Performance improvement for organic light emitting diodes by changing the position of mixed-interlayer

Abstract

Organic Light-Emitting Diode (OLED) is presently the most sought-after display technology. It provides low-cost, flexible, rollable displays in addition to wide viewing angles and excellent colour qualities. Still, the organic displays have not reached at their best performance and there is a lot of scope for improvement in their performance. In addition to the injection layer, emission layer, transport layer, etc, researchers are looking forward to the charge carrier transport layer, spacer layer, mixed interlayer, etc. to further enhance the device performance. In this article, a depth analysis related to the impact of the position of the mixed interlayer is performed to analyze the impact on device performance. It is observed that on shifting mixed interlayer (MI) towards the cathode; luminescence and current density depict depreciation. However, on shifting MI towards anode there is a significant performance improvement. The complete analysis includes seven device structures, wherein the position of MI is varied. The best performing device depicts luminescence of 17139 cd/m² and a current density of 84.6 mA/cm², which is 40.05% higher for luminescence and 111.5% for current density than that of reference device. Additionally, the internal analysis of device structure is thoroughly evaluated using the cut line method to better understand the internal device physics in terms of the electric field, electron concentration, total current density, Langevin’s recombination rate, and Singlet exciton density.

Keywords

Organic light-emitting diode (OLED)Electron block layer (EBL)Hole block layer (HBL)Mixed-interlayer (MI)Current Density (CD)Luminance

1 Introduction

Organic electronics was developed in the 1950s, with the discovery of the first organic molecule by H. Inokuchi and his colleagues. From this discovery, it was gathered that organic molecules could be semiconductors [1], a term generally designated for silicon, germanium, and other elements. This technology created new research areas by offering an alternative medium for the construction of lightweight [2], flexible [3], large-area [4], wider colour range [5], and economically feasible electronic devices [6]. The major applications of organic electronics include; displays devices [7, 8], wearable fabrics [9], microsensors, memory [6], biosensors [5, 10], Visual Light Communication (VLC) [11], Organic Light Emitting Diode (OLED) [4, 5], and the construction of new power generation circuits.

The Organic Light Emitting Diode is a light-transmitting technology that is flat and slim. This is accomplished by sandwiching an organic material thin film between two conducting materials. OLED in combination with OTFTs are used as displays [12] and possess a multi-purpose quality. These are efficient and contains a clear and high image quality. Adequate flexibility, low power consumption [13], good luminescence [14], viewing angle [4], low-cost fabrication, and thin dimension, are some of the attractive features of the OLEDs. They appear to be the advanced technology [15] for all types of displays. It does have some disadvantages including; a limited lifespan, manufacturing issues, a large energy gap, and color stability.

Numerous researchers are working to improve the results pertaining to OLED. To enhance the performance of OLEDs, different methods have been studied, and the researchers have used many techniques, including modifying the architectural design of OLEDs [4]. Several types of layers [4, 16], such as, transport layer [17], injection layer, EBL (electron block layer), HBL (hole block layer) [4], DBL (double block layer), SL (spacer layers) [15], and MI (mixed-interlayer) [18] are used to significantly improve the charge carrier transport, charge carrier concentration in the emissive layer, owing to improve recombination rate and thereby its efficiency. Mixed-interlayer is used because improved efficiency is attributed to the controllable mixed interlayer, which has enhanced charge carrier transport, excitons transfer, and increased the harvestings of singlet as well as triplet excitons [18]. Gao et al. [18] proposed the F–I–P–I–F structure for fluorescence blue interlayer phosphorescence complementary color(s)-interlayer-fluorescence blue structure, which separates the fluorescent and phosphorescent materials by interlayers [18].

In this paper, the impact on the position of mixed interlayer (MI) is analyzed. CBP (hole-transporting material) and TPBi (electron-transporting material) [18] are selected as mixed host materials with the ratio of (3:2) in an emitting interlayer and located in various positions within the EML. On shifting mixed-interlayer towards the cathode, the recombination rate decreases and thus current density too. However, the case is reversed on shifting mixed-interlayer towards the anode. This paper also includes an internal analysis of Mixed-interlayer OLED. Internal device analysis is performed to examine the different internal processes associated with different layers in the structure. Additionally, it aids in understanding of the device’s physics and behavior. These analyses are demonstrated to validate the improved performance of Mixed-Interlayer OLEDs. The method of Internal analysis is carried out by drawing a cut line at the centre of Mixed-Interlayer OLED to examine the role of different layers for generating electric field, electron concentration and total current density [23].

