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 4T1 state of Mn2+
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- GR 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.