Polymeric Hole Transport Layer Research Articles

Interface engineering with polymer hole transport layer for 3D perovskite solar cells

Interface engineering with polymer hole transport layer for 3D perovskite solar cells

Green Solvent Selection for All Solution-Processed Inverted Quantum Dot Light Emitting Diode.

Quantum-dot light-emitting diodes (QD-LEDs) have gained attention as potential display technologies. However, the solvents used to dissolve a polymeric hole transport layer (HTL) are hazardous to both humans and the environment. Additionally, intermixing the HTL and QD layers presents a significant challenge when fabricating inverted QD-LEDs. Here, a green solvent selection procedure to achieve good device performance and environmental safety in QD-LEDs is established. This procedure utilizes Hansen solubility parameters and surface roughness to identify a set of solvents that do not lower the device performance by avoiding interlayer mixing or a rough interface. The CHEM21 solvent selection guide is used to screen for environmentally hazardous solvents. Finally, cyclopentanone (CPO) is selected as the optimal HTL solvent from among 16 candidates. Using CPO improves the maximum luminescence by ≈1.6 times and the maximum current efficiency by ≈12.6 times, compared to that of conventional devices using hazardous chlorobenzene. Solvent selection is critical for the fabrication of green and high-performance inverted QD-LEDs, particularly for large display panels that require n-type oxide thin-film transistors.

69‐3: Late‐News Paper: Enhanced Performance of Quantum Dot Light Emitting Diode via Optimizing PVK and Poly‐TPD Mixture Ratio for Hole Transport Layer

For high‐performance quantum‐dot light‐emitting‐diode (QD‐LED), it is imperative to resolve the charge imbalance. In this paper, the solution to resolve the charge imbalance is introduced to mix two polymer hole transport layer (HTL) materials.

Inverted Solution-Processed Quantum Dot Light-Emitting Devices with Wide Band Gap Quantum Dot Interlayers.

Despite its benefits for facilitating device fabrication, utilization of a polymeric hole transport layer (HTL) in inverted quantum dots (QDs) light-emitting devices (IQLEDs) often leads to poor device performance. In this work, we find that the poor performance arises primarily from electron leakage, inefficient charge injection, and significant exciton quenching at the HTL interface in the inverted architecture and not due to solvent damage effects as is widely believed. We also find that using a layer of wider band gap QDs as an interlayer (IL) in between the HTL and the main QDs' emission material layer (EML) can facilitate hole injection, suppress electron leakage, and reduce exciton quenching, effectively mitigating the poor interface effects and resulting in high electroluminescence performance. Using an IL in IQLEDs with a solution-processed poly(9,9-dioctylfluorene-alt-N-(4-sec-butylphenyl)-diphenylamine) (TFB), HTL improves the efficiency by 2.85× (from 3 to 8.56%) and prolongs the lifetime by 9.4× (from 1266 to 11,950 h at 100 cd/m2), which, to the best of our knowledge, is the longest lifetime for an R-IQLED with a solution-coated HTL. Measurements on single-carrier devices reveal that while electron injection becomes easier as the band gap of the QDs decreases, hole injection surprisingly becomes more difficult, indicating that EMLs of QLEDs are more electron-rich in the case of red devices and more hole-rich in the case of blue devices. Ultraviolet photoelectron spectroscopy measurements verify that blue QDs have a shallower valence band energy than their red counterparts, corroborating these conclusions. The findings in this work, therefore, provide not only a simple approach for achieving high performance in IQLEDs with solution-coated HTLs but also novel insights into charge injection and its dependence on QDs' band gap as well as into different HTL interface properties of the inverted versus upright architecture.

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.

