This paper is a demonstration and a guideline to perform and analyze in-house (with a laboratory X-ray instrument) in situ GISAXS experiments of drying inks on roll-to-roll slot-die coated, non-fullerene organic photovoltaics.
This protocol enables us to perform in situ GISAXS studies of the photoactive layer of organic solar cells at our home laboratory, which would otherwise only be possible at Using in situ grazing incidence small angle x-ray scattering, we can study the structure development of the donor-acceptor intermixed state under conditions that are similar to those of large-scale coating. The roll-to-roll slot die coating procedure is going to be performed outside of the GISAXS setup just as a demonstration. Later, it will be in order to show the full experiment.
For slot die coating, wind 18 meters of PET substrate foil onto a feeder roll and detach the free end of the substrate to the winder roll. Run the foil 0.2 meters to tighten the substrate and set the first hot plate of the roll-to-roll setup to 60 degrees Celsius and the second hot plate to 80 degrees Celsius to ensure that the film is dried when wound into the winder roll. When the hot plates have stabilized for about 15 minutes, mount a three milliliter syringe loaded with 2.2 milliliters of roll-to-roll coating ink onto a syringe pump and attach a tube from the syringe to the slot die coating head.
Adjust the horizontal translation stage so the coating head is positioned close to the end of the first hot plate and place the meniscus guide approximately five millimeters above the substrate, then set the syringe pump to a 0.08 milliliter per minute flow rate and a 12.7 millimeter syringe diameter. To control the thickness of the active layer, adjust the flow rate and the speed of the moving substrate according to the formula in which w is the width of the film and row is the density of the materials in the ink. When the pump parameters have been set, manually dispense the ink from the syringe and through the hose, stopping one centimeter before the ink reaches the coating head.
When the meniscus guide is five millimeters above the substrate, start the syringe pump. When a droplet has wet the entire width of meniscus guide, immediately lower the coating head to wet the substrate with the ink and raise the meniscus guide to the coating position two millimeters above the substrate, then start the motor that winds up the substrate and start coating with the ink. To stop the coating, stop the pump and the moving substrate and raise the coating head approximately 20 millimeters above the substrate.
To perform a GISAXS experiment, fasten the mini roll-to-roll coater to the goniometer and mount the goniometer with the roll-to-roll coater on the optical bench at the sample position. Fasten the three motor cables and the goniometer stage to the bench and position the flight tube as close to the mini roll-to-roll coater as possible. Align the sample with the coater and coat 10 centimeters of the ink onto the sample, then roll the film onto the beam.
To align the sample parallel to the beam, scan the summed intensity of the direct beam as a function of the vertical sample position and incidence angle and use the formula to calculate the angle reflected beam on the detector to allow the sample to be aligned to a 0.2 degree incidence angle. To optimize the intensity in the reflected beam, scan the height of the sample position using an incidence angle of 0.2 degrees. Install the beam stop just before the detector to extend the lifetime of the detector and use a circular beam stop for the direct beam.
Place a point suction to remove all of the gases from the evaporating solvents. Mount a three milliliter syringe loaded with 2.2 milliliters of ink onto the syringe pump. Place the coating head 120 millimeters from the x-ray beam along the moving direction of the foil to ensure a drying time of 12 seconds.
When the coating head is in place, position the meniscus guide five millimeters above the substrate and start the syringe pump. When the entire width of the meniscus guide has been wet, immediately lower the coating head to wet the substrate with ink before raising the meniscus guide to the coating position two millimeters above the substrate. When the guide is in place, start the motor that winds up the substrate to begin coating the ink.
Use a camera to monitor the quality of the coated film, looking for de-wetting effects of the film on the substrate and meniscus misalignments. Based on the fitting, it can be deduced that the Teubner-Strey model successfully describes the data for the P3HTEH-IDTBR and the P3HT O-IDTBR for both 12 and 3 seconds of drying. In these tables, the characteristic length scales based on Teubner-Strey model and their corresponding errors can be observed.
For all four fits, the domain size and correlation length for the highest scattering vector are close to the same value. For the large structures, there is a clear tendency for the structures to become larger as they dry. Noticeably, the correlation length is more pronounced after 3 seconds of drying than after 12 seconds of drying for the P3HTO-IDTBR while for P3HTEH-IDTBR, the correlation length is more pronounced after 12 seconds of drying than after 3 seconds of drying.
For the large structures, there is a clear tendency for the structures to become larger as they dry. With this experiment, we have shown that the drying process of EH and O-IDTBR differs on the nanoscale. This protocol can be used to study the new acceptors and to identify the coating parameters that can help us to improve the power combustion efficiency of our flexible solar cells.
In situ x-ray scattering may become an indispensable tool for optimizing industrial processes from the semiconductor to the biomedical industries.
