1 Introduction

In the path towards cleaner energy utilization, novel carbon materials with enhanced properties should be designed and developed. These should be of low-cost, facile synthesis, solution-based processing using environmental friendly solvents without thermal annealing and so on. Such carbon materials can be used for energy storage, electrode and inter-electrode materials, and selective adsorption of pollutants among other applications. Organic light-emitting diodes (OLEDs) have become a prominent technology as they are positioned at the cutting edge of a wide range of applications that require high-quality, energy-efficient displays and lighting [1]. Concerning a well-established technology with bright future prospects, these devices involve extending low power consumption, higher efficiency and flexibility in design beyond traditional light-emitting technologies [2]. Certain crucial key components in OLEDs are the emissive layer and the balanced charge transport and injection of both electrons and holes, as they are critical for achieving efficient electroluminescence, lower operating voltages, and reducing the probability of charge accumulation through the alignment of energy barriers [3].

The multilayer architecture of OLEDs provides several degrees of freedom and an extensive domain of investigation for engineers to optimize the efficiency of the device. To reduce charge injection barriers formed between the conductive electrodes and the emissive layer (EML) and improve the charge carrier balance, interlayer materials, such as an electron or a hole injection/transport layer (EIL/ETL or HIL/HTL, respectively) are applied [4]. These interlayers should possess appropriate energetics, not only to facilitate the efficient injection from the respective electrode to the emissive layer, but also to prevent carrier leakage. ETL material should possess a lowest unoccupied molecular orbital (LUMO) lying between that of the EML and the Fermi level of the cathode electrode to serve as electron cascade material, sufficiently deep highest occupied molecular orbital (HOMO) level to block holes from escape the device, and high mobility. Moreover, it should exhibit orthogonal processing with respect to the EML, good film-forming properties, and have excellent compatibility with other layers in the device [5, 6].

Various inorganic materials used as ETLs in efficient organic and perovskite optoelectronic devices including metal oxides, such as zinc (ZnO) [7,8,9], titanium (TiO2) [9, 10] and tin (SnO2) [9, 11, 12] oxides, as well as polyoxometalate compounds [13]. These ETLs exhibit n-type conductivity attributed to oxygen vacancy formation during deposition and are extremely robust to environmental induced degradation [14, 15]. However, they usually require high temperature post-annealing which may affect the stability of the underlayers. Organic semiconductor materials are solution-processable and can provide enhanced film quality and stability over time than metal oxide ETLs [16]. As a result, several classes of organic materials, mostly molecules bearing amine groups such as polyethylenimine (PEI), which induce the formation of negative dipoles at the cathode interface and reduce the electron injection barrier, have been explored [17, 18].

Porphyrins are a unique class of nature-inspired artificial green carbon materials with a characteristic macrocyclic structure that can act as energy and charge funnels in the photosynthetic processes. Due to these properties, they have gained significant attention in recent years as potential charge transport materials in optoelectronic devices. They can be processed from a variety of solvents to form thin films on top of different substrates with no thermal treatment requirements. Also, they can be easily modified to possess excellent charge transport characteristics, rendering them valuable as ETLs in OLEDs [19, 20]. Our group has previously investigated a variety of porphyrin molecules processed from water solutions to form ETLs in OLEDs with a conventional architecture (that is HTL/EML/ETL) [21]. In this work, we aim to combine the superior properties of porphyrin compounds with the ability of amine groups to reduce the electron injection barrier and prepare a Zn–porphyrin complex with an amidine group (termed as ZnP-amidine) to serve as an ETL material in OLEDs. ZnP-amidine is found to facilitate efficient electron injection/transport in OLEDs based on a green emitting co-polymer EML. Additionally, it can be processed from methanol solution at room temperature, hence being suitable for energy-efficient and low-cost manufacturing, which has become an area of active research and development in the field of organic electronics. The dependence of ETL thickness on the OLED performance was investigated, suggesting that the insertion of an ultra-thin ZnP-amidine film coated on top of the F8BT (poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo-{2,1’,3}-thiadiazole)) emissive layer improved the luminance and current density of the device, and thus the efficiency of the OLED.