Numerical Simulation of Nitrogen-Doped Titanium Dioxide as an Inorganic Hole Transport Layer in Mixed Halide Perovskite Structures Using SCAPS 1-D

Perovskite solar cells (PSCs) stand out as superior third-generation (III-gen) thin-film energy harvesting structures with high efficiency, optical properties and light transmission ability. However, the need to develop cost-effective, stable and sustainable PSCs is allied to the influence of the absorber layer and charge selective transport layers when achieving semi-transparent (ST) structures. Using SCAPS simulation software that can envisage the conceptuality in devising ST PSCs, this work explores and reports the electrical performance of different methylammonium (MA)-based perovskite structures (FTO/TiO2/PCBM/SnO2/MAPbI3/TiO2:N/PTAA/Spiro-OMeTAD/PEDOT: PSS/Ag). The influence of absorber thickness and defect density is analyzed with optimal parameters. This research reports a novel idea that replaces the polymeric hole transport layer (HTL), such as Spiro-OMeTAD, PEDOT: PSS and PTAA with an air-stable inorganic metal oxide, viz., nitrogen-doped titanium dioxide (TiO2:N). The simulation results depict an attainable power conversion efficiency of 9.92%, 10.11% and 11.54% for the proposed structures with the novel HTL that are on par with polymeric HTLs. Furthermore, the maximum allowable absorber thickness was 600 nm with a threshold defect density of 1 × 1015 cm−3. The optimized electrical parameters can be implemented to develop thin-film light transmission perovskite cells with rational power conversion efficiencies.

Improving efficiency of polymer hole transport layer based perovskite solar cells via interfacial modification

In triple cation perovskite solar cells (PSCs), polymer hole transport layers (HTLs) based cells usually obtained lower power conversion efficiency (PCE) than 2,2′,7,7′-tetrakis (N, N-di-p-methoxyphenylamine)− 9,9′-spirobifluorene (spiro-OMeTAD) HTLs based cells. In this work, perylene with excellent stability was used to modify the interface between perovskite and HTLs of poly(3-hexylthiophene) (P3HT). Our study proved that the insertion of perylene smoothed surface morphology of perovskite, decreased the trap density, assisted the charge extraction, and polished the interfacial contact. Improved photovoltaic performance and prolonged long-term stability were observed in perylene based cells. A champion PCE of 15.87 % was obtained in Perylene(3000) based cell, which was > 14 % higher than the PCE of 13.71 % obtained in reference cell. Meanwhile, the PSCs with perylene displayed better reproducibility than the reference cell. This study showed the potential of perylene like materials in efficient and stabile PSCs.

Nitrogen-Doped Titanium Dioxide as a Hole Transport Layer for High-Efficiency Formamidinium Perovskite Solar Cells.

Perovskite solar cells (PSCs) offer advantages over widely deployed silicon solar cells in terms of ease of fabrication; however, the device is still under rigorous materials optimization for cell performance, stability, and cost. In this work, we explore a version of a PSC by replacing the polymeric hole transport layer (HTL) such as Spiro-OMeTAD, P3HT, and PEDOT: PSS with a more air-stable metal oxide, viz., nitrogen-doped titanium dioxide (TiO2:N). Numerical simulations on formamidinium (FA)-based PSCs in the FTO/TiO2/FAPbI3/Ag configuration have been carried out to depict the behaviour of the HTL as well as the effect of absorber layer thickness (∆t) on photovoltaic parameters. The results show that the cell output increases when the HTL bandgap increases from 2.5 to 3.0 eV. By optimizing the absorber layer thickness and the gradient in defect density (Nt), the device structure considered here can deliver a maximum power conversion efficiency of ~21.38% for a lower HTL bandgap (~2.5 eV) and ~26.99% for a higher HTL bandgap of ~3.0 eV. The results are validated by reproducing the performance of PSCs employing commonly used polymeric HTLs, viz. Spiro-OMeTAD, P3HT, and PEDOT: PSS as well as high power conversion efficiency in the highly crystalline perovskite layer. Therefore, the present study provides high-performing, cost-effective PSCs using TiO2:N.

