D-A-π-A Motif Quinoxaline-Based Sensitizers with High Molar Extinction Coefficient for Quasi-Solid-State Dye-Sensitized Solar Cells

To meet the requirement of high molar extinction coefficient, broaden absorption spectrum and photo/thermal stable for sensitizers of quasi-solid dye-sensitized solar cells (QsDSSCs) with reduced film thickness, a novel D-A-π-A configuration organic sensitizer IQ22 was specifically designed, in which the conjugation bridge of cyclopentadithiophene (CPDT) unit was incorporated to widen the light response and enhance molar coefficients for increasing the shortcircuit current density (JSC), and the octane chain on CPDT was targeted for suppressing the charge recombination and improving the open-circuit voltage (VOC). As a result, the Qs-DSSC based on IQ22 exhibits very promising conversion efficiency as high as 8.76%, with a JSC of 18.19 mA cm , a VOC of 715 mV, and a fill factor (FF) of 0.67 under AM 1.5 illumination (100 mW cm ), standing out in the Qs-DSSCs utilizing metal-free organic sensitizers.


INTRODUCTION
In the fields of dye-sensitized, perovskite and other emerging solar cell technologies, [1][2][3][4][5][6] stability issue on cell devices is recognized as an urgent problem for a new generation of practical photovoltaic devices.Although perovskite solar cells can show efficiency of more than 20%, the superiority in environmental friendliness and stability of DSSCs can be more attractive for some applications. 6Accordingly, for low-cost, high-efficiency dye-sensitized solar cells (DSSCs), the development of acceptably stable quasi-solid-state or solid-state devices, without any volatile electrolyte solution, is critical at present.Due to facile diffusion in porous TiO2 and the large area contact with counter electrode, the quasi-solid state dye-sensitized solar cell (Qs-DSSC) is not only conducive to a high power conversion efficiency (PCE), [7][8] but also can significantly improve the device stability, thereby becoming a hot topic in the DSSCs field.2][13][14][15] In this regard, Qs-DSSCs always need thin TiO2 electrode, and accordingly an ideal sensitizer should exhibit strong light-harvesting capability.2][23][24][25][26] As an exemplary D-A-π-A featured dye with quinoxaline as additional unit, IQ4 showed high PCE of 9.24% with volatile iodine electrolyte. 27Herein we focused on how to increase molar extinction coefficients and broaden light-responsive region, thereby specifically developing targeted sensitizers for constructing high performance Qs-DSSCs.Based on IQ4, we report a new D-A-π-A DSSC sensitizer IQ22 (shown in Scheme 1), in which the conjugation bridge of cyclopentadithiophene (CPDT) unit was incorporated to widen the light response and enhance molar coefficients for increasing the short-circuit current density (JSC), the octane chain on CPDT was targeted for suppressing the reverse current and improving the open-circuit voltage (VOC).As demonstrated, a commercial I -/I3 -polymer gel electrolyte OPV-MPV-I was successfully exploited for fabricating Qs-DSSCs utilizing IQ22 and IQ4 as sensitizers, achieving high conversion efficiencies of 8.76% and 8.30% under 100 mW cm -2 illumination, respectively, which is an exhilarating PCE for Qs-DSSCs based on metal-free organic sensitizers.Furthermore, these devices showed excellent stability, almost maintaining the initial conversion efficiency even after 1000 h.

RESULTS AND DISCUSSION
Charge Transport Difference between Gel and Volatile Electrolytes.In order to quantify any bottleneck limiting the efficiency for Qs-DSSCs, we first performed conductivity (σ) measurements of gel and volatile electrolytes (See experimental section) with electrochemical impedance spectroscopy. 28This offers key information about the mobility of the ions, their interaction with the solvent and any ion-pairing phenomena.As shown in Figure 1a, all plots of ln σ against 1000/T give straight lines, which is typical ion-conducting behavior follow an Arrhenius relationship. 29In Figure 1a, the obvious difference between the gel and volatile electrolyte is the variation of the slope with measurement temperature.With increasing temperature, the conductivity of the gel electrolyte rises rapidly owing to the viscosity reduction, whereas the change for the volatile electrolyte shows a more gradual process.At room temperature, the conductivity of the gel and volatile electrolyte are 2.10 × 10 -3 S cm -1 and 3.0 × 10 -3 S cm -1 , respectively.1] In the Tafel and diffusion zone, the exchange current density (J0) and limiting diffusion current density (Jlim) values lie in the order of volatile > gel for IQ22 and IQ4, respectively, indicating that the gel electrolyte has some limitations in the charge-transfer and diffusion likely due to the greater viscosity. 32As shown in Table 1 however, the change of electrolyte did not cause a significant impact on the short-circuit current density (JSC) for IQ22 or IQ4.

