Electron Trap to Electron Storage Center in Specially Aligned Mn-Doped CdSe d-Dot: A Step Forward in the Design of...

Specially aligned surface-accumulated Mn-doped CdSe (MnCdSe) quantum dots (QDs) have been synthesized to study the effect of dopant atom on charge-carrier dynamics in QD materials. EPR studies suggest that the (4)T1 state of Mn(2+) lies above the conduction band of CdSe, and as a result no Mn-luminescence was observed from MnCdSe. Femtosecond transient absorption studies suggest that Mn atom introduces structural defects in surface-doped CdSe, which acts as electron trap center in doped QD for the photoexcited electron. Bromo-pyrogallol red (Br-PGR) were found to form strong charge-trasfer complex with both CdSe and MnCdSe QDs. Charge separation in both the CdSe/Br-PGR and MnCdSe/Br-PGR composites was found to take place in three different pathways by transferring the photoexcited hole of CdSe/MnCdSe QDs to Br-PGR, electron injection from photoexcited Br-PGR to the QDs, and direct electron transfer from the HOMO of Br-PGR to the conduction band of both the QDs. Hole-transfer dynamics are found to be quite similar (∼1.1 to 1.3 ps) for both of the systems and found to be independent of Mn doping. However, charge recombination dynamics was found to be much slower in the MnCdSe/Br-PGR system as compared with that in the CdSe/Br-PGR system, which confirms that the Mn dopant act as the electron storage center. As a consequence, the MnCdSe/Br-PGR system can be used as a better super sensitizer in quantum-dot-sensitized solar cell to increase efficiency further.