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Studying the change in organic light-emitting diode performance at various vacuum-deposition rates of hole and electron transport layers

AMIR MIKAEILI and EZEDDIN MOHAJERANI*

Laser and Plasma Research Institute, Shahid Beheshti University, Tehran 1983963113, Iran

*Author for correspondence (e-mohajerani@sbu.ac.ir) MS received 4 December 2019; accepted 29 June 2020

Abstract. The electroluminescence (EL) of classic and thermally activated delayed fluorescence (TADF) organic light- emitting diodes (OLEDs) at various vacuum-deposition rates of hole and electron transport layer (HTL and ETL) has been studied. The external quantum efficiency (EQE) measurements showed that the best performance devices were those with a high charge carrier balance inside the emitting layer, which was engineered using hole and electron current manipulation as a result of vacuum-deposition rate control. Changing the vacuum-deposition rate of HTL and ETL leads to a change in the maximum EQE (EQEmax) of the classic and TADF OLEDs without obvious changes in EQE roll-off ratio at high current density. We used a simple analytical model to clarify that the enhanced hole current in HTL at high deposition rates is dominated by high hole mobility attributed to the increased hole hopping rate due to the reduction of the intermolecular separation between horizontally oriented N,N0-diphenyl-N,N0-bis(1-naphthyl)-1,10-biphenyl-4,40-diamine (a-NPD) molecules. The increase in the electron current of tris-(8-hydroxyquinoline) aluminium (Alq3) ETL at low deposition rate was ascribed to high electron injection from cathode into ETL by the fabrication and comparison of J–Vcharacteristic of two electron-only devices with a difference at deposition rate of ETL near cathode interface. Finally, we introduced an OLED with novel gradient and barrier structures for the emitting layer in which high injected charge carriers recombined inside added recombination zone to raise radiative recombination and efficiency of the device. Our results demonstrated that EL efficiency of an OLED can be changed by controlling the vacuum-deposition rate of organic layers.

Keywords. Novel gradient and barrier structures; controlling the vacuum-deposition rate; electroluminescence (EL) efficiency; charge hopping rate; charge recombination zone.

1. Introduction

Besides the unique properties of organic semiconductors, such as ability of determining chemical structure manipu- lation, the potential for simplicity and safety in flexible thin film fabrication as well as low-cost processes, small mole- cule organic semiconductors, have provided a novel exclusive feature of manipulation of a layer structure during the vacuum-deposition process [1–10]. Charge hopping rate inside the layer and charge injection at the organic/organic and organic/metal interface can, in principle, be affected markedly as a result of changes in electrical performance of the layer [9–18].

Control of molecular arrangement and structure of layers during vacuum-deposition, which is an obvious merit of this thin film making process as opposed to others such as solution-based methods, is of intense interest [8,9,18–21].

The structural and electrical characteristics of the layer can be engineered. The understanding of physical mechanisms related to charge carrier transport and injection through an organic layer can be developed as well.

Despite comprehensive research on the effect of vacuum- deposition rate on organic layer characteristics which have been done during the last decade, attempts to comprehend the mechanisms governing on charge transport and injection seem to be insufficient [8,9,18–21]. Recently, the key role of molecular orientation in electrical performance ofN,N0- diphenyl-N,N0-bis(1-naphthyl)-1,10-biphenyl-4,40-diamine (a-NPD) thin films vacuum-deposited at various deposition rates has been demonstrated [22]. The a-NPD molecules were horizontally oriented relative to the substrate plane at high deposition rate which resulted in increasing the hole current. However, the mechanism of improving hole transport inside the layer was unclear. Regarding the

Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s12034-020-02189-1) contains supple- mentary material, which is available to authorized users.

https://doi.org/10.1007/s12034-020-02189-1

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electrons, tris-(8-hydroxyquinoline) aluminium (Alq3) as an electron transport layer (ETL) showed higher electron cur- rent at lower deposition rates, which was attributed to the reduced surface roughness at the Alq3/cathode interface [22]. We speculated that smooth interface between Alq3 layer and cathode facilitates electron injection.

