Hole Transport Layer Structure Research Articles

Solution-Processed Thick Hole-Transport Layer for Reliable Quantum-Dot Light-Emitting Diodes Based on an Alternatingly Doped Structure.

The operating lifetime of quantum-dot light-emitting diodes (QLED) is a bottleneck for commercial display applications. To enhance the operational stability of QLEDs, we developed a robust solution-processed highly conductive hole-transport-layer (HTL) structure, which enables a thick HTL structure to mitigate the electric field. An alternating doping strategy, which involves multiple alternating stacks of N4,N4'-di(naphthalen-1-yl)-N4,N4'-bis(4-vinylphenyl)biphenyl-4,4'-diamine and phosphomolybdic acid layers, could provide significantly improved conductivity; more specifically, the 90 nm-thick alternatingly doped HTL exhibited higher conductivity than the 45 nm-thick undoped HTL. Therefore, when applied to a QLED, the increase in the thickness of the alternatingly doped HTL increased device reliability. As a result, the lifetime of the QLED with a thick, alternatingly doped HTL was 48-fold higher than that of the QLED with a thin undoped HTL. This alternating doping strategy provides a new paradigm for increasing the stability of solution-based optoelectronic devices in addition to QLEDs.

Design of n-i-p and p-i-n Sb2Se3 solar cells: role of band alignment

Investigations into novel device architectures and interfaces that enhance charge transport and collection are necessary to increase the power conversion efficiency (PCE) of antimony selenide (Sb2Se3) solar cells, which have shown great promise as a low-cost and high-efficiency alternative to conventional silicon-based solar cells. The current work uses device simulations to design p-i-n and n-i-p Sb2Se3-based solar cell structures. The n-i-p configuration is investigated by comparing distinct electron transport layer (ETL) materials to get the best performance. While certain ETL materials may yield higher efficiencies, the J–V curve may exhibit S-shaped behavior if there is a misalignment of the bands at the ETL/absorber interface. To address this issue, a proposed double ETL structure is introduced to achieve proper band alignment and conduction band offset for electron transport. A PCE of 20.15% was achieved utilizing (ZnO/ZnSe) as a double ETL and Spiro-OMeTAD as a hole transport layer (HTL). Further, the p-i-n configuration is designed by proposing a double HTL structure to facilitate hole transport and achieve a proper valence band offset. A double HTL consisting of (CuI/CuSCN) is used in conjunction with ETL-free configuration to achieve a PCE of 21.72%. The simulation study is conducted using the SCAPS-1D device simulator and is validated versus a previously fabricated cell based on the configuration FTO/CdS/Sb2Se3/Spiro-OMeTAD/Au.

PbS-TBAI / MoSe2-grating as new hole transport layer in perovskite solar cells: Superior efficiency enhancement thru simulation and optimization

For the first time, we increased the power conversion efficiency (PCE) of a perovskite solar cell (PSC) from 13.82% to 19.45%, by presenting a new structure of hole transport layer (HTL) consisting of spherical PbS-QD layer treated with tetrabutylammonium iodide (PbS-TBAI) and MoSe2 in the form of grating. MoSe2 with smaller bandgap than perovskite (CH3NH3PbI3) when used instead of CuSCN in the hole transport layer (HTL), can help absorb higher wavelengths and generate electron-hole pairs, which increases Jsc (short-circuit current density), Voc (open-circuit voltage) and PCE respectively from 17.2 to 20.8 mA/cm2, 0.97–1.21 V and 13.82–17.67%. However, due to the reduction of fill factor (FF), the arrangement of different layers to block electrons was investigated, which we found that employing grating structure can improve FF by 4%, while the other parameters do not change much (PCE: 17.67→17.3). Then, a spherical PbS-QD layer treated with tetrabutylammonium iodide (PbS-TBAI) was implemented inside the MoSe2 layer in grating form structure with Spiro to enhance the absorption and consequently the PCE. As a result, this change caused the increase of Jsc, FF and PCE, from 20.82 to 24.15 mA/cm2, 0.73–0.79 and 17.3–19.45%, respectively.

