Matteo Pitaro1,2,Javier Alonso1,Lorenzo Di Mario1,karolina Tran1,Malin Johansson2,Erik Johansson2,Maria Antonietta Loi1
University of Groningen1,Uppsala University2
Matteo Pitaro1,2,Javier Alonso1,Lorenzo Di Mario1,karolina Tran1,Malin Johansson2,Erik Johansson2,Maria Antonietta Loi1
University of Groningen1,Uppsala University2
Highly performing mixed tin-lead perovskite materials are among the most promising options as an alternative active layer in perovskite solar cells to reduce Pb content. Moreover, these compounds open the possibility of fabricating full perovskite tandem devices, owing to their reduced band gap. The most efficient single junction mixed Sn/Pb perovskite solar cells have been fabricated using methylammonium cations (MA<sup>+</sup>), and a p-i-n structure where poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT: PSS) is implemented as hole transport layer (HTL).<sup>[1]</sup> The record devices were reported to show limited stability, this can be attributed to two major reasons: i) MA<sup>+</sup> cations easily desorb at high temperature (85 C°) from the perovskite surface, introducing MA vacancies; ii) PEDOT: PSS, due to its hygroscopic and acid nature, reacts with the perovskite active layer, affecting the long-term stability.<br/>In this work we employed a MA<sup>+</sup> free perovskite composition, namely, Cs<sub>0.25</sub>FA<sub>0.75</sub>Sn<sub>0.5</sub>Pb<sub>0.5</sub>I<sub>3</sub> and 2-(9H-carbazol-9-yl) ethyl) phosphonic acid (2-PACz), and [2-(3, 6-dibromo-9H-carbazol-9-yl) ethyl] phosphonic acid (Br-2PACz) as hole transport layers, with the aim of replacing PEDOT: PSS. Moreover, the fact that 2PACz and Br-2PACz can form a monolayer may allow reducing parasitic recombination.<br/>Cs<sub>0.25</sub>FA<sub>0.75</sub>Sn<sub>0.5</sub>Pb<sub>0.5</sub>I<sub>3</sub> deposited on SAMs showed absence of pinholes, higher crystallinity (XRD), and larger grain size (SEM) when compared with the layers of PEDOT: PSS.<br/><br/>The fabricated solar cells using PEDOT: PSS as HTL exhibited a champion device efficiency of 16.33%, while devices fabricated on 2PACz and Br-2PACz showed an improved efficiency of 18.44% and 19.57%, respectively. The 19.57% efficiency is the record for the aforementioned perovskite composition.<br/>It furthermore interesting to note that encapsulated Br-2PACz based solar cells retained 80% of the initial efficiency after 230 hours under continuous working condition, while device fabricated on PEDOT: PSS maintained 79% only for 72 hours. In addition, the shelf-life test in N<sub>2</sub> atmosphere showed much more stable devices when using Br-2PACz, with 89% of the initial efficiency after 42 days, when compared to PEDOT: PSS based devices, which solely preserve the 71% of the initial PCE. Moreover, the Br-2PACz-based device retained the 80% of the initial efficiency after 138 days in N<sub>2</sub> atmosphere.<br/><br/>[1] S. Hu, K. Otsuka, R. Murdey, T. Nakamura, M. A. Truong, T. Yamada, T. Handa, K. Matsuda, K. Nakano, A. Sato, K. Marumoto, K. Tajima, Y. Kanemitsu, A. Wakamiya, <i>Energy Environ. Sci.</i> <b>2022</b>, 15, 2096-2107.