Components control for high-voltage quasi-solid state dye-sensitized solar cells based on two-phase polymer gel electrolyte

Solar Energy 181 (2019) 130–136

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Components control for high-voltage quasi-solid state dye-sensitized solar cells based on two-phase polymer gel electrolyte Chen Li, Chenghao Xin, Liang Xu, Ya Zhong, Wenjun Wu

T

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Key Laboratory for Advanced Materials and Institute of Fine Chemicals, Shanghai Key Laboratory of Functional Materials Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Components control Hysteresis effect Quasi-solid state Dye-sensitized solar cells

For the quasi-solid-state electrolyte in dye-sensitized solar cells (DSSCs), the selection and optimization of components is a decisive process for high photovoltaic performance while maintaining stability. In this work, a two-phase polymer gel system based on PEO and PEGDME formed by chemical crosslinking is used as quasi-solid electrolyte in DSSCs. Through components of preparation solvent, iodine and GuSCN control, the photoelectric conversion efficiency of QS-DSSCs reaches 7.66%, especially the open-circuit voltage reaches 793 mV due to the self-assembly and ionic conductivity promotion. It lays a solid foundation for probing its potential value of quasisolid dye-sensitized solar cells.

1. Introduction In order to solve the stability of dye-sensitized solar cell (DSSC) based on liquid electrolyte, quasi-solidification and solidification are inevitable research area for its application development (Chiang et al., 2018; Huang et al., 2018; Senevirathne et al., 2018; Song et al., 2017; Suzuka et al., 2016; Wu et al., 2008; Xia et al., 2016; Yu et al., 2016a; Zheng et al., 2017). Although the power conversion efficiency (PCE) of quasi-solid dye-sensitized solar cells (QS-DSSC) is still lower than that of liquid ones, the polymer gel electrolyte has received widespread attention due to its three-dimensional network structure with higher ionic conductivity and good pore size permeability(Cho et al., 2015; Tao et al., 2015). In general, for obtaining higher PCE of QS-DSSCs, the researches (Akhtar et al., 2007; Huo et al., 2010) carried out on polymer gel electrolytes mainly involve: (1) the improvement of the ionic conductivity on account of their lower conductivity and mobility relatively to the liquid electrolytes; (2) the enhancement of their permeability for the photoanode mesoporous TiO2 membrane in view of the network structure; (3) the increasement of the electrical conductivity through change of their mechanical properties in virtue of the copolymerizing, grafting, or blending. The polymer gel electrolyte is usually a mixture of some redox components and low-molecular polymers such as polyethylene oxide (PEO or PEG) or polyacrylonitrile (PAN) forming viscous gel system. However, the ionic conductivity of polymer electrolytes mainly

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depends on the concentration and mobility of the charge carriers, which are essentially related to the interaction of cations and ligands in the polymers (Zhou et al., 2009). Recently, polyethylene oxide (PEO) has been widely studied as quasi-solid electrolyte for DSSCs due to its better chemical stability and mechanical properties. For example, the photoelectric properties of QS-DSSC devices based on PEO as a gelling agent, exhibit good stability (Akhtar et al., 2007; Seidalilir et al., 2015). In order to further improve the mechanical properties and electrical conductivity of PEO-based gel electrolytes, in this work, we got the twophase polymer through chemical crosslinking with PEO and polyethylene glycol dimethyl ether (PEGDME) as curing agent of DSSCs electrolyte. In virtue of self-assembly promotion and high-conductive diffusion, the optimal PCE of 7.66% was achieved and its open-circuit voltage (Voc) was close to 793 mV (Fig. 1a). A new approach was established for DSSCs to improve their photothermal stability. 2. Experimental details 2.1. Materials Valeronitrile and 1,3-dimethyl-3-propylimidazolium iodide (DMPII) were purchased from Sigma-Aldrich. 18-NRT transmission layer and 18NR-AO scattering layer TiO2 paste were purchased from Greatcellsolar Company, Australia. Lithium bis(trifluoromethanesulfonyl) imide (LITFSI) was from Alfa Aesar. All reagents are AR grades and used directly without further processing.

