Cu(In,Ga)Se2 (CIGS) is one of the most promising semiconductor compounds for large-scale production of efficient, low-cost thin film solar cells, and several research institutes have announced their plans for CIGS production lines. But for the CIGS technology to become a commercial success, a number of issues concerning manufacturability, product definition, and long-term stability require further attention. Several studies indicate that CIGS-based modules are stable over many years in field operation. At the same time, it is shown in the present work that they may have difficulties in passing standard accelerated lifetime test procedures like the IEC 1646 damp heat test. In particular, CIGS modules are sensitive to humidity penetrating through the module encapsulation, which will increase the resistive losses in the front contact and cause severe corrosion of the back contact. It is also shown that cells experience degradation in both voltage and fill factor, and the causes of these effects are addressed. By concentrating the light falling onto a solar cell, the device will deliver a higher power output per illuminated absorber area, which can lower the electricity production costs. For CIGS-based solar cells, low-concentrated illumination could be an economically viable approach. In this work it is shown that the yearly performance of a photovoltaic system with CIGS modules can be significantly improved at a moderate cost by using parabolic aluminum mirrors as concentrating elements. However, in order to avoid detrimental power losses due to high temperatures and current densities, the modules need to be designed for the higher light intensity and to be sufficiently cooled during operation. A design where the front contact of the module is assisted by a metal grid has shown promising results, not only for concentrated illumination but also for normal operation. The benefits are enhanced window processing tolerance and throughput, as well as improved degrees of freedom of the module geometry.

This research describes an investigation on the fabrication of CuInSe2-based thin-film solar cells with the device structure of Al/ZnO:Al/ZnO/CdS/CIS/Mo/SLG at the substrate temperature of 450oC, which is at least 100oC below the temperature currently used for depositing CIS thin films. A great advantage for the low temperature process is that the polymer material can be used as substrate and it is feasible to make lightweight and flexible thin-film solar cells. In this work, we used a co-evaporation technique with an introduction of Sb during the film deposition process to modify the film growth mechanisms and produce the CIS film with compact grain structure and smooth surface morphology. In most cases, there was only tiny amount of Sb existed in the film as a p-type dopant. In some cases, second phases of Sb compounds could be detected in the film as the Sb flux was kept too high during the film deposition stage. The I-V characteristics measured under the AM1.5 condition for the solar cell using a CIS:Sb film as the absorber showed that the open circuit voltage (Voc) was 0.364 V, short circuit current (Jsc) was 48.16 mA/cm2, fill factor (FF) was 44.5%, and energy conversion efficiency (£b) was 8%. The device with the same layer structure except the use of CIS film prepared without the addition of Sb and at a higher substrate temperature of 550oC had a comparable device performance but a slightly lower efficiency, i.e. Voc=0.325 V, Jsc=48.54 mA/cm2, FF=45.1%, £b=7.4%. It is clear that a lower temperature process using Sb to modify the growth process can be successful to obtain a device quality CIS layer. In addition, a CIGS thin-film solar cell was also fabricated and its device properties were Voc=0.392 V, Jsc=37.28 mA/cm2, FF=46.2%, and £b=7.0%. We see that the addition of Ga to increase the bandgap do increase the Voc and decrease the Jsc. However, a low efficiency of this cell indicates that further improvement in fill factor of the cell is a necessary.

