Explaining Morphological and Electrical Features of Boron-doped Zinc Oxide to Tailor New Electrodes for Photovoltaics

TCOs are a class of metal oxides that combine transparency to visible light with electrical conductivity. Each TCO is characterized by the trade-off of these two properties which can be tuned for a particular application. This thesis is dedicated to the investigation of one TCO material: boron doped zinc oxide (ZnO:B) deposited by low-pressure chemical vapor deposition (LP-MOCVD). Its main distinction is low absorptance which makes ZnO films deposited by LP-MOCVD ideal as transparent electrodes in thin-film solar cells. Although ZnO:B properties have previously been optimized for this application, the exact processes behind the film formation have not been fully described. This thesis substantially clarifies the processes of the film nucleation and growth evolution, and the mechanisms of incorporation of the B atoms into the ZnO. First, for non-intentionally doped ZnO we establish that the main deposition parameters influencing the film properties are the deposition temperature, the gas precursor ratio and the total gas flow. We experimented with these three parameters across a wide range of parameter values. The films deposited were characterized at different stages of their growth using atomic force microscopy, X-ray diffraction and automated crystallographic orientation mapping. These data reveal the dependence of the film preferential orientation on the deposition conditions. We propose a model based on the adsorbed atom mean free path to explain this dependence. Using deposition parameters learnt from this model, we control the preferential orientation during film growth to increase the grain sizes. Second, we analyze the conductivity to B-doped ZnO films. Boron atoms act as electron donors in ZnO, increasing the electrical conductivity and decreasing the transparency. This work quantifies how the B concentration and spatial distribution affect the film conductivity. Quantification of the B atoms incorporated in the film was performed using nuclear reaction analysis. We found that a high level of O precursor gas favors B incorporation in the film. Combining nano secondary ion mass spectroscopy and Kelvin probe force microscopy we demonstrate that the dopant atoms incorporate in only one of the two sides of each grain. This is a new observation because dopant atoms are commonly assumed to be uniformly distributed in the film. The transparency and electrical conductivity of ZnO depend also on the carrier mobility. The sources of carrier scattering are investigated for intrinsic (i.e. O vacancies and Zn interstitials) and extrinsic (B atoms) impurity concentrations. We observed that for the same B concentration the source of electron scattering depends on the concentration of intrinsic defects: for films with a lower concentration of intrinsic defects, the main source of scattering is the grain boundaries; for films with a higher concentration of intrinsic defects, the main sources are the mechanisms happening in the crystalline region. Finally, LP-MOCVD ZnO:B films were optimized using the previous results and successfully applied in four different types of solar cells: amorphous/microcrystalline silicon, amorphous silicon, copper indium gallium selenide and silicon heterojunction. The advantages of the developed films to these four cells include the simplification of the fabrication process, the reduction of reflectance and parasitic absorption, and a better understanding of carrier transport mechanisms through the device.

Study of radio-frequency plasma deposition of amorphous silicon for the improvement of solar cell production

