Micro Organic Light Emitting Diode Arrays by Patterned Growth on Structured Polypyrrole

Patterning organic light emitting diodes (OLEDs) with several micrometers resolution is of importance in applications such as micro displays, virtual and augmented reality devices. Here a fabrication method of micro OLEDs is reported, using conducting polymer polypyrrole (PPy) as nucleation layer. The photolithographically patterned PPy films are used for patterned growth of hole transport layer to locally confine carrier injection. High‐resolution red, green, and blue micro OLED arrays are demonstrated. The green micro OLEDs with stacked structure of Al/Alq3/NPB/PPy/ITO exhibit a maximum current efficiency of 3.41 cd A−1, which is 80% in comparison to those devices on indium tin oxide glass.

DOI: 10.1002/adom.201902105 metal mask bears deformation by accumulation of materials upon deposition and large area positional accuracy problems as the pixel size goes down to micrometers. [7] Other patterning methods such as stamp printing and vapor jet printing suffer from poor uniformity or low yield subsequently. Thus, challenges exist for cost effective pattering of OLEDs having feature size in micrometers, with high uniformity and yield over large areas. [8][9][10] Photolithography is considered as an ideal way for micro-patterning of materials to meet the requirements listed above. Unfortunately, the photolithography cannot simply be applied to pattern OLEDs, because the UV light, solution, and organic solvents involved in the procedure lead to the damage of molecular functional groups and degradation of device performances. [7] To avoid the damage of solvent to organic films, orthogonal lithography was developed to pattern OLEDs in micrometers, using supercritical carbon dioxide or highly fluorinated photoresist and solvents. [11][12][13] Patterned growth by vacuum deposition was developed as an alternatively photolithographical way to pattern micro-OLEDs with feature size down to sub-micrometers. The substrate such as indium tin oxide (ITO) glass is first patterned with a thin Au layer by standard photolithography procedure. The organic molecules are sublimated from evaporator and deposited onto the substrate surface at an elevated temperature. Under optimized conditions such as substrate temperature, deposition rate, and pattern dimensions, the molecules are able to diffuse at the substrate surface up to micrometers and completely controlled to nucleate on Au area, owing to larger binding energy of molecules with Au than that with substrate surface. [14][15][16] The controlled nucleation results in area selective growth of organic functional molecules to form patterned films with lateral dimensions as same as of the patterned Au. With the strategy, micro-OLED arrays were successfully demonstrated on both rigid and flexible substrates. [17,18] For OLEDs, light extraction is one of the crucial parameters for the performances. Theoretically, light extraction efficiency is only 20% in a standard device architecture on ITO glass, while the other 80% of the emitted light is lost through electrode absorption and plasmon at the metal electrode-organic interfaces, as well as the waveguide mode and substrate mode. [19,20] In the case, the electrode materials play a crucial role to improve the light extraction. To improve the light extraction of micro OLEDs fabricated by patterned growth, ideally the nucleation layer should compose high transparent Patterning organic light emitting diodes (OLEDs) with several micrometers resolution is of importance in applications such as micro displays, virtual and augmented reality devices. Here a fabrication method of micro OLEDs is reported, using conducting polymer polypyrrole (PPy) as nucleation layer. The photolithographically patterned PPy films are used for patterned growth of hole transport layer to locally confine carrier injection. High-resolution red, green, and blue micro OLED arrays are demonstrated. The green micro OLEDs with stacked structure of Al/Alq 3 /NPB/PPy/ITO exhibit a maximum current efficiency of 3.41 cd A −1 , which is 80% in comparison to those devices on indium tin oxide glass. and non-metal materials to minimize electrode absorption and interface plasmon, respectively. Although Au is an excellent material for patterned growth that extensively used for various materials including inorganic semiconductors, carbon nanotubes, and organic functional molecules, the non-transparency of Au directly leads to low light extraction of OLEDs. [21][22][23][24][25] For example, the transmittance of ITO glass decreases from 90% to 60% by coating 2 nm Cr and 8 nm Au film. [17] In this work, we report fabrication of blue, green, and red micro-OLED arrays using of conducting polymer PPy, molecular structure shown in Figure 1a, as nucleation layer for patterned growth of hole transport material. The devices show much improvement in efficiency in comparison to those using Au as nucleation layer, but a relatively high turn-on voltage owing to mismatch of energy diagram at carrier injection interface.
