Lead-Free (CH3NH3)2CuClxBr4-x Perovskites

DANIELE CORTECCHIA^a, HERLINA DEWI^b, SUBODH MAISALKAR^b, ANNALISA BRUNO^b, TOM BAIKIE^b, PABLO BOIX^b, Nripan Matthews^b, CESARE SOCI^c, JUN YIN^c, SHI CHEN^c

^aEnergy Research Institute @ NTU (ERI@N), Research Technoplaza, Nanyang Technological University, Singapore, Nanyang Drive, Singapore, Singapore

^bEnergy Research Institute @ NTU (ERI@N), Research Technoplaza, Nanyang Technological University, Singapore, Nanyang Drive, Singapore, Singapore

^cNTU Singapore - Nanyang Technological University, Physics and Appld Physics, Nanyang Avenue, 50, Singapore, Singapore

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, DANIELE CORTECCHIA, 198
Publication date: 5th February 2015

Organolead-halide-perovskite solar cells have reached an astonishing NREL-certified power conversion efficiency of 20.1% in just four years, revolutionizing the roadmap of third-generation photovoltaic devices.¹ Although the main sensitizer methylammonium lead iodide CH₃NH₃PbI₃is very attractive for the excellent absorption and transport properties, its high lead content is a stunning barrier toward commercialization due to toxicity and environmental issues. Development of alternative “green perovskites” based on earth abundant and environmentally friendly materials is therefore urgently needed. Hybrid perovskites with general formula (CH₃NH₃)₂MX₄ based on transition metals are amongst the most interesting candidates to replace the lead-based ones. ²^,³Here we present the synthesis and characterization of novel two-dimensional (2D) Cu-based hybrid perovskites (having layered structure as shown in Figure 1a) for photovoltaic applications as an alternative to 3D lead-based perovskites. The series with general formula (CH₃NH₃)₂CuCl_xBr_4-x was studied and the optoelectronic properties analyzed in relation to the Br/Cl ratio. In agreement with the predictions by our first-principle calculations, we found that the band-gap associated to metal-to-ligand charge transfer (CT) transitions can be tuned from 2.48 eV to 1.8 eV by increasing Br content, while d-d transitions further extend the optical absorption up to 900 nm (Figure 1b). The overall photovoltaic performance of these copper-based perovskites is found to depend strongly on solution deposition process and device architecture (e.g. solar cells based on sensitized mesoporous TiO₂ or inverted structures with flat heterojunctions). Strategies currently pursued to increase photovoltaic efficiency, such as control of Cl^- content and fluorine doping to stabilize Cu²⁺ against reduction and to optimize the thin film morphology will be proposed and discussed.
Figure 1- a) Crystal structure of the 2D hybrid perovskite MA2CuCl2Br2, consisting of alternating organic and inorganic layers; b) absorption coefficient for perovskites of the series MA2CuClxBr4-x showing strong CT bands below 650 nm and broad d-d transitions between 700 nm and 900 nm (inset).
1 Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The emergence of perovskite solar cells. Nat Photon 2014, 8, 506-514. 2 Mitzi D. B. Synthesis, Structure, and Properties of Organic-Inorganic Perovskites and Related Materials. Progress in Inorganic Chemistry. John Wiley & Sons, Inc., 2007, 1-121. 3 Cheng, Z.; Lin, J. Layered organic-inorganic hybrid perovskites: structure, optical properties, film preparation, patterning and templating engineering. CrystEngComm 2010, 12, 2646-2662.

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