Hole Transport Capability Research Articles

Advancements in Carbazole-Based Sensitizers and Hole-Transport Materials for Enhanced Photovoltaic Performance

Carbazole-based molecules play a significant role in dye-sensitized solar cells (DSSCs) due to their advantageous properties. Carbazole derivatives are known for their thermal stability, high hole-transport capability, electron-rich (p-type) characteristics, elevated photoconductivity, excellent chemical stability, and commercial availability. This review focuses on DSSCs, including their structures, working principles, device characterization, and the photovoltaic performance of carbazole-based derivatives. Specifically, it covers compounds such as 2,7-carbazole and indolo[3,2-b]carbazole, which are combined with various acceptors like benzothiadiazole, thiazolothiazole, diketopyrrolopyrrole, and quinoxaline, as reported over the past decade. The review will also outline the relationship between molecular structure and power-conversion efficiencies. Its goal is to summarize recent research and advancements in carbazole-based dyes featuring a D-π-A architecture for DSSCs. Additionally, this review addresses the evolution of carbazole-based hole-transport materials (HTMs), which present a promising alternative to the costly spiro-OMeTAD. We explore the development of novel HTMs that leverage the unique properties of carbazole derivatives to enhance charge transport, stability, and overall device performance. By examining recent innovations and emerging trends in carbazole-based HTMs, we provide insights into their potential to reduce costs and improve the efficiency of DSSCs.

Enhancing Conductivity of Nickel Oxide to Achieve Scalable Preparation for High-Efficiency Perovskite Solar Modules

NiOx, prepared via the sputtering method, exhibits low conductivity and energy level mismatch with the perovskite layer, thereby limiting further enhancements in the performance of perovskite solar modules (PSMs). Unlike traditional methods that enhance the performance of NiOx through reactive sputtering or directly doping NiOx targets with metal ions, both of which incur high costs and low efficiency, we employ an evaporation method using LiF to achieve efficient and low-cost doping of NiOx. Compared to the pristine NiOx, the incorporation of LiF significantly increases the conductivity of NiOx. Additionally, the incorporation of LiF enhances the quality of the deposited perovskite films, as well as the energy level alignment and symmetry between NiOx and the perovskite, effectively improving the hole extraction and transport capabilities between NiOx and the perovskite. As a result, the PSM (active area of 57.30 cm²) fabricated in air achieves an impressive efficiency of 19.54%. Furthermore, the unencapsulated PSM retains 80% of its initial efficiency after 700 h of continuous illumination, whereas the NiOx-based PSM drops to 80% after only 150 h. This study provides a simple and low-cost method for doping NiOx, which is of great significance for the further industrialization of PSMs.

Direction Modulation of Intramolecular Electric Field Boosts Hole Transport in Phthalocyanines for Perovskite Solar Cells.

Tuning the strength of intramolecular electric field (IEF) in conjugated molecules has emerged as an effective approach to boost charge transfer. While direction manipulation of IEF would be a potential way that is still unclear. Here, we leverage the control of peripheral substituents of conjugated phthalocyanines to chemically tune the spatial orientation of IEF. By analyzing the spatial swing of side chains using the Kolmogorov-Arnold representation and least squares algorithm, a comprehensive mathematical-physical model has been established. This model enables rapid evaluation of the IEF and maximum hole transport performance induced by spatial swings. The champion phthalocyanine as dopant-free hole transport material in perovskite solar cell realizes a record performance of 23.41%. Greatly device stability is also exhibited. This work affords a new way to enhance hole transport capabilities of conjugated molecules by optimizing their IEF vector for photovoltaic devices.

