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Efficient charge generation at low energy losses in organic solar cells: a key issue review
摘要： Light absorption generates strongly bound excitons in organic solar cells (OSCs). To obtain efficient charge generation, a large driving force is required, which causes a large energy loss (Eloss) and severely hinders the improvement in the power conversion efficiencies (PCEs) of OSCs. Recently, the development of non-fullerene OSCs has seen great success, and the resulting OSCs can yield highly efficient charge generation with a negligible driving force, which raises a fundamental question about how the excitons split into free charges. From a chemical structure perspective, the molecular electrostatic potential differences between donors and acceptors may play a critical role in facilitating charge separation. Although the Eloss caused by charge generation has been suppressed, charge recombination, particularly via non-radiative pathways, severely limits further improvements in the PCEs. In OSCs with negligible driving forces, the lowest excited state, a hybrid local exciton-charge transfer state, is believed to have a strong association with the non-radiative Eloss. This review discusses the efficient charge generation at low Eloss values in highly efficient OSCs and highlights the issues that should be tackled to further improve the PCEs to new levels (~ 20%).
关键词： energy loss,organic solar cells,non-fullerene acceptors,electrostatic potential,charge generation
更新于2025-09-23 15:21:01
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Density Functional Theory Study on the Donating Strength of Donor Systems in Dye-Sensitized Solar Cells
摘要： The electron-donating strength of donor (D) moieties in thirteen donor-π-acceptor systems (D1-π-A to D13-π-A wherein -π- and A represent butadiene and cyanoacrylic acid units, respectively) have been studied using B3LYP/cc-pVDZ level density functional theory (DFT) calculations. The selected D moieties are encountered as a part of an organic sensitizer molecule in dye-sensitized solar cell (DSSC) applications. When D moiety is joined with π-A, a certain amount of electron donation from D to A occurs leading to increase in electron density at the A site of D-π-A compared to A site of π-A. This electron reorganization is quantified in terms of the change in molecular electrostatic potential (MESP) minimum (ΔVmA) at the acceptor site, the CN group of the cyanoacrylic acid. The ΔVmA is always negative, in the range -11.0 to -2.6 kcal/mol which provides a quick assessment of the rank order of the electron-donating nature of the D moieties in the ground state of D-π-A. The optical, and photovoltaic properties of D and D-π-A systems are also determined at TD-CAM-B3LYP/cc-pVDZ//B3LYP/cc-pVDZ level. An absorption red shift (Δλmax) in the range 81 – 242 nm is observed when D moieties change to D-π-A systems. The ground state property ΔVmA showed a strong linear correlation with the excited state property Δλmax. Further, ΔVmA is found to be proportional to the open-circuit voltage (Voc). The resemblance of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies of the D-π–A system with the respective energies of donor and π-A systems shows that donor tunes HOMO, while π-A tunes LUMO. Among the thirteen D-π-A systems, N,N-dialkylaniline, and julolidine are rated as the best donors for the photovoltaic applications. This study shows that MESP based assessment of donating strength of donor systems offers a powerful rational design strategy for the development of efficient dyes for DSSC applications.
关键词： donor-π-acceptor systems,electron-donating strength,molecular electrostatic potential,dye-sensitized solar cells,density functional theory
更新于2025-09-23 15:21:01
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Efficient Exciton Dissociation Enabled by the End Group Modification in Non-Fullerene Acceptors
摘要： For organic photovoltaic (OPV) cells, in order to overcome the larger Coulombic binding energy between holes and electrons, an extra driving force is required for efficient exciton dissociation. Here, we report two non-fullerene acceptors named IO-4H and IO-4F for OPV cells. By employing the polymer PBDB-TF as a donor, PBDB-TF:IO-4H-based device only shows a power conversion efficiency (PCE) of 0.30% with a charge dissociation probability (Pdiss) of 13.3%. On the contrary, PBDB-TF:IO-4F-based device demonstrates a PCE of 7.85%, with a Pdiss of 81.3%. The photoelectric processes demonstrate that both devices have similar charge transport and charge recombination properties. The limitation of photovoltaic performance is the low exciton dissociation efficiency in the PBDB-TF:IO-4H-based device. The theoretical studies show the electrostatic potential (ESP) of IO-4H is negative in the end groups and similar to the ESP of PBDB-TF, while ESP of IO-4F is positive. PBDB-TF and the IO-4F may form a strong intermolecular electric field to assist the exciton dissociation. Our results suggest that increasing the ESP difference between donor and acceptor may be beneficial to promote exciton dissociation, thus improving photovoltaic performance.
关键词： organic photovoltaic,electrostatic potential,power conversion efficiency,exciton dissociation,non-fullerene acceptors
更新于2025-09-23 15:19:57
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Electrostatic potential dispersing pyrimidine-5-carbonitrile acceptor for high efficiency and long lifetime thermally activated delayed fluorescence organic light-emitting diodes
摘要： Pyrimidine-5-carbonitrile was developed as an electrostatic potential managing and strong acceptor moiety of thermally activated delayed fluorescence (TADF) emitters for high efficiency and long lifetime in devices. Two types of TADF emitters with the donor moiety extended from either the 2 or 4 position of the acceptor moiety were prepared to study the effect of the donor substitution position on the TADF characteristics of the TADF emitters. Comparison of the two types of TADF emitters suggested that the extension of the donor structure from the 4 position of the pyrimidine-5-carbonitrile acceptor is an effective way of enhancing the external quantum efficiency (EQE) and lifetime of the TADF devices. A high EQE of 19.8% and lifetime exceeding that of the state-of-the-art green TADF emitter were demonstrated using one of the pyrimidine-5-carbonitrile derived emitters through the uniformly distributed electrostatic potential.
