Tushar Debnath

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

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.

Extensive Reduction in Back Electron Transfer in Twisted Intramolecular Charge‐Transfer (TICT) Coumarin‐Dye‐Sensitized TiO2 Nanoparticles/Film: A Femtosecond Transient Absorption Study

We report the synthesis, characterization, and optical and electrochemical properties of two structurally similar coumarin dyes (C1 and C2). These dyes have been deployed as sensitizers in TiO2 nanoparticles and thin films, and the effect of molecular structure on interfacial electron-transfer dynamics has been studied. Steady-state optical absorption, emission, and time-resolved emission studies on both C1 and C2, varying the polarity of the solvent and the solution pH, suggest that both photoexcited dyes exist in a locally excited (LE) state in solvents of low polarity. In highly polar solvents, however, C1 exists in an intramolecular charge-transfer (ICT) state, whereas C2 exists in both ICT and twisted intramolecular charge-transfer (TICT) states, their populations depending on the degree of polarity of the solvent and the pH of the solution. We have employed femtosecond transient absorption spectroscopy to monitor the charge-transfer dynamics in C1- and C2-sensitized TiO2 nanoparticles and thin films. Electron injection has been confirmed by direct detection of electrons in the conduction band of TiO2 nanoparticles and of radical cations of the dyes in the visible and near-IR regions of the transient absorption spectra. Electron injection in both the C1/TiO2 and C2/TiO2 systems has been found to be pulse-width limited (<100 fs); however, back-electron-transfer (BET) dynamics has been found to be slower in the C2/TiO2 system than in the C1/TiO2 system. The involvement of TICT states in C2 is solely responsible for the higher electron injection yield as well as the slower BET process compared to those in the C1/TiO2 system. Further pH-dependent experiments on C1- and C2-sensitized TiO2 thin films have corroborated the participation of the TICT state in the slower BET process in the C2/TiO2 system.

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

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.

Unusually Slow Electron Cooling to Charge-Transfer State in Gradient CdTeSe Alloy Nanocrystals Mediated through Mn Atom.

We have synthesized Mn-doped CdTeSe gradient alloy nanocrystals (NCs) by a colloidal synthetic method, and charge carrier dynamics have been revealed through ultrafast transient absorption (TA) spectroscopy. Due to the reactivity difference between Te and Se, a CdTe-rich core and CdSe-rich shell have been formed in the CdTeSe alloy with the formation of a gradient type II core-shell structure. Electron paramagnetic resonance studies suggest Mn atoms are located in the surface of the alloy NCs. Steady-state optical absorption and emission studies suggest formation of a charge-transfer (CT) state in which electrons are localized in a CdSe-rich shell and holes are localized in a CdTe-rich core which appears in the red region of the spectra. Electron transfer in the CT state is found to take place in the Marcus inverted region. To understand charge-transfer dynamics in the CdTeSe alloy NCs and to determine the effect of Mn doping on the alloy, ultrafast transient absorption studies have been carried out. In the case of the undoped alloy, formation of the CT state is found to take place through electron relaxation to the conduction band of the CT state with a time of 600 fs and through hole relaxation (from the CdSe-rich state to the CdTe-rich state) to the valence band of the CT state with a time scale of 1 ps. However, electron relaxation in the presence of Mn dopants takes place initially via an electron transfer to the Mn 3d state (d(5)) followed by transfer from the Mn 3d state (d(6)) to the CT state, which has been found to take place with a >700 ps time scale in addition to the hole relaxation time of 2 ps. Charge recombination time of the CT state is found to be extremely slow in the Mn-doped CdTeSe alloy NCs as compared to the undoped one, where the Mn atom acts as an electron storage center.

Subpicosecond Exciton Dynamics and Biexcitonic Feature in Colloidal CuInS2 Nanocrystals: Role of In–Cu Antisite Defects

Charge carrier dynamics of multinary quantum dots like CuInS2 (CIS) nanocrystals (NCs) is not clearly understood, especially in ultrafast time scales. Herein we have synthesized colloidal CIS NCs that show defect-induced emission between donor (antisite) and acceptor (internal/surface) states as indicated from steady-state and time-resolved photoluminescence (PL) measurements. Subpicosecond transient absorption (TA) spectra of CIS NCs reveal a gradient of electronic states that exists above the conduction band edge. The electron cooling rate has been determined to be ∼0.1-0.15 eV/ps. The cascade of electron cooling dynamics was monitored after following the TA kinetics at different electronic states. Interestingly, the kinetics at the antisite state unveil a biexcitonic feature, which has been enlightened through a probe-induced biexciton mechanism. With progressively higher fluence (⟨N⟩), the biexciton binding energy increases, and the electron cooling to the antisite state considerably slows down. Extra energy released during Auger recombination of bi/multiexcitons are used to re-excite the electron to a further high energy level, resulting in longer electron cooling time to the antisite states.