Developing a fundamental understanding of photocurrent generation processes at organic-inorganic interfaces is critical for improving hybrid solar cell efficiency and stability. This dissertation explores processes at these interfaces by combining data from photovoltaic device performance tests with characterization experiments conducted directly on the device. The dissertation initially focuses on exploring how morphologically and chemically modifying the organic-inorganic interface, between poly(3-hexylthiophene) (P3HT) as the electron donating light absorbing polymer and titanium dioxide (TiO₂) as the electron acceptor, can result in stable and efficient hybrid solar cells. Given the heterogeneity which exists within bulk heterojunction devices, stable interfacial prototypes with well-defined interfaces between bilayers of TiO₂ and P3HT were developed, which demonstrate tunable efficiencies ranging from 0.01 to 1.6 %. Stability of these devices was improved by using Cu-based hole collecting electrodes. Efficiency values were tailored by changing TiO₂ morphology and by introducing sulfide layers like antimony trisulfide (Sb₂S₃) at the P3HT-TiO₂ interface. The simple bilayer device design developed in this dissertation provides an opportunity to study the precise role played by nanostructured TiO₂ surfaces and interfacial modifiers using a host of characterization techniques directly on a working device. Examples introduced in this dissertation include X-ray photoelectron spectroscopy (XPS) depth profiling analysis of metal-P3HT and P3HT-TiO₂ interfaces and Raman analysis of bonding between interface modifiers like Sb₂S₃ and P3HT. The incompatibility of TiO₂ with P3HT was significantly reduced by using P3HT derivatives with -COOH moieties at the extremity of a polymer chain. The role of functional groups like -COOH in interfacial charge separation phenomena was studied by comparing the photovoltaic behavior of these devices with those based on pristine P3HT. Finally, for hybrid solar cells discussed in this dissertation to become commercially viable, high temperature processing steps of the inorganic TiO₂ layer must be avoided. Accordingly, this dissertation demonstrates the novel use of electromagnetic radiation in the form of microwaves to catalyze growth of anatase TiO₂ thin films at temperatures as low as 150 °C, which is significantly lower than that used in conventional techniques. This low temperature process can be adapted to a variety of substrates and can produce patterned films. Accordingly, the ability to fabricate TiO₂ thin films by the microwave process at low temperatures is anticipated to have a significant impact in processing devices based on plastics. / text

In this thesis, a novel method of reducing the rear surface recombination in copper indium gallium (di) selenide (CIGS) thin film solar cells, using atomic layer deposited (ALD) Al2O3, has been evaluated via qualitative opto-electrical characterization. The idea stems from the silicon (Si) industry, where rear surface passivation layers are used to boost the open-circuit voltage and, hence, the cell efficiency. To enable a qualitative assessment of the passivation effect, Al/Al2O3/CIGS metal-oxide-semiconductor (MOS) devices with 3-50 nm oxide thickness, some post-deposition treated (i.e. annealed), have been fabricated. Room temperature capacitance-voltage (CV) measurements on the MOS devices indicated a negative fixed charge density (Qf) within the Al2O3 layer, resulting in a reduced CIGS surface recombination due to field effect passivation. After annealing the Al2O3 passivation layers, the field effect passivation appeared to increase due to a more negative Qf. After annealing have also indications of a lower density of interface traps been seen, possibly due to a stronger or activated chemical passivation. Additionally, the feasibility of using ALD Al2O3 to passivate the surface of CIGS absorber layers has also been demonstrated by room temperature photoluminescence (PL) measurements, where the PL intensity was about 20 times stronger for a structure passivated with 25 nm Al2O3 compared to an unpassivated structure. The strong PL intensity for all passivated devices suggests that both the chemical and field effect passivation were active, also for the passivated as-deposited CIGS absorbers.

Thin-film solar cells are widely recognised to have the potential to compete with fossil fuels in the electricity market due to their low cost per peak Watt. The Thin-Film Group at the University of New South Wales (UNSW) is engaged in developing polycrystalline silicon (poly-Si) thin-film solar cells on glass using e-beam evaporation technology. We believe our solar cells have the potential of significantly lowering the manufacturing cost compared to conventional, PECVD-fabricated thin-film solar cells. After years of materials research, the focus of the Group??s work is now moving to the metallisation of evaporated solar cells. Minimising various kinds of losses is the main challenge of the cell metallisation procedure, within which the contact resistance is always a big issue. In this thesis, the contact resistance of aluminium contacts on poly-Si thin-film solar cells on glass is investigated. To the best of the author??s knowledge, this is the first ever contact resistance investigation of Al contacts on evaporated poly-Si material for photovoltaic applications. Various transmission line models (TLM) are employed to measure the contact resistance. An improved TLM model is developed to increase the measurement precision and, simultaneously, to simplify the TLM pattern fabrication process. In order to accommodate the particular requirements of poly-Si coated glass substrates, a TLM pattern fabrication process using photolithography is established. Furthermore, a Kelvin sense tester is set up to ensure an accurate measurement of the contact resistance. After establishment of the TLM technique at UNSW, it is successfully tested on singlecrystalline silicon wafer samples. The thermal annealing process of the contacts is also optimised. Then, the general behaviour of Al contacts on uniformly doped poly-Si films (i.e., no p-n junction) is investigated using the verified TLM technique. The long-term stability of the contacts is also studied. This is followed by an investigation of the contact resistance of the back surface field and emitter layers of different types of poly-Si thin-film solar cells. Finally, a novel contact resistance measurement model is proposed that is believed to be able to overcome the measurement bottleneck of the transmission line models.

