High‐Performance Förster Resonance Energy Transfer (FRET)‐Based Dye‐Sensitized Solar Cells: Rational Design of Quantum Dots for Wide Solar‐Spectrum Utilization

High‐performance Förster resonance energy transfer (FRET)‐based dye‐sensitized solar cells (DSSCs) have been successfully fabricated through the optimized design of a CdSe/CdS quantum‐dot (QD) donor and a dye acceptor. This simple approach enables quantum dots and dyes to simultaneously utilize the wide solar spectrum, thereby resulting in high conversion efficiency over a wide wavelength range. In addition, major parameters that affect the FRET interaction between donor and acceptor have been investigated including the fluorescent emission spectrum of QD, and the content of deposited QDs into the TiO₂ matrix. By judicious control of these parameters, the FRET interaction can be readily optimized for high photovoltaic performance. In addition, the as‐synthesized water‐soluble quantum dots were highly dispersed in a nanoporous TiO₂ matrix, thereby resulting in excellent contact between donors and acceptors. Importantly, high‐performance FRET‐based DSSCs can be prepared without any infrared (IR) dye synthetic procedures. This novel strategy offers great potential for applications of dye‐sensitized solar cells.     

Raman and photoluminescence investigation of CdS/CdSe quantum dots on TiO2 nanoparticles with multi-walled carbon nanotubes and their application in solar cells

Raman spectroscopy and photoluminescence (PL) were used to investigate the improved short-circuit current density (J_SC) of CdS/CdSe quantum dot (QD)-sensitized solar cells with multi-walled carbon nanotubes (MWCNTs). Raman and PL experiments were carried out in order to explore the hot-electron and cold-electron injections, respectively. The experimental results showed that the concentration of MWCNTs influences the hot-electron and cold-electron injections from CdS/CdSe QDs to TiO₂ nanoparticles. Therefore, the improved J_SC in CdS/CdSe QD-sensitized solar cells can be explained as due to the better electron injections.     

CdSe Quantum Dots and N719‐Dye Decorated Hierarchical TiO2 Nanorods for the Construction of Efficient Co‐Sensitized Solar Cells

Three‐dimensional hierarchical TiO₂ nanorods (HTNs) decorated with the N719 dye and 3‐mercaptopropionic or oleic acid capped CdSe quantum dots (QDs) in photoanodes for the construction of TiO₂ nanorod‐based efficient co‐sensitized solar cells are reported. These HTN co‐sensitized solar cells showed a maximum power‐conversion efficiency of 3.93 %, and a higher open‐circuit voltage and fill factor for the photoanode with 3‐mercaptopropionic acid capped CdSe QDs due to the strong electronic interactions between CdSe QDs, N719 dye and HTNs, and the superior light‐harvesting features of the HTNs. An electrochemical impedance analysis indicated that the superior charge‐collection efficiency and electron diffusion length of the CdSe QD‐coated HTNs improved the photovoltaic performance of these HTN co‐sensitized solar cells.     

The performance of coupled (CdS:CdSe) quantum dot-sensitized TiO2 nanofibrous solar cells

Highly porous networks and reduced grain boundaries with one-dimensional (1-D) nanofibrous morphology offer enhanced charge transport in solar cells applications. Quantum dot (QDs) decorated TiO₂ nanofibrous electrodes, unlike organic dye sensitizers, can yield multiple carrier generations due to the quantum confinement effect. This paper describes the first attempt to combine these two novel approaches, in which CdS (～18 nm) and CdSe (～8 nm) QDs are sensitized onto electrospun TiO₂ nanofibrous (diameter ～80–100 nm) electrodes. The photovoltaic performances of single (CdS and CdSe) and coupled (CdS/CdSe) QDs-sensitized TiO₂ fibrous electrodes are demonstrated in sandwich-type solar cells using polysulfide electrolyte. The observed difficulties in charge injection and lesser spectral coverage of single QDs-sensitizers are solved by coupling (CdS:CdSe) two QDs-sensitizers, resulting in a enhanced open-circuit voltage (0.64 V) with 2.69% efficiency. These results suggest the versatility of fibrous electrodes in QDs-sensitized solar cell applications.     