Direction Modulation of Intramolecular Electric Field Boosts Hole Transport in Phthalocyanines for Perovskite Solar Cells

Tuning the strength of intramolecular electric field (IEF) in conjugated molecules has emerged as an effective approach to boost charge transfer. While direction manipulation of IEF would be a potential way that is still unclear. Here, we leverage the control of peripheral substituents of conjugated phthalocyanines to chemically tune the spatial orientation of IEF. By analyzing the spatial swing of side chains using the Kolmogorov‐Arnold representation and least squares algorithm, a comprehensive mathematical‐physical model has been established. This model enables rapid evaluation of the IEF and maximum hole transport performance induced by spatial swings. The champion phthalocyanine as dopant‐free hole transport material in perovskite solar cell realizes a record performance of 23.41%. Greatly device stability is also exhibited. This work affords a new way to enhance hole transport capabilities of conjugated molecules by optimizing their IEF vector for photovoltaic devices.

Regulation of Intermolecular and Interfacial Interactions by Functionalization of Substituent Groups in Hole Transport Materials for Perovskite Solar Cells: A Density Functional Theory and Experimental Study

Developing potential hole transport materials (HTMs) is an effective approach to reduce carrier nonradiative recombination and improve the efficiency of perovskite solar cells (PSCs). To investigate the intermolecular and interfacial interactions of HTMs in PSC devices, two molecules (RQ7 and RQ8) with substituent group functionalization are designed by isomerizing ‐F and ‐SMe in the para‐ and ortho‐positions on the side chain of carbazole diphenylamine‐based HTMs. Theoretical simulations indicate that the functionalization of substituent groups in HTMs alters their dipole moments and electronic structures, as well as their intermolecular interactions, hole transport capabilities, and interfacial interactions at the HTMs/perovskite interface. Experimental results show that RQ8 exhibited higher hole mobility, better film‐forming ability, and reduced electron‐hole recombination, resulting in RQ8‐based PSC devices achieving a higher power conversion efficiency than those based on RQ7. Herein, it is demonstrated that isomerous functionalization of substituent groups in carbazole diphenylamine‐based HTMs is a feasible strategy for controlling and regulating the intermolecular and interfacial interactions of HTMs in PSC applications.

Enhancing Efficiency of Industrially‐Compatible Monolithic Perovskite/Silicon Tandem Solar Cells with Dually‐Mixed Self‐Assembled Monolayers

AbstractThe antisolvent‐assisted spin‐coating still lags behind the thermal evaporation method in fabricating perovskite films atop industrially textured silicon wafers in making monolithic perovskite/silicon solar cells (P/S‐TSCs). The inhomogeneity of hole‐selective self‐assembled monolayers (SAMs) often arises from the insufficient bonding between hygroscopic phosphonic acid anchors and metal oxide. To address this, a mixed‐SAM strategy (Mx‐SAM) is proposed to enhance the adsorption energy of SAMs on the ITO surface, facilitate the formation of dense and humidity‐resistant hole‐selective layer (HSL) on substrates, and improve hole transport capabilities. With the aid of the Mx‐SAM strategy, the optimized wide‐bandgap PSCs achieved an impressive power conversion efficiency (PCE) of 22.63% with an exceptionally high fill factor (FF) of 86.67% using the 1.68 eV perovskite. Moreover, they exhibited enhanced stability under damp‐heat conditions (ISOS‐D‐3, 85% RH, 85 °C) with a T90 of 900 h for encapsulated PSCs, representing one of the best performances for wide‐bandgap PSCs. When further extending the Mx‐SAM strategy to making P/S‐TSCs using silicon wafers from industry, a remarkable efficiency of 28.07% is reached while upholding outstanding reproducibility. This strategy holds significant promise for the feasibility of fabricating industrially‐compatible P/S‐TSCs.

