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Letter

Carbon Counter Electrode-Based Quantum Dot Sensitized Solar Cells with Certified Efficiency Exceeding 11% Zhonglin Du, Zhenxiao Pan, Francisco Fabregat-Santiago, Ke Zhao, Donghui Long, Hua Zhang, Yixin Zhao, Xinhua Zhong, Jong-Sung Yu, and Juan Bisquert J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b01356 • Publication Date (Web): 25 Jul 2016 Downloaded from http://pubs.acs.org on July 28, 2016

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The Journal of Physical Chemistry Letters

Carbon Counter Electrode-Based Quantum Dot Sensitized Solar Cells with Certified Efficiency Exceeding 11% Zhonglin Du,†,# Zhenxiao Pan,†,# Francisco Fabregat-Santiago,‡,# Ke Zhao,† Donghui Long,§ Hua Zhang,† Yixin Zhao,ǁ Xinhua Zhong,*,† Jong-Sung Yu,┴ and Juan Bisquert‡ †

Key Laboratory for Advanced Materials, Institute of Applied Chemistry, East China

University of Science and Technology, 200237 Shanghai, China ‡

Photovoltaic and Optoelectronic Devices Group, Department de Física, Universitat Jaume I,

12071 Castelló, Spain §

School of Chemical Engineering, East China University of Science and Technology, 200237

Shanghai, China ǁ

School of Environmental Engineering, Shanghai Jiaotong University, 200240 Shanghai, China

┴

Department of Energy Systems Engineering, DGIST, 42988 Daegu, Republic of Korea

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ABSTRACT The mean power conversion efficiency (PCE) of quantum dot-sensitized solar cells (QDSCs) is mainly limited by the low photovoltage and fill factor (FF), which are derived from the high redox potential of polysulfide electrolyte and the poor catalytic activity of counter electrode (CE), respectively. Herein, we report that this problem is overcome by adopting Ti mesh supported mesoporous carbon (MC/Ti) CE. The confined area in Ti mesh substrate not only offers robust carbon film with sub-millimeter thickness to ensure high catalytic capacity, but also provides an efficient three-dimension electrical tunnel with better conductivity than state-of-art Cu2S/FTO CE. More importantly, the MC/Ti CE can down shift the redox potential of polysulfide electrolyte to promote high photovoltage. In all, MC/Ti CEs boost PCE of CdSe0.65Te0.35 QDSCs to a certified record of 11.16% (Jsc = 20.68 mA/cm2, Voc = 0.798 V, FF = 0.677), an improvement of 24% related to previous record. This work thus paves a way for further improvement of performance of QDSCs.

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Table of Content (TOC)

Carbon Counter Electrode-Based Quantum Dot Sensitized Solar Cells with Certified Efficiency Exceeding 11% Zhonglin Du, Zhenxiao Pan, Francisco Fabregat-Santiago, Ke Zhao, Donghui Long, Hua Zhang, Yixin Zhao, Xinhua Zhong*, Jong-Sung Yu and Juan Bisquert

Key words: Quantum dots sensitized solar cells; mesoporous carbon; counter electrode

