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In this review, a discussion on renewable sources of energy with clear focus on solar cell applications is presented. Especially, possible future directions for development of dye-sensitized solar cells (DSSCs) are discussed. Dye-sensitized solar cells have become an important topic of research due to its high importance in energy conversion. Current DSSCs are based on either metal dye sensitizers, metal-free organic dyes or natural dyes. They have been extensively studied due to their low cost, simple preparation methodology, low toxicity, and ease of production. Still there is a need to find more abundant DSSC materials that at same time exhibit long-term stability. Computational studies have been an important ally for developing/designing new dye sensitizers. They are reviewed here with a special emphasis on the benefit of such studies. The conceptual understanding of development and working principle of photoactive DSSC materials are the primary feature of the review followed by examples of studies on different dye sensitizers using scarce to abundant metal based dyes and metal free organic dyes with donor-π-acceptor geometries for both n- and p-type DSSCs. The proper choice of organic dyes including donor, spacer, or acceptor is discussed and a prospective on dual donor based dyes is presented.
Computational studies using DFT and TDDFT approaches have been performed to study a series of metal free donor-л-linker- acceptor (D-л-A) dyes. Taking the novel organic D5 dye (3-(5-(4-(diphenylamino)styryl)thiophen-2-yl)-2-cyanoacrylic acid) as the primary structure, with triphenylamine as the donor, cyanoacrylic acid as the acceptor and a thiophene group as the л-linker, we have studied the effect of different electron withdrawing and electron donating groups at different positions on the dye efficiency upon substitution by analyzing the geometry, charge distribution, electron injection and absorbance spectra. Electron injection has also been addressed for the substituted dyes immobilized on TiO2. Our calculations show that adding electron withdrawing groups at specific positions will lead to a higher efficiency of the dye.
A series of different simple and fused conjugated structures as additional spacers for dyes with triphenylamine (TPA) and cyanoacrylic acid (CAA) as the donor and acceptor units have been designed and investigated with DFT/TDDFT studies to understand the consequence on efficiency for p-DSSCs. According to the calculated charge-transfer properties, Light-harvesting efficiencies and driving forces like hole-injection, dye-regeneration and charge-recombination; benzene and complex fused-ring spacers show effective increase in efficiencies of such dyes. Our analysis thus provides an assistance in designing more efficient p-type photo-sensitizers which contributes to the tandem DSSCs.
There has recently been a growing interest in dye sensitized solar cells (DSSCs) based on ruthenium metal, but due to the scarcity and high price of ruthenium, design of better and cheaper light adsorbent dyes based on more abundant metal ions is one of the key issues for future development of the DSSCs. Using density functional theory (DFT) and time‐dependent DFT we have studied the properties of new and abundant metal ion‐based polypyridyl dyes for p‐type DSSCs and compared with ruthenium and other scarce metal ions. Molecular geometries, electronic structures, and optical absorption spectra have been calculated using an implicit solvent corresponding to acetonitrile. The calculated fair light harvesting efficiency, high hole injection efficiency and Gibbs free energy for the hole injection and longer excited state lifetime (important for reflecting the efficiency of solar cells) for the new abundant metal ions (V³⁺ and Cr²⁺) based dyes could provide promising sensitizers for efficient next generation DSSC's for p‐SC.
We have synthesized a new photoactive ruthenium(II) complex having a pendant catechol functionality (K(2)[Ru(CN)(4)(L)] (1) (L is 4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol) for studying the dynamics of the interfacial electron transfer between nanoparticulate TiO(2) and the photoexcited states of this Ru(II) complex using femtosecond transient absorption spectroscopy. Steady-state absorption and emission studies revealed that the complex 1 showed a strong solvatochromic behavior in solvents or solvent mixtures of varying polarity. Our steady-state absorption studies further revealed that 1 is bound to TiO(2) surfaces through the catechol functionality, though 1 has two different types of functionalities (catecholate and cyanato) for binding to TiO(2) surfaces. The longer wavelength absorption band tail for 1, bound to TiO(2) through the proposed catecholate functionality, could also be explained on the basis of the DFT calculations. Dynamics of the interfacial electron transfer between 1 and TiO(2) nanoparticles was investigated by studying kinetics at various wavelengths in the visible and near-infrared region. Electron injection to the conduction band of the nanoparticulate TiO(2) was confirmed by detection of the conduction band electron in TiO(2) ([e(-)](TiO(2))(CB)) and cation radical of the adsorbed dye (1(*+)) in real time as monitored by transient absorption spectroscopy. A single exponential and pulse-width limited (<100 fs) electron injection was observed. Back electron transfer dynamics was determined by monitoring the decay kinetics of 1(*+) and [e(-)](TiO(2))(CB). This is the first report on ultrafast ET dynamics on TiO(2) nanoparticle surface using a solvatochromic sensitizer molecule.
We have synthesized a new photoactive rhenium(i)-complex having a pendant catechol functionality [Re(CO)(3)Cl(L)] (1) (L is 4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol) for studying the dynamics of the interfacial electron transfer between nanoparticulate TiO(2) and the photoexcited states of this Re(i)-complex using femtosecond transient absorption spectroscopy. Our steady state absorption studies revealed that complex 1 can bind strongly to TiO(2) surfaces through the catechol functionality with the formation of a charge transfer (CT) complex, which has been confirmed by the appearance of a new red-shifted CT band. The longer wavelength absorption band for 1, bound to TiO(2) through the proposed catecholate functionality, could also be explained based on the DFT calculations. Dynamics of the interfacial electron transfer between 1 and TiO(2) nanoparticles was investigated by studying kinetics at various wavelengths in the visible and near infrared regions. Electron injection into the conduction band of the nanoparticulate TiO(2) was confirmed by detection of the conduction band electron in TiO(2) ([e(-)](TiO(2)(CB))) and the cation radical of the adsorbed dye (1˙(+)) in real time as monitored by transient absorption spectroscopy. A single exponential and pulse-width limited (<100 fs) electron injection was observed. Back electron transfer dynamics was determined by monitoring the decay kinetics of 1˙(+) and .
The energy conversion efficiency of dye-sensitized solar cells derived from organic dye molecules has seen immense interest recently. In this work, we report a series of organic donor molecules with enhanced energy conversion efficiency using π-spacers and cyanoacrylic acid as an anchoring group (2−6). Density functional theory (DFT) and time-dependent DFT calculations of these molecules have been performed to examine their electronic structures and absorption spectra before and after binding to the semiconductor titanium dioxide surface. The computational results suggest that dyes 4 and 6 have a larger driving force (ΔG inject = −1.66 and −1.80 eV, respectively) and light-harvesting efficiency (LHE = 0.99) in the series of donor molecules studied. Thus, these dyes should possess a larger short-circuit photocurrent density (J sc) compared to the other examined dyes. The reported ΔG inject (−1.62 eV) and LHE (0.98) for compound 1, calculated with the same level of theory, were lower than those of the designed 4 and 6 dyes. Furthermore, the DFT calculations showed that the open-circuit photovoltage (V oc) is improved with the vertical dipole moment and number of photoinjected electrons for 4 and 6. Dyes 4 and 6 are expected to exhibit high solar-energy-to-electricity conversion.