Introduction

Organic electronics is a part of electronics that deals with organics semiconductor materials [1,2,3,4,5,6,7,8,9,10]. Organics semiconductor-based diode was discovered 40 years ago but day by day it is becoming very useful and popular. When Henry by anodized aniline in sulfuric acid, obtained a partially conductive substance, which led to the discovery of electrical conduction in organic materials in 1862. Heeger, MacDiarmid, and Shirakawa investigated new material in the 1970s that after certain modifications, the polymer polyacetylene was made conductive. Tang and VanSlyke [2] realized a breakthrough by demonstrating a LED based on two organic molecules, tris (8-hydroxyquinoline) aluminium (Alq3) and diamine [2].

The primary setup of OLED involves a hole injection layer (HIL), electron injection layer (EIL), and a luminescent layer. The luminescent layer, referred to as the host material or layer, comprises organic substances like Alq3. The hole injection layer serves the primary function of transporting holes, while the electron injection layer (EIL) consists of an amorphous diamine film where the sole mobile carrier is electrons. OLEDs have more advantages for lighting as well as displays with the feature of light weight, flexibility, transparency, colour quality, broad size, less power consumption, and viewing angle. With the development of this modern technology of OLED, the demand of consumers is increasing, especially in the field of display devices [5].

Capitalizing on these advantages, organic electronics emerges as a preferable option for researchers investigating the subsequent technology after silicon. Consequently, researchers are actively working towards enhancing performance for optimal utilization. Various approaches are being implemented to enhance overall performance, including integrating a static aspect into material design, utilizing the vibrant properties of charge block layers in architecture, and investigating alternative materials for electrodes. Due to its large area and low power coverage; all display devices have been replaced [6,7,8]. Negi et al. [8] investigate how different layers in an OLED affect its performance. It is observed that the luminescence performance of the device and efficiency are being enhanced by the hole and electron block layer. According to the concept of Langevin recombination in organic semiconductors (OSC), Negi et al. [8] assessed that the substantial concentrations of electrons and holes in close proximity to the emission layer are also responsible for the heightened luminescence observed in multi-layered OLEDs.

These analyses emphasise the influence of various OLED layers, opening up new possibilities for further performance enhancement in these devices [7, 8]. The copper thiocyanate (CuSCN) thin film is introduced by Hwang et al. in 2022 as the HIL in an OLED device. The CuSCN thin film is widely recognized as an improvement in hole injection layer (HIL) and transport layer in diverse optoelectronic devices. This is due to its notable attributes, including high conductivity, a broad band gap, and cost-effective manufacturing [9,10,11,12,13,14].

The CuSCN layer lowers the cut-in voltage of the OLED [15]. Due to the low cut-in voltage of devices, there is better hole transportation in OLEDs. Due to its outstanding features like high hole mobility (ranging from 0.01 to 0.1 cm2/V s), optical transparency with a wide bandgap (3.5 eV), low-temperature processing needs, and strong chemical stability, CuSCN is seen as a very promising material for hole transport applications [16,17,18,19,20,21,22]. The study of the multi-layered OLED's characteristic performance and its evaluation through experimentally and modeling are reported in [23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53], where various properties and performance parameters are evaluated and extracted.

In this research paper, the effectiveness of previously reported multilayer OLEDs is assessed by incorporating a layer of CuSCN inorganic material utilizing the Silvaco Technology Computer-Aided Design (TCAD) software. CuSCN material is used as a Hole Transporting material. Using this layer, the parameters of OLED such as Current density and Luminescence power have been improved.

Architecture of the copper thiocyanate (CuSCN) based OLED device

The OLED consists of multilayer Organic semiconductor (OSC) material between two electrodes anode and a cathode. In essence, an OLED operates as a semiconductor device where the emissive layer, composed of organic materials such as QAD and Alq3, emits light when an electric voltage is applied across the electrodes. In this paper, the OLED consists of a total of 9 layers among which 5 layers are organic materials and one inorganic layer CuSCN which behaves like hole transport layer between two electrodes. The five organic semiconductor layers are m-MTDATA, NPB, Alq3, QAD and Alq3.

In this configuration, the electron and hole charge carriers initiate movement due to the influence of the applied electric field between the anode and cathode electrodes. Electrons coming out from the cathode and holes coming out from the anode both recombine in emissive player and generate excitons. Due to the unstructured light discharge, some excitons disintegrate relative to the ground state. Therefore, the electrical energy delivered between the anode and cathode produces light. The OLEDs' lifespan is a crucial factor to take into account in addition to the device’s efficiency.

