Recent Advances in Hole Transport Layer Engineering for High-Performance Quantum Dot Light-Emitting Diodes

Abstract

Owing to their superior color purity and solution-processability, quantum dot light-emitting diodes (QLEDs) have garnered significant attention as promising candidates for next-generation displays. However, overcoming efficiency degradation and limited operational lifetime, which stem from charge transport imbalance, remains a critical challenge. Herein, we systematically discuss the role of the hole transport layer (HTL) in mitigating this imbalance. We classify HTL engineering strategies into four key categories: energy-level alignment, hole mobility and carrier concentration control, improvement in solvent stability, and multilayer architectures. Furthermore, we discuss hybrid HTL architectures that integrate multiple strategies to achieve synergistic improvement. Ultimately, this review highlights the pivotal role of HTL engineering in realizing high-performance QLEDs and provides insightful perspectives on future research directions.

1. Introduction

Quantum dot light-emitting diodes (QLEDs) have emerged as promising candidates for next-generation displays, owing to their narrow emission bandwidth, tunable emission spectra, high color purity, and solution-processability. Although significant progress has been achieved in enhancing the external quantum efficiency (EQE) of QLEDs in recent years, fundamental challenges—such as efficiency loss, limited operational lifetime, and poor driving stability—persist. These issues stem primarily from the intrinsic structural characteristics of QLEDs, most notably the charge transport asymmetry between the electron transport layer (ETL) and the hole transport layer (HTL), which remains a major obstacle to the commercialization [1,2,3,4].

Most QLEDs employ ETLs characterized by high electron mobility and excellent electron injection properties. While these ETLs facilitate robust electron transport, they often lead to excessive electron injection relative to hole injection. In contrast, hole transport remains relatively suppressed due to the limited mobility and injection capability of the hole injection layer (HIL) and HTL, placing QLEDs in an intrinsically electron-rich operating regime. This imbalance extends beyond simple current asymmetry; it induces localized charge accumulation within the quantum dot emissive layer (QD EML), thereby activating non-radiative recombination pathways. In particular, the charging of quantum dots (QDs) significantly enhances Auger recombination, simultaneously accelerating efficiency roll-off and device degradation. As a result, charge imbalance is directly linked to both reduced efficiency and shortened operational lifetime [4,5,6,7,8,9].

Recent studies demonstrate that optimizing the hole injection and transport architecture (HIL/HTL) can reduce turn-on voltage while enhancing current efficiency, EQE, and operational stability. These findings indicate that the hole transport architecture plays a crucial role in determining overall device performance, identifying HTL engineering as one of the most direct and effective strategies for mitigating charge imbalance [5,6,7,8,9].

Accordingly, this review provides a comprehensive overview of the role of the HTL in mitigating charge imbalance in QLEDs. We systematically categorize HTL engineering strategies into four key domains: solvent and processing stability; energy-level alignment; hole mobility and carrier concentration control; and multilayer HTL architectures. For each category, we discuss representative material systems and analyze the underlying physical mechanisms governing charge transport and injection. Through this framework, we systematically explore the correlations between HTL design strategies and device efficiency and stability. Finally, we conclude by discussing remaining challenges and offering perspectives on future research directions for high-performance QLEDs.

2. Fundamental Challenges of Hole Transport Layers in QLEDs

Typically, QLEDs feature a multilayer architecture comprising a substrate, anode, HIL, HTL, QD EML, ETL, and cathode. Under bias, electrons and holes are injected from the cathode and anode, respectively, and transport to the QD EML, where they recombine to generate light (Figure 1a). However, QLEDs inherently suffer from charge transport imbalance arising from the distinct transport characteristics of electrons and holes, which critically determine efficiency and operational stability (Figure 1b) [1,2,7,10].

A significant challenge lies in the energy-level mismatch between the highest occupied molecular orbital (HOMO) of the HTL/HIL and the valence band of the QD EML, which creates a substantial hole injection barrier (Figure 1c) [3,5,6,11]. This barrier necessitates a higher driving voltage to achieve effective hole injection, thereby increasing the turn-on voltage [1,5,11]. As a result, the elevated internal electric field induces thermal and electrical stress within the device, accelerating degradation and shortening the operational lifetime [4,5,10].

