Screening Mixed-Metal Sn₂M(III)Ch₂X₃ Chalcohalides for Photovoltaic Applications

Quaternary mixed-metal chalcohalides (Sn2M(III)Ch2X3) are emerging as promising lead-free, perovskite-inspired photovoltaic absorbers. Motivated by recent developments of a first Sn2SbS2I3-based device, we used density functional theory to identify lead-free Sn2M(III)Ch2X3 materials that are structurally and energetically stable within Cmcm, Cmc21, and P21/c space groups and have a band gap in the range of 0.7–2.0 eV to cover outdoor and indoor photovoltaic applications. A total of 27 Sn2M(III)Ch2X3 materials were studied, including Sb, Bi, and In for the M(III)-site, S, Se, and Te for the Ch-site, and Cl, Br, and I for the X-site. We identified 12 materials with a direct band gap that meet our requirements, namely, Sn2InS2Br3, Sn2InS2I3, Sn2InSe2Cl3, Sn2InSe2Br3, Sn2InTe2Br3, Sn2InTe2Cl3, Sn2SbS2I3, Sn2SbSe2Cl3, Sn2SbSe2I3, Sn2SbTe2Cl3, Sn2BiS2I3, and Sn2BiTe2Cl3. A database scan reveals that 9 of 12 are new compositions. For all 27 materials, P21/c is the thermodynamically preferred structure, followed by Cmc21. In Cmcm and Cmc21, mainly direct gaps occur, whereas indirect gaps occur in P21/c. To open up the possibility of band gap tuning in the future, we identified 12 promising Sn2M(III)1–aM(III)′aCh2–bCh′bX3–cX′c alloys, which fulfill our requirements, and an additional 69 materials by combining direct and indirect band gap compounds

Monitoring Charge Carrier Diffusion across a Perovskite Film with Transient Absorption Spectroscopy

We have developed a new noninvasive optical method for monitoring charge carrier diffusion and mobility in semiconductor thin films in the direction perpendicular to the surface which is most relevant for devices. The method is based on standard transient absorption measurements carried out in reflectance and transmittance modes at wavelengths below the band gap where the transient response is mainly determined by the change in refractive index, which in turn depends on the distribution of photogenerated carriers across the film. This distribution is initially inhomogeneous because of absorption at the excitation wavelength and becomes uniform over time via diffusion. By modeling these phenomena we can determine the diffusion constant and respective mobility. Applying the method to a 500 nm thick triple cation FAMACs perovskite film revealed that homogeneous carrier distribution is established in few hundred picoseconds, which is consistent with mobility of 66 cm2 (V s)−1

Multicomponent Molecularly Controlled Langmuir−Blodgett Systems for Organic Photovoltaic Applications

The capability of Violanthrone-79 (V-79) and N,N′-bis(2,5-di-tert-butylphenyl)-3,4:9,l0-perylenebis(dicarboximide) (PDI) to act as electron acceptors, with respect to poly(3-hexylthiophene) (PHT) and to the photoinduced fullerene anion of porphyrin−fullerene (P−F) dyad, was demonstrated in Langmuir−Blodgett (LB) films by the time-resolved Maxwell displacement charge method. The introduction of V-79 and PDI in oriented multilayered films led to improved light harvesting and increased lifetime of the charge separation, enhancing the photocurrent generation measured using a three-electrode photoelectrochemical cell. The best solar cell performance was achieved for the multifunctional film structure where efficient PHT-phthalocyanine heterojunction (PHT|ZnPH4) was combined with the P−F|V-79 system

Synthetic Control over the Surface Chemistry of Blue-Emitting Perovskite Nanocrystals for Photocatalysis

