Hole Transfer Research Articles

Constructing n/n- Type Perovskite Homojunctions to Achieve High-Efficiency and Stable Printable Mesoscopic Perovskite Solar Cells.

In recent years, carbon-based printable mesoscopic perovskite solar cells (p-MPSCs) without hole transport layers have garnered considerable interest because of their outstanding benefits in terms of stability and cost. However, the use of carbon electrodes instead of hole transport materials and noble metal electrodes leads to energy level mismatch, which limits the power conversion efficiency (PCE) of p-MPSCs. In this work, a molecular doping strategy is proposed employing cyclopentylmethanamine to passivate surface and subsurface crystal defects in perovskite layers while inducing an energy shift toward the p-type in the perovskite region within carbon electrodes. This approach facilitates the formation of a perovskite homojunction at carbon micro-interfaces between carbon electrodes and perovskites. Results demonstrate that the formation of this homojunction optimizes the internal energy level alignment of devices, thereby increasing driving force for hole transfer to carbon electrodes. Ultimately, the devices optimized through this strategy increase the PCE from 17.50% to 19.50% while retaining over 92% of the initial PCE after over 150 days in air ambiance. This study provides a straightforward and effective approach for designing high-efficiency and stable p-MPSCs.

Insight into Charge Transport Behaviors in Hematite Photoanodes via Potential-Dependent Impedance Spectroscopy and Machine Learning.

We employed machine learning (ML) techniques combined with potential-dependent photoelectrochemical impedance spectroscopy (pot-PEIS) to gain deeper insights into the charge transport mechanisms of hematite (α-Fe2O3) photoanodes. By the Shapley Additive exPlanations (SHAP) analysis from the ML model constructed from a small data set (dozens of samples) of electrical parameters obtained from pot-PEIS and the PEC performance, we identified the dominant factors influencing the electron transport to the back contact in the bulk and hole transfer to a solution at the hematite/electrolyte interface. The results revealed that shallow defect states significantly enhance electron transport, while deep defect states impede it, and also one of the surface states enhances the hole transfer to the electrolyte solution. The incorporation of a titanium dioxide underlayer had an effect to increase shallow defect states to promote electron transport and induce a new surface state to promote hole transfer. Cobalt phosphate cocatalyst improved charge separation, leading to enhance electron transport in the bulk, and the cocatalyst surface state promotes the hole transfer at the interface. The understanding of the band diagram based on the selected dominant descriptors provided critical insights into the electronic structure, elucidating the role of defect states and surface states in influencing the overall photoanode performance. This study showcases the potential of combining ML and pot-PEIS for material optimization.

Both Hole Transfer and Cocatalysis Exhibited by Organometallic Complexes for Enhancing Water-Splitting Performance of the Photoreducing BiVO4 Photoanode

Both Hole Transfer and Cocatalysis Exhibited by Organometallic Complexes for Enhancing Water-Splitting Performance of the Photoreducing BiVO₄ Photoanode

High‐Efficiency PbS Quantum Dots Infrared Solar Cells via Numerical Simulation and Experimental Optimization

AbstractLow‐bandgap lead sulfide quantum dots (PbS QDs) can efficiently harness the infrared (IR) light in the solar spectrum beyond 1100 nm, showing great application potential in the bottom subcells of tandem solar cells. However, achieving further efficiency improvements in PbS QDs IR solar cells still faces many challenges. In this work, the effects of the absorber layer thickness, the carrier mobility in the absorber layer, the defect density in the absorber layer and at the absorber/electron transfer layer (ETL) interface, and the doping density of the ETL and hole transfer layer (HTL) on the performance of PbS QDs (≈0.95 eV) IR solar cells are systematically investigated through SCAPS‐1D simulation. A theoretical efficiency of 16.95% and 2.15% is calculated for PbS QDs IR solar cells under AM 1.5 and 1100 nm‐filtered illumination, respectively. Based on the simulation results, the corresponding PbS QDs IR solar cells are fabricated with an efficiency of 11.53% under AM 1.5 illumination, a remarkable 1100 nm‐filtered efficiency of 1.30%, and a high external quantum efficiency of 70.50% at 1290 nm. Hence, these findings will accelerate the optimization of the performance of PbS QDs IR solar cells approaching their theoretical efficiency limit.