Also, there have been several papers endeavouring to clarify the organic light-emitting diode (OLED) perfor- mance at various organic layer(s) deposition rates by studying the changes in OLED characteristics [8,9,18–20].

Besides the charge carrier injection from electrodes to organic layers and charge carrier transport through organic layers towards emitting layer, the vacuum-deposition rate of emitting layer can affect charge carrier recombination zone, exciton production and annihilation, suppression of effi- ciency roll-off at high current densities, which are of intense interested parameters, particularly with regard to thermally activated delayed fluorescence (TADF) OLEDs [23–30].

Indeed, general comprehension of physical processes and performance of OLEDs with different structures associated with organic layer(s) vacuum-deposition rate still needs to be rendered.

In this study, a classic structure OLED with Alq3 as a classic fluorescent emitting layer and a TADF OLED structure were considered for evaluating their electrolumi- nescent characteristics at various vacuum-deposition rates of charge carrier transport and emitting layers. Due to the decomposition of TADF emitters at high vacuum-deposi- tion rates, the effect of emitting layer deposition rate on OLED performance was studied only in the classic struc- ture. Moreover, evaluating the TADF OLED performance at various deposition rates can be of intense interest. Next, our study focused on the electrical properties of organic layers at different vacuum-deposition rates to comprehend the physical mechanisms of changes in hole and electron injection, transport and recombination. Regarding the a- NPD hole transport layer (HTL), we tried to simulate the effect of molecular orientation on hole hopping rate by using non-adiabatic Marcus–Hush theory for the inter- molecular hole transfer rate and a simple geometrical model. Also, we demonstrated the facilitation of electron injection from cathode to the Alq3ETL by depositing a thin smooth Alq3layer at the interface of Alq3/cathode. Finally, we used these findings to introduce new structures for emitting layer in which we made an additional recombi- nation zone in order to produce more excitons and increase the efficiency of OLED.

2. Experimental

The classic and TADF OLED structures are shown in figure1a and b, respectively. The devices were fabricated using a vacuum-deposition system for depositing all organic and cathode layers under a pressure of 104Pa onto indium tin oxide (ITO)-coated glass substrates. ITO and silicon

substrate cleaning process were commenced by using an ultrasonic bath which was sequentially accompanied by pure water, acetone and isopropanol, respectively. The process was carried out by exposing ultraviolet light and ozone treatment.

a-NPD as the HTL and Alq3as the ETL were vacuum- deposited at three different deposition rates (0.01, 0.1 and 1 nm s1). The vacuum-deposition rates of 4,40-bis(N-car- bazolyl)-1,10-biphenyl (CBP):2,4,5,6-tetra(9H-carbazol- 9-yl)isophthalonitrile (4CzIPN) (6% by weight) and Alq3: 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)- 4H-pyran (DCM) (1% by weight) were set at about 0:16 and 1 nm s1, respectively. Other organic layers and LiF/alu- minium cathode were vacuum-deposited at deposition rates of 0.1, 0.01 and\0:33 nm s1, respectively.

In order to investigate the influence of smooth contact area at the Alq3/cathode interface forming at low vacuum- deposition rate of Alq3, on electron injection, we fabricated electron-only devices (EODs) with a structure of ITO anode (100 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) hole-blocking layer (20 nm)/Alq3 (200 nm)/LiF/Al cathode (0.5 nm/100 nm). The schematic structures of EODs at the organic/metal interface are shown in the inset of figure6b. Also, the schematics of gradient and barrier structures for Alq3, electron transport-emitting layer of classic OLEDs are depicted in figure7. These new struc- tures were fabricated using vacuum-deposition rate and shutter, in which the deposition rate at the interface of Alq3/cathode and Alq3/a-NPD was minimum, whereas it was maximum inside the Alq3layer. In addition, the vac- uum-deposition rate ofa-NPD HTL was 0:1 nm s1. So, the electron current near the Alq3/cathode and Alq3/a-NPD interface was maximum, while it was minimum inside the Alq3 layer [9,31]. Therefore, we named this a gradient structure. Additionally, we increased the thickness of low electron current region (deposited using high vacuum- deposition rate) in the barrier structure.

