Inorganic Cu₂ZnSnS₄ hole transport layer for perovskite light-emitting diodes

Abstract

Inorganic Cu₂ZnSnS₄ (CZTS) has garnered significant attention in the field of solar cells owing to its high hole mobility, suitable optical bandgap, cost-effective and earth-abundant elemental constituents. In particular, the deeper highest occupied molecular orbital (HOMO) of CZTS makes it an alternative for the traditional organic hole transport layer (HTL) material poly(3,4-ethyllenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), which can suppress acidic interface and hygroscopic problem in perovskite light-emitting diodes (PeLEDs). Herein, we demonstrate CZTS as a novel inorganic HTL for green FAPbBr₃ PeLEDs, achieving a peak external quantum efficiency (EQE) of 7.59% and a maximum luminance of 27 000 cd m⁻² (emitting at 529 nm). The fluorescence properties of the perovskite films and interfacial behaviors of CZTS/FAPbBr₃ were investigated through surface state regulation of CZTS HTL. High hole mobility (0–30 cm² V⁻¹ s⁻¹) of CZTS HTL and low hole injection barrier (∼0.13 eV) between CZTS and perovskite endow the CZTS-based PeLEDs with enhanced hole transport capability and suppressed interfacial carrier accumulation. In addition, an attractive cost-effectiveness advantage of about $0.24 per g synthesizing CZTS makes it a promising competitor to mainstream organic HTL materials such as PEDOT:PSS (∼$1.3 per g). An in-depth understanding of this novel CZTS HTL makes an essential step forward for the commercialization of PeLEDs.

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1. Introduction

In recent years, the research community has made remarkable achievements and quickly rocketed the external quantum efficiency (EQE) of perovskite light-emitting diodes (PeLEDs) to over 20%.^1–6 Nevertheless, the underlying hole transport layer (HTL) in PeLEDs always suffers from confined intrinsic stability, low hole mobility and high injection barrier, substantially creating extra nonradiative losses of carriers at the HTL/perovskite interface. For instance, poly(3,4-ethyllenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), as one of the mainstream HTL materials, is characterised by high hydrophilicity, conductivity and transparency.⁷ However, there are still several inherent limitations in PEDOT:PSS, including a relatively shallow highest occupied molecular orbital (HOMO) energy level (−5.3 eV), which results in a large energy offset with the valence band maximum (VBM) of the perovskite layer, the acidic and hygroscopic nature leading to severe buried interface deterioration of perovskite films and poor long-term stability of PeLEDs, high product costs (∼$1.3 per g) and large batch-to-batch variation severely degrading the cost-benefit ratio of device manufacturing.^8–10 Consequently, the development of novel, low-cost, efficient and stable HTL materials is essential for the resolution of these intractable issues to achieve high-performance PeLEDs.

Compared with organic transport materials, inorganic HTL materials are advantageous in terms of lower production cost, higher environmental stability and facile synthesis, collectively demonstrating the adaptability to the trend of large-scale commercial production of PeLEDs.¹¹ A few inorganic HTL materials have been incorporated into PeLEDs, including Cs_0.32WO₃, CuSCN, NiO_x and Ni_0.9Mg_0.1O_x, which afford comparable luminance and EQE.^12–16 However, there is still a shortage of P-type inorganic hole transport materials with both high mobility and deep HOMO energy levels to effectively match those mainstream electron transport layers with high electron mobility. Fortunately, the inorganic P-type semiconductor material Cu₂ZnSnS₄ (CZTS), commonly known as a light absorber material, shows great potential to act as a promising HTL material for its high hole mobility (0–30 cm² V⁻¹ s⁻¹) and deep HOMO energy level (−5.57 eV).^17–20 Besides, remarkably excellent chemical stability, non-toxic nature, high earth crust abundance and low cost (∼$0.24 per g) broaden its potential for commercial application in HTL materials.^4–6,18,21 As a high-performance active layer in solar cells, CZTS with good charge carrier transport capacity is easy to generate intriguing and heuristic interfacial carrier behavior when applied as HTL in PeLEDs.^19,22 Thus, it is of great significance to investigate the impact of CZTS HTL on the crystallographic quality of perovskite films and the behavior of interfacial carriers in CZTS-based PeLEDs.