Enhancement in Molar Extinction
Coefficients and Light-Harvesting Capability.In DSSCs, the sensitizers with high extinction coefficients allow reduced thickness of TiO2 electrode, which results in increase of the average optical power density within the film and decrease of the charge recombination sites, as the film becomes thicker the charges have more chance of recombining before influencing the potential at the electrodes. 337][38] We also used the standard group cyanoacetic acid as the acceptor/anchor unit.
For an ideal sensitizer, high molecular extinction coefficient and broad wavelength response are highly preferable for high-efficiency DSSCs.On the basis of the rational molecular design, the structure modification in this report was conducted by introducing the high conjugation building block of CPDT as π bridge (IQ22), instead of the thiophene unit (IQ4).In particular, dye IQ22 exhibits appropriate photo-physical properties, such as a higher molecular extinction coefficient and broad light response region.The addition of two methoxy groups in the auxiliary (quinoxaline) unit can fine-tune and optimize the EOX (The first oxidation potential) and E*RED (The excitedstate reduction potential), through certain electron-donating character.2][13][14][15] The synthetic route is presented in Figure S1.

Experimental and Calculated Absorption Properties.
To preliminarily estimate the effect of structure modification on the light-harvesting capacity for IQ22, we tested the UV-Vis absorption spectra of the two dyes in CH2Cl2 (Figure 2a) and their corresponding data are summarized in Table 1.Both dyes show two distinct absorption bands at about 325 and 540 nm, corresponding to the π-π* and ICT bands, respectively.With respect to IQ4, IQ22 presents a notable bathochromic shift in the maximum visible absorption wavelength from 531 to 555 nm, which arises from the introduction of a large π-linker CPDT unit.As expected, replacing the thiophene unit (in IQ4) with CPDT (in IQ22) is beneficial for greatly enhancing the molar extinction coefficient (up to 63200 M −1 cm −1 , 2.47-fold greater than that of the reference dye IQ4) and light-harvesting with red shift in absorption band, which are the preconditions for obtaining a high photocurrent output.Upon loading on TiO2 films (Figure 2b), both dyes show hypsochromic shift from 531 to 497 nm for IQ4 and from 555 to 516 nm for IQ22, due to the deprotonation of the cyanoacetic acid group.
Obviously, IQ22 shows a higher and broader spectrum in the visible region relative to IQ4.On the other hand, the broader peak width on the films indicate that aggregation inevitably occurs to some extent in these films for both dyes.The high molar extinction coefficient and wide absorption spectrum for IQ22 exactly cater to the reduced film thickness requirements of Qs-DSSCs since the charge recombination and diffusion problems in nano-porous membrane.
Electronic structures of IQ4 and IQ22 were calculated and investigated with DFT calculations.The selected Kohn-Sham (KS) molecular orbital distributions and energies of IQ4 and IQ22 are shown in Table S1 and Table S2, respectively.For both IQ4 and IQ22, the KS HOMO is mainly located on the strong electron-donating unit of indoline, while more centralized on indoline group for IQ22.The location of KS LUMO is on the electron-withdrawing unit of cyanoacetic acid, and was not affected by the alkane chains added.In Scheme 1, the energy level schemes of selected Kohn-Sham orbitals of IQ4 and IQ22 are shown, indicating good charge separation after excitation by photons.

