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Earth-Abundant Non-Toxic Titanium (IV) Based VacancyOrdered Double Perovskite Halides with Tunable 1.0 to 1.8 eV Bandgaps for Photovoltaic Applications Minggang Ju, Min Chen, Yuanyuan Zhou, Hector F Garces, Jun Dai, Liang Ma, Nitin P Padture, and Xiao Cheng Zeng ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01167 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017
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ACS Energy Letters
Earth-Abundant Non-Toxic Titanium (IV) Based Vacancy-Ordered Double Perovskite Halides with Tunable 1.0 to 1.8 eV Bandgaps for Photovoltaic Applications Ming-Gang Ju, †,# Min Chen, ‡ ,# Yuanyuan Zhou,‡,* Hector F. Garces,‡ Jun Dai,† Liang Ma,† Nitin P. Padture ‡,* and Xiao Cheng Zeng †,* †
Department of Chemistry, University of Nebraska-Lincoln, Nebraska 68588, United States
‡
School of Engineering, Brown University, Providence, Rhode Island 02912, United States
For: ACS Energy Letters (Manuscript ID nz-2017-01167m) Abstract The possibility of lead (Pb) contamination and the volatility of the organic cations in the prevailing Pb-based halide perovskite (HP) light-absorbers are the two most important issues of concern in the emerging perovskite solar cells (PSCs). The majority of the Pb-free HP candidates that are being explored for PSCs fall either suffer from instability issues and have unfavorable defect properties, or have unsuitable bandgaps for PSC applications. Here, we report the prediction of a promising new family of all-inorganic HPs based on the non-toxic, earthabundant, ultra-stable Ti(IV) for use in PSCs. We show that the Ti-based HPs possess a combination of several desirable attributes, including suitable bandgaps, excellent optical absorption, benign defect properties, and high stability. In particular, we show experimentally that representative members of the Ti-based HP family, Cs2TiIxBr6-x, have bandgaps that can be tuned between the ideal values of 1.38 eV and 1.78 eV for single-junction and tandem photovoltaic (PV) applications, respectively.
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Halide perovskites (HPs) are a family of semiconductor materials that have shown great promise as light-absorbers in the emerging perovskite solar cells (PSCs).1–4 Owing to the exceptional optoelectronic properties of HPs, the record power conversion efficiency (PCE) of PSCs has surpassed 22% within only a few years.4 Despite the rapid surge in the record PCE, there are two key issues regarding HP materials that need to be resolved before PSCs technology can be widely commercialized. First, the state-of-the-art HP materials in the high-PCE PSCs all contain toxic lead (Pb) — the use of Pb in devices is severely restricted in many countries and regions.5,6 Second, all the HPs in the highest-performing PSCs contain organic cations, such as formamidinium (FA+) or methylammonium (MA+),1,2,7 which are highly hygroscopic and volatile. This renders the state-of-the-art HPs intrinsically unstable, with inadequate tolerance for environmental (thermal, moisture) stresses.8 Thus, finding all-inorganic Pb-free HPs that can address both these issues, without sacrificing some of the key properties, e.g., bandgaps, optical absorption, benign defects, for PSCs is still a great challenge, although previous efforts in this area have met with some success.9,10 It is widely recognized that replacing MA+ and/or FA+ by cesium (Cs+) in HPs can significantly enhance their thermal/moisture stability.11 Also, it has been shown that Pb2+ can be replaced by other cations, such as tin (Sn2+),9,10,12 germanium (Ge2+),13 bismuth (Bi3+),14 antimony (Sb3+),15 indium (In+),16,17 and silver (Ag+).18-20 It is then
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ACS Energy Letters
