D-A-π-A Featured Sensitizers Bearing Phthalimide and Benzotriazole as Auxiliary Acceptor: Effect on Absorption and Charge Recombination Dynamics in Dye-Sensitized Solar Cells

Two organic D-A-π-A sensitizers <b>LS-2</b> and <b>WS-5</b> containing <i>N</i>-octyl substituted phthalimide and benzotriazole as auxiliary electron withdrawing units with similar dimension and structure architecture were systematically studied, focusing on photophysical and electrochemical as well as photovoltaic properties in nanocrystalline TiO₂-based dye-sensitized solar cells (DSSCs). Interestingly, with similar five-member benzo-heterocycles, the two auxiliary acceptors of phthalimide and benzotriazole play exactly different roles in absorption and intramolecular charge transfer: (i) in contrast with <b>WS-5</b> delocalized throughout the entire chromophore, the HOMO orbital of <b>LS-2</b> is mainly located at the donor part due to the twist conformation with the existence of two carbonyl groups in phthalimide; (ii) the dihedral angles of “D-A” plane and “A-π” plane in <b>LS-2</b> further suggest that the incorporation of phthalimide moiety results in curvature of electron delocalization over the whole molecule, in agreement with its blue-shifted, relatively narrow absorption spectra and low photocurrent density; (iii) in contrast with the beneficial charge transfer of benzotriazole in <b>WS-5</b>, the phthalimide unit in <b>LS-2</b> plays an oppositely negative contribution to the charge transfer, that is, blocking intramolecular electron transfer (ICT) from donor to acceptor to some extent; and (iv) in electrochemical impedance spectroscopy, the incorporated benzotriazole unit enhances electron lifetime by 18.6-fold, the phthalimide only increases electron lifetime by 5.0-fold. Without coadsorption of chenodeoxylic acid (CDCA), the DSSCs based on <b>WS-5</b> exhibited a promising maximum conversion efficiency (η) of 8.38% with significant enhancement in all photovoltaic parameters (<i>J</i>_SC = 15.79 mA cm^–2, <i>V</i>_OC = 791 mV, <i>ff</i> = 0.67). In contrast, with the very similar D-A-π-A feature changing the additional acceptor from benzotriazole to phthalimide unit, the photovoltaic efficiency based on <b>LS-2</b> was only 5.11%, decreased by 39%, with less efficient photovoltaic parameters (<i>J</i>_SC = 10.06 mA cm^–2, <i>V</i>_OC = 748 mV, <i>ff</i> = 0.68). Therefore, our results demonstrate that it is essential to choose proper subsidiary withdrawing unit in D-A-π-A sensitizer configuration for DSSCs

Near-Infrared Colorimetric and Fluorescent Cu²⁺ Sensors Based on Indoline–Benzothiadiazole Derivatives via Formation of Radical Cations

The donor–acceptor system of indoline–benzothiadiazole is established as the novel and reactive platform for generating amine radical cations with the interaction of Cu²⁺, which has been successfully exploited as the building block to be highly sensitive and selective near infrared (NIR) colorimetric and fluorescent Cu²⁺ sensors. Upon the addition of Cu²⁺, an instantaneous red shift of absorption spectra as well as the quenched NIR fluorescence of the substrates is observed. The feasibility and validity of the radical cation generation are confirmed by cyclic voltammetry and electron paramagnetic resonance spectra. Moreover, the introduction of an aldehyde group extends the electron spin density and changes the charge distribution. Our system demonstrates the large scope and diversity in terms of activation mechanism, response time, and property control in the design of Cu²⁺ sensors

Cosensitization of D‑A-π‑A Quinoxaline Organic Dye: Efficiently Filling the Absorption Valley with High Photovoltaic Efficiency

In the efficient cosensitization, the pure organic sensitizers with high molecular extinction coefficients and long wavelength response are highly preferable since the dye loading amount for each dye in cosensitization is decreased with respect to single dye sensitization. A D-A-π-A featured quinoxaline organic sensitizer <b>IQ21</b> is specifically designed. The high conjugation building block of 4<i>H</i>-cyclopenta­[2,1-<i>b</i>:3,4-<i>b</i>′]­dithiophene (CPDT) is introduced as the π bridge, instead of the traditional thiophene unit, especially in realizing high molecular extinction coefficients (up to 66 600 M^–1 cm^–1) and extending the light response wavelength. With respect to the reference dye <b>IQ4</b>, the slightly lower efficiency of <b>IQ21</b> (9.03%) arises from the decrease of <i>V</i>_OC, which offsets the gain in <i>J</i>_SC. While cosensitized with a smaller D-π-A dye <b>S2</b>, the efficiency in <b>IQ21</b> is further improved to 10.41% (<i>J</i>_SC = 19.8 mA cm^–2, <i>V</i>_OC = 731 mV, FF = 0.72). The large improvement in efficiency is attributed to the well-matched molecular structures and loading amounts of both dyes in the cosensitization system. We also demonstrated that coabsorbent dye <b>S2</b> can distinctly compensate the inherent drawbacks of <b>IQ21</b>, not only enhancing the response intensity of IPCE, making up the absorption defects around low wavelength region of IPCE, but also repressing the charge recombination rate to some extent

