Dye‐sensitized solar cells (DSCs) are a promising source of renewable energy. However, the power conversion efficiency (PCE) of devices has been limited largely by the difficulty of producing ...electricity using photons from the near‐infrared (NIR) spectral region. Metal‐free organic sensitizers frequently employ strong electron‐donating or ‐withdrawing moieties to tune the optical band gap to allow the absorption of lower energy wavelengths in charge‐transfer systems, whereas porphyrins and phthalocyanines use substituents to shift the Soret and Q bands toward lower energy absorption. Very few devices employing precious metal‐free dyes have achieved panchromatic and NIR photon conversion for electricity generation at wavelengths >750 nm despite a tremendous number of sensitizers published over the last 25 years. This Minireview seeks to compile a summary of these sensitizers to encourage assimilation, analysis, and development of efficient future sensitizers with absorption extending into the NIR. Herein, we discuss common synthetic strategies, optical properties, and electronic properties of the most successful panchromatic organic sensitizers.
Full spectrum DSCs: Structure–property relationships of organic near‐infrared‐absorbing dye‐sensitized solar cell devices are reviewed with an emphasis on the optical and electronic properties required to achieve precise energetics. General synthetic strategies and device properties are also included to encourage assimilation, analysis, and development of efficient future sensitizers with absorption extending into the NIR.
“The Hagfeldt donor” is a bulky triarylamine building block with four alkyl chains in a 3‐dimensional arrangement that is used with organic dyes in dye‐sensitized solar cells (DSCs) in over 140 ...publications. Many of the highest performing DSC devices in literature make use of this group due to exceptional TiO2 surface protection properties, which slows recombination of electrons in TiO2 with the electrolyte. Importantly, record‐setting cobalt and copper redox shuttle‐based DSCs require exceptional surface protection to slow a facile recombination of electrons to these positively charged redox shuttles. Several syntheses have emerged for the Hagfeldt donor due to the need for iterative aryl–halide cross‐ coupling reactions complicating a straightforward route. Six synthetic strategies found in literature are described along with the challenges of each route. A recent method that has been put forward in the literature as a scalable, regioisomerically pure route is highlighted.
Six routes, the good and the bad: This Minireview focuses on the synthesis of a molecular motif, the Hagfeldt donor, that has found widespread use in dye‐sensitized applications. This building block motif is both incredibly prevalent and has a diverse array of synthetic routes accessing it. Six distinct synthetic routes to this material have been identified in the literature, and each approach is discussed in terms of synthetic efficiency and apparent pitfalls.
Tin oxide (SnO2) is the most attractive alternative to titanium oxide (TiO2) with the aim of identifying a more positive conduction band material for dye‐sensitized solar cells (DSCs). This study ...puts forward a protocol based on grinding, sonication, and centrifuge to generate transparent SnO2 pastes to minimize light reflectance losses from the metal oxide. Under optimized conditions, a highly transparent film with substantially enhanced light penetration depth through active layer SnO2 is realized for efficient light harvesting from two different commercially available powders (18 and 35 nm nanoparticle sizes). A ruthenium sensitizer (B11) and two organic sensitizers (NL3 and MK2) are shown to achieve higher or comparable photocurrent densities with SnO2 relative to standard TiO2‐based DSCs. SnO2‐based DSCs show minimum recombination losses, comparable charge collection efficiencies, and minimal photovoltage losses relative to TiO2 DSCs. Thus, the option of a transparent metal oxide, which can facilitate high photocurrents (>16 mA cm−2 observed) and lower recombination rates than TiO2 is an attractive material for DSC applications.
High transparency: A method to achieve highly transparent and efficient SnO2 films as an alternative to TiO2 for panchromatic and NIR sensitizers in dye‐sensitized solar cells is reported (see figure, IPCE=incident photon‐to‐current conversion efficiency).
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•The importance of deep NIR sensitizers for maximal SSM-DSC performance is shown.•A SSM-DSC with a power conversion efficiency of >11% is realized for the first time.•Sensitizer and ...redox shuttle selection has enabled >2.3 V from a SSM-DSC at >11% PCE.•Minimal potential losses allow for applications in catalysis/solar fuel generation.•A versatile architecture compatible with various solar cell technologies is shown.
A sequential series multijunction dye-sensitized solar cell (SSM-DSC) designed to minimize optical and free energy losses to enable higher device performance is reported. Efficient photon management of solar spectrum irradiation was made possible by: controlling TiO2 film thickness in each subcell leading to balanced photocurrent, careful sensitizer selection based on absorption breadth and intensity for each subcell, and minimizing photon losses by application of an anti-reflective coating with an immersion oil between subcells. By using these strategies with a deep NIR absorbing sensitizer, an SSM-DSC device based on D35/B11/Black Dye demonstrated an overall power conversion efficiency of >11% with a photovoltage of 2.3 V from a single illuminated area device. The PCE of the SSM-DSC is significantly higher than any of the individual subcell in the mechanical stack. This approach has led to a higher performing device than the prior reported high efficiency SSM-DSC devices based on D35/B11/B11 or the all organic dye SSM-DSC based on D35/MK2/AP25 + D35.