Time-dependent DFT (TDDFT) calculations allow comparison of absorption spectroscopy
both experimentally and theoretically, and thus the electronic transitions were studied (Figure 2a).
The TDDFT calculations for IQ4 and IQ22 show broadly good agreement with experimental absorption spectra, and the pathways for excitation and electron injection process can be learned by studying the computational results.For IQ22, the first electronic transition, which is calculated to be at 521 nm, is characterized by HOMO → LUMO contribution (52%) and HOMO-1 → LUMO contribution (39%), and the absorption at 413 nm is mainly composed of HOMO → LUMO +1 (47%) and HOMO-1 → LUMO (29%).For IQ4, the calculated low-energy electronic transition (at 483 nm) is composed of HOMO → LUMO (69%) and HOMO -  38 Moreover, the other difference of CV curves for IQ22 and IQ4 is two pairs of submits were observed for IQ22.In previous investigations, for the sensitizer based on quinoxaline group with alkoxy chains 39 , a single oxidation peak appeared in its CV curve, but for that containing CPDT unit 40,41 , two obvious oxidation peaks were got.Therefore, two oxidation peaks in CV for IQ22 is owing to CPDT unit was involved in the redox process in addition to the electron donor indoline.As shown in Figure 4a, the light harvesting efficiency (LHE) spectra were calculated from the absorption spectra of the dye-loaded TiO2 films (LHE = 1-10 -α , where α is the intensity of the light absorption). 44For IQ22, the expanded conjugated system with CPDT improves the molar extinction coefficient and broadens the absorption spectrum, so its LHE showed significant enhancement in the visible range of 450-700 nm.To shed light on the contribution of absorption at different wavelengths to the JSC, we also measured the incident-photon-to-current conversion efficiency (IPCE) action spectra for the DSSCs based on the IQ22 and IQ4 with volatile or gel electrolyte (Figure 4b).As shown in Figure 4b, almost all the IPCE values between 300-800 nm slightly decrease for the gel electrolyte with IQ22 or IQ4, which is due to its higher viscosity hence lower conductivity.But for IQ22 compared with IQ4, the higher and broader IPCE spectrum with volatile or gel electrolyte can be attributed to its larger conjugated system and branched structure further improving the intramolecular charge separation and charge recombination inhibitory ability which is highly beneficial to the JSC of Qs-DSSC.1).

The Determinants of
To obtain insight into the electron recombination occurring between excited electrons in the conduction band and sensitizers or electrolyte, the electron lifetimes were explored as a function of potential bias (Figure 5c and d).At a given potential, the electron lifetime in cell sensitized with IQ22 was obviously longer than that with IQ4 based on gel (Figure 5c) or volatile electrolyte (Figure 5d).It further demonstrates that the branched alkyl chains for IQ22 effectively suppress the electron recombination on the TiO2 surface improving the electron lifetime.Especially, the major difference in lifetime for gel electrolytes (Figure 5c) with the serious charge recombination reflects the superior suppressing effect of the branched structure in IQ22.Therefore, the longer electron lifetime is another origins for the higher VOC of IQ22.The Stability of Photovoltaic Performance.To develop Qs-DSSCs, we mainly aimed at the improvement of their thermal and light-exposure stability.As presented in Figure 6, the photovoltaic performance of Qs-DSSCs exhibited excellent stability during a 1000 h accelerated aging for IQ22 and IQ4-based cells with gel electrolyte in a solar simulator under full intensity (100 mW cm -2 ) at 50 °C. 47In the long term light and thermal environment, the enhancement of JSC from 0 to 100h especially for IQ4 indicated the system changes gradually stabilized due to the improvement of the interfacial contact between TiO2 and electrolyte. 48The JSC increased significantly at this stage, resulting in an increase in efficiency, although VOC and FF changed slightly to lower values.Along with the increase in JSC, VOC decreased because more electron recombination can occur with passage of time.In the light-soaking from 100 to 1000 h, however, all four photovoltaic parameters remained almost constant, which shows not only the good stability of quasi-solid electrolyte, but also the excellent anti-decomposition property exposure to light and heat for IQs dyes.

CONCLUSIONS
In summary, we have demonstrated IQ22 as an effective sensitizer for Qs-DSSC devices, with a high power conversion efficiency.For the optimum Qs-DSSC based on IQ22, a metal-free organic sensitizer, the PCE reached 8.76 % with high JSC (18.19 mA cm -2 ).Due to the lower conductivity and higher charge recombination for the gel electrolyte, the superior performance can be attributed to the structural design with high conjugation unit (broadening the photoresponse spectrum) and branched alkyl chains (boosting the electronic recombination suppression).As a consequence, no matter which electrolyte we choose, the determining factors for the higher open-circuit voltage of IQ22 compared with IQ4 are its positive shift of conduction band and longer electronic lifetime.
Overall, we obtain efficient and stable DSSCs, using low-cost organic sensitizers for Qs-DSSCs with excellent energy matching properties, advancing the practical application of DSSCs.

