A protocol for the production of simple structured organic light-emitting diodes (OLEDs) is presented.
A method for producing simple and efficient thermally-activated delayed fluorescence organic light-emitting diodes (OLEDs) based on guest-host or exciplex donor-acceptor emitters is presented. With a step-by-step procedure, readers will be able to repeat and produce OLED devices based on simple organic emitters. A patterning procedure allowing the creation of personalized indium tin oxide (ITO) shape is shown. This is followed by the evaporation of all layers, encapsulation and characterization of each individual device. The end goal is to present a procedure that will give the opportunity to repeat the information presented in cited publication but also using different compounds and structures in order to prepare efficient OLEDs.
Organic electronics brings together all fields from chemistry to physics, going through materials science and engineering in order to improve the current technologies towards more efficient and more stable structures and devices. From this, organic light-emitting diodes (OLEDs) is a technology that has shown great improvements over the last few years, both in terms of efficiency and stability1,2. Reports say that the OLED industry for displays may increase from the 16 billion dollars in 2016 to around 40 billion dollars by 2020 and more than 50 billion by 20263. It is also finding its way into general lighting and head-mounted microdisplays for augmented-reality4. Applications like organic sensors for biomedical applications is more of a futuristic application at the moment, given the requirements for both high luminance and stability5. This trend confirms the need for improved device structures that includes more efficient molecules at less expense of natural resources. A better understanding of the inherent processes of the materials used for OLEDs is also of great importance when designing these.
An OLED is a multi-layered organic stack sandwiched between two electrodes, at least one of the latter transparent. Each layer, designed accordingly to their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and their intrinsic mobility, has a specific function (injection, blockage, and transport) in the overall device. The mechanism is based on opposite charge carriers (electrons and holes) travelling across the device where they meet in a specific layer, recombine to form excitons and from the deactivation of these excitons comes the emission of a photon6. This photon will be a characteristic of the layer where the deactivation is taking place7,8,9. So, pending molecular design strategies, different red, green and blue emitters can be synthesized and applied to the stack. Putting them together, white devices can also be produced10,11. The emitting layer of an OLED stack is usually based on the guest-host (G-H) system where the guest is dispersed into the host to avoid quenching of light9 and by-side reactions12.
There are several ways to push molecules to emit light, with thermally-activated delayed fluorescence (TADF) implemented more recently13,14,15. TADF allowed for the increase of the external efficiency of devices from 5% of a typical fluorescence emitter up to 30% by means of triplet harvesting through a small singlet-triplet energy-splitting in a process called reverse intersystem crossing (rISC). There are several ways to form efficient TADF-based OLEDs: one of the most common in literature is the G-H system where the emissive state is formed by a single molecule16,17,18. A second system uses an exciplex emitter formed between an electron donor (D) and an electron acceptor (A) molecules, which are simply called the donor-acceptor (D-A) system15,19,20,21; A small range of TADF materials and devices have been reported, yielding very high external quantum yields14, reaching a values of, for example, 19% EQE22, clearly indicating that very efficient triplet harvesting is occurring and that 100% internal quantum efficiency is possible. In these TADF-based OLEDs, care must be taken when choosing the proper host material as the polarity of the environment can change the charge transfer (CT) state away from the local excited (LE) state, therefore, reducing the TADF mechanism. The procedure to be taken into account is similar to other fluorescent emitters23. Such devices have relatively simple stack structures, typically 3 to 5 organic layers, and without the need of a p-i-n structure24, resulting in ultra-low turn-on voltages of the order of 2.7 V and a maximum thickness of around 130 nm for all organic layers to guarantee a good charge balance.
Apart from the materials' properties, the production of multi-layered stacks can be either be based on vacuum thermal evaporation (VTE) or spin-coating, the former more frequent for small molecules. It requires precise control over the temperature, pressure, environment, rate, and thickness of each layer. For emitting G-H layers, the rates of co-evaporation have to be controlled for the desired ratios to be obtained. Also of extreme importance is the cleaning of the substrates used for OLEDs which can result in non-working devices or uneven emissions throughout the emitting pixel25.
