Hole Transport Layer Film Research Articles

Systematic anode engineering enabling universal efficiency improvements in organic solar cells

Anode modification and optimization is crucial towards improving performance of organic solar cells (OSCs). PEDOT:PSS is the most common choice as a hole transport layer (HTL) material, but suffers from issues including low conductivity. In this work, three alkyl amine derivatives - methylamine hydrochloride (MA), ethylamine hydrochloride (EA) and propylamine hydrochloride (PA) are doped into the commercially available Al 4083 PEDOT:PSS to form PEDOT:PSS-MA, PEDOT:PSS-EA and PEDOT:PSS-PA, as modified HTLs. All these modified HTLs exhibit improved chemical and electrical properties including work functions (WF), conductivities and charge carrier motilities. The alkyl amine doping shows compatibility in both Small Molecular Acceptors and All-Polymer OSCs. With PEDOT:PSS-MA demonstrates a highest PCE of 18.49 % compared to the 17.84 % of OSC devices prepared with pristine PEDOT:PSS with the PM6:L8-BO system, while PM6:PY-IT all-polymer OSCs improve PCE from 14.53 % to 15.22 %. AFM characterizations reveal that the introduction of the dopants have smoothened the surface morphology of spin-coated HTL films, which contributes towards more efficient charge extraction. In summary, this study not only presents a method of improving OSC efficiencies, but also provides insight and further possible directions towards anode optimization of OSCs.

Enhancement of inverted structure perovskite solar cell by CZTS nanoparticles

Inverted structure perovskite solar cells have attracted much attention in recent years due to their reliable operational stability, low residual, and low-temperature fabrication process. In the past few years, to accelerate their commercialization, the focus of research on the inverted structure perovskite solar cells was on the power conversion efficiency increasing. In this study nanoparticles of Cu2ZnSnS4 (CZTS) were doped into the PEDOT:PSS film as the hole transport layer (HTL) and then the interface carrier recombination quenching was observed. Consequently, it leads the s to charge carrier's collection enhancement of the Inverted perovskite solar cells. Compared to other types of HTLs, the use of CZTS HTL reduces the amount of interaction of the HTL film and the perovskite film, which results in an increment of the stability of the solar cell over time.

Nondestructive Direct Patterning of Both Hole Transport and Emissive Layers for Pixelated Quantum-Dot Light-Emitting Diodes.

Considering the increasing demand for high-resolution light-emitting diodes (LEDs), it is important that direct fine patterning technologies for LEDs be developed, especially for quantum-dot LEDs (QLEDs). Traditionally, the patterning of QLEDs relies on resin-based photolithography techniques, requiring multiple steps and causing performance deterioration. Nondestructive direct patterning may provide an easy and stepwise method to achieve fine-pixelated units in QLEDs. In this study, two isomeric tridentate cross-linkers (X8/X9) are presented and can be blended into the hole transport layer (HTL) and the emissive layer (EML) of QLEDs. Because of their photosensitivity, the in situ cross-linking process can be efficiently triggered by ultraviolet irradiation, affording high solvent resistance and nondestructive direct patterning of the layers. Red QLEDs using the cross-linked HTL demonstrate an impressive external quantum efficiency of up to 22.45%. Through lithographic patterning enabled by X9, line patterns of HTL and EML films exhibit widths as narrow as 2 and 4 μm, respectively. Leveraging the patterned HTL and EML, we show the successful fabrication of pixelated QLED devices with an area size of 3 × 3 mm2, alongside the successful production of dual-color pixelated QLED devices. These findings showcase the promising potential of direct patterning facilitated by engineered cross-linkers for the cost-effective fabrication of pixelated QLED displays.

Reducing the Open-Circuit Voltage Loss of PbS Quantum Dot Solar Cells via Hybrid Ligand Exchange Treatment.

The interface VOC loss between the active layer and the hole transport layer (HTL) of lead sulfide colloidal quantum dot (PbS-CQD) solar cells is a significant factor influencing the efficiency improvement of PbS colloidal quantum dot solar cells (PbS-CQDSCs). Currently, the most advanced solar cells adopt organic P-type HTLs (PbS-EDT) via solid-state ligand exchange with 1,2-ethanedithiol (EDT) on the CQD top active layer. However, EDT is unable to altogether remove the initial ligand oleic acid from the quantum dot surface, and its high reactivity leads to cracks in the HTL film caused by volume contractions, which inevitably results in significant VOC loss. These flaws prompted this research to develop a method involving hybrid organic ligand exchange using 3-mercaptopropionic acid (MPA) and 1,2-EDT (PbS-Hybrid) to overcome these drawbacks of VOC loss. The results indicated that the new exchange strategy improved the quality of the HTL film and benefited from the enhanced passivation of the quantum dot surface and better alignment of energy levels, and the average VOC of PbS-Hybrid devices is increased by approximately 25 mV compared to control devices. With the enhanced VOC, the average power conversion efficiency (PCE) of the devices is improved by 10%, with the highest PCE reaching 13.24%.

