Electrical and photo-electrical properties of spin-coated nanocrystalline PbS thin films evaluated using thermoelectric effect

Abstract

Nanocrystalline (NC) PbS films have the potential to be used as tunable photodetectors and solar cells, but their electrical properties are not yet understood. The electrical and photo-electrical properties of NC PbS thin films deposited by spin-coating have been investigated. The films are p-type and photoconducting. Electrical conductivity (σ), thermoelectric power (αth), hole concentration (p), and mobility (μ) of the films in dark and under illumination were measured in the temperature range of 77–300 K. These parameters behaved differently in the low temperature region (LTR) of 77–150 K and the high temperature region (HTR) of 250–300 K. In LTR, dark p was constant (∼1014 cm−3) while thermally activated in HTR. In LTR, dark μ varied as T1.5 due to impurity scattering, while in HTR, μ varied as T−(4.2–9.2) due to lattice scattering. Under light, p was ∼1018 cm−3 in LTR while thermal quenching occurred in HTR suggesting the existence of electron traps below the conduction band, which act as sensitization centers. The quenching energy was 130–90 meV, which varied with film thickness. The μ of illuminated PbS films decreased with temperature as T−n due to lattice scattering. The NC PbS films showed high photoconductivity in LTR with a light to dark conductivity ratio of 103. Photoconductivity is explained on the basis of trapping of minority carriers by sensitization centers.