Highly Efficient Inverted Type-I CdS/CdSe Core/Shell Structure QD-Sensitized Solar Cells

Presynthesized high-quality CdS/CdSe inverted type-I core/shell structure QDs have been deposited onto TiO₂ electrodes after first coating with bifunctional linker molecules, mercaptopropionic acid (MPA), and the resulting quantum dot sensitized solar cells (QDSCs) exhibited record conversion efficiency of 5.32% (<i>V</i>_oc = 0.527 V, <i>J</i>_sc = 18.02 mA/cm², FF = 0.56) under simulated AM 1.5, 100 mW cm^–2 illumination. CdS/CdSe QDs with different CdSe shell thicknesses and different corresponding absorption onsets were prepared <i>via</i> the well-developed organometallic high-temperature injection method. MPA-capped water-dispersible QDs were then obtained <i>via</i> ligand exchange from the initial organic ligand capped oil-dispersible QDs. The QD-sensitized TiO₂ electrodes were facilely prepared by pipetting the MPA-capped CdS/CdSe QD aqueous solution onto the TiO₂ film, followed by a covering process with a ZnS layer and a postsintering process at 300 °C. Polysulfide electrolyte and Cu₂S counterelectrode were used to provide higher photocurrents and fill factors of the constructed cell devices. The characteristics of these QDSCs were studied in more detail by optical measurements, incidental photo-to-current efficiency measurements, and impedance spectroscopy. With the combination of the modified deposition technique with use of linker molecule MPA-capped water-soluble QDs and well-developed inverted type-I core/shell structure of the sensitizer together with the sintering treatment of QD-bound TiO₂ electrodes, the resulting CdS/CdSe-sensitized solar cells show a record photovoltaic performance with a conversion efficiency of 5.32%

Colorimetric and Ratiometric Near-Infrared Fluorescent Cyanide Chemodosimeter Based on Phenazine Derivatives

Two new near-infrared chemodosimeters for cyanide anion based on 5,10-dihexyl-5,10-dihydrophenazine were designed and synthesized. With dicyano-vinyl groups as the recognition site and electron-withdrawing groups on both sides, probe <b>1</b> exhibited an intramolecular charge transfer (ICT) absorption band at 545 nm and emission band at 730 nm, respectively, and thus showed an ICT block process and realized an “on–off” response after bilateral reaction with cyanide anions in CH₃CN. Probe <b>2</b> utilized an unreactive formyl group instead of one of the two reactive dicyano-vinyl groups as the electron-withdrawing component. Due to the unilateral recognition process the ICT of probe <b>2</b> was redirected and lead to a remarkably colorimetric and ratiometric near-infrared (NIR) fluorescent response for cyanine. Both probes provided high sensitivity and selectivity with apparent response signals which can be observed by naked eyes, even in the copresence of various other interference anions. Optical spectroscopic techniques, NMR titration measurements, and density functional theory calculations were conducted to rationalize the sensing mechanisms of these two probes

Pyrimidine-2-carboxylic Acid as an Electron-Accepting and Anchoring Group for Dye-Sensitized Solar Cells

We report a new dye (INPA) adopting pyrimidine-2-carboxylic acid as an electron-accepting and anchoring group to be used in dye-sensitized solar cells. IR spectral analysis indicates that the anchoring group may form two coordination bonds with TiO₂ and so facilitate the interaction between the anchoring group and TiO₂. The INPA-based cell exhibits an overall conversion efficiency of 5.45%, which is considerably higher than that obtained with cyanoacrylic acid commonly used as the electron acceptor

Diketopyrrolopyrrole-Based Ratiometric/Turn-on Fluorescent Chemosensors for Citrate Detection in the Near-Infrared Region by an Aggregation-Induced Emission Mechanism

This work reports two new diketoprrrolopyrrole-based fluorescent chemosensors (<b>DPP-Py1</b> and <b>DPP-Py2)</b> using symmetrical diamides as recognition groups for selective and fast detection of citrate in the near-infrared region. To our delight, <b>DPP-Py1</b> is a ratiometric sensor, whereas <b>DPP-Py2</b> is a turn-on fluorescent sensor. It is worth noting that <b>DPP-Py1</b> has higher accuracy and sensitivity with a relatively lower detection limit (1.8 × 10^–7 M) and better stability in different pH buffers than <b>DPP-Py2</b>. Scanning electron microscopy, dynamic light scattering analyses, ¹H NMR titration, and 2D-NOESY NMR suggested that the fluorescence increment of the probes <b>DPP-Py1</b> and <b>DPP-Py2</b> for citrate could probably originate from aggregation-induced emission (AIE) on the basis of the complexation of the pyridinium-based symmetrical diamides, DPPs, with carboxyl anions of citrate. Our work may provide a simpler and faster means for qualitative and quantitative analysis of citrate through an AIE mechanism