Positive to the core: Ullazine has both strong electron‐donating and weak accepting properties. This heterocycle was incorporated into sensitizers for dye‐sensitized solar cells (DSCs). One of these ...sensitizers demonstrated strong light absorption across the UV/Vis region. The corresponding DSC device has a maximum IPCE of 95 % at 520 nm, with a power conversion efficiency of 8.4 %.
Organic dyes that absorb and emit in the near-infrared (NIR) region are potentially noninvasive, high-resolution, and rapid biological imaging materials. Indolizine donor-based cyanine and squaraine ...dyes with water-solubilizing sulfonate groups were targeted in this study due to strong absorptions and emissions in the NIR region. As previously observed for nonwater-soluble derivatives, the indolizine group with water-solubilizing groups retains a substantial shift toward longer wavelengths for both absorption and emission with squaraines and cyanines relative to classically researched indoline donor analogues. Very high quantum yields (as much as 58%) have been observed with absorption and emission >700 nm in fetal bovine serum. Photostability studies, cell culture cytotoxicity, and cell uptake specificity profiles were all studied for these dyes, demonstrating exceptional biological imaging suitability.
Sequential series multijunction dye‐sensitized solar cells (SSM‐DSCs) can power solar‐to‐fuel processes with a single illuminated area device. Dye selection and strategies limiting photon losses are ...critical in SSM‐DSC devices for higher performance systems. Herein, an efficient and readily applicable spin coating protocol on glass surfaces with an antireflective fluoropolymer (CYTOP) is applied to an SSM‐DSC architecture. Combining CYTOP with the use of an immersion oil between glass spacers in a three subcell SSM‐DSC with judiciously selected TiO2 photoanode sensitizers and thicknesses, an overall power conversion efficiency (PCE) of 10.1% is obtained with an output of 2.3 V. Without external bias, this SSM‐DSC configuration shows an impressive overall solar‐to‐fuel conversion efficiency of 6% when powering IrO2 and Au2O3 electrocatalysts for CO2 and H2O to CO and H2 conversion in aqueous solution. The role of CYTOP, immersion oil, sensitizer selection, and film thickness on SSM‐DSC devices is discussed along with the stability of this system.
A simple‐to‐apply antireflective coating, immersion oil application, and chromophore selection enable a processable, ambiently stable solar‐to‐fuel electrolysis device at 6.6% efficiency with no added bias. The sequential series multijunction dye‐sensitized solar cell technology is improved in solar‐to‐electric conversion efficiency substantially by controlling optical loss pathways with a >10% efficiency and 2.3 V output from a single illuminated area.
The bulky triarylamine group commonly referred to as the “Hagfeldt donor” is a key building block found in many of the organic dyes used in dye‐sensitized applications such as dye‐sensitized solar ...cells (DSCs). This building block has gained popularity owing to its presence in many of the best‐performing DSC devices reported to date, which use dyes containing this donor group. The Hagfeldt donor provides a desirable 3‐dimensional structure that aids in surface protection of electrons injected into the semiconductor from oxidants in the electrolyte, allowing for record‐setting cobalt‐ and copper‐based redox shuttles to be utilized more frequently. However, the synthesis of this molecule has proven unreliable for many routes. This study concerns a novel, reliable and scalable five‐step synthesis of the Hagfeldt donor.
We're all going to dye: The bulky triarylamine group commonly referred to as the “Hagfeldt donor” is found in many of the organic dyes used in dye‐sensitized solar cells. However, the synthesis of this molecule has proven unreliable for many routes. This study concerns a novel, reliable and scalable five‐step synthesis of the Hagfeldt donor.
Near-infrared emissive materials with tunable Stokes shifts and solid-state emissions are needed for several active research areas and applications. To aid in addressing this need, a series of ...indolizine-cyanine compounds varying only the anions based on size, dipole, and hydrophilicity were prepared. The effect of the non-covalently bound anions on the absorption and emission properties of identical π-system indolizine-cyanine compounds were measured in solution and as thin films. Interestingly, the anion choice has a significant influence on the Stokes shift and molar absorptivities of the dyes in solution. In the solid-state, the anion choice was found to have an effect on the formation of aggregate states with higher energy absorptions than the parent monomer compound. The dyes were found to be emissive in the NIR region, with emissions peaking at near 900 nm for specific solvent and anion selections.
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