Juliette Ballutaud

Plasma enhanced chemical vapour deposition (PECVD) of thin films such as amorphous silicon has widespread applications especially in the field of photovoltaic solar cells and thin-film transistors for flat screen production. Industrial applications require high depositions rates over large areas with a good uniformity in layer thickness. In this thesis, some aspects of PECVD in large surface, industrial type, capacitive radio frequency reactor are investigated. The aim of this work is to study the plasma process conditions to increase the deposition rate of uniform, good quality, a-Si:H layer for solar cell application in a single chamber reactor. The studies realized during this thesis have necessitated the development and the comprehension of diagnostics such as deposition rate measurement by in-situ interferometry, plasma power measurement and layer density measurement by ellipsometry. During the thesis, we have also elaborated a matching-box circuit for using process frequencies of 27.12 MHz and 40.68 MHz. Deposition of a-Si:H in small electrode gap reactor has been studied. At present industrial reactors have a standard electrode gap of 2.4 cm. We modified a reactor to reach a small gap of 1.7 cm. It appears that we have obtained faster deposition rate in the small gap reactor but non-uniformity problems increase due to edge focusing and powder effects. One solution, based on a teflon jigsaw to keep the plasma away from the edge confined spaces, is proposed to suppress focusing effect but the operation parameter space is still reduced by the powder effect. Systematic measurements of a-Si:H layer density was also done by ellipsometry. It is shown that the layer density decreases when the deposition rate increases, independently of pressure, gas flow and frequency (27.12 MHz/40.68 MHz) of the plasma. At high deposition rate, 6 Å/s, only an increase of the process temperature from 200°C to 230°C can significantly improve the layer density. We have noted also a slight improvement of layer density for layers deposited in the small gap reactor. Nevertheless industrial constraints impose a process temperature of 200°C and a standard gap reactor. By optimising the process parameters, keeping the temperature process at 200°C, good quality, uniform, a-Si:H layer were deposited at 3 Å/s on 37 cm × 47 cm glass substrates at 40.68 MHz. A particular source of non-uniformity in large area reactor has been examined. In large area reactors, a perturbation in RF plasma potential, due to the electrode edge asymmetry, propagates towards the plasma center with a characteristic damping length λ. The variation of RF plasma potential at the edge implies a variation of the deposition rate across the reactor area and then a non-uniformity of the deposited layer. A theoretical study was done to understand of this phenomenon and experimental results confirmed the model. Finally, for solar cell applications, a study of the boron cross-contamination during solar cell deposition in a single chamber process has been done. During the deposition of the intrinsic layer, for a p-i-n cell, i-layer is contaminated by the residual boron radicals present in the reactor after the deposition of the boron-doped layer. This contamination decreases cell performance. Several reactor treatments have been tested to solve this contamination problem. The effectiveness of these treatments was evaluated by secondary ion mass microscopy (SIMS) measurements. It appears that an ammonia flush or a water vapour flush of a few minutes, between the deposition of the p-layer and i-layer, reduces the boron contamination at the p-i interface. The performance of cells made with these treatments, in a single chamber process, are comparable to performance of cells done in a multi-chamber process.

EPFL2004

A multi-technique approach to study the microstructural properties of tin-based transparent conductive oxides

Federica Landucci

Transparent conductive oxides (TCOs) are semiconductor-like materials that exhibit high electrical conductivity and high optical transparency combined. They are adopted in various applications ranging from gas sensors, to electrochromic windows, to photovoltaic cells. Indium-based TCOs represent the industry standard. Nevertheless, indium is among the less abundant elements in the earth crust and forecasts based on its current consumption envisage an urgent need to replace it. Tin-based TCOs are a promising alternative, since their opto- electronic characteristics mimic the ones of indium-based materials. This thesis aims to investigate the link between optoelectronic and microstructural properties of tin dioxide and zinc tin oxide (ZTO) with a composition Zn0.05Sn0.30O0.65 and their stability when submitted to thermal treatments. Indeed, lots of practi- cal applications require the TCO to operate in high temperature conditions. To conduct this study, a combination of analytical techniques, such as transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), X- ray diffraction (XRD), electron paramagnetic resonance (EPR) and differential scanning calorimetry (DSC) was employed. Amorphous SnO2 and ZTO were deposited by RF sputtering and annealed up to 1050°C in different atmospheres. The influence of annealing temperature and atmosphere were decoupled and led us to an in-depth comprehension of the mechanisms governing the optoelectronic properties of both materials. When annealed in air, between room temperature and 300°C, ZTO exhibits increased mobility and carrier concentration with respect to the as-deposited state. This increase, investigated with DSC, was ascribed to a structural relaxation that allows point defects to release electrons in conduction band. Between 300°C and 500°C atmospheric oxygen passivates oxygen vacancies, drastically decreasing the carrier concentration and therefore causing a large drop of the conductivity. EPR experiments allowed to ascribe the drop in conductivity to the decrease of carrier concentration, which occurs slightly before the phase change. At 570°C (and 930°C for the case of vacuum annealing) the phase change occurs and the ZTO crystallizes in the rutile form of SnO2. The material becomes completely insulating. When the temperature is increased to 1050°C, evaporation of zinc is observed. In order to improve the electrical conductivity of ZTO at high temperature, a doping strategy was implemented starting from DFT calculations conducted by a partner group, who screened among the entire periodic table, which elements are the best candidates to act as n-dopants for ZTO. Bromine and iodine were retained, since they were found to be the most energetically favorable to become substitutional defects for a tin site. An exploratory doping route is therefore presented and the treated samples analyzed with TEM, EDX and UV-VIS-IR spectroscopy. Finally, the structural properties of an indium-based TCO (zirconium-doped indium oxide) were investigated and used as a benchmark to propose a crystallization model for the tin-based, as well as the indium-based materials. The influence of pa- rameters such as the material thickness, annealing atmosphere and temperature and deposition pressure are discussed for both materials.

