Temperature dependence of carrier injection across organic heterojunctions

We present a theoretical and experimental study of carrier injection across organic heterojunctions of various barrier heights (0.4–1.0 eV) over a wide range of temperatures. An injection model with proposed escape probability function $wesc$ is formulated to include the total hopping frequencies at both sides of the heterojunction. The model predicts that the injection current at low temperature can be dramatically modified by an extremely small amount of deep trap states. More importantly, the temperature dependence of the injection current is found to decrease with increasing the barrier height. This suggests that extracting the barrier height from the $J$ versus 1/T plot, as commonly employed in literature, is problematic. Experimentally, hole-only heterojunction devices with injection barrier from 0.4 to 1.0 eV were fabricated by using various organic materials. $4,4′,4″$-tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine was chosen as the injecting layer. The accepting layer was $N,N′$-diphenyl-$N,N′$-bis(1-naphthyl)(⁠$1,1′$-biphenyl)-$4,4′$-diamine, tris(8-hydroxyquinoline) aluminum (Alq), $4,4′,4″$-tris(N-carbazolyl)-triphenylamine, or $2,2′,2″$-(1,3,5-phenylene) tris(1-phenyl-1H-benzimidazole). The measured electric field and temperature dependence of the injection currents of the heterojunction devices were in good agreement with the calculation results.