Constructing High-Efficiency D–A−π–A-Featured Solar Cell Sensitizers: a Promising Building Block of 2,3-Diphenylquinoxaline for Antiaggregation and Photostability

Controlling the sensitizer morphology on a nanocrystalline TiO₂ surface is beneficial to facilitating electron injection and suppressing charge recombination. Given that the grafted alkyl chain on a π-bridge thiophene segment for preventing π aggregation can deteriorate its intrinsic photostability, we incorporate a promising building block of 2,3-diphenylquinoxaline as the additional acceptor to construct a novel D–A−π–A-featured dye <b>IQ</b>_<b>4</b>, which exhibits several characteristics: (i) efficiently decreasing the molecular HOMO–LUMO energy gap by extending its absorption bands; (ii) showing a moderate electron-withdrawing capability for an ideal balance in both promising photocurrent and photovoltage; (iii) endowing an ideal morphology control with strong capability of restraining the intermolecular aggregation and facilitating the formation of a compact sensitizer layer via two twisted phenyl groups grafted onto the quinoxaline unit. The coadsorbent-free dye-sensitized solar cell (DSSC) based on dye <b>IQ</b>_<b>4</b> exhibits very promising conversion efficiency as high as 9.24 ± 0.05%, with a short-circuit current density (<i>J</i>_sc) of 17.55 mA cm^–2, an open-circuit voltage (<i>V</i>_oc) of 0.74 V, and a fill factor (FF) of 0.71 under AM 1.5 illumination (100 mW cm^–2). <b>IQ</b>_<b>4</b>-based DSSC devices with an ionic liquid electrolyte can keep constant performance during a 1000 h aging test under 1 sun at 60 °C. Because of spatial restriction, the two phenyl groups grafted onto the additional electron-withdrawing quinoxaline are demonstrated as efficient building blocks, not only improving its photostability and thermal stability but also allowing it to be a successful antiaggregation functional unit. As a consequence, the incorporated 2,3-diphenylquinoxaline unit can realize a facile structural modification for constructing organic coadsorbent-free D–A−π–A-featured sensitizers, thus paving a way to replace the common, stability-deleterious grafted alkyl chain on the thienyl bridge

Consecutive Morphology Controlling Operations for Highly Reproducible Mesostructured Perovskite Solar Cells

Perovskite solar cells have shown high photovoltaic performance but suffer from low reproducibility, which is mainly caused by low uniformity of the active perovskite layer in the devices. The nonuniform perovskites further limit the fabrication of large size solar cells. In this work, we control the morphology of CH₃NH₃PbI₃ on a mesoporous TiO₂ substrate by employing consecutive antisolvent dripping and solvent-vapor fumigation during spin coating of the precursor solution. The solvent-vapor treatment is found to enhance the perovskite pore filling and increase the uniformity of CH₃NH₃PbI₃ in the porous scaffold layer but slightly decrease the uniformity of the perovskite capping layer. An additional antisolvent dripping is employed to recover the uniform perovskite capping layer. Such consecutive morphology controlling operations lead to highly uniform perovskite in both porous and capping layers. By using the optimized perovskite deposition procedure, the reproducibility of mesostructured solar cells was greatly improved such that a total of 40 devices showed an average efficiency of 15.3% with a very small standard deviation of 0.32. Moreover, a high efficiency of 14.9% was achieved on a large-size cell with a working area of 1.02 cm²

Effect of a Long Alkyl Group on Cyclopentadithiophene as a Conjugated Bridge for D–A−π–A Organic Sensitizers: IPCE, Electron Diffusion Length, and Charge Recombination

