Photoinduced carrier transfer dynamics in a monolayer MoS2/PbS quantum dots heterostructure.

Two-dimensional molybdenum disulfide (MoS2) has been proven to be a candidate in photodetectors, and MoS2/lead sulfide (PbS) quantum dots (QDs) heterostructure has been used to expand the optical response wavelength of MoS2. Time-resolved pump-probe transient absorption measurements are performed to clarify the carrier transfer dynamics in the MoS2/PbS heterostructure. By comparing the carrier dynamics in MoS2 and MoS2/PbS under different pump wavelengths, we found that the excited electrons in PbS QDs can transfer rapidly (<100 fs) to MoS2, inducing its optical response in the near-infrared region, although the pump light energy is lower than the bandgap of MoS2. Besides, interfacial excitons can be formed in the heterostructure, prolonging the lifetime of the excited carriers, which could be beneficial for the extraction of the carriers in devices.

Energy transfer enhanced photoluminescence of 2D/3D CsPbBr3 hybrid assemblies.

Energy transfer has been proven to be an effective method to optimize optoelectronic conversion efficiency by improving light absorption and mitigating nonradiative losses. We prepared 2D/3D CsPbBr3 hybrid assemblies at different reaction temperatures using the hot injection method and found that the photoluminescence quantum yields (PLQYs) of these hybrids were greatly enhanced from 53.4% to 72.57% compared with 3D nanocrystals (NCs). Femtosecond transient absorption measurements were used to study the PLQY enhancement mechanisms, and it was found that the hot carrier lifetime improved from 0.36 to 1.88 ps for 2D/3D CsPbBr3 hybrid assemblies owing to the energy transfer from 2D nanoplates to 3D NCs. The energy transfer benefits the excited carrier accumulation and prolonged hot carrier lifetime in 3D NCs in hybrid assemblies, as well as PLQY enhancement in materials.

Non-Fully Conjugated Dimerized Giant Acceptors with Different Alkyl-Linked Sites for Stable and 19.13 % Efficiency Organic Solar Cells.

Achieving both high power conversion efficiency (PCE) and device stability is a major challenge for the practical development of organic solar cells (OSCs). Herein, three non-fully conjugated dimerized giant acceptors (named 2Y-sites, including wing-site-linked 2Y-wing, core-site-linked 2Y-core, and end-site-linked 2Y-end) are developed. They share the similar monomer precursors but have different alkyl-linked sites, offering the fine-tuned molecular absorption, packing, glass transition temperature, and carrier mobility. Among their binary active layers, D18/2Y-wing has better miscibility, leading to optimized morphology and more efficient charge transfer compared to D18/2Y-core and D18/2Y-end. Therefore, the D18/2Y-wing-based OSCs achieve a superior PCE of 17.73 %, attributed to enhanced photocurrent and fill factor. Furthermore, the D18/2Y-wing-based OSCs exhibit a balance of high PCE and improved stability, distinguishing them within the 2Y-sites. Building on the success of 2Y-wing in binary systems, we extend its application to ternary OSCs by pairing it with the near-infrared absorbing D18/BS3TSe-4F host. Thanks to the complementary absorption within 300-970 nm and further optimized morphology, ternary OSCs obtain a higher PCE of 19.13 %, setting a new efficiency benchmark for the dimer-derived OSCs. This approach of alkyl-linked site engineering for constructing dimerized giant acceptors presents a promising pathway to improve both PCE and stability of OSCs.

Determination of the refractive-index change in the excited state based on transient absorption microscopy.

Photoinduced excited-state carriers can affect both the absorption coefficient and refractive index of materials and influence the performance of photoelectric devices. Femtosecond time-resolved pump-probe transient absorption (TA) spectroscopy is usually used to detect carrier dynamics and excited-state absorption coefficients; however, measurements of transient refractive-index change are still difficult. We propose a method for determining the excited-state refractive-index change using TA microscopy. In TA measurements, a Fabry-Pérot cavity formed by the front and back surfaces of the sample could lead to interference of the probe light. As the wavelength of standing waves in the Fabry-Pérot cavity is closely related to the refractive index, the carrier-induced excited-state refractive-index change was obtained by comparing the transmission probe spectra between the ground and excited states. The proposed method was used to study the dynamics of excited-state refractive-index change in a perovskite film.