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Exploiting semiconductor quantum dots (QDs) as light-harvesting materials constitutes a promising approach towards low-cost third generation solar cells owing to their distinguished advantages such as band gap tunability, high absorption coefficient, low-cost facile preparation, solution processability, and multiple exciton generation potential.1−4 Benefited from the adoption of panchromatic QD sensitizers,5−8 high uploading sensitization approach,9−11 as well as barrier layer interface engineering for suppressing charge recombination,8,12−14 the best certified power conversion efficiency (PCE) of quantum dot sensitized solar cells (QDSCs) has been improved to 9.01%,15 with short-circuit current density (Jsc, ~ 20 mA cm-2) in a comparable value to those from other light absorbers (such as molecular dyes and even the popular organometal halide perovskites) based solar cells.16,17 Regretfully, an efficiency of 10% still seems a ceiling for QDSCs due to the relatively low open-circuit voltage (Voc < 0.7 V) and fill factor (FF < 0.65). The low Voc is mainly derived from the relative high redox potential of the commonly used polysulfide/sulfide (Sn2-/S2-) electrolyte,18 which requires a high overpotential for QD regeneration from electrolyte.19 To improve the Voc in QDSCs, modified polysulfide electrolyte has been exploited to down shift the redox potential.20,21 The promising results show that there is large room for improving the photovoltage of the current generation of QDSCs. FF in QDSCs is limited primarily by the mismatch between counter electrode (CE) and redox electrolyte.22,23 The role of a CE (consisting of catalyst supported on conducting substrate) is to collect electrons from external circuit and transfer them to electrolyte through catalytic reduction of oxidized species in electrolyte. Consequently, it has a critical effect on all the photovoltaic parameters (including FF, Jsc, and Voc) of a cell device. The impact of CE on the performance of QDSC is mainly derived from its conductivity (i.e sheet conductivity) and electrocatalytic activity toward electrolyte regeneration, which determines the overpotential and charge transfer resistance at CE/electrolyte interface as well as energy loss caused by the electron transport, thereby having a significant impact on the FF of QDSCs.22,23 Exploring ideal CEs has become a continuous and long ongoing effort in the development of QDSCs. Currently, brass foil and FTO (fluorine-doped SnO2) glass are the most commonly used substrates for CEs in QDSCs, but the former suffers from chemical corrosion by polysulfide electrolyte and the latter is limited by the relatively poor conductivity. Various promising electrocatalytic materials including noble metals and metal chalcogenides have been exploited in CE catalysts for QDSCs with the chemically instable Cu2S showing the superior catalytic activity for polysulfide reduction.24−26 Because of the low cost, high

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durability, and good electrical conductivity, carbonaceous materials (including carbon nanotubes, graphene, graphite, carbon black, activated carbon, porous carbon, and carbon composite materials etc.) have also been utilized as effective catalyst candidates in CEs with reasonable performance.27−33 Unfortunately, these previously reported carbon CEs are inferior to Cu2S based CEs for QDSCs application.24−33 Even though previous works on CEs based on different materials have brought forward considerable enhancement in performance of QDSCs, some issues including fair catalytic activity, large internal resistance, and poor stability of the CE are still existed due to the complexity of the redox chemistry of polysulfide electrolyte. Therefore, it is imperative to seek alternative CEs with low cost, high catalytic activity and conductivity, satisfactory stability for the construction of high efficiency QDSCs. Herein, Ti mesh or FTO glass has been investigated as the substrate for supporting carbon (including mesoporous carbon, and activated carbon) based CEs in QDSCs. The Ti mesh substrate possesses promising properties of low sheet resistance, and superior chemical stability. Furthermore, the confined area defined by the grid not only allows for effective deposition of strong carbon film with sub-millimeter thickness, but also provides a three-dimensional tunnel for fast electron transport from external circuit to carbon catalytic active sites. Due to the superior electrocatalytic activity, and excellent conductivity of the MC/Ti mesh CE, the resultant CdSeTe QD sensitized QDSCs show an extraordinary high PCE with certified value of 11.16% (Jsc = 20.68 mA/cm2, Voc = 0.798 V, FF = 0.677), which is among the best performance for all kinds of colloidal QD based solar cells (including QDSCs and depleted heterojunction quantum dot solar cells with the highest certified PCE of 10.6%).34−37

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Mesoporous carbon (MC) was synthesized based on a colloidal silica nanocasting route according to literature method, 38 and activated carbon (AC) was commercially available. The obtained MC exhibits randomly distributed interconnected mesopores of ~20 nm as shown in the transmission electronic microscopy (TEM) image in Figure 1a, and the detailed structural characterizations are available in Section 1 of Supporting Information (SI), Figure S1, 2, Table S1. The specific surface areas for MC and AC are 954 and 1541 m2 g-1, respectively. X-ray diffraction (XRD) analysis indicates the amorphous characteristics for both MC and AC. The carbon pastes were made by mixing the carbon materials with ethyl cellulose solution in terpineol. The carbon based CEs with different film thickness were obtained by screen printing the pastes on Ti mesh (a 80-mesh wire cloth woven from 100 µm diameter Ti wires) or FTO substrate with different number of screen printing cycles. Detailed procedure for preparation and characterization of CEs is available in Section 2 of Figure S3 to S8, Tables S2 and S3. For convenience, the AC-n or MC-n is referred to the n cycles of screen printing. Figure 1b, c show the top view and cross-section images of typical MC-4/Ti CE with corresponding photograph in the inset of Figure 1b. It is noted that MC film cannot be deposited on FTO or Ti foil substrate, while AC can be deposited on both Ti-mesh and FTO substrates. Therefore, the investigated carbon based CEs in this work include MC/Ti (MC deposited on Ti mesh), AC/Ti, and AC/FTO (AC deposited on Ti mesh or FTO, respectively). For comparison, the most commonly used Cu2S/FTO CE was also prepared according to literature method.39 a)

b) c)

