Charge Recombination Processes Research Articles

Theoretical study on the effect of end groups and core ring numbers in non-fullerene acceptors for organic solar cells

Modifying the skeleton, side chains and end groups of non-fullerene acceptors (NFAs) is important approach to improve the photovoltaic performance of organic solar cells. We selected PBDB-T as the donor and INIC, INIC1, INIC2, ITCC, ITCPTC and ITIC as the acceptors in order to study the difference in NFAs. The geometries, electronic structures, excited state properties, excited-state lifetimes and rate constants of charge transfer (CT), charge recombination (CR) and exciton dissociation (ED) processes of the monomers and PBDB-T:NFA complexes have been investigated by means of density functional theory and time-dependent density functional theory. The research reveals that fluorination of end groups, thienyl substitution and increase in the number of central rings result in the decrease in the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. Fluorine substitution and increase in central ring numbers cause redshift of the absorption spectrum for NFAs. NFAs with fluorine substitution have a larger maximum electrostatic potential, which facilitates CT at the donor:NFA heterojunction interfaces. Fluorine substitution and thienyl substitution decrease the NFA's CT distances, indicating that both fluorine and thienyl substitutions affect the CT rate. Fluorine substitution, thienyl modification of end groups and increase in central ring numbers also reduce the exciton binding energy, which is beneficial for decreasing CR and improving ED efficiency.

Controlled phase distribution of quasi-2D perovskite enables improved electroluminescence

Quasi-two-dimensional (quasi-2D) perovskites are increasingly explored for integration into light-emitting diodes (LEDs) as light-emissive layers. However, the quasi-2D perovskite films likely exhibit non-uniform dimensional phase distribution and irregular internal crystal structures. These characteristics are known to contribute to undesirable effects, including non-radiative recombination losses and radiative recombination in perovskites of various dimensions, impeding the realization of efficient electroluminescence and high color purity in LEDs. In this study, we present an investigation on the correlation between the dimensional distribution of quasi-2D perovskites and charge carrier behavior by modulating anti-solvent dripping during the film fabrication processes. We provide a comprehensive analysis of the impact of controlled dimensional distribution on charge injection and recombination processes associated with the performance of quasi-2D perovskite LEDs. Our work emphasizes the crucial role played by controlled dimensionality in quasi-2D perovskites in realizing efficient and stable perovskite-based LEDs.

Photogenerated carrier dynamics of Mn2+ doped CsPbBr3 assembled with TiO2 systems: Effect of Mn doping content.

In recent years, all-inorganic perovskite materials have become an ideal choice for new thin film solar cells due to their excellent photophysical properties and have become a research hotspot. Studying the ultrafast dynamics of photo-generated carriers is of great significance for further improving the performance of such devices. In this work, we focus on the transient dynamic process of CsPbBr3/TiO2 composite systems with different Mn2+ doping contents using femtosecond transient absorption spectroscopy technology. We used singular value decomposition and global fitting to analyze the transient absorption spectra and obtained three components, which are classified as hot carrier cooling, charge transfer, and charge recombination processes, respectively. We found that the doping concentration of Mn2+ has an impact on all three processes. We think that the following two factors are responsible: one is the density of defect states and the other is the bandgap width of perovskite. As the concentration of doped Mn2+ increases, the charge transfer time constant shows a trend of initially increasing, followed by a subsequent decrease, reaching a turning point. This indicates that an appropriate amount of Mn2+ doping can effectively improve the photoelectric performance of solar cell systems. We proposed a possible charge transfer mechanism model and further elucidated the microscopic mechanism of the effect of Mn2+ doping on the interface charge transfer process of the CsPbBr3/TiO2 solar cell system.

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.

