New Avenues for Organic Solar Cells Using Intrinsically Charge-Generating Materials

The molecular electron acceptor material Y6 has been a key part of the most recent surge in organic solar cell sunlight-to-electricity power conversion efficiency, which is now approaching 20%. Numerous studies have sought to understand the fundamental photophysical reasons for the exceptional performance of Y6 and its growing family of structural derivatives. Though significant uncertainty about several details remains, many have concluded that initially photogenerated excited states rapidly convert into electron-hole charge pairs in the neat material. These charge pairs are characterized by location of the electron and hole on different Y6 molecules, in contrast to the Frenkel excitons that dominate the behavior of most organic semiconductor materials. Here, we summarize the current state of knowledge regarding Y6 photophysics and the key observations that have led to it. We then link this understanding to other advances, such as the role of quadrupolar fields in donor-acceptor blends, and the importance of molecular interactions and organization in providing the structural basis for Y6's properties. Finally, we turn our attention to ways of making use of the new photophysics of Y6, and suggest molecular doping, crystal structure tuning, and electric field engineering as promising avenues for future exploration.

Constructing B─N─P Bonds in Ultrathin Holey g-C3N4 for Regulating the Local Chemical Environment in Photocatalytic CO2 Reduction to CO

The lack of intrinsic active sites for photocatalytic CO2 reduction reaction (CO2RR) and fast recombination rate of charge carriers are the main obstacles to achieving high photocatalytic activity. In this work, a novel phosphorus and boron binary-doped graphitic carbon nitride, highly porous material that exhibits powerful photocatalytic CO2 reduction activity, specifically toward selective CO generation, is disclosed. The coexistence of Lewis-acidic and Lewis-basic sites plays a key role in tuning the electronic structure, promoting charge distribution, extending light-harvesting ability, and promoting dissociation of excitons into active carriers. Porosity and dual dopants create local chemical environments that activate the pyridinic nitrogen atom between the phosphorus and boron atoms on the exposed surface, enabling it to function as an active site for CO2RR. The P-N-B triad is found to lower the activation barrier for reduction of CO2 by stabilizing the COOH reaction intermediate and altering the rate-determining step. As a result, CO yield increased to 22.45 µmol g-1 h-1 under visible light irradiation, which is ≈12 times larger than that of pristine graphitic carbon nitride. This study provides insights into the mechanism of charge carrier dynamics and active site determination, contributing to the understanding of the photocatalytic CO2RR mechanism.

Defect Engineering Simultaneously Regulating Exciton Dissociation in Carbon Nitride and Local Electron Density in Pt Single Atoms Toward Highly Efficient Photocatalytic Hydrogen Production

The high exciton binding energy (Eb) and sluggish surface reaction kinetics have severely limited the photocatalytic hydrogen production activity of carbon nitride (CN). Herein, a hybrid system consisting of nitrogen defects and Pt single atoms is constructed through a facile self-assembly and photodeposition strategy. Due to the acceleration of exciton dissociation and regulation of local electron density of Pt single atoms along with the introduction of nitrogen defects, the optimized Pt-MCT-3 exhibits a hydrogen production rate of 172.0 µmol h-1 (λ ≥ 420 nm), ≈41 times higher than pristine CN. The apparent quantum yield for the hydrogen production is determined to be 27.1% at 420 nm. The experimental characterizations and theoretical calculations demonstrate that the nitrogen defects act as the electron traps for the exciton dissociation, resulting in a decrease of Eb from 86.92 to 43.20 meV. Simultaneously, the stronger interaction between neighboring nitrogen defects and Pt single atoms directionally drives free electrons to aggregate around Pt single atoms, and tailors the d-band electrons of Pt, forming a moderate binding strength between Pt atoms and H* intermediates.

Sequentially Processed Bulk-Heterojunction-Buried Structure for Efficient Organic Solar Cells with 500 nm Thickness

Large-area printing fabrication is a distinctive feature of organic solar cells (OSCs). However, the advance of upscalable fabrication is challenged by the thickness of organic active layers considering the importance of both exciton dissociation and charge collection. In this work, a bulk-heterojunction-buried (buried-BHJ) structure is introduced by sequential deposition to realize efficient exciton dissociation and charge collection, thereby contributing to efficient OSCs with 500 nm thick active layers. The buried-BHJ distributes donor and acceptor phases in the vertical direction as charge transport channels, while numerous BHJ interfaces are buried in each phase to facilitate exciton dissociation simultaneously. It is found that buried-BHJ configurations possess efficient exciton dissociation and rapid charge transport, resulting in reduced recombination losses. In comparison with traditional structures, the buried-BHJ structure displays a decent tolerance to film thickness. In particular, a power conversion efficiency of 16.0% is achieved with active layers at a thickness of 500 nm. To the best of the authors' knowledge, this represents the champion efficiency of thick film OSCs.

Organic Donor-Acceptor Systems for Photocatalysis

Organic semiconductor materials are considered to be promising photocatalysts due to their excellent light absorption by chromophores, easy molecular structure tuning, and solution-processable properties. In particular, donor-acceptor (D-A) type organic photocatalytic materials synthesized by introducing D and A units intra- or intermolecularly, have made great progress in photocatalytic studies. More and more studies have demonstrated that the D-A type organic photocatalytic materials combine effective carrier separation, tunable bandgap, and sensitive optoelectronic response, and are considered to be an effective strategy for enhancing light absorption, improving exciton dissociation, and optimizing carrier transport. This review provides a thorough overview of D-A strategies aimed at optimizing the photocatalytic performance of organic semiconductors. Initially, essential methods for modifying organic photocatalytic materials, such as interface engineering, crystal engineering, and interaction modulation, are briefly discussed. Subsequently, the review delves into various organic photocatalytic materials based on intramolecular and intermolecular D-A interactions, encompassing small molecules, conjugated polymers, crystalline polymers, supramolecules, and organic heterojunctions. Meanwhile, the energy band structures, exciton dynamics, and redox-active sites of D-A type organic photocatalytic materials under different bonding modes are discussed. Finally, the review highlights the advanced applications of organic photocatalystsand outlines prospective challenges and opportunities.