Computational Insights Into Betanin for Dsscs: Unraveling Deprotonation Variations and Identifying Optimal Anchoring Sites on TiO2

AbstractBetanin (Bn), a natural dye in the Betalains family, predominantly takes on a cationic form known as Bn+. However, it exists in a neutral state as Bn_C2, Bn_C15, and Bn_C17 by losing an H+ from one of its carboxylic acids. Density functional theory (DFT) and Time‐dependent density functional theory (TD‐DFT) studies evaluate the efficiency of each betanin form and pinpoint the most probable anchoring point to TiO2. The Bn_C17 variant stands out as a highly promising candidate for DSSC cells, demonstrating a distinctive combination of electron injection efficiency, electrochemical performance, hole transport capabilities, and photovoltaic behavior. Considering factors like adsorption energy, binding mode, structural compatibility, electronic properties, and absorption characteristics, Bn_C17@TiO2 emerges as the most favorable dye@TiO2 complex among the studied betanin forms for DSSC applications. Contrastingly, the C2‐COOH anchoring point presents challenges with monodentate binding, a different orientation, and potential load distribution issues. This behavior, resembling that of a p‐type dye, differs from the n‐type behavior exhibited by the C15‐COOH and C17‐COOH forms, making the latter two more suitable as sensitizers. Consequently, C2‐COOH may not be the optimal anchoring point for TiO2 in the investigated betanin forms, especially when compared to the more favorable C17‐COOH anchoring point.

Design tungsten oxides hole transport layers (HTLs) to modify the back interface in all-inorganic carbon-based CsPbBr3 solar cells

To enhance the inferior photovoltaic performance of all-inorganic carbon-based perovskite solar cells (C-PSCs) caused by inefficient charge extraction and transport processes of the CsPbBr3/carbon back interface, three types of tungsten oxides (WO3, WO2.72, WO2) are synthesized as low cost and stable hole transport layers (HTLs) to modify the back interface by tunning the energy level alignment. Among the three kinds of tungsten oxides, WO3 shows the best hole extraction and transport capability and the C-PSCs using WO3 HTL generates a power conversion efficiency (PCE) of 7.90% with a high open circuit voltage (Voc) of 1.307 V, indicating a great PCE enhancement of 43.9%, as compared with the device without HTL (5.49%). Meanwhile, WO2.72 and WO2 also are also proved to be qualified HTLs and the devices show PCE values of 7.57% and 6.24%, respectively. Importantly, WO3 and WO2.72 based device show excellent stabilities with PCE retention over 90% of the initial PCE values under ambient conditions for 60 days.

Expansion strategy of carbazole connecting unit in linear hole transport materials for perovskite solar cells

Hole transport materials (HTMs) play an important role in perovskite solar cells (PSCs), and the regulation of their molecular structures can promote the further development of power conversion efficiency (PCE) and stability. In this work, through exploiting the expansion strategy at the molecular level, a new linear HTM, named H580, was designed and synthesized by expanding the carbazole connecting unit. The results show that the introduction of dibenzol structure can significantly improve the photophysical and electrochemical characteristics of the molecule while preserving the passivating effect of the cyano group. The results show that a higher efficiency and a better stability are obtained after using H580 as the HTM. Significantly, compared with using spiro-OMeTAD alone, the additional introduction of H580 as an interface material with hole transport capability and defect passivation effect has achieved an excellent performance of PSCs.

Modeling and enhancement of double perovskite solar cell using WO3 as electron transporting material and SrCu2O2 as hole transporting material

There is a growing necessity to develop renewable energy with cost-effective materials. In this work, a double perovskite-based solar cell design has been proposed. The proposed design is numerically simulated, and investigation of solar cell parameters has been carried out using SCAPS-1D simulation software. (FA)2BiCuI6 is an organic–inorganic double perovskite material employed as an absorber layer. SrCu2O2 is used as hole transporting material and WO3 is used as electron transporting material in proposed device model FTO/WO3/(FA)2BiCuI6/SrCu2O2/W. SrCu2O2 absorbs lower light due to its wide bandgap and provides better hole transport capability which reduces the recombination. WO3 has tunable bandgap and absorb light in visible spectrum and offer good conductivity and thus this combination improve the power conversion efficiency. The analysis and optimization of the various parameters of absorber layer such as thickness, shallow acceptor density, and operating temperature are done. At 500 nm absorber thickness for shallow acceptor density of 1.0 × 1018 cm−3 and 300 K operating temperature, the optimized results have been observed. The photo-voltaic parameters: open circuit voltage 1.04 V, short circuit current density 25.97 mA/cm2, fill factor 88.23 % and power conversion efficiency of the device 23.64 % are observed. The proposed configuration is a promising candidate for efficient, low-cost double perovskites-based solar cells.

