Low-Pressure Chemical Vapor Deposited Zinc Oxide Films

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.

Optical Layers for Thin-film Silicon Solar Cells

Peter Cuony

In this work we develop and analyze optical layers for use in Micromorph solar cells, a tandem configuration with an amorphous silicon top cell and a microcrystalline silicon bottom cell. The morphology of the front electrode has a decisive role in maximizing the efficiency of a solar cell. To reach a better understanding of the requirements for the front electrode surface, we present a wide range of morphologies that can be obtained with as-grown rough zinc oxide (ZnO) and post-deposition argon plasma surface treatments. We correlate the morphological parameters to light scattering in transmission and reflection, and identify the inclination angles of ZnO pyramids as the most pertinent parameter for thin-film silicon solar cells. We show that there are no reflection losses at the interface between as-grown rough ZnO and silicon, and we quantify the reflection losses for smoother interfaces. With titanium dioxide, produced by reactive magnetron sputtering, we demonstrate that for flat interfaces reflection losses of up to 7.4% can be canceled at 550 nm wavelength. We show that in microcrystalline silicon cells p-type silicon sub-oxide (SiOx) can also be used to reduce reflection losses, and at the same time can act as field-creating window layer. We develop a wide range of mixed-phase SiOx layers and with energy-filtered transmission electron microscopy we reveal the filamentous nanostructure of films produced with high hydrogen dilution of the precursor gas mixture. A partial decoupling of electrical and optical parameters is achieved for such films, where the transparency and the refractive index are determined mostly by the silicon oxide matrix, while sufficient transverse conductivity for the use in solar cells is maintained by few-nanometer-wide silicon filaments. In thin-film silicon solar cells, p-type SiOx~0.5 shows the best results as a window layer with enhanced transparency, while strongly phosphorous-doped n-type SiOx~1 is best suited for use as an intermediate reflecting layer in tandem cells due to a low refractive index of 1.8 with sufficient transverse conductivity. The tunable resistance of doped SiOx layers can be used at various locations in a Micromorph cell to quench shunts or bad diodes in spatially non-uniform devices. Implementation of SiOx with these three functionalities in Micromorph cells has significantly contributed to increasing the best initial efficiency from 11.8% in 2006 to 13.5% in 2010. While rough ZnO front electrodes and SiOx-based intermediate reflecting layers both help to improve Micromorph cell efficiencies, they render the devices more sensitive to water-vapor-induced degradation. With lock-in thermography measurements we demonstrate that water contamination affects mainly the cell borders resulting in non-uniform distribution of the local open circuit voltage. Finally we show how free-carrier absorption and surface plasmon polariton absorption can reduce the reflectivity of the back reflector, and we present preliminary results aimed toward the realization of an absorption-free back reflector making use of polished ZnO and sputtered silver layers.

EPFL2011

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

Sylvie Faÿ

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.

EPFL2003

Defect passivation in zinc tin oxide: improving the transparency-conductivity trade-off and comparing with indium-based materials

Oscar Esteban Rucavado Leandro

Transparent conductive oxides (TCOs) are essential in technologies coupling light and electricity. Due to their good optoelectronic properties and the production scalability, Sn-doped indium oxide (In2O3:Sn) is the preferred TCO in industrial applications. Nonetheless indium is scarce in the Earth's crust and its availability might be compromised over the next decades. To address this issue, this doctoral project aims to (i) improve the optoelectronic properties of In-free TCOs seeking to match those of In2O3:Sn, and (ii) refine the optoelectronic properties of In2O3-based films to decrease the In consumption in applications. To accomplish this, we investigated the links between the defects, the microstructure and the optoelectronic properties of two families of TCOs, indium- and tin-based oxides. First, we studied the evolution in the optoelectronic properties and microstructure of amorphous zinc tin oxide (a-ZTO) when annealed up to 500C in oxidizing, neutral, and reducing atmospheres. We show that annealing in atmospheric pressure at temperatures > 300C decreases the detrimental subgap absorptance while increasing the electron mobility (mu). Thermal treatments in reducing atmospheres increase the free-carrier density (Ne) and the subgap absorptance. None of the thermal treatments resulted in important changes in the amorphous microstructure. Combining these results and density functional theory (DFT) calculations, oxygen deficiencies (VO) were identified as the source of detrimental subgap absorption. VO can act as donors but also as electron scattering centres. Based on these results, a-ZTO with ÎŒ up to 35 cm2/Vs, is demonstrated by high-temperature defect passivation. Profiting from the microstructure stability of a-ZTO, this material was used as a recombination junction in a tandem solar cell which require a high-temperature step in its processing. The passivation scheme might be problematic for temperature-sensitive technologies. Therefore, we demonstrated an alternative low-temperature passivation method, which relies on co-sputtering a-ZTO with SiO2 . Using two Sn-based oxides with different composition and microstructure: a-ZTO and SnO2 , and results of DFT calculations, we demonstrate that SiO2 contribution is twofold. (i) The oxygen from SiO2 passivate the VO in SnO2 and a-ZTO. (ii) The formation energy of the ionized VO is lowered by the silicon-atoms, enabling defects that do not contribute to the subgap absorptance. This passivation scheme improves the optical properties without affecting the electrical conductivity, overcoming the optoelectronic trade-off in Sn-based TCOs. Finally, we study an In-based high-ÎŒ TCO: Zr-doped In2O3. Films of In2O3:Zr with thickness from 15 nm-100 nm were sputtered in the amorphous state and annealed in different atmospheres. Annealing in air yields fully crystalline films with high transparency and a high-mu limited by phonon and ionized impurity scattering. 15 nm-thick films exhibit an average absorptance of < 0.5 % (between 390 nm-2000 nm) and an mu of 50 cm2/Vs increasing to 105 cm2/Vs for 100 nm-thick films. Alternatively, annealing in a neutral or reducing atmospheres result in higher mu for films thinner than 50 nm as a high Ne is maintained. The demonstration of thickness reduction while keeping high lateral mu makes In2O3:Zr is an alternative to reduce In in applications such as flexible displays, solar cells and light emitting diodes.

EPFL2019