2 Simulation setup and device structure

In this work, fabricated devices are analyzed with respect to the change in position of the mixed interlayer thus enhancing the efficiency of OLED. The position of mixed-interlayers is shifted towards the two electrodes; anode (positive side) and cathode (negative side) one by one and thus effect on the performance of OLED is observed. Primarily, a fabricated reported device is simulated using the Silvaco (Atlas 2019) tool [19]. The Silvaco ATLAS tool is a 2-dimensional numerical simulator used to analyses organic material based devices. Two models; Poole–Frenkel (PF) mobility model and Langevin’s recombination are generally used for OLED. These are used to determine the electrical and luminescence characteristics of the organic LED respectively [19].

2.1 Poole frenkel mobility model

It examines the characteristics of organic devices using the Poole and Frenkel mobility model (ATLAS 2019). The model can be described using given equation (i) as [19]: $μ (E) = μ_{0} exp ⌈ - \frac{Δ}{KT} + (\frac{β}{KT} - α) \sqrt{E} ⌉$ (1)

In this, field-dependent mobility is denoted by μ (E), which varies in accordance with the applied electric field. Zero/null-field mobility is denoted by μ₀ and electric field by E while, Δ represent as activation energy at the null field. Further, T symbolizes for temperature and K for Boltzmann constant (J/K). Here α denotes the curve fitting-parameter. Hole Poole-Frenkel factor; β can be understood as. $β = q \sqrt{q / π ɛ ɛ_{0}}$ (2)

2.2 Langevin’s recombination model

This model relates the recombination of electrons and holes inside OLED. To investigate carrier recombination and excitons formation in OLEDs, this model holds the relation as [16]. $R_{L} (n, p) = r_{1} (x, y, t) (np - n_{1}^{2})$ (3)

Where, r₁ denotes Langevin’s recombination rate coefficient, p and n represents hole and electron concentration respectively and n_i is the intrinsic carrier concentration. The Langevin’s recombination rate coefficient [19] can be defined as follows: $r_{1} (x, y, t) = \frac{q μ (E)}{ɛ_{r} ɛ_{0}}$ (4)

Here, a charge of the electron is denoted by q, ɛ_r/ɛ₀ is represented as a relative/ absolute permittivity.

Figure 1(a) illustrates the stacked OLED architecture representing different layers, associated materials and device dimensions, and Fig. 1(b) represents the simulated OLED structure. Here, anode is of Indium tin oxide (ITO) material (on a glass substrate) and cathode is of Aluminum (Al). Further, Molybdenum trioxide (MoO₃) is a p-type doping material which is used for both HIL (hole injection layer) and HTL (hole-transporting layer) [20]. The lithium-doped TPBi (2,2^′,2^″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)) layer acts as electron transport layer and injection layer. The TCTA (tris(4-carbazoyl-9-ylphenyl) amine) is used for HIL (hole injection layer) and EBL (exciton-blocking layer). Intrinsic TPBi acts as HBL (self-hole-block layer) as well as a self-triplet exciton-block layer. There are two materials used for host and interlayer spacer; one is CBP (4,4^′-Bis(N-carbazolyl)-1,1^′-biphenyl) and the other is TPBi. Additionally, two dopant materials are used; one is BCzVBi (4,4 -Bis (9-ethyl-3- carbazovinylene)-1,1 -biphenyl) for blue fluorescent dopant, and other is PO-01(iridium (III)bis(4-phenylthieno[3,2-c] pyridinato-N, C2) acetylacetonate) for yellow phosphorescent dopant. CBP is hole predominated materials and TPBi is electron predominated materials (mixing ratio 3:2) that has been used as mixed-interlayer [18]. This experimental device is taken as a reference device for further analysis. It was fabricated and analyzed by Gao et al. [18]. Hereafter, this device is known as Device A.

