New Highly-Efficient and Stable non-Pb quasi-Perovskite Solar Cells based on Iodobismuthates with Rudorffite Structure
Ivan Turkevych a, Eisuke Ito a, Toshiyuki Urano a, Hideo Yamagishi a, Hiroshi Tomiyasu a, Said Kazaoui b, Koji Yamada c
a Chemical Materials Evaluation and Research Base (CEREBA), Higashi 1-1-1, AIST Central 5-2, Tsukuba, Ibaraki, 305-8565
b Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology, Higashi 1-1-1, AIST Central 5-2, Tsukuba, Ibaraki, 305-8565
c Department of Applied Molecular Chemistry, Nihon University, Izumi-cho 1-2-1, Narashino, Chiba, 275-8575
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of International Conference Asia-Pacific Hybrid and Organic Photovoltaics (AP-HOPV17)
Yokohama-shi, Japan, 2017 February 2nd - 4th
Organizers: Tsutomu Miyasaka and Iván Mora-Seró
Oral, Ivan Turkevych, presentation 119
Publication date: 7th November 2016

We firstly introduce here an FTO/c-TiO2/m-TiO2/Ag3BiI6/PTAA/Au solar cell fabricated via a solution process at ambient conditions that achieves 4.3% efficiency for the best cell. Our utterly new iodobismuthates are another family of photovoltaic halides with a general formula of AaBbXx (where A=Ag, Cu; B=Bi, Sb; X=Br, I; and x=a+3b) and a three-dimensional lattice that is based on edge-shared AX6 and BX6 octahedral units. We propose to name them "RUDORFFITES", since their prototype oxide NaVO2 was first reported by Rudorff in 1954. Methylammonium lead iodide perovskite has all properties that make it ideal photovoltaic material: solution processability, direct and moderate bandgap, defect tolerance and long electron diffusion length, except it is extremely toxic. Pb-free alternatives based Sn and Ge are highly unstable due to easy oxidation of Sn2+ and Ge2+ cations to 4+ valence states. Unfortunately, there are no other non-toxic divalent cations that are large enough to satisfy Goldschmidt tolerance rule for the formation of perovskite lattice with Iodine. Although a combination of monovalent and trivalent cations can result in the formation of C2ABX6 double-perovskites (where C=Cs, CH3NH3; A=Ag; B=Bi, Sb; X=Br, I) with alternating corner-shared AX6 and BX6 octahedral units, they are characterized by indirect bandgaps of over 2 eV. In contrast to perovskites, the rudorffite structure is tolerant to large mismatches of ion sizes. Also, it can accommodate vacant sites in the cation sublattice that allows a wide variation of stoichiometry without breaking the charge neutrality rule. Rudorffites belong to R-3m space group with cation sublattice described as joint populations of monovalent (A: Ag, Cu), trivalent (B: Bi, Sb) and neutral vacant sites with different occupancies. We studied several members of this family, such as Ag3BiI6, Ag2BiI5, AgBiI4, AgBi2I7, CuBiI4, Cu2BiI5, though, we will show that many other stoichiometries and cation substitutions are possible. These materials feature direct bandgaps in the range of 1.79-1.83eV that corresponds to a theoretical photoconversion efficiency of ~27% while practical ~18% is possible by assuming open circuit voltage of 1.2V and typical assumptions about optical losses and fill factor for optimized solar cells. We would like to encourage attention to this new family of highly stable and promising photovoltaic rudorffites. We will present our experimental studies of their structural and optoelectronic properties accompanied by theoretical DFT calculations, as well as detailed performance characterization of solar cell devices.

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