1 mm 200µm

Figure 1. MC material and derivative MC/Ti CEs. (a) Representative TEM image, (b) top view

image of the front side of a MC-4/Ti CE with the photograph in the inset, (c) SEM image of cross-section of MC-4/Ti CE. One of prerequisites for ideal CEs is good conductivity (i.e. low sheet resistance), which favors electron transport to the catalyst active sites and decreases the energy loss coming from the inherent resistance, and therefore benefits the FF, and PCE of the resulting cell devices. The conductivity of a CE is determined by the conductivity of both substrate and

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catalyst in CEs, and the contact resistance of the catalyst/substrate interface. The availability of AC/Ti and AC/FTO CEs offers the convenience to highlight the advantage of Ti mesh in comparison with FTO in serving as substrate in CEs. Experimental results (shown in Table 1, and Table S3) indicate that the Ti mesh supported CEs exhibit better conductivity with nearly one order of magnitude lower sheet resistance compared to the FTO supported ones (1.5 vs 9.2 Ω/square). This is ascribed to the better conductivity of Ti mesh in comparison with FTO, and the closer contact in the AC/Ti interface related to AC/FTO interface. Therefore, the utilization of Ti mesh substrate of high conductivity in CE paves a convenient and effective way for decreasing the series resistance and leads to high FF for QDSCs. Table 1. Characteristics of investigated CEs under their optimum conditions, and EIS parameters (series resistance Rs, and charge transport resistance Rct) of the symmetric cells assembled from two identical CEs. film thickness (µm) Cu2S/FTO 3.6 CEs

sheet resistance (Ω/sq) 9.2

Rs (Ω cm2) 9.38

Rct (Ω cm2) 2.76

AC/FTO

64

7.7

9.56

46.67

AC/Ti

292

1.5

0.71

4.33

MC/Ti

294

2.2

0.84

2.82

Next, the impact of carbon film thickness on the photovoltaic performance of the resultant cells was studied using MC/Ti CE in CdSe0.65Te0.35 (simplified as CdSeTe henceforth) model QDSCs as a representative example. It is noted that over 9% certified efficiency has been obtained based on this model QDSC.15 The corresponding photovoltaic performances are shown in Figure 2a, and Figure S9, Table S4. It is found that with the increase of carbon film thickness from MC-1 to MC-4, corresponding FF, Jsc, and PCE were enhanced systematically (increasing from 0.648, 19.83 mA cm-2, 10.18% to 0.686, 20.67 mA cm-2, 11.39%, respectively), while Voc kept nearly constant (0.793 to 0.803 V). With the further increase of MC film thickness (MC-5), the corresponding PCE decreased slightly to 11.0%. This may be ascribed to the worsening bonding at MC film/Ti mesh substrate with the increased film thickness. These data demonstrate that thicker carbon film with more active sites can enhance the catalytic capacity of CE toward polysulfide reduction, therefore results in low charge transfer resistance and high FF, Jsc and PCE of the resulting cell devices, but the carbon film with excessive thickness would deteriorate the photovoltaic performance. From these results,