Promoting the Near-Infrared-II Fluorescence of Diketopyrrolopyrrole-Based Dye for In Vivo Imaging via Donor Engineering

Small-molecule dyes for fluorescence imaging in the second near-infrared region (NIR-II, 900–1880 nm) hold great promise in clinical applications. Constructing donor–acceptor–donor (D–A–D) architectures has been recognized to be a feasible strategy to achieve NIR-II fluorescence. However, the development of NIR-II dyes via such a scheme is hampered by the lack of high-performance electron acceptors and donors. Diketopyrrolopyrrole (DPP), as a classic organic optoelectronic material, enjoys strong light absorption, high fluorescence quantum yield (QY), and facile derivatization. Nevertheless, its application in the NIR-II imaging field has been hindered by its limited electron-withdrawing ability and the aggregation-caused quenching (ACQ) effect resulting from the planar structure of DPP. Herein, with DPP as an electron acceptor and through donor engineering, we have successfully designed and synthesized a DPP-based dye named T-27, in which the strong D–A interaction confers excellent NIR absorption and high-brightness NIR-II fluorescence tail emission. By strategically introducing long alkyl chains on the donor unit to increase intermolecular spacing and reduce the influence of solvent molecules, T-27 exhibits an improved anti-ACQ effect in aqueous solutions. After being encapsulated into DSPE-PEG2000, T-27 nanoparticles (NPs) show a relative NIR-II fluorescence QY of 3.4% in water, representing the highest value among the DPP-based NIR-II dyes reported to date. The outstanding photophysical properties of T-27 NPs enable multimode NIR-IIa bioimaging under 808 nm excitation. As such, the T-27 NPs can distinguish mouse femoral vein and artery and achieve cerebral vascular microscopic imaging with a penetrating depth of 800 μm, demonstrating the capability for high-resolution deep-tissue imaging. This work holds significant potential in the field of bioimaging and provides a new strategy for developing bright NIR-II dyes

Studies of Excited-State Properties of Multibranched Triarylamine End-Capped Triazines

Electron donor–acceptor types of multibranched triarylamine end-capped triazines have been systematically investigated by steady-state electronic spectroscopy, electrochemistry, femtosecond fluorescence anisotropy and solvent relaxation dynamics. The results, together with computational approach, have gained in-depth insight into their excited-state properties, especially the interactions between branches. Among different branched triarylamines of one, two and three arms, the interbranch interaction between each arm is weak, as evidenced by their nearly identical absorption spectral profile and frontier orbitals analyses. Upon S₀ → S₁ excitation, the electronic delocalization in the three-branched triarylamine end-capped triazine is resolved to be 680 ± 130 fs, followed by a slow (28 ± 3 ps) electronic localization into one branch and consequently a rotational depolarization of 2.0 ± 0.1 ns. Similar delocalization dynamics was resolved for the two-branched triarylamine end-capped triazine (electronic delocalization, 500 ± 90 fs; twisting localization, 21 ± 5 ps; rotational depolarization, 700 ± 30 ps). The comparable electron delocalization and solvent relaxation time scale may set up a new paradigm to investigate their specific correlation in the early time domain

Comparative Study on Pyrido[3,4‑<i>b</i>]pyrazine-Based Sensitizers by Tuning Bulky Donors for Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) with cobalt electrolytes have gained increasing attention. In this Research Article, two new pyrido­[3,4-<i>b</i>]­pyrazine-based sensitizers with different cores of bulky donors (indoline for <b>DT-1</b> and triphenylamine for <b>DT-2</b>) were designed and synthesized for a comparative study of their photophysical and electrochemical properties and device performance and were also analyzed through density functional theory calculations. The results of density function theory calculations reveal the limited electronic communication between the biphenyl branch at the cis-position of <i>N</i>-phenylindoline and the indoline core, which could act as an insulating blocking group and inhibit the dye aggregation and charge recombination at the interface of TiO₂/dye/electrolyte. As expected, DSSCs based on <b>DT-1</b> with cobalt redox electrolyte gained a higher photoelectric conversion efficiency of 8.57% under standard AM 1.5 G simulated sunlight, with <i>J</i>_sc = 16.08 mA cm^–2, <i>V</i>_oc = 802 mV, and FF = 0.66. Both electrochemical impedance spectroscopy (EIS) and intensity-modulated photovoltage spectroscopy (IMVS) suggest that charge recombination in DSSCs based on <b>DT-1</b> is much less than that in their counterparts of <b>DT-2</b>, owing to the bigger donor size and the insulating blocking branch in the donor of <b>DT-1</b>

