Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

The overall goal of this experiment is to demonstrate the printing fabrication of organic thin film solar cells and to observe the thin film morphology evolution during solvent evaporation. This method help answer key questions in functional thin films such as in thin film solar cells and polymer coatings. The main advantage of this technique is that industry processing and synchrotron techniques are well merged to demonstrate the fundamental aspect of the material science.

The implication of this method help understand the morphology evolution of adjoining thin films, which is crucial in determining the final morphology and device performance. So this technique can give us information in the morphology of thin film organic photovoltaics. It could also give us information in other materials, for example, light-emitting diodes or functional thin film materials.

To load the substrate, put the prepared PEDOT:PSS coated indium-tin-oxide substrate onto the base plate of the mini-slot die coater. Using the linear manipulator beneath the substrate plate, adjust the position of the substrate to put it right beneath the printer head. Adjust the head tilting using the two-dimensional tilting manipulator that holds the printing head.

Make sure that the head stands vertically on top of the loaded substrate. Tune the head to substrate distance to zero. The vertical motor is coupled with a force sensor.

When the printing head is floating, a constant force reading will be obtained from the weight of the printing head and the tilting manipulator assemblies. Once the printer head touches the substrate, the reading will reduce, marking the zero position. Set a head to substrate value to run the experiment.

In this experiment, set the head to substrate gap to 100 microns. Next, adjust the linear translational stage motor that will be used to print. Set the 10 millimeter motor position as the starting point, and the 80 millimeter motor position as the end point.

Set the printing speed to 10 millimeters per second by using the motor controlling software interface. Set the motor acceleration speed to 100 meters per second. Load the OPV ink into a one milliliter syringe, and mount the syringe to the syringe pump that is connected to the slot die printer.

Set the printing parameters in the controlling software. To start the printing experiment, type the starting point position in the position window in the controlling software. This action will move the substrate to the starting point.

Start to pump the solution into the slot die head by clicking start in the syringe pump software. Quickly start the translational motor when the solution starts coming out from the printing head. The substrate will move to the end position.

Stop the syringe pump and lift the printing head by using the vertical motor. Turn the vacuum off, and take the substrate off the base plate. Load the printed substrate into a vacuum oven for three to five hours to remove residual solvent.

Next put a Petri dish beneath the printing head. Pump 10 milliliters of chloroform into the printing head to clean it. Collect the contaminated chloroform solution with the Petri dish.

Set up a helium box to suppress air scattering in the x-ray measurement. Mount the mini slot die coater into the helium box. Mount an optical interferometer onto the printing machine to monitor the thickness change over the solvent evaporation.

Put the PEDOT:PSS coated wafer substrate onto the substrate holder of the printer, and adjust the head and substrate position as before. Purge the helium box to remove the air. The oxygen level should be less than 0.3%which can be monitored by an oxygen sensor.

Align the substrate at the position where the x-ray impinges on the substrate, and set the incidence angle, which is 0.16 degrees in this case. Set the x-ray exposure time in the data acquisition method. Here, use two seconds as the exposure time, followed by three seconds of delay time to avoid server beam damage.

Thus, each experiment period is five seconds. Carry out a continuous queue of 100 repeats taking 100 pictures. Name the experiment and choose the data path to save the experimental files.

Move the substrate to the starting position by entering the starting position in the motor controlling software. Start the x-ray shutter, and the detector will continuously record diffraction and scattering signals. Next, start the syringe pump to feed solution into the printing head.

When the solution begins to eject from the printing head as monitored by a surveillance camera, quickly start the printing process. When the pre-chosen measurement position is reached, the two-dimensional detector will capture the scattering signal from the solution. Film thickness will be monitored by an interferometer.

Thus, the thin film morphology evolution will be recorded. Lift up the printer head and clean the head when the experiment is done. Shown here is an ITO substrate coated with conjugated polymer PCBM blends.

The film is quite smooth visually. The beginning and end of the coated film is not always uniform due to the formed meniscus and the drying from the edges. The freshly coated substrate is eventually loaded into shadow masks.

The mask is loaded into an evaporator to deposit the cathode thin layer. Shown here is a completed device after the cathode layer deposition. The device performance is measured using a solar simulator under 100 milliwatts per square centimeter.

Here is a representative current voltage curve of a mini-slot die coated device. An average power conversion efficient of 5.2%is achieved for slot die coated devices, which is close to that achieved by spin coating. This figure represents a typical in situ grazing incidence small angle scattering experiment during solvent drying.

The time evolution is color coded. In the earlier stage of drying where an excess of solvent existed, a red scattering curve is seen and the blends mixed well. A scattering peak gradually developed at around 0.02 inverse angstroms, indicating approximately 60 nanometers of phase separation.

This information, when coupled with in situ x-ray diffraction results, reveals the kinetics of polymer crystallization and phase separation. Once mastered this technique can be done in about five minutes if conducted properly. While attempted these experiments, it is important to remember that for each material system we have to find the ideal condition.

This has to be optimized for every experiment. This method paved the way for the researchers in thin film solar cells to explore the structure and property relationship in industrial manufacturing. After watching this video, you should have a very good understanding of how to conduct an in situ printing experiment at a synchrotron beamline.