Making OLEDs and OPVs: A Quickstart Guide
Organic photovoltaic cells (OPVs) or organic light emitting diodes (OLEDs) can be easily manufactured using Ossila's pre-patterned ITO substrates and a few simple spin coating and evaporating steps. This article, and its companion video, will guide you through this process and offer hints and tips for how to get the best devices.
This quickstart video guide demonstrates all the processes and steps required to fabricate organic photovoltaic (solar cell) and organic light emitting diode devices.
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Cleaning substrates is an important part of device manufacture in order to remove surface contaminants. This ensures good wetting of spin-coated layers, and will lead to the best final device performance. The cleaning procedure outlined here involves sonication in Hellmanex III, deionized (DI) water and isopropyl alcohol (IPA) but other materials will also clean effectively, such as sodium hydroxide, acetone, methanol and various dilute acids.
- Place the ITO patterned substrates in a substrate rack to clean them. Placing all the substrates facing the same way will speed up the following steps.
- The substrates should be first cleaned in Hellmanex III. This is an alkaline cleaning solution that helps to remove dirt particles from the substrates, a process that is assisted by sonication. Diluting the hellmanex in boiling water and/or using a heated bath for sonication will improve the cleaning process.
- After sonication in hellmanex, substrates should be sonicated in DI water, to rinse off the hellmanex and further remove dirt particles. It can be helpful to dunk rinse the substrates in DI water after removal from the hellmanex, before placing them in new DI water to sonicate, to ensure complete rinsing.
- After sonication in DI water, substrates should be sonicated in IPA in order to remove further organic contaminants and water. After all sonication steps substrates should be dried with a nitrogen gun.
- Following cleaning, substrates should be UV ozone treated. This both removes surface contaminants and increases surface energy to improve wetting of spin coated layers. 15 minutes of UV Ozone treatment in the Ossila UV Ozone Cleaner is sufficient to give a good surface in most cases.
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Spin Coating the Charge Transport & Active Layers
The charge transport and active layers for an OPV or OLED can be evaporated or spin coated depending on the material used. This video shows conventional architecture OPVs, which means the hole transport layer (HTL) is the first layer coated onto the ITO after cleaning.
- The next step is to spin coat the HTL. Here AI 4083 PEDOT:PSS is used, which should be filtered through a hydrophilic filter before deposition, to remove aggregates.
- To achieve a layer of around 30 nm of AI 4083 PEDOT:PSS, around 40 µL of solution should be dispensed dynamically whilst the substrate is spun at 6000 rpm for 30 seconds.
- Annealing is important to remove solvent from the film. PEDOT:PSS should typically be annealed for 15 minutes at around 110 °C.
- Ossila’s pre-patterned substrates need to be swabbed to expose the appropriate ITO areas. This can be done with a cleanroom swab dipped in an appropriate solvent (such as water or methanol for PEDOT:PSS).
- The substrate needs to be swabbed so that the ITO fingers are exposed but not the central ITO pad (as shown below). This can be done before or after annealing with PEDOT:PSS, but some charge transport layers (such as metal oxides) may convert on annealing and so will need to be swabbed before heating steps.
- Substrates should then be moved into a lab glove box. An optional secondary annealing step of 110 °C for 15 minutes can be performed for PEDOT:PSS inside the glove box, to remove any residual water.
- The active layer should then be spin coated after the first charge transport layer. In this case a non-fullerene acceptor based blend (PBDB-T:ITIC) was spin coated dynamically at 2000 rpm for 40 seconds, using 30 µL of solution. A solid concentration of 20mg/mL in chlorobenzene was used.
- In most cases the active layer should then be annealed. In our case the substrates were heated at 160 °C for 10 minutes inside the glove box.
- The active layer also needs to be swabbed to expose the ITO fingers. This can be done with a cleanroom swab and appropriate solvent in the same manner as the charge transport layer, or by scraping off the layer with a razorblade.
- The final charge transport layer can be spin coated or evaporated. For our OPVs an electron transport layer (ETL) of PFN-Br was spin coated on top of the active layer at 3000 rpm for 30 seconds. A solid concentration of 0.5 mg/mL in methanol was used. The final layer should be swabbed in the same manner as the other layers, but in some cases the active layer and final layer can be swabbed/scraped off at the same time.
Evaporating the Electrode
After all the necessary layers have been coated the final electrode should be evaporated. In this case the OPVs were of a conventional architecture, so the final electrode was the cathode and silver was used, but aluminium or gold can also be used depending on the context.
- Ossila’s pre-patterned substrates can be completed using an Ossila deposition mask. The mask should be filled with the active layer facing down and the glass facing up. The electrode can then be evaporated in a thermal evaporator under vacuum. In the OPVs used in this video a 100 nm silver cathode was evaporated at 1 Å/s.
In most cases the final OPV or OLED should be encapsulated to prevent rapid degradation on exposure to air. Ossila’s encapsulation epoxy should be placed in between the active area and a glass coverslip and cured to a solid using UV light.
- A glass pipette without a pipette bulb can be used to deposit a drop of epoxy onto the substrate. A glass coverslip can then be placed on top of this, pressing down slightly to ensure the epoxy is evenly spread underneath it. The coverslip should be placed so it perfectly covers the active area of the cell, but will not interfere with testing (as shown below).
- The epoxy can then be cured by exposure to UV light. The exposure time will depend on the intensity, but wavelengths up to 350 nm can be used and a 15 minute exposure time was used in the manufacture process shown in the video.
After encapsulation the devices are now ready to test. The procedure for this will depend on if OPVs or OLEDs have been manufactured. For a video guide to testing OLED performance please refer to Ossila’s guide.
To test the finished OPVs, the Ossila I-V Test System can be used. The cell should be placed face down so that the ITO can contact with the metal contacts in the test board. An aperture mask should be placed on top of the substrate to define the active area. For an 8 pixel cell this gives an active area of 2.56 mm2 per pixel. Use a solar simulator to generate reliable and controlled radiation, to the industry standard of AM1.5G.
To test the finished LEDs, the Ossila LED Measurement System can be used.
- The I-V curve software can be used to run JV sweeps and automatically calculate parameters, which will be saved in a handy .csv file for further analysis. For a more in-depth guide to testing using Ossila system, please refer to the manual & automatic video guides.