After fabrication, in order to protect thin layers against oxygen and moisture, the devices were immediately encapsulated with glass lids using epoxy glue in a nitrogen- filled glove box (O2\0:1 ppm and H2O\0:1 ppm) without exposure to air.

The J–V–luminance characteristics of OLEDs and J–V characteristics of EODs were evaluated using a computer- controlled Keithley 2400 source meter and an absolute external quantum efficiency (EQE) measurement system (C9920-12, Hamamatsu Photonics, Japan). In addition, an Ocean Optics 4000 spectrometer was implemented to measure the electroluminescence (EL) spectra.

Also, Alq3neat films with a thickness of about 100 nm were vacuum-deposited on clean silicon substrates at 0.01 and 0.1 nm s1. Atomic force microscope (AFM) images and the surface roughness of Alq3neat films were measured using a SPA400 atomic force microscope (AFM) (Seiko Instrument).

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3. Results and discussion

3.1 Classic and TADF OLED performances

The detailed classic and TADF OLED performances are depicted in figures2and3and tables1and2, respectively.

The hole–electron recombination zone and emitting region of the classic structure are pinned near the interface ofa-NPD (HTL) and Alq3 (ETL-EL) due to low hole and electron mobility in ETL and HTL, respectively [9,32,33], whereas in the TADF structure, 10 nm T2T blocks holes to penetrate into Alq3layer and recombination zone is inside the CBP:4CzIPN layer. Regarding classic OLEDs, the structures in which a-NPD (HTL) and Alq3(ETL-EL) vacuum-deposition rates were 0.01:0.1 nm s1 and 0.1:0.01 nm s1 had the best performance (EQEmax= 1.09 and 1.08%, respectively). The change in EQE of OLED can be originated from the change in: I – the light out-coupling efficiency (gout), II – the charge balance factor (geh), III – the singlet and triplet exciton generation efficiency (gexciton) and IV – the photolumines- cence (PL) quantum yield (gPL), according to equation EQE¼goutgehgexcitongPL[9,34,35]. The observed changes in EQE of OLEDs at different vacuum-deposition rates of HTL, ETL and EL are assumed to be independent of light out-coupling factor, exciton generation efficiency and PL quantum yield [9]. However, it is demonstrated that organic layer characteristics can be changed at various vac- uum-deposition rates resulting in charge carrier mobility changes [9,15,31]. Concerning a-NPD, hole mobility increases at high vacuum-deposition rates [9]. Recently, we showed that the increase in hole mobility ofa-NPD layer at high deposition rate can be attributed to horizontally oriented molecules. Furthermore, reduced surface roughness of Alq3

layer can facilitate electron injection from cathode and cau- ses high electron current [22]. Also, in the classic structure OLED at equala-NPD:Alq3deposition rate (0.1:0.1 nm s1), the hole is the majority carrier [36,37]. So, in this condition charge carrier imbalance could reduce EQE. Accordingly, the increase in EQE ata-NPD:Alq3vacuum-deposition rates of 0.01:0.1 nm s1and 0.1:0.01 nm s1 can be raised from higher charge carrier balance at the recombination zone.

Moreover, EL peak remained unchanged at different vac- uum-deposition rates because of no change in recombination zone position. Also, there was no meaningful change in EQE roll-off ratio at various HTL, ETL and EL deposition rates.

