Unraveling the cation dependent carrier cooling and transient mobility in lead-free A3Sb2I9 perovskites.

Lead halide perovskites (LHPs) have gained prominence for their exceptional photophysical properties, holding promise for applications in high-end optoelectronic devices. However, the presence of lead is one of the major obstacles to the commercialization of LHPs in the field of photovoltaics. To address this, researchers have explored environment friendly lead-free perovskite solar cells by investigating non-toxic perovskite materials. This study explores the enhancement of photophysical properties through chemical engineering, specifically cation exchange, focusing on the crucial photophysical process of hot carrier cooling. Employing femtosecond transient absorption spectroscopy and optical pump terahertz probe spectroscopy, we have probed the carrier relaxation dynamics in A3Sb2I9 with cesium and rubidium cations. This study unravels that the carrier relaxation is found to be slower in Rb3Sb2I9; along with this, the transient mobility decay is found to be retarded. Overall, this study suggests that an antimony-based Rb3Sb2I9 perovskite could be a substantial lead-free perovskite in photovoltaics. These findings provide valuable insights into cation engineering strategies, aiming to improve the overall performance of lead-free-based photovoltaic devices.

Efficient Hot Electron Transfer and Extended Separation of Charge Carriers at the 1P Hot State in Sb2Se3/CdSe p-n Heterojunction.

Utilization of hot carriers is very crucial in improving the efficiency of solar energy devices. In this work, we have fabricated an Sb2Se3/CdSe p-n heterojunction via a cation exchange method and investigated the possibility of hot electron transfer and relaxation pathways through ultrafast spectroscopy. The enhanced intensity of the CdSe hot excitonic (1P) bleach in the heterostructure system confirmed the hot electron transfer from Sb2Se3 to CdSe. Both the 1S and 1P signals are dynamically very slow in the heterosystem, validating this charge migration phenomenon. Interestingly, recovery of the 1P signal is much slower than that of 1S. This is very unusual as 1S is the lowest-energy state. This observation indicates the strength of hot electron transfer in this unique heterojunction, which helps in increasing the carrier lifetime in the hot state. Extended separation of charge carriers and enhanced hot carrier lifetime would be extremely helpful in extracting carriers and boost the performance of optoelectronic devices.

Chemically Engineered Avenues: Opportunities for Attaining Desired Carrier Cooling in Perovskites.

Hot carrier extraction-based devices are presently being persuaded as the most revolutionary means of surpassing the theoretical thermodynamic conversion efficiency limit (∼67 % for a model hot carrier solar cell). However, for practical realisation, there stand various hurdles that need to be surmounted, a major among all being the rapid hot carrier cooling rate. Though, the perovskite family has already demonstrated itself to exhibit slower cooling in contrast to the prototypical semiconductors. Decelerating this entire process of cooling further can prove to be a crucial stride in this regard. Quite contrarily, for the optoelectronic applications the situation is entirely conflicting where quick rate of cooling is a chief prerequisite. In the recent times, there have been various key developments that have targeted altering this cooling rate by various chemically engineered strategies. This review highlights such blueprints that can be utilized towards the advantageous alteration of the carrier cooling in accordance with the device requirements.

Probing ultrafast hot charge carrier migration in MoS2 embedded CdS nanorods.

Efficient utilization of hot charge carriers is of utmost benefit for a semiconductor-based optoelectronic device. Herein, a one-dimensional (1D)/two-dimensional (2D) heterojunction was fabricated in the form of CdS/MoS2 nanorod/nanosheet composite and migration of hot charge carriers was being investigated with the help of transient absorption (TA) spectroscopy. The band alignment was such that both the electrons and holes in the CdS region tend to migrate into the MoS2 region following photoexcitation. The composite system is composed of optical signatures of both CdS and MoS2, with the dominance of CdS nanorods. In addition, the TA signal of MoS2 is substantially enhanced in the heterosystem at the cost of the diminished CdS signal, confirming the migration of charge carrier population from CdS to MoS2. This migration phenomenon was dominated by the hot carrier transfer. The hot carriers in the high energy states of CdS are preferentially migrated into the MoS2 states rather than being cooled to the band edge. The hot carrier transfer time for a 400 nm pump excitation was calculated to be 0.21 ps. This is much faster than the band edge electron transfer process, occurring at 2.0 ps time scale. We found that these migration processes are very much dependent on the applied pump photon energy. Higher energy pump photons are more efficient in the hot carrier transfer process and place these hot carriers in the higher energy states of MoS2, further extending charge carrier separation. This detailed spectroscopic investigation would help in the fabrication of better 1D/2D heterojunctions and advance the optoelectronic field.