Unidirectional Sidechain Engineering to Construct Dual-Asymmetric Acceptors for 19.23 % Efficiency Organic Solar Cells with Low Energy Loss and Efficient Charge Transfer.

Achieving both high open-circuit voltage (Voc ) and short-circuit current density (Jsc ) to boost power-conversion efficiency (PCE) is a major challenge for organic solar cells (OSCs), wherein high energy loss (Eloss ) and inefficient charge transfer usually take place. Here, three new Y-series acceptors of mono-asymmetric asy-YC11 and dual-asymmetric bi-asy-YC9 and bi-asy-YC12 are developed. They share the same asymmetric D1 AD2 (D1 =thieno[3,2-b]thiophene and D2 =selenopheno[3,2-b]thiophene) fused-core but have different unidirectional sidechain on D1 side, allowing fine-tuned molecular properties, such as intermolecular interaction, packing pattern, and crystallinity. Among the binary blends, the PM6 : bi-asy-YC12 one has better morphology with appropriate phase separation and higher order packing than the PM6 : asy-YC9 and PM6 : bi-asy-YC11 ones. Therefore, the PM6 : bi-asy-YC12-based OSCs offer a higher PCE of 17.16 % with both high Voc and Jsc , due to the reduced Eloss and efficient charge transfer properties. Inspired by the high Voc and strong NIR-absorption, bi-asy-YC12 is introduced into efficient binary PM6 : L8-BO to construct ternary OSCs. Thanks to the broadened absorption, optimized morphology, and furtherly minimized Eloss , the PM6 : L8-BO : bi-asy-YC12-based OSCs achieve a champion PCE of 19.23 %, which is one of the highest efficiencies among these annealing-free devices. Our developed unidirectional sidechain engineering for constructing bi-asymmetric Y-series acceptors provides an approach to boost PCE of OSCs.

Nonlinear optical limiting property of the carboxyl-functionalized Ti3C2 MXene nanosheets.

Nonlinear optical limiting (OL) properties of carboxyl-functionalized Ti3C2 nanosheets (COOH-MXene) were studied using the nanosecond laser Z-scan technology. COOH-MXene showed excellent broadband OL properties with OL thresholds of 0.34 J/cm2 at 532 nm and 0.58 J/cm2 at 1064 nm, and the OL mechanism was mainly attributed to the reverse saturable absorption effect. Femtosecond time-resolved transient absorption measurements were used to clarify the ultrafast carrier dynamics in the OL process, and the results revealed that excited states absorption (ESA) in MXene was enhanced by introducing more carboxyl group terminations. When COOH-MXene was irradiated by laser pulses, excited electrons in the conduction band of MXene could transfer to the carboxyl groups and induce the ESA in the surface functional groups, resulting in the excellent OL property of COOH-MXene.

Nonlinear optical limiting property and carrier dynamics in tin phthalocyanine porous organic frameworks.

The nonlinear optical limiting (OL) property of tin phthalocyanine porous organic frameworks (Sn-Pc-POFs) dispersion in the nanosecond regime was studied, which showed excellent dispersibility and stability as well as a low OL threshold. To clarify the nonlinear optical response mechanisms in the material, the energy level structure of Sn-Pc-POFs was simulated using the density functional theory calculation, and the photoinduced carrier dynamics was studied using femtosecond time-resolved transient absorption spectroscopy. The results indicated that the large absorption cross section and long lifetime of the excited state were responsible for the excellent OL property of the material.

Control of Hot Carrier Relaxation in CsPbBr3 Nanocrystals Using Damping Ligands.

In photon-conversion processes, rapid cooling of photo-induced hot carriers is a dominant energy loss channel. We herein report a 3-fold reduced hot carrier cooling rate in CsPbBr3 nanocrystals capped with a cross-linked polysiloxane shell in comparison to single alkyl-chain oleylamine ligands. Relaxation of hot charge carriers depends on the carrier-phonon coupling (CPC) process as an important channel to dissipate energies in nanostructured perovskite materials. The CPC strengths in the two samples were measured through cryogenic photoluminescence spectroscopic measurements. The effect of organic ligands on the CPC in CsPbBr3 nanocrystals is elucidated based on a damped oscillation model. This supplements the conventional polaron-based CPC model, by involving a damping effect on the CPC from the resistance of the ligands against nanocrystal lattice vibrations. The model also accounts for the observed linear temperature-dependence of the CPC strength. Our work enables predictions about the effect of the ligands on the performance of perovskite nanocrystals in future applications.