Low Oxygen Content MoOx and SiOx Tunnel Layer Based Heterocontacts for Efficient and Stable Crystalline Silicon Solar Cells Approaching 22% Efficiency

AbstractIn crystalline silicon (c‐Si) solar cells, the hole transport layer (HTL) made of high oxygen content MoOx (x > 2.85, H‐MoOx) evaporating from molybdenum trioxide is not ideal due to low optical bandgap and interface reaction effects. This limits the power conversion efficiency (PCE) and stability of c‐Si solar cells. To improve this, low oxygen content MoOx (x < 2.85, L‐MoOx) with a wide bandgap of 3.87 eV, deposited using molybdenum dioxide (MoO2), is explored and implemented. The c‐Si/SiOx (FGA, forming gas annealing)/L‐MoOx heterojunction has a low contact resistivity of ≈15.06 mΩ cm2, which is almost one order of magnitude lower than that of c‐Si/SiOx(FGA)/H‐MoOx heterojunction. Using L‐MoOx as the HTL, a c‐Si solar cell based on the SiOx passivation layer shows a fill factor of 84.38% and PCE of 21.75%, representing the highest efficiency for MoOx‐based p‐type c‐Si solar cells. Scanning transmission electron microscopy results show that the L‐MoOx HTL effectively enhances the stability of c‐Si solar cells when exposed to air by reducing Ag and Si element diffusion into MoOx. This successful preparation of efficient and stable MoOx HTL films, while preserving their field‐effect passivation ability, provides valuable insights into the development of high‐performance HTL.

Study on the Fabrication of PEDOT:PSS/Si Hybrid Solar Cells Incorporated with F4TCNQ and VTMO

Poly(3,4-ethylenedioxythiophene) (PEDOT):polystyrenesulfonate (PSS)/Si hybrid heterojunction solar cells (HHSCs) are fabricated by a solution-based synthesis method which combines the advantages of PEDOT:PSS and the superior performance of crystalline silicon. The optimized PEDOT:PSS–2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) hole transport layer (HTL) was used to prepare organosilicon solar cells with a short-circuit current density (Jsc) of 33.33 mA/cm2 and a photoelectric conversion efficiency (η) of 13.23%. The HTL films were found to have increased conductivity by the addition of an electron-accepting dopant (F4TCNQ), obtaining well-defined crystallinity and few defects. In addition, an one-step combination of F4TCNQ with a silane coupling agent (vinyltrimethoxysilane, VTMO) was used to improve the textured device, with a notable efficiency of 14.46% and a Jsc of 33.88 mA/cm2. Our study provides an alternative and easy way to establish organic–inorganic hybrid applications with excellent optoelectronic properties and to gain a deeper understanding of organic semiconductors.

Suppressing Glass‐Transition and Lithium‐Ions Migration in Hole Transport Layer by V2O5 Decorated Graphite Carbon Nitride Nanosheets for Thermally Stable Perovskite Solar Cells

The undesired glass transition of 2,2’,7,7’‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9’‐spirobifluorene (spiro‐OMeTAD) and the lithium‐ions migration are the intractable factors affecting the stability of perovskite solar cells (PSCs). Herein, the 2D graphite carbon nitride nanosheets (CNVx) with various loading masses of V2O5 are developed as additives for spiro‐OMeTAD to achieve stable and efficient PSCs. The optimized CNVx possesses quantitative and controllable oxidation ability to spiro‐OMeTAD under the inert atmosphere, yielding significantly improved conductivity and hole mobility of the modified hole transport layer (HTL) film. As a result, a power conversion efficiency (PCE) of 21.10% is obtained in the CNVx modified device with enhanced open‐circuit voltage of 1.114 V and fill factor of 0.80. Furthermore, the glass transition temperature suppression in the CNVx‐modified HTL film and effective Li+ ions capture significantly improve the stability of the unencapsulated device, which maintains 82% and 90% of the original PCE after aging at 85 °C in the N2 atmosphere and storing in an ambient atmosphere with a relative humidity of 40% for 720 h, respectively. These results provide a deep understanding of PSCs stability enhancement with 2D carbon nanosheets for realizing high‐performance PSCs.

