Propriétés électriques et optiques de couches minces de WS2 et MoS2 en vue d'applications photovoltaïques

Single crystals of layered semiconductors such as WS2 and MoS2 have already proven their efficiency as active elements in photovoltaic cells. Due to their high optical absorption coefficient in the visible range, these materials could be used in the form of thin films in photovoltaic devices. In this work, we explore the potential of the sputtering technique to prepare semiconducting films of WS2 and MoS2. The influence of thermal treatments on the films is also examined. The study focuses on the electrical and optical properties of the films. They are determined by conventional conductivity, Hall effect, photoconductivity and reflection-transmission measurements. With the help of scanning probe microscopes like the scanning tunneling microscope (STM) or the atomic force microscope (AFM), the local electrical and photovoltaic properties are measured down to the nanometer scale and the relationships with the properties measured over macroscopic distances are established. Reactive sputtering (with H2S reactive gas) from a WS2 or a MoS2 target yields polycristalline WSx (0.7 < x < 1.95) and MoSx (0.9 < x < 2.1) films, at deposition temperatures between 70° and 600°C. The structural analyses, based on transmission electron microscopy (TEM), X-ray diffraction (XRD) and STM, reveal a maximum grain size of 20–30 nm, as well as a high density of stacking faults. An original STM study of the initial growth of sputtered films shows a three-dimensional growth mode including some spiral growth. The grains grow in the shape of trigonal pyramids, with step heights of 0.6 nm corresponding to the thickness of one single WS2 molecular layer. In the sputtered films, the variation of the electrical conductivity versus the temperature shows a typical semiconductor behavior with an activation energy of up to 90 meV at room temperature (RT). The high carrier concentrations (n > 1025 m-3) indicate an important level of doping and the Hall mobilities (µH < 0.1 × 10-4 m2V-1s-1) are smaller than those measured in single crystals against (µH ≈ 200 × 10-4 m2V-1s-1). STM current-voltage (I-V) spectroscopy on the sputtered film surface is typical of a degenerate semiconductor or of a high density of surface states. The films are therefore not suitable for the preparation of junctions. In order to improve the semiconducting properties of the film, a new preparation method is developed. The fabrication process, which can easily be scaled up, produces WS2 and MoS2 films with electronic properties close to those of single crystals. Firstly, an amorphous WS3-4 film is sputter-deposited at low temperature (0°C) on a substrate coated with a thin (10 nm) layer of nickel or cobalt. Secondly, the film is annealed for one hour under an argon flow between 750°C and 950°C. By predepositing Ni or Co the cristallinity, the grain size and the texture of the films are spectacularly improved. Elements such as Ni or Co are hereafter named crystallization promoter. After annealing, they remain in film in the form of NiSx or CoSx droplets. Films obtained by this sputtering/annealing process show large (1–5µm) and thick (50–200 nm) grains with their c axis oriented perpendicular to the substrate. Optical reflection and transmission coefficients are similar to those measured on WS2 single crystals, with excitonic absorption peaks of same intensity at 1.94 and 2.36 eV. The films are photoconductive when illuminated with photons whose energy is superior to 1.35 eV, which corresponds to the indirect bandgap of WS2. The conductivity is of p type with a carrier concentration of about 1023 m-3 and a Hall mobility of 5–10 × 10-4 m2V-1s-1at RT. The Hall mobility is thermally activated with an activation energy of 60–90 meV between 200 et 320K. The transport properties (mobility, photoconductivity) are mainly controlled by the potential barriers at grain boundaries. STM I-V spectroscopy with and without illumination shows that the flat (002) surfaces of the WS2 cristallites in the films have a low density of surface states, similarly to the single crystal (002) surfaces. Measurements using an AFM mounted with a conductive tip indicate that the NiSx phases in the film are metallic while the WS2 grain edges are typical of a degenerate semiconductor. In order to characterize more quantitatively the local electronic properties of the films an innovative characterization technique is introduced. A lattice of triangular gold electrodes, each electrode having a typical area of 0.2 µm2, is evaporated on the p-type WS2 film. With the help of an AFM, the current-voltage characteristics of the contacts between the gold electrodes and the WS2 film are measured. The electrodes deposited on flat WS2 crystallites form rectifying diodes with the underlying grains. Barrier heights of 0.56–0.74 eV and diode ideality factors between 1.15 and 2 are determined. Under illumination, open-circuit voltages of up to 500 mV can be measured on some contacts. The photodiodes collect all carriers photo-excited on a surface of about 20 µm2, i.e. the typical size of the WS2 cristallites. The various experiments performed on the micro-contacts clarify the respective roles of the cristallites and the grain boundaries in the macroscopic measurements (photoconductivity, Hall effect, diodes). The use of crystallization promoters during the annealing of WS3-4 films therefore proves fruitful for the preparation of semiconducting films. The WS2 crystallites inside these films are suitable for the preparation of solid-state photodiodes, what is a first step towards the realization of photovoltaic devices based on WS2 or MoS2 thin films.