Self-Assembled Monolayer Doping for MoTe2 Field-Effect Transistors: Overcoming PN Doping Challenges in Transition Metal Dichalcogenides.

Transition metal dichalcogenides (TMDs) have gained significant attention as next-generation semiconductor materials that could potentially overcome the integration limits of silicon-based electronic devices. However, a challenge in utilizing TMDs as semiconductors is the lack of an established PN doping method to effectively control their electrical properties, unlike those of silicon-based semiconductors. Conventional PN doping methods, such as ion implantation, can induce lattice damage in TMDs. Thus, chemical doping methods that can control the Schottky barrier while minimizing lattice damage are desirable. Here, we focus on the molybdenum ditelluride (2H-MoTe2), which has a hexagonal phase and exhibits ambipolar field-effect transistor (FET) properties due to its direct band gap of 1.1 eV, enabling concurrent transport of electrons and holes. We demonstrate the fabrication of p- or n-type unipolar FETs in ambipolar MoTe2 FETs using self-assembled monolayers (SAMs) as chemical dopants. Specifically, we employ 1H,1H,2H,2H perfluorooctyltriethoxysilane and (3-aminopropyl)triethoxysilane as SAMs for chemical doping. The selective SAMs effectively increase the hole and electron charge transport capabilities in MoTe2 FETs by 18.4- and 4.6-fold, respectively, due to the dipole effect of the SAMs. Furthermore, the Raman shift of MoTe2 by SAM coating confirms the successful p- and n-type doping. Finally, we demonstrate the fabrication of complementary inverters using SAMs-doped MoTe2 FETs, which exhibit clear full-swing capability compared to undoped complementary inverters.

Methoxy management of side chains on the carbazole-diphenylamine derivatives based hole transport materials for perovskite solar cells: A theoretical design and experimental research

Molecular design is the important approach for realizing the improvement of performance for hole transport materials (HTMs) in perovskite solar cells (PSCs). In this work, starting from the carbazole-diphenylamine derivatives based HTMs, three molecules (CX1-CX3) are designed through methoxy management on the side chains. The calculated results indicated that CX1-CX3 yield different molecular arrangements through methoxy management on the side chains, resulting in adjusting the intermolecular coupling strength and hole transport capabilities. In comparison with the designed CX2 and CX3, CX1 exhibits a higher hole mobility, stronger interfacial interactions and more efficient charge transfer at perovskite/HTMs interface. Hence, under the same conditions, PSC devices based on CX1 yields a PCE of 22.13% higher than those of other molecules CX2 (20.96%) and CX3 (19.72%). These findings highlight the significance of methoxy management on the side chains in HTMs for the photovoltaic performance of PSCs, and reveal the potential of molecular engineering for the design of efficient HTMs.

Solution-processed vanadium oxide by low-temperature annealing for silicon solar cells with hole selective contact

Transition metal oxides (TMOs) with high work function (V2O5, MoO3, WO3) are very promising hole selective materials for high-efficiency crystalline silicon solar cells, as they can provide excellent hole transport and electron blocking capabilities simultaneously. However, the TMOs are typically obtained using vacuum deposition techniques (thermal evaporation, atomic layer deposition or sputtering), for which the preparation process is complex and the requirement for the equipments is relatively high. In this work, we prepared vanadium oxide (V2O5-X) films using low-cost solution method as the hole-selective transport layer for silicon heterojunction solar cells. The low temperature post-annealing treatment at 150 °C reduces the surface roughness of film and improves the passivation contact performance with p-Si. Furthermore, annealing in an oxygen-deficient atmosphere (N2/H2 = 95/5) improves the carrier transport due to increased oxygen vacancies and enhances the surface passivation ascribed to the additional role of hydrogen. Combined with the annealing treatments and optimized device processes, the V2O5-X/p-Si heterojunction solar cells have achieved a conversion efficiency of 17.23%, which is the highest reported so far based on solution-processed V2O5-X for crystalline silicon solar cells.

