High-Efﬁciency Nonfullerene Organic Solar Cells Enabled by Atomic Layer Deposited Zirconium-Doped Zinc Oxide Geedhika K. Poduval, Leiping Duan, Md. Anower Hossain, Borong Sang, Yu Zhang, Yingping Zou,* Ashraf Uddin,* and Bram Hoex* 1. Introduction Organic solar cells (OSCs) have received considerable attention for their ease of fabrication, lightweight, semi-transparency, ﬂexibility, and low-cost. [1–4] The progress in the development of novel organic materials, surface morphology, and suppressed saturation current have boosted the power conversion efﬁciency (PCE) of OSCs from 12% to over 17% in the last two years. [5–8] Recently, Zou and co-workers synthesized state-of-the-art nonful- lerene material Y6 (more details can be found in ref. [4]). [8] Y6 shows a near-infrared region (NIR) absorption with a peak at around 880 nm and has the lowest unoccupied molecular orbital (LUMO) and a highest occupied molecular orbital (HOMO) level of 4.10 and 5.65 eV, respectively. By combining Y6 with the widely used polymer donor materials poly[(2,6-(4,8-bis(5-(2-ethylhexyl- 3-ﬂuoro)thiophen-2-yl)-benzo[1,2-b:4,5-b 0 ] dithiophene))-alt-(5,5-(1 0 ,3 0 -di-2-thienyl-5 0 , 7 0 -bis(2-ethylhexyl)benzo[1 0 ,2 0 -c:4 0 ,5 0 - c 0 ] dithiophene-4,8-dione)] (PM6) in a conven- tional device structure of ITO/PEDOT: PSS/PM6:Y6/PNDIT-F3N/Ag, efﬁciencies over 15% have been achieved. [8] Zou and co-workers further modiﬁed Y6 into another novel advanced nonfullerene acceptor material N3. [8,9] With consider- ation of its advanced photovoltaic (PV) properties, N3 is currently the most prom- ising nonfullerene acceptor material for large-scale commercial fabrication of OSCs. A record PCE of 15.8% has been achieved by PM6:N3-based OSCs in a conventional device structure. [8] However, as the conventional device structure usually exhibits poor device stability, it is worth investigating the PM6:N3 combination in the more stable inverted device structure, which has not yet been explored. [10,11] The OSC usually consists of three different layers including an active layer in the middle and the electron transport layer (ETL) and hole transport layer (HTL) on each side to help the charge transport (CT). The ETL and HTL are required to effectively transfer the electrons and holes generated from the active layer to each electrode. [12] In OSCs with an inverted device structure, the most commonly used ETL is zinc oxide (ZnO). ZnO is an II–VI material with widespread applications ranging from solar cells, light-emitting diodes, sensors to piezoelectric devices, and photocatalyst. [13] Its high electron conductivity and relatively low hole conductivity makes it an appealing ETL for thin-ﬁlm solar cells. [14] It should, however, be noted that ZnO has shown to exhibit a high photosensitivity caused by the presence of surface defects. [15–17] In OSCs, the ZnO ETL is fabricated using methods such as spin-coating, thermal evaporation, and chemical vapor deposi- tion. [18–20] Although these techniques are faster and have a better growth rate, atomic layer deposition (ALD) has recently gained signiﬁcant interest. This is due to its self-limiting nature, which results in precise control over the thickness, uniformity, and its G. K. Poduval, L. Duan, Dr. M. A. Hossain, B. Sang, Y. Zhang, Prof. A. Uddin, Prof. B. Hoex School of Photovoltaic and Renewable Energy Engineering University of New South Wales Sydney, NSW 2052, Australia E-mail: a.uddin@unsw.edu.au; b.hoex@unsw.edu.au Prof. Y. Zou College of Chemistry and Chemical Engineering Central South University Changsha 410083, P. R. China E-mail: yingpingzou@csu.edu.cn The ORCID identiﬁcation number(s) for the author(s) of this article can be found under https://doi.org/10.1002/solr.202000241. DOI: 10.1002/solr.202000241 Organic solar cells (OSCs) are promising photovoltaic devices and zinc oxide (ZnO) is a commonly used electron transport layer (ETL) in OSCs. However, the conventional spin-coating ZnO layer is currently limiting its efﬁciency potential. Herein, it is shown for the ﬁrst time that atomic layer deposition (ALD), which allows for controlled thin ﬁlm growth with atomic-scale control, can effectively be used to optimize the ZnO for nonfullerene OSCs. First, density functional theory (DFT) calculations are discussed to show the impact of doping ZnO with zir- conium (Zr) on its density of states and detail the synthesis of Zr doped ZnO ﬁlms by ALD using a supercycle approach. A 2.4% Zr concentration is found to be optimal in terms of optoelectronic properties and sufﬁciently low defect density. The champion efﬁciency of 14.7% for a PM6:N3-based nonfullerene OSC with Zr-doped ZnO ETL are obtained, which is 1% absolute higher compared to a device with an undoped ZnO ETL. This improvement is attributed to a lower series resistance, a suppressed surface recombination, and an enhanced current extraction resulting from the Zr-doped ZnO. This work demonstrates the potential of atomic-scale engineering afforded by ALD towards achieving the ultimate efﬁciency of OSCs. FULL PAPER www.solar-rrl.com Sol. RRL 2020, 2000241 2000241 (1 of 14) © 2020 Wiley-VCH GmbH