Tuning the photoconductivity of Co3O4 thin films by defect engineering

Nanocomposite Co3O4-ZnO Thin Films for Photoconductivity Sensors

“…In this case, the impurity atoms do not interact with each other and form narrow levels in the band gap with the formation of charge carrier transitions with different energies [ 31 ]. In addition, the transition energy in the range of 1.8–2.2 eV can correspond to the transitions of charge carriers between O 2− and Co 2+ , and the transition energy of 1.5–1.6 eV corresponds to the charge transfer between O 2− and Co 3+ [ 18 ]. Transitions of about 1 eV correspond to the charge transfer between Co 2+ and Co 3+ [ 5 ].…”

“…To check the response rate of the derived photosensors, measurements of the film photoconductivity were carried out under a pulsed exposure to LED radiation with wavelengths of 400 nm and 660 nm with a frequency of 5 Hz. To compare the parameters of sensors based on Co 3 O 4 -ZnO films, the normalized photoresponse (Δ I n ) was calculated using Formula (5) [ 18 , 42 ]: …”

“…At the same time, it was shown in the literature [ 16 ] that increasing the concentration of Co from 1 at.% up to 4 at.% in the ZnO structure forces its band gap to extend from 3.3 eV to 3.52 eV. It is also known that the band gap study of Co 3 O 4 -ZnO films by optical methods should be carried out extremely carefully, because the presence of Co 2+ and Co 3+ ions in the spinel structure leads to the existence of several possible energy transitions and, in fact, to a decrease in the band gap [ 17 , 18 ]. Another interesting fact is the formation of p-n junctions in the structure of the Co 3 O 4 -ZnO film with the corresponding appearance of internal electric fields [ 19 ].…”