Tetrathienopyrrole-based hole-transporting materials for highly efficient and robust perovskite solar cells

Developing coplanar π-extended backbone is essential for simultaneously achieving high hole transport capability and thermal stability for hole-transporting material (HTMs). In this work, the large coplanar tetrathienopyrrole (TTTP) building block is employed to construct two novel linear organic HTMs (WH01 and WH02). With respect to the dithieno[3,2-b:2′,3′-d]pyrrole core, the extended π-conjugation of the TTTP endows low-lying HOMO energy levels, improved interfacial contact and defect passivation, as well as enhanced hole extraction/transport capacity for TTTP-based HTMs. As a result, the TTTP core shows good compatibility to either planar or twisted donor. Perovskite solar cells (PSCs) based on both WH01 and WH02 realize power conversion efficiencies (PCEs) of around 21%. The optimized PSCs adopting WH01 exhibit a champion PCE of 21.54%, which significantly outperforms the reference M130 (18.85%). Furthermore, the un-encapsulated devices with TTTP-series HTMs demonstrate excellent operational stability under ambient air. This systematic study highlights that TTTP-based molecules are promising potential candidates as HTMs for achieving highly efficient and stable PSCs.

Enhancing Hole Transport of Quantum-Dot Light-Emitting Diodes by a Cruciform Oligothiophene for Effective p-Type Doping.

Effective p-type doping is essential to enhance hole transport and balance electron-hole injection in quantum dot light-emitting diodes (QLEDs). Here, an oligothiophene material is adopted as a p-type dopant in the hole-transport layer, considering its cruciform cross-center structure, precise molecular weight, and high purity. Compared with the dopant-free counterpart, hole transport capability at the optimal doping level exhibits a significant improvement, producing a boosted external quantum efficiency (EQE) and luminance up to 20.8%, 213439cdm-2 , respectively, among the highest reported on the red-light emission. The work indicates the potential applications of oligothiophene material in red light-emitting devices.

Molecular Engineering of Star-Shaped Indoline Hole Transport Materials: The Influence of Planarity on the Hole Extraction and Transport Processes.

As the key properties of perovskite solar cells (PSCs), the hole extraction and transport capabilities of the hole transport material (HTM) affect the photovoltaic performance of PSCs to a considerable extent, while both capabilities can be adjusted by molecular planarity. Therefore, in this work, the molecular planarity of the HTM is systematically optimized to regulate the hole extraction and transport capabilities. Along with the improvement in planarity, the HTM's HOMO level is increased, leading to the enhancement of hole extraction capability. Meanwhile, the hole transport capability can also be improved due to the intensification of molecular stacking during the film formation. As a result, the planar HTM achieves a relatively high efficiency of 18.48 %, which is higher than that of spiro-OMeTAD. Accordingly, the molecular planarity presents an important impact on the photovoltaic performance of PSCs, providing us with a promising strategy for further optimization of efficient HTMs.

Improved Stability and Efficiency of Inverted Perovskite Solar Cell by Employing Nickel Oxide Hole Transporting Material Containing Ammonium Salt Stabilizer