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we can conclude that MC-4/Ti CE with ~294 µm carbon film is the optimum thickness corresponding to best performance. For convenience, the MC/Ti is specially referred to the MC-4/Ti CEs hereafter. Similarly, the photovoltaic performance of the resultant cells dependent on the film thickness in AC/Ti and AC/FTO CEs was also investigated (shown in Table 1, and Table S5). Experimental results indicated that the optimum thickness of AC/Ti, and AC/FTO CEs were 292, and 64 µm, respectively, while the value for Cu2S/FTO CEs was 3.6 µm as determined in previous report.39 It is noted that the observed optimum film thickness (about 300 µm) for Ti mesh supported carbon CEs are much greater than those (tens of micrometers) from conventional FTO supported ones as in the case of AC/FTO CEs in this study and in previous reports, wherein the optimal film thickness is in the level of 10-40 µm for porous carbon,27,40−42 and 1-2 µm for carbon nanotubes.43,44 This highlights the effectiveness of the mesh structure in favoring the diffusion of electrolyte in CE film, and accelerating the mass transfer. On the other hand, the confined space in the mesh structure offers stronger holding for the supported carbon film and results in mechanically stable carbon film with thickness up to sub-millimeter. This can effectively overcome the commonly occurred poor adhesion between carbon materials and sheet substrates as above mentioned FTO or Ti foil (Figure S8).27 Furthermore, the selected Ti mesh substrate possesses promising properties of low sheet resistance, good flexibility, and superior corrosion resistance toward polysulfide electrolyte. Henceforth, when series number is not indicated, the studied CEs are referred to their optimum thicknesses, thus AC/FTO, AC/Ti and MC/Ti CEs are specifically referred to the AC-4/FTO, AC-4/Ti and MC-4/Ti corresponding to the best performance CEs. The MC/Ti, AC/Ti, AC/FTO electrodes together with the conventional Cu2S/FTO electrodes under their optimum conditions were adopted as CEs for assembling CdSeTe QDSCs according to previous approach.7,12,15,45 For each CE, average photovoltaic performance based on five cells under standard conditions (100 mW/cm2, AM 1.5 G) was analyzed and listed in Table 2. Current-voltage (J–V) curves for champion cells in each group are shown in Figure 2b. The detailed photovoltaic parameters of individual cells are shown in Tables S5, 6, and Figure S10. It is noted that the average PCE (8.79%) obtained from the reference cells based on Cu2S/FTO CEs in this work is on the order of the state-of-the-art.12,15,46 It is found that all the cell samples based on different CEs present similar Jsc value within 10% dispersion, and the MC/Ti and Cu2S/FTO based cells offer the best Jsc. The similar Jsc values was further verified by external quantum yield measurement (EQE, shown in Figure 2c and Table S7), which gave integration current values consistent 8


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with those from J–V curves (Figure 2b). The photovoltaic performances of cells with Ti mesh substrate in CE (9.77% for AC/Ti and 11.39% for MC/Ti CEs) are superior to that from the cells based on conventional Cu2S/FTO CEs (8.79%). this performance improvement is mainly derived from the enhancement of Voc and FF. Just changing FTO to Ti mesh substrate in AC based CEs, the corresponding cell efficiencies can be dramatically improved from 6.31% to 9.77% thanks to enhancements of 18% in Voc and 25% in FF. Furthermore, MC/Ti CE based cells show better photovoltaic performance than AC/Ti CE based ones (11.39% vs. 9.77%). Based on these results, we can draw a primary conclusion that both substrate and catalyst in CEs have a crucial effect on both FF and Voc, but a slight influence on Jsc of the resulting cell devices, and the performance of Ti mesh is superior to conventional FTO in serving as supporting substrate for CEs. Notably, QDSCs champion cell based on MC/Ti CEs provided a laboratory PCE of 11.51% (Jsc = 20.69 mA cm-2, Voc = 0.807 V, FF = 0.689) with certified efficiency of 11.16 % (Jsc = 20.68 mA cm-2, Voc = 0.798 V, FF = 0.677), Figure 2d, for active area of 23.6 mm2 by an accredited photovoltaic calibration laboratory (National Center of Supervision and Inspection on Solar Photovoltaic Products Quality of China, CPVT, see detailed information in Section 5 of SI). This result is among the best photovoltaic performance in all kinds of colloidal QD based solar cells (including QDSCs and depleted heterojunction quantum dot solar cells with the highest certified PCE of 10.6%),34−37,46 and also pushes the performance of QDSC to the same level of its analogue DSC for the first time.16,47

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15

MC-1 MC-2

10

MC-3 MC-4 MC-5

5

0 0.0

0.2

0.4

0.6

20 15 10 5

MC/Ti AC/Ti Cu2S/FTO AC/FTO

0 -5 0.0

0.8

0.2

0.4

0.6

0.8

Potential (V)

d)

