Promoting exciton dissociation by metal ion modification in polymeric carbon nitride for photocatalysis

Carbon nitride,a typical low-dimensional conjugated polymer photocatalyst,features a high exciton binding energy due to the weak dielectric screening and the strong Coulombic attraction of photogenerated electrons and holes.The reduction of the exciton binding energy of carbon nitride to promote the conversion from excitons into free carriers is the first priority for the improvement of charge-transfer-dependent photocatalytic reaction activity.In this paper,by introducing a variety of polar metal cations to carbon nitride,it is demonstrated that the charge distribution of the heptazine ring can be improved by ion polarization,which effectively promotes the dissociation of excitons into electrons and holes.The sodium ion shows the best modification effect,which enhances the rate of both photocatalytic hydrogen and hydrogen peroxide production by about 50%.Characterization shows that the introduction of strongly polar metal cations contributes to the reduction of the exciton dissociation energy of carbon nitride.This study provides a new perspective and a convenient method for the exciton modulation engineering of low-dimensional photocatalysts.

Exciton dissociation dynamics and light-driven H2 generation in colloidal 2D cadmium chalcogenide nanoplatelet heterostructures

Solar-to-H2 conversion is attracting much research attention as a potential approach to meet global renewable energy demands. Although significant advances have been made using metal-tipped colloidal cadmium chalcogenide zero-dimensional （0D） quantum dots and one-dimensional （1D） nanorod heterostructures in solar-to-H2 conversion, their efficiency may be further enhanced using an emerging class of colloidal cadmium chalcogenide nanocrystals, namely two-dimensional （2D） nanoplatelets （NPLs）, because of their unique properties. In this review, we summarize the recent advances on exciton dissociation dynamics and light-driven H2 generation performance of colloidal nanoplatelet heterostructures. Following an introduction on the electronic structure of 2D NPLs, we discuss the dynamics of exciton dissociation by electron transfer to molecular acceptors. The exciton quenching dynamics of CdS NPL-Pt and CdSe NPL-Pt heterostructures are compared to highlight the effect of material properties on the relative contributions of the energy-transfer and electron- transfer pathways. Representative solar-to-H2 conversion performances of 2D NPL-metal heterostructures are discussed and compared with those of 1D nanorod-metal heterostructures. Finally, we discuss the challenges in further improving the solar-to-fuel conversion efficiencies of these systems.

Analytical Study on Piezoelectric Effects on Exciton Dissociation

We analytically and numerically compute the Onsager dissociation rate(exciton dissociation)on an interface induced by a piezoelectric potential in an inorganicorganic hybrid p-n junction system(ZnO+(poly(p-phenylene vinylene));PPV).When a positive piezoelectric potential is created at the interface region owing to the deformation of the system,free electrons accumulate at the interface.Hence,screening effects are observed.It is assumed that the electron layer formed at the interface then attracts free holes from the p-type PPV region,which leads to exciton formation,possibly via the Langevin recombination process.The increased exciton density can then contribute to the Onsager dissociation rate,which is maximum around the interface.This paper focuses on the role of piezoelectric effects in promoting exciton formation at the interface and its relation with the exciton dissociation rate.

Design of ultranarrow-bandgap acceptors for efficient organic photovoltaic cells and highly sensitive organic photodetectors

The fabrication of multifunctional electronic devices based on the intriguing natures of organic semiconductors is crucial for organic electronics.Ultranarrow-bandgap materials are in urgent demand for fabricating high-performance organic photovoltaic(OPV)cells and highly sensitive near-infrared organic photodetectors(OPDs).By combining alkoxy modification and an asymmetric strategy,three narrowbandgap electronic acceptors(BTP-4F,DO-4F,and QO-4F)were synthesized with finely tuned molecular electrostatic potential(ESP)distributions.Through the careful modulation of electronic configurations,the optical absorption onsets of DO-4F and QO-4F exceeded 1μm.The experimental and theoretical results suggest that the small ESP of QO-4F is beneficial for achieving a low nonradiative voltage loss,while the large ESP of BTP-4F can help obtain high exciton dissociation efficiency.By contrast,the asymmetric acceptor DO-4F with a moderate ESP possesses balanced voltage loss and exciton dissociation,yielding the best power conversion efficiency of 13.6%in the OPV cells.OPDs were also fabricated based on the combination of PBDB-T:DO-4F,and the as-fabricated device outputs a high shot-noise-limited specific detectivity of 3.05×10^(13) Jones at 850 nm,which is a very good result for near-infrared OPDs.This work is anticipated to provide a rational way of designing high-performance ultranarrow-bandgap organic semiconductors by modulating the molecular ESP.