Involvement of Sub-Bandgap States in Subpicosecond Exciton and Biexciton Dynamics of Ternary AgInS2 Nanocrystals

We have synthesized three AgxInS2 (AIS) ternary nanocrystals (NCs), where x varies from 0.25 to 1, and reported their biexcitonic feature which depends on the stoichiometry ratio of Ag/In. The broadening of absorption band and dual photoluminescence in different AIS NCs indicates the existence of Ag-related sub-bandgap (S-states) and antisite states. Ultrafast charge carrier dynamics in AIS NCs that involve multiple states like higher excited state, band-edge, Ag-related sub-bandgap, and antisite states have been carried out by employing femtosecond transient absorption spectroscopy, which strongly depends on Ag/In ratio. The probe-induced biexcitonic feature that originated from antisite state has been observed in these AIS NCs even at low pump fluency ( = ∼0.2). The enhancement of binding energy of biexciton and slow down of electron cooling dynamics has been demonstrated by gradual increment of pump fluence as well as with different stoichiometry of Ag and In.

An Insight into the Interface through Excited-State Carrier Dynamics for Promising Enhancement of Power Conversion Efficiency in a Mn-Doped CdZnSSe Gradient Alloy

To explore the significance of impurity doping in power conversion efficiency, quaternary gradient CdZnSSe alloy nanocrystals (NCs) and its Mn-doped analogues were synthesized by high-temperature pyrolysis. The undoped and Mn-doped CdZnSSe alloy NCs have been characterized by employing high-resolution TEM, X-ray diffraction, energy-dispersive X-ray spectroscopy, and electron paramagnetic resonance spectroscopy measurements. A low-temperature injection of chalcogens led to a gradient interface in the alloy, comprised of a CdSe/CdS/ZnSe/ZnS nanostructure. Both steady-state and ultrafast time-resolved absorption studies suggested the formation of a charge-transfer (CT) state due to the inner quasi-type II CdSe/CdS part of the gradient CdZnSSe alloy NCs, in which electrons are delocalized throughout the conduction band (CB) of both CdSe and CdS. The CT-state bleach recovery kinetics gave an additional slow electron cooling component (8 ps) in the undoped alloy NCs, which has been assigned to electron equilibration in the delocalized CB before recombination (or trapping). Interestingly, in the presence of dopant Mn, the slow electron cooling component became even more sluggish at 10 ps due to Mn-mediated electron cooling, in which Mn acts as an electron storage center. An unprecedented increase in the photocurrent conversion efficiency (PCE) of approximately 30 % from (3.3±0.11) to (4.29±0.07) % was observed in the Mn-doped gradient alloy compared with the undoped alloy.

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

Charge-transfer processes from photoexcited CdSe quantum dots (QDs) to phenol derivatives with electron- donating (4-methoxy) and -withdrawing (4-nitro) moieties have been demonstrated by using steady-state and time- resolved emission and femtosecond transient absorption spectroscopy. Steady-state and time-resolved emission studies suggest that in the presence of both 4-nitrophenol (4NP) and 4-methoxyphenol (4MP) CdSe QDs luminescence is quenched. Stern-Volmer analysis suggests both static and dynamic mechanisms are active for both the QD/phenol composites. Cyclic voltammetric analysis recommends that photoexcited CdSe QDs can donate electrons to 4NP and holes to 4MP. To reconfirm both electron- and hole-transfer mechanisms, CdSe/CdS quasi-type II and CdSe/CdTe type II core-shell nanocrystals were synthesized and photoluminescence quenching was monitored in the absence and presence of both 4NP and 4MP, for which hole and electron transfer were systematically restricted. Results suggest that indeed electron and hole transfer take place from photoexcited CdSe to 4NP and 4MP, respectively. To monitor the charge-transfer dynamics in both systems on an early timescale, femtosecond transient absorption spectroscopic techniques have been employed. Electron and hole transfer and charge-recombination dynamics are discussed and the effect of electron-donating and -withdrawing groups has been demonstrated.

Restriction of molecular twisting on a gold nanoparticle surface.

To understand the photophysical properties of intramolecular charge transfer (ICT) and twisted intramolecular charge transfer (TICT) states on a gold nanoparticle (Au NP) surface, we have designed and synthesized a new coumarin molecule (C3) that exists both as ICT and TICT states in its excited state in a polar environment. On a Au NP surface, an excited C3 molecule only exists as an ICT state owing to restricted molecular rotation of a diethylamino group; as a result, no conversion from the ICT to TICT state was observed. Selection of the preferential state of a molecule with dual emitting states can be helpful for selected biological applications.

Slow Electron Cooling Dynamics of Highly Luminescent CdSxSe1-x Alloy Quantum Dot

Ultrafast Electron cooling dynamics of highly luminescent oleic acid caped CdSxSe1-x alloy quantum dot (QD) is investigated by femtosecond transient absorption studies and found to be much slower as compared to pure CdSe and CdS QDs.