Photoinduced Charge Separation in Semiconductor-Quantum-Dot/Organic-Molecule Hybrids

Charge Transfer Kinetics in Halide Perovskites: On the Constraints of Time-Resolved Spectroscopy Measurements

Understanding photophysical processes in lead halide perovskites is an important aspect of optimizing the performance of optoelectronic devices. The determination of exact charge carrier extraction rate constants remains elusive, as there is a large and persistent discrepancy in the reported absolute values. In this review, we concentrate on experimental procedures adopted in the literature to obtain kinetic estimates of charge transfer processes and limitations imposed by the spectroscopy technique employed. Time-resolved techniques (e.g., transient absorption-reflection and time-resolved photoluminescence spectroscopy) are commonly employed to probe charge transfer at perovskite/transport layer interfaces. The variation in sample preparation and measurement conditions can produce a wide dispersion of the measured kinetic parameters. The selected time window and the kinetic fitting model employed introduce additional uncertainty. We discuss here evaluation strategies that rely on multiexponential fitting protocols (regular or stretched) and show how the dispersion in the reported values for carrier transfer rate constants can be resolved.

Charge Transfer from Quantum-Confined 0D, 1D, and 2D Nanocrystals

The properties of colloidal quantum-confined semiconductor nanocrystals (NCs), including zero-dimensional (0D) quantum dots, 1D nanorods, 2D nanoplatelets, and their heterostructures, can be tuned through their size, dimensionality, and material composition. In their photovoltaic and photocatalytic applications, a key step is to generate spatially separated and long-lived electrons and holes by interfacial charge transfer. These charge transfer properties have been extensively studied recently, which is the subject of this Review. The Review starts with a summary of the electronic structure and optical properties of 0D-2D nanocrystals, followed by the advances in wave function engineering, a novel way to control the spatial distribution of electrons and holes, through their size, dimension, and composition. It discusses the dependence of NC charge transfer on various parameters and the development of the Auger-assisted charge transfer model. Recent advances in understanding multiple exciton generation, decay, and dissociation are also discussed, with an emphasis on multiple carrier transfer. Finally, the applications of nanocrystal-based systems for photocatalysis are reviewed, focusing on the photodriven charge separation and recombination processes that dictate the function and performance of these materials. The Review ends with a summary and outlook of key remaining challenges and promising future directions in the field.

B-Site Co-Alloying with Germanium Improves the Efficiency and Stability of All-Inorganic Tin-Based Perovskite Nanocrystal Solar Cells

Colloidal lead-free perovskite nanocrystals have recently received extensive attention because of their facile synthesis, the outstanding size-tunable optoelectronic properties, and less or no toxicity in their commercial applications. Tin (Sn) has so far led to the most efficient lead-free solar cells, yet showing highly unstable characteristics in ambient conditions. Here, we propose the synthesis of all-inorganic mixture Sn-Ge perovskite nanocrystals, demonstrating the role of Ge2+ in stabilizing Sn2+ cation while enhancing the optical and photophysical properties. The partial replacement of Sn atoms by Ge atoms in the nanostructures effectively fills the high density of Sn vacancies, reducing the surface traps and leading to a longer excitonic lifetime and increased photoluminescence quantum yield. The resultant Sn-Ge nanocrystals-based devices show the highest efficiency of 4.9 %, enhanced by nearly 60 % compared to that of pure Sn nanocrystals-based devices.

Carbon dots functionalized papers for high-throughput sensing of 4-chloroethcathinone and its analogues in crime sites

Sensitive and selective assays are demanded for quantitation of new psychoactive substances such as 4-chloroethcathinone that is a π-conjugated keto compound. Carbon dots (C-dots) prepared from L-arginine through a hydrothermal route have been used for quantitation of 4-chloroethcathinone in aqueous solution and on C-dot-functionalized papers (CDFPs). To prepare CDFPs, chromatography papers, each with a pattern of 8 × 12 circles (wells), are first fabricated through a solid-ink printing method and then the C-dots are coated into the wells. π-Conjugated keto or ester compounds induce photoluminescence quenching of C-dots through an electron transfer process. At pH 7.0, the CDFPs allow screening of abused drugs such as cocaine, heroin and cathinones. Because of poor solubility of heroin and cocaine at pH 11.0, the C-dot probe is selective for cathinones. The C-dots in aqueous solution and CDFPs at pH 11.0 allow quantitation of 4-chloroethcathinone down to 1.73 mM and 0.14 mM, respectively. Our sensing system consisting of a portable UV-lamp, a smartphone and a low-cost CDFP has been used to detect cathinones, cocaine and heroin at pH 7.0, showing its potential for screening of these drugs in crime sites.

Charge transfer dynamics in CsPbBr3 perovskite quantum dots-anthraquinone/fullerene (C60) hybrids

An advantage of colloidal quantum dots, particularly perovskite quantum dots (PQDs), as photoactive components is that they easily form complexes with functional organic molecules, which results in hybrids with enriched photophysical properties. Herein, we demonstrate the formation of stable ground state complexes of CsPbBr3 PQD with two widely used molecular electron acceptors, fullerene (C60) and anthraquinone, (AQ) which contain carboxylic anchor groups. Dynamics of the photo-induced electron transfer in the hybrids were compared. The use of carboxylic groups for binding results in stable complex formation and their photophysical properties depend on the ratio of components but not the absolute concentrations (up to micromolar concentrations). Time-resolved transient absorption (TA) spectroscopy shows that in both cases, a charge separated (CS) state is formed. Data analysis was aimed to evaluate the CS time constant in ideal one-to-one complexes and was found to be in the range of 30-190 ps. The CS state of PQD-AQ complexes recombines directly to the ground state in roughly one microsecond. Recombination of the CS state of PQD-C60 is more complex and points to strong inhomogeneity of these complexes. Majority of the CS states relax by first forming the C60 triplet state.

Photoinduced Energy Transfer in ZnCdSeS Quantum Dot-Phthalocyanines Hybrids

In this article, interaction between ZnCdSeS quantum dot (QD) and phthalocyanines with variable linker has been reported. Steady-state and time-resolved spectroscopic investigation reveals that only photoinduced energy transfer occurs from QD to phthalocyanines. To evaluate quantitatively the energy transfer, the Poisson statistics of QD-dye complex formation was used in the analysis of steady-state and time-resolved emission quenching, which allows to estimate the energy transfer rate constant for an ideal one-to-one complex. The measured rate constants are compared to the rates evaluated based on the classic Förster theory, which shows roughly 1 nm discrepancy in the energy transfer distance estimation, or one order in magnitude discrepancy in the transfer rate constants.