Similarly, TADF OLEDs with a-NPD as hole-transport and Alq3as electron transport layers fabricated at vacuum- deposition rates of 0.01:0.1 and 0.1:0.01 nm s1, respec- tively, showed the best efficiencies (EQE = 11.5 and 12.0%

at 10 mA cm2and EQE = 7.1 and 7.1% at 100 mA cm2).

Additionally, the latter showed the best maximum EQE.

Interestingly, it seemed that HTL and effect on ETL vacuum-deposition rate has no effect on the EL peak;

however, as it can be seen from figure3d, the EQE roll-off ratio was the minimum for the mentioned structures.

Although the change in the EQE of OLEDs was small, especially concerning TADF OLEDs probably because of the unchanged vacuum-deposition rate of the emitting layer, it will increase noticeably by using novel gradient and barrier structures as we will show later.

3.2 Origin of charge carrier mobility changes

Recently, we have demonstrated that current density of a-NPD hole-only devices (HODs) increases at high Figure 1. Schematic structure of (a) classic and (b) TADF OLEDs and energy diagrams of organic layers.

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vacuum-deposition rates, whilst current density associated with Alq3 EODs decreases. Our studies showed that the increase in hole current of a-NPD (HODs) was due to a horizontal orientation of a-NPD molecules relative to a substrate plane at high vacuum-deposition rates [22].

However, the mechanism of current increase remained unclear. In this study, to gain insight into the observed increase of hole current, the current density–electric field (J–E) of HODs was analysed using an injection-limited current model, which is given as follows [17,38–40]:

J¼4N0w2eEexp eUB

kBT

expf0:5

lð Þ0 expcE0:5

;

ð1Þ whereN0 is the density of chargeable sites in organic thin film, e is the elementary charge, UB is the injection barrier height, kB is the Boltzman constant, T is the temperature, lð Þ0 is the zero field mobility and c is the Poole–Frenkel factor. Also, f is the reduced electric field which is defined as f ¼e3E=4pe0erkB2T2, where e0 and er

are the vacuum and relative permittivity, respectively.

Furthermore, w can be defined as a function of f: w¼f1þf0:5f11þ2f0:50:5

:

Fitting was carried out with zero field mobility lð Þ0 and Poole–Frenkel factor c as variables in the high-voltage region where we can disregard the influence of the bend (see the fitting details and supplementary figure S1).

Figure4 shows zero field mobility lð Þ0 and Poole–

Frenkel factor cof a-NPD thin films fabricated at various vacuum-deposition rates and they were obtained by fitting the J–E characteristics of a-NPD HODs reported in ref.

[22]. The results reveal that zero field mobility lð Þ0 increases with vacuum-deposition rate, whilst Poole–Fren- kel factor cdecreases.

In order to elucidate the change in mobility parameters, we presented a simple analytical model to describe hopping conductivity and mobility changes in a-NPD thin films at different deposition rates. Our model is based on non-adi- abatic Marcus–Hush theory for estimating a-NPD inter- molecular hole hopping rate,k, which is defined as follows [41–43]:

ki;j¼2p

h Ji;j2 1 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4pkkBT

p exp DGi;jk2

4kkBT

( )

; ð2Þ

where theJi;j2 is the electronic coupling transfer integral,kis the molecular reorganization energy, i.e., the free energy which is needed to completely reorganize the local atomic Figure 2. Device performance of classic OLEDs: (a) current density–voltage–luminance (J–V–L) characteristics, (b) EQEvs.current density (EQE–J), (c) EL spectra and related photo of the resulting OLED and (d) EQE, drive voltage and EQE roll-off ratio vs.a- NPD:Alq3deposition rate at different current densities and brightness.

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reorganization of one state to the another state without charge transfer which is equal to half the polaron binding energy, kB is the Boltzmann’s constant, T is the tempera- ture, andDGi;j is the difference of free energy between the initial and final sites.