Lead halide perovskite nanocrystals (NCs) are particularly suitable for light-emitting and photocatalysis applications, where their potential can be maximized by controlling the surface composition of their organic shell. In this study, the preparation of CsPbClxBr3–x NCs at room temperature in toluene is described. Three differently structured surfactants are utilized for the synthesis, each with a specific function, namely, the solubilization of the lead precursor (n-Hept4NBr), the surface passivation with halide modification (dimethyldioctadecylammonium chloride), and the protection of the surface-active sites (octanoic acid) for photocatalysis. Under these conditions, nearly monodispersed blue-emitting nanocubes are selectively obtained in a one-pot synthesis by combining specific amounts of the perovskite precursors. As supported by thermogravimetric analysis (TGA) and Fourier transform infrared (FT-IR) spectroscopy investigations, the organic shell of the obtained NCs is composed of electrostatically bound dimethyldioctadecylammonium ions, granting robustness to the corresponding NCs, and octanoic acid molecules, interacting with the nanoparticle surface through weaker secondary bonds. The obtained NCs exhibit a high photoluminescence quantum yield (PLQY = 72 ± 3%) notwithstanding multiexponential recombination dynamics of the excited state, resulting from the different passivation modes at the NC surface. Moreover, the NCs show a remarkable optical stability after exposure to high temperatures and to water contact due to the high surface density of the multifunctional organic ligands. The introduction of 4-tert-butylphenyl thiol promotes a charge transfer process at the NC/thiol interface formed upon removal of the labile ligands (octanoic acid) at the NC surface. In these conditions, the NCs are prone to the photoinduced conversion of the aromatic thiol into the corresponding disulfide without varying the optical properties of the perovskite photocatalyst upon the substrate conversion. Therefore, the obtained results cast light on the versatility of the surface engineering of lead halide perovskite NCs for efficient blue emission and photocatalysis with improved stability

Moisture-Assisted near-UV Emission Enhancement of Lead-Free Cs₄CuIn₂Cl₁₂ Double Perovskite Nanocrystals

Lead-based halide perovskite nanocrystals (NCs) are recognized as emerging emissive materials with superior photoluminescence (PL) properties. However, the toxicity of lead and the swift chemical decomposition under atmospheric moisture severely hinder their commercialization process. Herein, we report the first colloidal synthesis of lead-free Cs4CuIn2Cl12 layered double perovskite NCs via a facile moisture-assisted hot-injection method stemming from relatively nontoxic precursors. Although moisture is typically detrimental to NC synthesis, we demonstrate that the presence of water molecules in Cs4CuIn2Cl12 synthesis enhances the PL quantum yield (mainly in the near-UV range), induces a morphological transformation from 3D nanocubes to 2D nanoplatelets, and converts the dark transitions to radiative transitions for the observed self-trapped exciton relaxation. This work paves the way for further studies on the moisture-assisted synthesis of novel lead-free halide perovskite NCs for a wide range of applications

Surface-Engineered Cesium Lead Bromide Perovskite Nanocrystals for Enabling Photoreduction Activity

In recent years, colloidal lead halide perovskite (LHP) nanocrystals (NCs) have exhibited such intriguing light absorption properties to be contemplated as promising candidates for photocatalytic conversions. However, for effective photocatalysis, the light harvesting system needs to be stable under the reaction conditions propaedeutic to a specific transformation. Unlike photoinduced oxidative reaction pathways, photoreductions with LHP NCs are challenging due to their scarce compatibility with common hole scavengers like amines and alcohols. In this contribution, it is investigated the potential of CsPbBr3 NCs protected by a suitably engineered bidentate ligand for the photoreduction of quinone species. Using an in situ approach for the construction of the passivating agent and a halide excess environment, quantum-confined nanocubes (average edge length = 6.0 ± 0.8 nm) are obtained with a low ligand density (1.73 ligand/nm2) at the NC surface. The bifunctional adhesion of the engineered ligand boosts the colloidal stability of the corresponding NCs, preserving their optical properties also in the presence of an amine excess. Despite their relatively short exciton lifetime (τAV = 3.7 ± 0.2 ns), these NCs show an efficient fluorescence quenching in the presence of the selected electron accepting quinones (1,4-naphthoquinone, 9,10-phenanthrenequinone, and 9,10-anthraquinone). All of these aspects demonstrate the suitability of the NCs for an efficient photoreduction of 1,4-naphthoquinone to 1,4-dihydroxynaphthalene in the presence of triethylamine as a hole scavenger. This chemical transformation is impracticable with conventionally passivated LHP NCs, thereby highlighting the potential of the surface functionalization in this class of nanomaterials for exploring new photoinduced reactivities