Herein, we employed CZTS as a novel inorganic HTL in FAPbBr₃ PeLEDs, which reached a peak EQE of 7.59% and exhibited a pure green luminescence at 529 nm with a maximum luminance of 27 000 cd m⁻². In addition, we reveal the relationship of carrier transport and recombination at the CZTS/FAPbBr₃ interface with surface state regulation of CZTS HTL. Owing to the deep HOMO energy level and high hole mobility, an appropriate energy offset of 0.13 eV for the CZTS/FAPbBr₃ interface dramatically enhanced hole injection capacity and reduced interfacial carrier accumulation in PeLEDs. This study affords a cost-effective inorganic HTL material and highlights the significant potential of inorganic CZTS HTL in future commercial applications of PeLEDs.

2. Experimental section

2.1. Materials

Lead bromide (PbBr₂, 99%), copper sulphate (CuSO₄·5H₂O), zinc sulphate (ZnSO₄·7H₂O), stannous sulfate (SnSO₄), sulphur powder, and isopropyl alcohol ((CH₃)₂CHOH) were obtained from Shanghai Macklin Co, Ltd. Keshi provided ethanol, toluene (99.9%), octane (98%) and N,N-dimethylformamide (DMF, 98%). Additionally, molybdenum trioxide (MoO₃, 99.95%), oleylamine (OLA, 90%), oleic acid (OA, 90%) and LiF were sourced from Shanghai Aladdin Industrial Corporation Co., Ltd. Xi'an Polymer Light Technology Corp supplied formamidine bromide (FABr), 2,2′,2′′-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi, 99.9%) and PEDOT:PSS (Clevios Al 4083). Lastly, Guangdong Poly Optoelectronics Co. Ltd supplied ZnMgO. No further purifications of any materials were required before use.

2.2. Preparation of FAPbBr₃ QDs

The synthesis of FAPbBr₃ QDs was carried out using a modified method based on the literature. The FAPbBr₃ precursor (150 μL), prepared by dissolving PbBr₂ and FABr in DMF with a molar ratio of 1 : 2.2 and Pb²⁺ concentration at 0.2 M, was injected into a vial containing a crystallization-inducing solution composed of 5 mL of toluene, 2 mL of 1-butanol, 500 μL of oleic acid, and 30 μL of n-diacylamine. The solution was vigorously stirred for 10 minutes after the injection, following which it was centrifuged at 13 000 rpm for 5 minutes and the supernatant was discarded. The precipitate was redispersed in 1 mL of n-hexane and then centrifuged at 2000 rpm for 2 minutes. The resulting supernatant was filtered through a 0.22 μm PTFE filter to collect FAPbBr₃ QDs. Prior to the injection of the precursor, the crystallization-inducing solution was precooled to 8–10 °C.

2.3. Preparation of CZTS (W₀/W₁/W₂/W₅) films

The colloidal CZTS nanocrystals were synthesized using the hot injection method. A mixture of 0.15 M copper sulphate (CuSO₄·5H₂O), 0.075 M zinc sulphate (ZnSO₄·7H₂O), 0.075 M stannous sulphate (SnSO₄), and 10 mL of oleylamine (OLA) was prepared in a three-neck flask at room temperature. The resulting solution was stirred constantly and heated to 130 °C for 40 minutes. Then, the reaction temperature was raised to 240 °C before rapidly injecting 5 mL of our previously prepared sulfur precursor solution (3 M) into the preheated Cu–Zn–Sn–OLA complex solution. After half an hour the reaction was complete. W₀, W₁, W₂ and W₅ inks were purified once, twice, three times and five times respectively, W₀/W₁/W₂/W₅ films were obtained by spinning W₀/W₁/W₂/W₅ inks on the ITO substrate.