Corresponding author. E-mail address: [email protected] (W. Wu).

https://doi.org/10.1016/j.solener.2019.01.072 Received 26 September 2018; Received in revised form 20 January 2019; Accepted 22 January 2019 0038-092X/ © 2019 Published by Elsevier Ltd on behalf of International Solar Energy Society.

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Fig. 1. The schematic of the component synergistical action in quasi-solid electrolyte (a), photocurrent density-voltage (J-V) curves of QS-DSSCs based on different preparation solvents (b) and different content of GuSCN and liquid electrolyte (c), and temperature dependence of ionic conductivity of gel and liquid electrolyte (d).

Preparation of liquid electrolyte: 0.5 M tetra-butylpyridine (TBP), 0.05 M iodine, 0.1 M lithium iodide, 0.1 M 1,3-dimethyl-3-propylimidazolium iodide (DMPII), 0.6 M 1-butyl-3-methylimidazolium iodide (BMII), 0.1 M guanidinium thiocyanate (GuSCN) were dissolved in a mixture of acetonitrile and valeronitrile (85:15, V:V). Preparation of gel polymer electrolyte: First, a clean 50 mL round bottom flask was charged with 1.4 g of PEO solid particles, followed by 2.1 g of PEGDME and 10 mL anhydrous preparation solvent and heated and stirred in an oil bath at 55° C until forming a fluid, viscous and transparent gel. 1.33 g of BMII, 0.51 g of I2, 0.035 g of LiI, and 0.185 mL of TBP were further added and stirred overnight. Then, the excess solvent was removed by rotary evaporation to finally obtain a twophase polymer electrolyte.

5 min, 375 °C for 5 min, 450 °C for 15 min and 500 °C for 15 min, cooled to room temperature, treated with a dilute solution of TiCl4 (40 mM, 75 °C for 30 min) and sintered at 500 °C for 30 min. The electrodes were allowed to cool (80–100 °C) and immersed into an ethanol solution of N719 (3 mM) overnight. (4) Preparation of quasi-solid DSSC: The dye-covered TiO2 electrode and Pt-counter electrode (CE) 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 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 fabrication of liquid device was reported in our previous works (Yang et al., 2015a).

2.3. Preparation of quasi-solid DSSC

3. Results and discussion

(1) The FTO conductive glass was washed with some detergent, water, acetone and ethanol in an ultrasonic cleaner for 30 min in sequence. (2) Preparation of a compact layer: First, a 0.04 M di(isopropylacetonato) titanate solution in (di(acetylacetonyl) diisopropyl titanate (400 μl) + 30 mL of anhydrous ethanol) was prepared as a precursor. Then, the compact layer on the conductive surface was formed by spraying under 450 °C and then recurs every 30 s for a total of 6 times. After completion, keep the instrument annealed at 450 °C for 30 min and turn off the instrument until it cools to room temperature. (3) Preparation of work electrodes: Titanium dioxide paste 18NR-T and 18NR-AO were deposited onto clean FTO conductive glass (FTO, 3 mm thick, 8 Ω sheet resistance, AGC) by screen printing, respectively. The TiO2 electrodes (area = 0.25 cm2) were gradually heated in muffle furnace with programmed temperature at 325 °C for

3.1. Effect of preparation solvent of quasi-solid electrolyte (QS-electrolyte) on DSSC performance

2.2. Preparation of liquid electrolyte and gel electrolyte

In this process, the solvent added in advance is critical to the dispersibility, conductivity and photovoltaic performance of the QS-electrolyte. The solvents tried to use here are ethanol, acetonitrile, acetonitrile/valeronitrile (AN/VN, V/V = 85/15) and acetonitrile/3methoxypropionitrile (AN/3-MPN, V/V = 85/15), respectively. The specific photoelectric data following with solvent is listed in Table 1, and the relevant J-V characteristic curve is shown in Fig. 1b. As depicted in Table 1 and Fig. 1b, under same test conditions (AM 1.5, 100 mW cm−2), the hysteresis phenomenon (the difference between the photovoltaic data (especially PCE) got from forward and reverse scanning process) was significantly reduced and the QS-DSSC device exhibited superior photovoltaic performances with acetonitrile 131

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Table 1 Photovoltaic performance of the DSSCs based on different solvents.