Furthermore, the disparity in carrier mobility, where electron mobility in the ETL typically far exceeds hole mobility in the HTL, exacerbates this imbalance [1,7,10]. This transport asymmetry leads to localized electron accumulation, which significantly reduces external quantum efficiency (EQE) and aggravates efficiency roll-off at the high-luminance regime [4,7,12].

Another critical issue arises from the solution-based fabrication process. Since HTLs are predominantly formed via solution-based processes, they often lack sufficient resistance to aromatic solvents (e.g., toluene and octane) used for depositing the subsequent QD layer. This solvent vulnerability can lead to interfacial erosion, increased surface roughness, and localized electric-field concentration, which in turn result in increased leakage current and degraded device reliability [6,8,10]. Therefore, optimizing energy-level alignment, charge transport properties, and interfacial solvent stability is paramount for enhancing the performance and lifetime of QLEDs [1,5,10].

3. HOMO Level Alignment at the Interface Between HTL and QD

One of the primary factors limiting device efficiency and operational stability in QLEDs is the hole injection barrier at the HTL/QD interface. Typically, the valence band of CdSe-based quantum dots, which are widely used, lies in the range of −6.0 to −7.0 eV. In contrast, representative polymeric HTLs, such as (poly(N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine)) (poly-TPD) and poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB), exhibit HOMO levels at approximately −5.2 eV and −5.3 eV, respectively. This mismatch in energy offset forms a hole injection barrier on the order of 1 eV, resulting in suppressed hole injection relative to electron injection [13,14,15,16,17].

Under such charge-imbalanced conditions, excess electrons accumulate within the QD EML, leading to the formation of charged QDs and the activation of Auger non-radiative recombination [15,17]. These loss mechanisms not only accelerate efficiency roll-off at high current densities but also induce localized heating and interfacial degradation, directly shortening the operational lifetime. Therefore, alleviating this injection bottleneck via HOMO-level engineering is a fundamental requirement for high-performance QLEDs [16].

To address these issues, Chen et al. introduced an organic–inorganic hybrid HTL by incorporating solution-processed WOx nanoparticles into poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), forming a WOx:PEDOT:PSS mixed hole injection and hole transport layer. From an energy-level perspective, the use of WOx leads to more favorable interfacial conditions for hole injection at the PEDOT:PSS (HOMO ≈ −5.20 eV) and TFB (HOMO ≈ −5.40 eV) interfaces. The deep-lying energy level of WOx at approximately −6.5 eV induces an interfacial dipole, which effectively reduces the hole injection barrier (Figure 2b).

Hole-only devices based on TFB/PEDOT:PSS:WOx exhibited increased current density at high voltages, confirming improved hole injection and transport capability in the HTL. As a result, QLEDs employing the optimized WOx:PEDOT:PSS HTL demonstrated significantly enhanced device performance, with the maximum current efficiency increasing from 9.8 to 13.1 cd A⁻¹ and the maximum luminance improving from 46,000 to 62,000 cd m⁻². In addition, the operational lifetime was nearly doubled, reaching an extrapolated T₅₀ lifetime of approximately 7071 h at an initial luminance of 100 cd m⁻², compared to 3162 h for conventional PEDOT:PSS-based HTL devices (Figure 2c,d). These results demonstrate that improved hole injection through hybrid inorganic–organic HTL engineering can simultaneously alleviate charge imbalance and enhance device efficiency and operational stability [13].

In a parallel approach, Yang et al. employed a p-type doping-based HTL by introducing B(C₆F₅)₃—a strong Lewis acid—into poly-TPD, which deepened the HOMO level from −5.2 eV to −5.6 eV, thereby enabling optimized HOMO alignment at the HTL/QD interface (Figure 2e). Owing to this optimized alignment at an optimal doping concentration of 8%, the maximum luminance, current efficiency, and external quantum efficiency (EQE) of the QLEDs were improved by approximately 26.1%, 35.4%, and 26.4%, respectively, while the turn-on voltage was reduced from 2.6 V to 2.3 V (Figure 2f–i). Furthermore, as the charge injection imbalance was mitigated, the operational stability of the devices was enhanced, resulting in a delayed onset of efficiency degradation [14].