For two sites with a separation distance,Dri;j, inside the applied electric field,E, the free energy difference between them is given by [43]

DGi;j¼eDri;jE; ð3Þ

Figure 3. Device performance of TADF OLEDs: (a) current density–voltage–luminance (J–V–L) characteristics, (b) EQEvs.current density (EQE–J), (c) EL spectra and related photo of the resulting OLED and (d) EQE, drive voltage and EQE roll-off ratio vs.a- NPD:Alq3deposition rate at different current densities and brightness.

Table 1. Detailed device performance of classic OLEDs.

a-NPD:Alq3deposition rate (nm s1)

kELmaxa

(nm) Vb(V)

Bc

(Cd m2) EQEd(%) gPe(lm W1) gCf(Cd A1)

Roll-off ratiog (%) 0.1:0.1 523 2.9/3.9/5.5 4691 0.94/0.87/0.94 2.35/1.73/1.29 4.80/4.25/4.71 11.2/2.1 0.01:0.1 527 3.2/4.5/6.2 5507 1.09/0.96/1.08 1.79/1.68/1.31 5.56/4.87/5.52 11.8/1.0 1:0.1 527 2.7/3.9/5.6 5249 0.99/0.89/0.98 2.50/1.79/1.31 5.01/4.56/4.96 11.0/2.0 0.1:1 527 2.6/3.7/5.2 4347 0.87/0.85/0.86 2.60/1.77/1.25 4.35/4.14/4.27 5.7/2.3 0.1:0.01 527 2.8/4.1/6.0 5792 1.08/1.08/1.09 2.47/1.71/1.34 5.61/4.71/5.51 17.1/1.8

aValues collected at the current density of 100 mA cm2.

bVoltage at onset, 100 Cd m2, 1000 Cd m2.

cBrightness at 100 mA cm2.

dExternal quantum efficiency: maximum, value at 10 mA cm2, value at 100 mA cm2.

ePower efficiency: maximum, value at 10 mA cm2, value at 100 mA cm2.

fCurrent efficiency: maximum, value at 10 mA cm2, value at 100 mA cm2.

gEQE roll-off ratio at 10 mA cm2, 100 mA cm2.

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whereeis the magnitude of the electron charge. The hop- ping rate reaches to its maximum at which the free energy difference between sites is equal to the molecular reorga- nization energy (calculated and reported 0.29 eV fora-NPD molecules) [44,45], which can be presented as follows:

kmaxi;j ¼ 2pJi;j2

h ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4pkkBT

p : ð4Þ

Also, the electronic coupling transfer integral, J, which represents the strength of electronic interaction between sites is strongly dependent on intermolecular separation and

the charge carrier mobility can be estimated using the sum of the hopping rates weighted by the relative step length along the electric field direction [43,46]. However, the average of intermolecular separation between a-NPD molecules (Din figure5a) inside the organic layer can be calculated through the layer density, which takes a value of 1:8 nm according to the experimental data presented in ref. [47]. Also, the structure ofa-NPD molecule was sim- ulated and optimized using Gaussian 09W software. Then, we estimated molecule dimensions (see supplementary figure S2 of supporting information which represents the optimized structure ofa-NPD molecule and its dimensions).

Table 2. Detailed device performance of TADF OLEDs.

a-NPD:Alq3deposition rate (nm s1)

kELmaxa

(nm) Vb(V)

Bc

(Cd m2) EQEd(%)

gPe

(lm W1) gCf(Cd A1)

Roll-off ratiog (%) 0.1:0.1 519 4.9/7.0/8.9 24,932 13.1/10.4/6.1 22.7/12.0/5.8 75.4/63.4/39.0 23.6/55.0 0.01:0.1 515 4.9/7.5/10.0 7652 12.9/11.5/7.1 22.7/10.4/4.6 74.0/56.5/32.8 17.0/48.2 1:0.1 515 5.7/8.3/10.8 6033 13.7/11.1/6.2 18.8/10.3/4.4 71.0/58.1/30.6 17.5/55.5