Boosting exciton dissociation and charge transfer in CsPbBr3 QDs via ferrocene derivative ligation for CO2 photoreduction

Photo-catalytic CO2 reduction with perovskite quantum dots (QDs) shows potential for solar energy storage, but it encounters challenges due to the intricate multi-electron photoreduction processes and thermodynamic and kinetic obstacles associated with them. This study aimed to improve photo-catalytic performance by addressing surface barriers and utilizing multiple-exciton generation in perovskite QDs. A facile surface engineering method was employed, involving the grafting of ferrocene carboxylic acid (FCA) onto CsPbBr3 (CPB) QDs, to overcome limitations arising from restricted multiple-exciton dissociation and inefficient charge transfer dynamics. Kelvin Probe Force Microscopy and XPS spectral confirmed successfully creating an FCA-modulated microelectric field through the Cs active site, thus facilitating electron transfer, disrupting surface barrier energy, and promoting multi-exciton dissociations. Transient absorption spectroscopy showed enhanced charge transfer and reduced energy barriers, resulting in an impressive CO2-to-CO conversion rate of 132.8 μmol g-1 h-1 with 96.5% selectivity. The CPB-FCA catalyst exhibited four-cycle reusability and 72 h of long-term stability, marking a significant nine-fold improvement compared to pristine CPB (14.4 μmol g-1 h-1). These results provide insights into the influential role of FCA in regulating intramolecular charge transfer, enhancing multi-exciton dissociation, and improving CO2 photoreduction on CPB QDs. Furthermore, these findings offer valuable knowledge for controlling quantum-confined exciton dissociation to enhance CO2 photocatalysis.

Ternary Organic Solar Cells by Small Amount of Efficient Light Absorption Polymer PSEHTT as Third Component Materials

We prepared ternary organic solar cells (OSCs) by incorporating the medium wavelength absorption polymer PSEHTT into the PM6:L8-BO binary system. The power conversion efficiency (PCE) is improved from 15.83% to 16.66%. Although the fill factor (FF) is slightly reduced, the short-circuit current density (JSC) and open-circuit voltage (VOC) are significantly increased at the same time. A small amount of PSEHTT has a broad absorption spectrum in the short wavelength region and has good compatibility with PM6, which is conducive to fine-tuning the photon collection and improving the JSC. In addition, the highest occupied molecular orbital (HOMO) energy level of PSEHTT is deeper than that of PM6, which broadens the optical bandgap. This study provides an effective method to fabricate high-performance ternary OSCs by using a lower concentration of PSEHTT with PM6 as a hybrid donor material, which ensures a better surface and bulk morphology, improves photon collection, and broadens the optical bandgap.

Boosting Exciton Dissociation and Charge Transfer in Triazole-Based Covalent Organic Frameworks by Increasing the Donor Unit from One to Two for the Efficient Photocatalytic Elimination of Emerging Contaminants

As novel photocatalysts, covalent organic frameworks (COFs) have potential for water purification. Insufficient exciton dissociation and low charge mobility in COFs yet restricted their photocatalytic activity. Excitonic dissociation and charge transfer in COFs could be optimized via regulating the donor-acceptor (D-A) interactions through adjusting the number of donor units within COFs, yet relevant research is lacking. By integrating the 1,2,4-triazole or bis-1,2,4-triazole unit with quinone, we fabricated COF-DT (with a single donor unit) and COF-DBT (with double donor units) via a facile sonochemical method and used to decontaminate emerging contaminants. Due to the stronger D-A interactions than COF-DT, the exciton binding energy was lower for COF-DBT, facilitating the intermolecular charge transfer process. The degradation kinetics of tetracycline (model contaminant) by COF-DBT (k = (12.21 ± 1.29) × 10-2 min-1) was higher than that by COF-DT (k = (5.11 ± 0.59) × 10-2 min-1) under visible-light irradiation. COF-DBT could efficiently photodegrade tetracycline under complex water chemistry conditions and four real water samples. Moreover, six other emerging contaminants, both Gram-negative and Gram-positive strains, could also be effectively eliminated by COF-DBT. High tetracycline degradation performance achieved in a continuous-flow system and in five reused cycles in both laboratory and outdoor experiments with sunlight irradiation showed the stability and the potential for the practical application of COF-DBT.

Exciton Nature of Plasma Phase Transition in Warm Dense Fluid Hydrogen: ROKS Simulation

The transition of warm dense fluid hydrogen from an insulator to a conducting state at pressures of about 20-400 GPa and temperatures of 500-5000 K has been the subject of active scientific research over the past few decades. However, various experimental and theoretical methods do not provide consistent results. In this work, we have applied the restricted open-shell Kohn-Sham (ROKS) method for first principles molecular dynamics of dense hydrogen after thermal excitation to the first singlet excited state. The Wannier localization method has allowed us to analyze the exciton dynamics in this system. The model shows that a key mechanism of the transition is associated with the dissociation of electron-hole pairs, which allows explaining several stages of the transition of fluid H₂ from molecular state to plasma. This mechanism is able to give a quantitative description of several experimental results as well as to resolve the discrepancies between experimental studies.

Electron Lifetime of Over One Month in Disordered Organic Solid-State Films

Understanding intrinsic carrier lifetime in disordered organic solid-state semiconductors is essential for improving device performance in not only molecule-based optoelectronic devices such as organic solar cells (OSC) but also photocatalysts used for producing solar fuel cells. Carriers in disordered films are generally thought to have short lifetimes on a scale ranging from nanoseconds to milliseconds. These short carrier lifetimes cause loss of charges in OSCs and low quantum yields in photocatalysts and impede the future application of organic semiconductors to, for example, charge-storage-based memory devices. This study reports an ultralong intrinsic carrier lifetime of more than one month in a disordered film of an organic semiconductor stored at room temperature without external power. This extraordinary lifetime, which is several orders of magnitude longer than that generally believed possible in conventional organic semiconductors, arises from carrier stabilization by spontaneous orientation polarization, excited spin-triplet recycling, and blocking of recombination processes in disordered films.