Enhanced Photocurrent Density by Spin-Coated NiO Photocathodes for N‑Annulated Perylene-Based p‑Type Dye-Sensitized Solar Cells

The low photocurrent density of p-type dye-sensitized solar cells (p-DSSCs) has limited the development of high-efficiency tandem cells due to the inadequate light-harvesting ability of sensitizers and the low hole mobility of semiconductors. Hereby, two new “push-pull” type organic dyes (PQ-1 and PQ-2) containing N-annulated perylene as electron donor have been synthesized, where the PQ-2-based p-DSSCs show higher photoelectric conversion efficiency (PCE) of 0.316% owing to the higher molar extinction compared to of that PQ-1. Additionally, the photocurrent densities were remarkably increased from 2.20 to 5.85 mA cm^–2 for PQ-1 and 2.45 to 6.69 mA cm^–2 for PQ-2 by spin-coated NiO photocathode based-p-DSSCs, respectively. This results are ascribed to the enhancement of hole transport rate, dye-loading amounts and transparency of NiO films in comparison to that prepared by screen-printing method. Electrochemical impedance spectroscopy and theoretical calculations studies indicate that the molecular dipole moment approaching closer to the NiO surface shifts the quasi-Fermi level to more positive levels, improving open-circuit voltage (<i>V</i>_oc). Intensity-modulated photocurrent spectroscopy illustrates that the hole transit time in NiO films prepared in spin-coating is shorter than that prepared by screen-printing method

Efficient Dye-Sensitized Solar Cells with Voltages Exceeding 1 V through Exploring Tris(4-alkoxyphenyl)amine Mediators in Combination with the Tris(bipyridine) Cobalt Redox System

Tandem redox electrolytes, prepared by the addition of a tris­(<i>p</i>-anisyl)­amine mediator into classic tris­(bipyridine)­cobalt-based electrolytes, demonstrate favorable electron transfer and reduced energy loss in dye-sensitized solar cells. Here, we have successfully explored three tris­(4-alkoxyphenyl)­amine mediators with bulky molecular structures and generated more effective tandem redox systems. This series of tandem redox electrolytes rendered solar cells with very high photovoltages exceeding 1 V, which approaches the theoretical voltage limit of tris­(bipyridine)­cobalt-based electrolytes. Solar cells with power conversion efficiencies of 9.7–11.0% under 1 sun illumination were manufactured. This corresponds to an efficiency improvement of up to 50% as compared to solar cells based on pure tris­(bipyridine)­cobalt-based electrolytes. The photovoltage increases with increasing steric effects of the tris­(4-alkoxyphenyl)­amine mediators, which is attributed to a retarded recombination kinetics. These results highlight the importance of structural design for optimized charge transfer at the sensitized semiconductor/electrolyte interface and provide insights for the future development of efficient dye-sensitized solar cells

Influence of the Donor Size in D−π–A Organic Dyes for Dye-Sensitized Solar Cells

We report two new molecularly engineered push–pull dyes, i.e., <b>YA421</b> and <b>YA422</b>, based on substituted quinoxaline as a π-conjugating linker and bulky-indoline moiety as donor and compared with reported <b>IQ4</b> dye. Benefitting from increased steric hindrance with the introduction of bis­(2,4-dihexyloxy)­benzene substitution on the quinoxaline, the electron recombination between redox electrolyte and the TiO₂ surface is reduced, especially in redox electrolyte employing Co­(II/III) complexes as redox shuttles. It was found that the open circuit photovoltages of <b>IQ4</b>, <b>YA421</b>, and <b>YA422</b> devices with cobalt-based electrolyte are higher than those with iodide/triiodide electrolyte by 34, 62, and 135 mV, respectively. Moreover, the cells employing graphene nanoplatelets on top of gold spattered film as a counter electrode (CE) show lower charge-transfer resistance compared to platinum as a CE. Consequently, <b>YA422</b> devices deliver the best power conversion efficiency due to higher fill factor, reaching 10.65% at AM 1.5 simulated sunlight. Electrochemical impedance spectroscopy and transient absorption spectroscopy analysis were performed to understand the electrolyte influence on the device performances with different counter electrode materials and donor structures of donor−π–acceptor dyes. Laser flash photolysis experiments indicate that even though the dye regeneration of <b>YA422</b> is slower than that of the other two dyes, the slower back electron transfer of <b>YA422</b> contributes to the higher device performance