Transparent Liquid Crystal Hole-Transporting Material for Stable Perovskite Solar Cells Qurat Ul Ain, Jianxing Xia,* Hiroyuki Kanda, Imanah Rafieh Alwani, Xiao-Xin Gao, Habib Ur Rehman, Guang Shao, Vygintas Jankauskas, Kasparas Rakstys, Ammar Ahmed Khan,* and Mohammad Khaja Nazeeruddin* 1. Introduction Organic lead halide perovskites have proven to be competitive light-harvesting materials for photovoltaic devices owing to their superior photoelectric properties. [1–6] Within a short period of development since first reported in 2009, the certified power conversion efficiency (PCE) of perovskite solar cells (PSCs) has soared to 25.7%. [7] In the state-of-the-art n–i–p PSCs, the 2,2 0 ,7,7 0 -tetrakis[N,N- di(4-methoxyphenyl)amino]-9,9 0 -spirobi- fluorene (spiro-OMeTAD) is widely used as hole-transporting materials (HTMs), which facilitate hole extraction from the perovskite absorber. [8] Spiro-OMeTAD are commonly doped with bis(trifluorome- thane)sulfonamide lithium salt (Li-TFSI) and 4-tert-butylpyridine (t-BP) to improve their hole mobility and change the increased highest occupied molecular orbital (HOMO) level via p-doping. [9] Despite the significant improvement in PCE of PSCs employed with the complex dopants, some drawbacks exist in the stability of devices that make the most challenging issue and limit its commercialization. The Li-TFSI is a hydrophilic compound that easily adsorbs water from ambient air, resulting in rapid degradation of the hole-transportation layer (HTL) and decomposition of the perov- skite absorber via molecular diffusion. [10,11] In addition, the Li þ ion migrates to other functional layers of PSCs due to low migra- tion barriers through perovskite, causing random doping and alteration of the charge equilibrium in PSCs, making the internal degeneration of PSCs, usually displayed at continuous output- ting and thermal stability. The interfacial t-BP will also cause chemical decomposition of the perovskite by forming [PbI 2 -t- BP] coordinated complex over time, which further diminishes the long-term stability of PSCs. [12,13] To solve the obstacles, several groups have reported on alter- native hydrophobic organic p-dopants based on ionic liquids, [10,14] organic salts, [9,15] fullerene derivatives, [16] and metal–organic frameworks [17,18] which is to improve the hydro- phobicity and the stability of PSCs. However, the reported effi- ciency is far away from the devices employing Li-TFSI and t-BP due to the low conductivity of the organic molecule than the metal Li þ ion. Another way, the dopant-free HTMs-included small molecule and the polymer have been designed, [19–24] but Q. Ul Ain, J. Xia, H. Kanda, I. R. Alwani, X.-X. Gao, M. K. Nazeeruddin Institute of Chemical Sciences and Engineering Ecole Polytechnique Federale de Lausanne (EPFL) CH-1951 Sion, Switzerland E-mail: Jianxing.xia@epfl.ch; mdkhaja.nazeeruddin@epfl.ch Q. Ul Ain, A. Ahmed Khan Department of Physics Lahore University of Management Sciences Lahore, Punjab 54792, Pakistan E-mail: ammar.ahmed@lums.edu.pk Q. Ul Ain, H. U. Rehman Department of Chemistry and Chemical Engineering Lahore University of Management Sciences Lahore, Punjab 54000, Pakistan G. Shao School of Chemistry Sun Yat-Sen University Guangzhou 510006, P. R. China V. Jankauskas, K. Rakstys Department of Organic Chemistry Kaunas University of Technology 50254 Kaunas, Lithuania The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/solr.202200920. DOI: 10.1002/solr.202200920 Hexakis(hexyloxy)triphenylene (HAT6) discotic liquid crystal is employed as a transparent hole-transporting material (HTM) for perovskite solar cells (PSCs) and a power conversion efficiency (PCE) of 15.7% is obtained, which is the highest using HAT6 type of HTMs. Despite lower PCE than spiro-OMeTAD- based devices (20.3%), the PSCs based on HAT6 exhibit much higher ambient and thermal stability. A fused polyaromatic core with six alkyl chains leads to high hydrophobicity, and the π-stacked molecular columns of the HAT6 shield the bis(trifluoromethane)sulfonamide lithium salt dopant migration into the perov- skite absorber. The PSCs (under N 2 condition) exhibit superior stability compared to the devices employing spiro-OMeTAD, retaining nearly 92% of their initial efficiency after 1200 h operation. Under ambient conditions, HAT6-based hole- transportation layer devices retain 93% of the initial efficiency for 690 h. Under continuous thermal stress of 85 °C, the devices based on HAT6 retain 95% of the initial PCE. The results demonstrate the time applicability of liquid crystal for stable PSCs fabrication. RESEARCH ARTICLE www.solar-rrl.com Sol. RRL 2022, 2200920 2200920 (1 of 7) © 2022 Wiley-VCH GmbH