The option of using conjugated π-linkers is critical for rational molecular design toward an energy-level strategy for organic sensitizers. To further optimize photovoltaic performance, methyl- and octyl-substituted 4<i>H</i>-cyclopenta­[2,1-<i>b</i>:3,4-<i>b</i>′]­dithiophene (CPDT) are introduced into D–A−π–A featured sensitizers. Along with CPDT, instead of thiophene as conjugated bridge, <b>WS-39</b> and <b>WS-43</b> exhibit an extended spectral response due to the excellent conjugation and coplanarity of CPDT. Specifically, we focused on the critical effect of length of the alkyl group linked to the bridging carbon atoms of CPDT on the photovoltaic performances. Octyl-substituted <b>WS-39</b> shows a broader IPCE onset with an enhanced photovoltage relative to the analogue <b>WS-5</b>. In contrast, <b>WS-43</b>, with methyl substituted on the CPDT moiety, presents a relatively low quantum conversion efficiency within the whole spectral response region, along with low photocurrent density. <b>WS-43</b> displays a distinctly low IPCE platform, predominately arising from the short electron diffusion length with significant electron loss during the electron transport. The relative movement of the conduction band edge (<i>E</i>_CB) and charge transfer resistance as well as lifetime of injected electrons are studied in detail. Under standard AM 1.5 conditions, <b>WS-39</b>-based solar cells show a promising photovoltaic efficiency of 9.07% (<i>J</i>_SC = 16.61 mA cm^–2, <i>V</i>_OC = 770 mV, FF = 0.71). The octyl chains attached on CPDT can provide <i>dual protection</i> and exhibit a high propensity to prevent binding of the iodide–triiodide redox couple, producing an efficient shielding effect to retard the charge recombination and resulting in improvement of <i>V</i>_OC. Our research paves the way to explore more efficient sensitizers through ingenious molecular engineering

Rational Molecular Engineering of Indoline-Based D‑A-π‑A Organic Sensitizers for Long-Wavelength-Responsive Dye-Sensitized Solar Cells

Indoline-based D-A-π-A organic sensitizers are promising candidates for highly efficient and long-term stable dye-sensitized solar cells (DSSCs). In order to further broaden the spectral response of the known indoline dye <b>WS-2</b>, we rationally engineer the molecular structure through enhancing the electron donor and extending the π-bridge, resulting in two novel indoline-based D-A-π-A organic sensitizers <b>WS-92</b> and <b>WS-95</b>. By replacing the 4-methylphenyl group on the indoline donor of <b>WS-2</b> with a more electron-rich carbazole unit, the intramolecular charge transfer (ICT) absorption band of dye <b>WS-92</b> is slightly red-shifted from 550 nm (<b>WS-2</b>) to 554 nm (<b>WS-92</b>). In comparison, the incorporation of a larger π-bridge of cyclopentadithiophene (CPDT) unit in dye <b>WS-95</b> not only greatly bathochromatically tunes the absorption band to 574 nm but also largely enhances the molar extinction coefficients (ε), thus dramatically improving the light-harvesting capability. Under the standard global AM 1.5 solar light condition, the photovoltaic performances of both organic dyes have been evaluated in DSSCs on the basis of the iodide/triiodide electrolyte without any coadsorbent or cosensitizer. The DSSCs based on <b>WS-95</b> display better device performance with power conversion efficiency (η) of 7.69%. The additional coadsorbent in the dye bath of <b>WS-95</b> does not improve the photovoltaic performance, indicative of its negligible dye aggregation, which can be rationalized by the grafted dioctyl chains on the CPDT unit. The cosensitization of <b>WS-95</b> with a short absorption wavelength dye <b>S2</b> enhances the IPCE and improves the η to 9.18%. Our results indicate that extending the π-spacer is more rational than enhancing the electron donor in terms of broadening the spectral response of indoline-based D-A-π-A organic sensitizers

Molecular Engineering of Quinoxaline-Based D–A−π–A Organic Sensitizers: Taking the Merits of a Large and Rigid Auxiliary Acceptor

The continuing efforts of creating novel D–A−π–A structured organic sensitizers with excellent optoelectronic properties have resulted in substantial improvement of power conversion efficiency (PCE) as well as stability of dye-sensitized solar cells (DSSCs). Here, we report a new molecular engineering strategy for enhancing optical gain and improving excited-state features in D–A−π–A structured organic sensitizers by improving the conjugation size and rigidity of the auxiliary acceptor functional group. A series of phenanthrene-fused-quinoxaline (PFQ)-based D–A−π–A organic sensitizers (<b>WS-82</b>, <b>WS-83</b>, and <b>WS-84</b>) are designed and synthesized for applications in DSSCs. Compared to 2,3-diphenylquinoxaline (DPQ)-based dye <b>IQ-4</b>, PFQ dyes show extended absorption spectra and improved open-circuit voltage performance. Upon a systematical engineering of alkyl chains and π-spacer structure, the unfavorable issues of PFQ dyes including low solubility and high energy barrier in intramolecular charge transition are successfully eliminated. When applied in iodine electrolyte-based DSSCs, the best performing PFQ dye <b>WS-84</b> shows a PCE of 10.11%, which is much higher than that of our previous champion DPQ dye <b>IQ-4</b> under the same conditions

Additional file 1 of Characterization of a wheat stable QTL for spike length and its genetic effects on yield-related traits

Supplementary Material