A Medium-Bandgap Nonfullerene Acceptor Enabling Organic Photovoltaic Cells with 30% Efficiency under Indoor Artificial Light

The correlation between molecular structure and photovoltaic performance is lagging for constructing high-performance indoor organic photovoltaic (OPV) cells. Herein, this relationship is investigated in depth by employing two medium-bandgap nonfullerene acceptors (NFAs). The newly synthesized NFA of FTCCBr exhibits a similar bandgap and molecular energy level, but a much stronger dipole moment and larger average electrostatic potential (ESP) compared with ITCC. After blending with the polymer donor PB2, the PB2:ITCC and PB2:FTCCBr blends exhibit favorable bulk-heterojunction morphologies and the same driving force, but the PB2:FTCCBr blend exhibits a large ESP difference. In OPV cells, the PB2:ITCC-based device produces a power conversion efficiency (PCE) of 11.0%, whereas the PB2:FTCCBr-based device gives an excellent PCE of 14.8% with an open-circuit voltage (V_OC ) of 1.05 V, which is the highest value among OPV cells with V_OC values above 1.0 V. When both acceptor-based devices work under a 1000 lux of 3000 K light-emitting diode, the PB2:ITCC-based 1 cm² device yields a good PCE of 25.4%; in contrast, the PB2:FTCCBr-based 1 cm² device outputs a record PCE of 30.2%. These results suggest that a large ESP offset in photovoltaic materials is important for achieving high-performance OPV cells.

Spontaneous exciton dissociation in organic photocatalyst under ambient conditions for highly efficient synthesis of hydrogen peroxide

SignificanceHydrogen peroxide is a highly competitive ready-to-use product for solar energy transformation. Nevertheless, the contemporary photosynthetic systems are not efficient enough, due to severe charge recombination caused by high activation energy and binding energy of the exciton. Herein, we achieve spontaneous exciton dissociation at room temperature. Moreover, the photosynthesis of H2O2 reaches between 9,366 and 12,324 µmol·g-1 from 9 AM to 4 PM in ambient conditions, that is, sunlight irradiation, real water including fresh water and seawater, room temperature, and open air. The ultrahigh photocatalytic efficiency in ambient conditions allows the solar-to-chemical conversion in a real cost-effective and sustainable way, which represents an important step toward real applications.

High-performance ternary organic photovoltaics with NC70BA as the third component material enabling thickness-insensitive photoactive performance

In this work, a thinner (100 nm) and thicker (150 and 200 nm) ternary organic photovoltaic (OPV) are fabricate by D18 as donor, Y6 as acceptor and NC₇₀BA as third component materials. The addition of the hollow 3D spherical structure of NC₇₀BA into D18:Y6 binary films is helpful for improving phase separation and smooth surface of ternary photoactive layer, and form more continuous electron transport channels in ternary photoactive layers. It is enhance photovoltaic performance under not only thinner photoactive layer thickness but also thicker photoactive layer thickness. Our results demonstrate the feasibility of employing D18:Y6 as a binary photovoltaic layer and fullerene derivative NC₇₀BA as a third component material and has construct high-efficiency thickness-insensitive ternary OPVs; this approach would promote the development of thicker photoactive layer ternary OPVs to fulfill the requirements of solution coating processes.

Acoustically Driven Stark Effect in Transition Metal Dichalcogenide Monolayers

The Stark effect is one of the most efficient mechanisms to manipulate many-body states in nanostructured systems. In mono- and few-layer transition metal dichalcogenides, it has been successfully induced by optical and electric field means. Here, we tune the optical emission energies and dissociate excitonic states in MoSe₂ monolayers employing the 220 MHz in-plane piezoelectric field carried by surface acoustic waves. We transfer the monolayers to high dielectric constant piezoelectric substrates, where the neutral exciton binding energy is reduced, allowing us to efficiently quench (above 90%) and red-shift the excitonic optical emissions. A model for the acoustically induced Stark effect yields neutral exciton and trion in-plane polarizabilities of 530 and 630 × 10^-5 meV/(kV/cm)², respectively, which are considerably larger than those reported for monolayers encapsulated in hexagonal boron nitride. Large in-plane polarizabilities are an attractive ingredient to manipulate and modulate multiexciton interactions in two-dimensional semiconductor nanostructures for optoelectronic applications.

Modulating Local Charge Distribution of Carbon Nitride for Promoting Exciton Dissociation and Charge-Induced Reactions

The sustainable light can generate reduction and oxidation centers in situ through the generation of photoexcited electrons and holes in the presence of photocatalyst. However, the photoexcited electrons and holes have huge Coulombic attraction and high exciton binding energy due to the weak screening effect and dielectric properties in many low-dimensional conjugated polymers, such as carbon nitride. Reducing the exciton binding energy of carbon nitride and promoting the conversion of excitons into free charge carriers are necessary for improving the activity of photocatalytic reactions but still very challenging. Here, by introducing amino-cyano functional groups into carbon nitride, it is demonstrated that excitons can be effectively dissociated into electrons and holes by finely controlling the charge distribution of heptazine ring. It is found that carbon nitride with heptazine rings of positive charge distribution can greatly reduce the exciton binding energy to 24 from 71 meV. Compared with heptazine ring having negative charge distribution, heptazine ring with positive charge distribution can increase photocatalytic hydrogen production of carbon nitride by up to ten times. This work provides an easy way to promote the dissociation of excitons in carbon nitride by regulating the charge distribution.

