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 IQ21 is specifically designed. The high conjugation building block of 4H-cyclopenta[2,1-b:3,4-b']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 IQ4, the slightly lower efficiency of IQ21 (9.03%) arises from the decrease of VOC, which offsets the gain in JSC. While cosensitized with a smaller D-π-A dye S2, the efficiency in IQ21 is further improved to 10.41% (JSC = 19.8 mA cm(-2), VOC = 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 S2 can distinctly compensate the inherent drawbacks of IQ21, 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.

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Quinoxaline derivatives show great potential in recent organic photovoltaics, not only as polymer acceptors for bulk heterojuction (BHJ) solar cells but also as molecular sensitizers for dye-sensitized solar cells (DSSCs). This work focuses on the effect of π-linkers on photovoltatic performances of D–A−π–A quinoxaline-based sensitizers used for DSSCs. The extension of π-linkers is one of the viable tactics to improve the molar absorption coefficient and red-shift the absorption peak, which is beneficial to light harvesting. With respect to IQ4, a series of quinoxaline sensitizers IQ6, IQ7, and IQ8 were synthesized on the basis of a promising building block of 2,3-diphenylquinoxaline with π-linker modification. Dye IQ8, with an additional thienyl unit near the anchor group, shows little change in absorption spectra and energy levels, while in IQ6 and IQ7, the additional thienyl group close to the donor group obviously red-shifts the absorption band and positively shifts the HOMO levels. In the series of sensitizers, their adsorption amounts on the TiO2 surface are slightly decreased by introduction of a thienyl unit near the donor part and/or the introduction of alkyl chains. Their photovoltaic performances are well evaluated by the electron collection length values (L col), first-principles calculations, the conduction band edge (E CB), and the fluctuation of electron density or charge recombination rate in DSSCs. Instead of the electron injection efficiency (Φinj), the low charge collection efficiency (Φcol) of IQ6, IQ7, and IQ8 results in their unsatisfactory incident photon-to-current conversion efficiency (IPCE) plateaus. Also the difference of V oc among these dyes mainly arises from the fluctuation of TiO2 electron density, which is closely related to the recombination resistance. Upon increasing the thiophene number, the electron collection lengths of IQ6, IQ7, and IQ8 based DSSCs become shorter, which dramatically decreases their photocurrent with an unbeneficial preferable photovoltaic performance. As demonstrated, it is essential to have a judicious design on π-linker modification for high-performance D–A−π–A quinoxaline-based sensitizers.

Dye-sensitized solar cell (DSSC) is considered as a feasible route to the clean and renewable energy conversion technique. The commercial application requires further enhancements on photovoltaic efficiency and simplification on the device fabrication. For avoiding the unpreferable trade-off between photocurrent (JSC) and photovoltage (VOC), here we report the molecular engineering and comprehensive photovoltaic characterization of three cyclopentadithiophene-bridged D-A-π-A motif sensitizers with a change in donor group. We make a careful choice on the donor and conjugation bridge for synergistically increasing JSC and VOC. Comparing with the reference dye WS-2, the photovoltaic efficiency with the single component dye of WS-51 increases by 18%, among one of the rare examples in pure metal-free organic dyes exceeding 10% in combination with traditional iodine redox couples. Moreover, WS-51 exhibits several prominent merits on potentially scale-up industrial application: i) facile synthetic route to target molecule, ii) simple dipping procedure without requirement of co-sensitization, and iii) rapid dye adsorption capability.

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A series of new D–A−π–A indoline-based metal-free dyes, which are derived from the reported dye WS-9, have been designed and characterized theoretically for application in future dye-sensitized solar cells (DSSCs). The absorption, light harvesting efficiency, kinetics of charge injection, and recombination for all chromophores have been systematically investigated via first-principles calculations. We find that featuring the quinoxaline auxiliary acceptor instead of benzothiadiazole and modification on the −A acceptor (2-cyanoacrylic acid) with the −A1 acceptor (2,3-dicyanoacrylic acid) could result in a positive influence in dye CDQ-A1 with respect to WS-9, which induces a remarkable enhancement of the parameters in close connection with the short-circuit current density (J sc) as well as the open-circuit photovoltage (V oc). In comparison to another CDQ dye, CDQ-A4, which differs from CDQ-A1 by only the acceptor group, CDQ-A1 displays outstanding performance due to its key parameters (i.e., absorption wavelength, light harvesting efficiency, dipole moment, less electron recombination) to achieve a balance between competing factors.

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 4H-cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) are introduced into D-A-π-A featured sensitizers. Along with CPDT, instead of thiophene as conjugated bridge, WS-39 and WS-43 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 WS-39 shows a broader IPCE onset with an enhanced photovoltage relative to the analogue WS-5. In contrast, WS-43, 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. WS-43 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 (E(CB)) and charge transfer resistance as well as lifetime of injected electrons are studied in detail. Under standard AM 1.5 conditions, WS-39-based solar cells show a promising photovoltaic efficiency of 9.07% (J(SC) = 16.61 mA cm(-2), V(OC) = 770 mV, FF = 0.71). The octyl chains attached on CPDT can provide dual protection 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 V(OC). Our research paves the way to explore more efficient sensitizers through ingenious molecular engineering.

The cocktail co-sensitization of WS62 and WS64 with S2 can compensate the peak valley of IPCE adsorbed by electrolyte near 400 nm and compact the surface of TiO2 to retard charge recombination, essentially for the optimization of photovoltaic performances.