Identifying Field Effect Passivation as the Origin of the Voltage Enhancing LiF Interlayer at the Perovskite/C₆₀ Interface

In metal halide perovskite (MHP) solar cells with p-i-n architecture, the electron selective contact is often realized using the fullerene C₆₀. However, the C₆₀/perovskite interface is known to limit the open circuit voltage (<i>V</i>_oc) of the solar cells. This might be caused by non-radiative recombination, an unfavorable conduction band offset at the interface, or a combination thereof [1]. It has been shown that with an ultra-thin interlayer of LiF the <i>V</i>_OC can be enhanced by around 50 meV [2], due to reduced non-radiative recombination [3]. Yet, the exact passivation mechanism is still a question of ongoing research.<br/>In this contribution we investigate the effect of such a thin LiF interlayer on the interface of a state-of-the-art multi cation multi halide perovskite composition, Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃ (CsMAFA, <i>E</i>_G = 1.63 eV), and thermally evaporated C₆₀ by means of photoelectron spectroscopy [4]. MHPs exhibit a characteristically low density of states at the valence band maximum (VBM) [5]. To access the energetic position of the VBM and defect states within the band gap, we employ a rarely used technique: near-UV photoemission spectroscopy in constant final state mode (CFSYS). CFSYS allows to directly trace the density of occupied states with a very high dynamic range of up to seven orders of magnitude. Previously, we were thus able to reveal the VBM and to quantify occupied states in the band gap of CsMAFA [6]. Furthermore, with the low photon energies used here, the information depth is strongly enhanced to 5 – 10 nm and thus, it is possible to study the buried interface of CsMAFA and C₆₀. <br/><br/>We refine our description of the perovskite’s valence band [6] to also model the superposition of the perovskite and C₆₀ density of occupied states for thin C₆₀layers. From a sample with 5 nm C₆₀ thickness, we can then directly determine the offset between the perovskite VBM and the C₆₀ HOMO-edge to 0.55 eV. Furthermore, considering the band gap of the MHP and C₆₀, the LUMO edge is found 0.26 eV below the perovskite CBM. This unfavorable line-up potentially contributes to the <i>V</i>_OC limitation by the C₆₀ interface. Interestingly, we find that introducing the LiF interlayer leads to an <i>enhanced</i> defect density in the first monolayers of C₆₀, which would lead to an increased trap assisted recombination, thus lower <i>V</i>_oc in a cell. Our CFSYS data shows, that this is overcompensated by a reduced hole density in the vicinity of the perovskite/C₆₀ interface, which can be caused by a small dipole and probably a positive fixed charge. Linking to established solar cell technologies, we propose that the LiF evokes a field effect passivation reducing the absolute non-radiative charge carrier recombination and leading to the improved <i>V</i>_oc. <br/> <br/>[1] Stolterfoht <i>et al.</i>, 2019, DOI: 10.1039/C9EE02020A<br/>[2] Al-Ashouri <i>et al.</i>, 2020, DOI: 10.1126/science.abd4016<br/>[3] Wolff<i> et al.</i>, 2019, DOI: 10.1002/adma.201902762<br/>[4] Menzel <i>et al.</i> 2022, accepted for publication in Advanced Energy Materials<br/>[5] Zu <i>et al.</i>, 2019, DOI: 10.1021/acs.jpclett.8b03728<br/>[6] Menzel<i> et al.</i>, 2021, DOI: 10.1021/acsami.1c10171