Therefore, this article aims at all steps of preparation, production, and characterization of organic devices and intends to help new specialists on the careful protocol required for high efficiency and evenness of emission. It involves the use of DPTZ-DBTO2 (2,8-Bis(10H-phenothiazin-10-yl)dibenzothiophene-S,S-dioxide) as emitting guest in a TADF G-H system16,26. Similar methods can be also implemented for the formation of an exciplex based D-A systems using DtBuCz-DBTO2 (2,8-Bis(3,6-di-tert-butyl-9H-carbazol-9-yl)dibenzothiophene-S,S-dioxide) in TAPC (4,4′-Cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine])15, where the main difference in the procedure is the concentration ratio of the emissive layer but it significantly changes the nature of emission (single molecule CT emission vs exciplex CT emission). The G-H system described here has a single molecule CT emitter and involves the evaporation of 5 layers with 3 organic, and 2 inorganic materials. The device is composed of indium tin oxide (ITO) as the anode, 40 nm of N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) as the hole transport layer (HTL), and a total 20 nm of 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) with 10% of DPTZ-DBTO2 as the emitting layer based on the G-H system. 60 nm of 2,2′,2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi) is then used as the electron transport layer (ETL) and 1 nm of Lithium Floride (LiF) as electron injection layer (EIL). 100 nm of aluminum (Al) finalizes the device as a cathode. A diagram of the whole procedure can be found in Figure 1. The thicknesses of organics were chosen to be similar to other devices used in the literature. The mobility of each layer must be carefully examined as to ensure good carrier balance inside the layer. The operation of LiF is based on a tunnelling effect, i.e., carriers travel through the tunnels of a packed LiF, ensuring a better injection to the transport layers. This means thin layers (between 0.8 and 1.5 nm) are required27. The layer of Al must be thick enough to prevent any oxidation (70 nm is a minimal requirement).
CAUTION: The following procedure involves the use of different solvents, so proper care must be taken when using them. Please use fume and personal protective equipment (gloves, lab-coat). To ensure the quality of the devices evaporated, it is recommended that all the procedure is done in a clean environment (such as a clean room and/or a glovebox). The safety datasheets must be consulted before the use of each equipment/material.
1. ITO Patterning
2. Substrate Cleaning
3. Preparation of the Evaporation Chamber
4. Evaporation of the Organic Layers
NOTE: For all organics, do not exceed the evaporation rate of 2 Å/s as this results in increased roughness and decreased uniformity of the layers. To a certain point, this may result in non-uniform emissions and even shorts.
5. OLED Encapsulation
NOTE: This section is not mandatory for the analysis of OLEDs though it is highly recommended. To secure their quality, it is also important that this section is done in a controlled environment.
6. OLED Characterization
The data presented in Figure 3 is a good example of the different information one can get by the analysis of this type of OLEDs. From Figure 3a, the turn-on voltage (voltage at which the detector starts detecting light on the device) can be determined. In this case, it is 4 V. Device degradation due to high voltages is seen when luminance decreases substantially (around 13 V). Degradation occurs when carriers injected into the device react with the organic layers resulting in the breaking of bonds and molecules. Also, electrical stress can be associated with device degradation. The maximum luminance of this device is around 17000 cd/m2. From Figure 3b, the maximum E.Q.E. (around 7%) and roll-off, a measure of the device electrical stability, are determined. The roll-off of a device is also defined as the drop in efficiency with the current flowing through it. To compare the roll-off of different devices, the values of EQE at the standard luminance of 100 and 1000 cd/m2 are usually given6. In this case, 6.1 and 5.5%, respectively which represents a drop of 9% and 20% of its maximum value. This represents a poor roll-off. Good values should be between 0 and 5% until high levels of luminance. The other values of efficiency are shown in Figure 3c, as other means of comparison with similar types of devices. Finally, the EL is shown peaking at 573 nm, a typical green-yellow emission (inset of Figure 3d). The EL at different voltages can help giving insights into optical stability i.e., where the emission is taking place. In this case, as this seemingly does not change with the applied voltage, one can assume that the device is optically-stable. Checking the CIE coordinates (inset of Figure 3b with voltage is another way to measure the optical stability.