Topological Design of Highly Anisotropic Aligned Hole Transporting Molecular Bottlebrushes for Solution-Processed OLEDs.

Polyvinyl polymers bearing pendant hole transport functionalities have been extensively explored for solution-processed hole transport layer (HTL) technologies, yet there are only rare examples of high anisotropic packing of the HT moieties of these polymers into substrate-parallel orientations within HTL films. For small molecules, substrate-parallel alignment of HT moieties is a well-established approach to improve overall device performance. To address the longstanding challenge of extension from vapor-deposited small molecules to solution-processable polymer systems, a fundamental chemistry tactic is reported here, involving the positioning of HT side chains within macromolecular frameworks by the construction of HT polymers having bottlebrush topologies. Applying state-of-the-art polymer synthetic techniques, various functional subunits, including triphenylamine (TPA) for hole transport and adhesion to the substrate, and perfluoro alkyl-substituted benzyloxy styrene for migration to the air interface, were organized with exquisite control over the composition and placement throughout the bottlebrush topology. Upon assembling the HT bottlebrush (HTB) polymers into monolayered HTL films on various substrates through spin-casting and thermal annealing, the backbones of HTBs were vertically aligned while the grafts with pendant TPAs were extended parallel to the substrate. The overall design realized high TPA π-stacking along the out-of-plane direction of the substrate in the HTLs, which doubled the efficiency of organic light-emitting diodes compared with linear poly(vinyl triphenylamine)s.

Scalable Preparation of High‐Performance ZnO–SnO2 Cascaded Electron Transport Layer for Efficient Perovskite Solar Modules

Perovskite solar cells are the fastest‐growing photovoltaic technology in recent years. However, together with the stability, the low‐cost and high‐quality preparation of large‐area modules still limits their commercialization process. Herein, a scalable and high‐performance ZnOSnO2 cascade double‐layer electron transport layer (ETL) for efficient and stable perovskite modules is reported. The cascaded ETL is fabricated using a simple spray pyrolysis coating combined with the blade coating process, which not only effectively improves the interface stability by avoiding the protonation of ZnO to maintain its high electron mobility, but also provides a much smoother surface for the crystallization of perovskites. In addition, the well‐matched conduction band level between SnO2 and perovskites ensures the improvement of open‐circuit voltage. Subsequently, combined with the blade‐coated perovskite layer and hole transport layer film, large‐area planar perovskite modules are successfully prepared. These high‐quality films enable the perovskite solar modules to achieve impressive efficiencies of 17.8% in the module size 6 × 6 cm2 and 16.6% in a size of 10 × 10 cm2. The obtained module also shows excellent reproducibility and stability. The high‐performance ETL and the related deposition method developed in this work are promising for applications in the industrial scalable perovskite modules’ fabrication.

Enhancing the efficiency of green perovskite-QDs-based light-emitting devices by controlling interfacial defects with diamine molecules

Inorganic perovskite quantum dots (QDs) have emerged as promising materials for light-emitting devices by merits of their high photoluminescence quantum yield (PLQY), narrow full width at half-maximum and tunable emission spectrum. However, fluorescence quenching and trap-mediated nonradiative recombination between hole transport layer (HTL) and QD films are prejudicial to the performance of perovskite-QDs-based light-emitting devices (QLEDs). Here, HTL was modified with diamine molecules (1,4-butanediamine (4-DA), 1,8-octyldiamine (8-DA) and 1,11-aminoundecylamine (11-DA)) through a simple self-assembly method. QD films based on modified poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)-benzidine] (Poly-TPD) exhibited higher PL intensity and lower fluorescence quenching properties. Meanwhile, diamine molecules do favor to the flatness of Poly-TPD film and contribute to the smoothness of QD films. Moreover, the exciton quenching and the density of defects at the interface between HTL and QD films were effectively reduced. Particularly, QLEDs based on HTL modified with 8-DA was endowed with a maximum luminance of 78634 cd/m2, a peak current efficiency of 30.77 cd/A, a maximum external quantum efficiency of 8.90% and T50 of 16.8 min when the initial brightness is about 1000 cd/m2.

Hydrophobic Cu2O Quantum Dots Enabled by Surfactant Modification as Top Hole-Transport Materials for Efficient Perovskite Solar Cells.

