Morphology Control for Fully Printable Organic–Inorganic Bulk-heterojunction Solar Cells Based on a Ti-alkoxide and Semiconducting Polymer

The overall goal of this experiemnt is to fabricate fully-printable, fullerene-free bulk-heterojunction solar cells using bulky titanium alkoxides as morphology-controlling electron acceptors. This method can help answer key questions in the bulk heterojunction solar cell field about controlling the phase-separation structure of the photoactive layer of organic-inorganic hybrid bulk heterojunction solar cells. The main advantage of this technique is that it controls the phase-separation structure by hindering self-organization with bulky molecules without the solubility limitations of the conventional solvent method.

The implications of this technique extend to obtaining higher overall solar cell efficiency because charged separations and charged transfer occur always in the active layer. So, this method can provide insight into the PFO-DVT electron donor system. It can also be operated with semiconducting polymers such as P3HT or PTV7.

To begin the procedure, using a glass cutter, cut a 1.1-millimeter-thick ITO glass substrate into two-centimeter by two-centimeter squares. Identify the conductive side of each square with a digital multimeter. Cover the conductive side of each square with two strips of masking tape, leaving a two-millimeter by two-centimeter area exposed in the center.

Place a few drops of one-molar hydrochloric acid onto the exposed, conductive area of each square. Allow the squares to sit for three minutes to etch a stripe in the conductive layer. Then, wipe off the hydrochloric acid with a cotton swap, and remove the masking tape.

Place the squares in a glass container, and fill the container with deionized water. Place the container in an ultrasonic cleaner water bath. Run the ultrasonic cleaner at 42 kilohertz for 15 minutes.

Then, clean the squares in acetone and isopropanol for 15 minutes each. Dry the squares under a stream of dry air. Then, clean the dry squares in an ultraviolet ozone cleaner for 30 minutes.

To prepare the precursor solutions, dissolve 0.5 milligrams of PFO-DBT and 1.0 milligrams of the chosen titanium alkoxide in one milliliter of chlorobenzene. Wrap the vial in aluminum foil to exclude light. Heat the precursor mixture to 70 degrees Celsius while stirring at 700 RPM.

Stir the mixture at that temperature for about 20 minutes. Once the solution appears clear, allow it to cool to room temperature. Dissolve 0.5 milligrams of PFO-DBT and 1.0 milligrams of 60-PCBM in one milliliter of chlorobenzene as a reference standard.

Heat, stir, and cool the reference standard under the same conditions as the precursors. To prepare for a spin coating, pre-heat the precursor or reference solution on a hot plate at 70 degrees Celsius while stirring at 700 RPM for 10 minutes. Heat a square of etched ITO glass substrate on a ceramic hot plate at 70 degrees for five minutes.

Then, place the warmed ITO glass square on the center of the spin coater vacuum stage. Reheat this square to 70 degrees with a heat gun, and then turn on the vacuum pump to fix the square in place. Draw 0.5 milliliters of warm precursor or reference solution into a one milliliter syringe, and immediately place the solution on the square.

Then, run the spin coater at 2, 000 to 6, 000 RPM for 60 seconds in the air to create a 50-nanometer-thick film. Allow the surface to dry for 10 minutes at room temperature in the absence of light. Then, use a cotton swab, wetted with chlorobenzene, to remove excess photoactive material.

Dry the surface again under the same conditions. To analyze the phase-separation structure, stop the preparation here, and use an optical or scanning electron microscope to obtain images of the photoactive layer. To print an organic electrode on a photoactive-material-coated ITO glass square, use a screen printer with a 50-micrometer-thick metal mask to print a five-millimeter by 20-millimeter rectangle of PDOT-PSS on the surface of the square.

Allow the electrode to dry for 30 minutes in air at room temperature in the absence of light. Then, cut out a 1.5-centimeter by 2.5-centimeter piece of 1.2-millimeter-thick glass substrate with a diamond cutter. Spread epoxy resin on the glass piece with a plastic spatula.

Place the glass epoxy-side down on the sample, leaving one side of the sample surface exposed. Clean the supporting electrodes with acetone and a cotton swab. Use a ultrasonic soldering iron to solder the supporting electrodes onto the remaining exposed surface of the square.

Measure the current voltage characteristics of the cell with a solar simulator. Repeat this procedure for each titanium alkoxide to be tested and for PFO-DBT as a reference. Organic-inorganic bulk heterojunction solar cells were fabricated using four different titanium alkoxides as electron acceptors.

The current voltage characteristics were significantly affected by the alkoxide properties, which was attributed to differences in the phase-separation structure of the photoactive layer. The short-circuit current densities of the cells using titanium four isopropoxide, ethoxide, and butoxide were much higher than that of the cell using a butoxide polymer. Scanning electron microscopy of the photoactive layers showed acceptable phase-separation structures for the titanium four ethoxide and isopropoxide cells.

The titanium four butoxide cell showed somewhat worse phase separation, which was attributed to the bulkiness of the butoxide. The even-greater bulk of the titanium four butoxide polymer was prohibitive to optimal phase separation, resulting in poor charge generation in the photoactive layer. After each development, this technique paved the way for researchers in the field of organic-inorganic hybrid solar cells to explore the combinations of various materials for highly-efficient photoactive layers.