20

2

Current density (mA/cm )

c)

80

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b)

2


2

a)

20

Potential (V)

60

EQE (%)

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40

MC/Ti AC/Ti Cu2S/FTO

20

AC/FTO

0 300

450

600

750

900

15

10

5

0 0.0

Certified Performance PCE = 11.16% Voc = 0.798 V Jsc = 20.68 mA/cm2 FF = 0.68 Area = 0.236 cm2

0.2

0.4

0.6

0.8

Potential (V)

Wavelength (nm)

Figure 2. Photovoltaic properties of CdSeTe QDSCs based on different CEs. (a) J–V curves of QDSCs corresponding to MC/Ti CEs with different thicknesses. (b) J–V curves of champion QDSCs using different CEs measured under AM 1.5 G full one sun irradiation (solid lines) and in the dark (dashed lines). (c) EQE of QDSCs corresponding to different CEs. (d) Certified efficiency of CdSeTe QDSCs based on MC/Ti CE. Table 2. Average photovoltaic performance and standard deviation for five CdSeTe QDSCs with different CEs in parallel under one full sun illumination. The numbers in parentheses are the values for champion cells in each group. CEs

Jsc (mA cm-2)

Voc (V)

FF

PCE (%)

MC-1/Ti

19.83 (19.96)

0.793 (0.798)

0.648 (0.650)

10.18±0.12 (10.35)

MC-2/Ti

20.21 (20.42)

0.796 (0.801)

0.660 (0.661)

10.62±0.14 (10.81)

MC-3/Ti

20.44 (20.48)

0.802 (0.805)

0.677 (0.684)

11.11±0.10 (11.27)

MC-4/Ti

20.67 (20.69)

0.803 (0.807)

0.686 (0.689)

11.39±0.09 (11.51)

MC-5/Ti

20.34 (20.24)

0.798 (0.804)

0.678 (0.684)

11.00±0.11 (11.13)

Cu2S/FTO

20.61 (20.47)

0.698 (0.701)

0.612 (0.618)

8.79±0.07 (8.87)

AC/FTO

18.77 (18.87)

0.634 (0.640)

0.530 (0.529)

6.31±0.05 (6.39)

AC/Ti

19.68 (19.72)

0.750 (0.754)

0.662 (0.665)

9.77±0.08 (9.88)

Certified cell

20.68

0.798

0.677

11.16

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In order to unravel the mechanism of photovoltaic performance of QDSCs dependent on the corresponding CEs, electrochemical properties of isolated CEs were quantified by conducting electrochemical impedance spectroscopy (EIS) and Tafel polarization measurements using symmetric dummy cells consisting of two identical CEs and filled with the polysulfide electrolyte as used in the complete cells. This symmetric cell configuration can rule out the contribution on device performance from other components in complete cells.24,25 Standard equivalent circuit for carbon based CEs as shown in Figure S11 was used to analyze the obtained EIS data.48−50 In the Nyquist plots (shown in Figure 3a and extracted EIS parameters in Table 1), the intercept of fitted curves on the real axis represents the collecting electrode resistance (Rs), and the first semicircle at high frequency is thought to be related to the resistance and capacitance (R1 and C1) of solid contact between substrate and carbon,40,49,50 while the next semicircle at lower frequencies corresponds to the charge transfer resistance (Rct) and capacitance (Cce) at the CE/electrolyte interface. 3

50

10 2


a) 4

40

MC/Ti AC/Ti Cu2S/FTO

2

-Z'' (Ω cm )

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30

2 0 0

4

8

12

AC/FTO

20 10 0 0

20

40

60 2

Z' (Ω cm )

80

b)

2

10

J0

1

10

0

MC/Ti AC/Ti Cu2S/FTO AC/FTO

10

-1

10

-2

10 -0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Voltage (V)

