Name: development of efficient oleds from solution deposition
Uploaded: 2022-11-04T12:30:00
Description: Watch this Scientific Journal Video about Development of Efficient OLEDs from Solution Deposition at JoVE.com

The authors would like to acknowledge the &#8220;EXCILIGHT&#8221; Project from the European Union&#8217;s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No 674990. This work was also developed within the scope of the project i3N, UIDB/50025/2020 &#38; UIDP/50025/2020, financed by national funds through the FCT/MEC.

CAUTION: The following steps involve the use of different solvents and organic materials, so proper care must be taken when handling. Use the fume hood and protective equipment such as lab glasses, face masks, gloves, and lab coats. Weighing of the materials should be done precisely using a high precision scale machine. To ensure cleanliness of the substrates, solution deposition of thin films, and evaporation, it is recommended that all the procedures be performed in a controlled environment or glovebox. Before the use ...

The protocol used here to fabricate an efficient OLED in a simple device structure is relatively simple. The electrical mobility is not only modulated by the material composition of a device layer but also critically depends on film morphology. Preparation of the solutions and a suitable choice of solvent and concentration are important. No material aggregation can occur, implying complete solubility at the nanometric scale. It is also important to observe the viscosity of the solution. A high viscosity leads to a high c...

The increase in organic electronics used in daily life has become an unsurpassed reality. Among several organic electronic applications, OLEDs are perhaps the most attractive. Their image quality, resolution, and color purity have made OLEDs a primary choice for displays. Moreover, the possibility to achieve large area emission in extremely thin, flexible, lightweight, and easy color-tunable OLEDs has applications in lighting. However, some technological issues associated with the fabrication process in large area emitters have postponed further application.
With the first OLED working at low applied voltages1, new paradigms for solid-state lighting have been designed, though with low external quantum efficiency (EQE). The OLED EQE is obtained by the ratio of emitted photons (light) to injected electrical carriers (electrical current). A simple theoretical estimate for the maximum expected EQE is equal to &#951;out&#160;x &#951;int&#160;2. The internal efficiency (&#951;int) can be approximated by &#951;int&#160;= &#947; x <img alt="Equation 1" src="/files/ftp_upload/61071/61071eq01.jpg" style="margin: auto;" /> x &#934;PL, where &#947; corresponds to the charge balance factor, &#934;PL&#160;is the photoluminescence quantum yield (PLQY), and <img alt="Equation 1" src="/files/ftp_upload/61071/61071eq01.jpg" style="margin: auto;" /> is the efficiency of emissive exciton (electron hole pair) generation. Finally, &#951;out is the outcoupling efficiency2. If outcoupling is not considered, attention is focused on three topics: (1) how efficient the material is in creating excitons that radiatively recombine, (2) how efficient the emissive layers are, and (3) how efficient the device structure is in promoting a well-balanced electrical system3.
A purely fluorescent organic emitter has only 25% internal quantum efficiency (IQE). According to spin rules, the radiative transition from a triplet to a singlet (T&#8594;S) is forbidden4. Therefore, 75% of excited electrical carriers do not contribute to the emission of photons5. This issue was first overcome using transition metals in organic emitter phosphorescence OLEDs6,7,8,9,10, where the IQE was reportedly close to 100%11,12,13,14,15,16. This is due to the spin-orbit coupling between the organic compound and heavy transition metal. The disadvantage in such emitters is their high cost and poor stability. Recently, reports on the chemical synthesis of a pure organic compound with low energy separation between the excited triplet and singlet states (&#8710;EST) by Adachi17,18 have given rise to a new framework. Although not new19, the successful employment of the TADF process in OLEDs have made it possible to obtain high efficiencies without using transition metal complexes.
In such metal-free organic emitters, there is a high probability for the excited carriers in a triplet state to populate to the singlet state; therefore, the IQE can achieve a theoretical limit of 100%5,20,21,22. These TADF materials provide excitons that can radiatively recombine. However, these emitters require dispersion in a matrix host to avoid emission quenching3,20,21,23,24 in a host-guest concept. Additionally, its efficiency depends on how the host (organic matrix) is appropriated to the guest (TADF) material25. Also, it is necessary to idealize the device structure (i.e., thin layers, materials, and thickness) to achieve an electrically balanced device (equilibrium between holes and electrons to avoid loss)26. Achieving the best host-guest system for an electrically balanced device is fundamental to increase the EQE. In TADF-based systems, this is not simple, due to the changes in the electrical carrier mobilities in EML that are not easily tuned.
With TADF emitters, EQE values greater than 20% are easy to obtain26,27,28,29. However, the device structure is typically comprised of three to five organic layers (hole transport/blocking and electron transport/blocking layers, HTL/HBL and ETL/EBL, respectively). Additionally, it is fabricated using a thermal evaporation process that is high in cost, technologically complex, and almost only for display applications. Depending on the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) levels, electrical mobility of carriers, and thickness, each layer can inject, transport, and block electrical carriers and guarantee recombination in the emissive layer (EML).
