Lead-free halide perovskites, as environment-friendly materials, have received critical interest in photovoltaic applications. In this regard, the bismuth halide perovskites demonstrate better stability under ambient conditions than lead halide perovskites and consequently remain one of the critical areas for the development of lead-free absorber materials. The steady-state optical properties are widely investigated in these bismuth halide perovskites, but excited-state charge carrier dynamics such as hot carrier relaxation remain elusive. However, it is crucial to investigate the rapid relaxation of above band gap “hot” carriers as it restricts the fundamental efficiency limit in the perovskite solar cells. Here, we demonstrate the cation-dependent hot carrier cooling in the lead-free A3Bi2I9 [A = FA (formamidinium), MA (methylammonium), and Cs (cesium)] perovskite by using femtosecond transient absorption spectroscopy. These lead-free perovskites were fabricated from gamma-butyrolactone (γ-GBL) solvent to ensure uniformity and continuity of the as-grown film and were well characterized by XRD, SEM, and steady-state absorption and photoluminescence spectroscopy. With varying A-cations, we observe that the hot-hole relaxation is slowest in the all-inorganic perovskite Cs3Bi2I9 (12.83 ps) and hot electron relaxation is slowest in the hybrid MA3Bi2I9 perovskite (6.42 ps) at the same excitation energy. The observed strong dependence of carrier cooling on cation composition is explained by the interaction between the different organic cations (A = FA, MA, and Cs) with the Pb–Br frameworks. Our study provides an opportunity to understand the effect of cations on the excited-state carrier dynamics, especially the hot carrier relaxation in the bismuth halide perovskites. This will pave the way for designing hot carrier-based high-efficient lead-free perovskite photovoltaic devices.

We report a large Stokes shift and broad emission band in a Mn-based organic–inorganic hybrid halide, (guanidinium)6Mn3Br12 [GuMBr], consisting of trimeric units of distorted MnBr6 octahedra representing a zero-dimensional compound with a liquid like crystalline lattice. Analysis of the photoluminescence (PL) line width and Raman spectra reveals the effects of electron–phonon coupling, suggestive of the formation of Frenkel-like bound excitons. These bound excitons, regarded as the self-trapped excitons (STEs), account for the large Stokes shift and broad emission band. The excited-state dynamics was studied using femtosecond transient absorption spectroscopy, which confirms the STE emission. Further, this compound is highly emissive with a PL quantum yield of ∼50%. With chloride ion incorporation, we observe enhancement of the emissive properties and attribute it to the effects of intrinsic quantum confinement. Localized electronic states in flat bands lining the gap and their strong coupling with phonons are confirmed with first-principles calculations.

Herein, we have fabricated self-assembled semiconducting organic nanomaterials with various morphologies (1Dfiber, 2D-flakes, and 2D-nanosheets) made of small conjugated oligomer 2,2′:5′,2″:5″,2‴-quaterthiophene (α-QTH) by a simple solution-based coprecipitation method. By simply varying the good-solvent-to-bad-solvent ratio, we can critically tune the selfassembly process and eventually can control the intermolecular interactions of the constituent molecules in these self-assembled nanostructures. Different types of self-assembled nanostructures have been utilized for photocatalytic solar H 2 production. The H 2 production efficiencies directly depend on the morphology of selfassembledselfassembled nanomaterials as well as intermolecular interactions of QTH molecules. The overall photocatalytic properties are further correlated with the ongoing photophysical properties by means of detailed steady-state and time-resolved fluorescence spectroscopy and dimer-based time dependent-density functional theory (TD-DFT) calculations. Furthermore, femtosecond transient absorption spectroscopy has been utilized to explore the detailed photoinduced exciton dynamics by global analysis of spectrally resolved pump−probe traces. In addition to that, the overall photocatalytic activities are further supported by an in-depth electrochemical study. Finally, a boost in photocatalytic H 2 production has been observed by using 4-methylbenzyl alcohol (4-MBA) as a specific hole scavenger for the completion of the redox cycle. Therefore, the present system can be utilized for simultaneous solar H 2 production and the specific organic transformation through a green and cost-efficient approach.

Serendipitous observations offer newer insights into materials properties. Here we describe the g-C3N4 nanosheets exhibiting remarkably blue-shifted photoluminescence within the 390-580 nm range centring at 425 nm which matches more...

In recent times, layered double perovskites have attracted considerable attention due to their nontoxic nature, structural stability in ambient conditions, and ability to tune optoelectronic properties through the interplay between two metal ions. To better comprehend the utility of this promising class of materials to be used as absorber materials in solar cells, it is important to understand the nature of band-gap and excited-state dynamics. In this work, we present a comprehensive study on the microcrystals of Cs4CuSb2Cl12, a relatively new class of double perovskites, which have emerged as a propitious contender. Using dispersion-corrected density functional theory, we study the nature of the band structure and identify the structural and energetic parameters that are also tested experimentally. Further, using femtosecond transient absorption spectroscopy, we show that depending on the excitation wavelength, the excited-state relaxation mechanism involves either excitons or free charge carriers. One crucial observation is the solvent dependence of the relaxation rates of carriers, opening up the possibilities of solvent control of charge carrier dynamics.

Among the all-inorganic lead halide perovskites, CsPbI3 has emerged as a competent photovoltaic material because of its enhanced stability and comparable efficiency to that of organic–inorganic hybrid perovskites, but the main constraint lies in the phase instability of the active cubic α-CsPbI3 perovskite at room temperature as it degrades to nonperovskite yellow-colored phase. Herein, we describe the synthesis of the active cubic α-CsPbI3 perovskite along with orthorhombic in the presence of surface capping agent poly-vinylpyrrolidone (PVP) inside a mesoporous alumina film, which restricts its interaction with air and moisture, leading to significantly enhanced stability of the composite film. Moreover, the conversion rate from the active cubic (α) to inactive yellow δ (orthorhombic) phase is found to be nominal in a time period of minimum 8 months. The as-synthesized composite CsPbI3–alumina film is found to be stable at ambient condition. To examine the charge-transport property of this stable composite film in a thin film device setup, electron and hole transport layers are used and femtosecond transient absorption spectroscopy is employed, all at room temperature and ambient condition, to investigate the charge-transfer kinetics of PVP-capped CsPbI3 in mesostructured alumina. The spectral data confirms the efficient charge transfer occurring from CsPbI3 to charge-conducting layers, and the electron and hole transfers happen in 40 ps and 600 fs, respectively. This study is expected to encourage new possibilities of using a surface capping agent as well as a mesostructured layer to synthesize and confine stable active perovskite nanocrystals useful for practical photovoltaic applications.

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