关键词： organic light-emitting diodes,Pyrimidine-5-carbonitrile,thermally activated delayed fluorescence,electrostatic potential,high efficiency,long lifetime
更新于2025-09-16 10:30:52
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Determination of electronic band structure by electron holography of etched-and-regrown interfaces in GaN <i>p-i-n</i> diodes
摘要： The electrostatic potential variation across etched-and-regrown GaN p-i-n diodes for power electronics has been studied using electron holography in a transmission electron microscope. The potential profiles have been correlated with the composition profiles of Mg, Si, and O obtained by secondary ion mass spectroscopy. Electronic charges obtained from the potential profiles correlate well with the presence of Si and O impurities at regrown interfaces. The overlap of Mg and Si when Mg doped GaN is grown directly over an etched undoped GaN surface results in the formation of a highly doped p-n junction. The introduction of a thin undoped layer over the etched GaN surface prevents the formation of such a junction as the regrowth interface is moved away from the Mg-doped GaN, and results in diodes with improved reverse leakage currents, close to the best values of continuously grown p-i-n diodes. Potential profiles of continuously grown (not etched) p-i-n diodes are compared to those of etched-and-regrown diodes.
关键词： Mg,O impurities,regrown interfaces,Si,secondary ion mass spectroscopy,electrostatic potential,GaN p-i-n diodes,electron holography
更新于2025-09-12 10:27:22
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Impact of the Coulomb Potential and the Electrostatic Potential on the Eigen-Frequencies of the Coupled Plasmons
摘要： In this paper, a coupled oscillator model is employed to describe the eigen-frequencies of the coupled plasmons, which are dominated by the coupled coefficient and the frequency offset defined by us. From this, we present a novel physical model to describe the influence of the Coulomb potential and the electrostatic potential (CPEP) on the coupled coefficient and the frequency offset. Based on the finite element method, we simulate numerically the evolution of the absorption spectra of the microstructure unit composed of a simply metal nanostructure. The simulated results show that two resonant absorption peaks are asymmetrical with the change of the coupling distance between the nano-elements. With the CPEP model, we not only explain successfully the asymmetrical behaviors of the peaks values of the absorption spectra with the change of the coupling distance but also obtain the resonant frequencies for different plasmons. The absorption spectra are simulated by the harmonic oscillator model, in which their calculated parameters originate from the CPEP model, and the results are very well association with those of the numerical simulation.
关键词： Eigen-frequencies,Harmonic oscillator model,Frequency offset,Coulomb potential and electrostatic potential
更新于2025-09-11 14:15:04
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Self-consistent procedure including envelope function normalization for full-zone Schr?dinger-Poisson problems with transmitting boundary conditions
摘要： In the quantum mechanical simulation of exploratory semiconductor devices, continuum methods based on a k·p/envelope function model have the potential to significantly reduce the computational burden compared to prevalent atomistic methods. However, full-zone k·p/envelope function simulation approaches are scarce and existing implementations are not self-consistent with the calculation of the electrostatic potential due to the lack of a stable procedure and a proper normalization of the multi-band envelope functions. Here, we therefore present a self-consistent procedure based on a full-zone spectral k·p/envelope function band structure model. First, we develop a proper normalization for the multi-band envelope functions in the presence of transmitting boundary conditions. This enables the calculation of the free carrier densities. Next, we construct a procedure to obtain self-consistency of the carrier densities with the electrostatic potential. This procedure is stabilized with an adaptive scheme that relies on the solution of Poisson’s equation in the Gummel form, combined with successive underrelaxation. Finally, we apply our procedure to homostructure In0.53Ga0.47As tunnel field-effect transistors (TFETs) and staggered heterostructure GaAs0.5Sb0.5/In0.53Ga0.47As TFETs and show the importance of self-consistency on the device predictions for scaled dimensions.
关键词： electrostatic potential,k·p/envelope function model,semiconductor devices,self-consistent procedure,tunnel field-effect transistors,quantum mechanical simulation
更新于2025-09-10 09:29:36
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Surface enhanced Raman spectroscopy with methyl-orange on Ag-TiO2 nanocomposites: Experimental and theoretical investigation
摘要： Ag-TiO2 nanocomposites of different Ag nanoparticle (NP) concentrations are experimentally prepared and their UV-vis and surface enhanced Raman scattering characteristics are determined. The enhancement of the Raman signal for the ligand methyl-orange (MO) adsorbed onto the nanocomposite system, is observed. To investigate the influence of changing Ag concentration in this nanocomposite system, molecular dynamics (MD) simulations are conducted with both a fixed as well as varying number of the surfactant MO adsorbed onto the nanocomposite. Density functional theory (DFT) simulations are performed to investigate the conditions for charge transfer from the MO surfactant via the highest occupied molecular orbitals (HOMO), lowest unoccupied molecular orbitals (LUMO) as well as the electrostatic potentials. It is shown that the bonding mode of the surfactant contributes greatly to the observed Raman scattering enhancement.
关键词： HOMO-LUMO,Surface Enhanced Raman Scattering,density functional theory,molecular dynamics,Ag-TiO2 nanocomposite,electrostatic potential
更新于2025-09-09 09:28:46