Fabrication of high-temperature tilted fiber Bragg gratings using a femtosecond laser.

We developed a new method to fabricate tilted fiber Bragg gratings (TFBGs) by using a femtosecond laser with a phase mask. During the laser processing, the fiber was obliquely moved at a tilt angle, in which the laser beam and the phase mask were fixed. The peak loss of the cladding modes with a tilt angle of 4.9° reaches to ~-8 dB, and the insertion loss is less than -0.2 dB. The TFBG was stable at temperature up to 700°C and slight degraded at 800 °C. The temperature sensing of the TFBG was demonstrated at a high temperature up to 800°C. The temperature sensitivities of the Bragg mode, the ghost mode, and the cladding mode were measured to be 15.72 pm/°C, 15.56 pm/°C, and 15.52 pm/°C, respectively. The refractive index response of the TFBGs was also measured.

Optical limiting properties and mechanisms of single-layer graphene dispersions in heavy-atom solvents.

The optical limiting (OL) properties of single-layer graphene dispersions in different solvents were studied using a nanosecond pulse laser. The graphene dispersions, especially in heavy-atom solvents, showed much better OL properties compared with referenced C60-toluene solution. The dependences of OL thresholds and nonlinear scattering (NLS) intensities on the solvent surface tensions indicated that, NLS effect played an important role in the OL process of graphene dispersions, while nonlinear absorption (NLA) effect might also contribute in solvents with heavy atoms. The NLA measurements further demonstrated the contribution of NLA effect to the excellent OL property of graphene dispersions in heavy-atom solvents.

High-frame-rate observation of single femtosecond laser pulse propagation in fused silica using an echelon and optical polarigraphy technique.

We have demonstrated high-frame-rate observations of a single femtosecond laser pulse propagating in transparent medium using the optical polarigraphy technique and an echelon. The echelon produced a spatially encoded time delay for the probe pulse to capture directly four successive images of an intense propagating pulse with picosecond time interval and femtosecond time resolution. Using this method, we observed the propagation process of a single femtosecond laser pulse in fused silica. The influence of pulse-energy fluctuation on the spatial and temporal distribution of the single laser pulse was visualized using the single-shot measurements.

Enhanced Optical Limiting of Gold Nanoparticles/Porous Carbon Nanocomposites.

With the wide application of laser weapons, the requirements of laser protection technology are becoming more and more strict. Therefore, it is important to find ideal optical limiting (OL) materials to protect human eyes and detectors. In this work, the nonlinear optical responses of gold nanoparticles/porous carbon (Au NPs/PC) nanocomposites prepared by the reduction method were studied using the nanosecond Z-scan technique. Compared with porous carbon, the Au NPs/PC nanocomposites show a lower damage threshold, a bigger optical limiting index and a wider absorption spectrum. The interaction between gold nanoparticles and porous carbon enhances the nonlinear scattering effect of suspended bubbles. These results indicate that Au NPs composites have potential applications in the protection of human eyes and detectors.

Exceptional Stability against Water, UV Light, and Heat for CsPbBr3@Pb-MOF Composites.

All-inorganic lead halide perovskite nanocrystals (NCs) have been widely applied in optoelectronic devices owing to their excellent photoluminescence (PL) properties. However, poor stability upon exposure to water, UV light or heat strongly limits their practical application. Herein, CsPbBr3@Pb-MOF composites with exceptional stability against water, UV light, and heat are synthesized by ultrasonic processing the precursors of lead-based MOF (Pb-MOF), oleylammonium bromide (OAmBr) and cesium oleate (Cs-OA) solutions at room temperature. Pb-MOF can not only provide the lead source for the in situ growth of CsPbBr3 NCs, but also the protective layer of perovskites NCs. The formed CsPbBr3@Pb-MOF composites show a considerable PL quantum yield (PLQY) of 67.8%, and can maintain 90% of the initial PL intensity when immersed in water for 2 months. In addition, the outstanding PL stability against UV light and heat is demonstrated with CsPbBr3 NCs synthesized by the conventional method as a comparison. Finally, a green (light-emitting diode) LED is fabricated using green-emitting CsPbBr3@Pb-MOF composites and exhibits excellent stability without packaging when immersed in water for 30 days. This study provides a practical approach to improve the stability in aqueous phase, which may pave the way for future applications for various optoelectronic devices.