2.4. Device fabrication

FAPbBr₃ QDs were spin-coated at 1000 rpm for 60 seconds on the W₀/W₁/W₂ films and then quickly transferred into a high-vacuum evaporator without annealing. Afterward, in a low-vacuum environment of 8 × 10⁻⁵ Pa, a sequential deposition of 50 nm TPBi, 1 nm of LiF, and 80 nm of Al was carried out. The hole-only device was fabricated by sequentially depositing 3 nm of MoO₃ and 80 nm of Ag onto the perovskite film. The electron-only device was fabricated by replacing the CZTS (W₀/W₁/W₂) with the ZnMgO film. The ZnMgO film was prepared by spin-coating ZnMgO (16 mg mL⁻¹ in ethanol) at 4000 rpm for 40 seconds on the N₂-plasma-treated ITO and annealed at 120 °C for 10 minutes.

2.5. Characterization

The crystalline structure and quality of the CZTS nanocrystal films were investigated by X-ray diffraction (XRD; X'Pert ProX-ray diffractometer) employing a Cu Kα (λ = 1.5418 Å) and Raman spectroscopy (Renishaw) using an Argon-ion laser (wavelength of 514.5 nm) at room temperature. Raman spectra were acquired at a constant incident laser power of 2.5 mW. A JEOL JSM-7800 Prime scanning electron microscopy (SEM) with an accelerating voltage of 5 kV and atomic force microscopy (AFM, Bruker NanoScope V), with a peak force tapping mode using silicon tips (TESP, Veeco), were used to characterize the morphologies of the films. The UV absorption spectra were obtained using the UV-2500 (Shimadzu Corporation). The PL spectra, TRPL spectra, and temperature-dependent photoluminescence spectra were investigated using an FLS980 (Edinburgh Instruments) spectrometer with a 450 W Xenon lamp. The room-temperature absolute PLQYs of the films were measured on an Edinburgh-FLS 980 spectrometer using an integrating sphere. Fourier transform infrared (FTIR) spectroscopy patterns were obtained to determine the types and changes of functional groups in materials using a TENSOR II spectrometer (Bruker) with a spectral resolution of 4 cm⁻¹ at a scanning frequency of 1 spectra min⁻¹. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo Scientific ESCALAB 250 Xi instrument. UV photoelectron spectroscopy (UPS) measurements were performed at room temperature using the He-discharge lamp (He I line at 21.2 eV) as the excitation source in normal emission with a resolution of 0.06 eV in the ESCALAB 220iXL spectrophotometer. The characterization of hole-only and electron-only devices and low- (f = 5 K) and high-frequency (f = 100 K) capacitance-voltage curves of PeLEDs was performed using the Keithley 4200 device. The Keithley 2400 source meter combined with a QE-Pro spectrometer (Ocean Optics), was employed to characterize the performances of PeLEDs in the glove box. The intensity of electroluminescence was measured using a FOIS-1 integration sphere.

