain Concentrator: Design and Limits

This is the author accepted manuscript. The final version is available from IET via the DOI in this recordThere is another ORE record for this publication: http://hdl.handle.net/10871/19792In this study, we present the conjugate refractive reflective homogeniser (CRRH) to be used in a 500× Cassegrain photovoltaic concentrator. The CRRH is a dielectric crossed v-trough lined with a reflective film whilst maintaining an air gap between them. This air gap between the two surfaces helps in trapping the scattered light from the refractive geometry and ensures both total internal reflection and standard reflection of the escaped rays. A 10-42% drop in optical efficiency has been shown to occur due to varying the surface roughness of the homogeniser in these ray trace simulations for the Cassegrain setup. The CRRH increased the overall optical efficiency by a maximum of 7.75% in comparison with that of a standard refractive homogeniser simulated within the same concentrator system. The acceptance angle and flux distribution of these homogenisers was also investigated. The simple shape of the CRRH ensures easy manufacturing and produces a relatively uniform irradiance distribution on the receiver. The theoretical benefit of the CRRH is also validated via practical measurements. Further research is required but a 6.7% power increase was measured under a 1000 W/m2 solar simulator at normal incidence for the experimental test.Department of Science and Technology (DST), IndiaEngineering and Physical Sciences Research Council (EPSRC

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

This is the final version of the article. Available from Elsevier via the DOI in this record.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.This research work was supported partially through the funding support received from EPSRC UK–India programme APEX (EP/H040218/1) and partially supported by Excitonic Supergen (EPSRC (EP(G03101088/1)) programme

Electronic structure, structural and optical properties of thermally evaporated CdTe thin films

Copyright © 2007 Elsevier. NOTICE: this is the author’s version of a work that was accepted for publication in Physica B: Condensed Matter. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Physica B: Condensed Matter (2007), DOI: 10.1016/j.physb.2006.04.008Thin films of CdTe were deposited on glass substrates by thermal evaporation. From the XRD measurements itis found that the films are of zinc-blende-type structure. Transmittance, absorption, extinction, and refractive coefficients are measured. Electronic structure, band parameters and optical spectra of CdTe were calculated from ab initio studies within the LDA and LDA+U approximations. It is shown that LDA underestimates the band gap, energy levels of the Cd-4d states, s-d coupling and band dispersion. However, it calculates the spin-orbit coupling correctly. LDA+U did not increase much the band gap value, but it corrected the s-d coupling by shifting the Cd-4d levels towards the experimentally determined location and by splitting the LDA-derived single s peak into two peaks, which originates from admixture of s and d states. It is shown that the sd coupling plays an important role in absorption and reflectivity constants. The calculated optical spectra fairly agree with experimental data. Independent of wave-vector scissors operator is found to be a good first approximation to shift rigidly the band gap of CdTe underestimated by LDA.Research Council of NorwayAcademy of Sciences of UzbekistanUniversity Grants Commission (UGC), Indi

Fully spray-coated organic solar cells on woven polyester cotton fabric for wearable energy harvesting applications

This paper presents the novel use of spray coating to fabricate organic solar cells on fabrics for wearable energy harvesting applications. The surface roughness of standard woven 65/35 polyester cotton fabric used in this work is of the order of 150 µm and this is reduced to few microns by a screen printed interface layer. This pre-treated fabric substrate with reduced surface roughness was used as the target substrate for the spray coated fabric organic solar cells that contains multiple layers of electrodes and active materials. A fully spray coated photovoltaic (PV) devices fabricated on fabric substrates has been successfully demonstrated with comparable power conversion efficiency to the glass based counterparts. All PV devices are characterised under simulated AM 1.5 conditions. Device morphologies were examined by scanning electron microscopy (SEM). This approach is potentially suitable for the low cost integration of PV devices into clothing and other decorative textilesThis work was supported by Sensor Platform for HEalthcare in a Residential Environment (SPHERE) project (EP/K031910/1). Professor S. P. Beeby acknowledges EPSRC support through his Fellowship ‘Energy Harvesting Materials for Smart Fabrics and Interactive Textiles’ (EP/I005323/1). Professor P. J. Skabara thanks the Royal Society for a Wolfson Research Merit Award