Exciton diffusion and dissociation in organic and quantum-dot solar cells

For the process of photovoltaic conversion in organic solar cells(OSCs)and quantum-dot solar cells(QDSCs),three of four steps are determined by exciton behavior,namely,exciton generation,exciton diffusion,and exciton dissociation.Therefore,it is of great importance to regulate exciton behavior in OSCs and QDSCs for achieving high power conversion efficiency.Due to the rapid development in materials and device fabrication,great progress has been made to manage the exciton behavior to achieve prolonged exciton diffusion length and improved exciton dissociation in recent years.In this review,we first introduce the parameters that affect exciton behavior,followed by the methods to measure exciton diffusion length.Then,we provide an overview of the recent advances with regard to exciton behavior investigation in OSCs and QDSCs,including exciton lifetime,exciton diffusion coefficient,and exciton dissociation.Finally,we propose future directions in deepening the understanding of exciton behavior and boosting the performance of OSCs and QDSCs.

Deposition of Ag Nanoparticles on Carbon Microspheres Surface：Evaluation of Structures,Electrochemical and Optical Properties

A hybrid material of carbon microspheres（CMSs） with Ag decoration（Ag/CMSs） was developed.Poly（3-hexylthiophene）：Ag/CMSs composite film was prepared by spin-coating.Scanning electron microscopy,transmission electron microscopy,Fourier transform infrared spectrometry,X-ray diffraction,thermogravimetric analysis and cyclic voltammetry were employed to analyze the morphologies,structures,thermal properties and energy levels of Ag/CMSs.The optical property of the composite films was characterized by ultraviolet-visible spectrophotometry and fluorescent spectrometry.The results indicate that silver nanoparticles（AgNPs,d = 10-20 nm） are distributed on the surface of CMSs.LUMO and HOMO energy levels of Ag/CMSs are-3.97 and-5.52 e V,below the vacuum energy level,respectively,indicating that it is feasible to use Ag/CMSs as an electron acceptor.Ag NPs are blended into the active layer to trigger localized surface plasmon resonance,and consequently enhance light harvesting.The coupling of surface plasmons and excitons increased the probability of exciton dissociation.

Steric hindrance induced low exciton binding energy enables low-driving-force organic solar cells

Exciton binding energy(E_(b))has been regarded as a critical parameter in charge separation during photovoltaic conversion.Minimizing the E_(b) of the photovoltaic materials can facilitate the exciton dissociation in low-driving force organic solar cells(OSCs)and thus improve the power conversion efficiency(PCE);nevertheless,diminishing the E_(b) with deliberate design principles remains a significant challenge.Herein,bulky side chain as steric hindrance structure was inserted into Y-series acceptors to minimize the E_(b) by modulating the intra-and intermolecular interaction.Theoretical and experimental results indicate that steric hindrance-induced optimal intra-and intermolecular interaction can enhance molecular polarizability,promote electronic orbital overlap between molecules,and facilitate delocalized charge trans-fer pathways,thereby resulting in a low E_(b).The conspicuously reduced E_(b) obtained in Y-ChC5 with pinpoint steric hindrance modulation can minimize the detrimental effects on exciton dissociation in low-driving-force OSCs,achieving a remarkable PCE of 19.1%with over 95%internal quantum efficiency.Our study provides a new molecular design rationale to reduce the E_(b).

Pyrimidine donor induced built-in electric field between melon chains in crystalline carbon nitride to facilitate excitons dissociation

The strong intrinsic Coulomb interactions of Frenkel excitons in crystalline carbon nitride(CCN) greatly limits their dissociation into electrons and holes, resulting in unsatisfactory charges separation and photocatalytic efficiency. Herein, we propose a strategy to facilitate excitons dissociation by molecular regulation induced built-in electric field(BIEF). The electron-rich pyrimidine-ring into CCN changes the charge density distribution over heptazine-rings to induce BIEF between melon chains. Such BIEF is sufficient to overcome the considerable exciton binding energy(EBE) and reduce it from 38.4 meV to 16.4 meV,increasing the excitons dissociation efficiency(EDE) from 21.5% to 51.9%. Our results establish a strategy to facilitate excitons dissociation through molecular regulation induced BIEF, targeting the intrinsic high EBE and low EDE of polymer photocatalysts.