4. HTL with Enhanced Hole Mobility

While energy-level alignment addresses the injection barrier, the intrinsically low hole mobility of organic HTLs remains a fundamental bottleneck limiting QLED performance. Therefore, strategies that accelerate hole transport kinetics to match the robust electron transport are crucial for balancing charge flux and maximizing device efficiency [18,19,20,21].

Pan et al. proposed a strategy to enhance hole mobility by creating a composite HTL, blending high-mobility small molecules—di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC, hole mobility ~10⁻² cm² V⁻¹ s⁻¹)—into a PVK (hole mobility, ~10⁻⁶ cm² V⁻¹ s⁻¹) host (Figure 3a). By optimizing the PVK:TAPC volume ratio to 3:1, they significantly enhanced the hole transporting capability of the layer while maintaining favorable energy alignment (Figure 3b). Hole-only device (HOD) analysis revealed that the composite HTL achieved exhibited hole current densities comparable to those of electron-only devices, exhibited a low turn-on voltage of 3.3 V and approximately 60% enhancement in operational stability (Figure 3c–f) [18].

Moving beyond physical blending, Cai et al. reported a mobility enhancement strategy based on molecular modification of the polymer. They introduced an angular-shaped heteroarene based on cyclopentane[b]thiopyran (C8–SS) into PVK to form a PVK:C8–SS HTL (Figure 3g). In this system, mobility increase was driven not by increased carrier concentration (as in p-type doping) but by improved charge transport arising from strong π···π and S···π intermolecular interactions. Space-charge-limited current (SCLC) analysis showed that the hole mobility increased by two orders of magnitude, from 2.44 × 10⁻⁶ cm² V⁻¹ s⁻¹ (pristine PVK) to 1.73 × 10⁻⁴ cm² V⁻¹ s⁻¹ (PVK:C8–SS). This high hole mobility facilitated efficient injection into QDs, improving charge balance (Figure 3h). As a result, the PVK:C8–SS-based QLED exhibited a low turn-on voltage of 3.2 V and achieved a peak EQE of 19.02% and a peak power efficiency of 7.31 lm W⁻¹, which were enhanced by 78% and 165%, respectively, compared with the control PVK-based device (Figure 3i). The enhancement was attributed to accelerated hole transport and suppressed Auger recombination in the QD EML [19].

Crucially, this mobility-centered HTL design strategy is conceptually distinct from conventional p-type doping approaches that rely on increasing hole concentration. This strategy directly mitigates the fundamental issue of electron–hole transport imbalance by accelerating hole transport kinetics while avoiding issues associated with heavy doping, such as trap formation, energetic disorder, and interfacial instability [20,22]. Enhanced hole mobility facilitates hole transfer from the HTL to the EML, thereby reducing the injection mismatch and effectively suppressing electron accumulation within the QD EML. The suppression of charge accumulation reduces non-radiative loss pathways, including Auger recombination, which directly translates into mitigated efficiency roll-off and improved operational stability [21,22]. Accordingly, mobility-centered HTL design has emerged as a core design paradigm for overcoming the intrinsic performance limitations of QLEDs [18,19,20,21,22].

5. Solvent Stability Engineering of HTL

The re-dissolution of the HTL and interfacial mixing during the formation of multilayer structures are primary causes of device degradation. Since the QD EML is typically deposited using aromatic solvents such as toluene, solvent-induced erosion and morphological degradation can occur on top of conventional polymeric HTLs. Conventional organic HTLs are susceptible to solvent-induced erosion and morphological damage; therefore, securing the solvent resistance of the HTL is a critical design requirement for ensuring the interfacial integrity and operational stability of QLEDs [23,24,25,26,27,28,29,30,31]. Accordingly, strategies to secure the solvent stability of HTLs have emerged as a crucial design element for improving the efficiency and operational stability of QLEDs.