In this study, we utilized the 2D atlas simulator TCAD to conduct simulations on the structure. This software provides a broad range of capabilities for simulating organic devices. Step-by-step coding can be used to explain simulation. Using the syntax of Atlas, we first define a structural mesh. A series of horizontal and vertical lines, as well as the spaces between them, can be referred to as a structure's mesh. For accurate findings and to increase the model's mathematical potency, a decent mesh is required. Following the completion of meshing, the subsequent step involves assigning the layer type to each component of the device using LiF/Al and ITO. In the last, the electrodes are specified. The structure of OLED which is simulated is shown in Fig. 1a. Table 1 show the device performance parameters used in the analysis [6, 8, 15, 44].

Fig. 1
figure 1

a Simulated OLED structure b Langevin Recombination in OLED

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Table 1 Performance parameter of proposed OLED device used during simulations
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Analysis of the copper thiocyanate (CuSCN) based OLED device

The following sub-sections offer a comprehensive explanation and evaluation of properties of electrical and optical of the OLED device.

Analysis of electrical characteristics of OLED

The characteristics of electrical and optical of OLED that are connected to voltage bias situations are predicted by the ATLAS framework simulator. After defining the device, we provided the polymer material parameters, such as the true refractive index, permittivity, lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels, band gap, density of state, carrier’s concentration, and lifetime, in order to forecast of characteristics of electrical and optical of the OLED. The electrical and luminescence characteristics of OLEDs are analyzed by the Poole–Fraenkel mobility and the Langevin recombination model. The various models used in OLEDs are explained here:

Poole–Frenkel mobility model

The luminescence characteristics and electrical parameters of OLEDs are analyzed by the Langevin recombination model and Poole–Fraenkel mobility model. At each point of mesh of the device, the variation in mobility is calculated with the help of an effective electric field. The model can be obtained from the Eq. 1 given below [8]:

$$\upmu ({\text{E}})={\upmu }_{0 }{\text{exp}}\left[-\frac{\Delta }{{\text{KT}}} + \left(\frac{\upbeta }{{\text{KT}}}-\mathrm{\alpha }\right)\sqrt{{\text{E}}}\right]$$
(1)

In Eq. 1, μ(E) denotes the mobility at electric field E. The value of μ(E) varies with the changing the value of electric field E and μ0 indicates the mobility at zero or null electric field. K denotes the Boltzmann constant (J/K) and T denotes the absolute temperature (K). Furthermore, Δ represents the activation energy under zero electric fields, while α serves as a fitting parameter in the analysis. Finally, the applied electric field is E and β is the hole Poole–Frenkel factor and it is described as [8]:

$$\upbeta ={\text{q}}\sqrt{{\text{q}}/\mathrm{\pi \varepsilon }{\upvarepsilon }_{0}}$$
(2)

Langerin’s recombination model

This model represents the recombination process of the charge carriers holes and electrons within the OLED. The model is expressed in Eq. 3 [8].

$${{\text{R}}}_{{\text{L}}}\left({\text{n}},{\text{p}}\right)={{\text{r}}}_{1}\left({\text{x}},{\text{y}},{\text{t}}\right)\left({\text{np}}-{{\text{n}}}_{{\text{i}}}^{2}\right)$$
(3)

In Eq. (3) r1 denotes the recombination rate coefficient of Langevin and ni represents the concentration of intrinsic carrier. The p represents the hole concentrations and n represents the electrons concentrations.

Moreover, the coefficient of recombination rate of Langevin is determined using the provided expression in Eq. 4 [8].

$${{\text{r}}}_{1}\left({\text{x}},{\text{y}},{\text{t}}\right)=\frac{\mathrm{q\upmu }({\text{E}})}{{\upvarepsilon }_{{\text{r}}}{\upvarepsilon }_{0}}$$
(4)

where, q denotes the charge of carrier.\({\upvarepsilon }_{{\text{r}}}\) and \({\upvarepsilon }_{0}\) represents the relativity and absolute permittivity respectively. The evaluation of the device's effectiveness involves assessing luminescence power and current density under identical operating conditions, utilizing the same materials, and maintaining a consistent physical structure. Additionally, the analysis of the OLED necessitates consideration of various parameters, including the LUMO level, HOMO level, energy gap in the OSC materials, and the work function of the used electrode.

The examined characteristics of the suggested OLED, such as current density and luminescence power in relation to the applied anode voltage, are illustrated in Eqs. (3) and (4), respectively. At an input voltage of 18 V, the maximum current density reaches 743 mA/cm2, while the luminescent power achieves a peak value of 0.137 cd/m2.

