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Letter 2
Roles of Pseudo-Closed s Orbitals for Different Intrinsic Hole Generation Between Tl-Bi and In-Bi Bromide Double Perovskites Zewen Xiao, Yanfa Yan, Hideo Hosono, and Toshio Kamiya J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02949 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 27, 2017
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The Journal of Physical Chemistry Letters
Roles of Pseudo-Closed s2 Orbitals for Different Intrinsic Hole Generation between Tl−Bi and In−Bi Bromide Double Perovskites Zewen Xiao,† Yanfa Yan,‡,* Hideo Hosono,†,§ and Toshio Kamiya†,§,* †
Materials Research Center for Element Strategy, Tokyo Institute of Technology, Yokohama
226-8503, Japan ‡
Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and
Commercialization, The University of Toledo, Toledo, Ohio 43606, United States §
Laboratory for Materials and Structures, Tokyo Institute of Technology, Yokohama 226-8503,
Japan AUTHOR INFORMATION Corresponding Author *Y.Y.:
[email protected]; T.K.:
[email protected] ACS Paragon Plus Environment
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ABSTRACT: Although metal halide double perovskites A2B(I)B(III)X6 are expected as nontoxic alternatives for lead halide perovskites, recent studies have shown that only Tl(I)−Bi(III) and In(I)−Bi(III) bromides are thermodynamically stable and possess optoelectronic properties suitable for photovoltaic absorbers. Here, we show, through density functional theory calculations, that Tl−Bi and In−Bi bromide double perovskites exhibit significantly different semiconducting behaviors due to the different energy levels of the highest-occupied pseudoclosed s2 orbitals of Tl(I) and In(I). While Tl−Bi double perovskites can exhibit semiconducting p-type properties, In−Bi bromide double perovskites exhibit metallic p-type ones regardless of the synthesis condition due to the extremely low formation energy of In vacancy. Such difference makes Tl−Bi bromide double perovskites suitable for solar cell applications, but not In−Bi bromide double perovskites. Furthermore, there is a high probability for In to substitute a Bi site, forming a local In−In bromide double perovskite structure with a lower local conduction band minimum, detrimentally affecting the open circuit voltage of In−Bi bromide double perovskite-based thin film solar cells.
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The Journal of Physical Chemistry Letters
Despite the rapid improvement in the record power conversion efficiency (PCE),1,2 the commercialization of lead (Pb) halide perovskite (ABX3) solar cells is still facing serious challenges such as the toxicity of Pb and instability against moisture air & temperature. Extensive efforts have been paid to searching non/low-toxicity and air-stable halide perovskitebased solar cell absorbers.3 Mutating two Pb(II) by a pair of a monovalent and a trivalent cation to form B(I)−B(III) halide double perovskites is a rational approach to explore Pb-free perovskite alternatives.4–6 So far, several such Pb-free halide double perovskites have been synthesized and characterized;4–11 however, most of the thin-film solar cells using the metal halide double perovskite absorbers have not achieved practically high PCEs e.g. > 10 %.12 For example, Cs2AgBiBr6 double perovskite-based solar cells by Greul et al. shows reasonable PCEs ~2.5%, but are not satisfactory. A recent density functional theory (DFT) study has shown that among all possible B(I)−B(III) halide double perovskites, only those with B(I) = In, Tl (though Tl is much toxic than Pb) and B(III) = Sb, Bi show optical absorption and electronic properties suitable for thin-film solar cell applications.13 Theoretical studies have also shown that the superior photovoltaic properties of Pb halide perovskite absorbers are attributed to the 3D electronic dimensionality associated with the perovskite structure and the unique features of Pb(II), i.e., the Pb 6s2 states of the valence band maximum (VBM).3,14 Since the double perovskites with B(I) = In, Tl and B(III) = Sb, Bi also exhibit a 3D electronic dimensionality with ns2 VBM structures, they are expected to be absorbers for efficient solar cell applications. Furthermore, DFT calculations have shown that some of these double perovskites could be thermodynamically stable.15–17 In fact, (CH3NH3)2TlBiBr6 has been experimentally synthesized, which exhibits a bandgap of ~2.0 eV.8
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Besides optoelectronic properties, defect properties of the absorbers must be appropriate also for producing efficient solar cells. A promising absorber must exhibit a semiconductivity with sufficiently low majority carrier density of e.g.
