Low-cost, nontoxic, and earth-abundant photovoltaic materials are long-sought targets in the solar cell research community. Perovskite-inspired materials have emerged as promising candidates for this goal, with researchers employing materials design strategies including structural, dimensional, and compositional transformations to avoid the use of rare and toxic elemental constituents, while attempting to maintain high optoelectronic performance. These strategies have recently been invoked to propose Ti-based vacancyordered halide perovskites (A 2 TiX 6 ; A = CH 3 NH 3 , Cs, Rb, or K; X = I, Br, or Cl) for photovoltaic operation, following the initial promise of Cs 2 SnX 6 compounds. Theoretical investigations of these materials, however, consistently overestimate their band gaps, a fundamental property for photovoltaic applications. Here, we reveal strong excitonic effects as the origin of this discrepancy between theory and experiment, a consequence of both low structural dimensionality and band localization. These findings have vital implications for the optoelectronic application of these compounds while also highlighting the importance of frontier-orbital character for chemical substitution in materials design strategies.

Cesium titanium halide perovskites are novel lead-free optoelectronic materials that have attracted attention in the last two years for their application in solar cells with power conversion efficiencies reaching 3.3%....

The use of colloidal quantum dots (CQDs) as a gain medium in infrared laser devices has been underpinned by the need for high pumping intensities, very short gain lifetimes, and low gain coefficients. Here, PbS/PbSSe core/alloyed‐shell CQDs are employed as an infrared gain medium that results in highly suppressed Auger recombination with a lifetime of 485 ps, lowering the amplified spontaneous emission (ASE) threshold down to 300 µJ cm−2, and showing a record high net modal gain coefficient of 2180 cm−1. By doping these engineered core/shell CQDs up to nearly filling the first excited state, a significant reduction of optical gain threshold is demonstrated, measured by transient absorption, to an average‐exciton population‐per‐dot 〈Nth〉g of 0.45 due to bleaching of the ground state absorption. This in turn have led to a fivefold reduction in ASE threshold at 〈Nth〉ASE = 0.70 excitons‐per‐dot, associated with a gain lifetime of 280 ps. Finally, these heterostructured QDs are used to achieve near‐infrared lasing at 1670 nm at a pump fluences corresponding to sub‐single‐exciton‐per‐dot threshold (〈Nth〉Las = 0.87). This work brings infrared CQD lasing thresholds on par to their visible counterparts, and paves the way toward solution‐processed infrared laser diodes.

Cs2TiBr6 stands as a promising Pb-free alternative perovskite material having demonstrated its potential in photovoltaics. However, it is considered to be highly unstable in air, hindering further improvements and raising...

Low-cost, non-toxic and earth-abundant photovoltaic materials are a long-sought target in the solar cell research community. Perovskite-inspired materials have emerged as promising candidates for this goal, with researchers employing materials design strategies including structural, dimensional and compositional transformations to avoid the use of rare and toxic elemental constituents, while attempting to maintain high optoelectronic performance. These strategies have recently been invoked to propose Ti-based vacancy-ordered halide perovskites (A₂TiX₆; A = CH₃NH₃, Cs, Rb, K; X = I, Br, Cl) for photovoltaic operation, following the initial promise of Cs₂SnX₆ compounds. Theoretical investigations of these materials, however, consistently overestimate their band gaps – a fundamental property for photovoltaic applications. Here, we reveal strong excitonic effects as the origin of this discrepancy between theory and experiment; a consequence of both low structural dimensionality and band localization. These findings have vital implications for the optoelectronic application of these compounds, while also highlighting the importance of frontier-orbital character for chemical substitution in materials design strategies.