Authors: William R. Erwin ([email protected])
The fabrication and characterization of organic solar cells using the model system P3HT (Poly(3-hexylthiophene-2,5-diyl)) as the electron donating polymer and PCBM ([6,6]-Phenyl C61 butyric acid methyl ester) as the electron accepting molecule is described.
Begin by using as purchased 1 inch square ITO glass. Because the glass is clean upon arrival, the cleaning steps are limited to ten minutes in a plasma cleaner at medium power. This plasma treatment removes adventitious carbon from the ITO surface making the surface more hydrophilic for PEDOT:PSS deposition and improving the work function of the ITO. When purchasing ITO, pay special attention to the surface roughness; if the surface roughness is too high, it may cause shorting in the device.
Using as purchased PEDOT:PSS, pass the solution through a 0.45 micron PVDF filter using a syringe. Before deposition, blow the substrate off with nitrogen to remove any particulate. For a 40-50 nm layer, drop 250 microliters of solution onto glass and spin coat at 500 rpm for 15 seconds to allow the liquid to spread, followed by a 30 second spin at 3000 rpm. Following deposition, anneal the film on a hot plate at 150 C for 10 minutes to remove any water from the film. Immediately place substrate into tightly fitting petri dish, and move to an nitrogen atmosphere for further processing. The anneal and subsequent transfer to an inert atmosphere serve to ensure that all water is forced from the PEDOT:PSS film.
The active layer consists of P3HT and PCBM (1:1 by mass) in 1,2-dichlorobenzene at a total concentration of 40 mg/mL. The blend should be stirred at 40 C in the glove box overnight. Prior to spin coating, pass the mixture through a 0.45 micron PVDF filter. Drop 200 microliters of the solution onto the substrate and allow it to spread across the substrate. Spin coat using a two step process: 2000 rpm for 2 s followed by 800 rpm for 15 s. The first step removes bulk solvent, increasing overall uniformity of the film, while the second step allows for partial solvent evaporation. Immediately after spin coating, transfer the sample into a well sealed petri dish and allow it to solvent anneal. During the solvent anneal, the film turns from purple to orange, indicating that solvent is evaporating from the film. The solvent anneal is of crucial importance, as it allows for the formation of an ordered bulk heterojunction between the donor (P3HT) and acceptor (PCBM) phases. If spin coating is too long, and no solvent is left for a solvent anneal, the morphology will be “finely mixed,” inhibiting charge transport. If too much solvent is left after spin coating, pinholes may form in the active layer, increasing shorting in the device. After solvent anneal, thermally anneal the sample at 140 C for 10 minutes in an inert atmosphere. Store in a dark, inert atmosphere until cathode deposition.
Place sample face down into evaporation mask. Using a resistive evaporator evaporate 1 nm LiF at a rate of 0.1 Å/s, followed by a 100 nm layer of Al at a rate of 1.5 Å/s.
Test devices over a desired potential range (-1V:1V) under AM 1.5 conditions. Typical devices should exhibit short circuit current densities of 5-15 mA/cm2, open circuit voltages in the range of 0.6-0.7 V and a fill factor between 0.5 and 0.7, resulting in efficiencies up to ~6%.