Window Layers for Silicon Heterojunction Solar Cells

Johannes Peter Seif

Currently, crystalline silicon (c-Si) wafer-based solar cells dominate the photovoltaic market (80-90%). In this thesis we concentrate on silicon heterojunction (SHJ) solar cells that--in contrast to diffused homojunction cells--rely on the application of amorphous silicon (a-Si:H) thin films. Unlike standard homojunction devices, which are typically limited by their highly recombination-active semiconductor-to-metal contacts, SHJ devices exhibit excellent surface passivation enabled by intrinsic and doped a-Si:H films. These a-Si:H layers, however, entail drawbacks for optical performance and carrier transport, two topics that will be addressed in this work. To this end we investigate non-traditional materials for SHJ devices, with the goal of replacing the a-Si:H or the transparent electrodes. These materials include microcrystalline silicon (uc-Si:H) and organic semiconductors for contact formation; amorphous silicon suboxides (a-SiOx:H) for surface passivation; and transparent electrodes applied by atomic layer deposition (ALD) as protective layers against subsequent processing steps. Along with the optical and electrical properties of these materials, we study the impact on device performance associated with their deposition. For this we test the devices under standard testing conditions (25 °C) and at elevated temperatures closer to those encountered in the field. For the investigations on uc-Si:H layers, we vary process parameters (including temperature, pressure, power, excitation frequency and hydrogen dilution) as well as pre-treatments, gas variations and nucleation layers. We assess the suitability of these approaches for SHJ solar cells and apply selected measures in devices. Thereby we demonstrate a gain in short-circuit current density in the range of 0.5-1 mA.cm-2 and good fill factor values of up to 79.2% using either n- and p-type uc-Si:H layers. Furthermore, with the goal of reducing optical losses, we test wide-bandgap a-SiOx:H layers for passivation. In terms of current gain without negative side effects on transport, we argue that these layers are best applied to the electron-collecting contact--put at the front of the device--as their application to the hole-collecting contact introduces a transport barrier for holes. This barrier deteriorates the device performance at 25 °C, but shows a beneficial effect on the temperature coefficient of the device, yielding coefficients as low as -0.1%/°C. In some cases--compared to standard devices--devices with a-SiOx:H layers exhibit superior performance at elevated temperatures, which can be of interest in warmer climates. In parallel to these main topic we also study aluminium-doped zinc oxide (ZnO:Al) layers deposited by ALD as a protective layer against sputter-induced damage and organic semiconductors as transparent electrodes for the hole-collecting contact. In both cases we observe a gain in terms of surface passivation, which indicates that these materials may be beneficial for the contact formation in future device structures. In addition to these material-related investigations, we unravel the temperature-dependence of each individual cell parameter and present a brief comparison of state-of-the-art technologies and their respective temperature dependence.

EPFL2015

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

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 (

University of Neuchâtel2009

Window Layers for Silicon Heterojunction Solar Cells

Johannes Peter Seif

EPFL2015

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

Thomas Söderström

University of Neuchâtel2009

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

Jonas Geissbühler

EPFL2015