Conjugated Organic Cations to Improve the Optoelectronic Properties of 2D/3D Perovskites
Eva M. Barea a, Jesús Rodríguez-Romero a, Bruno Clasen Hames a, Iván Mora-Seró a
a Institute of Advanced Materials (INAM), Universitat Jaume I, 12006 Castellón de la Plana, Castellón, España, Castellón de la Plana, Spain
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Oral, Eva M. Barea, presentation 038
Publication date: 11th February 2019

Two-dimensional (2D) hybrid perovskites (HPVKs) are structures alternating organic and inorganic layers, arising from inclusion of a large organic cation providing Goldschmidt’s tolerance factor higher than 1.1 This fact generates separation of a determined number of inorganic layers (n), which can range from 1 to ∞, which corresponds to a 3D arrangement. A variation of this pure organic−inorganic structure can be obtained by addition of a small organic cation, MA (CH3NH3+) in most cases, providing a Goldschmidt’s tolerance factor adequate for perovskite formation, making it so the inorganic part becomes a hybrid structure. These organic−inorganic hybrid structures are called 2D/3D HPVKs. Actually, 2D/3D perovskite-based solar cells have emerged as an alternative to pure 3D perovskites with the aim to improve their long-term stability, which is a key factor in future device commercialization.2,3

In the last years, several studies have been reported on 2D/3D perovskites prepared using mainly two ammonium salts: (i) phenylethylammonium iodide (PEA), with 4.73% efficiency for a layered material (n = 3)4 and 15.3% for a quasi-3D material (n = 60)5 and (ii) butylammonium iodide (BAI) (4.02−12.52%), observed only in layered materials, i.e., n ≤5).6−8 Recently, 1 year stable 2D/3D perovskite-based devices fabricated using 5-ammonium valeric acid iodide with an efficiency up to 11% have been shown.2

The results obtained so far can be considered very limited because, in general, the studies have focused only on aliphatic nonconjugated ammonium salts. Although it is true that the moisture stability increases, it is also true that the photovoltaic performance of 2D/3D PVK materials is severely limited owing to quantum and dielectric confinement effects. Accordingly, it is necessary the synthesis and deep optical characterization of materials with an adequate management of dielectric contrast between the layers. In this study, we demonstrate the successful tuning of dielectric confinement by the inclusion of a conjugated molecule, anilinium cation, as a bulky cation, in the fabrication of the 2D/3D PVK material (C6H5NH3)2(CH3NH3)n-1PbnI3n+1, where n=3-5.

The absence of excitonic states related to n ≥ 1 at room temperature, as well as the very low concentration of excitons after 1 ps of excitation of samples in which n ≥ 3, provide strong evidence of an excellent ability to dissociate excitons into free charge carriers. As consequence films with low n, presenting higher stability than standard 3D perovskites, improved significantly their performance, showing one of the highest short circuit current density (Jsc=13.8) obtained to date for perovskite materials within the 2D limit (n < 10).

In this study, we demonstrate that the use of a conjugated anilinium cation, with a cloud of free and polarizable p-electrons, in the preparation of a 2D/3D PVK leads to the production of a material with improved optical and electrical properties. The determining factor is the tuning of the dielectric contrast between the inorganic and organic layers, which has a direct influence on the decrease in the exciton binding energy. Using steady-state and femtosecond time-resolved absorption experiments, we have proved the high efficiency of the dissociation of initially generated excitons, even in samples with a low n value (about 3), and thus the high potential of these structures in photovoltaics. And surprisingly, although the intrinsic nature of employed cation suggests a significant interaction with water, devices fabricated with a 2D/3D perovskite displayed higher stability (~70%) after 288 h than those based on a 3D one (<40%).

Pavel Galar and Abderrazzak Douhal. Departamento de Química Física, Facultad de Ciencias Ambientales y Bioquímica, and INAMOL, Universidad de Castilla-La Mancha, Avenida Carlos III S.N., 45071 Toledo, Spain.

Azhar Fakharuddin and Lukas Schmidt-Mende. Department of Physics, University of Konstanz, 78457 Konstanz, Germany.

Isaac Suarez and Juan P. Martínez-Pastor. UMDO, Instituto de Ciencia de los Materiales, Universidad de Valencia, Valencia 46071, Spain.

© Fundació Scito
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