One of the poly-Si thin-film cells developed at the University of New South Wales (UNSW) is the EVA cell. In this work, SiN films for EVA cells as an antireflection/barrier coating were investigated. In addition, the effect of hydrogenation pre-treatment of solid phase crystallisation (SPC) on grain size and open-circuit voltage (Voc) was investigated. The SiN films deposited by PECVD were examined for uniformity of the thickness and the refractive index of the films across the position of the samples in the PECVD deposition system. A spectrophotometric analysis was used to determine these film properties. It was found that these properties were very uniform over the deposition area. Good repeatability of the depositions was also observed. A series of SiN film depositions by reactive sputtering were also performed to optimize the deposition process. Parameters adjusted during the deposition were nitrogen flow rate, substrate bias, and substrate temperature. By investigating the deposition rate, refractive index, and surface roughness of the films, the three deposition parameters were optimised. The effects of post SiN deposition treatments (a-Si deposition, SPC, RTA, and hydrogenation) on thickness and refractive index of both SiN films deposited by PECVD and reactive sputtering were investigated by using samples which have the same structure as the EVA cells. The thickness of the PECVD SiN films decreased about 6 % after all the treatments. On the other hand, the thickness reductions of the reactively sputtered SiN films were very small. The refractive index of the PECVD SiN films increased about 0.6 % after the treatments, whereas that of the reactively sputtered SiN films decreased 1.3 % after the treatments. As a possible method to improve the performance of EVA cells, hydrogenation of a-Si was investigated as a pre-treatment of SPC process. There were no obvious differences in the grainsize and the Voc of the EVA cells with and without the hydrogenation. Therefore it is likely that the hydrogenation pre-treatment of SPC does not have a beneficial effect on the performance of EVA cells.

The cost of photovoltaic electricity needs to be significantly reduced in order to achieve a high electricity market penetration. Thin-film solar cells have good potential to achieve such cost savings though (i) large-area deposition onto low-cost foreign substrates, (ii) more streamlined processing, (iii) monolithic cell interconnection, and very efficient use of the expensive semiconductor material. Polycrystalline silicon (poly-Si) on glass is a promising technology for the cost-effective large volume production of PV modules since it (i) makes use of an abundant raw material, (ii) is non-toxic, (iii) does not suffer from light-induced degradation, and (iv) does not rely on TCO layers. Usually plasma enhanced chemical vapour deposition (PECVD) is used for the layer formation. This thesis explores the use of e-beam evaporation as deposition method since it is potentially much faster and cheaper than PECVD. The resulting solar cells are referred to as EVA (from EVAporation). Two inherent shunting mechanisms in EVA cells are demonstrated to be shunting through sub-micron sized pinholes when the back electrode is deposited and shunting between the emitter and the absorber layer at the glass-side electrode. Through the improved understanding of these shunting mechanisms it was possible to develop a suitable metallisation scheme for EVA cells using an aligned deposition of emitter and back surface field line contacts and a specially developed shunt mitigation etching technique. For the first time appreciable efficiencies of up to 5.2% were demonstrated on this material. It was also shown that only very lightly doped absorber layers can lead to the required high short-circuit currents in EVA cells. The resulting cells are currently completely limited by space charge region recombination occurring with comparatively low ideality factors of only ~ 1.4 This thesis also demonstrates the usefulness of Jsc-Suns measurements and investigates optical loss mechanisms in the current devices. Advanced modelling of distributed series resistance effects, influencing Suns-Voc, m-Voc and Jsc-Suns curves, is employed. PC1D modelling is used to extract relevant device parameters. In this work it was found that the diffusion length in the best EVA cells is longer than the absorber layer and that insufficient light trapping is currently the major hurdle to higher cell efficiencies. From the obtained results it can be concluded that EVA solar cells are promising candidates for the low-cost and high-volume production of solar modules.

