Mechanism for doping induced p type C₆₀ using thermally evaporated molybdenum trioxide (MoO₃) as a dopant

Thermally evaporated molybdenum trioxide (MoO₃) doped C₆₀ films, which could change n type features of pristine C₆₀ to form a p type mixed C₆₀ layer, are investigated by x-ray and ultraviolet photoelectron spectroscopy. It is found that C₆₀ HOMO progressively shifts closer to the Fermi level after increased MoO₃ doping concentration, and final onset of C₆₀ HOMO is pinned at binding energy of 0.20 eV, indicating the formation of p type C₆₀ films. It is proposed that in charge transfer induced p type C₆₀ formation, due to large electron affinity of MoO₃ (6.37 eV), electrons from HOMO of C₆₀ could easily transfer to MoO₃ to form cations and therefore increase hole concentration, which could gradually push C₆₀ HOMO to the Fermi level and finally form p type C₆₀ films. Moreover, clear different types of C₆₀ species have been confirmed from UPS spectra in highly doped films.