Fig. 1

(a) Mixed-interlayer OLED stacked structure and (b) Simulated device structure of mixed-interlayer OLED.

Device A is simulated using the Silvaco Atlas tool. The current density versus anode voltage graph for Device A are shown in Fig. 2 (a) and (b) respectively for experiment and simulated results. It is observed that simulated results are close to the reported experimental results, with the recorded current density of 40 mA/cm².

Fig. 2

Comparison of current density vs. anode voltage characteristics for Device A (a) Experimental results and (b) Simulated results with similar parameters as used in experimental conditions.

The applicability of an OLED in various fields depends on several parameters, such as; luminance, luminescent power, Langevin’s recombination rate, etc. The analysis of these parameters is useful to improve the performance of OLED. These parameters were not reported for the experimental device. Herein the luminescent power and luminance are also analyzed for Device A as depicted in Fig. 3 (a) and (b) respectively. The maximum value of these parameters is measured to be 50.8 W/μm and 12000 cd/m² respectively. The band diagram of this device is shown in Fig. 4, which highlights the HOMO (Highest Occupied Molecular Level) and LUMO (Lowest Unoccupied Molecular Orbital) of all the groups of materials utilized in the device [25].

Fig. 3

(a) Luminescent power vs. anode voltage and (b) Luminance vs. anode voltage graph for Device A.

Fig. 4

Band diagram of mixed-interlayer OLED.

2.3 Cutline and internal structure analysis of mixed-interlayer OLED Device A

The analysis of characteristic parameters of the OLED is followed by examining the internal device parameters. Cutline analysis methodology of Silvaco Atlas tool [21 –23] is used for this purpose. In this method, a cutline is drawn along the length of the device (through various layers) and variation of different parameters is observed throughout these layers [22]. This analysis is helpful as the impact of different layers on internal device parameters is evident.

Therefore, one can enhance the device’s performance by changing these layers and/or their position. Herein, Langevin’s recombination rate and as well as singlet exciton density are analyzed for Device A. This analysis is undertaken to highlight the reasons for the outstanding performance of the device. The results pertaining to these parameters are shown in Fig. 5 (a) and (b) respectively for Langevin’s recombination rate and singlet exciton density. A high recombination rate is observed in the device with a value of 14.5/cm³. Additionally, a high singlet exciton density is observed in the same region. However, the maximum Langevin’s recombination is observed at 0.25μm, i.e., near the electron block layer (TCTA) and hole transport layer (TCTA: MoO₃) [30 –34]. Since recombination is not observed within the emission layer, therefore, luminescence performance can be improved for this device. Thereafter, electric field, electron concentration, and total current density are also examined for this device. Results for these parameters are illustrated in Fig. 6.

Fig. 5

Cutline analysis of Device A; (a) Langvin Recombination Rate and (b) Singlet Exicton Density.

Fig. 6

Variation in (a) Electric field, (b) Electron concentration and (c) Total current density for Device A.

Figure 6 depicts a high electric field at 0.110μm, which is related to the injection barrier at the electrode-OSC interface. Thereafter, there is a rise in the electric field from 0.160μm to 0.260μm. This shows that the electric field is rising near the emission layer and continues till the electron block layer. Similar results can be observed for electron concentration, wherein the reported highest value is within the device at 0.160μm and 0.260μm. Thus, there is a high electron concentration in the emission and the electron block layers. However, recombination is taking place at the electron block layer. The only reason for this can be attributed to the electrons that are reaching at electron block layer before the holes reaching to the emission layer. Therefore, maximum recombination is taking place at the electron block layer. The present work tries to solve this problem by changing the position of MI (mixed interlayer). The Current density is constant throughout the device as depicted in Fig. 6 (c).