Dielectric Engineering Boosts the Efficiency of Carbon Nanotube Photodiodes

Carbon nanotube (CNT) photodiodes are a promising system for high-efficiency photocurrent generation due to the strong Coulomb interactions that can drive carrier multiplication. If the Coulomb interactions are too strong, however, exciton formation can hamper photocurrent generation. Here, we explore, experimentally and theoretically, the effect of the environmental dielectric constant (ε_env) on the photocurrent generation process in CNTs. We study individual ultraclean CNTs of known chiral index in a vacuum or dry nitrogen gas (ε_env = 1) and oil (ε_env = 2.15). The efficiency of photocurrent generation improves by more than an order of magnitude in oil. Two mechanisms explain this improvement. First, the refractive index of the environment optimizes the interference between incident and reflected light. Second, exciton binding energies are reduced in oil, changing the relaxation pathways of photoexcited carriers. We varied the axial electric field in the pn junction from 4 to 14 V/μm. Our measurements at high field indicate that autoionization of second-subband excitons can coexist with carrier multiplication. Dielectric screening makes this coexistence regime more accessible and allows us to reach photocurrent quantum yields greater than 100%.

Quantum Dynamics of Exciton Transport and Dissociation in Multichromophoric Systems

Due to the subtle interplay of site-to-site electronic couplings, exciton delocalization, nonadiabatic effects, and vibronic couplings, quantum dynamical studies are needed to elucidate the details of ultrafast photoinduced energy and charge transfer events in organic multichromophoric systems. In this vein, we review an approach that combines first-principles parameterized lattice Hamiltonians with accurate quantum dynamical simulations using advanced multiconfigurational methods. Focusing on the elementary transfer steps in organic functional materials, we address coherent exciton migration and creation of charge transfer excitons in homopolymers, notably representative of the poly(3-hexylthiophene) material, as well as exciton dissociation at polymer:fullerene heterojunctions. We emphasize the role of coherent transfer, trapping effects due to high-frequency phonon modes, and thermal activation due to low-frequency soft modes that drive a diffusive dynamics.

Coarse-Grained Quantum Theory of Organic Photovoltaic Devices

Understanding the exciton dissociation process in organic solar cells is a fundamental issue for the design of high-performance photovoltaic devices. In this article, a parameterized quantum theory based on a coarse-grained tight-binding model plus non-local electron-hole interactions is presented, while the diffusion and recombination of excitons are studied in a square lattice of excitonic states, where a real-space renormalization method on effective chains has been used. The Hamiltonian parameters are determined by fitting the measured quantum efficiency spectra and the theoretical short-circuit currents without adjustable parameters show a good agreement with the experimental ones obtained from several polymer:fullerene and polymer:polymer heterojunctions. Moreover, the present study reveals the degree of polymerization and the true driving force at donor-acceptor interface in each analyzed organic photovoltaic device.

Hybrid Nonfused-Ring Electron Acceptors with Fullerene Pendant for High-Efficiency Organic Solar Cells

The rapid advance of fused-ring electron acceptors (FREAs) has made them a potential substitute to fullerene-based acceptors and offered new avenues for the construction of organic solar cells (OSCs). Nonfused-ring acceptors (NFRAs) could significantly reduce the synthetic cost while achieving reasonable power conversion efficiencies (PCEs). Widely used fullerene acceptors have been applied as a second acceptor to regulate the morphology, absorption, and electron transport. To take full advantage of both nonfullerene and fullerene acceptors at the same time, we rationally designed and synthesized two novel NFRAs with phenyl-C₆₁-butyric acid methyl ester (PCBM) as the lateral pendent. With the incorporation of fullerene pendent in PCBM-C6 and PCBM-C10, varied UV-vis absorption and photoluminescence (PL) quenching behaviors were observed, and isotropic diffraction patterns were obtained via grazing incidence wide-angle X-ray scattering (GIWAXS) measurements. The bulky, spherical, and electronic isotropic fullerene pendent could effectively suppress severe molecular aggregation and form the preferred blend morphology. This strategy significantly improved the efficiencies for exciton separation and charge collection relative to the control acceptor CH₃COO-C6. Finally, the V_oc, J_sc, and fill factor (FF) of PCBM-C10-based devices were simultaneously improved and an enhanced PCE of 13.55% was accomplished.

Charge Energetics and Electronic Level Changes At the Copper(II) Phthalocyanine/Fullerene Junction Upon Photoexcitation

Energy offset at the donor (D)/acceptor (A) interface plays an important role in charge separation in organic photovoltaics. Its magnitude determines the charge separation process under illumination. Extensive studies have been carried out for elucidating the charge transfer (CT) process at different D/A junctions. These works lead to two different views: upon photoexcitation, energies would be (1) consumed in molecular polarization and orientation such that those opposite charges would overcome mutual Coulombic attractive potential at the interface and (2) spent on promoting charges to high-lying delocalized energy states (i.e., hot states), which is necessarily important prior to charge separation. Under these two scheme of studies, the electronic structures and the charge behaviors at the D/A interface should be different under photoexcitation, yet there is so far no direct experimental approach for probing the changes in electronics structures (i.e., Fermi level, vacuum level, frontier molecular orbitals, etc.) upon photoexcitation. Herein, a modified photoelectron spectroscopy (PES) system with an additional solar simulator is used to study the charge distributions and electronic interactions for a standard D/A heterojunction (i.e., copper phthalocyanine (CuPc)/ fullerene (C₆₀)) under photoexcitation. CT states formed as a result of photon energy transfer at the CuPc/C₆₀ junction. Subsequent superpositions of charge transfer and electron polarization effects increase the D/A energy level offsets from 0.75 (ground state measured in the dark) to 1.07 eV (high-lying state measured upon illumination). We showed that there is excess energy consumed for a subtle change in the energy level alignment of the CuPc/C₆₀ junction under illumination, suggesting a new insight for the energy loss mechanism during the photocharge generation processes.