Quantum dots co-sensitized solar cells: a new assembly process of CdS/CdSe linked to mesoscopic TiO2-nano-SiO2 hybrid film

A green and simple method was found to prepare CdS/CdSe co-sensitized photoelectrodes for the quantum dots sensitized solar cells application. All the assembly processes of CdS and CdSe quantum dots (QDs) were carried out in aqueous solution. CdS and CdSe QDs were sequentially assembled onto TiO₂-nano-SiO₂ hybrid film by two steps. Firstly, CdS QDs were deposited in situ over TiO₂-nano-SiO₂ hybrid film by the successive ionic layer adsorption and reaction (SILAR) process in water. Secondly, using 3-mercaptopropionic acid (3-MPA) as a linker molecule, the pre-prepared colloidal CdSe QDs (~3.0 nm) dissolved in water was linked onto the TiO₂-nano-SiO₂ hybrid film by the self-assembled monolayer technique with the mode of dropwise. The mode is simple and advantageous to saving materials and time. The results show that the photovoltaic performance of the cells is enhanced with the increase of SILAR cycles for TiO₂-nano-SiO₂/CdS photoelectrode. The power conversion efficiency of 2.15 % was achieved using the co-sensitization photoelectrode prepared by using 6 SILAR cycles of CdS plus CdSe (TiO₂-nano-SiO₂/CdS(6)/CdSe) under the illumination of one sun (AM1.5, 100 mW/cm²).     

Electrochemiluminescence resonance energy transfer between quantum dots (QDs) as the donor and Cy5 dye molecules as the acceptor in QD-Cy5 conjugates with biomolecules as the linker

Electrochemiluminescence resonance energy transfer (ECRET) between CdSe/Zns quantum dots (QDs) as the donor and cyanine dye (Cy5) molecules as the acceptor in QD-Cy5 conjugates with DNA or protein as the linker was reported. When a negative potential was applied, the excited-state CdSe/ZnS* was produced in 0.1 mol/L phosphate buffer (pH 7.4) containing 0.1 mol/L K₂S₂O₈ and 0.1 mol/L KNO₃ (PB-K₂S₂O₈). The CdSe/ZnS* went back to the ground-state CdSe/ZnS to emit light at 590 nm or to transfer energy to proximal ground-state Cy5 molecules. The resultant excited-state Cy5 molecules relaxed to their ground state by emitting a light at 675 nm. The ECRET between QDs and Cy5 was used to evaluate interactions between DNAs and to measure conformational changes of DNAs and proteins.     

A Potential Carcinogenic Pyrene Derivative under Förster Resonance Energy Transfer to Various Energy Acceptors in Nanoscopic Environments

Picosecond‐resolved Förster resonance energy transfer (FRET) from various vibronic bands in benzo[a]pyrene (BP) shows a strong dependency on the spectral overlap of an energy acceptor in a confined environment. Our study on the dipolar interactions between BP and different acceptors, including ethidium (Et), acridine orange (AO), and crystal violet (CV), at the surface of a model anionic micelle revealed that the Förster distance (R₀) and the rate of energy transfer is dependent on the individual spectral overlap of the vibronic bands of BP with the absorption spectra of the different energy acceptors. The differential behavior of the vibronic bands is compared with that of different dyes [quantum dots (QDs)] in a “dye‐blend” (mixture) under FRET to an energy acceptor. Comparison of the FRET of the QDs with that of BP confirmed the independent nature of the dipolar interaction of the vibronic bands with other organic molecules, and the use of deconvolution techniques in the interpretation of the donor–acceptor (D –A) distance was also justified. We also showed that the consideration of differential FRET from the vibronic bands of BP and from the QDs in the dye‐blend is equally acceptable in theoretical frameworks including the Infelta–Tachiya model and D –A distribution analysis in nanoenvironments.     

Extending spectrum response of squaraine-sensitized solar cell by Förster resonance energy transfer

To extend the spectral response region of squaraine dye (SQ)-sensitized solar cell, eosin Y (EY) is encapsulated in the SQ-sensitized nanocrystalline thin film. EY is first adsorbed on nanocrystalline TiO₂ thin film (n-TiO₂), then a thin layer of EY contained ZnO (EY-ZnO) is electrodeposited, and SQ dye is finally sensitized to form two dye-sensitized nanocrystalline thin film with a structure of n-TiO₂/EY/EY-ZnO/SQ. There is a perfect spectral overlap between the emission of EY and the absorption of SQ; EY as an energy donor simultaneously transfers both electron and hole to the energy acceptor SQ according to the Förster resonance energy transfer (FRET) process. EY shifts the spectral response edge of SQ-sensitized solar cell toward blue from 550 to 450 nm through the FRET process in this new structure. Two dye-sensitized nanocrystalline thin film demonstrates a significant enhancement in light harvesting and photocurrent generation due to the FRET process. The thickness of the EY-ZnO thin layer and spectral overlap between emission of donor dye and absorption of acceptor dye are two important factors that affect the FRET process between EY and SQ in the structure of n-TiO₂/EY/EY-ZnO/SQ.     

