Bipolar doping between a molecular organic donor-acceptor couple

We demonstrate bipolar doping between a molecular organic system consisting of two planar-stacking compounds: 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and bis(1,2,5-thiadiazolo)-p-quinobis(1,3-dithiole) (BTQBT). Due to the small ionization potential of BTQBT (4.8 eV) and the large electron affinity of PTCDA (4.4 eV), partial electron transfer from BTQBT to PTCDA is observed. Combining ultraviolet photoemission spectroscopy with in-plane conductivity and field-effect mobility measurements, we show that BTQBT molecules act as donors in PTCDA, while PTCDA molecules are acceptors when doped into BTQBT. The strong intermolecular interaction in these materials enables efficient separation of doping-generated charge carriers from the ionized dopant molecules. Infrared absorption spectroscopy shows that the degree of charge transfer between BTQBT and PTCDA is significantly less than one electron. This ultimately limits the overall doping efficiency, which may be improved by using a stronger donor-acceptor couple.