Dimerized Acceptors with Conjugate‐Break Linker Enable Highly Efficient and Mechanically Robust Organic Solar Cells

AbstractDesigning new acceptors is critical for intrinsically stretchable organic solar cells (IS‐OSCs) with high efficiency and mechanical robustness. However, nearly all stretchable polymer acceptors exhibit limited efficiency and high‐performance small molecular acceptors are very brittle. In this regard, we select thienylene‐alkane‐thienylene (TAT) as the conjugate‐break linker and synthesize four dimerized acceptors by the regulation of connecting sites and halogen substitutions. It is found that the connecting sites and halogen substitutions considerably impact the overall electronic structures, aggregation behaviors, and charge transport properties. Benefiting from the optimization of the molecular structure, the dimerized acceptor exhibits rational phase separation within the blend films, which significantly facilitates exciton dissociation while effectively suppressing charge recombination processes. Consequently, FDY‐m‐TAT‐based rigid OSCs render the highest power conversion efficiency (PCE) of 18.07 % among reported acceptors containing conjugate‐break linker. Most importantly, FDY‐m‐TAT‐based IS‐OSCs achieve high PCE (14.29 %) and remarkable stretchability (crack‐onset strain [COS]=18.23 %), significantly surpassing Y6‐based counterpart (PCE=12.80 % and COS=8.50 %). To sum up, these findings demonstrate that dimerized acceptors containing conjugate‐break linkers have immense potential in developing highly efficient and mechanically robust OSCs.

Dimerized Acceptors with Conjugate-Break Linker Enable Highly Efficient and Mechanically Robust Organic Solar Cells.

Designing new acceptors is critical for intrinsically stretchable organic solar cells (IS-OSCs) with high efficiency and mechanical robustness. However, nearly all stretchable polymer acceptors exhibit limited efficiency and high-performance small molecular acceptors are very brittle. In this regard, we select thienylene-alkane-thienylene (TAT) as the conjugate-break linker and synthesize four dimerized acceptors by the regulation of connecting sites and halogen substitutions. It is found that the connecting sites and halogen substitutions considerably impact the overall electronic structures, aggregation behaviors, and charge transport properties. Benefiting from the optimization of the molecular structure, the dimerized acceptor exhibits rational phase separation within the blend films, which significantly facilitates exciton dissociation while effectively suppressing charge recombination processes. Consequently, FDY-m-TAT-based rigid OSCs render the highest power conversion efficiency (PCE) of 18.07 % among reported acceptors containing conjugate-break linker. Most importantly, FDY-m-TAT-based IS-OSCs achieve high PCE (14.29 %) and remarkable stretchability (crack-onset strain [COS]=18.23 %), significantly surpassing Y6-based counterpart (PCE=12.80 % and COS=8.50 %). To sum up, these findings demonstrate that dimerized acceptors containing conjugate-break linkers have immense potential in developing highly efficient and mechanically robust OSCs.

Enhanced photocatalytic efficiency of sol-gel derived ZnS-rGO binary nanocomposite

A facile chemical route of synthesis of ZnS-rGO binary nanocomposites is reported here. The efficacy of such nanocomposites as a photocatalyst in degrading the common pollutant dye such as Methylene blue (MB), has been thoroughly investigated and the underlying mechanism is also presented. The standard characterization methods were applied to understand the structure, bonding, morphology, optical and elemental compositions. The results indicated that the ZnS nanoparticles were well dispersed into the rGO nanosheets which due to their 2D sheet structure, served as a favourable template for growth and control of morphology. Increase in rGO amount showed a direct impact on particle size confirmed by XRD and Raman both. The synthesized nanocomposites were utilized as photocatalyst for the degradation of MB dyes under UV irradiation. The optimal combination of ZnS and rGO (in the ratio of 3:2) exhibited enhanced photocatalytic activity. A higher rate constant of 7.01×10−3 min−1, and an approximate degradation efficiency of 75% were obtained after 90 min of degradation. The improvement in photocatalytic activity can be attributed to the enhancement in charge separation, suppressed recombination of electron–hole (e−–h+) carriers, and a possible longer electron lifetime due to the presence of higher amount of rGO. Here, rGO assisted the suppression of charge recombination process in ZnS-rGO and ignited hydroxyl radicals and super-oxide ions which further accelerated the degradation rate of dye. Based on the nature of the dye and its concentration, a significant amount of rGO was needed to maximize the photocatalytic efficiency of ZnS-rGO binary nanocomposites. In addition, the dark current variation with applied bias was explored and it depicted a reduction in dark current with optimized amount of rGO in nanocomposite. The nanocomposites have a strong potential to be utilized in water purification and nano-detectors.