AbstractNickel oxide (NiO) is one of the promising hole‐transporting materials for perovskite solar cells (PSCs). Despite the ongoing efforts to improve PSC performance with sol–gel NiO, there has been limited study on the usage and influences of stabilizers on NiO and the relevant performance of PSCs. Until now, most of the sol–gel NiO methods use chemical stabilizers based on strongly acidic or mild basic catalysts such as hydrochloric acid and monoethanolamine. However, it is evident that the remaining pH‐biased stabilizers in the film aggravate device stability. Therefore, it is imperative to develop a more stable and effective NiO, which can boost the performance of PSC. Here, the relatively neutral ammonium salt is utilized in NiO solution, which can improve the hole transport capability and stability of NiO. Under the optimum salt condition, energy level and hole conductivity are modulated favorably for hole transportation. Moreover, constructive interaction between the ammonium salt and perovskite enhances interfacial properties and reduces trap‐assisted recombination. Based on this novel NiO, the champion power conversion efficiency of 19.91% with an exceptionally high open‐circuit voltage of 1.13 V among the reported MAPbI3‐based PSCs is demonstrated. Furthermore, NiO with salt stabilizer secures long‐term device stability, maintaining 97% of initial power conversion efficiency (PCE) even after 800 h.

Effect of hygroscopicity of the hole transport layer on the stability of organic solar cells

Organic solar cells (OSCs) are in high demand due to the need to power micro-powered and wireless indoor electronic devices. However, OSCs have significant disadvantages, such as poor environmental stability, lower efficiency, and low photo stability. Consequently, they have not been commercialized. OSCs are highly dependent on the hole transport layer (HTL). Acidity, hygroscopicity, environmental stability, and the hole transport ability of the HTLs are strongly correlated with the overall performance of OSCs. Many studies have examined how acidity and hole transport capability of HTL affect OSC performance, but hygroscopicity has never been studied. Therefore, in this study, we examined the effect of HTL hygroscopicity on the stability of OSC. Two different materials with the same acidity level—polystyrene sulfonate (PSS)-doped polyaniline (PANI) and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)—were used as an HTL of poly (3-hexylthiophene): [6, 6]-indene-C60 bisadduct active material-based OSC. Thermogravimetric analysis confirmed that PANI:PSS was less hygroscopic than the PEDOT:PSS. Furthermore, the stability study of the OSCs revealed that PANI:PSS based OSC had a longer stability than PEDOT:PSS based OSC.

Novel Anthracene HTM Containing TIPs for Perovskite Solar Cells

Recently, perovskite solar cells have been in the spotlight due to several of their advantages. Among the components of PSCs, hole transporting materials (HTMs) re the most important factors for achieving high performance and a stable device. Here, we introduce a new D–π–D type hole transporting material incorporating Tips-anthracene as a π–conjugation part and dimethoxy-triphenylamine as a donor part (which can be easily synthesized using commercially available materials). Through the measurement of various optical properties, the new HTM not only has an appropriate energy level but also has excellent hole transport capability. The device with PEH-16 has a photovoltaic conversion efficiency of 17.1% under standard one sun illumination with negligible hysteresis, which can be compared to a device using Spiro_OMeTAD under the same conditions. Ambient stability for 1200 h shown that 98% of PEH-16 device from the initial PCE was retained, indicating that the devices had good long-term stability.

Unveiling the ambipolar carrier transport property of SnO2−X for multiple-functional interlayers in perovskite solar cells

Tin oxides are the most promising electron transport layers in perovskite solar cells. An ambipolar carrier transport property has been recently demonstrated which enables a simple interconnection structure for all-perovskite tandem solar cells. However, the underlying mechanism for its ambipolar behavior is unclear, which cannot be explained by the intrinsic defects in SnO2−x. Here, by using density functional theory calculations, we unveil the origin of the ambipolar carrier transport of non-stoichiometry SnO2−x with a structure of SnO embedded in the SnO2 matrix. The hybridization of O 2p and Sn 5s orbitals of SnO introduces mid-gap states in the bandgap of SnO2, enabling hole transport property for SnO2−x when x is > 0.2. Increasing the percentage of SnO in SnO2−x significantly enhances the hole transport capability of SnO2−x due to the enlarged Sn–O–Sn angles that increase orbital overlapping between O and Sn atoms, thus providing strategies for the further tuning of the carrier transport properties of SnO2−x by compositional and structural designs.