Impact of the Oxygen Flow during the Magnetron Sputtering Deposition on the Indium Tin Oxide thin films for Silicon Heterojunction Solar Cell

We report on the optoelectronic properties of ITO layers deposited by DC sputtering, using different oxygen to total flow ratios [r(O-2) = O-2/Ar, ranging from 1% to 8%], for silicon heterojunction (SHJ) solar cell application. The depth profiling of the various elements throughout the thicknesses and interfaces of the ITOs and thin films forming the SHJ device was determined by time-of-flight secondary ion mass spectrometry. Finally, the photovoltaic performance of the fabricated SHJ cells was evaluated with respect to the r(O-2) into the ITO layers. Lower r(O-2) was found to yield the best PV performance which is attributed to lower parasitic resistive losses.

Impact of the Oxygen Content on the Optoelectronic Properties of the Indium-Tin-Oxide Based Transparent Electrodes for Silicon Heterojunction Solar Cells

Brahim Aissa, Christophe Ballif, Mathieu Gérard Boccard, Jean Cattin, Jan Haschke, Akshath Raghu Shetty

Amorphous/crystalline silicon heterojunction (SHJ) solar cells technology is attracting tremendous attention in recent years due to its potential to achieve high power conversion efficiencies at low fabrication temperatures and using few process steps. However, the commercial mass production of this technology is still somehow restricted so far, which is mainly due to the high sensitivity of the SHJ solar cell parameters to the growth conditions. A significant distinctness between the SHJ configuration and the standard silicon wafer solar cell is the current collection scheme. Indeed, as the SHJ silicon wafer solar cell is limited by the low lateral conductivity of the thin-film-silicon layers used to form the contact, a transparent conductive oxide (TCO) is systematically employed to improve the carrier transport properties, whilst also acting as an antireflective coating (ARC) for the front side. From the variety of TCOs, indium tin oxide (ITO) is the most frequently used. In this work, we investigate the properties of ITO thin films grown by DC magnetron sputtering using different oxygen to total flow ratios [r(O-2) = O-2/(Ar+O-2)] ranging from 0.01 (1%) to 0.08 (8 %). Hall effect measurements together with optical spectrometry were carried out and were found to be drastically affected by the oxygen content. Furthermore, time of flight-secondary ion mass spectrometry (TOF-SIMS) was used to determine the depth profiling of indium, oxygen, tin, silicon, phosphorous, and hydrogen throughout the ITO and silicon layers forming the solar cell. Finally, silicon heterojunction devices were fabricated and the associated photovoltaic performance were evaluated as a function of the r(O-2) into the ITO electrodes. Lower oxygen flow ratio was found to yield the best performance which is attributed to lower parasitic resistive losses.