0.1:1 522 5.6/8.6/11.0 6426 13.9/10.6/6.0 21.1/9.1/4.3 78.6/52.5/31.2 32.2/59.7

0.1:0.01 519 5.6/8.4/10.8 6349 14.9/12.0/7.1 17.2/10.4/4.7 76.6/58.7/33.0 23.6/56.8

aValues collected at the current density of 100 mA cm2.

bVoltage at onset, 100 Cd m2, 1000 Cd m2.

cBrightness at 100 mA cm2.

dExternal quantum efficiency: maximum, value at 10 mA cm2, value at 100 mA cm2.

ePower efficiency: maximum, value at 10 mA cm2, value at 100 mA cm2.

fCurrent efficiency: maximum, value at 10 mA cm2, value at 100 mA cm2.

gEQE roll-off ratio at 10 mA cm2, 100 mA cm2.

Figure 4. Zero field mobility lð Þ0 and Poole–Frenkel factor c of a-NPD thin films fabricated at various vacuum-deposition rates. Thelð Þ0 andcvalues were calculated by fitting theJ–Echaracteristics ofa-NPD HODs reported in ref. [22].

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There are several research studies indicating that organic layer densification can increase charge carrier mobility and enhance charge carrier transport and injection [17,48–50]

which can be attributed to the strong electronic coupling between near molecules inside the densified layer resulting in charge hopping rate enhancement. Recently, we demonstrated that a change in horizontally orienteda-NPD molecules occurs at high vacuum-deposition rates and the layer density is approximately independent on deposition rate [22]. Therefore, as shown in figure5a and b, at high vacuum-deposition rates, the intermolecular separation between a-NPD molecules reduces even more. Tip to tip intermolecular separation (Lin figure5a) takes a minimum of 0.16 nm between two molecules, which are aligned together (u1¼u2¼U). Indeed, the reduced intermolecular separation between a-NPD molecules at high vacuum- deposition rates, as a result of the change in molecular orientation, is the origin of enhanced hole hopping rate between neighbouring sites which resulted in the increase of zero field mobility lð Þ0 and decrease of Poole–Frenkel factorc.

Regarding the increased electron current of Alq3 thin films fabricated at low vacuum-deposition rates, smooth contact area at the organic/cathode interface as a result of low surface roughness of organic layer is considered as a factor which can facilitate the electron injection [10,19,22].

To prove this, the surface roughness of Alq3 neat films vacuum-deposited on silicon substrate at 0.01 and 0.1 nm s1 was measured and depicted in figure 6a. The aver- age surface roughness of Alq3films increased, by a factor of approximately four as a result of the increase in vacuum- deposition rate. Additionally, we fabricated EODs with a different structure of organic layer at the interface with cathode. As it is shown in the inset of figure6b, in device I, we fabricated a smooth contact area using low vacuum- deposition rate (0.01 nm s1). But, in device II, the

deposition rate of Alq3remained constant (0.1 nm s1) even at the interface with cathode.

After device fabrication, we measured the current den- sity–voltage (J–V) characteristic of two devices. It is easily seen a clear increase in electron current of device I relative to device II which can be ascribed to increased electron injection from cathode into Alq3organic layer at low vac- uum-deposition rate.

3.3 Introduction of novel organic layer structures

As a result of our findings, we fabricated and introduced classic OLEDs with different structures for emitting layer using the vacuum-deposition rate and shutter. The detailed structures of the emitting layer are depicted in figure 7. In constant thermal evaporation (CTE) structure the vacuum- deposition rate remains constant, as we discussed before, the only recombination zone is pinned at the a-NPD/Alq3

interface, because of low electron and hole mobility.