Table 2 Photovoltaic performance of QS-DSSCs based on different transparent TiO2 layers.

Solvent

Scanning

Jsc/(mA cm-2)

Voc/(mV)

FF (％)

PCE (%)

NLa

Scanning

Jsc (mA/cm2)

Voc (mV)

FF (%)

PCE (%)

Ethanol

Reverse Forward

11.89 12.91

730 730

61.08 65.69

5.30 6.34

3+1

Acetonitrile

Reverse Forward

13.47 13.45

757 752

61.64 66.98

6.28 6.87

Reverse Forword

11.06 11.26

784 784

74.78 75.76

6.48 6.69

4+1

AN/VN

Reverse Forward

11.69 11.56

772 785

73.53 75.87

6.63 6.88

Reverse Forword

12.05 12.54

787 787

73.57 73.74

6.98 7.28

4+2

AN/3-MPN

Reverse Forward

11.56 12.05

781 785

75.74 75.70

6.84 7.16

Reverse Forword

11.57 11.87

783 783

74.67 75.43

6.76 7.01

5+1

Reverse Forword

11.43 11.69

772 772

72.59 73.53

6.41 6.64

a The number of the printing times is the number of the transmission layer, followed by the number of scattering layers. For example, 3+1 is 3 layers of transmission layer and 1 layer of scattering layer. The thickness of each layer is about 3 μm.

as a preparative solvent compared with ethanol. Based on acetonitrile, the optimum photoelectronic parameters including short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and PCE obtained by the forward scan reached 13.45 mA cm−2, 752 mV, 66.98%, and 6.87%, respectively. This forward-scan PCE was 8.4% higher than that based on ethanol as a solvent due to favorable solubility of the component in it and advantageous dispersion of the product obtained from it. In the research works of DSSCs, some prominent photovoltaic performances were achieved with mixed solvents beyond single solvent (Tedla and Tai, 2018). On this basis, we further optimized the QSelectrolyte with blend solvents AN/VN (V/V = 85/15) and AN/3-MPN (V/V = 85/15). As shown in Fig. 1b, using a mixed solvent AN/VN or AN/3-MPN, the Jsc drops and the Voc and FF increase significantly for the J-V curve relatively to single solvent. Also, with AN/VN, a more reduced hysteresis effect is obtained compared with acetonitrile improving the charge transport balance of the system possibly (Venkatesan et al., 2018; Yu et al., 2016b). In addition, for all solvents, the PCE got with forward-scanning is greater than that with reversescanning. It is worth mentioning that with AN/VN and AN/3-MPN, the open-circuit voltages reach 785 mV, which is related to the improvement of charge transfer mechanism. In especial, with AN/3-MPN, the Jsc, Voc, FF, and PCE obtained by the forward scanning were 12.05 mA cm−2, 785 mV, 75.70%, and 7.16%, respectively, showing the best performance. It may be that the addition of 3-methoxypropionitrile can better dissolve various components in the electrolyte system, thereby promoting the formation of a more uniform and stable gel throughout the electrolyte (Wang et al., 2013). On the other hand, it is beneficial to the dissolution and migration of the redox couple, which leads to the change of the quasi-Fermi level of titanium dioxide and increases the open-circuit voltage of the device. However, due to more serious electron recombination in the quasi-solid electrolyte, the photocurrent appears lower.

