Functionalization of two-dimensional perovskites by incorporating carboxy group
Ryosuke Arai a, Masahiro Yoshizawa-Fujita a, Yuko Takeoka a, Masahiro Rikukawa a
a Sophia University, 7-1 KIoi-cho, Chiyoda-ku, Tokyo, 1028554, Japan
NIPHO
Proceedings of International Conference on Perovskite Thin Film Photovoltaics, Photonics and Optoelectronics (ABXPV18PEROPTO)
Perovskite Thin Film Photovoltaics (ABXPV18). 27-28 Feb
Rennes, France, 2018 February 27th - March 1st
Organizer: Jacky Even
Poster, Ryosuke Arai, 100
Publication date: 11th December 2017

Organic-inorganic perovskite solar cells have attracted significant attention due to their high power conversion efficiency over 22% and low fabrication cost. The low stability is the most serious problem. Two-dimensional (2D) perovskites with the general formula, A2PbX4 (A = RNH3+; X = Cl, Br, or I), have attracted a great deal interest due to their high stability. 2D perovskites naturally form layered structures consisting of 2D sheets of [PbX6]4- octahedra and organic ammonium (RNH3+) layers by van der Waals interactions between organic ammoniums, and ionic hydrogen bonds between halide anions and ammonium cations. The ability to design various analogues is also an important feature of 2D perovskites. By changing amine species, it is expected that the balance of interactions will be delicately changed, and new functionality will be generated. In this study, 2D perovskites were fabricated by using alkylamines with carboxy groups at the ends in order to generate new functionalities on the structure and surface properties.

Perovskite solution was prepared by reacting stoichiometric amounts of HOOC(CH2)6NH3I with PbI2 in N,N-dimethylformamide. [HOOC(CH2)6NH3]2PbI4 film was fabricated on pre-heated glass substrates at 100°C by spin-coating at 2000 rpm. [CH3(CH2)6NH3]2PbI4 spin-coated film was also fabricated by similar procedure for a comparison. From the X-ray diffraction (XRD) measurements, formation of a layered perovskite structure was confirmed. It is considered that hydrogen bonds were formed between carboxy groups in [HOOC(CH2)6NH3]2PbI4.1 The orientations of the 2D structures were investigated by grazing incident XRD. It was suggested that the crystalline growth of [HOOC(CH2)6NH3]2PbI4 was random compared with highly oriented [CH3(CH2)6NH3]2PbI4. The random crystal growth is advantageous for charge transfer in the direction perpendicular to the substrate. Contact angles of films were measured to investigate the surface properties. The bonding works with porous TiO2 T/SP (W T/SP) and with spiro-OMeTAD (W spiro-OMeTAD) which were main components of perovskite solar cells were calculated by the Owens-Wendt theory. As a result, each value for [HOOC(CH2)6NH3]2PbI4 and [CH3(CH2)6NH3]2PbI4 was W T/SP = 146, 129 mJ m-2 and W spiro-OMeTAD = 143, 123 mJ m-2, respectively. It was suggested that the adhesion to T/SP and spiro-OMeTAD was improved by incorporating carboxy groups. Incorporating carboxy groups into 2D perovskites was allowed to give functionality that random crystal growth and surface properties which are suitable for perovskite solar cells.

1. R. Arai, et. al., ACS Omega, 2, 2333 (2017).

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