Photocatalytic Activity of Ag Nanoparticles Deposited on Thermoexfoliated g-C3N4.

The limited access to fresh water and the increased presence of emergent pollutants (EPs) in wastewater has increased the interest in developing strategies for wastewater remediation, including photocatalysis. Graphitic carbon nitride (g-C3N4) is a 2D non-metal material with outstanding properties, such as a 2.7 eV bandgap and physicochemical stability, making it a promising photocatalyst. This work reports the process of obtaining high-surface-area (SA) g-C3N4 using the thermal-exfoliation process and the posterior effect of Ag-nanoparticle loading over the exfoliated g-C3N4 surface. The photocatalytic activity of samples was evaluated through methylene blue (MB) degradation under visible-light radiation and correlated to its physical properties obtained by XRD, TEM, BET, and UV-Vis analyses. Moreover, 74% MB degradation was achieved by exfoliated g-C3N4 compared to its bulk counterpart (55%) in 180 min. Moreover, better photocatalytic performances (94% MB remotion) were registered at low Ag loading, with 5 wt.% as the optimal value. Such an improvement is attributed to the synergetic effect produced by a higher SA and the role of Ag nanoparticles in preventing charge-recombination processes. Based on the results, this work provides a simple and efficient methodology to obtain Ag/g-C3N4 photocatalysts with enhanced photocatalytic performance that is adequate for water remediation under sunlight conditions.

Dielectric multilayers impact on radiation-induced charge accumulation in highly sensitive oxide field effect transistors

Radiation dosimetry is crucial in many fields where the exposure to ionizing radiation must be precisely controlled to avoid health and environmental safety issues. Among solid state detectors, we recently demonstrated that Radiation sensitive OXide Field Effect Transistors (ROXFETs) are excellent candidates for personal dosimetry thanks to their fast response and high sensitivity to x rays. These transistors use indium–gallium–zinc oxide as a semiconductor, combined with a dielectric based on high-permittivity and high-atomic number materials. Here, we present a study on the ROXFET gate dielectric fabricated by atomic layer deposition, where we compare single- and multi-layer structures to determine the best-performing configuration. All the devices show stable operational parameters and high reproducibility among different detectors. We identified an optimized bi-layer dielectric structure made of tantalum oxide and aluminum oxide, which demonstrated a sensitivity of (63 ± 2) V/Gy, an order of magnitude larger than previously reported values. To explain our findings, we propose a model identifying the relevant charge accumulation and recombination processes leading to the large observed transistor threshold voltage shift under ionizing radiation, i.e., of the parameter that directly defines the sensitivity of the device.

Concepts behind the Redox Photocatalysis with Quantum Dots

AbstractIn this conceptual review, we present the optical and electronic characteristics of quantum dots for the designing of organic synthesis experiments through the photocatalysis. We discuss the control parameters associated to the charge recombination processes on the semiconductor interface. The positioning of bands and control of intragap states, such as charge donors and acceptors, can define the performance of faradaic processes. Electrochemical strategies, such as voltammetry, can be used as a powerful tool for the determining of the electronic band edges with good precision.

Structural and Optical Characteristics of Multilayer ZnO Nanorod:TiO2 by DC Magnetron Sputtering for Dye Sensitized Solar Cell (DSSC) Application

The development of Dye-Sensitized Solar Cell (DSSC) technology by utilizing various semiconductors has attracted attention. Among all types of semiconductors, the nanostructure of ZnO is selected due to their unique electrical properties and ease of preparation in various morphologies, and it has been considered a promising material to be applied in DSSC. In this research, the DC magnetron sputtering method was used to prepare ZnO thin films as an efficient alternative to press the charge recombination process that occurs in TiO2-based DSSC. Different thicknesses of TiO2 layers on the FTO conductive glass substrate were made through various sputtering deposition times, while the ZnO nanorod layers were made with a single layer using the hydrothermal method. We used XRD, SEM, UV-Vis, I-V meter, and EIS analysis. Based on these characterization we concluded that multilayer ZnO nanorod:TiO2 coating with a sputtering time of 60 minutes resulted the best performance of DSSC with an efficiency of 0.27%.