Molecular Orbital Tuning of Pentacene-Based Organic Semiconductors through N-Ethynylation of Dihydrodiazapentacene.

This study explores the concept of molecular orbital tuning for organic semiconductors through the use of N,N'-diethynylated derivatives of 6,13-dihydro-6,13-diazapentacene (2a and 2b). These novel molecules maintain the same molecular geometry and π-π stacking as their parent pentacene derivatives (1a and 1b), as confirmed by X-ray crystallography. However, they exhibit altered frontier molecular orbitals in terms of the phase, nodal properties, and energy levels. Theoretical calculations based on crystal structures indicate that 2a and 2b could significantly enhance the hole mobilities of the parent compounds by improving the hole transfer integral. Organic field-effect transistors (OFETs) of 1a and 2a were fabricated by using dip-coating and bar-coating methods. Both types of devices for 2a demonstrated a hole mobility exceeding 1 cm2 V-1 s-1, more than twice that of the respective devices for 1a. Additionally, unlike its pentacene parent, 2a is transparent to visible light and exhibits significantly enhanced environmental stability against light and air, making it a promising candidate for broader applications in organic electronic devices.

Exploring the Impact of Ionic Additives on the Structure and Energetics of Dye-Sensitized NiO Interfaces: Insights from Electrochemistry and First-Principles Calculations.

The efficient functioning of dye-sensitized solar cells (DSSCs) is governed by the interplay of three essential components: the semiconductor, the dye, and the electrolyte. While the impact of the electrolyte composition on the device's performance has been extensively studied in n-type DSSCs, much less is known about p-type-based devices. Here, we investigate the effect of potential-determining ions on the energetics and stability of dye-sensitized NiO surfaces by using electrochemical, ab initio molecular dynamics simulations, and ab initio electronic structure calculations. The experimental results indicate that the presence of Li+ leads to a ca. 100 mV positive shift in the potential of NiO, which is in good agreement with what is theoretically predicted (ca. 180 mV). Moreover, both experiments and calculations pinpoint that the presence of Li+ at the interface weakens the bonds between C343 and NiO, resulting in an accelerated desorption of the dye from the substrate. Computational remodeling of C343@NiO interfaces in the presence and absence of lithium salts (LiF) confirms that the interfacial energetics are very sensitive to the dynamical structure of C343@NiO and to the presence of anionic/cationic species. Indeed, although the presence of lithium at the interface has only a minor impact on the potential difference between NiO and the dye, it significantly influences the electronic coupling between the two interfacial components. This, in turn, leads to variations in the interfacial hole transfer rates.

Combining Cocatalyst and Oxygen Vacancy to Synergistically Improve Fe2O3 Photoelectrochemical Water Oxidation Performance

Considering the poor conductivity of Fe2O3 and the weak oxygen evolution reaction associated with it, surface hole accumulation leads to electron hole pair recombination, which inhibits the photoelectrochemical (PEC) performance of the Fe2O3 photoanode. Therefore, the key to improving the PEC water oxidation performance of the Fe2O3 photoanode is to take measures to improve the conductivity of Fe2O3 and accelerate the reaction kinetics of surface oxidation. In this work, the PEC performances of Fe2O3 photoanodes are synergistically improved by combining loaded an FeOOH cocatalyst and oxygen vacancy doping. Firstly, amorphous FeOOH layers are successfully prepared on Fe2O3 nanostructures through simple photoassisted electrodepositon. Then oxygen vacancies are introduced into FeOOH-Fe2O3 through plasma vacuum treatment, which reduces the content of Fe-O (OL) and Fe-OH (-OH), jointly promoting the generation of oxygen vacancies. Oxygen vacancy can increase the concentration of most carriers in Fe2O3 and form photo-induced charge traps, promoting the separation of electron holes and enhancing the conductivity of Fe2O3. The other parts of -OH act as oxygen evolution catalysts to reduce the reaction obstacle of water oxidation and promote the transfer of holes to the electrode/electrolyte interface. The performance of FeOOH-Fe2O3 after plasma vacuum treatment has been greatly improved, and the photocurrent density is about 1.9 times higher than that of the Fe2O3 photoanode. The improvement in the water oxidation performance of PEC is considered to be the synergistic effect of the cocatalyst and oxygen vacancy. All outstanding PEC response characteristics show that the modification of the cocatalyst and oxygen vacancy doping represent a favorable strategy for synergistically improving Fe2O3 photoanode performance.