Extremely Efficient Photocurrent Generation in Carbon Nanotube Photodiodes Enabled by a Strong Axial Electric Field

Carbon nanotube (CNT) photodiodes have the potential to convert light into electrical current with high efficiency. However, previous experiments have revealed the photocurrent quantum yield (PCQY) to be well below 100%. In this work, we show that the axial electric field increases the PCQY of CNT photodiodes. Under optimal conditions, our data suggest PCQY > 100%. We studied, both experimentally and theoretically, CNT photodiodes at room temperature using optical excitation corresponding to the S₂₂, S₃₃, and S₄₄ exciton resonances. The axial electric field inside the pn junction was controlled using split gates that are capacitively coupled to the suspended CNT. Our results give new insight into the photocurrent generation pathways in CNTs and the field dependence and diameter dependence of PCQY.

Revealing the Importance of Energetic and Entropic Contributions to the Driving Force for Charge Photogeneration

Despite significant recent progress, much about the mechanism for charge photogeneration in organic photovoltaics remains unknown. Here, we use conjugated block copolymers as model systems to examine the effects of energetic and entropic driving forces in organic donor-acceptor materials. The block copolymers are designed such that an electron donor block and an electron acceptor block are covalently linked, embedding a donor-acceptor interface within the molecular structure. This enables model studies in solution where processes occurring between one donor and one acceptor are examined. First, energy levels and dielectric constants that govern the driving force for charge transfer are systematically tuned and charge transfer within individual block copolymer chains is quantified. Results indicate that in isolated chains, a significant driving force of ∼0.3 eV is necessary to facilitate significant exciton dissociation to charge-transfer states. Next, block copolymers are cast into films, allowing for intermolecular interactions and charge delocalization over multiple chains. In the solid state, charge transfer is significantly enhanced relative to isolated block copolymer chains. Using Marcus Theory, we conclude that changes in the energetic driving force alone cannot explain the increased efficiency of exciton dissociation to charge-transfer states in the solid state. This implies that increasing the number of accessible states for charge transfer introduces an entropic driving force that can play an important role in the charge-generation mechanism of organic materials, particularly in systems where the excited state energy level is close to that of the charge-transfer state.

Vertically Aligned Porous Organic Semiconductor Nanorod Array Photoanodes for Efficient Charge Utilization

Because of inefficient charge utilization caused by localized π-electron conjugation and large exciton binding energy, the photoelectrochemical water-splitting efficiency of organic polymers is seriously limited. Taking the graphitic carbon nitride (g-CN) polymer as an example, we report a novel photoanode based on a vertically aligned g-CN porous nanorod (PNR) array prepared in situ, using a thermal polycondensation approach, with anodic aluminum oxide as the template. The g-CN PNR array exhibits an excellent photocurrent density of 120.5 μA cm^-2 at 1.23 V_RHE under one sun illumination, the highest reported incident photon-to-current efficiency of ∼15% at 360 nm, and an outstanding oxygen evolution reaction stability in 0.1 M Na₂SO₄ aqueous solution, which constitutes a benchmark performance among the reported g-CN-based polymer photoanodes without any sacrificial reagents. When compared with its planar counterpart, the enhanced performance of the PNR array results principally from its unique structure that enables a high degree of aromatic ring π-electron conjugation for higher mobility of charge carriers, provides a direct pathway for the electron transport to the substrate, produces a large portion of hole-accepting defect sites and space charge region to promote exciton dissociation, and also withstands more strain at the interface to ensure intimate contact with the substrate. This work opens a new avenue to develop nanostructured organic semiconductors for large-scale application of solar energy conversion devices.

Influence of Shell Thickness on the Performance of NiO-Based All-Inorganic Quantum Dot Light-Emitting Diodes

The effect of shell thickness on the performance of all-inorganic quantum dot light-emitting diodes (QLEDs) is explored by employing a series of green quantum dots (QDs) (Zn _xCd_{1- x}Se/ZnS core/shell QDs with different ZnS shell thicknesses) as the emitters. ZnO nanoparticles and sol-gel NiO are employed as the electron and hole transport materials, respectively. Time-resolved and steady-state photoluminescence results indicate that positive charging processes might occur for the QDs deposited on NiO, which results in emission quenching of QDs and poor device performance. The thick shell outside the core in QDs not only largely suppresses the QD emission quenching but also effectively preserves the excitons in QDs from dissociation of electron-hole pairs when they are subjected to an electric field. The peak efficiency of 4.2 cd/A and maximum luminance of 4205 cd/m² are achieved for the device based on QDs with the thickest shells (∼4.2 nm). We anticipate that these results will spur progress toward the design and realization of efficient all-inorganic QLEDs as a platform for the QD-based full-colored displays.

Quantum Confinement of Hybrid Charge Transfer Excitons in GaN/InGaN/Organic Semiconductor Quantum Wells

We investigate hybrid charge transfer exciton (HCTE) confinement in organic-inorganic (OI) quantum wells (QWs) comprising a thin InGaN layer bound on one side by GaN and on the other by the organic semiconductors, tetraphenyldibenzoperiflanthene (DBP) or 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP). A binding energy of 10 meV is calculated for the Coulombically bound free HCTE state between a delocalized electron in GaN and a hole localized in DBP. The binding energy of the HCTE increases to 165 meV when the electron is confined to a 1.5 nm In_0.21Ga_0.79N QW (HCTE_QW). The existence of the HCTE_QW is confirmed by measuring the voltage-dependent DBP exciton dissociation yield at the OI heterojunction in the QW devices that decrease with increasing In concentration and decreasing electric field, matching the trends predicted by Poole-Frenkel emission. Combining spectroscopic measurements with optical models, we find that 14 ± 3% of the excitons that reach the GaN/DBP heterojunction form HCTEs and dissociate into free charges, while the remainder recombine. A high nonradiative recombination rate through defect states at the heterointerface account for the lack of observation of HCTE_QW photoluminescence from GaN/InGaN/CBP QWs at temperatures as low as 10 K.