Cu(In,Ga)(Se,S)2 (CIGS) solar cells have attracted a lot of attention due to their high performance and the prospect for lower manufacturing costs over conventional crystalline silicon solar cells. All recent record efficiency CIGS absorbers have been deposited using vacuum processing which introduces high manufacturing costs. CIGS can also be compatible with low cost, atmospheric processing which can significantly reduce manufacturing costs. Recently, there has been some progress in developing atmospheric solution-based processes for CIGS. Among different solution approaches, deposition of molecular precursors can be advantageous in terms of simplicity and straightforward compositional control. Nonetheless, the developed methodologies involve highly toxic reagents or large impurity content in the device, limiting the potential for commercialisation. This thesis describes the development of a novel solution-based approach for the deposition of CIGS absorber layers. Metal chalcogenides are used as the starting precursors, which are free from detrimental impurities. These compounds contain strong covalent bonds and, consequently, they are insoluble in common solvents. Until recently, hydrazine, which is highly toxic and explosive, was the only solvent to effectively dissolve these types of precursors, limiting the feasibility of this approach for industrial applications. In this work, metal chalcogenides are dissolved in a safer solvent combination of 1,2-ethanedithiol and 1,2-ethylenediamine, completely eliminating hydrazine from the process. By using this solvent system, optically transparent solutions are formed which exhibit long-term stability. The precursor solutions are decomposed cleanly and they are converted to single phase CIGS upon selenisation. CuIn(S,Se)2 solar cells with power conversion efficiencies up to 8.0% were successfully fabricated by spray depositing the precursor solution, followed by a selenisation step. This progress has been made by continuously optimising the deposition, drying, and especially the selenisation configuration. Among other parameters, the working pressure during selenisation was found to have a dramatic effect on the material crystalline quality. Rapid thermal processing was also explored as an alternative selenisation configuration to tube furnace annealing and it was shown to improve the back contact/absorber interface. It has been demonstrated that Ga can easily be incorporated in the absorber for band-gap tuning and, consequently, for VOC enhancement of the solar cells. The structural properties of the films were investigated with Ga content, as well as the opto-electronic characteristics of the corresponding solar cells. The band-gap of the material was conveniently varied by simply adjusting the precursor ratio, allowing for fine compositional control. By using this technique, Cu(In,Ga)(Se,S)2 solar cells with conversion efficiencies of up to 9.8% were obtained. The solar cell performance in this work is limited by the porosity of the absorber and the back contact quality. Despite a significant improvement during the course of this work, the remaining porosity of the absorber causes selenium to diffuse towards the back forming a thick MoSe2 layer and causing a high series resistance in the device. A low cost, solution-based technique was also developed for the deposition of aluminium-doped zinc oxide films that can be used as the transparent conductive oxide layer in thin film solar cells. This methodology involves the use of an ultrasonic spray pyrolysis system, which is a very versatile and easily controlled deposition technique. Although the presence of oxygen makes the film closer to stoichiometric (fewer oxygen vacancies) good electronic and optical properties have been obtained by process optimisation. Films deposited with optimum conditions exhibited a sheet resistance of 23 Ω/sq, which can be further reduced by increasing the thickness with minimal transmittance losses. The simplicity, low toxicity and straightforward control make the proposed methodologies extremely potential for low cost and scalable deposition of thin film solar cells.