Enhanced Charge Carrier Separation and Improved Biexciton Yield at the p-n Junction of SnSe/CdSe Heterostructures: A Detailed Electrochemical and Ultrafast Spectroscopic Investigation.

Tin chalcogenides (SnX, X = S, Se)-based heterostructures (HSs) are promising materials for the construction of low-cost optoelectronic devices. Here, we report the synthesis of a SnSe/CdSe HS using the controlled cation exchange reaction. The (400) plane of SnSe and the (111) plane of CdSe confirm the formation of an interface between SnSe and CdSe. The Type I band alignment is estimated for the SnSe/CdSe HS with a small conduction band offset (CBO) of 0.72 eV through cyclic voltammetry measurements. Transient absorption (TA) studies demonstrate a drastic enhancement of the CdSe biexciton signal that points toward the hot carrier transfer from SnSe to CdSe in a short time scale. The fast growth and recovery of CdSe bleach in the presence of SnSe indicate charge transfer back to SnSe. The observed delocalization of carriers in these two systems is crucial for an optoelectronic device. Our findings provide new insights into the fabrication of cost-effective photovoltaic devices based on SnSe-based heterostructures.

Ultrafast Insights into High Energy (C and D) Excitons in Few Layer WS2.

High energy (C and D) excitons possess extraordinary influence over the optical properties of atomically thin transition metal dichalcogenides (TMDCs), and the comprehensive understanding of these would play a pivotal role in advancing research on 2D optoelectronics. Herein, we employed transient absorption spectroscopy to monitor the underlying photophysical processes involved with different excitonic features in few layer WS2, modeled as a TMDC representative. We observed a strong intervalley coupling across the momentum space and proposed the most plausible relaxation pathway for different excitons in few layer scenario. C and D exciton dynamics were significantly slower as compared to canonical A and B excitons, as a consequence of the indirect Λ-Γ relaxation in C and D and direct K-K combination in A and B. Most importantly, all four excitons emerge in the system and influence each other irrespective of the incident photon energy, which would be extremely impactful in fabricating wide range photonic devices.

Defect-Mediated Slow Carrier Recombination and Broad Photoluminescence in Non-Metal-Doped ZnIn2S4 Nanosheets for Enhanced Photocatalytic Activity.

Elemental doping has already been established to be one of the most effective approaches for band-gap engineering and controlled material response for improved photocatalytic activity. Herein atomically thin ZnIn2S4 (ZIS) nanosheets were doped with O and N separately, and the effects of doping were spectroscopically investigated for photocatalytic H2 evolution. Steady-state photoluminescence studies revealed an enhanced charge-carrier population in the doped systems along with a defect-state-induced broad peak in the red region of the spectra. Transient absorption (TA) spectroscopy demonstrated that the conduction-band-edge electrons are transferred on an ultrafast time scale to the inter-band-gap defect states. TA analysis suggests that O and N doping contributes to the defect state concentration and ensures an enhanced photocatalytic activity of the system. This detailed spectroscopic analysis uncovers the role of inter-band-gap defect states in the photocatalytic activity of ZIS and will open new avenues for the construction of nanosheet-based optical devices.

Ultrafast Charge Delocalization Dynamics of Ambient Stable CsPbBr3 Nanocrystals Encapsulated in Polystyrene Fiber.