Optimizing Optical Absorption, Exciton Dissociation, and Charge Transfer of a Polymeric Carbon Nitride with Ultrahigh Solar Hydrogen Production Activity

Polymeric or organic semiconductors are promising candidates for photocatalysis but mostly only show moderate activity owing to strongly bound excitons and insufficient optical absorption. Herein, we report a facile bottom-up strategy to improve the activity of a carbon nitride to a level in which a majority of photons are really used to drive photoredox chemistry. Co-condensation of urea and oxamide followed by post-calcination in molten salt is shown to result in highly crystalline species with a maximum π-π layer stacking distance of heptazine units of 0.292 nm, which improves lateral charge transport and interlayer exciton dissociation. The addition of oxamide decreases the optical band gap from 2.74 to 2.56 eV, which enables efficient photochemistry also with green light. The apparent quantum yield (AQY) for H₂ evolution of optimal samples reaches 57 % and 10 % at 420 nm and 525 nm, respectively, which is significantly higher than in most previous experiments.

Plasmonic Effects of Metallic Nanoparticles on Enhancing Performance of Perovskite Solar Cells

We report systematic design and formation of plasmonic perovskite solar cells (PSCs) by integrating Au@TiO₂ core-shell nanoparticles (NPs) into porous TiO₂ and/or perovskite semiconductor capping layers. The plasmonic effects in the formed PSCs are examined. The most efficient configuration is obtained by incorporating Au@TiO₂ NPs into both the porous TiO₂ and the perovskite capping layers, which increases the power conversion efficiency (PCE) from 12.59% to 18.24%, demonstrating over 44% enhancement, compared with the reference device without the metal NPs. The PCE enhancement is mainly attributed to short-circuit current improvement. The plasmonic enhancement effects of Au@TiO₂ core-shell nanosphere photovoltaic composites are explored based on the combination of UV-vis absorption spectroscopy, external quantum efficiency (EQE), photocurrent properties, and photoluminescence (PL). The addition of Au@TiO₂ nanospheres increased the rate of exciton generation and the probability of exciton dissociation, enhancing charge separation/transfer, reducing the recombination rate, and facilitating carrier transport in the device. This study contributes to understanding of plasmonic effects in perovskite solar cells and also provides a promising approach for simultaneous photon energy and electron management.

Designing the Interface of Carbon Nanotube/Biomaterials for High-Performance Ultra-Broadband Photodetection

Inorganic/biomolecule nanohybrids can combine superior electronic and optical properties of inorganic nanostructures and biomolecules for optoelectronics with performance far surpassing that achievable in conventional materials. The key toward a high-performance inorganic/biomolecule nanohybrid is to design their interface based on the electronic structures of the constituents. A major challenge is the lack of knowledge of most biomolecules due to their complex structures and composition. Here, we first calculated the electronic structure and optical properties of one of the cytochrome c (Cyt c) macromolecules (PDB ID: 1HRC ) using ab initio OLCAO method, which was followed by experimental confirmation using ultraviolet photoemission spectroscopy. For the first time, the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of Cyt c, a well-known electron transport chain in biological systems, were obtained. On the basis of the result, pairing the Cyt c with semiconductor single-wall carbon nanotubes (s-SWCNT) was predicted to have a favorable band alignment and built-in electrical field for exciton dissociation and charge transfer across the s-SWCNT/Cyt c heterojunction interface. Excitingly, photodetectors based on the s-SWCNT/Cyt c heterojunction nanohybrids demonstrated extraordinary ultra-broadband (visible light to infrared) responsivity (46-188 A W^-1) and figure-of-merit detectivity D* (1-6 × 10¹⁰ cm Hz^1/2 W^-1). Moreover, these devices can be fabricated on transparent flexible substrates by a low-lost nonvacuum method and are stable in air. These results suggest that the s-SWCNT/biomolecule nanohybrids may be promising for the development of CNT-based ultra-broadband photodetectors.

Side-Chain Fluorination: An Effective Approach to Achieving High-Performance All-Polymer Solar Cells with Efficiency Exceeding 7

Side-chain fluorination of polymers is demonstrated as a highly effective strategy to improve the efficiency of all-polymer solar cells from 2.93% (nonfluorinated P1) to 7.13% (fluorinated P2). This significant enhancement is achieved by synergistic improvements in open-circuit voltage, charge generation, and charge transport, as fluorination of the donor polymer optimizes the band alignment and the film morphology.

Mapping the Competition between Exciton Dissociation and Charge Transport in Organic Solar Cells

The competition between exciton dissociation and charge transport in organic solar cells comprising poly(3-hexylthiophene) [P3HT] and phenyl-C61-butyric acid methyl ester [PCBM] is investigated by correlated scanning confocal photoluminescence and photocurrent microscopies. Contrary to the general expectation that higher photoluminescence quenching is indicative of higher photocurrent, microscale mapping of bulk-heterojunction solar-cell devices shows that photoluminescence quenching and photocurrent can be inversely proportional to one another. To understand this phenomenon, we construct a model system by selectively laminating a PCBM layer onto a P3HT film to form a PCBM/P3HT planar junction on half of the device and a P3HT single junction on the other half. Upon thermal annealing to allow for interdiffusion of PCBM into P3HT, an inverse relationship between photoluminescence quenching and photocurrent is observed at the boundary between the PCBM/P3HT junction and P3HT layer. Incorporation of PCBM in P3HT works to increase photoluminescence quenching, consistent with efficient charge separation, but conductive atomic force microscopy measurements reveal that PCBM acts to decrease P3HT hole mobility, limiting the efficiency of charge transport. This suggests that photoluminescence-quenching measurements should be used with caution in evaluating new organic materials for organic solar cells.

