Excitation dependence of steady-state photoluminescence in CdSe nanocrystal films.
The excitonic and deep-level photoluminescence (PL) in CdSe nanocrystal (NC) films (wurtzite type) was studied under continuous-wave excitation as a function of excitation power, temperature, and time of photoaging. It was shown that the intensity-power dependencies are identical for excitonic and deep-level emissions in a wide temperature range. At low temperatures (80-100 K), both emissions were saturated at the laser power used, which generates more than one exciton per nanocrystal. A transition point from the linear to the saturated region was dependent on the temperature, size, and quality of the NCs. A clear inverse dependency between the intensities of excitonic and deep-level emissions was revealed at 80 K over the entire sample area. At room-temperature, the quantum yield dropped significantly and a higher laser power was needed to reach PL saturation. An increase in temperature led to worsening of the reverse dependence between excitonic and deep-level emissions, and at room-temperature, they became uncorrelated. These results can be explained by Auger recombination and also by an increase of nonradiative recombination in the surface states with increasing temperature.