Patent Application
20160126505
A portion of the disclosure of this patent document contains material, which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
The present invention relates to a polymer light emitting diode. More particularly, the present invention relates to a method for fabricating a polymer light emitting diode by depositing a thin layer of electron transport layer (ETL) by solution process.
Recently the development of polymer light-emitting diodes (PLED) focuses on enhancing the device efficiency and operating lifetime by multilayer device structure. In the multilayer PLED, electron transport layer (ETL) plays an important role that can provide efficient electron transport, reduce the potential barrier between the emission layer (EML) and the cathode, and prevent the cathode quenching effect by hole-blocking.
In addition, if there is no ETL, the device needs low work function or unstable cathodes, like Ca, Ba, or CsF/Al. That is one of the reasons why the lifetime of PLED is less than that of small molecular organic light-emitting diodes. The cathode LiF/Al, which is commonly used in small molecular organic light-emitting diodes, is know to be more stable than the low work function cathodes in PLED. Thus, the ETL plays an important role to provide attractive performance towards the PLED.
Concerning the fabrication of PLED, solution process is low-cost and more price-competitive than the high-cost thermal evaporation. Despite of some reports about solution-processed PLED, the dissolution problem between layers still exists in conventional solution-processed multilayer PLED, leading to mixing the layers together, and the device failing to function. Therefore, the ETL needs to be deposited by thermal evaporation.
Consequently, there is an unmet need to have a time-efficient and cost-effective manufacturing method to fabricate a PLED.
A first aspect of the presently claimed invention is to provide a method for fabricating a polymer light emitting diode.
In accordance with an embodiment of the presently claimed invention, a method for fabricating a polymer light emitting diode comprising: providing an emission layer (EML); dissolving at least one electron transport layer (ETL) material into an alcoholic solvent to form an ETL solution; coating the ETL solution on the EML by a first solution process to form an ETL wet film; and annealing the ETL wet film to form an ETL.
A second aspect of the presently claimed invention is to provide a polymer light emitting diode.
In accordance with an embodiment of the presently claimed invention, a polymer light emitting diode comprises a substrate, a hole transport layer, an emission layer, an electron transport layer, and a cathode. The electron transport layer is fabricated by a solution process.
The present invention provides a solution-processed PLED fabrication method, which is low-cost and time-efficient during manufacturing. More importantly, the method can avoid the dissolution problem existed between the emission layer and the electron transport layer, thus providing better performance in terms of lifetime and brightness.
Embodiments of the present invention are described in more detail hereinafter with reference to the drawings, in which:
In the following description, a PLED, and methods for fabricating the PLED are set forth as preferred examples. It will be apparent to those skilled in the art that modifications, including additions and/or substitutions, may be made without departing from the scope and spirit of the invention. Specific details may be omitted so as not to obscure the invention; however, the disclosure is written to enable one skilled in the art to practice the teachings herein without undue experimentation.
In the present invention, a solution-processed PLED including the ETL is fabricated that provides performance comparable to the one prepared by conventional vacuum deposition.
Some common small molecular electron transport materials including 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) can be dissolved in a polar solvent such as methanol, and the ETL can be formed by a spin coating method. The polar solvent can dissolve the electron transport materials only, but doesn't dissolve the EML. Preferably, the polar solvent is an alcoholic solvent. For example, methanol is a very weak solvent to the emissive layer such that the dissolution problem between ETL and EML is solved. For blade coating and inkjet printing, different alcoholic solvents can be further applied such as isopropanol, n-butanol, and mixed together to balance the surface tension, to obtain the better uniformity of the ETL during solvent evaporation, resulting in better performance of the layer.
Step 34 comprises the steps of dissolving an emission material in a non-polar solvent to form an EML solution, coating the EML solution on the HTL by a solution process to form an EML wet film, and annealing the EML wet film to form the EML. The non-polar solvent used therein is able to further reduce the dissolution problem during the deposition of the EML.
Preferably, the emission material comprises poly-(N-vinyl carbazole) (PVK), Poly(p-phenylene vinylene) (PPV), or spiro-bifluorene polymer. The non-polar solvent can be toluene, or chlorobenzene. The solution process can be a spin coating, an inkjet printing, or a blade coating.
Step 35 comprises the steps of dissolving an electron transport layer (ETL) material into an alcoholic solvent to form an ETL solution, coating the ETL solution on the EML by a first solution process to form an ETL wet film, and annealing the ETL wet film to form an ETL. As the solution process is used, instead of thermal evaporation, the method of the present invention is more cost-effective and time-efficient.
Preferably, the ETL material includes 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), 2(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxdiazole (PBD), or 3-(4-biphenyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole (TAZ). The alcoholic solvent is selected from the group consisting of methanol, isopropanol, n-butanol, ethylene glycol and combinations thereof. The choice and mixing of the alcoholic solvents depend on the solubility of the ETL material used. Preferably, a volume ratio of an alcoholic solvent mixture is 95% of methanol, 4.5% of n-butanol, and 0.5% of ethylene glycol. The alcoholic solvent mixture can avoid the coffee ring effect and assist in preparing a smooth, even thin film.
The ETL solution comprises 0.2-1 wt % of the ETL material. The ETL material can be small molecule based.
Preferably, the step of annealing the ETL wet film to form the ETL layer is performed at 90-120° C. for 5-15 min. The solution process can be a spin coating, an inkjet printing, or a blade coating. The ETL layer comprises a thickness from 10 to 40 nm.
Preferably, a spin coating rate is about 2-4000 rpm, depending on the required thickness. For example, for 0.5 wt % of TPBi, 2500 rpm is used to produce a TPBi layer with 10 nm.
As the non-polar solvent used in step 34 is not dissolved in the alcoholic solvent used in step 35, the dissolution problem between the EML and ETL is avoided.
A blue PLED was fabricated according to an embodiment of the presently claimed invention. The blue PLED comprised a multi-layered structure of ITO/MoO3 (10 nm)/blue EML: 10% FirPIc in PVK (25 nm)/TPBi (10 nm)/Al (150 nm). The TPBi of ETL layer was deposited by a spin coating, and annealed at 100° C. for 10 min.
A green PLED was fabricated according to an embodiment of the presently claimed invention. The green PLED comprised a multi-layered structure of ITO/MoO3 (10 nm)/green EML: 10% Ir(ppy)3 in PVK (25 nm)/TPBi (10 nm)/Al (150 nm). The TPBi of ETL layer was deposited by a spin coating, and annealed at 100° C. for 10 min.
A red PLED was fabricated according to an embodiment of the presently claimed invention. The red PLED comprised a multi-layered structure of ITO/MoO3 (10 nm)/red EML: 10% hex-Ir(piq)3 in PVK (25 nm)/TPBi (10 nm)/Al (150 nm). The TPBi of ETL layer was deposited by a spin coating, and annealed at 100° C. for 10 min.
A white PLED was fabricated according to an embodiment of the presently claimed invention. The white PLED comprised a multi-layered structure of ITO/MoO3 (10 nm)/white EML: spiro-bifluorene copolymer (50 nm)/TPBi (10 nm)/Al (150 nm). The TPBi of ETL layer was deposited by a spin coating, and annealed at 100° C. for 10 min.
A lifetime test was conducted with a PLED of the present invention. After working for more than 2000 hr, the brightness of the PLED was only dropped by 50%, indicating that even using a cost effective solution process, the performance of the PLED of the present invention is still guaranteed.
The foregoing description of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to the practitioner skilled in the art.
The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications that are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalence.
