Effective Hole-transporting Material Research Articles

Regio- and Chemo-selective C-H Arylation of 3-Bromothiophene: A Synthesis Shortcut to Versatile π-Conjugated Building Blocks for Optoelectronic Materials.

Unlike traditional multi-step synthetic approaches, we developed a single-step synthesis of versatile π-conjugated building blocks bearing post-functionalizable C-H and C-Br bonds. Direct C-H arylation of 3-bromothiophene with various iodo(hetero)aryls was successfully carried out with good regio- and chemo-selectivity. Under optimized reaction conditions, 20 new compounds were facilely prepared in yields up to 91%. One of the obtained compounds was demonstrated to further extend its conjugation length using a succinct synthetic plan to create two symmetrical oligo(hetero)aryls (MLC01 and MLC02) that were fabricated as effective hole-transporting materials (HTM) for perovskite solar cells (PSC). PSC devices utilizing MLC01 as hole-transport layer displayed promising power conversion efficiencies of up to 17.01%.

Achieving High Doping Density in pH-neutral Conjugated Polyelectrolyte Toward Effective Hole-Transporting Materials for Organic Solar Cells.

The lack of effective and non-corrosive hole-transporting layer (HTL) materials has remained a long-standing issue that severely restricts the performance of organic solar cells (OSCs). Most pH-neutral conjugated polyelectrolytes (CPEs) exhibit inferior performance to the acid-doped HTL materials due to their low doping density. In this study, a series of pH-neutral CPEs is designed and synthesized with high doping density as HTL materials. Through an elaborate synthetic route, two sulfonate-terminating alkoxyl side chains can be introduced into thiophene, by which the electron-rich, highly soluble, and chemically stable thiophene monomer is synthesized to enable the subsequent polymerization. The CPE PTT-F exhibit a remarkable self-doping property with an enhanced doping density from 2.01 × 1017 to 7.02 × 1018 cm-3. The high work function and the increased doping density of PTT-F-based HTL decrease the depletion region width from 38.4 to 8.1nm at the anode interface, which minimized the energy loss in hole transport. Consequently, a binary OSC modified by PTT-F-based HTL achieve a high PCE of 18.8%. To the best of the knowledge, this is the highest PCE for OSC employing CPE-based HTL. The results from this work demonstrate an encouraging achievement of realizing exceptional hole collection ability in pH-neutral CPEs.

Blending Self-Assembled Monolayers for Enhanced Band Alignment and Improved Morphology in p-i-n Perovskite Photodetectors.

Perovskite photodetectors, devices that convert light to electricity, require good extraction and low noise levels to maximize the signal-to-noise ratio. Self-assembling monolayers (SAMs) have been shown to be effective hole transport materials thanks to their atomic layer thickness, transparency, and energetic alignment with the valence band of the perovskite. While efforts are being made to reduce noise levels via the active layer, little has been done to reduce noise via SAM interfacial engineering. Herein, we report hybrid perovskite photodetectors with high detectivity by blending two different SAMs (2-PACz and Me-4PACz). We find that with a 1:1 2-PACz:Me-4PACz ratio (by weight), the devices achieved a low noise of 1 × 10-13 A Hz-1/2, a high responsivity of 0.41 A W-1 at 710 nm, and a specific detectivity of 6.4 × 1011 Jones at 710 nm at -0.5 V, outperforming its two counterparts. In addition to the improved noise levels in these devices, impedance spectroscopy revealed that higher recombination lifetimes of 0.85 μs were achieved for the 1:1 2-PACz:Me-4PACz-based photodetectors, confirming their low defect density.

N,N-Bis(9-methyl-3-carbazolyl)-4-anisidine as an Electroactive Material for Use in Perovskite Solar Cells.