In-Plane Bulk Photovoltaic Effect in a MoSe2/NbOI2 Heterojunction for Efficient Polarization-Sensitive Self-Powered Photodetection.

Two-dimensional ferroelectric materials can generate a bulk photovoltaic effect, making them highly promising for self-powered photodetectors. However, their practical application is limited by a weak photoresponse due to a weak transition strength and wide band gap. In this study, we construct a van der Waals heterojunction using NbOI2, which has significant in-plane polarization, with a highly absorbing MoSe2 layer. We observe ultrafast hole transfer from MoSe2 to NbOI2 within 0.4 ps and electron transfer in the opposite direction within 3.8 ps, facilitating efficient charge dissociation and extraction. Applying a direct current electric field poling modulates the ferroelectric domains in NbOI2, enhancing the bulk photovoltaic effect. This results in one of the highest responsivities for self-powered photodetectors (101.3 mA/W) at 0 V bias alongside excellent polarization sensitivity (∼7.58). This work advances the understanding of self-powering mechanisms via the bulk photovoltaic effect and proposes new strategies for future self-powered devices.

Excited-state hole transfer in quantum dot-ferrocene hybrids: influence of ligand-exchange chemistry on surface loading and charge-transfer dynamics

We investigated how the post-synthesis treatment of tetradecylphosphonic acid-capped CdTe (TDPA-CdTe) QDs with CdCl2 and oleylamine (OAm) affected subsequent ligand-exchange reactions with ferrocenylhexanethiol (FcC6SH) and the dynamics and yield of photoinduced hole transfer from CdTe to adsorbed ferrocenylhexanethiolate (FcC6S-). CdCl2/OAm treatment promoted an 8-fold increase of the average number of adsorbed FcC6S- ligands ( N FcC 6 S − ) per CdTe QD; N FcC 6 S − increased with the concentration of FcC6SH and reached a maximum of 198 ± 5. Higher per-QD loadings of FcC6S- increased the overall ensemble rate constant of CdTe-to-FcC6S- hole transfer (kNht ), which reached (3.0 ± 0.4) × 109 s−1, and the efficiency of hole transfer (ηht ), which reached (98.2 ± 0.3)%. Values of kNht increased monotonically with N FcC 6 S − ; at relatively low loadings of FcC6S−, kNht varied linearly with N FcC 6 S − , indicating that hole-transfer pathways were additive. The intrinsic rate constant of hole transfer per adsorbed FcC6S− (kht ), (2.75 ± 0.19) × 107 s−1, was 10-fold higher for CdCl2/OAm-treated CdTe QDs than for TDPA-CdTe QDs. Surface-anchoring thiolates did not trap photogenerated holes; instead, holes were transferred to the Fe(II) center of FcC6S−. Treatment of CdTe QDs with CdCl2 and OAm thus promoted ligand-exchange with FcC6S−, accelerated excited-state hole transfer, and afforded tunability of charge-transfer dynamics.

An Energy Level Alignment Study of 2PACz Molecule on Perovskite Device‐Related Interfaces by Vacuum Deposition

Self‐assembling molecules (SAM) have been widely used in inverted perovskite solar cells (PSC) as a hole transfer layer due to nearly lossless charge transfer giving excellent device performance. However, the energy level alignment between SAM‐ and PSC‐related interfaces has not been systematically studied. Herein, the 2PACz, a typical SAM with the largest dipole moment, is chosen as the model system and is studied by vacuum deposition. It is found that the energy level alignment is determined by the orientation of 2PACz molecules on a different substrate. The molecules are lying down on highly oriented pyrolytic graphite and giving nearly zero interface dipole. On solvent‐cleaned and plasma‐treated indium tin oxide (ITO) substrates, the SAM is vertically assembled with 0.22 and 0.13 eV work function increases, respectively. However, on sputtered ITO, SAM is assembled with upside down orientation, with 0.51 eV work function decrease. The change of orientation is due to strong interaction between oxygen vacancies in ITO substrate and carbazole head group of 2PACz. On perovskite film, SAM also shows a slightly upward orientation with additional passivation of free MA+ ions. Herein, it is confirmed that the energy level alignment of SAM plays an important role in hole extraction.

