Development of buffer layers and absorbers for next generation thin film solar cells

Solar energy is sought after to produce clean, renewable energy to combat climate change and photovoltaics is the way to convert the sunlight to electricity. Thin film photovoltaics is a major subject of research with aims of improving efficiency and cutting costs. This thesis explores the development of materials for use as either buffer layers or absorber layers for thin film PV. The first being magnetron sputtered copper-tungsten oxides, where it is explored for use as a potential absorber material or a transparent conducting oxide. The second being nickel oxide, where this thesis builds upon previous use as a back buffer layer for cadmium telluride (CdTe) films, but in this case, oxygen is varied during deposition of nickel oxide (NiO),to identify any improvements or setbacks since the characteristics of nickel oxide will differ. To add, it is also explored as a bifacial device, as cadmium telluride is yet to establish itself in this market. Lastly, tungsten diselenides (WSe2), an absorber material, has initial optimisations regarding a rapid thermal annealing method which is relatively unexplored.

Copper tungsten oxides are researched in this thesis due to the possibility of being used in photovoltaics as either an absorber or transparent conducting oxide (TCO). Films were magnetron sputtered from copper and tungsten targets in varied oxygen-argon environments, of oxygen between 25% - 37.5%. They were designed to produce a gradient across the substrate where each metal favoured an opposing side, to allow for variability. As oxygen content increased the transmission of the films increased, as did the band gaps. Conductivity types swapped from n-type to p-type on the tungsten rich side as further oxygen was introduced, and those films became highly resistive and transmissive. The large band gaps were identified as appropriate for TCO usage, but measured resistivities were too large. One sub-sample was identified as possibly being used as an absorber, with a band gap of 1.55eV, n-type conductivity, and low resistivity. X-ray spectroscopy measurements identified the films to be generally a mix of WO3, WO2, Cu2O, and CuO, in varied amounts.

Another material studied is nickel oxide. Nickel oxide was previously of interest due to improving the open circuit voltage (Voc) of CdTe based solar cells when used as a back buffer layer. Other works showcase the oxygen effects on NiO films in which electrical and optical characteristics differ, and this work agrees well. The characteristic differences offer a route of novelty as a back buffer layer with CdTe. Having CdTe with NiO back buffer layers improved the open circuit voltage relative to the control sample without. Introducing oxygen content improves efficiencies relative to NiO without oxygen and the control sample, however, larger oxygen contents are detrimental to current-voltage characteristics. All devices show rollover and greater oxygen samples, ≥40%, show s-kinks. SCAPS software proves the rollover and s-kink’s to be products of the valence band offset (VBO). Where the VBO value is greater there is a greater incidence of both occurring. The VBO was shown to be at its minimum at 20% oxygen during sputtering, and oxygen of 40% onwards increased the VBO. These devices are also studied for metastable effects which is a common observation in cadmium telluride solar cells. A light soaking method was conducted on these devices, which has previously shown CdTe metastable improvements. It improved the electrical characteristics to almost as deposited conditions. The cause of the metastability was investigated: carrier concentrations and activation energies were ruled out as the main causes. The specific cause is yet unknown, but altering the light soaking method, and further work, is recommended.

Bifacial devices are a relatively recent improvement to thin film PV, in which light is utilised at the front and back of the device to maximise light. A device structure using nickel oxide as a back buffer layer was investigated. Copper was included and varied between 0-20nm. Including 5nm and 10nm copper improved electrical characteristics, but 20nm had minimal improvements. The device successfully converts light into current on both sides but is limited under back illumination. The efficiency under front illumination reaches 3.15% but efficiency with back illumination only reaches 0.19%. The low back illuminated efficiency is identified to be due to the thickness of the CdTe and is suggested to be reduced to 2μm.

Absorber layers are an integral part of thin film photovoltaics. One such absorber material is tungsten diselenide. An initial investigation was conducted with the aim of producing films, using a rapid thermal annealing method, that adhere to the substrate as delamination is a common effect from selenisation. Rapid thermal annealing has the advantages of providing quick ramp rates to high temperatures. A temperature variation was investigated, but no reliable conclusion could be drawn of the temperature effects due to surface layer delamination. Scanning electron microscopy topography revealed cracks in the form of grains on the film left on the surface. Tungsten substrates were varied for the tungsten thickness and selenised. Thickness ≤134nm produced WSe2 without delamination and films ≥252nm had delaminated. SEM topography revealed large cracks on the latter films, showing that thickness is important for stable WSe2 films.