High Conversion Efficiency Carbon Nanotube-Based Barrier-Free Bipolar-Diode Photodetector

Conversion efficiency (CE) is the most important figure of merit for photodetectors. For carbon nanotubes (CNT) based photodetectors, the CE is mainly determined by excitons dissociation and transport of free carriers toward contacts. While phonon-assisted exciton dissociation mechanism is effective in split-gate CNT p-n diodes, the CE is typically low in these devices, approximately 1-5%. Here, we evaluate the performance of a barrier-free bipolar diode (BFBD), which is basically a semiconducting CNT asymmetrically contacted by perfect n-type ohmic contact (Sc) and p-type ohmic contact (Pd) at the two ends of the diode. We show that the CE in short channel BFBD devices (e.g., 60 nm) is over 60%, and it reduces rapidly with increasing channel length. We find that the electric-field-assisted mechanism dominates the dissociation rate of excitons in BFBD devices at zero bias and thus the photocurrent generation process. By performing a time-resolved and spatial-resolved Monte Carlo simulation, we find that there exists an effective electron (hole)-rich region near the n-type (p-type) electrode in the asymmetrically contacted BFBD device, where the electric-field strength is larger than 17 V/μm and exciton dissociation is extremely fast (<0.1 ps), leading to very high CE in the BFBD devices.

Utilizing Energy Transfer in Binary and Ternary Bulk Heterojunction Organic Solar Cells

Energy transfer has been identified as an important process in ternary organic solar cells. Here, we develop kinetic Monte Carlo (KMC) models to assess the impact of energy transfer in ternary and binary bulk heterojunction systems. We used fluorescence and absorption spectroscopy to determine the energy disorder and Förster radii for poly(3-hexylthiophene-2,5-diyl), [6,6]-phenyl-C61-butyric acid methyl ester, 4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine (DIBSq), and poly(2,5-thiophene-alt-4,9-bis(2-hexyldecyl)-4,9-dihydrodithieno[3,2-c:3',2'-h][1,5]naphthyridine-5,10-dione). Heterogeneous energy transfer is found to be crucial in the exciton dissociation process of both binary and ternary organic semiconductor systems. Circumstances favoring energy transfer across interfaces allow relaxation of the electronic energy level requirements, meaning that a cascade structure is not required for efficient ternary organic solar cells. We explain how energy transfer can be exploited to eliminate additional energy losses in ternary bulk heterojunction solar cells, thus increasing their open-circuit voltage without loss in short-circuit current. In particular, we show that it is important that the DIBSq is located at the electron donor-acceptor interface; otherwise charge carriers will be trapped in the DIBSq domain or excitons in the DIBSq domains will not be able to dissociate efficiently at an interface. KMC modeling shows that only small amounts of DIBSq (<5% by weight) are needed to achieve substantial performance improvements due to long-range energy transfer.

Electric-Field Induced Activation of Dark Excitonic States in Carbon Nanotubes

Electrical activation of optical transitions to parity-forbidden dark excitonic states in individual carbon nanotubes is reported. We examine electric-field effects on various excitonic states by simultaneously measuring photocurrent and photoluminescence. As the applied field increases, we observe an emergence of new absorption peaks in the excitation spectra. From the diameter dependence of the energy separation between the new peaks and the ground state of E11 excitons, we attribute the peaks to the dark excited states which became optically active due to the applied field. Field-induced exciton dissociation can explain the photocurrent threshold field, and the edge of the E11 continuum states has been identified by extrapolating to zero threshold.

Chemical Vapor Deposition Synthesized Atomically Thin Molybdenum Disulfide with Optoelectronic-Grade Crystalline Quality

The ability to synthesize high-quality samples over large areas and at low cost is one of the biggest challenges during the developmental stage of any novel material. While chemical vapor deposition (CVD) methods provide a promising low-cost route for CMOS compatible, large-scale growth of materials, it often falls short of the high-quality demands in nanoelectronics and optoelectronics. We present large-scale CVD synthesis of single- and few-layered MoS2 using direct vapor-phase sulfurization of MoO2, which enables us to obtain extremely high-quality single-crystal monolayer MoS2 samples with field-effect mobility exceeding 30 cm(2)/(V s) in monolayers. These samples can be readily synthesized on a variety of substrates, and demonstrate a high-degree of optoelectronic uniformity in Raman and photoluminescence mapping over entire crystals with areas exceeding hundreds of square micrometers. Because of their high crystalline quality, Raman spectroscopy on these samples reveal a range of multiphonon processes through peaks with equal or better clarity compared to past reports on mechanically exfoliated samples. This enables us to investigate the layer thickness and substrate dependence of the extremely weak phonon processes at 285 and 487 cm(-1) in 2D-MoS2. The ultrahigh, optoelectronic-grade crystalline quality of these samples could be further established through photocurrent spectroscopy, which clearly reveal excitonic states at room temperature, a feat that has been previously demonstrated only on samples which were fabricated by micro-mechanical exfoliation and then artificially suspended across trenches. Our method reflects a big step in the development of atomically thin, 2D-MoS2 for scalable, high-quality optoelectronics.

Concurrent Effects of Delocalization and Internal Conversion Tune Charge Separation at Regioregular Polythiophene-Fullerene Heterojunctions

Quantum-dynamical simulations are used to investigate the interplay of exciton delocalization and vibronically induced internal conversion processes in the elementary charge separation steps at regioregular donor-acceptor heterojunctions. Ultrafast internal conversion leads to efficient deexcitation within the excitonic and charge transfer manifolds, thus modifying the charge separation dynamics. We address a model donor-acceptor junction representative of regioregular P3HT-PCBM, using high-dimensional quantum dynamics simulations by multiconfigurational methods. While partial trapping into an interfacial charge separated state occurs, long-range charge-separated states are accessed as previously demonstrated in the work of Tamura and Burghardt [J. Am. Chem. Soc. 2013, 135, 16364]. For an H-aggregate type, stacked donor species, the initial bright state undergoes ultrafast internal conversion within the excitonic manifold, creating multiple charge transfer pathways before reaching the lowest-energy dark exciton, which is uncoupled from the charge transfer manifold. This process profoundly affects the charge separation mechanism and efficiency. For small energetic offsets between the interfacial excitonic and charge transfer states, a delocalized initial bright state proves less prone to electron-hole capture by the interfacial trap than a localized, vibronic wavepacket close to the interface. For both delocalized and localized initial states, a comparable yield of free carriers is obtained, which is found to be optimal for energetic offsets of the order of the Coulomb barrier to charge separation. Interfacial trapping is significantly reduced as the barrier height decreases with fullerene aggregation. Despite the high-dimensional nature of the system, charge separation is an ultrafast coherent quantum process exhibiting oscillatory features as observed in recent experiments.