Han et al. mitigated this issue by optimizing the thermal annealing conditions of a high-molecular-weight TFB film (Figure 4a). They demonstrated that increasing the annealing temperature to 150 °C sufficiently densified the TFB layer, thereby suppressing dissolution and interfacial intermixing between the HTL during subsequent QD deposition. Optical microscopy and Focused Ion Beam–Scanning Electron Microscopy (FIB-SEM) analyses confirmed that the optimized TFB film maintained its thickness and surface integrity after QD spin-coating. This robust solvent resistance enabled the formation of uniform QD films without dewetting defects. As a result, QLEDs employing the optimally annealed TFB HTL achieved a maximum luminance of 47,410 cd m⁻², a current efficiency of 17.69 cd A⁻¹, and a maximum EQE of 4.36%, representing a clear enhancement compared to devices fabricated with insufficiently hardened HTLs (Figure 4b,c) [23].

Alternatively, Lin et al. developed a chemical approach using a thermally crosslinkable HTL material, molecule 4,4′-bis(N-(1-naphthyl)-N-phenylamino)biphenyl-4-yl vinyl benzene (VB-FNPD) (Figure 4d,e). Upon thermal treatment at 170 °C, VB-FNPD forms a robust three-dimensional crosslinked network that exhibits near-complete insolubility against organic solvents. QLEDs employing this crosslinked HTL exhibited a maximum luminance of 7702 cd m⁻² and a maximum EQE of 1.64%, substantially higher than reference devices using conventional poly-TPD HTLs (luminance 3426 cd m⁻² and EQE 0.42%) (Figure 4f–i). Time-Resolved Photoluminescence (TRPL) and hole-only device analyses revealed these enhancements improved interfacial quality and reduced non-radiative recombination sites [24].

Collectively, securing solvent stability, whether through thermal optimization or molecular crosslinking, yields a threefold benefit: (i) suppression of HTL/QD interfacial mixing; (ii) improved uniformity of the QD active layer; and (iii) enhanced effective charge recombination. These improvements directly translate into a higher EQE and extended operational lifetime, establishing solvent stability as a prerequisite for high-performance solution-processed QLEDs [32,33,34,35,36].

6. Double-HTL Architectures for Energy-Level Alignment

Energy-level alignment engineering can improve hole injection by lowering the interfacial energy barrier; however, this approach has limitations in addressing the intrinsically low hole mobility of HTLs and the transport asymmetry relative to the ETL [13,14]. Similarly, approaches aimed at increasing the hole mobility can accelerate hole injection, thereby improving electron–hole balance. Nevertheless, when excessive doping or mixing is applied, film non-uniformity and increased roughness often occur, leading to increased leakage current and reduced device reliability [18,19]. Finally, solvent-stability engineering can suppress solvent-induced erosion and interfacial mixing, improving charge injection balance and the EQE. Yet, since thermal treatment is typically required to achieve solvent resistance, the resulting performance becomes strongly dependent on the annealing temperature and time, which can potentially damage the pre-deposited layers [23,24].

As a structural approach to overcome these collective limitations, a double-layer HTL strategy has been proposed. The double-HTL structure aims to reduce the hole injection barrier by sequentially stacking two HTLs with different HOMO levels and hole mobilities, forming a cascading energy-level alignment within the HTL [37,38,39,40].

The effectiveness of this strategy is evident in recent studies on solution-processed QLEDs (Figure 5a,b). Luo et al. implemented a cascade HTL architecture by stacking a TFB/2,7-dioctyl [1]benzothieno [3,2-b][1]benzothiophene (C8-BTBT) layer on a TFB layer in cadmium-free blue QLEDs. This double-HTL structure created a stepwise energy alignment that significantly reduced the hole injection barrier. Consequently, the device achieved a remarkable improvement in EQE, rising from 4.84% (single TFB) to 7.23% (double HTL), attributed to suppressed electron leakage and balanced charge injection (Figure 5c,d) [37].

