Research On The Doping Mechanism In Organic Semiconductors

In recent decades,organic semiconductor materials have attracted more attention from researchers because of their low cost,light weight,wide variety,and simple preparation process.It has made substantial progress till now in the display,lighting and other aspects of the commercial productions.However,compared with the inorganic semiconductor materials,there is still obvious difference in material's mobility and conductivity,Therefore,further study required.Doping is one of the effective methods to improve the carrier conductivity and mobility.However,the quantitative mechanism of the doping of the organic semiconductor material and related electronic structure requires to be further studied.In this thesis,we focus on the influence of doping mechanism on carrier transport properties in organic semiconductor.The research mainly includes the following sections:(1),Mechanism for doping induced p type Fullerenes(C60)using thermally evaporated molybdenum trioxide(MoO3)as a dopant.Thermally evaporated MoO3 doped C60 films,which could change n type features of pristine C60 to form a p type mixed C60 layer,namely the transmission properties have changed,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.(2),Quantitative Fermi level tuning in amorphous organic semiconductor by molecular doping:Toward full understanding of the doping mechanism.The doping mechanism in organic-semiconductor films has been quantitatively studied via ultrahigh-sensitivity ultraviolet photoelectron spectroscopy of a-NPD films doped with HAT(CN)6.We observed that HOMO of a-NPD shifts to the Fermi level(EF)in two different rates with the doping concentration of HAT(CN)6,but HOMO distributions of both pristine and doped amorphous a-NPD films are excellently approximated with a same Gaussian distribution without exponential tail states over?5×1018/(cm3·eV).From the theoretical simulation of the HAT(CN)6-concentration dependence of the HOMO in doped films,we show that the passivation of Gaussian-distributed hole traps,which peak at 1.1 eV above the HOMO onset,occurs at ultralow doping[HAT(CN)6 molecular ratio(MR)<0.01],leading to a strong HOMO shift of 0.40 eV towards EF,and MR dependence of HOMO changes abruptly at MR?0.01 to a weaker dependence for MR>0.01 due to future of the dopant acceptor level.