CsPbBr3 nanocrystals (NCs) encapsulated in a transparent polystyrene (PS) fiber matrix (CsPbBr3 @PS) have been synthesized to protect the NCs. The ultrafast charge delocalization dynamics of the embedded NCs have been demonstrated, and the results are compared with the pristine CsPbBr3 in toluene. The electrospinning method was employed for the preparation of CsPbBr3 @PS fibers by using a polystyrene solution doped with pre-synthesized CsPbBr3 and characterized by XRD, HRTEM, and X-ray photoelectron spectroscopy (XPS). Energy level diagrams of CsPbBr3 and PS suggest that CsPbBr3 @PS fibers make a type I core-shell structure. The carrier cooling for CsPbBr3 @PS fibers is found to be much slower than pure CsPbBr3 NCs. This observation suggests that photoexcited electrons from CsPbBr3 NCs get delocalized from the conduction band of the perovskite to lowest unoccupied molecular orbital (LUMO) of the PS fiber matrix. The CsPbBr3 @PS fibers possess remarkable stability under ambient conditions as well as in water over months. The clear understanding of charge carrier relaxation dynamics of CsPbBr3 confined in PS fibers could help to design robust optoelectronic devices.

Temperature-Dependent Interplay of Polaron Formation and Hot Carrier Cooling Dynamics in CsPbBr3 Nanocrystals: Role of Carrier-Phonon Coupling Strength.

Photovoltaic devices with CsPbBr3 nanocrystals as the working layer are generally subjected to wide variance of extreme temperatures during operation. Thus, the underlying photophysics, which is undeniably a temperature-dependent entity, demands intricate understanding. In this finding, with the assistance of temperature-dependent transient absorption spectroscopy, an endeavor has been made to disentangle the detrimental conflict between polaron formation and hot carrier relaxation for CsPbBr3 nanocrystal systems. Carrier relaxation pathways are seen to diverge drastically upon varying the lattice temperature from 300 to 5 K. Acquired results indicate the involvement of polarons for retarded carrier cooling dynamics observed at 300 K, whereas its absence at lower temperatures (<200 K) provides the basis for relatively quicker cooling. Additionally, despite the expected participation from the polaron due to the onset of strong carrier-longitudinal optical phonon coupling at 200 K, the reason for its absence in the cooling dynamics at 200 K has been revealed.

Effect of Confinement on the Exciton and Biexciton Dynamics in Perovskite 2D-Nanosheets and 3D-Nanocrystals.

The performance of the high-end optoelectronic devices is essentially influenced by the intrinsic relaxation mechanisms pursued by the hot carriers. Therefore, the key toward achieving progression in such fields lies in developing a complete understanding of the involved carrier cooling dynamics. In this work, an endeavor has been made to highlight the difference in the cooling mechanisms in 2D CsPbBr3 nanosheets (NSs) and their 3D counterpart nanocrystals (NCs) with the aid of femtosecond broad-band pump-probe spectroscopy, varying the excitation energies. The exciton and biexciton dynamics in both systems are found to be retarded upon increasing the excitation energy. However, in contrast to 3D NCs, carrier cooling is found to be faster in the 2D system, regardless of the excitation energy used, attributing this to less efficient charge screening by Fröhlich interaction in low-dielectric medium. A similar trend is replicated in the biexciton formation rate since the formation is also found to be faster in NSs compared to NCs.

Ni-Co bimetal nanowires filled multiwalled carbon nanotubes for the highly sensitive and selective non-enzymatic glucose sensor applications.

The facile, time and cost efficient and environmental benign approach has been developed for the preparation of Nickel (Ni)-Cobalt (Co) alloy nanowires filled multiwalled carbon nanotubes (MWCNTs) with the aid of mesoporous silica nanoparticles (MSN)/Ni-Co catalyst. The controlled incorporation of Ni-Co nanostructures in the three dimensional (3D) pore structures of MSN yielded the catalytically active system for the MWCNT growth. The inner surface of MWCNTs was quasi-continuously filled with face-centered cubic (fcc) structured Ni-Co nanowires. The as-prepared nanostructures were exploited as non-enzymatic electrochemical sensor probes for the reliable detection of glucose. The electrochemical measurements illustrated that the fabricated sensor exhibited an excellent electrochemical performance toward glucose oxidation with a high sensitivity of 0.695 mA mM-1 cm-2, low detection limit of 1.2 µM, a wide linear range from 5 µM-10 mM and good selectivity. The unprecedented electrochemical performances obtained for the prepared nanocomposite are purely attributed to the synergistic effects of Ni-Co nanowires and MWCNTs. The constructed facile, selective and sensitive glucose sensor has also endowed its reliability in analyzing the human serum samples, which wide opened the new findings for exploring the novel nanostructures based glucose sensor devices with affordable cost and good stability.