3. Results and discussion

Generally, inorganic compounds serve as hole transport materials in PeLEDs, and their carrier transport performance is significantly influenced by the abundant surface ligands (states).^23–25 Thus, the rational regulation of the surface states of CZTS is a fundamental prerequisite for the first application in PeLEDs as an efficient HTL material. CZTS nanocrystal synthesised by the hot injection method is typically capped with plentiful long-chain oleylamine (OLA) ligands, and their surface states can be effectively regulated through the purification process.^21,26Fig. 1a illustrates the preparation process of the three different CZTS nanocrystal films with varying washing times (W₀, W₁ and W₂), where toluene and isopropyl alcohol were employed to precisely control the quantity of OLA ligands, thereby effectively modulating the surface states of CZTS. The Tauc's plots demonstrate that the band gap of CZTS films is barely affected by washing times (Fig. S1†).^27,28 Additionally, The HOMO energy level of the three CZTS (−5.57 eV) was determined through ultraviolet photoelectron spectroscopy (UPS), which aligns well with the VBM of FAPbBr₃ films (−5.7 eV) and facilitates hole transport at the CZTS/FAPbBr₃ interface (Fig. S2†).^29,30 The distinct peaks of X-ray diffraction (XRD) patterns at 28.48°, 32.90°, 47.43° and 56.25° corresponding to (112), (103), (220), and (312) planes, respectively, of the kesterite CZTS phase are shown in Fig. S3,† demonstrating the successful synthesis of pure-phase CZTS.³¹ Moreover, Raman spectra exhibit two prominent peaks at 287 cm⁻¹ and 337 cm⁻¹, unequivocally confirming the formation of pure quaternary CZTS rather than binary or ternary impurities (Fig. 1b).³² The consistency observed in both XRD and Raman spectra suggests that the surface state regulation does not affect the crystal structure or orientation of the CZTS films. Furthermore, transmission electron microscopy (TEM) analysis demonstrates uniform dispersion of CZTS nanocrystal films within the toluene solvent with an average particle size of approximately about 6.7 nm (W₀), 6.4 nm (W₁) and 6.9 nm (W₂ sample) (Fig. S4†).³³ The excellent crystallinity and dispersibility of CZTS lay the solid foundation for the application of high-performance HTL in PeLEDs.

Fig. 1 Characteristics of W₀/W₁/W₂ films. (a) Synthetic schematic, (b) Raman spectra, (c) FTIR spectra, and (d) three-dimensional topographic AFM characterization of W₀/W₁/W₂ films.

X-ray photoelectron spectroscopy (XPS) was performed to confirm the ligand modifications on the surface of the W₀/W₁/W₂ films. As shown in Fig. S5,† the carefully calibrated N 1s peaks of W₀/W₁/W₂ samples were consistently distributed between 399–402 eV. Additionally, all samples exhibited a characteristic C-NH₂ peak at 400.86 eV, indicating attachment of the OLA ligands on the surface of CZTS films with different washing times.³⁴ The Fourier-transform infrared spectroscopy (FTIR) patterns of CZTS nanocrystal films are dominated by vibrational modes of bound ligand molecules (Fig. 1c). A noticeable decrease in the intensity of the signal peaks corresponding to C–H stretching vibration (2853 cm⁻¹ and 2923 cm⁻¹) and NH₂ bending vibration (1620 cm⁻¹) of the OLA ligands was observed when the CZTS nanocrystal films were purified twice, which suggests a large number of ligands are shed from the CZTS nanocrystals in the purification process and contributes to the surface states differentiation of CZTS.^35,36 Furthermore, the transmittance of the transport layer significantly impacts the efficiency of the photon extraction in PeLEDs. Fig. S6† demonstrates that the transmittance of CZTS films exceeds 90%, and this superior light output characteristic was maintained at a thickness of 120 nm, which means that CZTS has great potential as a novel HTL (Fig. S7†). The SEM images and EDS results shown in Fig. S8,† indicated that Cu, Zn, Sn and S components are uniformly distributed in CZTS films and all three samples are compact and pinhole-free, demonstrating that the varying purification times will not significantly change the morphology of the CZTS nanocrystal films.³³ Parallelly to the atomic force microscope (AFM) images of these three samples in Fig. 1d, the W₀ film without purification displays a low surface roughness with root mean square (RMS) of 5.63 nm, while the roughness of the W₂ film is increased two times than that of the initial W₀ film. Therefore, the surface state regulation method is feasible to optimize the quality of CZTS HTL, which is critical for the charge transport in PeLEDs.