As a conductive polymer, PPy is a promising material for various organic electronic devices due to high stability to water and oxygen, mild synthesis conditions, and thermal stability. Besides its excellent electrical properties, optical properties of PPy were also well investigated and depended on the means of synthesis. [26][27][28] Due to the free carrier and π-π* transitions, respectively, typically the PPy thin films have absorption bands above 800 nm and between 400 and 500 nm. [29] As here the PPy film is only for patterned growth of hole transport layer to confine the carrier injection of micro OLEDs, we focus on the transmittance of PPy films in thickness below 10 nm and wavelengths of visible light. Figure 1b shows the transmittance of PPy films in situ synthesized onto ITO glass in thickness of 10, 20, and 30 nm, respectively. Meanwhile, the bare ITO glass was also characterized as a reference to value the transmittance of PPy films. The PPy films were synthesized through a conventional chemical oxidation polymerization method with iron chloride as catalyst at 20 °C. [28] The thickness of the conducting PPy thin films is controlled by polymerization time. The samples with PPy films show good transmittance, as shown in Figure 1b. The all three samples show same transmittance to that of bare ITO glass below 500 nm in wavelengths. Above 500 nm, the transmittance decreases slightly when increasing the thickness from 10 to 30 nm. In particular for PPy film of 10 nm in thickness, the transmittance is almost the same as that of bare ITO glass, with less than 1% deviation in the measured range from 400-800 nm. Even for the PPy films of 150 nm, the absorption is kept below 13% in the wavelength of visible light and no photoluminescence emission is detected under 320 nm excitation ( Figure S1, Supporting Information).

Figure 2
schematically illustrates fabrication of micro OLEDs by patterned growth using PPy as nucleation layer. First a 10 nm Al 2 O 3 layer was deposited by atomic layer deposition onto ITO glass to electrically isolate the substrate from electron injection/transport layer upon deposition in the following steps. And then the samples were processed by standard photolithography to partially expose Al 2 O 3 (step I). The partially exposed Al 2 O 3 was removed by reactive ion etching to ITO underneath, leaving a conductive surface for OLED carrier injection (step II). The conductive PPy layer was then synthesized through the chemical oxidation polymerization method described above (step III). Followed by lift-off in acetone, the PPy on photo resist was removed, leaving patterned PPy film directly on partially exposed ITO (step IV). The samples with patterned PPy were dried with nitrogen flow and then transferred to a vacuum system for patterned growth of hole transport layer (step V). Experimentally we choose N,N′-bis-(1-naphyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) which is extensively used in OLEDs. [2] Due to the weak interaction of Al 2 O 3 with NPB, the molecules are selectively grown on the PPy area at elevated substrate temperature of 160 °C, forming patterned hole transport NPB layer. In comparison to inert surface of Al 2 O 3 , the presence of PPy provides additional π interaction and hydrogen bonding with NPB, which leads to area selective nucleation of the molecules during the vapor deposition. [30] Finally emitting layer and electron transport layer (such as tris-(8-hydroxyquinoline) aluminum, abbreviated as Alq 3 ), and cathode Al were subsequently deposited to form a complete OLED structure (step VI).
Following the procedure described in Figure 2, we first tested a classic OLED structure of ITO glass/PPy (10 nm)/ NPB (60 nm)/Alq 3 (70 nm)/Liq (1 nm)/Al (140 nm). The PPy thickness of 10 nm was chosen owing to the good transmittance that is shown in Figure 1b   characterized by conductive atomic force microscope (c-AFM) to evaluate the local conductivity over surfaces. Accordingly, the c-AFM images show that the injection current from AFM tip is confined to the patterned PPy area, with almost zero current outside owing to good electrical isolation of Al 2 O 3 . Even with deposition of continuous electron transport layer of Alq 3 , no obvious carrier diffusion outside the patterned PPy area is observed ( Figure S2, Supporting Information).