EPFL2019

Selective light induced chemical vapour deposition of titanium dioxide thin films

Estelle Wagner

Light Induced Chemical Vapour Deposition (LICVD) of titanium dioxide thin films is studied in this work. It is shown that this technique enables to deposit locally and selectively a chosen crystalline phase with a precise controlled thickness at low substrate temperature, allowing even the use of polymer substrates. A home made LICVD reactor was set up, consisting of a main chamber in which the substrate was placed on a temperature controlled plate and could be irradiated perpendicularly through a window by a 250 ns long pulse XeCl excimer laser (308 nm) with a controlled fluence (energy per surface area per pulse). A liquid precursor (titanium tetra-isopropoxide) was brought by an oxygen carrier gas flow in the chamber and a nitrogen flow was added to prevent deposition on the window. The total pressure in the chamber was maintained at 10 mbar. Projection of a mask image on the substrate was realized. Titanium dioxide films could be deposited locally by this technique with a thickness ranging from few nanometers to several micrometers. Deposits were realized at substrate temperatures as low as room temperature and with fluences between 1 and 400 mJ/cm2. The deposited films were characterized by profilometry, Atomic Force Microscopy, optical microscopy, Scanning Electron Microscopy, Transmission Electron Microscopy, X-ray diffraction, Raman spectroscopy, X-ray Photoelectron Spectroscopy, UV-vis spectroscopy and ellipsometry. A detailed and systematic study of the influence of some selected process parameters (substrate temperature and irradiation dose principally) on the deposition rate and material properties was realized on glass and silicon substrates. An empirical equation of the growth rate as a function of the different experimental parameters was derived and was tested successfully to obtain a precise control of the deposited thickness (a precision better than 10 nm was demonstrated for films below 300 nm), with growth rates up to 300 nm/min. From this equation, a deposition mechanism involving both a photolytic reaction and a thermal contribution of the substrate temperature was proposed. The deposited material was shown to consist of TiO2 with a possible very low additional carbon contamination. Depending on the deposition conditions, amorphous, anatase or rutile material was deposited. One parameter explaining the variation of the crystalline state of the material was the variation of the laser induced temperature rise which was simulated theoretically in parallel. Surface temperature rises in the order of several hundreds of degrees were calculated during the laser pulse, varying with the substrate, the fluence and the film thickness. The optical properties of the deposited material vary in a correlated way with the crystalline state of the material, and high indexes of refraction (up to 2.6 at 633 nm) were measured. Selective deposition in the irradiated area was studied. Provided substrate temperature was kept below 150°C, no thermal deposition was observed, and deposits were strictly located in the irradiated area. The edge resolution (around 10 micrometers) was demonstrated to be due to the optical aberrations of the mask projection set up. No effect of substrate temperature and irradiation conditions on the resolution could be evidenced, but resolution decreased with increasing deposited thickness due to optical effects. Thanks to the low substrate temperature required for the deposition process, deposition on thermo- fragile substrates (such as polymers, in particular PMMA) was demonstrated. However, in the case of polymer substrates, the fluence had to be kept below 15 mJ/cm2 not to damage or ablate the polymers, resulting in very low growth rates (in the order of 10-3 nm/pulse). In these conditions, only amorphous films with about 10% of carbon contamination could be obtained. Additionally, films were shown to crack above a certain deposited thickness, which is attributed to thermal effects. As a matter of fact, calculations showed a high temperature rise up to 70°C of the polymer/oxide interface. Deposition on polymers coated with transparent conductive oxide was also demonstrated.

EPFL2003