A semiconducting polymer film, dipped in a solution containing polyoxometalates such as phosphomolybdic acid, can be used to form a single layer organic solar cell, according to a new study. Such a solution-phase partial doping process could also be applied to other organic electronics, the researchers say.

One way to make an organic solar cell is with three layers of materials sandwiched between two electrodes. The middle layer is semiconducting that absorbs light and generates charge carriers. The material underneath the semiconductor helps transfer one type of carrier  (e.g., electrons) to one electrode, while the material atop the semiconductor helps transfer carriers of opposite sign (e.g., holes) to the other electrode. The functions of electron- and hole-collection can be implemented by separate materials or alternately by using a layer comprised of an electrically doped film of the semiconductor used in the light-absorbing layer. However, several layers of materials with varying functions are still needed for a working solar cell.

A common way to deposit each layer of an organic solar cell is to sequentially vacuum evaporate each material. A simpler way to create a layered device involves printing each layer using liquid inks or coating solutions of each material to create films. However, there are several challenges with solution phase manufacturing of organic electronics. Successive solvents can damage previously deposited layers. Mixing dopants with a semiconducting polymer in a printable solution often creates charged components that precipitate out of the liquid. And dopants diffused into a cast semiconductor film often penetrate the entire film, affecting the semiconducting properties needed for a functioning solar cell.

Bernard Kippelen, at the Georgia Institute of Technology, and his colleagues wanted a simpler way to partially dope a semiconducting polymer film. As reported recently in Nature Materials, the researchers accomplished this by dipping a 210-nm thick film of poly(3-hexylthiophene-2,5-diyl) (PH3T), a polymeric semiconductor common in organic electronics, in a solution of nitromethane containing phosphomolybdic acid for one minute. Phosphomolybdic acid is a polyoxometalate that contains 12 molybdenum atoms. In the most efficient organic solar cells, a layer of molybdenum oxide on top of the semiconducting layer helps transfer holes to one electrode. 

The researchers etched their dipped film with an ion beam to help obtain x-ray photoelectron spectra of the molybdenum atoms at various depths. The molybdenum signals were strongest at the surface of the film and decreased exponentially with depth, indicating that the phosphomolybdic acid partially diffused into the film. The transport may have been limited by the size of the polyoxometalate, Kippelen says. Solar cells produced using this method, and with a variety of semiconducting polymers, performed similarly to chemically identical devices constructed with hole-collecting layers deposited by vacuum deposition.

The various interfaces in a typical organic solar cell generate mechanical and chemical instabilities that impact the lifetime of the device. Having eliminated one layer through this partial doping process, the researchers wondered if they could get rid of the other layer. They mixed P3HT and ethoxylated polyethylenimine (PEIE), a polymer used to coat electrodes to help transfer electrons to the metal. As the solution dried into a film atop a metal electrode, the two polymers naturally phase segregated, with the PEIE sinking to the bottom of the film. Then the researchers partially doped the top of the film with molybdenum containing polyoxometalate and covered the material with another metal electrode. The resulting 500-nm thick, single layer solar cell had a power conversion efficiency of about 5.9% and was stable at 60°C for 280 hours. 

Karl Leo, at Technische Universität Dresden, thinks creating a simpler organic solar cell with reasonable efficiency using solution processing is an important step for polymer devices. He notes that although the partially doped devices show improved stability in these initial studies, he believes that the long-term stability under operative conditions needs to be further investigated. 

Read the abstract in Nature Materials.