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Amorphous TiO Compact Layers via ALD for Planar Halide Perovskite Photovoltaics In Soo Kim, Richard T. Haasch, Duyen H. Cao, Omar K. Farha, Joseph T. Hupp, Mercouri G. Kanatzidis, and Alex B. F. Martinson ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07658 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 7, 2016
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ACS Applied Materials & Interfaces
Amorphous TiO2 Compact Layers via ALD for Planar Halide Perovskite Photovoltaics In Soo Kim,a,b Richard T. Haasch,c Duyen H. Cao,b,d Omar K. Farha,b,d Joseph T. Hupp,a,b,d Mercouri G. Kanatzidis,a,b,d and Alex B. F. Martinsona,b* a
Materials Science Division, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, Illinois 60439, USA b Argonne-Northwestern Solar Energy Research (ANSER) Center, 2145 Sheridan Rd., Evanston, Illinois, 60208, USA c Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA d Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA KEYWORDS amorphous titanium dioxide, atomic layer deposition, low temperature processing, hybrid perovskites, solar energy conversion
ABSTRACT. A low temperature (< 120 °C) route to pinhole-free amorphous TiO2 compact layers may pave the way to more efficient, flexible, and stable inverted perovskite halide device designs. Toward this end, we utilize low-temperature thermal atomic layer deposition (ALD) to synthesize ultra-thin (12 nm) compact TiO2 underlayers for planar halide perovskite PV. While device performance with as-deposited TiO2 films is poor, we identify room temperature UV-O3 treatment as a route to device efficiency comparable to crystalline TiO2 thin films synthesized by higher temperature methods. We further explore the chemical, physical, and interfacial properties
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that might explain the improved performance through x-ray diffraction, spectroscopic ellipsometry, Raman spectroscopy, and x-ray photoelectron spectroscopy. These findings challenge our intuition about effective electron selective layers as well as point the way to a greater selection of flexible substrates and more stable inverted device designs.
Titanium dioxide (TiO2) has been widely adopted for applications in solar energy conversion including photovoltaics (PV)1-2 and (photo)electrochemical (PEC) devices3-4 owing to its transparency across the visible spectrum, excellent chemical stability, favorable electronic properties, as well as ability to passivate photoactive semiconductors against aqueous media.2, 5-6 It is not surprising, therefore, that TiO2 is also the most common choice for efficient extraction of electrons from halide perovskite based PV devices, in both compact (thin film) and nanoparticle (mesoscopic) forms.7-9 However, the utilization of TiO2 in these devices is subject to several limitations. First, the compact TiO2 layers are almost universally crystalline, as often induced by high temperature (500 °C) firing, which significantly limits its compatibility with low-thermalbudget substrates and inverted device designs (the halide perovskite absorber itself has a thermal budget less than 150 °C). The firing step crystallizes the compact TiO2 layer into anatase or rutile phases10-11 and empirically improves device efficiency, ostensibly by maximizing electron mobility and minimizing mid-gap defect states that may result in accelerated recombination. Alternative low temperature routes to crystalline TiO2 include chemical bath deposition of nanocrystallites12 which template as rutile TiO2 on FTO at 70 °C or spin-coating of pre-formed nanocrystals which are subsequently fired at 150 °C.13,14 However, high performance (> 10% efficient) devices with amorphous compact TiO2 layer are far more rare, but provide an intriguing route to fully flexible devices free from grain-induced inhomogeneities and diffusion pathways. The known methods for integrating amorphous TiO2 films into high efficiently planar
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halide perovskite devices require harsh deposition conditions including oxygen plasma15 or magnetron sputtering16 that are unlikely to be compatible with inverted device designs. In contrast, we utilize a thermal atomic layer deposition (ALD) process post-treated with UVgenerated O3 (UV-O3) ambient pressure and temperature to achieve high quality amorphous compact TiO2 layers in order to maintain future compatibility with the widest range of substrates, absorbers, and inverted device designs.
In addition to high temperatures, an additional limitation to most TiO2 compact layers is the relatively thick film (> 30 nm) required to avoid pinholes in crystalline films. Pinholes in the compact TiO2 layer allow for direct contact between the hybrid perovskite absorber and transparent conducting oxide (TCO), thereby short-circuiting the electron selective layer. As an electron selective layer, the compact TiO2 is necessarily electrically in series, and therefore results in resistance to collection of electrons in proportion to its thickness. To address these limitations, we utilize the low-temperature thermal ALD of ultrathin amorphous TiO2. Exceptionally low pinhole densities in ultra-thin films have previously been demonstrated via ALD17-19 by leveraging the inherent atomic scale precision of self-limiting surface synthesis. Herein, we investigate an ultra-thin (