2 Experimental section

ZnP-amidine preparation. ZnP-amidine was synthesized according to previously reported procedures [22].

OLED fabrication. Indium tin oxide (ITO) coated glass substrates purchased from Sigma-Aldrich (Athens, Greece) with sheet resistance of 15–25 Ω/sq served as the transparent anode electrode. Before the deposition of the hole transport layer on top, ITO substrates were placed into an ultrasonic bath for ten minutes in each solvent: deionized water, acetone and isopropyl alcohol (purchased from Sigma-Aldrich, Athens, Greece), they dried with nitrogen (N2), and then they were treated with UV-ozone for 20 min. First to be deposited was the hole transport layer (HTL) using the commercially available solution PEDOT: PSS (poly(3,4 -ethylenedioxythiophene)-poly(styrenesulfonate)) with 1.3 wt % dispersion in H2O (Sigma-Aldrich). PEDOT:PSS was passed through a 0.45 μm pore diameter polyvinylidene fluoride (PVDF) filter and spin-coated at 6000 rpm for 40 s forming a 40 nm thick layer. The substrates were then annealed at 110 °C for 30 min on a hotplate. A green-yellow copolymer called F8BT (poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo-{2,1′,3}-thiadiazole)), purchased from the American Dyes Source, Quebec, Canada, (ADS 233 YE) was used as the EML. F8BT solution with a concentration of 10 mg ml−1 in chloroform was passed through a 0.22 μm pore diameter PTFE filter, spin-coated at 1200 rpm for 40 s forming a 80 nm thick layer on top of the PEDOT:PSS layer, and then annealed at 85 °C for 10 min. Subsequently, ZnP-amidine serving as the ETL of the OLED was deposited atop. Porphyrin solutions were prepared in methanol in various concentrations, which were spin-coated at 2000 rpm for 40 s forming a thin layer. Last to be deposited was the aluminum cathode electrode, using a thermal evaporation system. The thickness of the Al was 150 nm thick and the active area of the prepared OLED was 12.56 mm2. Note that a reference device without ETL was also prepared for comparison reasons.

Characterization methods. Current density–voltage (J−V) measurements were performed using a Keithley 2400 (Vector Technologies, Athens, Greece). Luminance and electroluminescence (EL) spectra were taken with an Ocean Optics USB2000 fiber optic spectrometer. For luminance measurements a Lambertian emission profile was assumed. UV–vis absorbance spectra were recorded using a Perkin Elmer Lambda 40 UV–vis spectrometer (Vamvakas-Scientific Equipment, Athens, Greece). The surface morphology was investigated with an NT-MDT AFM system (LaborScience SA, Athens, Greece) operating in tapping mode. Wettability was determined by means of contact angle measurements using a Kruss DSA 100 system. Fourier-transform infrared (FTIR) spectra were obtained using a Bruker Tensor 27 spectrometer (Interactive, Athens, Greece), with a DTGS detector. Cyclic voltammetry was conducted with a VersaSTAT 4 potentiometer (Megalab, SA, Athens, Greece). Steady-state photoluminescence (PL) measurements were performed using a commercial platform (ARKEO—Cicci Research). In particular, the substrate was illuminated with a diode-pumped solid-state Nd:YVO4 + KTP laser (peak wavelength 532 nm ± 1 nm, optical power 1 mW on a circular spot of 2 mm of diameter 31 mW cm−2) at an inclination of 45°. The fluorescence on the opposite side of the substrate was focused on a bundle of fibers (10 mm in diameter) with an aspheric lens close to the substrate to maximize the PL. The bundle sent the signal to a CCD-based spectrometer. The same integration time and number of averages were maintained to better compare the results. X-ray diffractograms (XRD) were taken with a Smart Lab Rigaku (Japan) diffractometer (θ/θ scan) with Cu Kα radiation (3 kW).