The underlying CZTS nanocrystal films determine the quality of the perovskite (PVK) emission layer (EML).^37–39 The photoluminescence (PL) spectra of FAPbBr₃ quantum dots (QDs) on the W₀/W₁/W₂ films in Fig. 2a exhibit a homogeneous emission at 526 nm and a distinguishing attenuation of peak intensity. In particular, the CZTS(W₀)/FAPbBr₃ film shows the best luminescence properties and longest lifetime (τ_ave = 17.76 ns, PLQY = 66.37%) compared to the CZTS(W₁)/FAPbBr₃ film (τ_ave = 15.04 ns, PLQY = 45.48%) and CZTS(W₂)/FAPbBr₃ film (τ_ave = 11.23 ns, PLQY = 30.42%) (Fig. 2b and c). The radiation/non-radiation recombination rate (k_r, k_nr) was further calculated using formulas (1) and (2) and the obtained parameters are summarized in Table S1 (ESI†). The k_r of W₀/W₁/W₂ decreases sequentially while the k_nr increases consistently, certifying more OLA ligand retention and relatively flat CZTS film is conducive to maintaining the luminescence properties of the EML. The CZTS(W₅)/FAPbBr₃ film with further reduced OLA ligand retention shows a sharp drop of PLQY (15.28%) (Fig. S9†), which intuitively illustrates the decrease of ligands will expose a number of undesired defects on the CZTS/FAPbBr₃ interface and severely deteriorates the optical performance. Furthermore, it is also noteworthy that the transmittance of both W₀/W₁/W₂ films exceeds 90%, which indicates that the change of PLQY is independent of the photocoupling efficiency (Fig. S6†).

Fig. 2 Photophysical properties of the perovskite films on W₀/W₁/W₂ films. (a) PL spectra, (b) PLQY spectra, (c) TRPL spectra, (d, e and f) temperature-dependent PL spectra of the perovskite films on W₀/W₁/W₂, (g) FWHM, (h) integrated PL intensity as a function of reciprocal temperature, (i) UV-vis and Urbach energy spectra of the perovskite films on W₀/W₁/W₂ films.

It is worth mentioning that the bottom transport layer enables the easy dissociation of light-activated excitons in the perovskite layer, leading to relatively low PLQY and average PL lifetime. Temperature-dependent PL spectra provide insights into the non-radiative relaxation processes and exciton–phonon coupling behavior, revealing the influence of the CZTS transport layer on the luminescence properties of EML. The broadening of temperature-dependent PL spectra can explain phonon scattering in the perovskite films. The FWHM curve extracted from Fig. 2d–f was fitted, and the experimental data are in good consistency with the optimal fitting curve (R² > 0.990, Fig. 2g), the obtained parameters are summarized in Table S1.† The variation of FWHM broadening with temperature is described by formula (3):^37,40,41

Γ(T) = Γ₀ + Γ_AC + Γ_LO = Γ_inh + γ_ACT + γ_LO/[e^E_LO/k_BT − 1] (3)

where Γ₀ is an intrinsic inhomogeneous broadening term associated with material composition, size, and shape. Γ_AC results from acoustic scattering and Γ_inh stems from optical phonon scattering, both contributing to homogeneous PL emission broadening. γ_AC and γ_LO depict the strengths of exciton–phonon coupling, k_B is the Boltzmann constant and E_LO denotes the phonon energy. The contribution of the acoustic phonon scattering to PL broadening can be neglected for all FAPbBr₃ QDs, while the primary contribution of PL broadening comes from the optical phonon scattering. Thus, the higher E_LO represents better lattice stability of the FAPbBr₃ film, while the gradual decrease of E_LO indicates that less ligand-coated CZTS destroy the lattice stability of FAPbBr₃ QDs. E_B of the FAPbBr₃/CZTS film was fitted using the Arrhenius equation (formula (4)):^42–46where I₀ is the PL integral strength at the temperature of T₀, A is the proportional coefficient, and E_B is the exciton binding energy. As shown in Fig. 2h, the E_B of the FAPbBr₃/CZTS films decreases from 180.73 meV to 159.60 meV. Higher binding energy indicates a reduction of excitons dissociating into free carriers, suggesting more OLA ligands of the CZTS films can inhibit exciton dissociation and increase radiation recombination in emission films. The interface defects between the CZTS film with fewer ligands and EML increase, and exciton dissociation is easy to occur, which eventually leads to obvious degradation of optical properties.