Structurally Driven Ultrafast Charge Funneling in Organic Bulk Heterojunction Hole Transport Layer for Efficient Colloidal Quantum Dot Photovoltaics

AbstractNanoscopic packing structures crucially determine the charge conduction and the consequent functionalities of organic semiconductors including bulk heterojunctions (BHJs), which are dependent on various processing parameters. Today's high‐performance colloidal quantum dot photovoltaics (CQDPVs) employ functional organic semiconductors as a hole transport layer (HTL). However, the processing of those films replicates a protocol dedicated to high‐performance organic PVs, and thus little is known about how to control the molecular packing structures to maximize the hole extraction function of the HTLs. Herein, it is uncovered that the random‐oriented, but closer‐packed BHJ crystallites, constructed by 1,2‐dichlorobenzene (o‐DCB) as a solvent, allow exceptional charge conduction vertically across the film and restrict diffusion‐driven charge transfer process, enabling ultrafast hole funneling from CQD to BHJ to be extracted. As a result, a power conversion efficiency of 13.66% with high photocurrent >34 mA cm−2 is achieved by employing o‐DCB‐processed BHJ HTL, far exceeding the performance of the CQDPV solely employing neat polymer HTL. A charge conduction mechanism associated with the BHJ HTL structure suppressing the bimolecular recombination is proposed. This works not only suggests key principles to control the packing structures of organic HTLs but also opens a new avenue to boost optoelectronic performance.

Device Modeling of Efficient PBDB-T:PZT-Based All-Polymer Solar Cell: Role of Band Alignment

In this study, we present some design suggestions for all-polymer solar cells by utilizing device simulation. The polymer solar cell under investigation is formed by a photoactive film of a blend comprising PBDB-T as a polymer donor and PZT as a polymerized small molecule acceptor. The initial cell is based on a fabricated cell whose structure is ITO/PEDOT:PSS/PBDB-T:PZT/PFN-Br/Ag, which has a power conversion efficiency (PCE) of about 14.9%. A calibration procedure is then performed by comparing the simulation results with experimental data to confirm the simulation models, and the material parameters, implemented in the SCAPS (Solar Cell Capacitance Simulator) simulator. To boost the open circuit voltage, we investigate a group of hole transport layer (HTL) materials. An HTL of CuI or P3HT, that may replace the PEDOT:PSS, results in a PCE of higher than 20%. However, this enhanced efficiency results in a minor S-shape curve in the current density-voltage (J-V) characteristic. So, to suppress the possibility of the appearance of an S-curve, we propose a double HTL structure, for which the simulation shows a higher PCE with a suppressed kink phenomenon due to the proper band alignment. Moreover, the designed cell is investigated when subjected to a low light intensity, and the cell shows a good performance, signifying the cell's suitability for indoor applications. The results of this simulation study can add to the potential development of highly efficient all-polymer solar cells.

Hole injection of quantum dot light-emitting diodes facilitated by multilayered hole transport layer

We report a multilayered hole transport layer (HTL) structure to improve the efficiencies and lifetimes of quantum dot light-emitting diodes (QD-LEDs). The HTL structure was constructed by inserting molybdenum trioxide (MoO3) into an organic monolayer to enhance its hole conductivity. We then investigated the effects of the multilayered HTL on the performance and operating lifetimes of QD-LEDs. QD-LEDs with optimal HTL exhibit power efficiency, external quantum efficiency, and lifetime (T80) of 10.19 lm/W, 9.66%, and 336 h, respectively. Compared to the monolayer structure, the lifetime of the multilayered HTL was enhanced by a factor of 15. This novel architecture for QD-LEDs with multilayered HTL offers a new design strategy for next-generation high-efficiency QLED displays and solid-state lighting technologies.

A p-p+ Homojunction-Enhanced Hole Transfer in Inverted Planar Perovskite Solar Cells.

Perovskite solar cells (PSCs) have triggered a research trend in solar energy devices in view of their high power conversion efficiency and ease of fabrication. However, more delicate strategies are still required to suppress carrier recombination at charge transfer interfaces, which is the necessary path to high-efficiency solar cells. Here, a p-p+ homojunction was constructed on basis of NiO film to enhance hole transfer in an inverted planar perovskite solar cell. The homojunction was generated by fabricating a NiO/Cu:NiO bilayer film. The density functional theory calculation demonstrated the charge density difference in the two layers, which could generate a space charge region and a band bending at the junction, and the result was further proved by energy level structure analysis of NiO and Cu:NiO films. The designed homojunction could accelerate the hole transfer and inhibit carrier recombination at the interface between hole transfer layer and perovskite layer. Finally, the inverted planar perovskite solar cell with p-p+ homojunction showed an efficiency of 18.30 % and a high fill factor of 0.81, which were much higher than the counterpart of the PSCs individually using NiO or Cu:NiO as hole transfer layer. This work developed a new structure of hole transport layer to enhance the performance of PSCs, and also provided new ideas for design of charge transfer films.