CdS量子点与曙红Y间的荧光共振能量转移研究

在水溶液中,量子点与有机荧光染料之间可能发生荧光共振能量转移(FRET)。本文以发射波长470nm的Cd S量子点为供体,曙红Y为受体,建立了Cd S量子点-曙红Y的FRET体系,研究了该体系的FRET参数。该体系受体供体数目比为8,猝灭效率为45.6%,增强效率为20.1%;供体-受体间的距离为4.4nm;临界能量转移距离为2.4nm。     

High-Efficiency FRET Processes in BODIPY-Functionalized Quantum Dot Architectures

Efficient FRET systems are developed combining colloidal CdSe quantum dots (QDs) donors and BODIPY acceptors. To promote effective energy transfer in FRET architectures, the distance between the organic fluorophore and the QDs needs to be optimized by a careful system engineering. In this context, BODIPY dyes bearing amino-terminated functionalities are used in virtue of the high affinity of amine groups in coordinating the QD surface. A preliminary QD surface treatment with a short amine ligand is performed to favor the interaction with the organic fluorophores in solution. The successful coordination of the dye to the QD surface, accomplishing a short donor–acceptor distance, provides effective energy transfer already in solution, with efficiency of 76 %. The efficiency further increases in the solid state where the QDs and the dye are deposited as single coordinated units from solution, with a distance between the fluorophores down to 2.2 nm, demonstrating the effectiveness of the coupling strategy.     

Fluorescence quenching of CdS quantum dots by 4-azetidinyl-7-nitrobenz-2-oxa-1,3-diazole: a mechanistic study

Fluorescence quenching of CdS quantum dots (QDs) by 4‐azetidinyl‐7‐nitrobenz‐2‐oxa‐1,3‐diazole (NBD), where the two quenching partners satisfy the spectral overlap criterion necessary for Förster resonance energy transfer (FRET), is studied by steady‐state and time‐resolved fluorescence techniques. The fluorescence quenching of the QDs is accompanied by an enhancement of the acceptor fluorescence and a reduction of the average fluorescence lifetime of the donor. Even though these observations are suggestive of a dynamic energy transfer process, it is shown that the quenching actually proceeds through a static interaction between the quenching partners and is probably mediated by charge‐transfer interactions. The bimolecular quenching rate constant estimated from the Stern–Volmer plot of the fluorescence intensities, is found to be exceptionally high and unrealistic for the dynamic quenching process. Hence, a kinetic model is employed for the estimation of actual quencher/QD ratio dependent exciton quenching rate constants of the fluorescence quenching of CdS by NBD. The present results point to the need for a deeper analysis of the experimental quenching data to avoid erroneous conclusions.     

A Brief Overview of Some Physical Studies on the Relaxation Dynamics and Förster Resonance Energy Transfer of Semiconductor Quantum Dots

This article highlights some physical studies on the relaxation dynamics and Förster resonance energy transfer (FRET) of semiconductor quantum dots (QDs) and the way these phenomena change with size, shape, and composition of the QDs. The understanding of the excited‐state dynamics of photoexcited QDs is essential for technological applications such as efficient solar energy conversion, light‐emitting diodes, and photovoltaic cells. Here, our emphasis is directed at describing the influence of size, shape, and composition of the QDs on their different relaxation processes, that is, radiative relaxation rate, nonradiative relaxation rate, and number of trap states. A stochastic model of carrier relaxation dynamics in semiconductor QDs was proposed to correlate with the experimental results. Many recent studies reveal that the energy transfer between the QDs and a dye is a FRET process, as established from 1/d⁶ distance dependence. QD‐based energy‐transfer processes have been used in applications such as luminescence tagging, imaging, sensors, and light harvesting. Thus, the understanding of the interaction between the excited state of the QD and the dye molecule and quantitative estimation of the number of dye molecules attached to the surface of the QD by using a kinetic model is important. Here, we highlight the influence of size, shape, and composition of QDs on the kinetics of energy transfer. Interesting findings reveal that QD‐based energy‐transfer processes offer exciting opportunities for future applications. Finally, a tentative outlook on future developments in this research field is given.     

