Environmental stability of high-mobility indium-oxide based transparent electrodes

Large-scale deployment of a wide range of optoelectronic devices, including solar cells, critically depends on the long-term stability of their front electrodes. Here, we investigate the performance of Sn-doped In2O3 (ITO), H-doped In2O3 (IO:H), and Zn-doped In2O3 (IZO) electrodes under damp heat (DH) conditions (85 degrees C, 85% relative humidity). ITO, IO: H capped with ITO, and IZO show high stability with only 3%, 9%, and 13% sheet resistance (Rs) degradation after 1000 h of DH, respectively. For uncapped IO: H, we find a 75% Rs degradation, due to losses in electron Hall mobility (mu(Hall)). We propose that this degradation results from chemisorbed OH-or H2O-related species in the film, which is confirmed by thermal desorption spectroscopy and x-ray photoelectron spectroscopy. While mu(Hall) strongly degrades during DH, the optical mobility (mu(optical)) remains unchanged, indicating that the degradation mainly occurs at grain boundaries. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

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

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

Novel p-dopant toward highly efficient and stable perovskite solar cells

Seckin Akin, Michael Graetzel, Ulf Anders Hagfeldt, Hui-Seon Kim, Jiyoun Seo, Marko Stojanovic, Shaik Mohammed Zakeeruddin

Li-TFSI is the most common p-dopant for the hole conductor spiro-MeOTAD in the normal structure (n-i-p) of perovskite solar cells (PSCs), which consistently yield the highest power conversion efficiency (PCE) albeit at the risk of lower long-term operational stability. Here we successfully replace conventional Li-TFSI with Zn-TFSI2, which not only acts as a highly effective p-dopant but also enhances considerably both the photovoltaic performance and long-term stability. The incorporation of Zn-TFSI2 as a dopant for spiro-MeOTAD leads to an increase by one order in the hole mobility compared to Li-TFSI from 3.78 x 10(-3) cm(2) V-1 s(-1) to 3.83 x 10(-2) cm(2) V-1 s(-1). Furthermore, the device with Zn-TFSI2 showed an 80 mV higher built-in voltage and a bigger recombination resistance than the one with Li-TFSI, which were responsible for the striking increase in both the open-circuit voltage and fill factor, leading to a stabilized PCE of 22.0% for the best cells. Remarkably, the device employing Zn-TFSI2 demonstrated superb photo-stability, showing even a 2% increase in the PCE after 600 h light soaking at the maximum power point (mpp) under full sun, while the PCE of the device with Li-TFSI decreased by 20% under the same conditions. Similarly, the device with Zn-TFSI2 showed better operational stability at 50 degrees C resulting in a 21% decrease in the PCE after 100 h aging at the mpp under full sun while the Li-TFSI based one showed a 55% decrease. Moreover, the Zn-TFSI2 based device was capable of effectively resisting humidity compared to the one based on Li-TFSI from shelf stability monitoring (R.H. 40%) in the dark.