The performances of OLED devices using PPy as nucleation layer for patterned growth were further characterized. As comparisons, the other two devices, using 10 nm Au as nucleation layer and direct on bare ITO glass, were also fabricated and characterized, using same OLED structure ( Figure S3, Supporting Information). As shown in Figure 3a of normalized electroluminescence spectra, all three samples emit green light with maximum emission peaks located between 520 nm for OLEDs with Au and 540 nm for those with PPy and on bare ITO glass, indicating no obvious effects of interfacial modification by PPy or Au were observed. The blue shift of the maximum emission peak of OLEDs with Au layer can be ascribed to strong absorption of Au at long wavelengths. [31] By increasing the brightness from 19 to 1364 cd m −2 , the emission peak of the OLEDs on PPy keeps at around 520 nm, further confirming stability of the devices by presence PPy as the nucleation layer ( Figure S4, Supporting Information). Figure 3b-d display current efficiency, power efficiency, and external quantum efficiency (EQE) for the three devices, respectively. Generally, despite the ability to achieve high resolution micro OLEDs, the introducing of nucleation layer PPy and Au leads to a decrease of the efficiency. Specifically, current efficiency and EQE of the OLEDs with PPy layer are much improved in comparison to those with Au, close to those on bare ITO glass. In particular, the current efficiency is increased from 1.1 to 3.4 cd A −1 by using PPy as nucleation layer to replace Au, which is about 20% decrease in comparison to the devices on bare ITO glass. However, the both OLEDs with PPy and Au layer show low power efficiency of 1.07 and 0.62 lm W −1 , which is below that on bare ITO glass of 2.27 lm W −1 at 20 mA cm −2 .
To explain the low power efficiency of OLED using the PPy nucleation layer, we measured current density-voltageluminance characteristics of the three devices, as shown in Figure 4a. The devices with Au nucleation layer and on bare ITO glass have low turn-on voltage of 2.75 and 4.16 V at 0.2 mA cm −2 , respectively. The low turn-on voltage is consistent to work functions of Au (5.2 eV) and ITO (4.7 eV) which present low energy barrier for hole injection to highest occupied molecular orbit (HOMO) of NPB (5.2 eV). [19] Meanwhile, the devices with PPy nucleation layer have turn-on voltage of 8.52 V at 0.2 mA cm −2 , which is more than 3 times higher than those on bare ITO glass. Considering energy level diagram of OLEDs as schematically shown in Figure 4b, mismatch of HOMO between PPy (2.8 eV) to NPB (5.2 eV) results in a large injection barrier and the large turn-on voltage. [32] We believe that the turn-on voltage could be decreased by using conductive polymer with high HOMO to match that of NPB, thus in return to improve the device performances including power efficiency.
