Interface Modification Enabled by Atomic Layer Deposited Ultra-Thin Titanium Oxide for High-Efficiency and Semitransparent Organic Solar Cells Leiping Duan, Borong Sang, Mingrui He, Yu Zhang, Md Anower Hossain, Md Habibur Rahaman, Qingya Wei, Yingping Zou,* Ashraf Uddin,* and Bram Hoex* 1. Introduction Featuring lightweight, semitransparency, flexibility, and low costs, organic solar cells (OSCs) have attracted wide research interest. [1–4] In the recent 2 years, ascribing to significant devel- opments in new organic materials and device engineering strat- egies, OSCs are considered to reach its second golden age with the profoundly improved power conversion efficiency (PCE) and device stability. [5–7] Although the light- absorbing layer is essential, the charge transport layer also plays a significant role in achieving high device performance in OSCs, as it promotes the charge carrier transport and protects the active layer from the air penetration in the device. [8,9] Due to its high electron mobility, high transpar- ency, and the suitable energy level, zinc oxide (ZnO) is the most widely used elec- tron transport layer (ETL) for OSCs. [10,11] However, ZnO exhibits high energy radia- tion instability due to its high photosensi- tivity caused by the presence of surface defects. [12–14] Consequently, there is a need for further optimization of the ZnO ETL in OSCs. [15–17] Recently, Zou and co-workers modified the state-of-the-art acceptor material Y6 and synthesized a novel acceptor material N3. [18,19] We have previously reported a 14% PCE for the N3-based OSCs in an inverted device structure based on the ZnO layer. [20] The unencapsulated device showed around 50% of PCE loss after the burn-in degradation test for 5 h, where the instability of the interface layers and the electrode was found to be the leading cause. Herein, modi- fying the ZnO layer can be a feasible method to bring further performance and stability improvement to this type of device. An atomic layer deposition (ALD) technique is an intrinsic self-limiting process with ultimate process control and low sub- strate temperature, which is an appealing method for depositing ultrathin metal oxides to modify the interfacial layer. [21] To date, ALD metal oxide for the interface modification is widely demon- strated in silicon, dye-sensitized, perovskite, and chalcogenide thin-film solar cells to improve their device performance. [22–27] It may also be an effective method to modify the ETL for OSCs. For instance, Kim et al. used ALD to deposit an additional ultra-thin ZnO as the ETL in OSCs to enhance the PCE and sta- bility. [28] Vasilopoulou et al. demonstrated the passivation effect from the ALD deposited alumina (Al 2 O 3 ) and zirconia (ZrO 2 ) insulating nanolayers to OSCs and demonstrated a 30% PCE improvement. [29] Polydorou et al. used ultra-thin ALD deposited alumina (Al 2 O 3 ) and zirconia (ZrO 2 ) to modify the ZnO ETL in OSCs and demonstrated a significant improvement in both PCE and device stability. [30] Consequently, the ALD-assisted interface L. Duan, B. Sang, M. He, Y. Zhang, Dr. M. A. Hossain, M. H. Rahaman, 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 Q. Wei, 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.202000497. DOI: 10.1002/solr.202000497 Organic solar cells (OSCs) are considered to have reached a second golden age with profoundly improved power conversion efficiency (PCE) and device stability in recent years. The modification of the interface layer plays a significant role in achieving performance enhancement in OSCs. Herein, the use of the atomic layer deposition (ALD) ultrathin TiO x to modify the interface layer in OSCs is reported. The modification with only two TiO x ALD cycles not only effectively passivates the interface between the ZnO electron transport layer (ETL) and the active layer, but also reduces the series resistance and improves the charge transport process in the device. An absolute 1% increase in PCE with enhanced device stability for modified OSCs is achieved. Semitransparent OSCs are also fabricated by applying this interface modification strategy. The modification with two TiO x ALD cycles increases the electrical device performance without affecting the optical properties of the semitransparent device. An average PCE of 10.46% with an average visible transmittance (AVT) of 19.61% and a color rendering index (CRI) close to 100 is demonstrated for the fabricated semitransparent device with the modification. The ALD-assisted interface modification provides a straightforward way to realize high-performance semitransparent OSCs. FULL PAPER www.solar-rrl.com Sol. RRL 2020, 2000497 2000497 (1 of 12) © 2020 Wiley-VCH GmbH