CsSnI 3 is a representative all-inorganic and less toxic perovskite material. However, extreme structural and chemical instability of perovskite CsSnI 3 makes its optoelectronic applications highly challenging. Upon exposure to air and moisture, it immediately undergoes a phase transition to a thermodynamically more stable, but optoelectronically inactive, one-dimensional polymorph near ambient temperature and ultimately deforms into Cs 2 SnI 6 . To prohibit this undesirable process, perovskite CsSnI 3 has to be stored and treated restrictively in an inert atmosphere and encapsulated hermitically. Here, we demonstrate an unusual strategy to markedly enhance the air stability of perovskite CsSnI 3 . Namely, MI 3 (M = Sb, Bi) doping modifies the electronic band structure of perovskite CsSnI 3 . As a result, it is remarkable that its heat of formation reduces, being lower than that of its competing polymorph. Accordingly, otherwise thermodynamically unfavorable perovskite CsSnI 3 becomes more stable than the latter energetically, thereby preventing the undesirable phase transition. SbI 3 (3 mol %)-doped CsSnI 3 retains 96% of its perovskite structure, whereas pristine CsSnI 3 retains only 12% after 12 h of exposure to air with 45−55% relative humidity. MI 3 doping also reduces the energy band gap of perovskite CsSnI 3 . We employed first-principles density functional theory calculations to explain the origin of the enhanced stability and redshifted band gaps. Our current work demonstrates that electronic band structure engineering by chemical doping can be an effective means of controlling the phase stability of polymorphs, which are otherwise difficult to stabilize or unattainable. This strategy can be widely applied to materials with low stability but high technological importance.

Extraordinary properties of traditional hyperbolic metamaterials, not found in nature, arise from their man-made subwavelength structures causing unique light−matter interactions. However, their preparation requiring nanofabrication processes is highly challenging and merely provides nanoscale two-dimensional structures. Stabilizing their bulk forms via scalable procedures has been a sought-goal for broad applications of this technology. Herein, we report a new strategy of designing and realizing bulk metamaterials with finely tunable hyperbolic responses. We develop a facile two-step process: (1) self-assembly to obtain heterostructured nanohybrids of building blocks and (2) consolidation to convert nanohybrid powders to dense bulk pellets. Our samples have centimeter-scale dimensions typically, readily further scalable. Importantly, the thickness of building blocks and their relative concentration in bulk materials serve as a delicate means of controlling hyperbolic responses. The resulting new bulk heterostructured material system consists of the alternating h-BN and graphite/graphene nanolayers and exhibits significant modulation in both type-I and type-II hyperbolic resonance modes. It is the first example of real bulk hyperbolic metamaterials, consequently displaying the capability of tuning their responses along both in-plane and out-of-plane directions of the materials for the first time. It also distinctly interacts with unpolarized and polarized transverse magnetic and electronic beams to give unique hyperbolic responses. Our achievement can be a new platform to create various bulk metamaterials without complicated nanofabrication techniques. Our facile synthesis method using common laboratory techniques can open doors to broad-range researchers for active interdisciplinary studies for this otherwise hardly accessible technology.

Tin-based halide perovskite materials are promising ecofriendly light absorbers with similar optoelectronic properties to lead-based ones. However, their solar cells have suffered from considerably low open-circuit voltage (V OC ) arising from mismatched energy levels between tin-based perovskite and chargetransporting layers because they imitate typical device structures developed for lead-based ones. Herein, we report that the lowest unoccupied molecular orbital (LUMO) level of the electron-transporting layer (ETL) significantly affects V OC of tin-based perovskite solar cells (PSCs) in contrast to lead-based ones. The indene-C 60 bisadduct ETL with a much higher LUMO level than that of typical ETLs decreases an energy off-set with the conduction band minimum of the mixed formamidinium/phenylethylammonium tin iodide (FA 0.9 PEA 0.1 SnI 3 )-based perovskite material. The resultant reduced V OC loss at their interface gives a V OC of 0.651 V, the highest to date for FASnI 3 -based PSCs. The achieved champion power conversion efficiency reaches 7.05%. This result highlights the importance of redesigning device structures dedicated to tin-based PSCs.

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