To improve charge carrier injection into or extraction from organic optoelectronic devices, electrically doped layers are often employed. Whereas n-doping of organic semiconductors has been widely used in vacuum processed optoelectronic devices, adequate solution processes to enable future device printing are underdeveloped. In this work, we study n-doping of 1,3,5-tris(1 -phenyl-1H-benzimidazol-2-yl)benzene (TPBi) in a solution process, using sodium as the electron donor. Upon addition of elementary sodium to a clear TPBi/toluene solution, we observed a change in color, indicating charge carrier transfer between sodium and TPBi. The optical and electrical properties of doped and undoped TPBi were characterized in solution and in the corresponding thin-films. Electron Paramagnetic Resonance (EPR) measurements revealed an increase of the number of unpaired spins upon doping, indicating the presence of doping-induced charge carriers. Implementing TPBi : Na as electron extraction layers in organic solar cells, we found almost the same device performance as compared to state-of-the-art solar cells comprising zinc oxide electron extraction layers.
