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

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 (

Transparent Passivating Contacts for Front Side Application in Crystalline Silicon Solar Cells

Josua Andreas Stückelberger

In this thesis, we present the development and characterization of novel approaches to carrier-selective passivating contacts for crystalline silicon (c-Si) solar cells. In our first approach, we benefit from a wide-bandgap material, namely mixed-phase silicon oxide (SiOx). High hydrogen dilution during the plasma-enhanced chemical vapor deposition of a SiOx layer leads to the growth of vertically oriented silicon filaments embedded in a silicon oxide matrix. The resulting mixed-phase material combines transparency and vertical conduction through the layer. We propose a layered contact structure starting with an ultra-thin chemically grown SiOx (~1.2 nm) at the c-Si wafer surface, followed by a phosphorus-doped bilayer of the mixed-phase SiOx, finishing with a nanocrystalline silicon layer that ensures contact with the metallization. With such a stack, an electron-selective passivating contact with a saturation current density J0 of 5.3 fA/cm2 is achieved on n-type wafers with hydrogenation, whereas on p-type wafers the contact reached 5.8 fA/cm2 even without hydrogenation. The contact is analyzed in detail with respect to its structure and its change upon thermal treatment. Two different growth regions are detected: one with the vertically oriented silicon inclusions and a second one in which the silicon phases have more lateral growth. Crystallization and atomic redistribution are analyzed during in-situ annealing in an electron microscope, leading to a better understanding of the change in distribution of phosphorus and oxygen as well as the nucleation sites. The influence of the annealing temperatures and dwell times as well as the initial doping concentration on the resulting in-diffused region and the passivation quality is reported. We use simulations to relate the influence of these parameters on passivation to the different recombination mechanisms. From the analyzed phosphorus doping profiles, an interesting observation is reported. The chemical oxide varies in thickness and density depending on the wafer polarity as well as the base resistivity. Proof-of-concept cells including this novel structure demonstrate the suitability for electron-selective passivating contacts at the device level with conversion efficiencies of 19.0% on planar wafers and 20.1% on textured wafers. Promisingly high short-circuit current densities (Jsc) of 35 mA/cm2 (40 mA/cm2) on planar (textured) with low parasitic absorption indicate its potential as a front contact in next-generation solar cells. Additionally, the beneficial feature of the smooth refractive index change within the layers that leads to a low reflectance over a broad wavelength range, similar to a built-in anti-reflection coating, is presented. A second approach to reach highly transparent electron-selective passivating contacts is demonstrated by the use of a plasma process based on the precursor gases phosphine (PH3), silicon tetrafluoride (SiF4), hydrogen (H2), and argon (Ar). The beneficial effect of fluorine in terms of passivation without the need for additional hydrogenation processes is shown, reaching implied open-circuit voltages of 730 mV on planar surfaces and 707 mV on textured surfaces. These layers are implemented in first proof-of-concept working devices with efficiencies of 19.5% on single-side-textured wafers and Jsc values of up to 40 mA/cm2.

EPFL2018

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

Advanced Architectures and Processing for High-Efficiency Silicon Heterojunction Solar Cells

Jonas Geissbühler

Crystalline silicon solar cells currently represent the largest part of the photovoltaic market. In response to the demand for higher efficiency devices, silicon heterojunction technology, which merges a crystalline silicon wafer with thin amorphous silicon (a-Si:H) films enabling the achievement of passivating contacts, may be considered as one of the most promising approaches for the next generation of industrial solar cells. These cells demonstrate record energy conversion efficiency enabled by excellent surface passivation leading to open-circuit voltages close to the theoretical limit. Despite the remarkable electronic properties of a-Si:H, its narrow bandgap induces significant parasitic light absorption. The aim of this thesis is to mitigate this loss through novel cell architectures and new fabrication processes. The outcome of this work is four-fold: First, a new silicon heterojunction solar cell structure is investigated that decouples the optical and electrical properties of the window layers through localized front contacts. By using shadow masking for the film patterning, we demonstrate the feasibility of the new cell structure under the condition that a sufficiently high contact coverage be maintained. Second, for the patterning of a-Si:H layers, we develop a hydrogen plasma etching technique carried out in a PECVD reactor. In particular, we show that a critical thickness of the intrinsic a-Si:H layer is needed to provide sufficient shielding of the amorphous/ crystalline interface to ensure good passivation. This technique enables the patterning of a-Si:H layers with nanometric accuracy while preserving the surface passivation. A third important outcome of this thesis is the replacement of the front metallization¿usually made by screen-printing of a silver paste¿with a copper front grid formed by electrodeposition. To ensure sufficient adhesion of the metallic fingers, a first process based on a double electrodeposition of a nickel/copper stack is presented. Important improvements in light management are realized with this metallization scheme, reducing the finger width from 70 um to 15 um which leads to a short-circuit gain of 1.1 mA cm2. In a second step, a simplified process based on a copper seed layer is proposed, further improving the metallization adhesion and making this technique compatible with chemically sensitive transparent conductive oxides such as zinc-oxide-based materials. The fourth outcome of this work is the replacement of the p-doped a-Si:H hole collector with a highly transparent molybdenum oxide (MoOx) film. We show that this material is an efficient hole collector if the post-deposition processes are carried out below 130 °C. From an optical point of view, this layer enables an improvement equivalent to 0.8 mA cm2 of the cell¿s response in the blue part of the spectrum. Nevertheless, this gain is reduced by the modification of the MoOx film induced during the TCO sputtering process, which leads to broadband parasitic light absorption. Finally, by merging this MoOx hole collector with the electrodeposited copper front metallization, remarkable fill factors of 80.3% are obtained leading to a certified efficiency of 22.5% for a 4 cm2 solar cell.

EPFL2015