Figure 3. Electrochemical properties of investigated CEs. (a) Nyquist plots, and (b) Tafel polarization curves of symmetric cells based on identical CEs. From the extracted EIS data in Table 1, it is found that the Ti mesh supported carbon CEs (MC/Ti and AC/Ti CEs) show a similar low Rct value to that of the most effective Cu2S/FTO CE (2.82, 4.33 vs 2.76 Ω·cm2), and all of them exhibit one order of magnitude lower Rct than that from AC/FTO CE (46.67 Ω·cm2). The observed slightly higher Rct in AC/Ti relative to MC/Ti may be due to different structures in these two kind of carbon materials. As described above, MC material possess ~20 nm sized mesopores, which facilitates the diffusion of electrolyte through the carbon film while AC has only disordered micropores. None of the samples showed mass transfer limitations, neither in impedance spectra nor J–V curves of Figure 3. The improved regeneration rate of polysulfide observed with the lower charge transfer resistance value is attributable to the increase of the carbon area in contact with the

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electrolyte.30 Thus the ~5 times increase in porous film thickness from AC/FTO to AC/Ti samples produces a 10 times decrease in Rct, even improving expected values. The extra enhancement in Rct for the MC/Ti could be attributed to an even larger increase in porosity of material, what yields its value to a result very close to Cu2S/FTO. For the case of Rs values, the Ti mesh supported carbon CEs present one order of magnitude smaller Rs than those of FTO supported CEs (0.84, 0.71, vs 9.38, 9.56 Ω·cm2). The remarkably low Rs value in Ti mesh supported CEs should be ascribed to the better conductivity of the Ti mesh compared to FTO substrate as discussed above. For the identical catalyst AC based CEs, both Rs and Rct from AC/FTO CE (9.56, 46.67 Ω cm2) are one order of magnitude higher than those from AC/Ti CE (0.71, 4.33 Ω cm2). The higher Rct is derived from thinner AC film (64 µm in AC/FTO vs 292 µm in AC/Ti), which corresponds to less active sites for catalytic reduction of polysulfide. While, the higher Rs can be ascribed to the poor conductivity of FTO substrate as discussed above. Finally, R1 values for Ti mesh supported samples become negligible indicating much better contact of carbon electrodes than that in the case of FTO. Furthermore, the Tafel polarization measurements (Figure 3b) were used to cross-check the data obtained by EIS characterizations. It was observed that the deduced exchange current density (J0) was varied in the order of: MC/Ti ≈ Cu2S/FTO > AC/Ti > AC/FTO, which is consistent with the variation trend of Rct as obtained in the EIS measurement, and also consistent with the variation of Jsc values as observed in the J–V measurement. This is reasonable, since J0 = RT/nFRct, higher J0 in Tafel curves corresponds to lower Rct in EIS.24 Therefore, the Tafel polarization results give further support to the result observed from the EIS characterization. Known from the electrochemical properties characterization, it was found that both MC/Ti and AC/Ti CEs show comparable electrocatalytic activity toward polysulfide reduction to the most effective Cu2S/FTO CE, but exhibit better conductivity. This would favor the electron transfer at the CE/electrolyte interface, and consequently result in an increased FF, Jsc, and photovoltaic performance in the resulting cell devices. This expectation is in accordance with the observation in the photovoltaic measurement results as discussed above. Generally, the FF in a sensitized solar cell is largely affected by the internal series resistance (Rseries) of the cell device, which includes both the collecting electrode resistance and charge transfer resistance at CE/electrolyte interface (i.e. Rseries = Rs + Rct).23,51 High FF in MC/Ti and AC/Ti based QDSCs (0.686, 0.662, respectively) implies low internal series resistance in the devices. In our case as can be observed in Table 1, series resistance