2.4 Impact of changing the position of mixed-interlayer in the OLED structure

A Mixed interlayer of hole and electron predominant materials is an effective methodology to enable the movement of the necessary charge carriers required for recombination. The analysis related to the mixed interlayer includes the effect of changing the ratio of MI for improving the device performance. Gao et al. [18] in their work illustrated that changing the ratio of MI; CBP: TPBi can alter the performance of organic LEDs. The best device performance for F-MI-P (Fluorescence-MI-Phosphorescence) structure was observed for CBP: TPBi ratio of 2:3 i.e. Device A in Fig. 1 (a). However, it is believed that not only the ratio of hole predominant material to electron predominant material of the mixed interlayer affects the device performance. The position where the MI layer is placed also has an impact on the device performance.

Therefore, in the present work, the impact of changing the position of the mixed layer on the performance of OLED is analyzed. The MI is shifted towards the cathode first and thereafter towards the anode. For the complete analysis, seven devices are examined as shown in Fig. 7. The figure depicts the structure of each device highlighting the change in position of the mixed interlayer. First, the MI is shifted towards cathode as illustrated in Fig. 7 (a)–(c). Thereafter the position of MI is shifted in the direction of the anode; Fig. (d)–(g). However, in all these structures, the ratio of mixed interlayer used is CBP: TPBi (2:3) is similar to the best performing device illustrated by Gao et al. [18]. These devices are named Device B–H. On shifting mixed-interlayer towards the cathode, the recombination rate is decreasing and thereby current density too as charge carriers are not reaching properly to the emissive layer, Hence reduces the performance of the devices (a)–(d). However, on shifting mixed-interlayer towards the anode side, the recombination rate is increasing and a subsequent increase in the current density is observed as the charge carriers are reaching properly in the emissive layer. Hence improving the performance of the devices (f)–(h).

Fig. 7

Block diagram for Device B–H obtained by shifting position of mixed interlayer; (a) Device B, (b) Device C, (c) Device D, (d) Device E, (e) Device F, (f) Device G, (g) Device H.

3 Result and discussion

The devices shown in Fig. 7 are analyzed and their performance parameters: current density and luminance are observed. For ease of comparison, these parameters are plotted in a combined graph as shown in Fig. 8.

Fig. 8

Combine graph for (a) Current Density vs. anode voltage and (b) Luminance vs. anode voltage with changing the position of the mixed interlayer in Device B–H.

It is observed that when the mixed interlayer is shifted towards the cathode side (Device B to Device D), then there is a decrease in both current density as well as the luminance of these devices. On the contrary, when the MI is shifted towards the anode (Device E to Device H), these devices demonstrate an increment in the performance parameters. However, in both the cases, it is observed that, as the MI layer moves closer to the electrode, the general trend is depreciation in the performance. This is evident from the poor results for Device D and Device H, wherein the MI layer is close to cathode and anode, respectively. All these observations are also clear from Table 1, wherein, current density and luminance of these devices are tabulated. Additionally, luminescence power efficiency is also tabulated for these devices based on (5) [35] as: $η_{p} = L π / JV$ (5)

Table 1

Comparison of Performance parameter on changing the position of mixed-interlayer

Parameter	Current Density	Luminance	Luminescence Power
(at 5.32Volt).	(mA/cm²)	(cd/m²)	Efficiency (lm/W)
Device A.	40	12237	18.05
Device B.	32.5	9596	17.42
Device C	34.8	10426	17.6
Device D	29.81	8674	17.1
Device E	33	6408	11.4
Device F	84.6	17139	12.3
Device G	60.6	12195	11.8
Device H	61.1	6825	6.5

In the above equation, L is the luminescence, J is the current density and V is the operating voltage of the device. The highest value of current density, as well as luminance, is observed for Device F with the values of 84.6 mA/cm² and 17,139 cd/m² respectively. Internal device analysis is also performed for all these devices. The analysis aids in a thorough understanding of the device’s physics and behavior. In the present examination, Langevin’s recombination rate, as well as singlet exciton density, has been analyzed. The data in this respect is depicted in Fig. 9 for Device E, F, and H. It is observed that with the change in position of the mixed interlayer, the recombination region does get affected [34].

Fig. 9

Langevin’s recombination rate and singlet exciton density for (a) & (b) Device E, (c) & (d) Device F and (e) & (f) Device H.

