Micromorph thin-film silicon solar cells with transparent high-mobility hydrogenated indium oxide front electrodes

We investigate the performance of hydrogenated indium oxide as a transparent front electrode for micromorph thin-film silicon solar cells on glass. Light trapping is achieved by replicating the morphology of state-of-the-art zinc oxide electrodes, known for their outstanding light trapping properties, via ultraviolet nanoimprint lithography. As a result of the high electron mobility and excellent near-infrared transparency of hydrogenated indium oxide, the short-circuit current density of the cells is improved with respect to indium tin oxide and zinc oxide electrodes. We assess the potential for further current gains by identifying remaining sources of parasitic absorption and evaluate the light trapping capacity of each electrode. We further present a method, based on nonabsorbing insulating silicon nitride electrodes, allowing one to directly relate the optical reflectance to the external quantum efficiency. Our method provides a useful experimental tool to evaluate the light trapping potential of novel photonic nanostructures by a simple optical reflectance measurement, avoiding complications with electrical cell performance.

New Generation Transparent LPCVD ZnO Electrodes for Enhanced Photocurrent in Micromorph Solar Cells and Modules

Christophe Ballif, Mustapha Benkhaira, Mathieu Gérard Boccard, Grégory Bugnon, Matthieu Despeisse, Laura Ding, Sylvain Nicolay, Fanny Sculati-Meillaud

ZnO bilayer films were deposited by low-pressure chemical vapor deposition in a single process step by controlling the differential doping of the nucleation and bulk parts of the layers. The resulting 2-μm-thick films are characterized by low free-carrier absorption and electron mobility over 40 cm2 /Vs. They, therefore, combine high transparency in the infrared region and moderate sheet resistance that can be lowered below 20 Ω/sq. These properties make ZnO bilayers ideal candidates as electrodes for the development of micromorph thin-film solar cells with enhanced photogenerated current. The potential of such bilayer front electrodes for a further power improvement and cost reduction of industrial micromorph tandem modules is currently investigated at Oerlikon Solar. The first experiments already show a promising gain in the bottom μc-Si:H cell photogenerated current compared with the current generated with modules deposited on standard uniformly doped ZnO single-layer front contacts.

2012

Low-Pressure Chemical Vapor Deposited Zinc Oxide Films

Laura Ding

Polycrystalline zinc oxide (ZnO) films, prepared by low-pressure chemical vapor deposition are investigated in this thesis. ZnO belongs to the class of transparent conductive oxide materials, as it is transparent to light from the visible to the near-infrared range and is simultaneously electrically conductive. These two properties allow ZnO films to be integrated in thin-film silicon solar cells, as well as other opto-electronic devices. In particular, thin-film solar cells require a contact layer covering the whole active layer in order to efficiently collect the photogenerated electrical current, and therefore, the film electrode at the front of the cell has to be a window to the sunlight, in addition to being able to transport current. ZnO’s electrical conductivity is usually improved by doping with impurities like boron, but the film transparency is reduced, and consequently, the current that the solar cell can produce decreases. Thus, the first point investigated in this thesis is to find ways to relax the trade-off between transparency and conductivity. Instead of doping, which increases the electron concentration, the electron mobility has to be improved. To do so, the electron transport is first studied and modeled for standard films, in order to better define and understand its limitations. Then, a new way of preparing ZnO bilayer films is shown to lead to enhanced transparency and light scattering, for a maintained conductivity compared to uniformly doped films. Alternatively, various treatments (hydrogen plasma, vacuum annealing and ultraviolet exposure) are tested, all of which reduce the density of defects that cause electron scattering in the ZnO; treated films present an improved conductivity and very high mobilities close to 60 cm2V−1s−1, which corresponds to the mobility value in single crystals of equivalent doping. In addition, these films absorb less than 2% of the visible-to-infrared light, which makes them very promising ultra-transparent electrodes, for solar cells or other applications. For use in thin-film silicon solar cells, two additional criteria have to be met besides trans- parency and conductivity. First, because the front electrode is also the substrate for the deposition of the rest of the cell layers, its surface morphology has to be favorable to the growth of silicon. Moreover, this surface has to present a rough texture capable of diffusing and trapping the light into the cell, and increasing its chance of being absorbed. These charac- teristics can be incompatible, but are nevertheless necessary to reach thin-film solar cells with high efficiencies. So, the second point of interest of this thesis is to adapt the surface of ZnO layers by investigating their preparation method. Combinations of different growth regimes in multilayers are proposed to enable a smoother controllable of the surface without resort to additional etching treatments, leading to improved solar cell electrical performance. Finally, the stability of ZnO films exposed to various conditions that modify their electrical and optical properties is investigated. First, ZnO is exposed to different oxidizing conditions (storage and annealing in air, exposure to moisture, plasma of carbon dioxide). The loss in conductivity from the interaction of the oxygen species with ZnO is characterized by a new infrared spectroscopic method and a hydrogen plasma treatment is investigated as a means to reverse or partially prevent the degradation in conductivity. Then, the effect of simulated sunlight exposure on ZnO film properties in solar cells is investigated. The resistivity of the front ZnO electrode decreases due to ultraviolet light absorption, and the resistivity of the back electrode increases due to the chemisorption of oxygen species, in absence of ultraviolet light. The front electrode transparency is also degraded; this effect is more pronounced for doped ZnO and its impact on the current photogeneration of solar cells is demonstrated. Therefore, a specific optimization of the front and back electrodes appears necessary. This thesis contributes on the one side to an in-depth understanding of the opto-electrical properties of ZnO films prepared by low-pressure chemical vapor deposition. On the other side, the different proposed approaches enable more flexibility in the development of more efficient and stable electrodes, not only for solar cells, but for many other applications such as electroluminescent diodes, flat-panel displays, touch-screens or transparent transistors.