Urbach tail is the broadening effect of semiconductor band edges related to thermal and inherent structural disorder, quantified by the Urbach energy (E_u).^47,48 The E_u value of CZTS/FAPbBr₃ is increased from 86.43 meV to 127.71 meV, as shown in Fig. 2i, indicating that the presence of sufficient ligands of CZTS film can maintain the integrity of the FAPbBr₃ lattice. Additionally, the diffraction peaks for the CZTS/FAPbBr₃ films exhibit slight shifts to smaller angles, measured at 0.04°, 0.05°, and 0.07° in Fig. S10.†^49–51 This shift implies that Cu⁺ (76 pm) or Zn²⁺ (74 pm) with the smaller ionic radius than FA⁺ (263 pm) might be partially incorporated into the perovskite lattice in the presence of decreased OLA ligands, resulting in destroyed perovskite QDs and poor photophysical properties of EML.⁵² Therefore, the regulation of surface states in CZTS HTL significantly influences the photophysical behavior of perovskite materials.

To investigate the carrier behavior of CZTS-based PeLEDs, we utilized capacitor–voltage (C–V) characteristics to evaluate the injection, capture, and recombination behavior in PeLEDs.^53–55Fig. 3a depicts the low-frequency C–V characteristic diagram, segmented into four regions. At lower applied voltages, the similar barrier capacitance of these PeLEDs implies comparable carrier accumulation at the interface prior to injection into the EML (region 2). As the applied voltage surpasses a certain threshold in region 3, the diffusion capacitance reflects the ability of the majority carriers to transport between the transport layer and the EML layer. Notably, the diffusion capacitance (C₃) of W₀-based PeLEDs is 2.83 nF, which markedly surpasses that of the W₁-based PeLEDs (2.46 nF) and W₂-based PeLEDs (2.31 nF), suggesting that the presence of long-chain OLA ligands on CZTS substantially impedes hole transport between HTL and EML. Furthermore, a decline in region 4 indicates carrier recombination has occurred.^56,57 Particularly, there is a more pronounced drop in capacitance for W₁-based-PeLEDs compared to W₀/W₂-based PeLEDs, indicating intensified carrier radiative recombination in W₁-based PeLEDs. Furthermore, the presence of OLA ligands in W₀-based PeLEDs hampers carrier transport, leading to a reduced number of carriers capable of efficient radiative recombination during voltage increase. As a result, the capacitance prematurely increases after 7 V. Fig. 3b illustrates the differentiation of the three C–V curves in Fig. 3a, represented by capacitance-voltage derivative (dC/dV) and categorized into five distinct regions. A negative slope in dC/dV indicates a strong recombination region. In comparison, W₁-based PeLEDs exhibit a larger negative value in dC/dV compared to W₀/W₂ based-PeLEDs, suggesting that W₁-based PeLEDs possess stronger recombination ability.

Fig. 3 Carrier behavior in PeLEDs. (a) Low-frequency capacitance–voltage (C–V) plots (f = 5 K), (b) dC/dV–V curves, (c) high-frequency C–V plots (f = 100 K) of CZTS-based PeLEDs. (d) Nyquist plots of impedance spectra and (e) current density–voltage (J–V) characteristics curves of hole-only device and electron-only devices of CZTS-based PeLEDs. The inset shows the equivalent circuit diagram for fitting.

Under high-frequency conditions, trapped carriers are significantly suppressed, thereby our focus was directed toward investigating the relationship between carrier injection and recombination (Fig. 3c). A smaller capacitance at higher voltages indicates enhanced carrier recombination, with W₁-based PeLEDs exhibiting a diminished trend of negative dC/dV (Fig. S11†), which aligns with the observed intense carrier recombination in W₁-based PeLEDs as evidenced by low-frequency C–V curves. These findings indicate that an excess of OLA ligands on the CZTS surface impedes interface carrier injection and diffusion, while proper surface states promote carrier recombination.