Linear cross-linkers enabling photothermally cured hole transport layer for high-performance quantum dots light-emitting diodes with ultralow efficiency roll-off

In quantum-dot light-emitting diodes (QLEDs), poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt(4,4′-(N-(4-butylphenyl))] (TFB) has been one of the most widely used hole-transport layer (HTL) materials. However, for solution-processed especially ink-jet printed QLEDs, TFB normally suffers from low glass-transition temperature and inter-layer erosions hindering to achieve both high device efficiency and stability. In the present work, three novel linear bis-benzophenone cross-linkers (BPO4, BPO6 and BPO8) were carefully designed and synthesized for constructing compact HTL with ∼ 100 % solvent resistance. Under a mild photothermal curing condition of 120 °C and 365 nm UV with a power of 16 mW cm−2, the introduction of these cross-linkers into TFB film, even at a low weight concentration of 3.33 %, not only resulted in the HTL with reduced free film volumes and hence compact three-dimensional networks in QLEDs, but also enhanced the hole transport ability, so that better charge balance in QDs layer and improved overall QLED performance could be achieved. In the BPO series which have varying alky lengths, the BPO6 with a medium central alkyl length showed the best thin film state, and thus enabled to improve the QLED device performance with the highest EQE of 16.77 % (20.22 lm W−1, 22.72 cd A-1), which is 20% higher than that of pristine TFB based device. From the peak EQE to the EQE at 20000 cd m−2, an ultra-low deviation of 1.73 % was observed in the TFB:BPO6 based red QLEDs. The T50 lifetime of TFB:BPO6 based QLEDs is almost twice of that of the pristine TFB based device. Our strategy demonstrates that developing linear cross-linkers for polymer HTL can facilitate forming compact and dense HTL, achieving simultaneous improvements in QLEDs device efficiency and stability.

Showerhead-assisted chemical vapor deposition of CsPbBr3 films for LED applications

CsPbBr3 represents a highly attractive material for perovskite light-emitting diodes (PeLEDs) in the green spectral range. However, the lack of deposition tools for reproducible and scalable growth of perovskite films is one of the major obstacles hindering PeLED commercialization. Here, we employ the highly scalable showerhead-assisted chemical vapor deposition (CVD) method to produce uniform pinhole-free CsPbBr3 films for PeLED application. The precursors CsBr and PbBr2 are evaporated under low vacuum in N2 carrier gas. By adjusting the PbBr2 sublimation temperature, process conditions for CsBr-rich, stoichiometric, and PbBr2-rich CsPbBr3 layer growth have been developed. A substrate temperature of 160 °C enables direct growth of these CsPbBr3 films on a polymeric hole transport layer (HTL), finally yielding PeLEDs with a maximum luminance of 125 cd/m2. Although the device efficiency still lags behind solution-processed counterparts, our approach presents the first demonstration of PeLEDs containing CsPbBr3 films processed in a perovskite showerhead-assisted CVD reactor.Graphic abstract

Highly efficient and stable carbon-based perovskite solar cells with the polymer hole transport layer

Carbon-based perovskite solar cells are competitive candidate for photovoltaic applications in future due to their high-performance, low-cost and good stability. However, the hole transport materials are limited and their properties are not satisfying in carbon-based perovskite solar cells. Here, we report a typical p-type P3HT as hole transport layer for all low-temperature processed carbon-based perovskite solar cells with ZnO electron transport layer. Because P3HT has energy level matching with perovskite film and can facilitate the hole collection process, the power conversion efficiency of device can reach 16.05% that far higher than devices without P3HT hole transport layer. More importantly, due to good hydrophobicity and encapsulation function of P3HT, the stability of devices can also be significantly improved. The power conversion efficiency of device is able to retain over 90% of their initial value over 1200 h upon ambient air exposure without encapsulation. This present study provides a simple strategy for fabricating low-cost, highly efficient and stable perovskite solar cells.