The utilization of an inorganic hole‐transport layer (HTL) is one of the most effective methods to improve the stability and reduce the cost of perovskite solar cells (PSCs). However, achieving high‐quality inorganic HTL films, especially HTL films in n‐i‐p structures, via solution processes remains a big challenge. Here, a simple surface modification strategy for low‐cost and stable cuprous oxide (Cu2O) quantum dots is proposed, which utilizes a silane coupling agent. The modified Cu2O can be directly deposited on the perovskite film as the top HTL without decomposing the perovskite to maintain an n‐i‐p structure. The efficiency (18.9%) of PSCs with surface‐modified Cu2O as the HTL is significantly higher than that (11.9%) of PSCs with unmodified Cu2O, which is also the record efficiency for a Cu2O‐based perovskite solar cell in n‐i‐p structure. The enhanced performance of PSCs is attributed to the remarkably enhanced film properties achieved through surface modification. Moreover, because of the dopant‐free technology and hydrophobic surface, the Cu2O‐based PSCs have distinctly better stability than 2,2′,7,7′‐tetrakis[N,N‐di(4‐methoxyphenyl)amino]‐9,9′‐spiro‐bifluorene‐based PSCs.

Improved Surface Morphology of Crosslinked Hole Transport Films by a Mixture of Polymer for OLEDs

Solution process for multilayer organic light-emitting diodes (OLEDs) brings in the dissolution problems between two stacked layers. It can be tackled using crosslinkable organic materials, especially for a hole transport layer (HTL). Similar to noncrosslinked HTL films based on small molecules, however, there appears the aggregation phenomenon in crosslinked HTL films fabricated by spin coating. We have demonstrated that aggregation can be suppressed through a mixture of noncrosslinkable polymer and crosslinkable small molecule HTL materials. Although such a mixed HTL film results in a slight degradation in the solvent resistance and device performance, it ensures the film uniformity and thus OLED stability. When cured under ambient air, however, the mixed HTL films exhibit a number of microaggregates that increase in proportion to the concentration of the polymer chains. It is attributed that the moisture adsorption near the hybrid interfaces causes lateral phase separation. We have found that they disappear by carrying out the thermal curing under inert gas atmosphere or increasing the polymer concentration in ambient air.

Improved performance of organic solar cells with solution processed hole transport layer

This work is based on Cobalt Oxide as solution processed, inexpensive and effective hole transport layer (HTL) for efficient organic photovoltaic applications (OPVs). In Organic solar cell (OSC) devices ITO coated glass substrate used as a transparent anode electrode for light incident, HTL material Co3O4 dissolve in DMF solvent deposited on anode electrode, after that active layer material (donor/acceptor) deposited on to HTL and finally Al were deposited by thermal evaporation used as cathode electrode. These devices were fabricated with PCDTBT well known low band gap donor material in OSCs and blended with PC71BM as an acceptor material using simplest device structure ITO/Co3O4/active layer/Al at ambient conditions. The power conversion efficiencies (PCEs) based on Co3O4 and PEDOT:PSS have been achieved to up to 3.21% and 1.47% with PCDTBT respectively. In this study we reported that the devices fabricated with Co3O4 showed better performance as compare to the devices fabricated with well known and most studied solution processed HTL material PEDOT:PSS under identical environmental conditions. The surface morphology of the HTL film was characterized by (AFM). Lastly, we have provided Co3O4 as an efficient hole transport material HTL for solution processed organic photovoltaic applications.

All-room-temperature solution-processed new nanocomposites based hole transport layer from synthesis to film formation for high-performance organic solar cells towards ultimate energy-efficient fabrication

It is an ultimate goal of energy-efficient fabrication that photovoltaic devices only need low energy consumption processes covering both synthesis of material and fabrication of device. Regarding to charge transport layer, it is worthwhile to synthesize nanomaterials and deposit films all at room temperature while still have good electrical properties. Meanwhile, there are very limited studies on room temperature solution approaches for modification on nanocrystal colloidal for facilely forming high quality film with tunable electrical properties. In this work, we propose and demonstrate a new nanocomposites of maghemite and iron hydroxide through a low energy consumption approach which is all room-temperature solution processes from the synthesis of the nanocomposites to the formation of high quality hole transport layer (HTL). Strategically adjustment of acidity for the conversion of prepared precipitation is demonstrated to achieve a component controllable maghemite and iron hydroxide nanocomposites which contributes to in-situ tunable work function of the nanocomposites HTL from 4.70 eV to 5.16 eV. Simultaneously, since the nanocomposites synthesized from this approach have the features of ultra-small size of 6–10 nm and surfactant-free, high quality and efficient HTL films can be formed at room temperature. For organic solar cells using nanocomposite as HTL, the power conversion efficiency can be significantly improved by as much as 80% as compared with the optimized devices without HTL. Besides, both the efficiency and stability of the nanocomposite based organic solar cells are better than that of devices using poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS). Consequently, the work contributes to the fabrication of simple, low-cost, and stable optoelectronics promoting green photovoltaics and flexible electronics.

