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Parasitic absorption reduction in metal oxide-based transparent electrodes: application in perovskite solar cells Jérémie Werner, Jonas Geissbuehler, Ali Dabirian, Sylvain Nicolay, Monica Morales Masis, Stefaan De Wolf, Björn Niesen, and Christophe Ballif ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04425 • Publication Date (Web): 24 Jun 2016 Downloaded from http://pubs.acs.org on June 25, 2016
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Parasitic absorption reduction in metal oxide-based transparent electrodes: application in perovskite solar cells Jérémie Werner,1* Jonas Geissbühler,1,2 Ali Dabirian,1 Sylvain Nicolay,2 Monica MoralesMasis,1 Stefaan De Wolf,1 Bjoern Niesen,1, 2 and Christophe Ballif 1, 2 1
Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics
and Thin-Film Electronics Laboratory, Rue de la Maladie`re 71b, 2002 Neuchâtel, Switzerland. 2
CSEM, PV-Center, Jaquet-Droz 1, 2002 Neuchâtel, Switzerland
KEYWORDS molybdenum oxide, tungsten oxide, CO2, plasma treatment, perovskite, silicon heterojunction, tandem, solar cell
ABSTRACT Transition metal oxides (TMOs) are commonly used in a wide spectrum of device applications, thanks to their interesting electronic, photo- and electro-chromic properties. Their environmental sensitivity, exploited for gas and chemical sensors, is however undesirable for application in optoelectronic devices, where TMOs are used as charge injection or extraction layers. In this work, we first study the coloration of molybdenum and tungsten oxide layers, induced by thermal annealing, Ar plasma exposure or transparent conducting oxide overlayer deposition,
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typically used in solar cell fabrication. We then propose a discoloration method based on an oxidizing CO2 plasma treatment, which allows for a complete bleaching of colored TMO films and prevents any subsequent re-coloration during following cell processing steps. Then, we show that tungsten oxide is intrinsically more resilient to damage induced by Ar plasma exposure, as compared to the commonly used molybdenum oxide. Finally, we show that parasitic absorption in TMO-based transparent electrodes, as used for semitransparent perovskite solar cells, silicon heterojunction solar cells or perovskite/silicon tandem solar cells, can be drastically reduced by replacing molybdenum oxide with tungsten oxide and by applying a CO2 plasma pre-treatment prior to the transparent conductive oxide overlayer deposition.
1. Introduction Transition metal oxides (TMOs) have attracted considerable attention for use in gas and chemical sensors,1,2 and more recently, for thin-film transistors and optoelectronic devices, such as light emitting diodes and solar cells, including organic, perovskite, and silicon heterojunction solar cells.3–5 Their high work function, large band gap, efficient carrier selectivity, and high transparency make them viable and cost-effective candidates for these applications, where they are employed as protective buffer layers, 6–10 optical spacers 11–13 or charge transport layers. 14–21 TMOs are susceptible materials, which are sensitive to their environment, such as air or oxygen exposure,
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temperature,
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UV-light,
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UV-ozone
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or plasma treatments.
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This is
because many TMOs readily undergo redox reactions. Evaporated metal oxides are especially sensitive, as they are often sub-stoichiometric when as-deposited, due to an oxygen deficiency3,5. The presence of oxygen vacancies creates positively charged structural defect states in the bandgap, which enables the attractive hole injection properties of TMOs despite their n-type
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semiconductor character.3 Some of the defect states also act as localized color centers, resulting in absorption in the visible/near-infrared spectrum, spread at different wavelengths around 800 nm depending on their oxidation states. This sensitivity and the associated coloration are desirable for some applications such as gas and chemical sensing. TMOs were for these reasons widely studied specifically for their photochromic 34–36 and electrochromic 37,38 properties. However, for optoelectronic applications, where high transparency is required in charge transport layers, any color change could be detrimental and result in performance reduction. In particular, it was recently pointed out that the optical properties of TMO layers are strongly affected during sputter deposition of a transparent conducting oxide (TCO) overlayer, resulting in a more pronounced absorptance of the TMO/TCO stack than the expected sum of their individual absorptance values. 6,18 If such a stack is used as a front window electrode in a solar cell, this increased absorptance directly translates into a decreased photocurrent and, as a result, reduced device performance. It would therefore be highly interesting to find either a pre-treatment method to prevent the appearance of this absorptance increase or a post-treatment to recover sputter damage (discoloration). Inspiration for this can be taken from an early study on photochromism of amorphous transition metal oxides by Colton et al.
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They showed that TMO films can be
bleached (decolored) by thermal annealing at 300°C in an oxidizing atmosphere, preventing simultaneously any further (post) coloration. However, high temperature annealing is not desirable for many applications, due to the temperature sensitivity of underlying films, such as for perovskite solar cells,39 silicon heterojunction solar cells,40 or other organic optoelectronic devices.5 To date though, an alternative bleaching method at low temperature has yet to be reported.
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Here, we study the coloration of TMOs, induced by temperature, Ar plasma exposure and TCO overlayer sputter deposition and demonstrate a low-temperature bleaching treatment based on CO2 plasma exposure. We show how this treatment can prevent and recover TMO layer coloration when applied respectively before or after Ar plasma exposure. To illustrate this method, we investigate and compare two commonly used sub-stoichiometric metal oxides: molybdenum oxide (MoOx, x
