L'oxyde de zinc par dépôt chimique en phase vapeur comme contact électrique transparent et diffuseur de lumière pour les cellules solaires

Zinc oxide (ZnO) is a material that belongs to the family of Transparent Conductives Oxides (TCO). Its non-toxicity and the abundant availability in the Earth's crust of its components make it an ideal candidate as electrical transparent contact for thin-film amorphous and/or microcrystalline silicon solar cells. The Low-Pressure Chemical Vapour Deposition (LP-CVD) method allows one to obtain rough ZnO layers, which can effectively scatter the light that passes through them. This high scattering capacity increases the path of the light within the solar cell and therefore enhances its probability to be absorbed by the solar cell, and consequently the photogenerated current. This thesis work studies in detail the LP-CVD technology used for deposition of ZnO layers, which are employed as TCO layers for amorphous, microcrystalline and "micromorph" (microcrystalline/amorphous tandem) solar cells developped at the Institute of Microtechnology of the University of Neuchâtel. The chapter 3 is a detailed study of the growth of ZnO layers on a glass substrate. This study reveals a columnar microstructure of the ZnO layers, which are composed of large conical monocrystals. The tops of these monocrystals emerge at the surface of the ZnO layers as pyramids, which give to such layers their rough aspect and therefore their capacity of scattering the light. The two following chapters represent a study of the effect of variations in deposition parameters on the optical properties (transparency and scattering power), electrical properties (conductivity), and structural properties of ZnO layers. The deposition parameters varied thereby are the various gas flows, the substrate temperature, and the process pressure. In particular, a strong influence of the substrate temperature on the microstructure of the ZnO layers is observed. A variation of ten degrees of this deposition temperature leads to abrupt variations of the optical and electrical properties of the layers. From these various studies, a strong correlation is established, between the scattering power, the electrical properties, and the microstructure of the ZnO layers deposited by LP-CVD: The higher the pyramidal grains at the surface of the layers, the higher the scattering capacity of these layers. The larger the monocrystals within the bulk of the layers, the higher the mobility of the free electrons. This enhanced mobility leads to an increase of the conductivity of the layers. Finally, the chapter 6 of this thesis work describes the various situations where LP-CVD ZnO layers are used as transparent electrical contacts and light scattering layers in thin-film silicon solar cells. It is schown that, by incorporating ZnO layers developped in this work in the photovoltaic devices, an increase of 8% in the photogenerated current is obtained, compared to the performances of solar cells using standard commercially available TCO.

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

N/I buffer layer for substrate microcrystalline thin film silicon solar cell

Christophe Ballif, Franz-Josef Haug, Xavier Niquille, Martin Python, Thomas Söderström, Vanessa Terrazzoni-Daudrix

The influence of the substrate surface morphology on the performance of microcrystalline silicon solar cells in the substrate or n-i-p (nip) configuration is studied in this paper. The experiments are carried out on glass substrates coated with naturally textured films of ZnO deposited by low pressure chemical vapor deposition which serves as backcontact and as template for the light trapping texture. The film surface morphology can be modified with a plasma treatment which smoothens the V-shaped valleys to a more U- shaped form. We investigate, first, the influence of different substrates morphologies on the performance of microcrystalline (μc-Si:H) thin film silicon solar cells deposited by very high frequency plasma enhanced chemical vapor deposition. The V-shaped morphologies are found to have strong light trapping capabilities but to be detrimental for the μc-Si:H material growth and lead to degraded open circuit voltage (Voc) and fill factor (FF) of the solar cells. Hence, in Sec., we introduce a buffer layer with a higher amorphous fraction between the n doped and intrinsic layer. Our study reveals that the buffer layer limits the formation of voids and porous areas in the μc- Si:H material on substrates with strong light trapping capabilities. Indeed, this layer mitigates Voc and FF losses which enhances the performance of the μc-Si:H solar cell. Finally, by applying our findings, we report an efficiency of 9% for a nip μc-Si:H thin film silicon cell with a thickness of only 1.2 μm. © 2008 American Institute of Physics.

2008

Single and multi-junction thin film silicon solar cells for flexible photovoltaics

Thomas Söderström

This thesis investigates amorphous (a-Si:H) and microcrystalline (μc-Si:H) solar cells deposited by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) in the n-i-p or substrate configuration. It focuses on processes that allow the use of non transparent and flexible substrates such as plastic foil with Tg < 180°C like poly-ethylene-naphtalate (PEN). In the first part of the work, we concentrate on the light trapping properties of a variety of device configurations. One original test structure consists of n-i-p solar cells deposited directly on glass covered with low pressure chemical vapour deposition (LP-CVD) ZnO. For this device, silver is deposited below the LP-CVD ZnO or white paint is applied at the back of the substrate as back reflector. This avoids the parasitic plasmonic absorptions in the back reflectors, which are observed for conventional rough metallic back contacts. Furthermore, the size and morphology of the LP-CVD ZnO is varied and the relation between the substrate morphology and the short circuit current density (Jsc) is experimentally explored. As a result, the Jsc can be increased by 23% for a-Si:H and 28% for μc-Si:H solar cells compared to the case of flat substrate and the role of the size and shape can be clearly separated. We also explore the optical behavior of single and multi-junction devices prepared with different back and front contacts. The back contact consists either of a 2D periodic grid with moderate slope, or of LP-CVD ZnO with random pyramids of various sizes. The front contacts are either a 70 nm thick, nominally flat ITO or a rough 2 microns thick LP-CVD ZnO. We observe that, for a-Si:H, the cell performance is critically dependent on the combination of thin flat or thick rough front TCOs and the back contact. Indeed, for a-Si:H, a thick LP-CVD ZnO front contact provides more light trapping on the 2D periodic substrate. The Jsc relatively increases by 7 % with LP-CVD ZnO compared to ITO. Then, we study the influence of the thick and thin TCOs in conjunction with thick absorbers like triple junction or mc-Si:H solar cells. Because of the different nature of the optical systems, thick (> 1 micron) against thin (