Much effort has been devoted to the development of polymer solar cells because they can be made inexpensively and offer possibilities for mechanical flexibility. The properties of polymer solar cells depend critically on the electrical and optical properties of the polymer and, as a result, many researchers are looking for ways to tailor these properties for improved performance.
In conventional polymer solar cell architectures, the performance rapidly degrades because the top electrode is readily oxidized. An alternative, inverted structure has been developed that has an air-stable metal as the top contact. In this architecture, the interfacial electron- and hole-transporting materials are key to improving device efficiency. The electron-selective materials used in the devices should have frontier molecular orbitals suited to efficient electron extraction and hole blocking, be solvent resistant (so as not to erode during solution processing), have good conductivity, have small absorption coefficients for visible light, and make smooth films that adhere to the glass and active layers in the device.
Inorganic metal oxides are good candidates, but they absorb oxygen when illuminated, which decreases their stability. Organic semiconductors allow the design of better interfaces using synthetic chemistry approaches and they tend to be highly flexible. However, they usually lack solvent resistance, making them difficult to process. Alex K.-Y. Jen and co-workers have recently reported a way to overcome the problems previously limiting the use of polymer semiconductors through the in situ crosslinking and n-doping of semiconducting polymers, which results in high conductivity and increased solvent resistance.
In the study, thiophene and naphthalene diimide (NDI) copolymers, which are n-type semiconductors, were crosslinked using bis(perfluorophenyl) azide and doped with varying concentrations of an efficient organic n-dopant. The crosslinking prevents erosion during subsequent solution processing steps and the n-doping increases electrical conductivity. Solar cells made from the n-doped, crosslinked polymer showed increased power conversion efficiency with increasing dopant concentration. Overall the properties of the devices were similar to those of a control device made using zinc oxide. The power conversion efficiency for devices with the highest dopant concentrations even exceeded that of the zinc oxide based device. The electrical conductivity of organic thin-film transistors that incorporated the new material increased as the doping concentration was increased. Together these results offer a new step toward improving the performance of all inverted organic solar cells.