Figure 1: Diagram containing all steps represented in this protocol. All organic layers and LiF are evaporated using mask A. After metallization (evaporation of aluminium), two sets of devices can be produced using mask B: one with 2×4 cm2 and another with 4×4 cm2. The voltage will be applied between the ITO (anode: +) and aluminium (cathode: – ) and a current will be measured. A cross-section of the device structure is also shown. Please click here to view a larger version of this figure.
Figure 2: a) Diagram of the organic low temperature (black) and inorganic high-temperature sources (blue) to be placed in the vacuum chamber. Each material has to be put in the specified source with a specific heating number for the software as they were previously optimized for each material in question. b) QCM sensors arranged throughout the chamber. Please click here to view a larger version of this figure.
Figure 3: a) J-V-L, b) EQE-J, c) ƞP-V- ƞL, d) EL-λ at different voltages for the device in this study. The CIE coordinates change with voltage is shown on the inset of b) while a photograph of the device is shown in the inset of d). Please click here to view a larger version of this figure.
Curve | x | Scale | y1 | Scale | y2 | Scale |
J-V-L | V | linear | J | log | L | log |
ƞP-V-ƞL | ƞP | log | ƞL | log | ||
EQE-J | J | log | EQE | log | ||
EL- λ | λ | linear | EL | linear |
Table 1: Considered curves and related scale for the unification of the characterization of OLEDs.
The present protocol aims to present an effective tool for the patterning, production, encapsulation and characterization of OLEDs based on small molecular-weight TADF-emitting or exciplex-emitting layers. The organic vacuum thermal evaporation allows for the production of thin films (from a few Å to hundreds of nm) of both organic and inorganic materials and produce pathways for charge carriers to recombine from which light will be emitted. Although versatile, the device production is fairly limited to the evaporator i.e., the number of organic and inorganic sources available or the possibility of more than one evaporation at the same time (co- and tri-evaporations are very common, particularly in TADF devices). More advanced systems may allow for the evaporation of more than 3 sources at the same time, which may be useful for applications such as white-OLEDs28 for displays and general lighting. Nevertheless, a trade-off between the device complexity and its performance must be met. The multifunctionality of this evaporation procedure also allows doing different studies that go beyond this work. These include effects of layer thickness, dopant concentration, layer functionality or even study the inherent mobilities of new layers. The fine control over the rates of single and co-evaporated layers is also crucial since it allows for the formation of uniform films with controlled precise rations.
It is recommended that all steps of this protocol are done in a controlled environment and, more importantly for the encapsulation, inside a glovebox to avoid any ambient related degradation. Finally, an integrating sphere is most welcomed as it provides for a more detailed electrical and optical analysis. With this mind, all steps from theoretical introduction to production and characterization of TADF-based OLEDs were presented in this protocol highlighting all these different stages allowing the production of stable devices that, when encapsulated, can last for large periods of time.
The authors have nothing to disclose.
The authors would like to acknowledge the "Excilight project" which received funding from H2020-MSCA-ITN-2015/674990.
N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine | NPB | Sigma Aldrich | 556696 | Sublimed grade |
4,4′-Bis(N-carbazolyl)-1,1′-biphenyl | CBP | Sigma Aldrich | 699195 | Sublimed grade |
2,2′,2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) | TPBi | Sigma Aldrich | 806781 | Sublimed grade |
Lithium Floride 99.995% | LiF | Sigma Aldrich | 669431 | |
Aluminum 99.999% | Al | Alfa Aesar | 14445 | |
Acetone 99.9% | Acetone | Sigma Aldrich | 439126 | |
Isopropyl alcohol 99.9 % | IPA | Sigma Aldrich | 675431 | |
Photoresist | DOW Electronic Materials | Microposit S1813 | ||
Developer | DOW Electronic Materials | Microposit 351 | ||
Hydrochloric acid 37% | HCl | Sigma Aldrich | 435570 | |
Nitric acid 70% | HNO3 | Sigma Aldrich | 258113 | |
Encapsulation resin | Delo | Kationbond GE680 | ||
Encapsulation square glass 15x15mm | Agar | AGL46s15-4& | ||
ITO | Naranjo Substrates | Custom made |