Low Temperature, Vacuum-Processed Bismuth Triiodide Solar Cells with Organic Small-Molecule Hole Transport Bilayer

Herein, the preparation of fully vacuum-processed bismuth triiodide solar cells with low annealing temperature is reported. Planar n-i-p devices are prepared using a thin compact SnO2 layer as the electron extraction layer and an electron blocking/hole extraction bilayer consisting of an intrinsic and doped organic hole-transport molecule. Using this configuration, herein, higher fill-factors and overall power conversion efficiencies than with conventional solution-processed hole transport materials are achieved

Solvent-Free Synthesis and Thin-Film Deposition of Cesium Copper Halides with Bright Blue Photoluminescence

Non-toxic alternatives to lead halide perovskites are highly sought after for applications in optoelectronics. Blue-luminescent materials are especially demanded as they could be used to prepare white light-emitting diodes, with important potential applications in lighting systems. However, wide bandgap blue emitters with high photoluminescence quantum yields (PLQY) are typically more difficult to obtain as compared to green- or red-emitting ones. Here, we prepared two series of inorganic cesium copper halides, with the general formulas Cs3Cu2X5 and CsCu2X3 (X = Cl, Br, I, and mixtures thereof) by dry mechanochemical synthesis at room temperature. X-ray diffraction demonstrates quantitative conversion of binary precursors into the desired ternary structures and good halide mixing in single-phase compounds. We identified Cs3Cu2I5 as the most promising material as it maintains blue luminescence centered at 442 nm with high PLQY (>40%) after several days in air (Cs3Cu2Cl5 shows significantly higher PLQY over 80% but is unstable in air). Based on this, we fabricated homogeneous and pinhole-free Cs3Cu2I5 thin films by thermal single-source vacuum deposition. Crystalline phase and photoluminescence are maintained in the thin films, validating that these low-toxicity materials can be synthesized and processed by fully solvent-free routes for a widespread implementation in optoelectronic devices

Mechanochemical synthesis of inorganic halide perovskites: evolution of phase-purity, morphology, and photoluminescence

Dry mechanochemical ball-milling of halide precursor salts is a promising route for the synthesis of high-purity halide perovskites in a fast and solvent-free manner. However, there is a lack of information on the process mechanisms, kinetics, and possible side-effects. Here, we investigated in detail the mechanochemical synthesis of fully-inorganic CsPbBr3 by ball-milling of stoichiometric CsBr and PbBr2. Detailed structural, morphological and optical analyses reveal several beneficial and detrimental effects of milling as a function of time. Three stages are identified during the process: (i) at short milling times (t < 5 min) different ternary compounds are formed, including stoichiometric CsPbBr3 as well as Cs4PbBr6, and to a lesser extent, CsPb2Br5. Photoluminescence from "nano" and "bulk" CsPbBr3 species is observed, centered at 525 nm and 545 nm, respectively. (ii) At the optimum time (around 5 min for the present case) the complete transformation of all reactants and byproducts into phase-pure CsPbBr3 has occurred. Photoluminescence corresponds to bulk CsPbBr3; (iii) at much longer milling times (up to 10 hours) eventually smaller quantum-confined CsPbBr3 NCs are exfoliated from the bulk product leading to a broad and blue-shifted emission. At this stage the photoluminescence intensity is strongly reduced which is ascribed to the formation of surface defects induced by ball-milling in dry conditions

Vacuum-Deposited Cesium Tin Iodide Thin Films with Tunable Thermoelectric Properties

Most current thermoelectric materials have important drawbacks, such as toxicity, scarceness, and peak operating temperatures above 300 °C. Herein, we report the thermoelectric properties of different crystalline phases of Sn-based perovskite thin films. The 2D phase, Cs2SnI4, is obtained through vacuum thermal deposition and easily converted into the black β phase of CsSnI3 (B-β CsSnI3) by annealing at 150 °C. B-β CsSnI3 is a p-type semiconductor with a figure of merit (ZT) ranging from 0.021 to 0.033 for temperatures below 100 °C, which makes it a promising candidate to power small electronic devices such as wearable sensors which may be interconnected in the so-called Internet of Things. The B-β phase is stable in nitrogen, whereas it spontaneously oxidizes to Cs2SnI6 upon exposure to air. Cs2SnI6 shows a negative Seebeck coefficient and an ultralow thermal conductivity. However, the ZT values are 1 order of magnitude lower than for B-β CsSnI3 due to a considerably lower electrical conductivity