3 Results and discussion

3.1 Characterization of ZnP-amidine and F8BT/ZnP-amidine interfaces

ZnP-amidine, a zinc complex of an amidine-modified porphyrin, has been previously reported to participate in efficient excited energy transfer processes [22,23,24]. The chemical structure of the porphyrin is shown in Fig. 1a. The UV–Vis absorption spectra of both ZnP-amidine solution and film are presented in Fig. 1b, along with the absorption spectra of F8BT and F8BT/ZnP-amidine samples. In the case of the ZnP-amidine solution with a concentration of 10–4 M, a strong absorption band in the visible region centered at 425 nm appeared, corresponding to the Soret band. Weaker bands in the visible region corresponding to the Q-bands (557 nm and 599 nm) are also observed. For the ZnP-amidine film formed on a glass substrate from a methanol solution with concentration 0.5 mg ml−1, the Soret band is shifted to lower energies (centered at 437 nm), suggesting the formation of aggregates. The red-shifted absorption spectra is called J-aggregate absorption and corresponds to the porphyrin self-assembly aggregation through an edge-to-edge orientation of porphyrin molecules [25, 26]. In addition, ZnP-amidine thin film exhibits visible emission in the red region, as observed in the photoluminescence (PL) spectrum shown in Figure S1 (Supporting Information). On the other hand, F8BT film exhibits a characteristic absorption peak centered at 376 nm, while the F8BT/ZnP-amidine sample preserves both F8BT and porphyrin peaks. The specific porphyrin molecule was chosen because of its capability to form aggregates from methanol solutions. The representation of the porphyrin molecule in its aggregates on top of the emissive layer (F8BT) is shown at the Fig. 1c.

Fig. 1
figure 1

a Chemical structure of ZnP-amidine. b UV–Vis absorption spectra of ZnP-amidine in solution (red) and film (green) and F8BT and F8BT/ZnP-amidine films. c Schematic illustration of the planar interface of F8BT and aggregates of ZnP-amidine film

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In addition, these films were further analyzed by Fourier-transform infrared spectroscopy (FTIR) to determine the material's chemical composition, bonding, and interactions, as well as explore how the ZnP-amidine affects the F8BT, when deposited on top of it (Fig. 2a and b). For the pristine F8BT film, the peaks at 2926 cm−1 and 2854 cm−1 are relating to stretching mode of C–H bond, while the peak at 1459 cm−1 corresponds to the C=C stretching mode of an aromatic group. The stretching mode between the phenyl rings is identified at 1255 cm−1, the deformation of aliphatic chain bonds at 1106 cm−1, and the C–H rocking mode at 814 cm−1 [27]. In the case of ZnP-amidine film, stretching mode of C-H bonds is located at the 3500–2500 cm−1 region. Furthermore, the peaks appeared in the region 1300–1590 cm−1 are related to the stretching vibration of C=C bond and the weak peaks at around 1247 cm−1 and 1107 cm−1 correspond to C-N stretching of pyrrole groups. The deposition of ZnP-amidine on top of F8BT renders an increase in intensity and width of the peaks at 2000–1800 cm−1 without any noticeable changes. The spectrum retains both polymer and porphyrin peaks without major alterations suggesting that the molecules are intact and no deleterious effect occurs, thus making the porphyrin suitable for its successful incorporation in an OLED as ETL.

Fig. 2
figure 2

FTIR transmittance spectra of ZnP-amidine, F8BT and F8BT/ZnP-amidine films at the a 4000–2500 cm−1 and b 2500–500 cm−1 region

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Furthermore, surface morphology of the F8BT and F8BT/ZnP-amidine films was investigated with atomic force microscope (AFM) measurements. The 2 × 2 μm2 AFM images of F8BT and ZnP-amidine pristine films (Figure S2) show smooth surface with low root-mean-square (RMS) roughness values of RMS = 0.36 nm and RMS = 0.68 nm, respectively. Also, the F8BT/ZnP-amidine films with concentration of 0.25 mg ml−1 and 1 mg ml−1 present RMS = 1.96 nm and RMS = 0.69 nm, respectively (Fig. 3a, b and d, e). From these 2D and 3D topographies, it can be noticed that ZnP-amidine form a nano-aggregated layer on the F8BT film, which may facilitate electron injection from the ETL to the emissive layer resulting in enhancement of the OLEDs performance. Also, the hydrophobic nature of ZnP-amidine (Figure S2h) slightly improves the hydrophobicity of the F8BT film (Figure S2d), as observed by the contact angle measurements (Fig. 3c and f), which may influence the device stability.