Reducing the device complexity (e.g., a simple, two layer structure) usually results in a noticeable decrease of EQE, sometimes to less than 5%. This happens due to the different electron and hole mobility in the EML, and the device becomes electrically unbalanced. Thus, instead of the high efficiency of exciton creation, the efficiency of emission in the EML becomes low. Moreover, a noticeable roll-off occurs with a strong decrease of the EQE as the brightness increases, due to the high concentration of excitons at a high applied voltage and long excited lifetimes24,30,31. Overcoming such issues requires a strong capability to manipulate electrical properties of the emissive layer. For a simple OLED architecture using solution-deposited methods, electrical properties of the EML can be tuned by the solution preparation and deposition parameters32.
Solution deposition methods for organic-based devices have been previously used31. OLED fabrication, compared to the thermal evaporation process, is of great interest due to their simplified structure, low cost, and large area production. With high success in transition metal complexes OLEDs, the main goal is to increase the emitting area but keep device structure as simple as possible33. Methods such as roll-to-roll (R2R)34,35,36, inkjet printing37,38,39 and slot-die40 have been successfully applied in multilayer fabrication of OLEDs, which is a possible industrial approach.
Despite solution deposition methods for organic layers serving as a good choice for device architecture simplification, not all desired materials can be easily deposited. Two types of materials are used: small molecules and polymers. In solution deposition methods, small molecules have some drawbacks, such as poor thin film uniformity, crystallization, and stability. Thus, polymers are mostly used due to the ability to form uniform thin films with low surface roughness and on large, flexible substrates. Moreover, the materials should have good solubility in the appropriate solvent (mainly organic ones like chloroform, chlorobenzene, dichlorobenzene, etc.), water, or alcohol derivatives.
Besides the problem of solubility, it is necessary to guarantee that a solvent used in one layer should not act as one for the preceding layer. This allows a multi-layer structure deposited by the wet process; however, there are limitations41. The most typical device structure uses some solution-deposited layers (i.e., the emissive one) and one thermally evaporated layer (ETL). Additionally, thin film homogeneity and morphology strongly depend on the deposition methods and parameters. Electrical charge transport through these layers is completely governed by such morphology. Nevertheless, a tradeoff between the desired final device and compatibilities of the fabrication process should be judiciously established. Adjusting the deposition parameters is a key to success, despite being time-consuming work. For instance, the spin coating is not a straightforward technique. Although it seems simple, there are several aspects of thin film formation from a solution on top of a spinning substrate that require attention.
Besides film thickness optimization, manipulation of spinning velocity, and time (thickness is an exponential decay of both parameters), the experimenter&#8217;s actions must also be adjusted to obtain good results. Correct parameters also depend on the solution viscosity, deposition area, and wettability/contact angle of solution on the substrate. There are no unique sets of parameters. Only basic assumptions with specific adjustments to the solution/substrate yield the desired results. Moreover, the electrical properties that depend on the layer molecular conformation and morphology can be optimized for desired results, following the protocol described here. Once completed, the process is simple and feasible.
Nevertheless, decreasing the device structure complexity leads to a maximum EQE decrease; although, a compromise can be achieved in terms of efficiency vs. brightness. As such a compromise allows practical applications, the surplus of a simple, large area compatible, and low cost process can become a reality. This article describes these requirements and how to develop a recipe to handle the required issues.
The protocol focuses on a green TADF emitter 2PXZ-OXD [2,5-bis(4-(10H-phenoxazin-10-yl)phenyl)-1,3,4-oxadiazole]42 as a guest in a host matrix composed by PVK [poly(N-vinylcarbazole)] and OXD-7 [1,3-Bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene], which corresponds to the EML. An electron transport layer (ETL) of TmPyPb [1,3,5-Tri(m-pyridin-3-ylphenyl)benzene] is used. Both the anode&#8217;s and cathode&#8217;s work functions are optimized. The anode is comprised of ITO (indium tin oxide) with a high conductive polymer PEDOT:PSS [poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)], and the cathode is comprised of a double layer of aluminum and LiF (lithium fluoride).
Finally, both the PEDOT:PSS and EML (PVK: OXD-7: 2PXZ-OXD) are deposited by spin coating, whereas TmPyPb, LiF, and Al are thermally evaporated. Considering the conductive metal-like nature of PEDOT:PSS, the device is a typical &#8220;two organic layer&#8221; in the simplest structure possible. In the EML, TADF guest (10% wt.) is dispersed in the host (90% wt.) composed of PVK0.6+OXD-70.4.

