Contact formation of C 60 to thin films of formamidinium tin iodide
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Contact formation of C60 to thin films of formamidinium tin iodide Jonas Horn1,2, Derck Schlettwein1,2,a) 1
Institute of Applied Physics, Justus Liebig University Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany Center for Materials Research (LaMa), Justus Liebig University Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany a) Address all correspondence to this author. e-mail: [email protected] 2
Received: 30 June 2020; accepted: 11 September 2020
Lead-free perovskite layers may provide a good alternative to the commonly used lead-halide-based perovskite absorber layers in photovoltaics. Energy level alignment of the active semiconductor with contact layers is a key factor in device performance. Kelvin probe force microscopy was used during vapor deposition of C60 onto formamidinium tin iodide to investigate contact formation with detailed local resolution of these materials that are significant for photovoltaic cells. Significant differences dependent on the growth rate of C60 were detected. Sufficiently high deposition rates were essential to reach compact C60 films needed for good contact. A space charge layer larger than 90 nm within the C60 layer was established without indication of interfacial dipoles. The present analysis gives a clear indication of a well-functioning contact of fullerenes to formamidinium tin iodide that is suitable for the use in photovoltaic devices provided that thin compact fullerene films are formed.
Introduction During the last decade, organic–inorganic halide perovskites proved as a potential game changer in third-generation photovoltaics [1] and, further, showed potential as semiconductor for other optoelectronic devices, e.g., photodetectors [2] or lightemitting diodes [3, 4]. However, the presence of toxic Pb in most perovskite materials used in various devices is regarded as problematic [5], mainly because of water solubility of Pb (II) salts and possible leakage following mechanical damage [6], but also regarding problems of disposal or recycling after a device life cycle [7]. Hence, the substitution of Pb is highly desirable. Presently, two main approaches can be seen to mimic the beneficial structural and electronic properties of Pb: either the combination of Bi(III) and Ag(I) in double perovskites Cs2AgBiBr6 [8, 9] or more successfully the use of Sn(II) [10, 11]. Formamidinium tin iodide (FASnI3) showed impressive improvements of a power conversion efficiency, PCE = 9.47% [12]. With a slightly modified absorber using phenylethylammonium as an additive, even 12.4% were reached [13]. However, PCE and stability of such lead-free devices still are significantly lower than those reported for leadbased perovskite solar cells, surpassing the limit of 25% [1].
Further improvement of PCE of tin-based devices is needed not only to compete with lead-based cells in terms of efficiency but also regarding sustainability aspects of such lead-free systems [7]. One of the major issues concerning the use of tin-based perovskite materials is the oxidation of Sn(II) to S
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