The rate-limiting step of charge generation in charge-generation units (CGUs) composed of a p-doped hole-transporting layer (p-HTL), 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN) and n-doped electron-transporting layer (n-ETL), where 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane (TAPC) was used as the HTL is reported. Energy level alignment determined by the capacitance–voltage (C–V) measurements and the current density–voltage characteristics of the structure clearly show that the electron injection at the HATCN/n-ETL junction limits the charge generation in the CGUs rather than charge generation itself at the p-HTL/HATCN junction. Consequently, the CGUs with 30 mol% Rb₂CO₃-doped 4,7-diphenyl-1,10-phenanthroline (BPhen) formed with the HATCN layer generates charges very efficiently and the excess voltage required to generate the current density of ±10 mA cm⁻² is around 0.17 V, which is extremely small compared with the literature values reported to date.