Di(9-methyl-3-carbazolyl)-(4-anisyl)amine is presented as an effective hole-transporting material suitable for application in perovskite solar cells. It is obtained by a three-step synthesis from inexpensive starting compounds. It has a relatively high glass transition temperature of 93 °C and thermal stability with 5% weight loss at 374 °C. The compound exhibits reversible double-wave electrochemical oxidation below +1.5 V and polymerization at higher potential. A mechanism for its oxidation is proposed based on electrochemical impedance and electron spin resonance spectroscopy investigations, ultraviolet-visible-near-infrared absorption spectroelectrochemistry results, and density functional theory-based calculations. Vacuum-deposited films of the compound are characterized by a low ionization potential of 5.02 ± 0.06 eV and hole mobility of 10-3 cm2/(Vs) at an electric field of 4 × 105 V/cm. The newly synthesized compound has been used to fabricate dopant-free hole-transporting layers in perovskite solar cells. A power conversion efficiency of 15.5% was achieved in a preliminary study.

Dopant‐free NiOx Nanocrystals: A Low‐cost and Stable Hole Transport Material for Commercializing Perovskite Optoelectronics

AbstractAdvancing inverted (p‐i‐n) perovskite solar cells (PSCs) is critical for commercial applications given their compatibility with different bottom cells for tandem photovoltaics, low‐temperature processability (≤100 °C), and promising operational stability. Although inverted PSCs have achieved an efficiency of over 25 % using doped or expensive organic hole transport materials (HTMs), their synthesis cost and stability still cannot meet the requirements for their commercialization. Recently, dopant‐free and low‐cost non‐stoichiometric nickel oxide nanocrystals (NiOx NCs) have been extensively studied as a low‐cost and effective HTM in perovskite optoelectronics. In this minireview, we summarize the synthesis and surface‐functionalization methods of NiOx NCs. Then, the applications of NiOx NCs in other perovskite optoelectronics beyond photovoltaics are discussed. Finally, we provide a perspective for the future development of NiOx NCs for the commercialization of perovskite optoelectronics.

Dopant-free NiOx Nanocrystals: A Low-cost and Stable Hole Transport Material for Commercializing Perovskite Optoelectronics.

Advancing inverted (p-i-n) perovskite solar cells (PSCs) is critical for commercial applications given their compatibility with different bottom cells for tandem photovoltaics, low-temperature processability (≤100 °C), and promising operational stability. Although inverted PSCs have achieved an efficiency of over 25 % using doped or expensive organic hole transport materials (HTMs), their synthesis cost and stability still cannot meet the requirements for their commercialization. Recently, dopant-free and low-cost non-stoichiometric nickel oxide nanocrystals (NiOx NCs) have been extensively studied as a low-cost and effective HTM in perovskite optoelectronics. In this minireview, we summarize the synthesis and surface-functionalization methods of NiOx NCs. Then, the applications of NiOx NCs in other perovskite optoelectronics beyond photovoltaics are discussed. Finally, we provide a perspective for the future development of NiOx NCs for the commercialization of perovskite optoelectronics.

Molecular engineering of several butterfly-shaped hole transport materials containing dibenzo[b,d]thiophene core for perovskite photovoltaics

Several butterfly-shaped materials composed of dibenzo[b,d]thiophene (DBT) and dibenzo-dithiophene (DBT5) cores were designed as hole transporting materials (HTMs) and their properties were studied by density functional theory (DFT) computations for usage in mesoscopic n-i-p perovskite solar cells (PSCs). To choose suitable HTMs, it was displayed that both of lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energies of molecules were located higher than those of CH3NH3PbI3 (MAPbI3) perovskite as they were able to transfer holes from the MAPbI3 toward Ag cathode. Negative solvation energy (ΔEsolvation) values for all HTMs (within the range of − 5.185 to − 18.140 kcal/mol) revealed their high solubility and stability within CH2Cl2 solvent. The DBT5-COMe demonstrated the lowest values of band gap (Eg = 3.544) and hardness (η = 1.772 eV) (the greatest chemical activity) and DBT5-CF3 displayed the biggest η = 1.953 eV (maximum stability) that were predominantly valuable for effective HTMs. All HTMs presented appropriately high LHEs from 0.8793 to 0.9406. In addition, the DBT5 and DBT5-SH depicted the lowest exciton binding energy (Eb) values of 0.881 and 0.880 eV which confirmed they could produce satisfactory results for the PSCs assembled using these materials. The DBT5-SH and DBT5-H had maximum hole mobility (μh) values of 6.031 × 10–2 and 1.140 × 10–2 which were greater than those measured for the reference DBT5 molecule (μh = 3.984 × 10–4 cm2/V/s) and about 10 and 100 times superior to the calculated and experimental μh values for well-known Spiro-OMeTAD. The DBT5-COOH illustrated the biggest open circuit voltage (VOC), fill factor (FF) and power conversion efficiency (PCE) values of 1.166 eV, 0.896 and 23.707%, respectively, establishing it could be as the best HTM candidate for high performance PSCs.

