2000074 (1 of 8) © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-methods.com FULL PAPER Transparent Electrodes Consisting of a Surface-Treated Bufer Layer Based on Tungsten Oxide for Semitransparent Perovskite Solar Cells and Four-Terminal Tandem Applications Helen Hejin Park, Jincheol Kim, Geunjin Kim, Hyunmin Jung, Songhee Kim, Chan Su Moon, Seon Joo Lee, Seong Sik Shin, Xiaojing Hao, Jae Sung Yun, Martin A. Green, Anita W. Y. Ho-Baillie, Nam Joong Jeon, Tae-Youl Yang, and Jangwon Seo* Dr. H. H. Park, Dr. G. Kim, H. Jung, S. Kim, C. S. Moon, Dr. S. J. Lee, Dr. S. S. Shin, Dr. N. J. Jeon, Dr. T.-Y. Yang, Dr. J. Seo Advanced Materials Division Korea Research Institute of Chemical Technology (KRICT) Daejeon 34114, Korea E-mail: jwseo@krict.re.kr Dr. J. Kim, Prof. X. Hao, Dr. J. S. Yun, Prof. M. A. Green, Prof. A. W. Y. Ho-Baillie Australian Centre for Advanced Photovoltaics School of Photovoltaic and Renewable Energy Engineering University of New South Wales (UNSW) Sydney, NSW 2052, Australia Dr. J. Kim New and Renewable Energy Research Center Korea Electronics Technology Institute (KETI) Seongnam-si, Gyeonggi-do 13509, Korea Prof. A. W. Y. Ho-Baillie School of Physics and University of Sydney Nanoscience Institute The University of Sydney Sydney 2006, Australia The ORCID identifcation number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smtd.202000074. DOI: 10.1002/smtd.202000074 1. Introduction Inorganic–organic hybrid perovskite solar cells (PSCs) have rapidly developed within a decade starting of with power con- version efciencies (PCEs) of 3.8% in 2009 [1] and now currently certifed as 25.2% in 2019, [2] recognized as the fastest develop- ment compared to any other photovoltaic technology in history. For semitransparent devices with n-i-p structures, a metal oxide bufer material is commonly used to protect the organic hole transporting layer from damage due to sputtering of the transparent conducting oxide. Here, a surface treatment approach is addressed for tungsten oxide-based trans- parent electrodes through slight modifcation of the tungsten oxide surface with niobium oxide. Incorporation of this transparent electrode technique to the protective bufer layer signifcantly recovers the fll factor from 70.4% to 80.3%, approaching fll factor values of conventional opaque devices, which results in power conversion efciencies over 18% for the semitransparent perovskite solar cells. Application of this approach to a four-terminal tandem confguration with a silicon bottom cell is demonstrated. Inorganic–organic lead halide perovskites have high absorption coefcients and long difusion lengths with easily tunable band gaps through composition engi- neering. [3–10] High efciency, low material cost, and simple solution processing make PSCs a highly promising source of sus- tainable energy as either single-junction devices or in tandem structures on top of other solar cells with narrower band gap absorbers, such as silicon, chalcogenides, and Sn-containing materials. [11–20] In par- ticular, further improvement of silicon solar cell efciencies through silicon- perovskite tandems is realistic business models, considering that over 90% of the photovoltaic market is dominated by single-junction silicon solar cells. [21] Most silicon-perovskite tandem solar cells have been demon- strated through either four-terminal tandems, which commonly involve mechanically stacking the semitransparent perovskite top cell on the silicon bottom cell, or two-terminal tandems, which involve monolithically integrating the perovskite top cell onto the silicon bottom cell. To date, the highest efciencies reported for four-terminal and two-terminal silicon-perovskite tandem solar cells are 27.1% and 29.2% (26.0% in literature), respectively. [2,22,23] Advantages of the mechanically stacked four- terminal tandem confguration include the two cells required to be only optically coupled, eliminating the necessity of current or voltage matching, and thus, allowing independent develop- ment of the silicon and PSCs. [24] In order for light to pass through the perovskite top cell to the underlying silicon bottom cell, high transparency of the perovskite cell is crucial. Due to its excellent transparency and high conductivity, indium tin oxide (ITO) is industry’s standard transparent conducting oxide in optoelectronic devices, including smart windows, fat-panel displays, light-emitting diodes, and solar cells. [25] However, ITO is usually deposited by magnetron sputtering, which can damage any underlying layers through high kinetic energy of the sputtered particles, requiring a sputter bufer layer to protect the underlying layer. In the case of PSCs with n-i-p structures, organic hole transport materials (HTMs), such as poly(triarylamine) (PTAA) and 2,2′,7,7′-tetrakis- (N,N-di-4-methoxyphenylamino)-9,9′-spirobifuorene (spiro- OMeTAD), have shown high efciencies, and thus, when Small Methods 2020, 2000074