Validating the universality of this approach, Chen et al. (Figure 5e) applied a similar TFB/TPD double-HTL structure to a cadmium-free Cu-In-Zn-S (CIZS)-based QLED. A double-HTL structure combining polymeric TFB and small-molecule TPD was introduced, enabling simultaneous improvement in energy-level alignment and hole transport characteristics compared to single-HTL devices (Figure 5f,g). Consequently, the EQE increased from 2.58% to 3.87%, and through optimization of processing conditions, a maximum efficiency of up to 5.61% was achieved (Figure 5h–k). These findings indicate that the double-HTL strategy represents a generally applicable structural design principle, independent of specific emission colors or QD materials [38].

Overall, the double-HTL structure has emerged as a core design strategy for precise control of energy-level alignment in QLEDs, transcending a simple increase in layer number or material combination. This approach provides structural degrees of freedom that allow hole injection and charge transport requirements to be optimized separately, paving the way for the realization of high-efficiency and high-stability QLEDs [39,41,42,43,44,45,46].

7. Double-HTL Architectures with Buffer Layer

More recently, buffer or transition-layer-based double-HTL strategies have been proposed to mitigate charge-accumulation-induced degradation [47]. Unlike conventional alignment strategies, this approach focuses on structurally suppressing charge accumulation at the HTL/QD interface to prevent HTL oxidation (Figure 1 and Figure 6a).

This strategy is particularly critical for blue QLEDs. Due to the deep valence band level of the QDs, a large hole injection barrier typically forms at the HTL/QD interface, leading to excessive hole accumulation within the HTL. Such hole accumulation accelerates the electrochemical oxidation of the HTL material, serving as a primary degradation mechanism that increases driving voltage and shortens device lifetime (Figure 6b) [47]. In contrast, green and red QLEDs are less susceptible to these injection limitations due to their relatively smaller interfacial energy barriers [47].

Addressing this issue, Zhang et al. reported a buffer-layer-based architecture by inserting a thin layer of poly(p-phenylene benzobisoxazole) (PBO), a polymer material with high oxidation stability, between the TFB HTL and the QD EML. The PBO transition layer effectively suppressed hole pile-up at the interface, resulting in a reduction in the electrical stress and prevention of HTL oxidation (Figure 6c–e) [47].

The introduction of this buffer layer directly translated into significant performance enhancement. Pure blue QLEDs employing the PBO-based double-HTL structure achieved an EQE exceeding 20% and an operational lifetime (T₅₀) at 100 cd m⁻² reaching up to 41,000 h (an average value > 31,000 h) (Figure 6f–l). These results indicate that controlling interfacial accumulation and mitigating electrical stress are just as vital as energy-level alignment for prolonging device lifetime [47].

By directly controlling charge accumulation and degradation mechanisms at the HTL–QD interface, this strategy provides structural flexibility to simultaneously achieve high efficiency and extended operational lifetime. This capability is particularly critical for blue QLEDs, where achieving both remains a significant challenge [48]. Accordingly, double-HTL design strategies are evolving beyond simple energy-level alignment to encompass interfacial charge management and stability enhancement [48,49,50,51].

However, despite these district advantages, double-HTL architecture faces inherent limitations. Employment of an additional HTL or buffer layer increases process complexity, complicating thickness control and reproducibility. Furthermore, the requirement for extra materials and deposition steps inevitably leads to higher fabrication costs.

8. Hybrid HTL Architectures with Multiple Strategies

Various strategies—ranging from energy-level modulation, solvent resistance, hole mobility enhancement, and double-layer stacking—have addressed specific performance bottlenecks in HTL. However, relying on a single strategy often necessitates trade-offs, making it challenging to simultaneously optimize efficiency, luminance, and long-term stability under practical operating conditions. To overcome these limitations, hybrid HTL architecture that integrates multiple design rationales into a single cohesive framework has garnered significant attention (Figure 7a,b,g) [52,53,54,55].