2,3-Diphenylthieno[3,4-b]pyrazines as Hole-Transporting Materials for Stable, High-Performance Perovskite Solar Cells

High-performance and durable perovskite solar cells (PSCs) have advanced rapidly, enabled in part by the development of superior interfacial hole-transporting layers (HTLs). Here, a new series of 2,3-diphenylthieno[3,4-b]pyrazine (DPTP)-based small molecules containing bis- and tetrakis-triphenyl amino donors (1–3) was synthesized from simple, low-cost, and readily available starting materials. The matched energy levels, ideal surface topographies, high hole mobilities of 8.57 × 10–4 cm2 V–1 S–1, and stable chemical structures of DPTP-4D (3) make it an effective hole-transporting material, delivering a PCE of 20.18% with high environmental, thermal, and light-soaking stability when compared to the reference HTL materials, doped Spiro-OMeTAD and PTAA in PSC n-i-p planar devices. Overall, these DPTP-based molecules are promising HTM candidates for the fabrication of stable PSCs.

Pd-Free synthesis of dithienothiophene-based oligoaryls for effective hole-transporting materials by optimized Cu-catalyzed annulation and direct C–H arylation

For the first time, [Cu]-catalyzed annulation and direct C–H/C–I couplings were used as key transformations to access new DTT-based hole-transporting materials (HTMs), CHC05–07. Perovskite solar cells with CHC07 display PCEs of up to 14.7%.

Simple 9,10-dihydrophenanthrene based hole-transporting materials for efficient perovskite solar cells

It is a very important task of searching for novel hole-transporting materials (HTMs), along with effective building blocks, on the way towards commercialization of the perovskite solar cells (PSCs). 9,10-Dihydrophenanthrene (DHP) is a rigid conjugated system, whose skeleton is closely analogous to the widely used fluorene unit in developing HTMs. Herein, for the first time, DHP employed as a promising building block in the construction of HTMs for PSCs is presented. Two readily-available DHP-cored small molecules, namely K-DHP-1 and K-DHP-2, were synthesized and characterized. An attractive bridged ketal pattern of the two molecules was revealed by the X-ray crystallography of K-DHP-2. Afterwards, the two compounds were applied as HTMs in conventional PSCs with a FA0.9MA0.1Pb(I0.9Br0.1)3 • 0.05 CsI perovskite, respectively. Thereinto, the champion device based on doped K-DHP-1 exhibits a power conversion efficiency (PCE) of up to 20.52% with Jsc of 23.96 mA/cm2, Voc of 1.09 V and FF of 78.6%, rivalling the performance of device fabricated with spiro-OMeTAD (20.97%). More encouragingly, the K-DHP-1-based PSC displays good long-term stability, remaining 88% of its original performance after storing in atmosphere (30%~50% relative humidity) for 45 d without encapsulation, which is better than the spiro-OMeTAD-based PSC. These results indicate that K-DHP-1 can be an effective HTM to realize high efficiency and stable PSCs, and furthermore DHP can serve as an excellent building block for the development of new HTMs in the future.