Cu(In,Ga)Se2 (CIGS) is a semiconductor material and the basis of the promising thin-film photovoltaic technology with the same name. The CIGS film has a typical thickness of 1-2 mm, and solar cells based on CIGS technology has recently reached efficiencies of 23.3%. Ultra-thin CIGS solar cells use sub-micrometer thick films that require significantly less material and can be manufactured in a shorter amount oftime than films with typical thicknesses. With decreasing thickness, both electrical and optical losses get more significant and lower the overall performance. Electrical losses can be decreased by increasing the overall film quality and by utilising a graded bandgap throughout the CIGS layer. The band gap can be changed by varying the[Ga]/([Ga]+[In]) (GGI) ratio. Higher overall film quality and a higher band-gap towards the back of the absorber are expected to increase the performance. In this work, sputtered CIGS solar cells were made with different CIGS layer thicknesses, that ranged between 550-950 nm. Increased heat during deposition was examined and shown to increase the film quality and performance for all thicknesses. Two different ways of doping CIGS with Na was examined and it was found that higher Na content lead to an increasing predominance of the (112) plane. The bandgap was graded by varying the GGI composition throughout the CIGS layer and depth profiles were made with Glow-Discharge Optical Emission Spectroscopy (GDOES). It was found that a sputtered CuGaSe2 (CGS)layer below the CIGS-layer lead to a steep increase of the GGI near the back contact. When CGS made up 10% of the total CIGS layer thickness, a significant increase in performance was observed for all thicknesses. CIGS-absorbers with a less graded region with low GGI, making up 30% or 60% of the total CIGS layer thickness were made. A decrease in GGI in that region, was shown to increase the current but lower the voltage. No substantial increase in total performance compared to a fully graded CIGS layer was seen regardless of layer thickness. For further work the optical losses needs to be addressed and work on increasing the optical path in the CIGS layer needs to be done.

Thin-film solar cells are potentially low-cost devices to convert sunlight into electricity. Improvements in the conversion efficiencies of these cells reduce material utilization cost and make it commercially viable. Solar cells from the Thin-Film Physics Group, ETH Zurich, Switzerland and the Florida Solar Energy Center (FSEC), UCF were characterized for defects and other microstructural features within the thin-film structure and at the interfaces using transmission electron microscopy (TEM). The present thesis aims to provide a feedback to these groups on their deposition processes to understand the correlations between processing, resulting microstructures, and the conversion efficiencies of these devices. Also, an optical equipment measuring photocurrents from a solar cell was developed for the identification of defect-prone regions of a thin-film solar cell. The focused ion beam (FIB) technique was used to prepare TEM samples. Bright-field TEM along with scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS) including elemental distribution line scans and maps were extensively used for characterizing the absorber layer and interfaces both above and below the absorber layer. Energy-filtered transmission electron microscopy (EFTEM) was applied in cases where EDS results were inconclusive due to the overlap of X-ray energies of certain elements, especially molybdenum and sulfur. Samples from ETH Zurich were characterized for changes in the CIGS (Cu(In,Ga)Se2) microstructure due to sodium incorporation from soda-lime glass or from a post-deposition treatment with NaF as a function of CIGS deposition temperature. The CIGS-CdS interface becomes smoother and the small columnar CIGS grains close to the Mo back contact disappear with increasing CIGS deposition temperature. At 773 K the two sodium incorporation routes result in large differences in the microstructures with a significantly larger grain size for the samples after post-deposition Na incorporation. Porosity was observed in the absorber layer close to the back contact in the samples from FSEC. The reason for porosity could be materials evaporation in the gallium beam of the FIB or a processing effect. The porosity certainly indicates heterogeneities of the composition of the absorber layer near the back contact. A Mo-Se rich layer (possibly MoSe2) was formed at the interface between CIGS/CIGSS and Mo improving the quality of the junction. Other chemical heterogeneities include un-sulfurized Cu-Ga deposits, residual Se from the selenization/ sulfurization chamber in CIGS2 and the formation of Cu-rich regions which are attributed to decomposition effects in the Ga beam of the FIB. Wavy absorber surfaces were observed for some of the cells with occasional discontinuities in the metal grids. The 50 nm thick CdS layer, however, remained continuous in all the samples under investigation. For a sample with a transparent back contact, a 10 nm Mo layer was deposited on ITO (indium tin oxide) before deposition of the CIGS2 (Cu(In,Ga)S2) layer. EFTEM maps indicate that a MoS2 layer does not form for such a Mo/MoS2-ITO back contact. Instead, absorber layer material diffuses through the thin Mo layer onto the ITO forming two layers of CIGS2 on either side of Mo with different compositions. Furthermore, an optical beam induced current (OBIC) system with micron level resolution was successfully developed and preliminary photocurrent maps were acquired to microscopically identify regions within a thin-film solar cell with undesirable microstructural features. Such a system, when fully operational, will provide the means for the identification of special regions from where samples for TEM analysis can be obtained using the FIB technique to study specifically the defects responsible for local variations in solar cell properties.