Demonstrating the efficacy of this integrated approach, Ha et al. reported a hybrid HTL based on a low-temperature processed PTAA layer combined with UV–ozone surface treatment to improve hole injection and electrical stability in flexible QLEDs (Figure 7c,d). In this work, the corresponding QLED exhibited a maximum luminance of approximately 8.9 × 10⁴ cd m⁻² on a glass substrate and approximately 5.4 × 10⁴ cd m⁻² on a flexible PEN substrate, with a maximum EQE of approximately 3.5% (Figure 7e,f). These results indicate that integrating band alignment engineering, hole concentration control, and a low-temperature processed multilayer HTL can effectively enhance device efficiency and operational stability [52].

Further advancing this concept, Zhang et al. reported a hybrid HTL that combines crosslinking and blending strategies to secure solvent stability and charge transport properties. They constructed a composite HTL by blending a discotic molecule, 3,6,10,11-tetrakis(pentyloxy)triphenylene-2,7-diyl bis(2,2-dimethylpropanoate) (T5DP-2,7), with a cross-linked CBP-V host, forming robust hole-transport channels and a solvent-resistant cross-linked network (Figure 7g). With the optimized HTL, the turn-on voltage decreased from 3.91 V to 3.42 V, and this optimization significantly improved device performance by enhancing hole injection and achieving more balanced carrier transport within the device (Figure 7h–j) [53].

Hybrid HTL strategies represent a holistic evolution in device engineering, moving beyond a single material parameter optimization to simultaneously address energy level alignment, transport kinetics, interfacial stability, and process stability. Such integrated strategies are poised to play an important role in the development of next-generation QLEDs, particularly for blue-emitting devices where multiple stability and efficiency challenges intersect [52,53,54,55].

9. Conclusions

The HTL is increasingly recognized as a crucial layer that governs not only hole injection and charge transport balance but also device operational stability. Due to the intrinsically electron-dominant nature of QLEDs and their vulnerability to interfacial degradation, performance limitations originating from the hole transport characteristics remain a significant challenge. Recent research suggests that the role of the HTL extends beyond simple energy-level alignment to include the regulation of charge transport, control of interfacial charge accumulation, and securing of process-related and chemical stability. This indicates that the HTL is not a secondary auxiliary element but a key player for the realization of high-performance QLEDs.

For future improvements in QLED performance, it is necessary to adopt an integrated approach that considers energy-level alignment, hole mobility, device stability, and structural design. Conventional energy-level engineering based on HOMO level tuning has been effective in alleviating the hole injection barrier; however, it has been insufficient to completely resolve the fundamental limitations of HTLs, namely, the intrinsically low hole mobility and charge transport asymmetry relative to the ETL. To overcome these limitations, mobility optimization strategies that can practically enhance hole transport are essential, in addition to improving hole injection.

However, conventional mobility enhancement methods based on doping or blending are often accompanied by issues such as film inhomogeneity, increased surface roughness, and leakage currents, which in turn deteriorate device reliability. Therefore, structural approaches utilizing the control of molecular orientation, enhancement of crystallinity, or inorganic and organic–inorganic hybrid HTL materials that intrinsically possess high hole mobility are expected to play an important role. At the same time, the thermal and chemical stability of the HTL should be considered as a key design factor; solvent-stable HTLs, cross-linkable HTLs, or inorganic HTLs can serve as effective alternatives for suppressing interfacial degradation and improving long-term operational lifetime.

In particular, multilayer and hybrid HTL structures provide the advantage of integrating hole injection, hole transport, and interfacial stabilization functions. They offer a realistic design direction for simultaneously achieving energy-level alignment, enhanced mobility, and improved stability. In this context, multilayer and hybrid HTL structures should be regarded as practical solutions that structurally overcome the limitations of single-HTL strategies and as a key research direction for future QLED HTL design.

As QLED technology advances, the strategic importance of the HTL is paramount, particularly for blue QLEDs, which face severe stability bottlenecks due to deep valence bands and high interfacial stress. Since relying solely on emissive layer improvements is insufficient to overcome these hurdles, holistic HTL designs that simultaneously master energy alignment, transport kinetics, and interfacial stability are imperative. Such systematic engineering will be the cornerstone for realizing next-generation high-performance QLEDs with both high efficiency and extended operational lifetimes.