Gorkem Gunbas^1,2,Ummugulsum Gunes^1,2,3,Figen Varlioglu Yaylali^1,2,Zeynep Gozukara Karabag^1,2,3,Aliekber Karabag^1,2,Xiao-Xin Gao³,Olga Syzgantseva⁴,Bensu Cel Yildirim^1,2,Naoyuki Shibayama⁵,Hiroyuki Kanda³,Alwani Imanah Rafieh³,Liping Zhong³,Andreas Züttel³,Joseph Paul Dyson³,Selcuk Yerci^1,2,Mohammad Khaja Nazeeruddin³

ODTU-GUNAM¹,Middle East Technical University²,École Polytechnique Fédérale de Lausanne³,Lomonosov Moscow State University⁴,University of Hyogo⁵

Abstract

ODTU-GUNAM¹,Middle East Technical University²,École Polytechnique Fédérale de Lausanne³,Lomonosov Moscow State University⁴,University of Hyogo⁵

Perovskite solar cells (PSCs) became a household name in the solar energy community in less than a decade, and rightly so, by combining ease of fabrication and high efficiencies. However, the long-term stability and reproducibility are far from ideal for the commercial implementation of the technology. Stability of PSCs are dependent on several factors and encapsulation can only be a remedy, to a certain extent, for moisture and oxygen related degradations. Interfacial molecular dissociation and ion migration are main concerns for perovskites apart from environmental factors and several approaches have been pursued to address these issues. Surface passivation with 2D perovskites (2DPs) became an impactful strategy to achieve improved stability accompanied with enhanced performance. Phenylethyl ammonium iodide (PEAI) and its derivatives are the mostly commonly utilized cations for 2DPs, although various other cations have been implemented. We envisioned that strongly electron releasing groups are the clear choice as substituents on PEAI, due to their potential to contribute hole extraction and favorable interaction with uncoordinated Pb2+ ions. We concentrated on the electron releasing methoxy (-OMe) group substituted at -ortho, -meta, and -para positions of the phenyl ring of PEAI to explore not only the effect of the chemical identity but also the effect of substituent position. We developed a three step, straightforward synthetic methodology, which gave us easy access to all target cations. XRD, GIWAXS and SEM analyses confirmed the formation of 2D films on 3D perovskites, independent of the substituent position. PL and TRPL studies revealed that for the o-OMe-PEAI treated surfaces, strong passivation, hence, lower defect densities were achieved, compared to reference along with m-OMe and p-OMe analogues. Device results showed that o-OMe-PEAI based cells outperformed the reference, m-OMe and p-OMe based ones, showing PCEs over 23.3% with excellent Voc(1.2V) and FF(80%). Highest efficiencies for m-OMe-PEAI, p-OMe-PEAI, and reference cells were 22.2%, 22.6% and 21.4% respectively. Detailed DFT analyses attributed the outlier performance of o-OMe-PEAI incorporated cells to the favorable formation energy and desired vertical orientation along with optimal surface coating of the 2DP layer. One major advantage of 2DP passivation is the increased hydrophobicity of the surface due presence of large organic cations, thus higher stability. Contact angle analyses showed that while all salts showed more hydrophobic character compared to reference, o-OMe-PEAI treated surfaces were far superior. Hence, unique orientation of the o-OMe-PEAI on 3D perovskite surface improves performance and provides best protection for the 3D perovskite layer against moisture, simultaneously. Long term stability studies are currently underway however, as of 300 hours, o-OMe-PEAI based devices preserve 95% of their initial efficiencies whereas reference cells only retain 60%. Encouraged by these results, we pursued a larger set of cations to gain further insight to the effect of chemical identity and position of substituents on 2DP passivation and device performance. Here, we synthesized 9 salts, with fluorine, chlorine and bromine as the substituents at -ortho, -meta, and -para positions. Interestingly, independent of the halogen, salts substituted at -meta position outperformed their -ortho and -para counterparts (averages = m-XPEAI: 23.2%, o-XPEAI: 22.5%, p-XPEAI: 22.1, ref.: 20.8%). Among different halogens at -meta position, m-BrPEAI gave only slightly higher PCE. DFT analyses revealed that lower formation energies and higher interfacial dipoles achieved by meta-substituted derivatives was responsible for the enhanced performance. We are currently creating a larger library of ammonium cations with electronically distinct substituents to device a general set of design principles that could hopefully lead to the optimal material for commercial 2D/3D PSCs.