Fig. 3
figure 3

a, b 2 × 2 μm2 AFM surface topographies (3D-left and 2D-right height images) of ITO/PEDOT:PSS/F8BT/Zn-amidine (0.25 mg ml−1), c Contact angle of a droplet of deionized water deposited on ITO/PEDOT:PSS/F8BT/Zn-amidine (0.25 mg/ml), d, e 2 × 2 μm2 AFM surface topographies (3D-left and 2D-right height images) of ITO/PEDOT:PSS/F8BT/Zn-amidine (1 mg ml−1) films, f Contact angle of a droplet of deionized water deposited on ITO/PEDOT:PSS/F8BT/Zn-amidine (1 mg ml−1)

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The crystallinity of the proposed porphyrin compound was also investigated. XRD measurements presented in Figure S3a show that ZnP-amidine is a polycrystalline material exhibiting characteristic peaks at angles 2θ = 14.25°, 37.46°, 43.61°, and 61.79°. It is noted that the peak at 2θ = 33.07° refers to the silicon substrate. Figure S3b also shows the XRD diffractograms for the pristine F8BT and the F8BT coated with the ZnP-amidine. It is observed that both samples exhibit a diffraction peak at 2θ = 37.49° corresponding to a lattice spacing of d = 0.24 nm (according to Bragg law, nλ = 2*d*sinθ, where λ = 0.154 nm). More interestingly, in the case of the F8BT/ZnP-amidine the full width at half maximum (FWHM) of the peak is larger compared to that of the F8BT sample, indicating smaller crystalline size (43.26 nm for the F8BT and 36.75 nm for the F8BT/ZnP-amidine sample, according to Scherrer formula,\(L = \frac{0.9 \cdot \lambda }{{FWHM \cdot \cos \theta }}\)). The decrease in the crystalline size of the emitter, F8BT, may enhance exciton confinement in the emissive layer leading to improved radiation probability.

3.2 OLED fabrication and characterization

To accomplish a detailed research of ZnP-amidine as an ETL material, it is necessary to determine substantial information of the material, such as the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. As cyclic voltammetry constitutes one of the prominent techniques to investigate the oxidation and reduction processes of molecular species, the corresponding cyclic voltammograms of ZnP-amidine films deposited on indium tin oxide (ITO)/glass substrates were recorded and presented in Fig. 4a and b, respectively. Oxidation and reduction processes enabled the HOMO and LUMO calculation, respectively, taking advantage of the empirical formulas as follows [28,29,30]:

$${{\text{E}}}_{{\text{HOMO}}}= -\,({{\text{E}}}_{{\text{ox}},{\text{onset}}} + 4.4)\mathrm{ eV}$$
(1)
$${{\text{E}}}_{{\text{LUMO}}}= -\,({{\text{E}}}_{{\text{red}},{\text{onset}}} + 4.4)\mathrm{ eV}$$
(2)

where Eox,onset and Ered,onset correspond to the oxidation and reduction potential onset, respectively, and described as the position where the current begins to deviate from the baseline.

Fig. 4
figure 4

Cyclic voltammetry of ZnP-amidine film coated on indium-tin-oxide (ITO)/glass substrate at a scan rate of 0.1 V s−1 in a 0.1 M LiClO4 aqueous electrolyte solution. Cyclic voltammograms for a oxidation and b reduction process. The estimated HOMO and LUMO levels for ZnP-amidine are − 4.85 and − 3.15 eV, respectively. c The OLED architecture and d the corresponding energy level diagram

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The potential onsets were determined as Eox,onset = 0.45 eV and Ered,onset = − 1.25 eV and they result in the EHOMO = − 4.85 eV and ELUMO = − 3.15 eV, respectively. HOMO (− 5.96 eV) and LUMO (− 3.40 eV) levels of the F8BT were also estimated using cyclic voltammetry (Figure S4). Consequently, the energy gaps for ZnP-amidine and F8BT are 1.70 eV and 2.56 eV, respectively.