Overcoming Kinetic Limitations of Electron Injection in the Dye Solar Cell via Coadsorption and FRET

A new, extremely simple concept for the use of energy transfer as a means to the enhancement of light absorption and current generation in the dye solar cell (DSC) is presented. This model study is based upon a carboxy‐functionalized 4‐aminonaphthalimide dye (carboxy‐fluorol) as donor, and (NBu₄)₂[Ru(dcbpy)₂(NCS)₂] (N719) as acceptor chromophores. A set of three different devices is assembled containing either exclusively carboxy‐fluorol or N719, or a mixture of both. This set of transparent devices is characterized via IV‐measurements under AM1.5G and monochromatic illumination and their light‐harvesting and external quantum efficiencies (LHE and EQE, respectively) are determined as well. It is shown that the device containing only the donor chromophore has a marginal power conversion efficiency, thus indicating that carboxy‐fluorol is a poor sensitizer for the DSC. Cyclovoltametric measurements show that the poor sensitization ability arises from the kinetic inhibition of electron injection into the TiO₂ conduction band. Comparing the spectral properties of the DSCs assembled presently, however, demonstrates that light absorbed by carboxy‐fluorol is almost quantitatively contributing to the photocurrent if N719 is present as an additional sensitizer. In this case, N719 acts as a catalyst for the sensitization of TiO₂ by carboxy‐fluorol in addition to being a photosensitizer. Evaluation of the maximum output power under blue illumination shows that the introduction of an energy‐donor moiety via coadsorption, leads to a significant increase in the monochromatic maximum output power. This result demonstrates that energy transfer between coadsorbed chromophores could be useful for the generation of current in dye‐sensitized solar cells.     

The photophysical studies of a mixture of CdTe quantum dots and negatively charged zinc phthalocyanines

Fluorescence resonance energy transfer (FRET) studies were carried out with quantum dots capped with thioglycolic acid (TGA) and 2-mercaptoethanol (2-ME) and negatively charged phthalocyanines {Zn tetracarboxy (ZnTCPc), Zn octacarboxy (ZnOCPc) and Zn tetrasulfo (ZnTSPc) phthalocyinines} in a 0.1 NaOH:EtOH (50:50) solvent mixture. The best overlap between emission spectra of the donor (QDs) and the absorption spectra of the acceptor (ZnPc derivatives) was observed for TGA capped QDs, very little overlap was obtained for 2-ME QDs. ZnTSPc gave the highest FRET efficiency (0.3), with ZnOCPc and ZnTCPc giving a FRET efficiency of 0.2. The Φ_T values of the MPcs generally decreased in the presence of the QD whereas the triplet lifetimes (τ_T) of the ZnPc derivatives were higher in the presence of QDs.     

Impact of FRET between Molecular Aggregates and Quantum Dots

Energy transfer has been employed in third‐generation solar cells for the conversion of light into electrical energy. Long‐range nonradiative energy transfer from semiconductor quantum dots (QDs) to fluorophores has been demonstrated by using CdS QDs and thiophene?BODIPY (boron dipyrromethene, abbreviated as TG2). TG2 shows a broad photoluminescence (PL) spectrum, which varies with concentration. At very low concentrations, monomeric units are present; then, upon increasing the concentration, these monomers form a mixed (J‐/H‐)aggregated state. Energy transfer between the CdS QDs and TG2 was confirmed by separately investigating the interactions between CdS and the monomer of TG2 and between CdS and the aggregated states of TG2. Size‐dependent PL quenching confirmed that nonradiative Förster resonance energy transfer (FRET) from photoexcited CdS QDs to the J‐aggregate state of TG2 was the major energy‐relaxation channel, which occurred on the timescale of hundreds of fs. These results have broad applications in the field of light harvesting based on the assembly of molecular aggregates.     

Enhancing the photoresponse by CdSe-Dye-TiO2-based multijunction systems for efficient dye-sensitized solar cells: A theoretical outlook

This work presents theoretical modeling of some systems, using density functional theory (DFT), for enhancing the photoresponse of a dye-sensitized solar cell. The optimization of the dye (NKX 2587) as well as the dye derivatives was carried out using B3LYP and 6-311g (d,p) level of theory, using DFT as incorporated in Gaussian 03 level of programming. The HOMO–LUMO energy gaps are lower for (CdSe)₁₃-Dye-(TiO₂)₆ multijunction systems in comparison with both the isolated dyes as well as dye-TiO₂ systems. The absorption peaks were found to be mostly red-shifted for (CdSe)₁₃-Dye-(TiO₂)₆ multijunction systems with respect to the Dye-TiO₂ systems, indicating the enhancement of the absorption behavior of the dye sensitizer by its interaction with the CdSe framework. The results thus indicate some sort of co-sensitization of the TiO₂ by the dye sensitizer as well as the CdSe quantum dot and are hence expected to increase the efficiency of the solar device. © 2019 Wiley Periodicals, Inc.     