Internal analysis of OLED

The mathematical evaluation of the multilayer OLED is according to some common equations. These equations are employed together for the numerical assessment of the internal parameters of the OLED. The electric field within the device is computed through Poisson's equation, and the examination of charge carrier concentration and current density is performed using the drift–diffusion equation. The mobility of carriers is calculated by Poole–Frenkel model. Both models are discussed in this section. The equation of Poisson is very helpful in the measurement of the electric field inside the multilayer OLEDs. The application of voltage leads to the emergence of an electric field, which aligns with the distribution of charge carrier density within the device. The equation that describes this phenomenon is provided below [8]:

$${\text{E}}\left({\text{z}}+\frac{\Delta {\text{z}}}{2},{\text{t}}\right)=\mathrm{ E}\left(\mathrm{z }-\frac{\Delta {\text{z}}}{2},\mathrm{ t}\right)+{{\text{N}}}_{{\text{D}}}\left({\text{z}}\right)-{{\text{N}}}_{{\text{A}}}\left({\text{z}}\right)+\nabla {\text{z}}\frac{{\text{q}}}{\upvarepsilon }\left\{\mathrm{ p}\left({\text{z}},{\text{t}}\right)-\mathrm{ n}\left({\text{z}},{\text{t}}\right)\right\}$$
(5)

In the above Eq. (5), the width of each mesh is Δz. The unit of width is in centimetres.t is the frame time for which the measurements are done. All the calculations were done in these mesh/cells of the device. E(z, t) represents the electric field at specific mesh points, while 'q' and 'ɛ' symbolize the charge of holes and permittivity, respectively. n (z, t) and p(z, t) denote the concentrations of electrons and holes with respect to z and t. Additionally, ND(z) and NA(z) stand for the concentrations of donor and acceptor ions at a given point z within the device. Hence, the updated dielectric field is calculated for each mesh point within the device. The units for parameters such as n(z, t), p(z, t), ND(z), and NA(z) are expressed in per cubic centimetre, indicating their densities within the device.

The migration of these charge carriers in the device occurs through the processes of drift and diffusion. Due to this migration of charge carrier the changes establish in the current as well as in electric field in the device. Equation 6 is the equation to calculate the hole density [8]:

$${\text{p}}\left( {{\text{z}},{\text{t}} + \Delta {\text{t}}} \right) = {\text{p}}\left( {{\text{z}}, {\text{t}}} \right) + {\text{r}}\left( {{\text{z}}, {\text{t}}} \right){\text{n}}\left( {{\text{z}}, {\text{t}}} \right){\text{p}}\left( {{\text{z}}, {\text{t}}} \right) - \Delta {\text{t}}\left\{ {\frac{1}{{\text{q}}}\frac{{{\text{J}}_{{\text{p}}} ({\text{z}} + \left( {\Delta {\text{z}}/2} \right),{\text{t}} - {\text{J}}_{{\text{p}}} ({\text{z}} - \left( {\Delta {\text{z}}/2} \right),{\text{t}}}}{{\Delta {\text{z}}}}} \right\}$$
(6)

However, Eq. (7) is the equation to calculate the electron density [8]:

$${\text{n}}\left( {{\text{z}},{\text{t}} + \Delta {\text{t}}} \right) = {\text{n}}\left( {{\text{z}}, {\text{t}}} \right) + {\text{r}}\left( {{\text{z}}, {\text{t}}} \right){\text{n}}\left( {{\text{z}},{\text{t}}} \right){\text{p}}\left( {{\text{z}}, {\text{t}}} \right) - \Delta {\text{t}}\left\{ {\frac{1}{{\text{q}}}\frac{{{\text{J}}_{{\text{n}}} ({\text{z}} + \left( {\Delta {\text{z}}/2} \right),{\text{t}} - {\text{J}}_{{\text{n}}} ({\text{z}} - \left( {\Delta {\text{z}}/2} \right),{\text{t}}}}{{\Delta {\text{z}}}}} \right\}$$
(7)

where in both equations Δt is the time step taken for calculation of charge carrier density. Further, r(z, t) is the rate of recombination of Langevin. \({{\text{J}}}_{{\text{p}}}\) indicates the hole current density and \({{\text{J}}}_{{\text{n}}}\) indicates the electron current density. Equation (8) represents the hole current density:

$${{\text{J}}}_{{\text{p}}}\left(\mathrm{z }+ \frac{\Delta {\text{z}}}{2},\mathrm{ t}\right)={\text{q}}{\upmu }_{{\text{p}}}\left(\mathrm{z }+ \frac{\Delta {\text{z}}}{2},\mathrm{ t}\right)\frac{{\text{p}}\left({\text{z}},{\text{t}}\right)+{\text{p}}({\text{z}}+\Delta {\text{z}},{\text{t}})}{2}{\text{E}}\left(\mathrm{z }+ \frac{\Delta {\text{z}}}{2},\mathrm{ t}\right)-{\text{KT}}{\upmu }_{{\text{p}}}\left(\mathrm{z }+ \frac{\Delta {\text{z}}}{2},\mathrm{ t}\right)\frac{{\text{p}}\left({\text{z}}+\Delta {\text{z}},{\text{t}}\right)-{\text{p}}\left({\text{z}},{\text{t}}\right)}{\Delta {\text{z}}}$$
(8)

However, Eq. (9) represents for the electron density.

$${{\text{J}}}_{{\text{n}}}\left(\mathrm{z }+ \frac{\Delta {\text{z}}}{2},\mathrm{ t}\right)={\text{q}}{\upmu }_{{\text{n}}}\left(\mathrm{z }+ \frac{\Delta {\text{z}}}{2},\mathrm{ t}\right)\frac{{\text{n}}\left({\text{z}},{\text{t}}\right)+{\text{n}}({\text{z}}+\Delta {\text{z}},{\text{t}})}{2}{\text{E}}\left(\mathrm{z }+ \frac{\Delta {\text{z}}}{2},\mathrm{ t}\right)-{\text{KT}}{\upmu }_{{\text{n}}}\left(\mathrm{z }+ \frac{\Delta {\text{z}}}{2},\mathrm{ t}\right)\frac{{\text{n}}\left({\text{z}}+\Delta {\text{z}},{\text{t}}\right)-{\text{n}}({\text{z}},{\text{t}})}{\Delta {\text{z}}}$$
(9)

where \({\upmu }_{{\text{n}}}\) is the mobility of electron and \({\upmu }_{{\text{p}}}\) is the mobility of hole. The Poole–Frenkel mobility defined as:

$$\upmu \left({\text{E}}\left({\text{z}},{\text{t}}\right)\right)={\upmu }_{0}{\text{exp}}\left(\sqrt{\frac{E\left(z,t\right)}{{E}_{0}}}\right)$$
(10)

The characteristics of internal analysis of the proposed structure are shown in Fig. 2.

Fig. 2
figure 2

Characteristic plots a Electric Field, b Singlet exciton density, c Electron concentration, d Hole concentration, e Total Current density.

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Electric field in the device

An electric field is the driving force for the charged carrier which flows in the electric circuit. Without the electric field, there is no any charged carrier flow in circuit. Experimental measurements and numerical device modeling are employed to investigate the internal electric field of both single and multilayer organic light-emitting diodes. This strategy produces a detailed understanding of how a multilayer OLED works.

Singlet exciton density

The concepts of an exciton play a role of paramount significance in the physics of organic semiconductor and interaction in organic materials. Excitons are two types: single exciton and triplet excitons. The presence of an electron–hole pair with opposing spin characterised a singlet exciton. When an electron and a hole get together due to their mutual Coulomb attraction, they create a state known as exciton. The electron releases energy after recombination in order to return to the ground state. The electron undergoes an energy loss, and this lost energy is absorbed by a photon, which then carries it away, adhering to the principle of conservation of energy. The nature of the semiconductor material significantly influences the amount of energy released in this process. In Fig. 2b shows the singlet excitons in each layer of the simulated OLED device due to the flow of current.

Electron concentration

Multiplying the density of states by the probability of occupancy, denoted as f(E), yields the number of electrons per unit volume within the specified energy range dE. Thus, the integral of the entire conduction energy band is the overall electron concentration. In the Fig. 2c shows the concentration of electrons in each layer of the simulated OLED device due to the flow of current. This figure shows the peaks and troughs of electrons concentration, illustrating the different layers encountered during current flow in the device.

Hole concentration

The quantity of holes within a given energy range, denoted as dE, can be determined by multiplying the states density by the probability of occupancy per unit volume. Thus, the integral of the entire conduction energy band is the overall hole concentration.

In Fig. 2d shows the concentration of holes in each layer of the simulated OLED device due to the flow of current. This figure shows the peaks and troughs of holes concentration, illustrating the different layers encountered during current flow in the device. Amount of energy released in this process. In the Fig. 2b shows the singlet excitons in each layer of the simulated OLED device due to the flow of current.

