In this paper, we used thermal evaporation method to deposit metal oxide films as inorganic buffer layer in Organic light-emitting diodes (OLEDs), and investigated how the K2CO3-doped NiO (KNO) buffer layers affect OLEDs' performances. The mechanisms of enhanced hole injection into the hole transport layer NPB (alpha-naphthylphenylbiphenyl diamine) via NiO:K2CO3 anode buffer layers in Alq(3)-based and TADF-based OLEDs were studied by means of J-V-L characteristics, secondary ion mass spectroscopy (SIMS), and admittance spectroscopy measurements. For Alq(3)-based OLEDs, devices' opto-electrical performances showed increased of luminance from 8780 cd/m(2) to 22940 cd/m(2), the decreased of turn-on voltage from 3.4 V to 3.1 V at 1 mA/cm(2), and increased of current efficiency from 4.3 cd/A to 3.48 cd/A when 1-nm-thick K2CO3-doped NiO film with doping concentration of 1 mol% was inserted into OLEDs. TADF-based OLEDs' current efficiency increased from 21.7 cd/A to 29.7 cd/A with similar anode buffer layer parameters. SIMS depth profiles on OLEDs suggested higher indium (In) diffusion from ITO anode onto TADF-based OLEDs upon device operation leading to higher roll-off phenomenon. Deposition of NiO:K2CO3 as anode buffer layer onto OLEDs device reduced the diffusion of In in organic layers during device operation. Investigation of the capacitance as function of voltage and frequency yield information about carrier injection characteristics for both Alq(3) and TADF-based OLEDs. Due to better carrier recombination capability, TADF-based OLEDs' slope in V > V-bi area has fewer shifts and more stable response, leading to higher amplitude in current efficiency enhancement in comparison with fluorescents (Alq(3)) material.