The photovoltaic properties under simulated sunlight of AM1.5 (100 mW cm−2) following with film thickness are listed in Table 2, while the J-V characteristics are shown in Fig. 2d. With 4+1 layers, the optimum PCE 7.28% was obtained with Jsc of 12.54 mA/cm2, Voc of 787 mV and FF of 73.74%. As shown in Table 2 and Fig. 2d, the Jsc increases from 11.26 mA/cm2 to 12.54 mA/cm2 as the printing number of TiO2 from 3+1 to 4+1 attributed to the increase of the specific surface area and adsorption sites for the amount of dye. With 4+2 photoanode, the photoelectric parameters, especially Jsc, decline dramatically because the thick scattering layer greatly increase the effective distance between the electrolyte and the sensitizer, and further limiting the regeneration rate of dye. As the thickness of the transparent layer continues to increase, the Jsc, Voc and PCE decrease to 11.69 mA cm−2, 772 mV and 6.64% due to the aggregation of the dye and increase of the trap and defect states. Interestingly, we have achieved an open circuit voltage of up to 787 mV through film thickness optimization. In addition, the change of film thickness hardly betters the hysteresis effect. In order to demonstrate the effect of the number of prints on the film thickness, we selected 3+1, 4+1 and 5+1 film for SEM cross-section characterization (Fig. 2a–c). Except that the film thickness increased obviously with the number of prints, the compactness of the transparent layer undergoes a certain degree of change, which may affect the adsorption amount and aggregation degree of the sensitizer, thereby affecting the photovoltaic performances of the devices. Composition optimization of I2 content in polymer quasi-solid electrolyte also has a great influence on the performance of DSSC. In the electrolyte of the DSSC device, I2 and I3- are the most essential components that carry the regeneration of the oxidation-state sensitizer and the effect on the electron transport of the electrode. Its concentration not only affects the reduction rate of the sensitizer, but also determines the recombination probability of conduction band electrons and I3− (Wang et al., 2016). As in the general DSSC electrolyte formation process, I- is significant excess, its concentration is at least 10 times that of I2, so in the control of photovoltaic performance, iodine concentration is a controlling factor. According to reference (Kawano and Watanabe, 2003), when the concentrations of I- and I3- were comparable, the charge transport of the I-/I3- redox couple in QS-electrolyte follows Grotthus exchange mechanism. Therefore, we conducted an optimization study on the amount of iodine in the solid electrolyte. The quasi-solid electrolytes with iodine concentrations of 0.1, 0.2 and 0.4 M were prepared, numbered 1-1, 2-1, and 3-1, respectively, and the effects of different iodine concentrations on the photoelectric properties of solid DSSCs were explored. For the quasi-solid electrolyte system based on different iodine concentrations, the photoelectric properties of the devices were tested under standard simulated light. The specific current-voltage

3.2. Screening of TiO2 film thickness for high-voltage devices As is well known, film thickness has a direct relationship with electron recombination due to the distribution of charge generation and recombination sites (Snaith et al., 2006). Especially for quasi-solid DSSC, it is more sensitive to the thickness of the film, because of the charge transport characteristics in viscous bodies. Different film thickness determines on the amount of dyes adsorbed. The thinner TiO2 film, the fewer active sites, and therefore the less dye adsorption, the short-circuit current density will naturally be smaller (Jiang and Boschloo, 2018). With the number of TiO2 layers increase, the amount of dye adsorption increases, but it aggravates the molecular aggregation and the probability of electronic recombination, which is unfavorable to the current increase. Therefore, the screening of optimal TiO2 thickness is extremely important for DSSCs, especially quasi-solid devices. We printed the working electrodes with transparent adsorption layer and scattering layer of 3+1, 4+1, 4+2 and 5+1 layers respectively, and assembled them into DSSC photovoltaic devices. 132

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Fig. 2. Cross-section SEM view image of dye-sensitized TiO2 with layers of transmission + scattering layer of (a) 3+1, (b) 4+1, (c) 5+1 and (d) J-V curves for QSDSSCs devices based on different number of TiO2 layers.

b

790

12.8

Voltage(V)

-2

Photocurrent (mA.cm )

a 12.4 12.0 11.6

785 780 775

11.2 0.1

0.2

0.3

770

0.4

0.1

0.2

76.0

d

0.3

0.4

7.6 7.4

75.5

PCE(%)

Fill Factor/%

0.4

I2(M)

I2(M)

c

0.3

75.0

7.2 7.0 6.8 6.6

74.5

0.1

0.2

0.3

0.1

0.4

0.2

I2(M)

I2(M)

Fig. 3. Photovoltaic parameters (a) PCE, (b) photocurrent, (c) voltage and (d) fill factor change of QS-DSSCs based on different content of iodine.