Moreover, in spite of lower hole mobility in Alq3 layer, some hole carriers pass through the emitting layer even near cathode without radiative recombination since they are major carriers. In other structures, in order to create high electron injection from cathode into organic layer, Alq3 was vacuum-deposited at the minimum deposition rate (0.01 nm s1). Then, we increase deposition rate to 1 nm s1, resulting in low electron current inside the emit- ting layer. Finally, for achieving high electron and hole current near the a-NPD, the deposition rate again was decreased. So, in these two novel structures (gradient and barrier structures), the electron current is high near the interfaces with cathode and HTL and is low inside the emitting layer.

After device fabrication, the OLED performance was measured and the results are shown in figure8and table3.

Figure 5. (a) A pair ofa-NPD molecules with a length of‘, which are randomly oriented representing a relative angleDu¼u2u1, angleUwhich indicates relative position of molecules, tip to tip intermolecular separationLand the average intermolecular separationD. (b) Tip to tip intermolecular separation between twoa-NPD moleculesvs.

their relative angle (Du) at different relative positional angles (U) in the case ofu1¼0. The minimum tip to tip separation (0.16 nm) occurs between two molecules which are aligned together (u1¼u2¼U).

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As it is shown in current density–voltage–luminance (J–V–L) diagrams, both current–density and luminance were high in gradient structure and low in barrier structure. Also, EQE results showed that gradient structure showed the best OLED performance (EQEmax¼1:00% and EQE = 0.98% at 100 mA cm2), and barrier structure showed the worst OLED performance (EQEmax¼0:78% and EQE = 0.74% at 100 mA cm2). Also, the EQE roll-off ratio of gradient structure at 100 mA cm2 was measured (about 1%) which was half the value of CTE structure and for barrier structure was about 3.9% which showed an increase by a factor of about 4. The increased EQE and suppressed EQE roll-off ratio of gradient structure can be attributed to some reasons: first, electron

mobility of Alq3layer increases at lower vacuum-deposition rates [31] and consequently the rate of exciton generation enhances. Second, the facilitation of electron injection at the Alq3interface with cathode increases the electron density inside the recombination zone resulting in enhanced exciton generation. Finally, because of a relatively thin layer with low electron current inside the gradient structure, a second recombination zone (broadening the recombination zone) is likely formed by escaping holes from the first recombination zone and electrons injected from cathode. In barrier structure, we noticeably increased the thickness of barrier region in which electron current is low, so that electron density in the first recombination zone reduces which results in low EQE.

Figure 6. (a) AFM images of Alq3neat films vacuum-deposited on silicon substrate at 0.01 and 0.1 nm s1.Ra,Rq

andRzare the average surface roughness, root mean squared roughness and maximum roughness, respectively. Also, Sis the measured area. (b) The current density–voltage (J–V) characteristics of EODs. The detailed schematic structure of Alq3organic layer at the interface of cathode is shown in the inset.

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Also, high electron density inside the first recombination zone can enhance the exciton-charge quenching [51–53]

which can lead to a drop in EQE and an enhanced roll-off ratio. However, it can be easily seen from our results that it

is not the case in gradient structure mainly due to the second recombination zone and alleviated exciton quenching in a broadened emissive region [54], as opposed to other structures.

Figure 7. The schematic structures of the Alq3emitting layer fabricated by its vacuum-deposition rate manipulation and using a shutter named as barrier, gradient and CTE structures.

Figure 8. Device performance of classic OLEDs with CTE, gradient and barrier structures for the emitting layer:

(a) current density–voltage–luminance (J–V–L) characteristics, (b) EQEvs.current density (EQE–J), (c) EL spectra and related photo of the resulting OLED and (d) EQE, drive voltage and EQE roll-off ratio for barrier, CTE and gradient structures at different current densities and luminance.

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Table 3. Detailed device performance of classic OLEDs with CTE, gradient and barrier structures for the emitting layer.