Low temperature processed inverted planar perovskite solar cells by r-GO/CuSCN hole-transport bilayer with improved stability

Low temperature processed Perovskite solar cells (PSCs) are popular due to their potential for scalable production. In this work, we report reduced Graphene Oxide (r-GO)/copper (I) thiocyanate (CuSCN) as an efficient bilayer hole transport layer (HTL) for low temperature processed inverted planar PSCs. We have systematically optimized the thickness of CuSCN interlayer at the r-GO/MAPbI3 interface resulting in bilayer HTL structure to enhance the stability and photovoltaic performance of low temperature processed r-GO HTL based PSCs with a standard surface area of 1.02 cm2. With matched valence band energy level, the r-GO/CuSCN bilayer HTL based PSCs showed high power conversion efficiency of 14.28%, thanks to the improved open circuit voltage (VOC) compared to the only r-GO based PSC. Moreover, enhanced stability has been observed for the r-GO/CuSCN based PSCs which retained over 90% of its initial efficiency after 100 h light soaking measured under continuous AM 1.5 sun illumination.

Copper iodide-PEDOT:PSS double hole transport layers for improved efficiency and stability in perovskite solar cells

The hole transport layer (HTL) plays an important role in perovskite solar cells (PSCs). We demonstrate the cupper iodide (CuI)/poly (3,4-ethylenedioxythiophene) poly (styrenesulphonate) (PEDOT:PSS) double layers as the anode buffer layer for efficient hole transport in PSCs. An obvious enhancement of open-circuit voltage and power conversion efficiency (PCE) is observed, and a final PCE of 14.3% is achieved. While the single PEDOT:PSS based device’s PCE is 12.9%. The inorganic CuI buffer layer significantly enhanced the hole extraction from the perovskite film, as revealed from the photoluminescence spectra. The average exciton lifetime is reduced to 2.7 ns. Moreover, the devices with double HTLs structure exhibit better long time stability. The PCE remains 88% of the initial value after 720 h’ storage.

Enhancement of external quantum efficiency and reduction of roll-off in blue phosphorescent organic light emitt diodes using TCTA inter-layer

The improved external quantum efficiency (EQE) and reduced roll-off properties of blue phosphorescent organic light-emitting diodes (PHOLEDs), were fabricated with structure, ITO/NPB (400Å)/TCTA (200Å)/mCP:FIrpic (7%)(300Å)/TPBi (300Å)/Liq (20Å)/Al (800Å) by incorporating an 4,4′,4′′-tris(carbazol-9-yl)-triphenylamine (TCTA) interlayer. We compared the properties of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) as the electron transport layer (ETL) with a typical structure of hole transport layer (HTL)/emissive layer (EML)/ETL in OLEDs and utilized inter-layer in the optimized structure to enhance EQE to 52% at 5.5V, also stabilize the roll-off of 23%. The use of inter-layer in blue PHOLEDs exhibits a current efficiency of 10.04cd/A, an EQE of 6.20% at 5.5V and the highest luminance of 10310cd/m2 at 9.5V. We have identified the properties of electroluminescence through the inter-layer in blue PHOLEDs which can be divided into singlet excitons and triplet excitons which emit fluorescence of N,N′-bis(1-naphthalenyl)-N,N′-bis-phenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) at 420nm and phosphorescence of Iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′] picolinate(FIrpic) at 470nm, 494nm, respectively.

Performance Improvement of Flexible Organic Light-Emitting Diodes with Double Hole Transport Layers by Spin-Coating and Evaporation

In this study N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPB) was dissolved in tetrahydrofuran (THF) solvent, and the resulting solution was spin-coated onto an indium tin oxide (ITO)-coated plastic substrate to form the first hole transport layer (HTL). A second hole transport layer of NPB was then deposited by thermal evaporation on the first HTL to produce a flexible organic light-emitting diode (OLED) with a double HTL structure. The luminance efficiency of a device with a double HTL structure is thereby improved. Utilizing spin-coated (NPB+THF)/evaporated NPB as the double HTL structure, the efficiency can reach 3.86 cd/A, and the lifetime is extended by 14.4% compared with that of the conventional single thermally evaporated HTL.