AMER INST PHYSICS2019

Electronic properties of nano-crystalline titanium dioxide thin films

Alain Bally

Titanium dioxide is a cheap, chemically stable and non-toxic material. However its electrical properties are unstable and it is a modest semiconductor and a mediocre insulator. For several applications it would be interesting to make it either more insulating or more conducting. The goal of this work was to modify the electrical properties of nano-crystalline TiO2 thin films deposited by reactive sputtering and to understand the mechanism leading to these modifications. The principal factors that influence the electrical conductivity are on the one hand the concentration and nature of the chemical impurities incorporated in TiO2, and on the other hand the morphology of the thin films. The study was split into two parts. The fist part describes the modifications of the sputtered material obtained by chemical doping of the TiO2 thin films. Niobium, cerium, iron, and fluorine were incorporated successfully in TiO2. The second part describes the modifications obtained by modifying the reactive gas used during the sputtering process: thus oxygen was substituted with water vapor. Several analysis techniques have been used to characterize the TiO2 thin films. They are essentially divided in four categories. The chemical analyses included electron probe microanalyses, x-ray photoemission spectroscopy, and secondary ion mass spectrometry. The structure and morphology analyses of thin films were carried out with x-ray diffraction and atomic force microscopy. The electrical properties in dc or ac mode were measured between room temperature and 350°C. Finally optical transmission provided information on the electronic states and morphology of the thin films. With an appropriate choice of impurities, titanium dioxide can be made more insulating (TiO2:Ce), more conducting with an n-type electrical conductivity (TiO2:Nb) or p-type electrical conductivity (TiO2:Fe). Chemical doping is also the cause of important structure and morphology changes, for example it can force the transformation from the anatase to the rutile structure. It is shown in particular that dopant atoms generate defects such as oxygen vacancies. These defects impede the variation of the electrical conductivity produced by the impurities, thus, in spite of dopant concentration in the percent range, the variation of the electrical conductivity is an increase or a decrease of three order of magnitude at most when compared with undoped TiO2. Thin films deposited with water vapor as the reactive gas present an increase in the electrical conductivity by height order of magnitude compared to samples prepared with oxygen in similar conditions. No atom other than titanium or oxygen could be detected in the thin films in significant concentration. The dramatic conductivity increase is due to the injection inside TiO2 grains of electrons donated by unsaturated titanium atoms lying on the grain surface. A nanometric grain size is a requirement to make such high doping levels possible. Depending on the impurities or reactive gas selected, titanium dioxide can be made either into a good insulator with a high breakdown field and a high permittivity or into a reasonably conducting, transparent film. The electrical conductivity of the samples prepared for this study cover the range from 10-10 to 103 S m-1.

EPFL1999

Free carriers in nanocrystalline titanium dioxide thin films

Simon Springer

This thesis analyzes the properties of heavily doped nanocrystalline titanium dioxide. Thin films were deposited by magnetron sputtering with either water vapor as reactive gas or a periodically interrupted oxygen supply. The samples were at the same time electrically conducting and transparent. They consisted of mixtures of rutile, anatase and amorphous phases. Titanium dioxide is chemically stable, hard, non-toxic, transparent, and inexpensive. Due to a high refractive index it is often found in optical applications as a thin film coating. For many purposes it is interesting to take advantage of a combination of good transparency and electrical conductivity. An even larger field of applications would open if the electric conductivity of TiO2 could be increased in a controlled manner. Conventional bulk doping is limited, however, by low solubility limits. Highly conducting nanocrystalline TiO2 thin films have been obtained recently by sputtering in presence of water vapor. The present project intends to elucidate the doping mechanisms at work in such films. In addition, an alternative process to achieve grain-boundary doping of TiO2 was explored. Spectrophotometry over a wide spectral range (0.05 to 5 eV) was the main tool in probing the mobile electrical charge carriers. The interpretation of these optical measurements required appropriate models. The models developed took many structural properties into account, such as thickness, surface roughness, density, anisotropy and crystalline phase composition. The phase and morphology of the films were analyzed by X-ray diffraction and reflection, scanning probe microscopy, and electron microscopy. The chemical analyses included electron-probe microanalyses, Rutherford backscattering and elastic recoil detection analysis. The DC electrical properties were measured between room temperature and 250 °C. Most water-deposited samples showed semiconducting behavior with an activation energy of the order of 100 meV. The optical measurements revealed a broad absorption band around 1 eV. The absorption coefficient scaled with conductivity. It was successfully modelled as a Lorentzian. Similar absorption bands have been reported for reduced rutile and anatase single crystals. Water-deposited samples containing anatase exhibited a metallic behavior. In these samples the thermal activation energy was reduced to about 30 meV and the charge carriers displayed a high mobility (3 cm2V-1s-1). The infrared absorption band was about 10 times less important than in the samples devoid of anatase. Thin films of TiO2 intercalated with the equivalent of nanometer-thick layers of Ti were prepared. By changing the period in the oxygen supply the grain size, and thereby the film properties, could be modified. The films obtained in this way had many properties in common with water-deposited films. They showed the same absorption band in the infrared, had comparable charge carrier densities and were electrically conductive (102 to 104 Sm-1 at room temperature). To our knowledge, this was the first time that such high electrical conductivities were achieved by sputtering in an oxygen plasma.

EPFL2004