Fluorinated triphenylamine-based dopant-free hole-transporting material for high-performance inverted perovskite solar cells

A fluorinated methoxytriphenylamine-based small molecule H2 is demonstrated as an effective hole-transporting material (HTM) to simultaneously enhance the efficiency and stability of the dopant-free inverted perovskite solar cells (PSCs). The fluorine-methoxytriphenylamine hybrid strategy in H2 is found to improve energy level alignment, hole extraction, and most importantly, the film morphology, which enables the growth of high quality perovskite active layer and superior interfacial contact. Consequently, the dopant-free H2-based inverted PSCs achieve an excellent efficiency of 18.69% with negligible hysteresis and good ambient stability, representing one of the best dopant-free organic-small-molecule HTMs for conventional perovskite MAPbI3 PSCs reported thus far. These results demonstrate the promising applications of fluorine-methoxytriphenylamine hybrid strategy in designing efficient and stable HTMs for dopant-free PSCs.

Vapor-Phase Formation of a Hole-Transporting Thiophene Polymer Layer for Evaporated Perovskite Solar Cells.

Homogeneous layer formation on textured silicon substrates is essential for the fabrication of highly efficient monolithic perovskite silicon tandem solar cells. From all well-known techniques for the fabrication of perovskite solar cells (PSCs), the evaporation method offers the highest degree of freedom for layer-by-layer deposition independent of the substrate's roughness or texturing. Hole-transporting polymers with high hole mobility and structural stability have been used as effective hole-transporting materials (HTMs) of PSCs. However, the strong intermolecular interactions of the polymers do not allow for a layer formation via the evaporation method, which is a big challenge for the perovskite community. Herein, we first applied a hole-transporting terthiophene polymer (PTTh) as an HTM for evaporated PSCs via an in situ vapor-phase polymerization using iodine (I2) as a sublimable oxidative agent. PTTh showed high hole mobility of 1.2 × 10-3 cm2/(V s) and appropriate energy levels as HTM in PSCs (EHOMO = -5.3 eV and ELUMO = -3.3 eV). The PSCs with the in situ vapor-phase polymerized PTTh hole-transporting layer and a co-evaporated perovskite layer exhibited a photovoltaic conversion efficiency of 5.9%, as a proof of concept, and high cell stability over time. Additionally, the polymer layer could fully cover the pyramidal structure of textured silicon substrates and was identified as an effective hole-transporting material for perovskite silicon tandem solar cells by optical simulation.

Inorganic CuFeO2 Delafossite Nanoparticles as Effective Hole Transport Materials for Highly Efficient and Long-Term Stable Perovskite Solar Cells.

The regular architecture (n-i-p) of perovskite solar cells (PSCs) has attracted increasing interest in the renewable energy field, owing to high certified efficiencies in the recent years. However, there are still serious obstacles of PSCs associated with spiro-OMeTAD hole transport material (HTM), such as (i) prohibitively expensive material cost (∼150-500 $/g) and (ii) operational instability at elevated temperatures and high humidity levels. Herein, we have reported the highly photo, thermal, and moisture-stable and cost-effective PSCs employing inorganic CuFeO2 delafossite nanoparticles as a HTM layer, for the first time. By exhibiting superior hole mobility and additive-free nature, the best-performing cell achieved a power conversion efficiency (PCE) of 15.6% with a negligible hysteresis. Despite exhibiting a lower PCE as compared to the spiro-OMeTAD-based control cell (19.1%), nonencapsulated CuFeO2-based cells maintained above 85% of their initial efficiency, while the PCE of control cells dropped to ∼10% under continuous illumination at maximum power point tracking after 1000 h. More importantly, the performance of control cells was quickly degraded at above 70 °C, whereas CuFeO2-based cells, retaining ∼80% of their initial efficiency after 200 h, were highly stable even at 85 °C in ambient air under dark conditions. Besides showing significant improvement in stability against light soaking and thermal stress, CuFeO2-based cells exhibited superior shelf stability even at 80 ± 5% relative humidity and retained over 90% of their initial PCE. Overall, we strongly believe that this study highlights the potential of inorganic HTMs for the commercial deployment of long-term stable and low-cost PSCs.