Tuning the Optical Absorption of Sn-, Ge-, and Zn-Substituted Cs2AgBiBr6 Double Perovskites: Structural and Electronic Effects

Lead-free halide double perovskites (DPs) are highly tunable materials in terms of chemical composition and optical properties. One of the most widely reported DPs is Cs2AgBiBr6, which is envisaged as a promising absorber for photovoltaics. Nevertheless, its bandgap (around 1.9−2.3 eV) remains too large for common tandem solar cells. In this work, we report the mechanochemical synthesis of Sn-, Ge-, and Zn-substituted Cs2AgBiBr6 in powder form; their bandgaps reach 1.55, 1.80, and 2.02 eV, respectively. These differences are rationalized through density functional theory calculations, demonstrating combined electronic and structural (disorder) effects introduced by the divalent metal-cation substituents. Finally, we present the first vacuum-deposited thin films of the Sn-substituted DP, which also show a notable narrowing of the bandgap, and this paves the way toward its implementation in photovoltaic solar cells

Pulsed Laser Deposition of Cs2AgBiBr6: from Mechanochemically Synthesized Powders to Dry, Single-Step Deposition

Cs2AgBiBr6 has been proposed as a promising lead-free and stable double perovskite alternative to hybrid and lead-based perovskites. However, the low solubility of precursors during wet synthesis, or the distinct volatility of components during evaporation, results in complex multistep synthesis approaches, hampering the widespread employment of Cs2AgBiBr6 films. Here, we present pulsed laser deposition of Cs2AgBiBr6 films as a dry, single-step and single-source deposition approach for high-quality film formation. Cs2AgBiBr6 powders were prepared by mechanochemical synthesis and pressed into a solid target maintaining phase purity. Controlled laser ablation of the double perovskite target in vacuum and a substrate temperature of 200 °C results in the formation of highly crystalline Cs2AgBiBr6 films. We discuss the importance of deposition pressure to achieve stoichiometric transfer and of substrate temperature during PLD growth to obtain high-quality Cs2AgBiBr6 films with grain sizes > 200 nm. This work demonstrates the potential of PLD, an established technique in the semiconductor industry, to deposit complex halide perovskite materials while being compatible with optoelectronic device fabrication, such as UV and X-ray detectors

Highly Luminescent Transparent Cs2AgxNa1−xBiyIn1−yCl6 Perovskite Films Produced by Single-Source Vacuum Deposition

Thermal deposition of halide perovskites as a universal and scalable route to transparent thin films becomes highly challenging in the case of lead-free double perovskites, requiring the evaporation dynamics of multiple metal halide sources to be balanced or a single-phase precursor preliminary synthesized to achieve a reliable control over the composition and the phase of the final films. In the present Letter, the feasibility of the single-source vacuum deposition of microcrystalline Cs2AgxNa1-xBiyIn1-yCl6 double perovskites into corresponding transparent nanocrystalline films while preserving the bulk spectral and structural properties is shown. The perovskite films produced from the most emissive powders with x = 0.40 and y = 0.01 revealed a photoluminescence quantum yield of 85%, highlighting thermal evaporation as a promising approach to functional perovskite-based optical materials

Vacuum-Deposited Cesium Tin Iodide Thin Films with Tunable Thermoelectric Properties

Most current thermoelectric materials have important drawbacks, such as toxicity, scarceness, and peak operating temperatures above 300 °C. Herein, we report the thermoelectric properties of different crystalline phases of Sn-based perovskite thin films. The 2D phase, Cs2SnI4, is obtained through vacuum thermal deposition and easily converted into the black β phase of CsSnI3(B-β CsSnI3) by annealing at 150 °C. B-β CsSnI3is a p-type semiconductor with a figure of merit (ZT) ranging from 0.021 to 0.033 for temperatures below 100 °C, which makes it a promising candidate to power small electronic devices such as wearable sensors which may be interconnected in the so-called Internet of Things. The B-β phase is stable in nitrogen, whereas it spontaneously oxidizes to Cs2SnI6upon exposure to air. Cs2SnI6shows a negative Seebeck coefficient and an ultralow thermal conductivity. However, the ZT values are 1 order of magnitude lower than for B-β CsSnI3due to a considerably lower electrical conductivity