Aminated graphene quantum dots/CdS nanobelts for enhanced photocatalytic degradation of RhB dye under visible light.

CdS nanoparticles have wide applications as photocatalysts for degradation of organic pollutants, but due to their limited turnover number and off-pathway charge recombination processes, their degradation efficiency is low. Herein, aminated graphene quantum dots/CdS (GQDs/CdS) nanobelts were successfully fabricated by solvothermal and hydrothermal processes. The prepared GQDs/CdS were characterized by physical methods to investigate their structure, morphology, optical properties, specific surface area, element composition, and chemical state. GQDs/CdS materials promoted efficient charge separation, and showed high efficiency in the photocatalytic degradation of the organic dye Rhodamine B (RhB) under visible light. The degradation efficiency of RhB samples over 0.05 g of catalysts reached 97.40% after 150 min, a much higher efficiency in comparison to pure CdS. Electron paramagnetic resonance (EPR) spectroscopy provided direct evidence for ˙OH and ˙O2- as the reactive oxidative species using DMPO as a spin trap. Consistent with the experimental results, a possible mechanism of RhB photocatalytic degradation by GQDs/CdS under visible light was proposed. This work may provide environmentally friendly photocatalysts for degrading organic dyes and purifying water.

Theoretical Study on Functionalizing A–D–A Type Non‐Fullerene Acceptor by Fused Rings and Side Chains for Organic Solar Cells

AbstractThe modification of fused rings in the central backbone, side chains, and end groups of the non‐fullerene acceptor (NFA) for organic solar cells (OSCs) can modulate properties and photovoltaic performance. In order to investigate the effect of fused rings and side chains on photovoltaic performance, PBDB‐T is selected as electron donor and IDTIC, IDIC, IDIC‐PhC6, IDIC‐C4Ph, 4TIC, ITIC‐OE, IDTTIC, ITC6‐IC, ITIC, and C8‐ITIC are selected as NFAs. Based on quantum chemistry calculations, the geometries, electronic structures, excitation properties, excited‐state lifetimes, and electrostatic potentials (ESPs) of the monomer and the PBDB‐T:NFA complexes are studied, and the rate constants of charge transfer (CT), exciton dissociation (ED) and charge recombination (CR) processes are analyzed. The results show that the increase of fused‐ring in NFA's backbone by substituting phenyl or thienyl with bithiophene can elevate the highest occupied molecular orbital energies, reduce the gap of frontier molecular orbital energies, induce red‐shift of absorption spectrum, increase CT energy, improve CT and ED rates. This work provides a detailed understanding of tuning optoelectronic properties and photovoltaic performance by modifying NFA's side chains and extending backbones.

Resolving the Puzzle of Charge Carrier Lifetime in ZnO by Revisiting the Role of Oxygen Vacancy.

Zinc oxide (ZnO) is a wide bandgap prototypical n-type semiconductor due to the presence of intrinsic oxygen vacancies (VO). The VO can readily transfer to the most energetically favorable +2 charged VO (VO2+) by losing two electrons mediated by the metastable VO1+ defect. Nevertheless, the influence of charged VO on the charge dynamics in ZnO and the underlying mechanisms remain elusive. By performing nonadiabatic molecular dynamics simulations of the charge trapping and recombination processes, we show that both VO1+ and VO2+ slow down the nonradiative electron-hole recombination via assisted defect states and, thus, extending charge carrier lifetime compared to pristine ZnO. Our study contributes to identifying the different recombination pathways that take place in VO1+ and VO2+ of n-type ZnO systems, providing useful guidance for designing high-performance ZnO-based devices.