Molecular weight effect of poly-TPD hole-transporting layer on the performance of inverted perovskite solar cells

Inverted planar perovskite solar cells (PSCs) with poly(N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine) (PTPD) layer as hole-transporting layer (HTL) exhibit high power conversion efficiencies (PCEs) over 20%. However, the influence of PTPD’s molecular weight (Mw) on the performance of PSCs has not been reported yet. Herein, we employ PTPD with various molecular weights as HTL to fabricate highly efficient inverted planar PSCs. Our results show that PTPD with 75 kDa possesses superior conductivity, which can quicken the charge carrier extraction from perovskite to HTL. In addition, the MAPbI3 perovskite film atop the 75-kDa PTPD is smoother with higher crystallinity and lower defect density. As a result, inverted planar PSCs with 75-kDa PTPD as HTL deliver a highest PCE of 19.24%. Our work provides a beneficial reference for future preparation of stable and highly efficient inverted PSCs with polymer HTL.

Photocross-Linkable Hole Transport Materials for Inkjet-Printed High-Efficient Quantum Dot Light-Emitting Diodes.

Efficient approach based on the photochemistry of benzophenone has been developed for the cross-linking of the polymer hole-transporting layer (HTL). The cross-linked poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl) (TFB) thin films showed high solvent stability, smooth surface morphology, and improved charge-carrier mobility. The solution-processed red, green, and blue (RGB) quantum dot light-emitting diodes (QLEDs) based on the cross-linked HTLs showed much better performances than the corresponding devices based on the pristine TFB HTLs. The spin-coated red QLEDs based on the cross-linked HTLs showed the maximum current efficiency (CE), the maximum power efficiency (PE), and the peak external quantum efficiency (EQE) of 32.3 cd A-1, 42.3 lm W-1, and 21.4%, respectively. The inkjet-printed red QLEDs with the cross-linked HTLs exhibited the CE, PE, and EQE of 26.5 cd A-1, 37.8 lm W-1, and 18.1%, respectively. The high-performance HTLs were obtained by significantly reducing the amount of cross-linking agents.

Interfacial engineering and down-conversion of ultraviolet light for efficient perovskite solar cells

The addition of a tiny amount of fluorescent organic material N, N′-di (naphtha-1-yl)-N, N′-diphenyl-benzidine (NPB) into polymeric hole transport layer (HTL) is shown to yield a significant enhancement in down-conversion of ultraviolet (UV) light into visible light. As a result, NPB incorporated HTL based devices exhibit enhancement in power conversion efficiency (PCE) of 18.2% compared to 15.1% for pristine devices. This work demonstrated that fluorescent down-conversion of UV light based on doped HTLs is a novel and effective approach for enhanced performance of perovskite solar cells (PSCs). In addition, the incorporated fluorescent material NPB provides a favorable energetic alignment at the interface for low voltage loss. These results are further supported by time-resolved photoluminescence (TRPL) measurements that are performed on perovskite layers deposited on top of NPB doped Poly (triaryl amine) (PTAA), which shows an improvement in lifetime of the charge carriers with increasing concentration of NPB. An increase in the lifetime of the charge carriers is observed on the perovskite layer that is deposited on top of 2.5 wt% NPB doped PTAA layer. The lifetime enhancement is due to additional charge carriers that are generated from the conversion of high energy photons to low energy photons. These additional charge carriers that are generated in HTL occupy the charge transport states after filling the trap states in the perovskite layer and hence the lifetime of the charge carriers increases. The TRPL lifetime decreases with further increase in the NPB concentration more than 2.5 wt% due to efficient charge carrier extraction.