配体密度控制促进CsPbBr_(3)量子点激子解离与能量转移实现高效CO_(2)光催化还原

Perovskite quantum dots(PQDs) hold immense potential as photocatalysts for CO_(2) reduction due to their remarkable quantum properties,which facilitates the generation of multiple excitons,providing the necessary high-energy electrons for CO_(2) photoreduction.However,harnessing multi-excitons in PQDs for superior photocatalysis remains challenging,as achieving the concurrent dissociation of excitons and interparticle energy transfer proves elusive.This study introduces a ligand density-controlled strategy to enhance both exciton dissociation and interparticle energy transfer in CsPbBr_(3) PQDs.Optimized CsPbBr_(3) PQDs with the regulated ligand density exhibit efficient photocatalytic conversion of CO_(2) to CO,achieving a 2.26-fold improvement over unoptimized counterparts while maintaining chemical integrity.Multiple analytical techniques,including Kelvin probe force microscopy,temperaturedependent photoluminescence,femtosecond transient absorption spectroscopy,and density functional theory calculations,collectively affirm that the proper ligand termination promotes the charge separation and the interparticle transfer through ligand-mediated interfacial electron coupling and electronic interactions.This work reveals ligand density-dependent variations in the gas-solid photocatalytic CO_(2) reduction performance of CsPbBr_(3) PQDs,underscoring the importance of ligand engineering for enhancing quantum dot photocatalysis.

Critical Role of Molecular Electrostatic Potential on Charge Generation in Organic Solar Cells

Revealing the charge generation is a crucial step to understand the organic photovoltaics. Recent development in non-fullerene organic solar cells （OSCs） indicates efficient charge separation even with negligible energetic offset between the donor and acceptor materials. These new findings trigger a critical question concerning the charge separation mechanism in OSCs, traditionally believed to result from sufficient energetic offset between the polymer donor and fullerene acceptor. We propose a new mechanism, which involves the molecular electrostatic potential, to explain efficient charge separation in non-fullerene OSCs. Together with the new mechanism, we demonstrate a record efficiency of -12% for systems with negligible energetic offset between donor and acceptor materials. Our analysis also rationalizes different requirement of the energetic offset between fullerene-based and non-fullerene OSCs, and paves the way for further design of OSC materials with both high photocurrent and high photovottage at the same time.

Understanding the roles of carbon in carbon/g-C_(3)N_(4)based photocatalysts for H2 evolution

Coupling graphitic carbon nitride(CN)with carbonaceous materials is an effective strategy to improve photocatalytic performance,but the contributions of carbonaceous materials are not fully understood.Herein,a new type of carbon/CN(CCN)complex photocatalyst is synthesized with a 6-fold enhancement of H2 evolution rate compared to that of pristine CN.The role of carbon in photocatalytic H2 evolution reaction is systemically studied and it is experimentally and theoretically revealed that carbon mainly contributes to the improved capability of exciton dissociation and enhanced electric conductivity for charge transfer,leading to an increased population of photo-carriers for photocatalytic reactions.Interestingly,the enhanced light absorption originated from carbon barely generates charge carriers for H2 evolution activity.These new findings will inspire the rational design of carbon-based photocatalysts for efficient solar fuel production.

Improved efficiency of ternary the blend polymer solar cells by doping a narrow band gap polymer material

A series of P3HT：PC71BM polymer solar cells （PSCs） with different PIDTDTQx doping concentrations were fabricated to in- vestigate the effect of the PIDTDTQx as a complementary electron donor on the performance of PSCs. The power conversion efficiency （PCE） of the optimized ternary blend PSCs （with 2 wt% PIDTDTQx） reached 3.87%, which is 28% higher than that of the PSCs based on P3HT：PCvlBM （control cells）. The short-circuit current density （J^c） was increased to 10.20 mA/cm2 compared with the control cells. The PCE improvement could be attributed to more photon harvest and charge carrier transport by appropriate doping PIDTDTQx. The energy transfer from P3HT to PIDTDTQx was demonstrated from the 650 nm emis- sion intensity decrease and the red-shifted emission peaks from 725 nm to 737 nm along with the increase of PIDTDTQx dop- ing concentrations.