Electrical impedance spectroscopy (EIS) measurements confirmed the disparity at the interface of PeLEDs.^58–61 CZTS-based PeLEDs devices were subjected to 4 V bias within the frequency range of 0.1 Hz to 1 MHz. As depicted in Fig. 3d, the electron transfer impedance (or resistance) of CZTS/FAPbBr₃ was determined by fitting an equivalent circuit diagram derived from the Nyquist curve. Utilizing the illustrated equivalent circuit (Fig. 3d), it is evident that W₀-based PeLEDs exhibit a higher series resistance value (R_s) of 13.5 kΩ compared to W₁-based PeLEDs (9.8 kΩ) and W₂-based PeLEDs (4.3 kΩ). This observation suggests a more efficient carrier transfer from W₂-CZTS HTL to EML, which aligns with the impedance characteristics displayed in Fig. S12† for different batches of CZTS. Furthermore, the single-carrier device measurement in Fig. 3e also demonstrates the inhibitory effect of OLA ligands on the charge transport.

According to the variation of photophysical characteristics and interfacial carrier behavior of CZTS-based PeLEDs, Fig. 4 presents a schematic diagram illustrating the carrier transport and recombination mechanism at the CZTS/FAPbBr₃ interface. Owing to the insulating nature, excessive OLA ligands hinder carrier transport at the CZTS/FAPbBr₃ interface and increase the series resistance, meanwhile, the large carrier accumulation increases the non-radiative recombination. Conversely, a lack of OLA ligands may permit Cu⁺ or Zn²⁺ ions to infiltrate into EML breaking the integrity of the perovskite lattice, which is detrimental to radiative recombination. Satisfyingly, the appropriate ligands on the CZTS (W₁) surface are the best solution to balance the optical property of EML and carrier transport in PeLEDs. Optimal surface state regulation can effectively passivate interface defects and minimize their direct contact with FAPbBr₃, thereby stabilizing the perovskite lattice, maintaining the optical properties of FAPbBr₃ and facilitating carrier transport/recombination.

Fig. 4 Schematic diagram of carrier behaviour at CZTS/FAPbBr₃ interface.

