Investigating the Mechanism by Which Intragap States Tailor Light Emission: Case of Copper Deficient and Zn Doped Cu−In−Se Quantum Dots**
Graphical By changing Cu/In molar ratios of Cu−In−Se QDs from 0.3 : 1 to 0.9 : 1, the photoluminescence spectra displayed a remarkable red-shift from 700 nm to 1050 nm. Although there was a blue-shift after the introduction of Zn 2+ dopants. in Cu−In−Se QDs, a significant red-shift occurred from 660 nm to 760 nm when the Zn/Cu molar ratios decreased from 0.7 : 0.3 to 0.3 : 0.7. These results confirmed the strong effect of the Cu/In and Zn/Cu molar ratios on the intrinsic and dopants states related-transition channels, including the band-edge, conduction band-intragap acceptor, as well as the donor-acceptor. The large structural tolerance of I–III–VI group quantum dots (QDs) to off-stoichiometry allows their photoluminescence properties to be adjusted via doping, thereby enabling application in different fields. However, the photophysical processes underlying their photoluminescence mechanism remain significantly unknown. In particular, the transition channels of CuInSe 2 QDs, which are altered by intrinsic and extrinsic intragap states, remain poorly reported. Herein, we investigated the photophysical processes associated with intragap states via electrochemical and optical techniques by using copper deficient Cu−In−Se QDs as well as Zn doped Cu−In−Se QDs. When the Cu/In molar ratios of Cu−In−Se QDs increased from 0.3 : 1 to 0.9 : 1, the photoluminescence spectra displayed a red-shift from 700 nm to 1050 nm. Although there was a blue-shift after the introduction of Zn 2+ dopants in Cu−In−Se QDs, a significant red-shift occurred (from 660 nm to 760 nm) when the Zn/Cu molar ratios decreased from 0.7 : 0.3 to 0.3 : 0.7. The Gaussian deconvolution results of the photoluminescence spectra and the band gap derived from absorption spectra by fitting supported the fact that the optical transition channels are dependent on the Cu/In and Zn/Cu molar ratios. After the introduction of the Zn 2+ ions, the alloyed-resultant blue-shift of the emission spectra was observed, primarily due to the enlarged band gap; however, the radiative recombination of prominent intrinsic intragap states is still observed; and only a small proportion of the band-edge exciton undergoes recombination for the sample with low Zn content. Cyclic voltammetry measurements revealed well-defined extrinsic Zn Cu intragap states (Zn substitution on Cu sites) and intrinsic Cu x (x= 1+/2+) states in the band gap. The results presented here provide a better understanding of the varying effects of dopant on photoluminescence in terms of I–III–VI group QDs.
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