Solution-Processed p-Dopant as Interlayer in Polymer Solar Cells

We report here an original approach to dope the semiconducting polymer-metal interface in an inverted bulk-heterojunction (BHJ) organic solar cell. Solution processed 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F-4-TCNQ), is deposited on top of a P3HT:PC61BM layer before deposition of the top electrode. Doping of P3HT by F-4-TCNQ occurs after thermally induced diffusion at 100 degrees C of the latter into the BHJ. Diffusion and doping are evidenced by XPS and UV-vis-NIR absorption. XPS highlights the decrease in Fluorine concentration on top of the BHJ after annealing. In the same time, a charge transfer band attributed to doping is observed in the UV-vis-NIR absorption spectrum. Inverted polymer solar cells using solution-processed F-4-TCNQ exhibit power conversion efficiency of nearly 3.5% after annealing. This simple and efficient approach, together with the low annealing temperature required to allow diffusion and doping, leads to standard efficiency P3HT:PC61BM polymer solar cells, which are suitable for printing on plastic flexible substrate.