Electron concentration

Multiplying the density of available electron states by the probability of occupancy, denoted as f(E), gives the number of electrons present per unit volume within the energy interval dE. Thus, the integral of the entire conduction energy band is the overall electron concentration. In the Fig. 2c shows the concentration of electrons in each layer of the simulated OLED device due to the flow of current. This figure shows the peaks and troughs of electrons concentration, illustrating the different layers encountered during current flow in the device.

Total current density

The amount of total current flowing through one unit of a cross-sectional area is referred to as the current density. In Fig. 2e shows the total current density in each layer of the simulated OLED device due to the flow of current. This figure shows the peaks and troughs of total current density, illustrating the different layers encountered during current flow in the device.

Results and discussion

This section entails the analysis and discussion of the internal behaviour of OLEDs. Following the incorporation of the CuSCN layer into the OLED structure, enhancements in parameters such as current density, luminescence power, and electron and hole recombination rates are observed and discussed. The extracted results are shown in Table 2.

Table 2 Performance outcomes of simulated multilayered OLED
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Rate of change of current density with anode voltage

Current density is the most important parameter of OLED. The examination of current density in relation to anode voltage is widely acknowledged as crucial for the proper functioning of OLEDs. Studies indicate that the rate of change of current density corresponds to the rate of carrier injection per unit area within the device structure. The anode voltage represents the forward-directed positive operational voltage applied between the anode and cathode of the OLED.

Once the electric voltage between the anode and cathode exceeds the knee voltage, the current density relative to the anode voltage starts to rise exponentially, as depicted in Fig. 3. In this study, the inclusion of a CuSCN layer in simulated devices leads to a decrease in the driving voltage of OLEDs. CuSCN is recognized for its function as a hole transport material, thanks to its noteworthy hole mobility falling within the range of 0.01 to 0.1 cm2/V s. The use of CuSCN as a hole injection layer (HIL) has been shown to reduce the operational voltage of manufactured OLEDs by 1.0 to 1.5 V compared to the control device [45]. Due to decreased driving voltage of the OLEDs, result in enhancement of rate of change of current density. The extracted current density of the proposed device is enhanced to 743 mA/cm2 at an anode voltage of 18 V.

Fig. 3
figure 3

Current density versus anode voltage for a Reported device b Proposed device and Luminescence power with respect to anode voltage c Reported device d Proposed device

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Rate of change of luminance power with respect to anode voltage

The effectiveness of optical devices is significantly influenced by luminance power. This segment offers a brief summary of how luminance power relates to the anode voltage in the suggested OLED. In this context, optical power refers to the brightness or intensity of light emitted from the surface of the devices. The luminescent power is influenced by factors such as the types of materials used, the number of excitons present, and the applied voltage on the OLED. Moreover, light frequency also affects the luminescent power [15].

In Fig. 3, the graph illustrates the relationship between luminescence power and applied voltage. Examining the device depicted in Fig. 1a, there is a consistent improvement in luminance power from anode voltages of 0V to 18V. Beyond 18V, the luminance power reaches a plateau, indicating a constant level. Notably, the effectiveness of the anode voltage is observed in the range of 6V to 18V for the proposed device. This suggests that the device performs optimally within this voltage range. Consequently, it can be concluded that the proposed structure exhibits superior performance with respect to the work of Negi et al. [8]. The heightened brightness illustrated in the graph shown in Fig. 2b arises from the inclusion of several electron and hole blocking layers, each having an optimal thickness, along with a thinner emissive layer. This enhancement positively impacts the luminescent output of the OLED. The extra HBL directly speeds up the electron–hole recombination rate within the emissive layer [13]. Relative to the control device, the introduction of the CuSCN layer lowers the operational voltage of the OLEDs under consideration. With a decrease in the applied voltage, the recombination of electrons and holes becomes more pronounced within the emission layer, resulting in an increase in exciton density. Consequently, with the escalating exciton density, the luminance power also experiences a corresponding increase.

Conclusion

OLED is an organic material-based diode, widely used in display applications in mobile and television at lower cost and light weight. The commonly used electrical parameters of OLED are current density, luminescent power, and electron (hole) recombination rate etc. This research work investigated the enhancement in electrical performance of multilayered OLED device by incorporating a Copper Thiocyanate (CuSCN) interfacial layer, as the hole injection and transport layer. CuSCN material offered a distinct advantage in OLED performance that resulted in lowered the drive voltage. It has also increased luminescence of multilayered OLEDs due to augmenting the concentrations of electrons and holes near the emission layer. It was also verified by Langevin recombination principal. The entire device behaviour is analyzed using the Silvaco Atlas tools, where, the parameters of device, including luminescent and current density are extracted. This research is precisely applicable for low-cost lighting. The significant improvement in OLED performance is intended the application in display devices.