Fast Dissociation and Reduced Auger Recombination of Multiple Excitons in Closely Packed PbS Nanocrystal Thin Films

Exciton decay dynamics in chemically treated PbS quantum-dot (QD) films have been studied using femtosecond transient-absorption (TA) spectroscopy. In photoconductive QD films, a decay component with a lifetime of a few nanoseconds appeared in the TA signals because of exciton dissociation under weak excitation. Increasing excitation fluence resulted in additional fast-decay components corresponding to the lifetimes of multiple excitons, which decreased with increasing photoconductivity of the closely packed QD films. Auger recombination in photoexcited QDs was suppressed in highly photoconductive films. Our findings clearly show that the carrier transfer between the QDs dominates the lifetimes of single and multiple excitons.

Improving polymer/nanocrystal hybrid solar cell performance via tuning ligand orientation at CdSe quantum dot surface

Achieving superior solar cell performance based on the colloidal nanocrystals remains challenging due to their complex surface composition. Much attention has been devoted to the development of effective surface modification strategies to enhance electronic coupling between the nanocrystals to promote charge carrier transport. Herein, we aim to attach benzenedithiol ligands onto the surface of CdSe nanocrystals in the "face-on" geometry to minimize the nanocrystal-nanocrystal or polymer-nanocrystal distance. Furthermore, the "electroactive" π-orbitals of the benzenedithiol are expected to further enhance the electronic coupling, which facilitates charge carrier dissociation and transport. The electron mobility of CdSe QD films was improved 20 times by tuning the ligand orientation, and high performance poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT):CdSe nanocrystal hybrid solar cells were also achieved, showing a highest power conversion efficiency of 4.18%. This research could open up a new pathway to improve further the performance of colloidal nanocrystal based solar cells.

High performance organic photovoltaics with plasmonic-coupled metal nanoparticle clusters

Performance enhancement of organic photovoltaics using plasmonic nanoparticles has been limited without interparticle plasmon coupling. We demonstrate high performance organic photovoltaics employing gold nanoparticle clusters with controlled morphology as a plasmonic component. Near-field coupling at the interparticle gaps of nanoparticle clusters gives rise to strong enhancement in localized electromagnetic field, which led to the significant improvement of exciton generation and dissociation in the active layer of organic solar cells. A power conversion efficiency of 9.48% is attained by employing gold nanoparticle clusters at the bottom of the organic active layer. This is one of the highest efficiency values reported thus far for the single active layer organic photovoltaics.

Direct optical generation of long-range charge-transfer states in organic photovoltaics

Direct optical excitation of long-range charge-transfer (CT) states in organic photovoltaics is shown to be feasible, a fact that is ascribed to the very low but non-vanishing oscillator strength of each individual transition and the much higher density of states (DOS) as compared with their short-range counterparts. This finding provides a new framework to interpret the low-energy absorption spectra of photovoltaic devices and to correlate this property with the optoelectronic conversion process in working devices.

Simultaneous study of exciton diffusion/dissociation and charge transport in a donor-acceptor bilayer: pentacene on a C60 -terminated self-assembled monolayer

In-situ photoinduced threshold voltage measurement is a sensitive probe for exploring charge transport and exciton diffusion simultaneously, the main determinants of the power conversion efficiency in organic photovoltaic devices. The exciton diffusion length in a pentacene film deposited onto a C60 -terminated self-assembled monolayer is measured. The methodology and analysis presented here can be applied in the design and engineering of electron/donor acceptor interfaces for photovoltaic devices..

Tuning the Interfacial Electronic Structure at Organic Heterojunctions by Chemical Design

Quantum-chemical techniques are applied to assess the electronic structure at donor/acceptor heterojunctions of interest for organic solar cells. We show that electrostatic effects at the interface of model 1D stacks profoundly modify the energy landscape explored by charge carriers in the photoconversion process and that these can be tuned by chemical design. When fullerene C60 molecules are used as acceptors and unsubstituted oligothiophenes or pentacene are used as donors, the uncompensated quadrupolar electric field at the interface provides the driving force for splitting of the charge-transfer states into free charges. This quadrupolar field can be either enhanced by switching from a C60 to a perylene-tetracarboxylic-dianhydride (PTCDA) acceptor or suppressed by grafting electron-withdrawing groups on the donor.

Photoluminescence quenching in carbon nanotube-polymer/fullerene films: carbon nanotubes as exciton dissociation centres in organic photovoltaics

Photoluminescence quenching is observed in acid functionalized multiwall carbon nanotubes incorporated polymer: fullerene films, suggesting efficient charge transfer, believed to be due to nanotubes acting as exciton dissociation centres. The fabricated photovoltaic devices with triple heterojunction interfaces show increased short circuit current density compared to the device without nanotubes.

Femtosecond Dynamics in Single Wall Carbon Nanotube/Poly(3-Hexylthiophene) Composites

Femtosecond transient absorption measurements on single wall carbon nanotube/poly(3-hexylthiophene) composites are used to investigate the relaxation dynamics of this blended material. The influence of the addition of nanotubes in polymer matrix on the ultrashort relaxation dynamics is examined in detail. The introduction of nanotube/polymer heterojunctions enhances the exciton dissociation and quenches the radiative recombination of composites. The relaxation dynamics of these composites are compared with the fullerene derivative-polymer composites with the same matrix. These results provide explanation to the observed photovoltaic performance of two types of composites.