133

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2012; Nath et al., 2016). This is because the ions ([CH6N3]+ or Gu+) adsorbed on the surface of TiO2 can lead to negatively charged surface negative displacement and increase the open-circuit voltage of the device. On the other hand, the change of band gap will have a significant effect on the I3- charge recombination on the TiO2 conduction/electrolyte interface. Therefore, under the adsorption of nitrogen ions on the surface of TiO2, GuSCN is also found to slow down the electronic recombination and increase the voltage of the device. As shown in Table 4, from G1 to G3, with the increase of GuSCN content, both Jsc and Voc increase from 11.94 mA cm−2 and 778 mV to 12.56 mA cm−2 and 793 mV, respectively, indicating the inhibition action of GuSCN on the recombination of injected electrons with the oxidized sensitizer or electrolyte. As the concentration continues to increase, it may be the accumulation of guanidine ions on the surface of the photoanode, which affects the reduction rate of the oxidized dye, and the photoelectric conversion efficiency begins to decrease. In order to better study the response in different visible region of iodine and QS electrolyte, we also tested their IPCE spectra shown in Fig. 4a. As we can see from Fig. 4a, with a darker color, the quasi-solid electrolyte exhibits more obvious absorption defect at 400 nm, but significantly improves the photoelectric conversion performance in the ultraviolet region (300–350 nm). However, the limitation of charge transport in quasi-solid electrolytes limits the regeneration rate of the sensitizer to a certain extent, and thus the photocurrent density of QSelectrolyte is lower than that of liquid one in the visible light region. The maximum IPCE value of the liquid DSSC device can exceed 80%, and the maximum value of the QS-electrolyte is about 70%. The highest value includes the presence of liquid devices at 540 nm, and the response interval of IPCE is also limited to 300–770 nm, indicating that the replacement of the electrolyte does not affect the distribution of the sensitizer on the surface of the photo anode. Only because of the components change, the IPCE value is impacted due to the charge recombination and the conduction band migration. In order to consider the limitation of the device performance due to light harvest by electrolyte, UV–Visible absorption spectra (Fig. S2) for quasi-solid and liquid electrolyte were characterized with dummy cells (FTO|electrolyte|FTO). The absorption range is only 350–500 nm, and the absorption intensity of quasi-solid is much smaller than that of the liquid one. Therefore, the lower PCE of the QS electrolyte is independent of its light absorption. In this work, we obtained 793 mV remarkable Voc for QS-DSSC based on two-phase polymer of PEO and PEGDME as electrolyte. We know that the photovoltage Voc of dye-sensitized solar cells is determined by the quasi Fermi level (EFn) of TiO2 and the redox potential (Ered) of electrolytes. Here, the liquid and quasi-solid electrolyte use the same oxidation-reduction pair (I-/I3-). Therefore, in addition to the effect of the solvent, the open-circuit voltage of the photovoltaic device is mainly related to the position and charge of the titanium dioxide. In order to further explore the relationship between the electrolyte and the photoelectric properties of QS-DSSC for probing the deep-level cause of high open-circuit voltage, we have studied the electron and ion transmission characteristics applied different bias in the darkness by the electrochemical impedance spectroscopy (EIS). The film thickness, iodine and GuSCN concentration of the quasi-solid electrolyte devices are the best values.

Table 3 Photovoltaic performance of QS-DSSCs based on different content of iodide. Electrolyte

I2 (M)

Jsc (mA cm−2)

Voc (mV)

FF (%)

PCE (%)