Tetrabenzotriazacorrole and its derivatives as undoped hole transporting materials for perovskite solar cells: Synthesis, device fabrication, and device performance

Abstract Phosphorous tetrabenzotriazacorrole (TBC) and its two soluble derivatives (TBC-1 and TBC-2) were synthesized and used for the first time as undoped hole transporting materials (HTMs) in MAPbI3 perovskite solar cells (PSCs). Their performance in PSCs was measured and compared with that of Spiro-OMeTAD and phthalocyanine precursor. The fundamental properties related to HTMs are also examined. These novel HTMs are easily prepared, cost-effective, and solution processable. The materials exhibited much higher hole transport mobility and broader light absorption than pristine Spiro-OMeTAD and phthalocyanine precursor. They can work efficiently in the absence of any dopant for devices composed of FTO/cp-TiO2/mp-TiO2/MAPbI3/HTM/Au. The undoped mesoscopic solar cell devices based on TBC exhibited a promising power conversion efficiency (PCE) of up to 16.2% (measured at 100 mW cm2 illumination, AM 1.5 G), together with good long-term stability under ambient conditions. This PCE of 16.2% observed using TBC is remarkably higher than the 11.2% observed using undoped Spiro-OMeTAD and also much better than the 8.70% observed using its phthalocyanine precursor. As to substitution effects, α-substituted TBC-1 was found to be a better HTM than β-substituted TBC-2 (PCE 11.4%) and unsubstituted TBC-3 (PCE 6.81%) under the same conditions. These results provide the basis for further exploiting TBC compounds as a new type of low-cost and effective HTM for PSCs.

Planar starburst hole-transporting materials for highly efficient perovskite solar cells

There is a requirement to develop more effective hole-transporting materials (HTMs) than commonly used 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) to fabricate highly efficient and stable perovskite solar cells. Herein, we reported a new HTM of N [2],N2,N [5],N5,N [11],N11-hexakis(4-methoxyphenyl)indolo[3,2,1-jk]carbazole-2,5,11-triamine (DCZ-OMeTAD) by employing indolo[3,2,1-jk]carbazole (DCZ) as a central building block. In addition, another DCZ-based HTM named as 4,4′,4''-(indolo[3,2,1-jk]carbazole-2,5,11-triyl)tris(N,N-bis(4-methoxyphenyl)aniline) (DCZ-OMeTPA) with different arylamines as the electron-rich branch was also synthesized for comparison. CH3NH3PbI3 and (NH2CHNH2PbI3)1-x(CH3NH3PbI3)x based perovskite solar cells (PSCs) by utilizing the low-cost DCZ-OMeTAD as HTM exhibited the power conversion efficiency (PCE) of 19.81% and 21.66%, respectively, which were significantly higher than those of Spiro-OMeTAD based devices (18.06% and 20.08%). Unfortunately, DCZ-OMeTPA based PSCs presented unsatisfied device performance compared with Spiro-OMeTAD based devices. We ascribe it to the inferior charge-extraction capability and poor hole mobility of DCZ-OMeTPA. What's more, DCZ-OMeTAD based device also delivered the best cell stability among three HTMs based PSCs, indicating that the newly designed concept by replacing spirobifluorene with DCZ has good potential for developing effective HTMs for high-performance PSCs.

The fabrication of homogeneous perovskite films on non-wetting interfaces enabled by physical modification

Abstract Conjugated polymers are commonly used as effective hole transport materials (HTMs) for preparation of high-performance perovskite solar cells. However, the hydrophobic nature of these materials renders it difficult to deposit photovoltaic perovskite layers on top via solution processing. In this article, we report a generic surface modification strategy that enables the deposition of uniform and dense perovskite films on top of non-wetting interfaces. In contrast to the previous proposed chemical modifications which might alter the optoelectronic properties of the interfacial layers, we realized a nondestructive surface modification enabled by introducing a layer of insulating mesoporous aluminum oxide (Al2O3). The surface energies of the typical non-wetting hole-transport layers (PTAA, P3HT, and Poly-TPD) were significantly reduced by the Al2O3 modification. Benefiting from the intact optoelectronic properties of the HTMs, perovskite solar cells deposited on these interface materials show full open-circuit voltages (VOC) with high fill factors (FF) up to 80%. Our method provides an effective avenue for exploiting the full potential of the existing as well as newly developed non-wetting interface materials for the fabrication of high-performance inverted perovskite solar cells.