To demonstrate the merit of using photolithographically patterned PPy as nucleation layer, we further fabricated red, green, and blue micro OLED arrays. The OLED arrays were processed as described in Figure 2. The NPB molecules were selectively Adv. Optical Mater. 2020, 8,1902105  grown on PPy dot arrays with diameter of 10 µm (pixel size of the OLEDs) and periodicity of 12 µm. We note that the area selective growth of NPB on PPy is completely reproducible under optimized substrate temperature and beam flux, which results in high repeatability of micro OLED array fabrication by upon depositing of emissive layer, electron transport layer, and cathode layer. For simplification, the green micro OLEDs use Alq 3 as electron transport layer and emitting layer as tested above. The blue micro OLEDs take 1,3,5-tri(1-phenyl-1 H-benzo[d]imidazol-2-yl)phenyl (TPBi) as electron transport layer. The interface of p-type NPB and n-type TPBi forms an efficient exciplex system for blue light emitting region. [33] Benefiting from energy transfer, the red micro OLEDs simply apply green ones with doping of 2% DCM into Alq 3 by co-deposition. [34] All the micro OLED arrays were fabricated by subsequently stacking of emitting layer/electron transport layer and Al cathode on patterned NPB via vacuum deposition. Figure 5 shows microscope images of green, blue, and red electroluminescent micro OLED arrays achieved by patterned growth of NPB using transparent PPy as nucleation layer. The strategy is to pre-pattern the substrate with conductive nucleation layer which provides a strong interaction with hole transporting molecules. Typically, polymers and self-assembly layers can meet the requirements through π interaction and hydrogen bonding with the hole transporting molecules. [15,30,35] The uniform and defect free micro OLED arrays confirm the possibilities to extend the nucleation layer to various materials for device performance improvements, for example, surface engineering to improve light extraction. Owing to the lithographical procedure developed, the OLED arrays in pixel size of sub-micrometer can be achieved with high yield, and the lateral dimensions of the micro OLEDs, for example, shape and size, are free to design pixel for further applications.
In summary, to improve the light extraction, we use lithographically patterned conducting polymer PPy as nucleation layer for patterned growth of organic hole-transport film. The classic green OLEDs consisted of Al/Alq 3 /NPB/PPy exhibits a maximum current efficiency of 3.41 cd A −1 which is about 80% in comparison to that on bare ITO glass. The high turn-on voltage of the devices using PPy suggests that the performances could be further improved by using polymers with high HOMO to decrease carrier injection barrier. Micro OLED arrays in red, green, and blue color are further demonstrated, confirming potentiality of the strategy toward a photolithography compatible technique with high resolution and high uniformity over large areas. Furthermore, when using hetero-patterns to control carrier recombination regions, multiple emissions in micro OLED arrays could be achieved based on patterned growth, which would enable the full color integration with the technique. [36,37]
Photolithography: The ITO glass with 10 nm Al 2 O 3 was spin-coated with photoresist at 4000 r min −1 , and baked at 100 °C for 3 min. The photoresist was partially exposed to UV light under photomask for 2 s and developed in developer for 15 s.
PPy Synthesis and Liftoff: 113.4 µL pyrrole was dispersed in 6 mL deionized water by mechanical vibration, followed by adding 3 mL aqueous solution containing 30 mg FeCl 3 , and then the ITO glass with patterned photoresist were immediately put into this solution. After keeping the reaction temperature at 20 °C for 6 min, the samples were taken out with a layer of PPy on the surface. Then the PPy on photoresist was removed by liftoff under sonication in acetone at low power for 5 min. The sample was ultrasonically cleaned in ethanol and water for 3 min, and then dried under nitrogen flow.
Patterned Growth of NPB: The samples with patterned PPy were loaded into a home-made vacuum chamber. The organic molecules were sublimed from quartz crucible and deposited onto the sample surfaces at substrate temperature of 160 °C and deposition rate of 0.1 nm min −1 .
OLED Device Fabrication: After patterned growth of the NPB, the sample was kept in the vacuum with substrate temperature and cooled down to room temperature. Then 70 nm Alq 3 layer was deposited with deposition rate of 0.2 nm s −1 . Then the samples were transferred to another vacuum chamber to deposit 140 nm Al as cathode with the deposition rate of 0.5 nm s −1 .
Characterization: The AFM measurements were conducted with a Bruker Dimensional Icon in contact mode. The electroluminescence images were taken with the Olympus microscope BX-FLA (made in Hamburg, Germany) equipped with charge-coupled device (CCD) camera and an objective. The transmittance spectra were measured by a UV-vis/near-IR spectrophotometer (PerkinElmer Lambda 750). The electroluminescence (EL) and current density-voltage (J-V) characteristics of the devices were measured using a constant current source meter unit (Keithley 2400s Source Meter) equipped with a photometer (Photo Research PR 655 spectrophotometer). All the measurements were conducted in ambient air.