Modulating Charge Transfer Kinetics along Poly Adenine: Chemical Modifications, Temperature, and Conformational Effects.

The charge transfer (CT) reactions in nucleic acids are crucial for genome damage and repair and nanoelectronics using DNA as a molecular conductor. Previous experimental and theoretical works underlined the significance of nucleic acid structural dynamics on CT kinetics, requiring models that incorporate the dynamics of the nucleic acid, solvents, and counterions. Here, we investigated hole transfer kinetics in poly adenine single and double strands at various temperatures and the rate enhancement due to adenine-to-7-deazaadenine mutation by means of a QM/MM approach. We found that the hole transfer rate in poly adenine double strands increases with temperature while the helix conformation is retained, whereas single strands exhibit the opposite thermal response. Additionally, the positive charge migrates more efficiently in poly-7-deazaadenine double strands. Our results, consistent with experimental data, suggest that a thermally induced hopping model can accurately describe CT kinetics in these sequences. The approach is transferable for studying CT reactions in other nucleic acid strands.

Layer-number-dependent photoswitchability in 2D MoS2-diarylethene hybrids.

Molybdenum disulfide (MoS2) is a notable two-dimensional (2D) transition metal dichalcogenide (TMD) with properties ideal for nanoelectronic and optoelectronic applications. With growing interest in the material, it is critical to understand its layer-number-dependent properties and develop strategies for controlling them. Here, we demonstrate a photo-modulation of MoS2 flakes and elucidate layer-number-dependent charge transfer behaviors. We fabricated hybrid structures by functionalizing MoS2 flakes with a uniform layer of photochromic diarylethene (DAE) molecules that can switch between closed- and open-form isomers under UV and visible light, respectively. We discovered that the closed-form DAE quenches the photoluminescence (PL) of monolayer MoS2 when excited at 633 nm and that the PL fully recovers after DAE isomerization into the open-form. Similarly, the electric conductivity of monolayer MoS2 is drastically enhanced when interacting with the closed-form isomers. In contrast, photoinduced isomerization did not modulate the properties of the hybrids made of MoS2 bilayers and trilayers. Density functional theory (DFT) calculations revealed that a hole transfer from monolayer MoS2 to the closed-form isomer took place due to energy level alignments, but such interactions were prohibited with open-form DAE. Computational results also indicated negligible charge transfer at the hybrid interfaces with bilayer and trilayer MoS2. These findings highlight the critical role of layer-number-dependent interactions in MoS2-DAE hybrids, offering valuable insights for the development of advanced photoswitchable devices.

Accelerated interfacial hole transfer over Au/TiO2 photocatalysts for highly efficient oxidative coupling of methane.

Accelerated interfacial hole transfer over Au/TiO2 photocatalysts facilitates a highly efficient oxidative coupling of methane, achieving a C2 hydrocarbon production rate of 3.3 mmol g-1 h-1 and selectivity of up to 97%.

Enhanced Charge Generation and Transport by Incorporating a Benzotriazole Unit for Low-Cost and High-Efficiency Organic Solar Cells.

The photovoltaic performance of organic solar cells (OSCs) has reached the threshold for industrial applications, but the cost of most high-performance organic photovoltaic molecules is too high to meet the needs of industrialization. Herein, two low-cost thiophene-alt-quinoxaline (TQ)-based polymers, PTQ16-10 and PTQ16-20, are designed and synthesized by incorporating a benzotriazole (BTA) unit into the PTQ10 backbone, with the consideration of expanding the chemical modifiability of PTQ10 and thus optimizing its photovoltaic properties. The incorporation of BTA induces improved light absorption, up-shifted energy levels, more orderly molecular π-π packing, enhanced molecular crystallinity, and better charge transport capacity of the two polymers. Consequently, PTQ16-10:K2-based OSCs achieve enhanced charge generation and transport with faster and more efficient hole transfer, suppressed carrier recombination, and higher charge mobilities, resulting in an impressive power conversion efficiency (PCE) of 18.83%. Meanwhile, by introducing the second acceptor K6 into the PTQ16-10:K2 host blend, the ternary device achieves an excellent PCE of 19.52%, which is among the highest PCEs of OSCs based on low-cost photovoltaic materials. More importantly, this device is highly cost-effective for industrial-scale production, with an estimated minimum sustainable price of only 0.377 $ Wp-1 (Wp = peak-Watt), which is appreciably lower than that of reported high-performance OSCs. This work offers a rational guide in the development of low-cost and high-performance organic photovoltaic molecules and suggests the great potential of PTQ16-10 in cost-effective OSCs for industrialization.