Solution-processed Cu2ZnSnS4 nanoparticle film as efficient hole transporting layer for stable perovskite solar cells

Compact Cu2ZnSnS4 (CZTS) nanoparticle films are in-situ prepared on ITO-coated glass by a novel molecular precursor solution method at a low temperature (300 °C) and a short annealing time. The CZTS films exhibit a band gap of 1.48 eV and absorption coefficient of > 104 cm−1. It is found that the performance of CH3NH3PbI3 perovskite solar cells with CZTS film as hole transporting material (HTM) strongly depends on CZTS thickness due to that its absorption of photons significantly limits the photon-to-electron transfer below 550 nm, and the CZTS film thinner than 100 nm is a more effective HTM for efficient MAPbI3-based solar cells; furthermore, the CZTS/MAPbI3 solar cells are much more efficient and stable than the counterpart devices with poly(3,4-ethylenedioxy-thiophene):poly(styrene sulfonate) as HTM.

2D-MoS2 nanosheets as effective hole transport materials for colloidal PbS quantum dot solar cells.

Herein, we demonstrate for the first time matrix-free deposition of two dimensional (2D) MoS2 nanosheets as an efficient hole transport layer (HTL) for colloidal lead sulfide (PbS) quantum dot (QD) solar cells. We have developed all-solution-processed n–p–p+ architecture solar cells where ZnO nanoparticles were used as an n-type window layer, a PbS QD layer acted as a light absorbing p-type layer and 2D-MoS2 nanosheets acted as a p+-type hole transport layer. The MoS2 nanosheets allow better interface with the PbS QD layers. The incorporation of the MoS2 hole transport layer leads to superior fill factor, higher open circuit voltage and better performance in colloidal PbS QD solar cells. These results show that one layer of MoS2 nanosheets improves the power conversion efficiency of the device from 0.92% for a hole transport material free device to 2.48%. The present work reveals the development of 2D-MoS2 nanosheets as a new hole transport layer for the fabrication of cost-effective, durable and efficient colloidal PbS quantum dot solar cells.

Efficient planar perovskite solar cells with a conjugated random terpolymer as a novel hole-transporting material

We introduce that a conjugated random terpolymer (LGC-D013) with N-alkylthieno[3,4-c]pyrrole-4,6-dione (TPD, the acceptor) and benzodithiophene (BDT) and terthiophene (the donor units) can efficiently function as a novel hole-transporting material (HTM) for high-performance planar perovskite solar cells (PeSCs). The roles of the LGC-D013 HTM on PeSC-performances were examined using filed-emission scanning electron microscope, atomic force microscope, photoluminescence, hole mobility, energy-level diagram, and device-resistances. The LGC-D013 based device produced a high power conversion efficiency (PCE) of 12.22%, which was 22% enhancement compared to the reference poly (3-hexylthiophene) (P3HT) based device with a PCE of 9.53%. These results suggest that the random terpolymer LGC-D013 can be considered as an effective HTM to produce a high-efficiency PeSC.

A new thermal-stable truxene-based hole-transporting material for perovskite solar cells

Developing effective hole-transporting material is critical to achieve high efficiency CH3NH3PbI3 perovskite solar cells. A new thermal-stable truxene-based material, 2,7,12-tri(N,N-di(4-methoxyphenyl)amino)-5,5′,10,10′,15,15′-hexapropyltr-uxene, has been synthesized by a facile route. A reference compound, 1,3,5-tris(di-p-anisylamino)benzene, was prepared as well. Their optical, electronic properties, thermal stability and photovoltaic performances were investigated. This new truxene-based material exhibits a high decomposition temperature (432 °C), a high glass transition temperature (134 °C), and a suitable highest occupied molecular orbital level well-matched with the valence band of CH3NH3PbI3. The device based on this new material in conjunction with a carbon counter electrode achieves a power conversion efficiency of 3.18% under the illumination of 100 mW cm−2, which is better than that of the reference compound (0.93%). These results indicate truxene core is a promising building block for hole-transporting material.