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			<td height="21" style="height:21px;width:507px;">2PXZ-OXD (2,5-bis(4-(10H-phenoxazin-10-yl)phenyl)-1,3,4-oxadiazole)</td>
			<td style="width:207px;">Lumtec ltd</td>
			<td style="width:119px;">1447998-13-1</td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:507px;">Aluminum (99.999%)</td>
			<td style="width:207px;">Alfa Aesar</td>
			<td style="width:119px;">7429-90-5</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:507px;">Acetone (99.9%)</td>
			<td style="width:207px;">Sigma Aldrich</td>
			<td style="width:119px;">67-64-1</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:507px;">Hellmanex</td>
			<td style="width:207px;">Ossila</td>
			<td style="width:119px;">7778-53-2</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:507px;">Isopropyl alcohol</td>
			<td style="width:207px;">Sigma Aldrich</td>
			<td style="width:119px;">67-63-0</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:507px;">ITO patterned substrates</td>
			<td style="width:207px;">Ossila</td>
			<td style="width:119px;">65997-17-3</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:507px;">Lithium Fluoride (99.99%)</td>
			<td style="width:207px;">Sigma Aldrich</td>
			<td style="width:119px;">7789-24-4</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:507px;">OXD-7 (1,3-Bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene)</td>
			<td style="width:207px;">Ossila</td>
			<td style="width:119px;">138372-67-5</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">PEDOT: PSS (Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate)</td>
			<td style="width:207px;">Ossila</td>
			<td style="width:119px;">155090-83-8</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:507px;">PVK (Polyvinlycarbazole) (average Mn 25,000-50,000)</td>
			<td style="width:207px;">Sigma Aldrich</td>
			<td style="width:119px;">25067-59-8</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:507px;">TmPyPb (1,3,5-Tri(m-pyridin-3-ylphenyl)benzene)</td>
			<td style="width:207px;">Ossila</td>
			<td style="width:119px;">138372-67-5</td>
			<td></td>
		</tr>
	</tbody>
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development of efficient oleds from solution deposition

The use of highly efficient organic emitters based on the thermally activated delayed fluorescence (TADF) concept is interesting due to their 100% internal quantum efficiency. Presented here is a solution deposition method for the fabrication of efficient organic light-emitting diodes (OLEDs) based on a TADF emitter in a simple device structure. This fast, low-cost, and efficient process can be used for all OLED emissive layers that follow the host-guest concept. The fundamental steps are described along with necessary information for further reproduction. The goal to establish a general protocol that can be easily adapted for principal organic emitters currently under study and development.

Figure 5 shows the main results for the fabricated device. The turn-on voltage was extremely low (~3 V), which is an interesting result for a two organic layer device. The maximum brightness was around 8,000 cd/m2 without using an integrating sphere. The maximum values for &#951;c, &#951;p, and EQE were around 16 cd/A, 10 lm/W, and 8%, respectively. Although the results are not the best figures of merits for this TADF emitter, they were the best found in such...

Watch this Scientific Journal Video about Development of Efficient OLEDs from Solution Deposition at JoVE.com

Development of Efficient OLEDs from Solution Deposition

Presented here is a protocol for the fabrication of efficient, simple, solution-deposited organic light-emitting diodes with low roll-off.

The use of highly efficient organic emitters based on the thermally activated delayed fluorescence (TADF) concept is interesting due to their 100% ...

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