natural to combine the two ion-substitution strategies in searching for non-toxic, all-inorganic HPs for PSCs. However, majority of the recently developed Pb-free, all-inorganic HPs have bandgaps that are not suitable for single-junction and/or tandem PV applications. CsSnI3 HP with a bandgap of ~1.3 eV is an exception, which is why it has drawn the most attention.21–23 Despite substantial effort, the development of efficient CsSnI3-based PSCs is severely hampered by the metallic conductivity of the CsSnI3 HP.15,22,23 Furthermore, Sn2+ is extremely sensitive to moisture and oxygen in the ambient.21 Thus, preventing CsSnI3 HP from degrading is much more challenging than its Pb-based HP counterparts. Furthermore, there is certain level of toxicity in Sn-based HPs, as reported by Babayigit et al.24 In this context, the compound Cs2SnI6 with a special double-perovskite crystal structure, containing ordered vacancies due to the +4 oxidation state of Sn, has also been proposed as a candidate for use in PSCs.25 However, Cs2SnI6 is found to exhibit insufficient stability, and it contains intrinsically deep defects that are detrimental to the PSC performance.26 In fact, many non-toxic transition-metals possess stable +4 oxidation state, and, thus, there are opportunities for finding new promising HP materials for PSCs by replacing Sn4+ in Cs2SnI6 by other transition-metal cations. This is confirmed by a recent study by Sakai et al.27 who reported Cs2PdBr6 as a possible HP for use in PSCs. However, Cs2PdBr6based PSCs has not been demonstrated as yet, and it contains the noble element Pd, making it potentially unaffordable. In this integrated theoretical and experimental study, we unravel a promising new family of vacancy-ordered halide double perovskites based on the earth-abundant, non-toxic, and ultrastable Ti(IV) for use in PSCs. This HP family has a general chemical formula A2TiX6 (A = K+, Rb+, Cs+, In+, MA+, or FA+; X = Cl, Br, or I. Our first-principles density functional theory (DFT) calculations show that several materials in this HP family possess desired optical/electronic
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properties, e.g. a suitable bandgap within the range 1.0-1.8 eV, as well as tunable defect properties. We have also synthesized experimentally a series of Cs2TiIxBr6-x (x = 0, 2, 4, 6) HPs using the melt-crystallization method. Both theoretical computations and experimental measurements show that the bandgap of Cs2TiIxBr6-x HPs can be tuned continuously from 1.02 to 1.78 eV. In particular, Cs2TiI2Br4 and Cs2TiBr6 and HPs exhibit bandgaps of 1.38 eV and 1.78 eV, which are ideal for application in single-junction PSCs and tandem PVs, respectively. The stability and processability of the newly synthesized HPs are demonstrated, attesting to the promise of their use in PSCs.
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ACS Energy Letters
Figure 1. (A) Schematic crystal structure of A2TiX6 HPs. (B) Computed bandgaps of various HPs using HSE06 functional. The range of optimal bandgaps for solar-cell materials is highlighted by the horizontal band. (C) Computed optical absorption spectra (based on the HSE06 functional) of several predicted HPs, compared with absorption spectra of Si and MAPbI3 HP. The absorption coefficient of MAPbI3 is computed using the PBE functional without considering the SOC effect. Note that the computed absorption spectrum of MAPbI3 is coincidentally in good agreement with the experimental spectrum due to the cancellation of errors by using the PBE functional without considering SOC. (D) Computed band structure of Cs2TiI6 basing on primitive unit cell (Fig. S1) using PBE functional; here Γ (0.0, 0.0, 0.0), W (0.5, 0.25, 0.75), L (0.5, 0.5, 0.5), X (0.5, 0.0, 0.5) and K (0.375, 0.375, 0.75) refer to the high-symmetry special points in the first Brillouin zone. (E) Computed DOS and projected DOS of Cs2TiI6 using HSE06 functional.
Like Cs2SnI6, all the optimized structures of vacancy-ordered A2BX6 HPs considered here are K2PtCl6-type (see Fig. 1A).25,28 The A2BX6 HPs may be viewed as a derivative structure of the conventional ABX3 HPs in which every other B2+ cation is missing. The interrupted solidstate framework results in isolated [BX6]2- octahedra and discrete anions, akin to molecular salts.25 Since the bandgap of the light-absorbing material in PVs is one of the most important properties, the computed bandgaps for all HPs considered here were computed (Fig. 1B). For the same A-site cation, the bandgap increases gradually in the order I