Efficiency Improvement of Planar Inverted Perovskite Solar Cells by Introducing Poly 9,9‐Dioctyfluorene‐co‐benzothiazole into Polytriarylamine as Mixed Hole‐Transport Layer

The hole‐transport layer (HTL) is a crucial component for influencing the crystallization of a perovskite layer, which has a close relationship with the photovoltaic efficiency and stability of perovskite solar cells (PVSCs). Herein, planar inverted PVSCs using polytriarylamine (PTAA) HTL mixed with a polymeric material of poly 9,9‐dioctyfluorene‐co‐benzothiazole (F8BT) are fabricated, and the effect of mixed polymer HTL on the device performance is investigated. After variation in the F8BT ratio in the mixed HTL, average power conversion efficiency (PCE) of 14.88% with negligible hysteresis is achieved and the champion device exhibits a PCE of 15.41% due to the increased charge carrier extraction and transmission, and the optimized crystallization properties of the perovskite. Unsealed planar p‐i‐n PVSCs with mixed polymer HTL show a 28.8% increase in average PCE (14.88% vs 11.55%) and over 30% enhancement in stability at ambient conditions for 2 weeks with respect to control due to the improvement in the crystallinity of the perovskite layer and conductivity of the polymer layer. This work provides an effective strategy for the development of highly efficient planar PVSC fabricated on mixed polymer HTL.

Molecular engineering of a conjugated polymer as a hole transporting layer for versatile p–i–n perovskite solar cells

Along with the development of perovskite materials, which have enormous potential for optoelectronics such as solar cells and light-emitting diode devices, numerous organic semiconductor polymers, which have been critically adopted into the hole and electron transporting layers, have been synthesized and studied. In n–i–p-structured perovskite solar cells, various outstanding polymer materials have been successfully applied. However, in p–i–n-structured solar cells, the hydrophobic nature of the polymers makes the sequential deposition of a perovskite thin film difficult. Several destructive methods have been proposed; however, a more efficacious and fundamental method is urgently needed. Here, we present a nondestructive polymer hole-transporting layer (HTL) thin-film formation process based on molecular engineering via a simple solvent process. When we used various solvents with different volatilities, perovskite film formation was achieved on polymer thin films formed from highly volatile solvents. In addition, we elucidated the structure and orientation of the molecules in the films and revealed that the molecular structure of face-on orientation for the horizontally aligned hydrophobic alkyl groups induced a lower surface energy of the film, as determined by grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. Furthermore, the tilt angle of the molecules, which was calculated from the results of quantitative near-edge X-ray absorption fine structure (NEXAFS) analysis, was found to correlate with the surface energy. This result provides guidance for polymer-orientation and surface-energy studies, and perovskite solar cells fabricated using the polymer HTL demonstrated good durability and flexibility. We expect that our approach represents a new route for fabricating p–i–n-structured solar cells and that numerous valuable p-type conjugated polymers will be developed via our proposed molecular engineering process.

Overcoming the Electroluminescence Efficiency Limitations in Quantum‐Dot Light‐Emitting Diodes

AbstractDevelopment of quantum dots (QDs) based light‐emitting diodes (QLEDs) is driven by attractive properties of these fluorophores such as precise Gaussian distribution, tunable emission, and facile solution processability. The performance of QLED devices is limited by intrinsic factors such as luminance quenching in quantum dots due to imbalanced carrier injection predominantly caused by a large hole injection barrier as well as by extrinsic processes such as nonradiative recombination at active layer interfaces. The Auger recombination problem is overcome by charge siphoning at the interfaces between QDs and charge‐transporting material. A simplest trilayer (p–i–n) LED structure is fabricated using an all‐solution processing method: a carefully engineered p‐type polymeric hole transport layer with a gradient work function is incorporated. The gradient work function creates the cascading energy levels from the moderate Fermi level anode to the deep‐lying valence band level of QDs. As a result, the QLEDs exhibit significantly improved external quantum efficiencies and luminous efficiencies of 15.9% and 31.8 cd A−1, 17.4% and 59.3 cd A−1, and 12.8% and 14.4 cd A−1 for red, green, and blue light‐emitting devices, respectively. It is expected that the concept demonstrated here will facilitate the design and development of efficient solution‐processible QLEDs for full‐color displays.