Next, ZnP-amidine used as EIL/ETLs in fluorescent OLEDs spin-coated on top of the green-yellow polymer, F8BT. Figure 4c illustrates the architecture of the prepared OLEDs with the structure ITO/PEDOT:PSS/F8BT/ZnP-amidine/Al, where various concentrations of the porphyrin were investigated. Note that, a reference device without the ETL was also fabricated for comparison reasons. Figure 4d represents the energy level diagram of the materials used for the OLEDs fabrication, where the HOMO and LUMO levels of F8BT and ZnP-amidine were estimated by cyclic voltammetry, and work function values for ITO, PEDOT:PSS and Al were obtained from literature [31].

Figure 5a and b shows the current density–voltage (J-V) and luminance-voltage (L-V) characteristics of the F8BT based OLEDs without and with the ZnP-amidine ETLs, respectively. The OLEDs performance characteristics, as derived from batches of 12 identical devices of each type, are also summarized in Table 1. It was observed that the devices with ZnP-amidine at concentrations 0.25 mg ml−1 and 0.5 mg ml−1 present higher current efficiencies compared with the reference device without the porphyrin layer. In particular, the F8BT/Zn-amidine 0.25 mg ml−1-based device exhibits higher luminance of about 6930 cd m−2, current density of 1996 A m−2 and a maximum luminous efficiency (LE) of approximately 9.0 cd A−1 (Fig. 5a–c), compared with the reference device without the ETL (L = 5344 cd A−1, J = 1329 A m2, and LE = 4.3). This device also exhibits the highest external quantum efficiency (EQE) of 3.7% compared to 1.7% of the reference device (Figure S5). This suggests that efficient charge carrier balance is achieved in the ZnP-amidine based devices in respect with the reference OLED. In addition, the incorporation of the ZnP-amidine in the OLED didn’t induce the emission of the F8BT layer, when coated on top of it, as clearly seen in the electroluminescence (EL) spectra of Fig. 5d. Notably, the device using the 0.25 mg ml−1 ZnP-amidine interlayer outperforms that using the widely applied commercially available cesium carbonate (Cs2CO3) electron transport material (Figure S6). The same device exhibited better stability compared to the reference device without any ETM (Figure S7). On the other hand, OLEDs based on thicker ZnP-amidine ETLs with concentration > of 0.25 mg ml−1 showed lower luminance values and poorer luminous efficiencies, which may be attributed to the increased size of the nano-aggregates in the porphyrin layer.

Fig. 5
figure 5

a Current density–voltage and b luminance—voltage characteristics of the fabricated OLEDs with and without the ZnP-amidine. c Luminous efficiency and d normalized electroluminescence (EL) spectra of the same devices

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Table 1 Representative performance characteristics OLEDs with and without the ZnP-amidine as extracted from a batch of 12 identical devices of each type
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4 Conclusions

In conclusion, we have investigated the potential of a novel material, ZnP-amidine, as an electron transport layer (ETL) in organic light-emitting diodes (OLEDs). ZnP-amidine porphyrin was investigated for its optical, electrochemical and structural properties. The possible interaction with the organic semiconductor used as an emissive layer in OLEDs was also studied. Our experimental results show that ZnP-amidine can provide better coverage and adhesion to the underlying layer and reduce the risk of delamination. In addition, the efficiency of OLEDs was significantly influenced by the ETL material inserted in the device and specifically, its thickness and morphology were critical to device performance. Particularly, ZnP-amidine of low concentration (0.25 mg ml−1 in methanol) exhibited excellent electron transport properties, leading to enhanced current density and luminance, and thus improved OLED performance, suggesting the beneficial role of the proposed porphyrin compound in organic optoelectronic devices.