Electron
Trap to Electron Storage Center in Specially
Aligned Mn-Doped CdSe d‑Dot: A Step Forward in the Design of
Higher Efficient Quantum-Dot Solar Cell
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Abstract
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 <sup>4</sup>T<sub>1</sub> state of Mn<sup>2+</sup> 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