Unraveling the competition between charge and energy transfer in 0D/2D nanographene-graphene heterojunctions

The charge and energy transfer processes in photoexcited 0D/2D donor/graphene heterojunctions occur through multiple different pathways. A donor deexcitation event occurring in the most prevalent Förster energy transfer mechanism (strongly favored over Dexter transfer in van der Waals heterojunctions) prevents charge transfer from taking place, thus creating a competition between the two processes. By applying a robust computational approach, we describe the two processes from first principles and quantify their rates using Förster and Marcus theories. We consider nanojunctions where the donor are nanographenes with varying size and symmetry, and discern important trends, e.g., the symmetry-induced quenching, or the enhancement due to increased size. We observe that heterojunctions where nanographenes do not have a center of symmetry show decreased photoinduced hole and energy transfer rates, which can then be recovered by increasing the delocalization length, whereas for centrosymmetric nanographenes both hole and energy transfer processes are enhanced. Nevertheless, the hole transfer rate dominates over the energy transfer process, providing a new computation-driven design principle for obtaining a high-charge transfer junction with minimized contribution of the competing energy transfer.

Noble-Metal-Free Cocatalysts Reinforcing Hole Consumption for Photocatalytic Hydrogen Evolution with Ultrahigh Apparent Quantum Efficiency.

Achieving efficient and sustainable hydrogen production through photocatalysis is highly promising yet remains a significant challenge, especially when replacing costly noble metals with more abundant alternatives. Conversion efficiency with noble-metal-free alternatives is frequently limited by high charge recombination rates, mainly due to the sluggish transfer and inefficient consumption of photo-generated holes. To address these challenges, a rational design of noble-metal-free cocatalysts as oxidative sites is reported to facilitate hole consumption, leading to markedly increased H2 yield rates without relying on expensive noble metals. By integrating femtosecond transient absorption spectroscopy with in situ characterizations and theoretical calculations, the rapid hole consumption is compellingly confirmed, which in turn promotes the effective separation and migration of photo-generated carriers. The optimized catalyst delivers an impressive photocatalytic H2 yield rate of 57.84 mmol gcat -1 h-1, coupled with an ultrahigh apparent quantum efficiency reaching up to 65.8%. Additionally, a flow-type quartz microreactor is assembled using the optimal catalyst thin film, which achieves a notable H2 yield efficiency of 0.102 mL min-1 and maintains high stability over 1260 min of continuous operation. The strategy of reinforcing hole consumption through noble-metal-free cocatalysts establishes a promising pathway for scalable and economically viable solar H2 production.

Fusion Modes and Number of Fused Rings: Their Impact on Acceptor-Donor-Acceptor Nonfullerene Acceptors in Organic Solar Cells.

The power conversion efficiency (PCE) of organic solar cells (OSCs) devices has surpassed 19% owing to the blooming of fused-ring nonfullerene acceptors (NFAs), especially for acceptor-donor-acceptor (A-D-A) type NFAs. However, the structural effect of the angular/linear fusion mode and number of fused rings for A-D-A type NFAs on the photovoltaic performance in OSCs devices remains unclear. Herein, the A-D-A type NFAs (F-0Cl, IDIC8-H, and ITIC) have been selected to obtain the intrinsic role of structural design strategies including the angular/linear fusion mode and the number of fused rings. The results indicate that compared to the linear fusion mode in ITIC, the angular fusion mode in F-0Cl effectively diminishes electronic vibrational coupling within the low-frequency range, leading to lower charge reorganization during the exciton diffusion process. Meanwhile, it facilitates the generation of multiple charge transfer mechanisms at the donor/acceptor (D/A) interface and increases the rates of hole transfer. On the other hand, the decreased number of fused rings of NFAs could inhibit the exciton decay and charge recombination but increase the rates of exciton diffusion and exciton dissociation for individual NFAs and the rates of electron separation for D/A interface. This work provides theoretical insights into structural design strategies, such as linear/angular fusion, and the number of fused rings of NFA, which presents a promising outlook for further enhancing PCE of high-performance A-D-A type NFAs.