Pushing the heterometal doping limit while preserving long-lived charge separation in a Ti-based MOF photocatalyst

This study explores the nature, dynamics, and reactivity of the photo-induced charge separated excited state in a Fe3+-doped titanium-based metal organic framework (MOF), xFeMIL125-NH2, as a function of iron concentration. The MOF is synthesized with doping levels x = 0.5, 1 and 2 Fe node sites per octameric Ti-oxo cluster and characterized by powder x-ray diffraction, UV-vis diffuse reflectance, atomic absorption, and steady state Fe K-edge X-ray absorption spectroscopy. For each doping level, time-resolved X-ray transient absorption spectroscopy studies confirm the electron trap site role of the Fe sites in the excited state. Time scan data reveal multiexponential decay kinetics for the charge recombination processes which extend into the microsecond range for all three concentrations. A series of dye photodegradation studies, based on the oxidative decomposition of Rhodamine B, demonstrates the reactivity of the charge separated excited state and the photocatalytic capacity of these MOF materials compared to traditional heterometal-doped semiconductor photocatalysts.

Photocatalytic reduction of hexavalent chromium using Zn2SnO4–ZnO modified g-C3N4 composite

In this study, a novel hierarchical g-C3N4/Zn2SnO4–ZnO heterojunction system was reported as an efficient photocatalyst for the reduction of Cr(VI). The fabrication of the composite involved the synthesis of the complex metal oxide (Zn2SnO4–ZnO), followed by the in-situ integration into graphitic carbon nitride. The structure, morphology, and optical properties of the as-prepared tertiary composite were determined using various analytical techniques. The results indicated the improvement in surface area and the electronic structure of the semiconductor heterostructures. Photocatalytic measurement showed the efficiency of the ternary composite to cause a reduction of the band gap energy, delayed charge recombination process and enhancement of the visible light absorption. Reaction parameters including the solution pH, photocatalyst (g-C3N4/ZTO-ZnO) dosage, and the initial concentration of Cr(VI) on the degradation efficiency of the photocatalyst were evaluated. The solution pH was varied from 2 to 8, and pH 2 displayed the highest removal of Cr(VI) with 99.2 % removal efficiency for the g-C3N4/ZTO-ZnO, while the ZTO-ZnO exhibited 72 % efficiency. Finally, the kinetic study of the photocatalytic reaction showed an increase in the overall rate constant k with the increase in initial concentration of Cr(VI). The outstanding performance of the ternary composite compared to the bare complex metal oxide makes the innovative g-C3N4/Zn2SnO4–ZnO composite a promising material for the removal of Cr(VI) from aquatic environment.

Balancing charge recombination and hole transfer rates in hematite photoanodes by modulating the Co2+/Fe3+ sites in the OER cocatalyst

This work investigates the roles of Co and Fe sites in a composite cocatalyst on the performance of hematite photoanodes for photoelectrochemical (PEC) water splitting. The cobalt/iron-based composite (Co-Fe-O) cocatalyst, consisting of adjustable Co2+/Fe3+ratios, was synthesized using a one-step hydrothermal method. It reveals that Co2+ sites with a robust capacity for low-bias hole capture, which is insignificantly affected by partial substitution by Fe3+, decelerate the charge recombination process. However, it also leads to a slower charge transfer, with slower oxygen-evolution kinetics on Co sites than on Fe sites. Consequently, the modulation of the Co2+/Fe3+ ratio facilitates the redistribution of surface strap states, striking a delicate balance between charge recombination and charge transfer rates. This optimization led to the highest low-bias photocurrent density of 1.6 mA cm−2 at 1.0 V vs. RHE (a 2.4-fold increase) for the cocatalyst with a Co2+/Fe3+ ratio of 1:2 (CoFe2O4 nanoparticles). Additionally, the cocatalyst with a Co2+/Fe3+ ratio of 1:4 (mixture of CoFe2O4 and Fe2O3 nanoparticles, demonstrated an impressive high-bias photocurrent density of 3.8 mA cm−2 at 1.6 V vs. RHE (a 2.3-fold increase). This study emphasizes the promising potential of modulating active sites within a cocatalyst to achieve efficient PEC water splitting on a hematite-based photoanode.