Optical Limiting Response of Porous Carbon Dispersions.

With the wide application of intense lasers, the protection of human eyes and detectors from laser damage is becoming more and more strict. In this paper, we study the nonlinear optical limiting (OL) properties of porous carbon with a super large specific surface area (2.9 × 103 m2/g) using the nanosecond Z-scan technique. Compared to the traditional OL material C60, the porous carbon material shows an excellent broadband limiting effect, and the limiting thresholds correspond to 0.11 J/cm2 for 532 nm and 0.25 J/cm2 for 1064 nm pulses, respectively. The nonlinear scattering experiments showed that the OL behavior was mainly attributed to the nonlinear scattering effect, which is caused by the rapid growth and expansion of bubbles in the dispersion induced by laser irradiation, and the scattered light distribution is consistent with the results of Mie's scattering. These results suggest that porous carbon materials are expected to be applied to the field of laser protection in the future to further protect the human eye and precision optical instruments.

Halogenated Dibenzo[f,h]quinoxaline Units Constructed 2D-Conjugated Guest Acceptors for 19% Efficiency Organic Solar Cells.

Halogenation of Y-series small-molecule acceptors (Y-SMAs) is identified as an effective strategy to optimize photoelectric properties for achieving improved power-conversion-efficiencies (PCEs) in binary organic solar cells (OSCs). However, the effect of different halogenation in the 2D-structured large π-fused core of guest Y-SMAs on ternary OSCs has not yet been systematically studied. Herein, four 2D-conjugated Y-SMAs (X-QTP-4F, including halogen-free H-QTP-4F, chlorinated Cl-QTP-4F, brominated Br-QTP-4F, and iodinated I-QTP-4F) by attaching different halogens into 2D-conjugation extended dibenzo[f,h]quinoxaline core are developed. Among these X-QTP-4F, Cl-QTP-4F has a higher absorption coefficient, optimized molecular crystallinity and packing, suitable cascade energy levels, and complementary absorption with PM6:L8-BO host. Moreover, among ternary PM6:L8-BO:X-QTP-4F blends, PM6:L8-BO:Cl-QTP-4F obtains a more uniform and size-suitable fibrillary network morphology, improved molecular crystallinity and packing, as well as optimized vertical phase distribution, thus boosting charge generation, transport, extraction, and suppressing energy loss of OSCs. Consequently, the PM6:L8-BO:Cl-QTP-4F-based OSCs achieve a 19.0% efficiency, which is among the state-of-the-art OSCs based on 2D-conjugated Y-SMAs and superior to these devices based on PM6:L8-BO host (17.70%) and with guests of H-QTP-4F (18.23%), Br-QTP-4F (18.39%), and I-QTP-4F (17.62%). The work indicates that halogenation in 2D-structured dibenzo[f,h]quinoxaline core of Y-SMAs guests is a promising strategy to gain efficient ternary OSCs.

Visualization of Hot Carrier Dynamics in a Single CsPbBr3 Perovskite Microplate Using Femtosecond Kerr-Gated Wide-Field Fluorescence Spectroscopy.

Lead halide perovskites (LHPs) have excellent semiconductor properties. They have been used in many applications such as solar cells. Recently, the hot carrier dynamics in this type of material have received much attention as they are useful for enhancing the performance of optoelectrical devices fabricated from it. Here, we study the ultrafast hot carrier dynamics of a single CsPbBr3 microplate using femtosecond Kerr-gated wide-field fluorescence spectroscopy. The transient photoluminescence spectra have been measured under a variety of excitation fluences. The temporal evolution of bandgap renormalization and the competition between hot carrier cooling and the recovery of the renormalized bandgap are clearly revealed.