Langevin’s exciton rate has not been affected much in all three devices, even though it has shifted slightly towards the emission layer for Device F. However, on the other hand, there is a major shift in singlet exciton density for Device F. This positional shift in excitons towards the emission layer is a major reason for good luminescence performance of the device.

The internal analysis also includes examining the electric field and electron concentration for these devices. The results for these findings are illustrated in Fig. 10. These results are very similar to the previous devices which depicted a high electric field near 0.10μm, owing to the injection barrier. Thereafter, over a range of 0.21–0.09μm there is a continuous increment in the electric field. The devices that perform better such as Device A and Device F have high values much closer to 0.22μm as compared to device which does not perform too well; Device D and H.

Fig. 10

Variation in combine devices (A–G) electric field and electron concentration Device a and Device b.

Almost similar results are observed for electron concentration as highlighted in Fig. 10 (b). A high electron concentration is observed near to 0.10μm due to the lower injection barrier and thereafter it varies throughout the device. For Device A and F, electron concentration is high near 0.22μm and thereafter it falls rapidly. This suggests that for these devices, recombination is occurring much closer to the EML layer as compared to Device D and H. For these latter devices, the recombination zone is somewhere near 0.27–0.29μm. These results reaffirm the belief that by changing the position of mixed interlayer (MI) device performance can be enhanced [32].

Data pertaining to these parameters are tabulated in Table 2. The data in the table shows that even though there is not a significant variation related to the values of these different parameters still the performance varies by a large margin. This signifies the importance of recombination region as well as exciton generation within the vicinity of emission layer as earlier suggested by Negi et al. [24] as well.

Table 2

Internal structure analysis of mixed-interlayer through cutline all devices

Parameter	Recombination	Singlet Exciton	Electron Current
	Rate (cm^–3s)	Density (cm^–3)	Density (Acm^–2)
Device A	14.5	1.9e^–08	0.0188
Device B	14.1	1.9e^–08	0.0153
Device C	14.3	1.92e^–08	0.0163
Device D	13.9	1.86e^–08	0.014
Device E	14.6	1.96e^–08	0.0156
Device F	13.8	1.95e^–08	0.0284
Device G	13.1	2.04e^–08	0.0285
Device H	14.7	1.76e^–08	0.0286

4 Conclusion

In this work, the impact of changing the position of mixed interlayer is investigated. This mixed-interlayer structure position variation is introduced and output is evaluated by adjusting the mixing ratio (3:2) of predominated materials hole (CBP) and electron (TPBi). On the basis of position of mixed-interlayer total, seven devices are analyzed (Device A to Device G), wherein the MI layer is shifted towards cathode side (Device B–D) first and then towards the anode side (Device E–H). Shifting towards cathode side illustrated poor performance in terms of current density and luminance. On the contrary, the performance od OLED is much improved while MI layer is shifted towards the anode. Additionally, internal device parameters, such as; electric field, electron concentration, total current density, Langvin’s recombination rate are analyzed to validate the performance dependence of OLED onto the position of MI layer. The obtained extracted results showed the highest current density value of 84.6 mA/cm² and luminance value of 17139 cd/m² for Device F. Subsequently, the performance enhancement is recorded as 111.5% and 40.5% higher as compared to reference device; Device A. Therefore, this device can be proven more useful for displays and light panel applications as light detectors as well as Bio-medical applications.

Funding statement

This research work did not receive a financial support.

Conflict of interest

The authors declare that there is no conflict of interest regarding the publication of this paper.

Author contribution

Authors have made substantial contributions to the conception and design or acquisition of data, analysis, and interpretation of data; have been involved in drafting the manuscript or revising it critically for important intellectual content; and have given final approval of the version to be published. Author has participated sufficiently in the work to take public responsibility for appropriate portions of the content. Author read and approved the final manuscript.

Availability of data and material

The data and material are available within the manuscript.

Compliance with ethical standards

The author declares that all procedures followed were in accordance with the ethical standards.

Consent to participate

Author declares their consent to participate in this research article.

Consent for publication

Author declares their consent for publication of the article on acceptance.

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