High-Efficiency Light-Emitting Diodes Based on Formamidinium Lead Bromide Nanocrystals and Solution Processed Transport Layers

Perovskite nanocrystal light-emitting diodes (LEDs) employing architecture comprising a ZnO nanoparticles electron-transport layer and a conjugated polymer hole-transport layer have been fabricated. The obtained LEDs demonstrate a maximum external-quantum-efficiency of 6.04%, luminance of 12998 Cd/m2 and stable electroluminescence at 519 nm. Importantly, such high efficiency and brightness have been achieved by employing solution processed transport layers, formamidinium lead bromide nanocrystals (CH(NH2)2PbBr3 NCs) synthesized at room-temperature and in air without the use of a Schlenk line, and a procedure based on atomic layer deposition to insolubilize the NC film. The obtained NCs show a photoluminescence quantum yield of 90% that is retained upon film fabrica-tion. The results show that perovskite NC LEDs can achieve high-performance without the use of transport layers deposited through evapo-ration in ultra-high-vacuum.

Polymer Assisted Small Molecule Hole Transport Layers Toward Highly Efficient Inverted Perovskite Solar Cells

In this paper, inverted perovskite solar cells (PSCs) employing a novel polymer‐assisted small molecule layer as hole transport layer (HTL) are reported and the effect of mixed HTL on the device performance is investigated. It is the first time that the small molecule HTL is doped with a polymer HTL. The introduction of appropriate content of polymer into the small molecule layer will lead to a much smoother surface for the mixed HTL and largely reduced charge recombination, and most importantly, the energy level alignment is more matched with that of the perovskite via optimization of the doping content. Therefore, the hole transfer property is largely improved for the perovskite/mixed HTL composites. After the optimization of the polymer content in the mixed HTLs, an average power conversion efficiency (PCE) of 19.03 ± 0.53% is achieved, and the champion device exhibits a PCE of >21%. This work provides an effective strategy for the development of highly efficient inverted PSCs based on small molecule HTLs.

High-efficiency solution-processed CdTe nanocrystal solar cells incorporating a novel crosslinkable conjugated polymer as the hole transport layer

Solution-processed nanocrystal (NC) solar cell are attracting much attention for prospective application in low cost energy source; however, the worse collection of hole carriers at the hole interface limits the device performance further improvement. Here, a novel crosslinkable conjugated polymer poly(diphenylsilane-co-4-vinyl-triphenylamine) (Si-TPA) has been applied as the hole transport layer (HTL) for solution-processed CdTe nanocrystal (NC) solar cells. To the best of our knowledge, it is the first report with a crosslinkable conjugated polymer as the HTL in NC solar cells. It is found that the devices with Si-TPA HTL show much better performance than that of the devices without the HTL or with other polymer HTLs such as poly(9-vinylcarbazole) (PVK) and poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). The improvement with incorporating Si-TPA is attributed to the better energy level matching between the NC film and the Au anode with a good ohmic contact among each interface. Interestingly, it is also observed that introducing Si-TPA can produce a higher open-circuit voltage compared to that without Si-TPA, originating from the interface dipole effect produced by Si-TPA. The improved fill factor value illustrates that the hole and electron carriers are more balanced in the devices with Si-TPA. The champion solar cell with a configuration of ITO/ZnO/CdSe/CdTe/Si-TPA/Au shows a PCE of 8.34%, which is a record for solution-processed CdTe NC solar cells to date with an inverted device structure. Furthermore, for another type of the solution-processed CdS NC device, a high PCE of 7.27% is also obtained with incorporating crosslinkable Si-TPA HTL, which is two times higher than that without Si-TPA, demonstrating the remarkable universality through this method. This work provides a new facile strategy in further increasing solution-processed NC solar cells performance by incorporating organic crosslinkable conjugated polymers as the HTL.