SYNTHESIS AND CHARACTERIZATION OF COMPOUNDS:
Synthesis of IQ22a.The unpurified indoline borate THF solution was reacted with 5,8-dibromo-  [50][51] were carried out using Gaussian 09 program with PCM in Dichloromethane.CAM-B3LYP functional 52 was used, and total of 70 lowest singlet electronic transitions were calculated and further processed with GaussSum software package. 53

Scheme 1 .
Scheme 1. a) The molecular structures of IQ22 and IQ4 and b) their calculated energy-level diagram and major electron-transfer absorption processes.

Figure 1 .
Figure 1.Temperature dependence of ionic conductivity (a) and Tafel polarization curves of symmetric dummy cells with two platinum electrodes at room temperature (b) for volatile and gel electrolytes.

Figure 2 .
Figure 2. Absorption spectra of IQ22 and the reference dye IQ4.a) The experimental spectra (in CH2Cl2) are shown as continuous lines and the theoretical electronic transitions are shown as bars for both IQ22 (red) and IQ4 (black).Theoretical data were computed using TD-DFT (CH2Cl2) and b) The experimental spectra on 4 μm TiO2 thin film.
Performances.To compare the photovoltaic performance of Qs-DSSCs with the traditional volatile-electrolyte iodine-based devices, we prepared a set of devices sensitizing 8 µm (4 µm transparent layer + 4 µm scattering layer) mesoporous TiO2 films with sensitizer IQ22 or IQ4.The current-voltage (J-V) curves of devices measured under Am 1.5G illumination (100 mW cm -2 at 298 K) are shown in Figure3, with the corresponding photovoltaic parameters listed in

c 43 TheFigure 3 .
Figure 3.The J-V curves for DSSCs based on IQ22 (a) and IQ4 (b) with volatile and gel electrolyte.

Figure 4 .
Figure 4. a) LHE spectra calculated from the absorption spectra of dye-loaded TiO2 film; b) IPCE spectra of DSSCs sensitized by IQ22 and IQ4 with gel or volatile electrolyte.

Figure 5 .
Figure 5.Chemical capacitance and electron lifetime as a function of bias potential obtained through electrochemical impedance spectroscopy carried out on devices with gel electrolyte (a, c) and volatile electrolyte (b, d), respectively in the dark.

Figure 6 .
Figure 6.Stability test of photovoltaic parameters (VOC, PCE, FF and JSC) variation with aging time for the devices based on IQ22 (blue line) and IQ4 (red line) with quasi-solid-state electrolyte during 1 sun visible-light soaking at 50 °C.
The working electrode was composed of an 8 µm thick TiO2 film, including a 4 µm transparent layer with 18 NRT and 4 µm scattering layer with 18NR-AO.The dye solutions were 0.3 mM in chloroform/ethanol (3/7) and the photoanodes underwent dipping for 12 h to complete the loading with sensitizers.The dye-covered TiO2 electrode and Pt-counter electrode were assembled into a sandwich type cell and sealed with a hot-melt gasket of 45 μm thickness made of the ionomer Surlyn 1702 (DuPont) with a heat sealing machine.The size of TiO2 electrodes used was 0.25 cm 2 (i.e., 5 mm×5 mm).For the liquid state device, a drop of the electrolyte was put on the hole in the back of the counter electrode.It was introduced into the cell via vacuum backfilling.The hole in the counter electrode was sealed by an aluminum foil tape.For the Qs-DSSCs, the electrolyte was spreaded on the TiO2 film before packaging with Surlyn ring and Pt electrode and then hot-pressed.Both conductivity and Tafel polarization curves were recorded by assembling symmetric dummy cells consisting of Pt CE|electrolyte|Pt CE.The volatile iodine electrolyte contained: 0.5 M BMII (1-butyl-3-methylimidazolium iodide), 0.1 M DMPII (1, 2-dimethyl-3-propylimidazolium iodide), 0.05 M I2, 0.1 M LiI, 0.1 M GuSCN (guanidinium thiocyanate) and 0.5 M 4-tert-butylpyridine in a mixture of acetonitrile and valeronitrile (volume ratio, 85 : 15).The polymer gel electrolyte (OPV-MPV-I) was a product of Yingkou OPV Tech New Energy Co, Ltd. (Liaoning, China), and it contains polymer, LiI, 3-methoxypropionitrile (MPN), I2, guanidine thiocyanate (GuSCN) and 4-tert-butylpyridine (TBP).[9][10]

Table 1 .
Photophysical and electrochemical properties of sensitizers and photovoltaic parameters of DSSCs based on IQ22 and IQ4 with volatile and gel electrolyte.