ETL-EL structure kELmaxa(nm) Vb(V) Bc(Cd m2) EQEd(%) gPe(lm W1) gCf(Cd A1) Roll-off ratiog(%) CTE 523 2.9/3.9/5.5 4691 0.94/0.87/0.94 2.35/1.73/1.29 4.80/4.25/4.71 11.23/2.11 Gradient 521 2.6/3.2/3.8 5132 1.00/0.95/0.98 2.64/1.91/1.35 4.71/4.46/4.62 4.04/1.01 Barrier 524 3.1/4.3/6.2 3815 0.78/0.75/0.74 2.02/1.32/0.92 3.85/3.68/3.71 3.85/3.85

aValues collected at the current density of 100 mA cm2.

bVoltage at onset, 100 Cd m2, 1000 Cd m2.

cBrightness at 100 mA cm2.

dExternal quantum efficiency: maximum, value at 10 mA cm2, value at 100 mA cm2.

ePower efficiency: maximum, value at 10 mA cm2, value at 100 mA cm2.

fCurrent efficiency: maximum, value at 10 mA cm2, value at 100 mA cm2.

gRoll-off ratio at 10 mA cm2, 100 mA cm2.

Figure 9. (a) The schematic structures of doped and undoped classic OLEDs with the gradient emitting layer and the photo of resulting OLEDs. (b) EL spectra of doped and undoped OLEDs and PL spectra of DCM dissolved in THF measured by using Alq3 OLED and 405 nm laser. (c) Current density–voltage (J–V) characteristics of doped and undoped OLEDs.

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In order to further improve our understanding regarding the charge transport mechanism inside the emitting layer and elucidate the electron–hole recombination process and region in our new structures, we fabricated another gradient device in which 1% by weight DCM was doped in the high deposition rate region, according to figure 9a. Next, we measured the EL spectra and compared with conventional device. Figure 9b illustrates EL spectra for undoped and doped gradient devices representing a single peak and a double peak, respectively. The second peak in red region is attributed to electroluminescent radiation of DCM indicated another recombination zone inside the Alq3emitting layer.

Moreover, the shapes of PL spectra associated with the sample of DCM dissolved in tetrahydrofuran (THF) at a concentration of 0.1 mg ml-1 which was excited using undoped classic OLED and 405 nm solid state laser, are depicted in figure9b. The comparison between EL spectra of doped gradient OLED and PL spectra of DCM solution excited by undoped classic OLED indicates that the DCM peak related to emission spectra of doped gradient OLED is generated probably due to the EL emission rather than the PL emission.

Therefore, we speculate that fabrication of a second recombination region for electrons and holes in our novel structures only by controlling the deposition rate during device fabrication is possible. However, more studies are needed to be performed to characterize our new structures.

4. Conclusion

In this paper we evaluated the influence of HTL and ETL vacuum-deposition rate on classic and TADF OLED per- formance, respectively. We concluded from EQE mea- surements that charge carrier balance factor had a key role in changing device efficiency because deposition rate manipulation changes charge carrier mobility and conse- quently, the charge density inside the recombination zone.

Moreover, our analytical model associated witha-NPD thin films revealed that horizontally oriented molecules caused a decrease in intermolecular separation regardless of organic layer density, resulting in enhanced hole hopping rate.

Additionally, we demonstrated that high electron current density at low deposition rate in Alq3ETL is attributed to high electron injection from cathode as a result of low surface roughness of organic layer at the interface with cathode. Finally, we fabricated and introduced new gradient and barrier emitting layer structures using vacuum-deposi- tion rate control to create an extra-recombination zone which resulted in increasing radiative recombination of electrons and holes and device efficiency. Our findings can open a new way to enhance the efficiency of OLEDs by controlling the vacuum-deposition rate of organic layers during the deposition process. It can also reduce the time of fabrication process using organic layers with high electrical characteristics at high deposition rates.

Acknowledgements

Classic and TADF OLEDs were fabricated and character- ized in the Centre for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan. We would like to appreciate Professor Chihaya Adachi and Toshinori Mat- sushima for their sincere advice.

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