Thin film solar cells based on cadmium telluride (CdTe) are complex devices which have great potential for achieving high conversion efficiencies. Lack of understanding in materials issues and device physics slows down the rapid progress of these devices. This paper combines relevant results from the literature with new results from a research programme based on electro-plated CdS and CdTe. A wide range of analytical techniques was used to investigate the materials and device structures. It has been experimentally found that n-, i-and p-type CdTe can be grown easily by electroplating. These material layers consist of nano-and micro-rod type or columnar type grains, growing normal to the substrate. Stoichiometric materials exhibit the highest crystallinity and resistivity, and layers grown closer to these conditions show n → p or OPEN ACCESSCoatings 2014, 4 381 p → n conversion upon heat treatment. The general trend of CdCl 2 treatment is to gradually change the CdTe material's n-type electrical property towards i-type or p-type conduction. This work also identifies a rapid structural transition of CdTe layer at 385 ± 5 °C and a slow structural transition at higher temperatures when annealed or grown at high temperature. The second transition occurs after 430 °C and requires more work to understand this gradual transition. This work also identifies the existence of two different solar cell configurations for CdS/CdTe which creates a complex situation. Finally, the paper presents the way forward with next generation CdTe-based solar cells utilising low-cost materials in their columnar nature in graded bandgap structures. These devices could absorb UV, visible and IR radiation from the solar spectrum and combine impact ionisation and impurity photovoltaic (PV) effect as well as making use of IR photons from the surroundings when fully optimised.

Conducting films are becoming increasingly important for the printed electronics industry with applications in various technologies including antennas, RFID tags, photovoltaics, flexible electronics, and displays. To date, expensive noble metals have been utilized in these conductive films, which ultimately increases the cost. In the present work, more economically viable copper based conducting films have been developed for both glass and flexible PET substrates, using copper and copper oxide nanoparticles. The copper nanoparticles (with copper(I) oxide impurity) are synthesized by using a simple copper reduction method in the presence of Tergitol as a capping agent. Various factors such as solvent, pH, and reductant concentration have been explored in detail and optimized in order to produce a nanoparticle ink at room temperature. Second, the ink obtained at room temperature was used to fabricate conducting films by intense pulse light sintering of the deposited films. These conducting films had sheet resistances as low as 0.12 Ω/□ over areas up to 10 cm(2) with a thickness of 8 μm.

A cheap and more environmentally friendly method for the synthesis of high quality PbS nanoparticles in olive oil at 60 C has been developed. Carefully controlling the conditions of reactions leads to PbS nanoparticles with well-defined sizes, and band gaps between 1.72 and 0.88 eV. The nanoparticles were characterized by XRD, HRTEM, NIR absorption spectroscopy, X-ray photoelectron spectroscopy (XPS) and a fluorescence lifetime experiment based on the time-correlated single photon counting (TCSPC) technique. Photoelectrochemical study carried out by steady-state current-voltage measurements of self-assembled PbS nanoparticles on ZnO-SnO 2 electrodes in 1 M Na 2 SO 3 electrolyte solution showed that as-prepared PbS nanoparticles were photochemically active.

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Current solar cells under research and development utilise mainly one absorber layer limiting the photon harvesting capabilities. In order to develop next generation solar cells, research should move towards effective photon harvesting methods utilising low-cost solar energy materials. This will lead to reduce the $W −1 figure for direct solar energy conversion to electrical energy. In this work, a graded bandgap solar cell has been designed to absorb all photons from the UV, visible and IR regions. In addition, impurity PV effect and impact ionisation have been incorporated to enhance charge carrier creation within the same device. This new design has been experimentally tested using the most researched MOCVD grown GaAs/AlGaAs system, in order to confirm its validity. Devices with high Voc ~ 1175 mV and the highest possible FF ~ (0.85-0.87) have been produced, increasing the conversion efficiency to ~20% within only two growth runs. These devices were also experimentally tested for the existence of impurity PV effect and impact ionisation. The devices are PV active in complete darkness producing over 800 mV, Voc indicating the harvesting of IR radiation from the surroundings through impurity PV effect. The quantum efficiency measurements show over 140% signal confirming the contribution to PV action from impact ionisation. Since the concept is successfully proven, the low-cost and scalable electrodeposited semiconducting layers are used to produce graded bandgap solar cell structures. The utilisation of nano-and micro-rod type materials in graded bandgap devices are also presented and discussed in this paper. Preliminary work on glass/FTO/n-ZnS/n-CdS/n-CdTe/Au , showing the high potential of these devices in achieving higher efficiencies. The detailed results on these low-cost and novel graded bandgap devices are presented in a separate publication.