Reduction of pulse-to-pulse fluctuation in laser pulse energy using the optical Kerr effect.

An all-optical stabilization method of laser pulse energy is proposed using the optical Kerr effect (OKE). The method uses the OKE induced by a portion of the laser pulse as a power controller. The decrease (increase) in the throughput of the optical setup for OKE compensates for the increase (decrease) in pulse energy, thereby stabilizing the pulse-to-pulse fluctuation in pulse energy. The validity of this principle was proven by experiments with a femtosecond laser.

Two Compatible Acceptors as an Alloy Model with a Halogen-Free Solvent for Efficient Ternary Polymer Solar Cells.

A ternary strategy of halogen-free solvent processing can open up a promising pathway for the preparation of polymer solar cells (PSCs) on a large scale and can effectively improve the power conversion efficiency with an appropriate third component. Herein, the green solvent o-xylene (o-XY) is used as the main solvent, and the non-fullerene acceptor Y6-DT-4F as the third component is introduced into the PBB-F:IT-4F binary system to broaden the spectral absorption and optimize the morphology to achieve efficient PSCs. The third component, Y6-DT-4F, is compatible with IT-4F and can form an "alloy acceptor", which can synergistically optimize the photon capture, carrier transport, and collection capabilities of the ternary device. Meanwhile, Y6-DT-4F has strong crystallinity, so when introduced into the binary system as the third component can enhance the crystallization, which is conducive to the charge transport. Consequently, the optimal ternary system based on PBB-F:IT-4F:Y6-DT-4F achieved an efficiency of 15.24%, which is higher than that of the binary device based on PBB-F:IT-4F (13.39%).

Ultrafast Kinetics of Chlorinated Polymer Donors: A Faster Excitonic Dissociation Path.

Halogen-substituted donor/acceptor materials are widely regarded as a promising strategy toward improved power-conversion efficiencies (PCEs) in polymer solar cells (PSCs). A chlorinated polymer donor, PClBTA-PS, and its non-chlorinated analogue, PBTA-PS, are synthesized. The PClBTA-PS-based devices show significant enhancements in terms of open-circuit voltage (VOC = 0.82 V) and fill factor (FF = 76.20%). In addition, a PCE of 13.20% is obtained, which is significantly higher than that for the PBTA-PS-based devices (PCE = 7.63%). Grazing incident wide-angle X-ray scattering shows that the chlorinated polymer enables better π-π stacking in both pure and blend films. DFT and TD-DFT calculations as well as ultrafast photophysics measurements indicate that chlorinated PClBTA-PS has a smaller bonding energy and a longer spontaneous-emission lifetime. The results also reveal that the charge-transfer-state excitons in PClBTA-PS:IT4Cl blend films split into the charge-separated (CS) state via a faster dissociation path, which produces a higher yield of the CS state. Overall, this study provides a deeper understanding of how a halogen-substituted polymer can improve PSCs in the future.

Harvesting the Triplet Excitons of Quasi-Two-Dimensional Perovskite toward Highly Efficient White Light-Emitting Diodes.

Utilization of triplet excitons, which generally emit poorly, is always fundamental to realize highly efficient organic light-emitting diodes (LEDs). While triplet harvest and energy transfer via electron exchange between triplet donor and acceptor are fully understood in doped organic phosphorescence and delayed fluorescence systems, the utilization and energy transfer of triplet excitons in quasi-two-dimensional (quasi-2D) perovskite are still ambiguous. Here, we use an orange-phosphorescence-emitting ultrathin organic layer to probe triplet behavior in the sky-blue-emitting quasi-2D perovskite. The delicate white LED architecture enables a carefully tailored Dexter-like energy-transfer mode that largely harvests the triplet excitons in quasi-2D perovskite. Our white organic-inorganic LEDs achieve maximum forward-viewing external quantum efficiency of 8.6% and luminance over 15 000 cd m-2, exhibiting a significant efficiency enhancement versus the corresponding sky-blue perovskite LED (4.6%). The efficient management of energy transfer between excitons in quasi-2D perovskite and Frenkel excitons in the organic layer opens the door to fully utilizing excitons for white organic-inorganic LEDs.