EPFL2013

Promotion électrochimique des catalyseurs à base de rhodium et d'iridium

Justyna Eaves

In this thesis, the electrochemical promotion of lr and Rh based catalysts was studied for the complete combustion of ethylene and propylene. The study was performed by using two types of electrocatalytical cells: a classical, pellet type cell and a tubular cell. This work has two major goals. The first one is the optimization of the employed electrocatalytical systems from electrochemical point of view. The second one concerns the study of the mechanism of the electrochemical promotion. The major part of this work concerns the electrochemical characterization of two cell types: a pellet type cell and a tubular cell. On the basis of the obtained results the modélisation of the potential and current distribution was performed, providing key parameters for the development of more efficient cell configurations. On one hand, these configurations ensure an uniform current distribution on the working electrode and enable correct catalyst potential measurement. On the other hand, it minimizes the current bypass in the bipolar tubular cell. The aim of the investigation in a two-compartment reactor was to evaluate and to compare the Au and Ir electrodes used in this work. Under 400°C, gold was found to be much worse material than lrÛ2 both as reference and counter electrode. Its potential was affected by oxygen and propylene adsorption and no equilibrium of electrochemical reaction was obtained at such low temperature. On the base of this knowledge, several materials better adapted to electrochemical promotion conditions have been proposed. Using the two-compartment reactor potential measurements under steady state conditions allowed to verify the mechanism of the propylene combustion on Rh and that of ethylene combustion on lr based catalysts proposed in the literature. Ameliorations of the existent kinetic models were proposed. The kinetic measurements were then analyzed with the additional help of ex situ XPS analysis. This study allowed to link the oxidation state of the catalyst surface with the extent of electrochemical promotion of the catalysts. It also allowed the ex situ observation of the appearance of new oxygen (oxide) species on the gas exposed surface of Ir catalyst. This new type of oxygen was different from the oxygen of Ir02 oxide, and it can represent the promoting species, according to backspiïïover theory. The same oxygen species was observed on well oxidized Ir02 catalyst surface, even before the polarization of this catalyst. Its presence was associated to the catalyst-support effect. It explains the high activity of Ir02 catalyst by comparison with the catalytic activity of lr metal. XPS analysis of Rh based catalysts indicated the possibly formation of an inferior Rh oxide (close to RhO) due to polarization. This may be a possible explanation of the observed permanent electrochemical promotion. Voltammetric charge measurements in very narrow potential window were performed while changing the catalyst thickness. The charge was demonstrated to be proportional to the catalyst thickness, suggesting the presence of the charged species on the gas exposed catalyst surface. The analogy between this situation and that of the emerged electrodes in liquid state electrochemistry was emphasized. The voltammograms obtained in this study were interpreted in the quite new, original way, which allowed to calculate the capacity of the different catalyst interfaces, the gas/catalyst interface, and the triple phase boundary, in good agreement with reference values from impedance spectroscopy measurements. Finally, the reaction rate transients during galvanostatic polarization of lr based catalyst have been moralized. The model fits well the experimental results.