Synthesis and spectroscopic properties of silica-dye-semiconductor nanocrystal hybrid particles

We prepared silica-dye-nanocrystal hybrid particles and studied the energy transfer from semiconductor nanocrystals (= donor) to organic dye molecules (= acceptor). Multishell CdSe/CdS/ZnS semiconductor nanocrystals were adsorbed onto monodisperse Sto?ber silica particles with an outer silica shell of thickness 2-23 nm containing organic dye molecules (Texas Red). The thickness of this dye layer has a strong effect on the energy transfer efficiency, which is explained by the increase in the number of dye molecules homogeneously distributed within the silica shell, in combination with an enhanced surface adsorption of nanocrystals with increasing dye amount. Our conclusions were underlined by comparison of the experimental results with numerically calculated FRET efficiencies and by control experiments confirming attractive interaction between the nanocrystals and Texas Red freely dissolved in solution.     

PbS/CdS Core–Shell Quantum Dots Suppress Charge Transfer and Enhance Triplet Transfer

A sub‐monolayer CdS shell on PbS quantum dots (QDs) enhances triplet energy transfer (TET) by suppressing competitive charge transfer from QDs to molecules. The CdS shell increases the linear photon upconversion quantum yield (QY) from 3.5 % for PbS QDs to 5.0 % for PbS/CdS QDs when functionalized with a tetracene acceptor, 5‐CT . While transient absorption spectroscopy reveals that both PbS and PbS/CdS QDs show the formation of the 5‐CT triplet (with rates of 5.91±0.60 ns⁻¹ and 1.03±0.09 ns⁻¹ respectively), ultrafast hole transfer occurs only from PbS QDs to 5‐CT . Although the CdS shell decreases the TET rate, it enhances TET efficiency from 60.3±6.1 % to 71.8±6.2 % by suppressing hole transfer. Furthermore, the CdS shell prolongs the lifetime of the 5‐CT triplet and thus enhances TET from 5‐CT to the rubrene emitter, further bolstering the upconverison QY.     

Computational studies on optoelectronic and charge transfer properties of some perylene‐based donor‐π‐acceptor systems for dye sensitized solar cell applications

We report DFT studies on some perylene‐based dyes for their electron transfer properties in solar cell applications. The study involves modeling of different donor‐π‐acceptor type sensitizers, with perylene as the donor, furan/pyrrole/thiophene as the π‐bridge and cyanoacrylic group as the acceptor. The effect of different π‐bridges and various substituents on the perylene donor was evaluated in terms of opto‐electronic and photovoltaic parameters such as HOMO‐LUMO energy gap, λ_max, light harvesting efficiency(LHE), electron injection efficiency (Ø_inject), excited state dye potential (E^dye*), reorganization energy(λ), and free energy of dye regeneration (). The effect of various substituents on the dye–I₂ interaction and hence recombination process was also evaluated. We found that the furan‐based dimethylamine derivative exhibits a better balance of the various optical and photovoltaic properties. Finally, we evaluated the overall opto‐electronic and transport parameters of the TiO₂‐dye assembly after anchoring the dyes on the model TiO₂ cluster assembly.     

Förster resonance energy transfer in poly(methyl methacrylates) copolymers bearing donor–acceptor 1,3‐thiazole dyes

The Förster resonance energy transfer (FRET) properties in poly(methyl methacrylate) copolymers containing 2‐(pyridine‐2‐yl) thiazole dyes were studied upon systematic variation of the donor‐to‐acceptor ratio. To this end, 2‐(pyridine‐2‐yl) thiazole dyes specially designed for the usage as energy donor and acceptor molecules were incorporated within one polymer chain. Poly(methyl methacrylate) copolymers containing these donor and acceptor dyes were synthesized using the RAFT polymerization method. Copolymers with a molar mass (M_n) of nearly 10,000 g/mol were achieved with dispersity index values (?) under 1.3. The presented copolymers act as a model system for the FRET investigation. Förster resonance energy transfer properties of the copolymers are characterized by steady state as well as time resolved fluorescence spectroscopy. The results indicate that the energy transfer rates and the transfer efficiencies are tunable by variation of the donor‐acceptor‐ratio. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4765–4773