Super Sensitization: Grand Charge (Hole/Electron) Separation in ATC Dye Sensitized CdSe, CdSe/ZnS Type‐I, and CdSe/CdTe Type‐II Core–Shell Quantum Dots

Abstract

Ultrafast charge-transfer dynamics has been demonstrated in CdSe quantum dots (QD), CdSe/ZnS type-I core-shell, and CdSe/CdTe type-II core-shell nanocrystals after sensitizing the QD materials by aurin tricarboxylic acid (ATC), in which CdSe QD and ATC form a charge-transfer complex. Energy level diagrams suggest that the conduction and valence band of CdSe lies below the LUMO and the HOMO level of ATC, respectively, thus signifying that the photoexcited hole in CdSe can be transferred to ATC and that photoexcited ATC can inject electrons into CdSe QD, which has been confirmed by steady state and time-resolved luminescence studies and also by femtosecond time-resolved absorption measurements. The effect of shell materials (for both type-I and type-II) on charge-transfer processes has been demonstrated. Electron injection in all the systems were measured to be <150 fs. However, the hole transfer time varied from 900 fs to 6 ps depending on the type of materials. The hole-transfer process was found to be most efficient in CdSe QD. On the other hand, it has been found to be facilitated in CdSe/CdTe type-II and retarded in CdSe/ZnS type-I core-shell materials. Interestingly, electron injection from photoexcited ATC to both CdSe/CdTe type-II and CdSe/ZnS type-I core-shell has been found to be more efficient as compared to pure CdSe QD. Our observation suggests the potential of quantum dot core-shell super sensitizers for developing more efficient quantum dot solar cells.

Effect of Molecular Coupling on Ultrafast Electron‐Transfer and Charge‐Recombination Dynamics in a Wide‐Gap ZnS Nanoaggregate Sensitized by Triphenyl Methane Dyes

“…In our previous investigation, we have demonstrated ultrafast interfacial electron‐transfer dynamics with a series of TPM dyes with different molecular structures and coupling . We have also reported CT dynamics with TPM‐dye‐sensitized CdSe and CdS QDs and found that a TPM‐dye–QD composite can act as a super‐sensitizer in a QD solar cell …”

Cascading electron and hole transfer dynamics in a CdS/CdTe core–shell sensitized with bromo-pyrogallol red (Br-PGR): slow charge recombination in type II regime

“…In addition to that lower QD loading on TiO 2 surface (as compared to dye loading in DSSC) resulting direct electron transfer from TiO 2 film to the oxidized redox couple in photo anode. However exciton dissociation through hole transfer is not widely reported in literature except few recent reports by us [26][27][28][29] and Kamat & co-workers 30 . Electron injection is much faster as compared to hole transfer, so for efficient devices hole has to extracted from QD surface as fast as possible 20 .…”

Tuning Hole and Electron Transfer from Photoexcited CdSe Quantum Dots to Phenol Derivatives: Effect of Electron‐Donating and ‐Withdrawing Moieties

“…Also, in the case of CdSe/CdTet ype II coreshell particles, emission quenching takes place in the presence of 4NP (electron quencher), in which electrons are supposed to localize in the CdSe core (Figure 4f and f'). In our earlier investigation, we have observed that both electron and hole transfer processes from photoexcited QDs to the molecular adsorbatesc an take place on an ultrafast timescale, [24,44,45] which cannot be accounted for by time-resolved emissionq uenching studies. The emission spectrum of the CdSe/ CdTet ype II core-shell particles is broader,r elative to that of CdSe/CdSq uasi-type II core-shell particles, which showse mission maximaa tl = 575 nm with as houlder at l = 555 nm.…”

“…[44] It shouldb en oted that, althoughe lectrons are delocalized throughout the CBs of both CdSe and CdS in the NC, electron wavefunction overlap between CdSe and 4-NP will be much higher than that of CdSe/CdS and 4-NP composite. B) Schematic illustration of cascading hole transfer from the CdTeshell to 4MP and also cascading from the CdSe core to the CdTes hell and finally to 4MP,and electron transfer from the CdTes hell to 4NP and also leaking from the CdSe core through the CdTes hell to 4NP.…”

“…Charge separation has also been demonstrated in type II coreshell nanostructures, in whicho ne of the carrierl eakages to the shell leads to charge separation. [44,45] In QDs, ah igh fraction of atoms reside on the surface ( % 56 %a toms for 3nmQ Ds), which interacts with the surroundings. [42,43] Exciton dissociation in the core-shell structure in the presence of am olecular adsorbate has also been reported by us.…”