1-1 2-1 3-1

0.1 0.2 0.4

11.64 12.49 11.94

779 785 784

75.79 75.34 75.09

6.88 7.39 7.03

characteristic (J-V) curves are shown in Fig. 3 and photovoltaic data are listed in Table 3. As shown in Fig. 3 and Table 3, with the change of iodine concentration, the photocurrent density and the photoelectric conversion efficiency have changed obviously. When the iodine concentration increases from 0.1 to 0.2 M, the Jsc, Voc and PCE increase from 11.64 mA/cm2, 779 mV and 6.88% to 12.49 mA/cm2, 785 mV and 7.39%, respectively. This is because with the increase of iodine content, more I3- can be formed and the conductivity of the electrolyte is increased (Cho et al., 2014). While continuing to increase the iodine concentration to 0.4 M, on the one hand, iodine will dissociate to generate iodide ions in organic solvents, but excess iodine will generate iodide ions and polyiodides, such as I3- and I5- (Lan et al., 2012). Therefore, more photoelectrons are needed to reduce I5- or I3- to I-, resulting in a decrease in photocurrent, from 12.49 mA/cm2 to 11.84 mA/cm2. On the other hand, it may also be because too much iodine has not been dissolved and it has not played its due role. Based on above results, we used 0.2 M concentration of iodine in subsequent works. For DSSC, its electrolyte additives are usually involved in the TiO2/ electrolyte interface modification, affecting the quasi-Fermi level of the photoanode, and achieving the regulation of the photovoltaic performance. Guanidine thiocyanate (GuSCN) as an electrolyte additive can greatly improve the performance of DSSCs (Lee et al., 2007; Wang et al., 2015). This can be explained by the further adsorption of the guanidinium salt cations at the dye-sensitized photoanode, further promoting the self-assembly of the dye molecules, so as to either inhibit electron recombination or positively movement of the conduction band edge of the TiO2. Therefore, we introduced the GuSCN, as an important additive component in the liquid electrolyte, into our quasi-solid electrolyte system. The specific experimental results are shown in Fig. 1c and Table 4. GuSCN was added at concentrations of 0, 0.05, 0.1, and 0.2 M, and numbered G1, G2, G3, and G4, respectively. As presented in Fig. 1c and Table 4, the photoelectric performance of the DSSC devices changed significantly with the change in the amount of GuSCN. With G3 (0.1 M GuSCN), the device showed the best photoelectric performance 7.66% (Jsc = 12.56 mA cm−2, Voc = 793 mV, FF = 76.88%). Under the same conditions, the PCE of the DSSC device based on liquid iodine electrolyte was 8.03% (Jsc = 16.36 mA cm−2, Voc = 720 mV, FF = 68.18%). Compared with the DSSC with liquid electrolyte, the Voc has been significantly improved from 720 to 793 mV based on our twophase polymer electrolyte system. Using GuSCN, the PCE is further approached to that of liquid electrolyte, and an even higher open-circuit voltage of 793 mV is obtained. It is also generally believed that GuSCN is considered to have an impact on the forward transfer of the TiO2 conduction band, resulting in higher open-circuit voltage (Voc) (Asghar et al., 2016; Lin et al.,

3.3. Characterization of conductivity and charge transfer properties Table 4 Photovoltaic performance of QS-DSSCs based on different content of GuSCN. Electrolyte

GuSCN (M)

Jsc (mA cm

G1 G2 G3 G4 Liquid

0 0.05 0.1 0.2 0.1

11.94 12.48 12.56 12.85 16.36

−2

)

Voc (mV)

FF (%)

PCE (%)

778 787 793 758 720

75.34 74.81 76.88 72.68 68.18

7.00 7.35 7.66 7.07 8.03

The capacitance and electron lifetime curves obtained by EIS are shown in Fig. 4b, c. As depicted in Fig. 4c, the chemical capacitance (Cchem) of DSSC based on liquid and quasi-solid-state electrolyte increases with the bias, and the overall trend is positive. As shown in Fig. S1, the Rct (Charge Transfer Resistance) increases obviously for the QSelectrolyte due to its smaller charge diffusion. The higher Rct is conducive to suppressing the electron recombination between the anode and electrolyte and enhancing the open-circuit voltage. 134

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a

IPCE(%)

80 60 40

b

10

Lifetime/s

C. Li et al.

1

Fig. 4. (a) IPCE Spectrogram, (b) electron lifetime (τ), (c) cell capacitance (Cchem), (d) Tafel polarization curves of DSSCs devices based on quasi solid state electrolyte (red line) and liquid electrolyte (black line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

0.1

20 0 300

c

0.01 400

500

600

700

800

900

0.55

0.60

0.65

0.70

0.75

0.80

Voltage/V

Wavelength/V

d

1E-3

Cchem/F

2

logj (A/cm )