Wet-chemical synthesis of sponge-like porous Zn-doped copper oxide ceramic as an efficient solar-light triggered photocatalyst for multiple applications

Abstract This work presents a simple co-precipitation method for making Zn-doped copper oxide (C1−x (Zn) x O) that is nanostructured, porous, active in visible light, and highly conductive. The combined impacts of doping, structural modifications, and nanoscaled synthesis contribute to the development of a novel catalyst that has inherent characteristics. Physicochemical studies confirm the coexistence of all desired features in the C1−x (Zn) x O photocatalyst. The formed photocatalyst’s dye-destroying and antimicrobial properties were carefully examined and compared to those of pristine copper oxide (PCO) that had not been doped. When tested against Escherichia coli (a “negative strain”) and Staphylococcus aureus (a “positive strain”), the antimicrobial properties of the C1−x (Zn) x O photocatalyst were better than those of PCO and on par with those of commercially available drugs. Also, C1−x (Zn) x O photocatalyst gets rid of reactive orange 4 (RO-4) dye more effectively and faster (0.023 min−1) than PCO photocatalyst (0.11 min−1) by using simple sorption and photocatalytic annihilation. The C1−x (Zn) x O photocatalyst eliminated 90.25 % of RO-4 dye under visible light irradiation. Under identical circumstances, the PCO photocatalyst removed 65.12 % of the RO-4 dye. The boosted bactericidal and photocatalytic activity of the C1−x (Zn) x O photocatalyst may be attributed to its larger surface area (56.5 m2 g−1), good electrical conductivity (2.33 × 10−3 S m−1), low bandgap (1.98 eV), and doping-induced structural defects. The developed features increase the light-capturing sites on the catalyst surface, improve the charge transport kinetics, enable the catalysts to harvest visible light, and limit the charge recombination process, allowing our photocatalyst to show exceptional bactericidal and dye annihilation activities. This study opens new avenues for developing metal-substituted metal oxides with porous nanostructures for environmental and ecological protection.

Color-Tunable Dual-Mode Organic Afterglow for White-Light Emission and Information Encryption Based on Carbazole Doping.

Dual-mode emission materials, combining phosphorescence and delayed fluorescence, offer promising opportunities for white-light afterglow. However, the delayed fluorescence lifetime is usually significantly shorter than that of phosphorescence, limiting the duration of white-light emission. In this study, a carbazole-based host-guest system that can be activated by both ultraviolet (UV) and visible light is reported to achieve balanced phosphorescence and delayed fluorescence, resulting in a long-lived white-light afterglow. Our study demonstrated the critical role of a charge transfer state in the afterglow mechanism, where the charge separation and recombination process directly determined the lifetime of afterglow. Simultaneously, an efficient reversed intersystem crossing process was obtained between the singlet and triplet charge transfer states, which further boosted the delayed fluorescence properties of the doping system. As a result, delayed fluorescence lifetime was successfully prolonged to approach that of phosphorescence. This work presents a delayed fluorescence lifetime improvement strategy via doping method to realize durable white-light afterglow.

Color‐Tunable Dual‐Mode Organic Afterglow for White‐Light Emission and Information Encryption Based on Carbazole Doping

AbstractDual‐mode emission materials, combining phosphorescence and delayed fluorescence, offer promising opportunities for white‐light afterglow. However, the delayed fluorescence lifetime is usually significantly shorter than that of phosphorescence, limiting the duration of white‐light emission. In this study, a carbazole‐based host–guest system that can be activated by both ultraviolet (UV) and visible light is reported to achieve balanced phosphorescence and delayed fluorescence, resulting in a long‐lived white‐light afterglow. Our study demonstrated the critical role of a charge transfer state in the afterglow mechanism, where the charge separation and recombination process directly determined the lifetime of afterglow. Simultaneously, an efficient reversed intersystem crossing process was obtained between the singlet and triplet charge transfer states, which facilitating the delayed fluorescence properties of host–guest system. As a result, delayed fluorescence lifetime was successfully prolonged to approach that of phosphorescence. This work presents a delayed fluorescence lifetime improvement strategy via doping method to realize durable white‐light afterglow.