Enhancing Interlayer Charge Transport of Two-Dimensional Perovskites by Structural Stabilization via Fluorine Substitution.

Two-dimensional lead-halide perovskites provide a more robust alternative to three-dimensional perovskites in solar energy and optoelectronic applications due to increased chemical stability afforded by interlayer ligands. At the same time, the ligands create barriers for interlayer charge transport, reducing device performance. Using a recently developed ab initio simulation methodology, we demonstrate that ligand fluorination can enhance both hole and electron mobility by 1-2 orders of magnitude. The simulations show that the enhancement arises primarily from improved structural order and reduced thermal atomic fluctuations in the system rather than increased interlayer electronic coupling. Arising from stronger hydrogen bonding and dipolar interactions, the higher structural stability decreases the reorganization energy that enters the Marcus formula and increases the charge transfer rate. The detailed atomistic insights into the electron and hole transfer in layered perovskites indicate that the use of interlayer ligands that make the overall structure more robust is beneficial simultaneously for chemical stability and charge transport, providing an important guideline for the design of new, efficient materials.

Mapping Binding Sites for Efficient Hole Extraction in Lead Halide Perovskites through Sulfur‐Based Ligand Engineering

AbstractLead halide perovskite nanocrystals (NCs) rapidly emerge as promising materials for photovoltaics. However, to fully harness their potential, efficient charge extraction is crucial. Despite rapid advancements, the specific active sites where acceptor molecules interact remain inadequately understood. Surface chemistry and interfacial properties are pivotal, as they directly impact charge transfer efficiency and overall device performance. This study identifies and maps binding sites for hole transporters, examining their influence on charge transfer dynamics through ligand engineering with 2,3‐dimercaptopropanol (DMP), a compound with a strong affinity for lead (Pb). DMP effectively passivates Pb sites in CsPbBr3 (CPB) NCs, enhancing photoluminescence (PL) by forming stable chelating bonds. DMP‐modified CPB nearly completely suppresses hole transfer to ─COOH‐functionalized ferrocene (FcA) and partially suppresses transfer to ─NMe2‐functionalized ferrocene (FcAm), suggesting an alternative hole extraction pathway for FcAm. This is further supported by enhanced hole transfer in bromine‐excess CPB (CPB‐Br(XS)) synthesized via SOBr2 treatment. The distinct binding interactions and charge transfer dynamics are validated through steady‐state and time‐resolved PL, along with transient absorption spectroscopy. These findings underscore the role of strategic ligand engineering in enhancing perovskite NC‐charge acceptor interactions, enabling better charge extraction, higher solar cell efficiency, and reduced lead toxicity through strong Pb binding.

Learning Intermolecular Electronic Coupling with Molecular-Orbital-Based Descriptors.

Electronic coupling is the key parameter to determine the rate of intermolecular electron transfer and energy transfer at excited states. When excited states are involved, the couplings are state-specific, as they originate from the interactions between different molecular orbitals (MOs). Based on the MO overlap description of the electron transfer (ET) and excitation energy transfer (EET) couplings taken as the domain knowledge, here we propose a graphic molecular orbital (MO) based descriptor to predict intermolecular electronic couplings. As the MOs are characterized by the spatial distribution of their wave functions, namely, the size and sign of the lobes, we transform the grid points of a MO into two feature vectors containing the quantum and spatial information. Then, inspired by the MO overlap description of the electronic couplings, we build the descriptors by multiplying the vectors for paired MOs. Together with a deep neural network (DNN) model, we learn the couplings of hole transfer (HT), electron transfer (ET), and Dexter energy transfer (DET). For the couplings of naphthalene dimers, high accuracy of learning is achieved by our approach compared with the results from quantum chemical (QC) calculations with a small size of training data. Therefore, the MO-pair-based descriptor shows the ability to characterize MO interaction for the high performance learning of electronic couplings and implies the potential of this strategy for other state-specific properties of excited molecular systems.