-2

1E-4

-3

-4

-5 1E-5

0.55

0.60

0.65

0.70

0.75

0.80

-0.4

-0.2

0.0

0.2

0.4

Voltage/V

Voltage/V

J0 = RT / nFR ct

With the increase of the conduction band position, the Density of Occupied States (DOS) can be reduced, while the DOS is proportional to the chemical capacitance (Wang et al., 2016). Therefore, the polymer electrolyte we prepared effectively enhanced the conduction band position of the anode, thus obtaining a higher open-circuit voltage (Barnes et al., 2013; Mathew et al., 2014). In addition, the electron lifetime (t) is the direct embodiment of the open-circuit voltage of the device. It can be obtained by using the formula:

where R is the gas constant, T is the temperature, n is the number of electrons involved in the reaction, F is the Faraday constant. As presented in Fig. 4d, J0 of QS and liquid electrolyte is almost same, indicating quasi-solid electrolytes have almost the same advantages as liquid electrolytes in participating in the electrolyte/Pt electrode interface reaction. Transfer resistance limited current density (Jlim) is determined by the diffusion of ionic carriers in the electrolyte, which can be used to evaluate the diffusion rate of ions in the electrolyte. The Jlim obtained from Tafel diagram is directly proportional to the diffusion coefficient (Dn) of the redox couple.

t = R ct Cμ where Rct is the charge transfer impedance which can be read from EIS. As the forward bias grow, the potential barrier of the conduction band (CB) gradually decreases, and the injection of unbalanced carriers increases the chance of recombination of electrons in CB and reduces the electron lifetime (in Fig. 4b). As shown in Fig. 4b, compared with liquid photovoltaic devices, the electronic lifetime of quasi-solid state devices is relatively larger under a certain voltage. This is related to the network structure of this two-phase polymer quasi-solid electrolyte, the auxiliary function of additional components and the optimization of other device processes. Therefore, its long electron lifetime is a controlling factor for high open-circuit voltage output. In order to further compare the efficiency limitations of QS DSSC, we also measured the conductivity of quasi-solid and liquid electrolytes by electrochemical impedance spectroscopy. As shown in the Fig. 1d, the corresponding curves of ln (σ) and 1000/T follow the Arenius relation, which is a typical ionic conduction behavior. The slope of the measured value varies with temperature shown in Fig. 1d, because the viscosity of quasi-solid electrolyte decreases with the temperature rise, its conductivity increases, while the change of liquid electrolyte is relatively flat (Ahmad et al., 2010; Wu et al., 2013). To calculate the charge transfer characteristics in different electrolytes, Tafel polarization measurements were performed using dummy cells with each of different electrolytes between two Pt electrode (Fig. 4d). The slope of Tafel polarity was used to evaluate the exchange current density (J0):

Jlim = 2nFCDn / l where Dn is the diffusion coefficient, l is the thickness of the spacer, n is the number of electrons involved in the redox couple, F is the Faraday constant and C is the electrolyte concentration(Jin et al., 2016; Li et al., 2018; Muthalif et al., 2018; Wang et al., 2018). In the diffusion zone in Fig. 4d, the Jlim values lie in the order of volatile > gel, indicating that the gel electrolyte has some limitations in the charge-transfer and diffusion likely due to the greater viscosity(Guo et al., 2013; Wang et al., 2009; Yang et al., 2015b). Because of the direct proportional relationship between Jlim and electrolyte diffusion coefficients, these results also show that I-/I3- has lower diffusion efficiency in quasi-solid electrolytes. It limits the further improvement of its photoelectric performance. 4. Conclusion A two-phase polymer was obtained by chemical crosslinking using PEO and PEGDME, and a high-performance polymer gel electrolyte was prepared by adding I2, BMII, LiI and GuSCN. Through the optimization of film thickness, preparation solvent, iodine concentration and GuSCN concentration, the best photoelectric conversion efficiency 7.66% (Jsc = 12.56 mA cm−2, Voc = 793 mV, FF = 76.88%) was obtained 135

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under standard light intensity of AM 1.5G (100 mW cm−2) which is close to that of 8.03% based on liquid ones. In particular, a high opencircuit voltage 793 mV (near 800 mV) is achieved via two-phase gel electrolyte. The EIS test shows that the high open-circuit voltage output can be attributed to the higher quasi Fermi level of the anode and the longer lifetime of the excited state. This work has laid a solid foundation for the development of high performance, especially potential quasi-solid electrolyte.

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