(TTF)Pb2I5: A Radical Cation-Stabilized Hybrid Lead Iodide with Synergistic Optoelectronic Signatures

Abstract

(TTF)Pb 2 I 5 : a Radical Cation-Stabilized Hybrid Lead Iodide with Synergistic Optoelectronic Signatures Hayden A. Evans, †,‡,§ Anna J. Lehner, § John G. Labram, ∥ Douglas H. Fabini, §,⊥ Omar Barreda, § Sara R. Smock, ‡ Guang Wu, ‡ Michael L. Chabinyc, ∥ , ⊥ * Ram Seshadri, †,‡,§,⊥ * and Fred Wudl ‡,§,⊥ * Mitsubishi Chemical Center for Advanced Materials, University of California, Santa Barbara, CA 93106 Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106 Materials Research Laboratory, University of California, Santa Barbara CA 93106 California NanoSystems Institute, University of California, Santa Barbara CA 93106 ⊥ Materials Department, University of California, Santa Barbara, CA 93106, USA. ABSTRACT: Hybrid organic inorganic materials offer the opportunity to combine functionalities of two distinct chemis- tries. Tetrathiafulvalene (TTF) is the precursor to some of the first organic metals, and perovskite lead iodides have re- cently proved their efficacy as photovoltaic materials. Here we describe a hybrid material that incorporates both, compris- ing TTF radical cations stabilizing an anionic layered lead–iodide network. Preparation, property analysis, and DFT calcu- lations are reported, verifying that (TTF)Pb 2 I 5 is a narrow gap semiconductor with optical properties synergistically influ- enced by the TTF +• radical cation. This radical cation contributes to states in the electronic gap formed by orbitals in the inorganic framework. Hybrid organic-inorganic materials can potentially combine the properties of highly tunable functional or- ganics with those of complementary inorganic networks in a single material. Distinct control over what is desired from each component becomes possible in such materi- als, allowing the functions of the organic and inorganic components to work synergistically. Tetrathiafulvalene (TTF), the organic component in the new hybrid material reported here, is a precursor to some of the early organic metals and was a clear choice when choosing a functional organic. It was first reported by Wudl et al. in the early 1970's, 1,2 and has been studied for its impressive electron donating abilities, 3,4 including derivatives displaying su- perconductivity. 5 Hybrid compounds containing TTF have also shown novel electronic and magnetic properties. Devic et al. 6,7 reported a ethylenedithio-1,2-diiodotetrathiafulvalene (EDT-TTF-I 2+• ) hybrid with metallic character, and showed its metal-insulator transition to be preserved within a hybrid compound with PbI 2 . Kondo et al. 8 simi- larly reported TTF +• hybrids containing Sn and Cl/Br, all which display semiconducting behavior due to interac- tions through the extended TTF networks. Magnetic compounds with TTF derivatives have also been made with transition metal halides, and found to be composed of two kinds of donor columns which magnetically couple with the anions through close contact between metal hal- ide interactions. 9 Metal-organic-frameworks (MOFs), a related class of hybrid materials, generally possess poor electronic functionality, but attempts to prepare electri- cally conductive MOFs have recently been made. Exam- ples include incorporating a conjugated molecule within the framework directly, 10 as a guest that electronically couples metal atoms in the framework using (TCNQ), 11 or post-synthetic modification of an insulating MOF via chemical leaching. 12,13 Stavila et al. 14 have recently re- viewed this topic. Layered lead halides, in their own right, are an attractive set of materials with rich optoelectronic and semiconducting behavior. The extensive work of Mitzi and coworkers 15,16 in the late 90’s first highlighted how amines and diammines combine with main group halides to form unique layered perovskite structures, which moti- vated the chemistry of organic ammonium cation “spac- ers” to flourish. Following the work of Kojima et al. 17 demonstrating the use of perovskite CH 3 NH 3 PbI 3 materi- als as solution deposited photovoltaics, there has been a huge flurry of research in this space to enhance the stabil- ity of the air unstable compound. This has included the use of ammonium spacer chemistry by Smith et al. 18 which improved stability of a layered perovskite incorpo- rating phenylethylammonium ions into the structure. Cao et al. 19 also explored this idea with butylammonium in- stead of phenylethylammonium, and reported that the extent of ammonium spacer incorporation as well as growth direction of subsequent films had significant im-