To explore the application of the CZTS HTL in PeLEDs, CZTS-based FAPbBr₃ PeLEDs devices were fabricated and the schematic structure and energy level diagram are illustrated in Fig. 5a and b. The current density–voltage (J–V) curves depicted in Fig. 5c exhibit an increase in the current density occurring at voltages lower than approximately 4 V, indicating that controlling the surface states of CZTS can effectively suppress leakage current, which aligns with our quality characterization analysis of perovskite films. Samples with a higher concentration of OLA ligand (W₀) exhibit an enhanced carrier accumulation (Fig. 3a), leading to increased non-radiative recombination and thus reduced device performance. Meanwhile, when the voltage is beyond 4 V, the accumulated carriers exhibit an augmented conduction current subsequent to conduction, thereby demonstrating a current density similar to W₁.^57,62,63 However, as the OLA ligand content decreases significantly (W₂) or is completely absent (W₅), severe damage occurs to the perovskite lattice with a substantial reduction of device series resistance in EIS analysis. Consequently, it can even resemble conductor-like characteristics leading to breakdown at high voltage (Fig. S14a).^64–66 As observed in Fig. 5d and e, W₀-based PeLEDs demonstrate poor performance with a maximum luminance of 8000 cd m⁻² and a maximum EQE of 2.61%, while the EQE of W₂ device is only 2.03% with luminance dropping to 6000 cd m⁻². Surprisingly, device W₁ achieves a peak EQE of 7.59% along with a high maximum luminance of 27 000 cd m⁻². It is noteworthy that W₁-based PeLEDs demonstrate a lower turn-on voltage (V_on) than other devices by 0.5 V, which is consistent with a low carrier injection barrier and effective carrier radiation recombination. To ensure data reliability and eliminate testing variability, we extracted the last ten instances of device performance and generated a statistical plot as shown in Fig. S13.† Remarkably, the average V_on of the W₁-based PeLEDs was found to be smaller than that of the other two samples (with a reduction ranging from 0 to 0.4 V). We propose that CZTS, being a novel inorganic HTL, abundant in metal ions, may disrupt the perovskite lattice upon direct contact with it, leading to a decrease in the radiation recombination efficiency of the W₂ device. V_on signifies the initiation of carrier radiative recombination, while excessive insulating ligands in the W₀-based PeLEDs impede carrier transport efficiency. As a result, the W₁ device has lower V_on than other devices in terms of carrier transport and perovskite lattice integrity. For the W₅-based PeLEDs (Fig. S14†), the luminance and peak EQE reach as low as 1350 cd m⁻² and 0.6%. This substantial fluctuation confirms that effective regulation of the surface states of CZTS HTL is an efficient approach to enhancing device performance. The inset in Fig. 5d displays a regular change in brightness of EL emission, providing visual evidence for this improvement. The EL spectra consistently exhibited emission at 529 nm as the bias increased, while maintaining a narrow full-width half-maximum (FWHM) of 20 nm at 6 V, suggesting that CZTS-based PeLEDs devices have superior spectral stability (Fig. S15† and Fig. 5f). Multiple batches of FAPbBr₃ PeLEDs were fabricated, and the histograms of peak EQE obviously depict the reproducibility and overall trend in device performance with varying CZTS HTLs (Fig. S16†); the average EQE of the W₀ device is 2.36%, W₁ device is 7.32% and W₂ device 2.09%. Additionally, the half-life (T₅₀) represents the time required for the luminance to decay from 100 cd m⁻² to 50 cd m⁻² under constant voltage in a device. It can be calculated using the formula L₀ⁿT₅₀ = a, where L₀ denotes the initial luminance, n is the exponential parameter characterizing the device, and a is a constant. In comparison with the PEDOT:PSS-based PeLEDs, CZTS equipment exhibits an increased T₅₀ from 15.91 min to 43.48 min, indicating improved device stability that can be attributed to the inherent strong stability of CZTS itself (Fig. S17†). Therefore, CZTS shows great potential as HTM for PeLEDs applications.

Fig. 5 Device structure and EL performance characteristics of W₀/W₁/W₂-based green PeLEDs. (a) Schematic device structure, (b) schematic energy levels, (c) current density–voltage (J–V) characteristics curves, (d) luminance–voltage characteristics curves (L–V), (e) EQE-voltage curves and (f) normalized EL spectra at 6V driving voltage of W₀/W₁/W₂-based PeLEDs.

4. Conclusions

In summary, we have successfully incorporated CZTS as a novel inorganic HTL with low toxicity, deep HOMO energy level, high hole mobility and cost-effectiveness into PeLEDs for the first time. The undesirable charge accumulation and non-radiative recombination at the CZTS/FAPbBr₃ interface are significantly suppressed by selectively optimizing the surface state distribution of CZTS nanocrystals. Thus, a pure green FAPbBr₃ PeLED device stably emitting at 529 nm with a peak EQE of 7.59% and a highest luminance of 27 000 cd m⁻² was fabricated; the stability of the CZTS-based PeLEDs is comparatively higher than that of the PEDOT:PSS device (T₅₀ increased from 15.91 min to 43.48 min). This study unveils the previously undiscussed impact of surface state regulation of transport layer materials on the interfacial carrier behavior in PeLEDs, maximizing the application value of CZTS as a novel inorganic HTL material in high-performance PeLEDs. This work paves the way for the development of low-toxicity and cost-effective HTL materials, which could facilitate the large-scale production and future commercialization of PeLEDs and other luminescence applications.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article or its ESI.† Moreover, the data that support the findings of this study are available from the corresponding author, upon reasonable request.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 52202188 and 11704050). The authors would like to thank the Analysis and Testing Center of Southwest Jiaotong University for supporting the TEM and XRD measurements.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi01186d

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