Pre- and post-treatments free nanocomposite based hole transport layer for high performance organic solar cells with considerably enhanced reproducibility

In this work, we demonstrate a one-step room-temperature ethanol-processed nickel oxide (NiOx):electron acceptor nanocomposite functioning as efficient hole transport layer (HTL). Specifically, one-step refers to the formation of the nanocomposite HTL films without extra steps of pre-treatments of ITO nor post-treatments of HTL films, and thus considerably reduce the fabrication complexity and cost. By varing the concentration of the electron acceptor, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), the work function (WF) of the nanocomposite films can be widely tuned from 4.73eV to 5.30eV, which favors its use for photovoltaic applications of organic donor materials with different highest energy occupied molecular orbital (HOMO) energy levels. Organic solar cells (OSCs) have been fabricated by using the NiOx:F4-TCNQ nanocomposite as HTL. The optimized average power conversion efficiency (PCE) of NiOx:F4-TCNQ based OSCs can be 18% better than that of one-step prepared poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) based OSCs. Remarkably, NiOx:F4-TCNQ based OSCs show better reproducibility as the deviation of PCE values can be improved by a greatest extent of 10 times. This work contributes towards simple and low-cost fabrication of high efficiency OSCs for practical photovoltaic applications.

Investigation on slot-die coating of hybrid material structure for OLED lightings

With an attempt to fabricate large-area OLED lighting panels, we investigate slot-die coating of a small molecule (SM) hole transport layer (HTL). It is observed that SM HTL films formed by spin coating exhibit pinhole-like surface, whereas the films by slot-die coating show micro-sized hillocks due to agglomeration. As the plate temperature of the slot coater is increased, smaller hillocks appear more densely. To tackle it, a small amount of a polymer HTL is added into the SM HTL (Hybrid HTL). By the aid of entangled polymer chains, small molecules are prohibited from migrating and thus agglomerations disappear. The peak-to-peak roughness of the slot-coated hybrid HTL films is measured to be about 11.5nm, which is slightly higher than that (~7nm) of the polymer HTL film, but much lower than that (~1071nm) of the SM HTL film. Similar results are also observed in spin-coated films. It is also addressed that OLED with the hybrid HTL shows higher luminous efficacy, compared to OLED with the SM HTL or the polymer HTL. We have further demonstrated that the dissolution problem occurring between two stacked layers with different solvents during slot-die coating can be suppressed to a great extent using such a combination of materials in hybrid structure.

New Triarylamine-Containing Polymers as Hole Transport Materials in Organic Light-Emitting Diodes: Effect of Polymer Structure and Cross-Linking on Device Characteristics

A series of poly(norbornenes) with pendant triarylamine (TPA) groups has been synthesized by ring-opening metathesis polymerization and investigated as hole transport materials in organic two-layer light-emitting diodes (LEDs). Efficient device fabrication through spin casting of the hole transport layer (HTL) was possible, since the polymers exhibited excellent film formation properties. LEDs of the form ITO/poly(norbornene)-TPA/Alq3/Mg (ITO = indium tin oxide, Alq3 = tris(8-quinolinato)aluminum) showed bright green emission with external quantum efficiencies of up to 0.77% (1.30 lm/W) for 20 nm thick HTL films. The length and polarity of the linker between the triarylamine functionality and the polymer backbone were varied systematically. The device performance was found to depend strongly on these structural differences. Substitution of ester groups by less polar ether functionalities greatly enhances external quantum efficiencies, lowers the operating voltage, and improves the stability of the device. Further improvement of the device characteristics is achieved by reducing the length of the alkyl linker. The HTL can be conveniently cross-linked by UV irradiation. Cross-linking was found to decrease device performance. A maximum external quantum efficiency of 0.37% was achieved for an Alq3-LED with cross-linked HTL.