Synthesis of Benzothiadiazole Derivatives by Applying C–C Cross‑Couplings

The benzothiadiazole moiety has been extensively exploited as a building block in the syntheses of efficient organic semiconducting materials during the past decade. In this paper, parallel synthetic routes to benzothiadiazole derivatives, inspired by previous computational findings, are reported. The results presented here show that various C–C cross-couplings of benzothiadiazole, thiophene, and thiazole derivatives can be efficiently performed by applying Xantphos as a ligand of the catalyst system. Moreover, improved and convenient methods to synthesize important chemical building blocks, e.g., 4,7-dibromo-2,1,3-benzothiadiazole, in good to quantitative yields are presented. Additionally, the feasibility of Suzuki–Miyaura and direct coupling methods are compared in the synthesis of target benzothiadiazole derivatives. The computational characterization of the prepared benzothiadiazole derivatives shows that these compounds have planar molecular backbones and the possibility of intramolecular charge transfer upon excitation. The experimental electrochemical and spectroscopic studies reveal that although the compounds have similar electronic and optical properties in solution, they behave differently in solid state due to the different alkyl side-group substitutions in the molecular backbone. These benzothiadiazole derivatives can be potentially used as building blocks in the construction of more advanced small molecule organic semiconductors with acceptor–donor–acceptor motifs

Role of Self-Trapped Excitons in the Broadband Emission of Lead-Free Perovskite-Inspired Cu₂AgBiI₆

The perovskite-inspired Cu2AgBiI6 (CABI) absorber shows promise for low-toxicity indoor photovoltaics. However, the carrier self-trapping in this material limits its photovoltaic performance. Herein, we examine the self-trapping mechanism in CABI by analyzing the excited-state dynamics of its absorption band at 425 nm, which is responsible for the self-trapped exciton emission, using a combination of photoluminescence and ultrafast transient absorption spectroscopies. Photoexcitation in CABI rapidly generates charge carriers in the silver iodide lattice sites, which localize into the self-trapped states and luminesce. Furthermore, a Cu–Ag–I-rich phase that exhibits similar spectral responses as CABI is synthesized, and a comprehensive structural and photophysical study of this phase provides insights into the nature of the excited states of CABI. Overall, this work explains the origin of self-trapping in CABI. This understanding will play a crucial role in optimizing its optoelectronic properties. It also encourages compositional engineering as the key to suppressing self-trapping in CABI

Nanostructured TiO₂ Grown by Low-Temperature Reactive Sputtering for Planar Perovskite Solar Cells

Low-temperature nanostructured electron-transporting layers (ETLs) for perovskite solar cells are grown by reactive sputtering at 160 °C with thickness in the range 22–76 nm and further stabilization in air at 180 °C to improve the lattice structure and to consequently reduce charge recombination during solar cell operation. In addition, the post-deposition treatment aims at leveling differences among samples to ensure material reproducibility. Nanostructured TiO2 has a further added value in promoting the structural coupling with the perovskite layer and establishing conformal interfaces in favor of the charge extraction from the active material. Nanostructuring of the ETLs also allows the shaping of the band gap width and position with a beneficial impact on the electrical parameters, as tested in standard architecture containing methylammonium lead iodide perovskites. A balance among parameters is achieved using a 40-nm-thick TiO2 ETL with a maximum efficiency of ∼15% reached without surface treatments or additional layers. The proposed growth methodology would be compatible with the use of flexible substrates after appropriated ETL structural adaptation. It can be likewise applied in perovskite/silicon-heterojunction tandem solar cells to fulfill the industrial demand for clean, solvent-free, reproducible, reliable, and high-throughput processes