Improving spectral modification for applications in solar cells: A review

The spectral mismatch between solar cells and incident radiation is a fundamental factor limiting their efficiencies. There exist materials and luminescent processes which can modify the incident sunlight’s properties to better suit the cell’s optimal absorption regions. This makes for an interesting area of research and promising technique for enhancing the efficiency of solar cells which is important for environmental reasons. It is intended for this review to provide the reader with historical and up-to-date developments of the application of spectral modification to solar cells and contribute to growing its impact on real-world PV devices. We concisely outline the underlying principles of three spectral modification processes: upconversion (UC), downconversion (DC) and luminescent downshifting (LDS). For each section we present up to date experimental results for applications to a range of solar PV technologies and discuss their drawbacks. With particular focus on UC, we then review how nanostructures or integrated optics might overcome these problems. Finally, we discuss practical challenges associated with advancing this approach for commercialisation and opportunities spectral modification presents; namely where future research should focus and via a cost analysis with a simple formula that can be used to determine financial viability for the deployment of this technology

Spray deposited copper zinc tin sulphide (Cu<inf>2</inf>ZnSnS<inf>4</inf>) film as a counter electrode in dye sensitized solar cells

Stoichiometric thin films of Cu2ZnSnS4 (CZTS) were deposited by the spray technique on a FTO coated glass substrate, with post-annealing in a H2S environment to improve the film properties. CZTS films were used as a counter electrode (CE) in Dye-Sensitized Solar Cells (DSCs) with N719 dye and an iodine electrolyte. The DSC of 0.25 cm2 area using a CE of CZTS film annealed in a H2S environment under AM 1.5G illumination (100 mW cm-2) exhibited a short circuit current density (JSC) = 18.63 mA cm-2, an open circuit voltage (VOC) = 0.65 V and a fill factor (FF) = 0.53, resulting in an overall power conversion efficiency (PCE) = 6.4%. While the DSC using as deposited CZTS film as a CE showed the PCE = 3.7% with JSC = 13.38 mA cm-2, VOC = 0.57 V and FF = 0.48. Thus, the spray deposited CZTS films can play an important role as a CE in the large area DSC fabrication. © the Partner Organisations 2014

Enhanced efficiency for building integrated concentrator photovoltaic modules based on rare earth doped optics

A major challenge facing silicon solar cells used in building-integrated concentrator photovoltaics (BICPV) is their reduced electrical response when exposed to light of short or long wavelengths. In an attempt to tackle this problem, single cell static CPV modules were fabricated with some of the devices containing rare earth doped compounds which were dispersed into the system in varying concentrations and geometries. Under a solar simulator at 1000 W/m2, the power conversion efficiency (PCE) of devices improved up to 11.1% relative through the addition of these materials. At lower irradiances and compared to cells without concentrators, the relative efficiency gains were more pronounced and external quantum efficiency (EQE) measurements suggested spectral conversion was responsible for these enhancements. For a large scale BICPV system, a simple analysis showed cost per watt could fall by up to 8.1% and power output increased from 25.7 to 28.4 W/m2 through this approach

A selective metasurface absorber with an amorphous carbon interlayer for solar thermal applications

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this recordData availability: Data shown in this paper is accessible via the University of Bristol data repository: 10.5523/bris.11twobtdyxfs1ib2fxhlvn107This paper presents fabrication, measurement and modelling results for a metal-dielectric-metal metasurface absorber for solar thermal applications. The structure uses amorphous carbon as an inter-layer between thin gold films with the upper film patterned with a 2D periodic array using focused ion beam etching. The patterned has been optimised to give high absorptance from 400-1200nm and low absorptance above this wavelength range to minimise thermal radiation and hence obtain higher temperature performance. Wide angle absorptance results are shown and detailed modelling of a realistic nanostructured upper layer results in excellent agreement between measured and modelled results. The use of gold in this paper is a first step towards a high temperature metasurface where gold can be replaced by other refractory metals such as tungsten or chrome.Engineering and Physical Sciences Research Council (EPSRC