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

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.

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

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

Perovskite/Silicon Tandem Solar Cells: Toward Affordable Ultra-High Efficiency Photovoltaics ?

Jérémie Werner

Nowadays the photovoltaics market is dominated by crystalline silicon solar cells, which are approaching their theoretical efficiency limit. Novel solutions have to be found for photovoltaics to further enhance its competitiveness with other energy sources. One of the most promising approaches lies in combining market-proven silicon solar cell technology, having a low optical band gap, with an efficient near-infrared transparent wide-band gap top cell to form a tandem cell. Organic-inorganic halide perovskite solar cells are promising candidates for top cells, showing high efficiencies with simple and cost-effective device fabrication. We first develop perovskite solar cells specifically for tandem applications. We develop and optimize a hybrid sequential deposition method combining thermal evaporation and solution processing to fabricate perovskite absorber materials with specifically tailored optoelectronic properties. These materials are systematically investigated for their optical, structural and electronic properties, and then applied in perovskite solar cells both in n-i-p and p-i-n configurations, for which we show several charge transport materials combinations. As a replacement for the standard metal opaque rear electrode of perovskite cells, we develop a TCO-based transparent electrode, for which sputtered IZO is presented as a good candidate thanks to its high carrier mobility and broadband transparency. Sputtering-induced damages are reduced by the introduction of a buffer layer: MoOx for n-i-p cells and SnO2 for p-i-n cells. The parasitic absorption losses in the MoOx are described in details. We then discuss the parasitic absorption losses in charge transport layers and TCOs, with experimental comparison of various materials. The optimisation of the perovskite absorber deposition method and the improvement of the charge transport layers and transparent electrode allows the fabrication 1cm2 area semitransparent perovskite solar cells with >16% efficiency. We then integrate the semitransparent perovskite cells in mechanically stacked 4-terminal tandem solar cells. The challenges in reducing the strong parasitic absorption and reflection losses are first discussed. We then demonstrate 4-terminal tandem measurements with >25% total efficiency with a small area top cell. With larger 1cm2 area top cells, we fabricate integrated 4-terminal tandem devices with both subcells having similar size and total efficiency >23%. The integration of perovskite solar cells in 2-terminal monolithically connected tandem solar cells with silicon heterojunction bottom cells is finally presented. First, we show the development of a TCO-based recombination layer and the important reflection losses and interference effects observed in all-flat devices. The origins of parasitic absorption losses in monolithic tandems are then explained and new architectures and materials are investigated supported by optical simulations. We then replace the polished wafers by fully textured silicon bottom cells for better light management. The development of a tandem device with the top cell conformally coated onto the textured bottom cell is explained, leading to >25% certified power conversion efficiency. The end of the thesis presents preliminary results on the up-scalability and light soaking stability of the developed textured tandems, as well as a proof-of-concept of a first perovskite/perovskite/silicon triple junction solar cell on textured wafers.

EPFL2018