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contributions obtained from impedance measurements over dummy cells (shown in Figure 3a), allow to correlate directly the decrease in total series resistance with the rise in FF. This decrease in total series resistance has two main responsible factors: (i) the substitution of FTO by Ti mesh diminishes largely (~ 1 order of magnitude) the sheet resistance of hole collecting electrode (i.e. CEs); (ii) the increase of thickness of the porous carbon layer increases its catalytic active sites until obtaining a charge transfer kinetics comparable to Cu2S electrode (reflected in a similar Rct for MC/Ti and AC/Ti to Cu2S/FTO CEs). Furthermore, Rct is a measure of the reaction barrier in the catalytic reduction of polysulfide by CE materials and its value is inversely proportional to the regeneration rate of redox couple electrolyte.23,24 The greater Rct values in the AC/FTO and AC/Ti CEs (46.67, 4.33 Ω cm2, respectively) result in a poorer regeneration rate for oxidized electrolyte, and concomitantly a lower charge transfer rate at the CE/electrolyte interface that creates a CE/electrolyte voltage drop and current bottleneck as realized in the lower Jsc (18.77 and 19.68 mA cm-2 respectively) found in the corresponding cells, see Figure 2b and Table 2.52 Simultaneously, high Rs deteriorates furthermore the FF and efficiency of the cells furthermore. These results imply that by simply replacing FTO with Ti mesh substrate, both Rct and Rs can be remarkably reduced in the resultant CEs, and therefore improve the FF, Jsc, and PCE of the resultant QDSCs substantially. The observed excellent electrochemical properties (low Rct and Rs values) of the MC/Ti CE derived from the EIS measurement on symmetric cells give a solid support to the observed high FF and Jsc values (0.686, 20.67 mA cm-2, respectively) in the resultant QDSCs, but the observed high Voc from cells corresponding to Ti supported CEs (0.803, 0.750 V for MC/Ti and AC/Ti, respectively) still lacks a reasonable interpretation. Theoretically, the maximum Voc in sensitized solar cells is determined by the energy level offsets between the quasi-Fermi-level of electron in TiO2 and redox potential of the redox couple in electrolyte.53 Therefore, Voc can be enhanced through either up shift the conduction band edge of TiO2 or down shift the redox potential of electrolyte,54 provided that there is still sufficient energy offset for photogenerated electron extraction and regeneration of the oxidized QDs. Herein, since the QD sensitized photoanode and redox mediator used in this study are identical, no change in the conduction band edge of TiO2 is expected and thus the first route is excluded.

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Table 3. Redox potential of polysulfide electrolyte dependence on electrodes. Electrodes potential vs SCE (V)

FTO

Ti mesh

Cu2S/FTO

AC/FTO

AC/Ti

MC/Ti

−0.699

−0.696

−0.757

−0.735

−1.020

−1.015

To explore the origin of Voc variation in cells corresponding to different CEs, the redox potential of electrolyte dependent on the used CEs was approximately determined by measuring the equilibrium potential of the investigated electrode versus that of a reference electrode.55 In the experiment, a three-electrode symmetrical cell consisting of the investigated electrodes as both working and counter electrodes, calomel electrode (SCE) as reference electrode was set to measure the potential in equilibrium and the results are shown in Table 3. Both Light and dark measurements were carried out, but the same Voc value was obtained for every tested electrode. From the results, we can find that bare electrodes (FTO, Ti mesh without catalyst) all presented a redox potential of −0.70 V vs SCE. When FTO was modified by carbon or Cu2S catalyst, the redox potential of polysulfide diminished ~50 mV, to −0.74 and −0.76 V, respectively. When Ti-mesh was modified with either AC or MC, the redox potential shifted extra −270 mV to −1.02 V. This change has two potential origin: (i) A change in new equilibrium concentration of polysulfide species in electrolyte induced by the thick carbon electrodes moving Eredox;56,57 (ii) the CEs equilibrate catalyzing a different reaction in polysulfide electrolyte providing a lower equilibrium redox potential in the electrolyte.58,59 Given the large shift in equilibrium potential matches what is found in previous works,58,59 the second option seems to be the most likely. Part of this large shift in redox potential is reflected in the increase of Voc as observed in the J−V measurements on complete cells. Regrettably, the actual reaction that the carbon based CEs reacts chemically with the redox electrolyte to change its redox potential is unclear currently due to the complexity of the electrochemistry of polysulfide, and further exploration is in progress. However, the procedure is reproducible, and the photovoltaic performance of the assembled cells can be stable for 1−2 hours. Meanwhile, as discussed below the stability of the carbon based CEs is superior to that of conventional Cu2S/FTO CEs. These results demonstrate that the Voc 14


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enhancement in resultant cell devices is not an artifact or transient. Furthermore, photovoltaic performance of zero under dark condition (as shown Figure 2b) excludes the contribution

100

10

50 0 -50

-100 -1.5

-1.0

-0.5

0.0

Voltage (V vs. SCE)

0.5

450

2

0

Cu2S/FTO

2

a)


2

150


from galvanic cells. Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


b)

0

AC/Ti

300

50 150 0 -150 -300 -1.5

-1.0

-0.5

0.0

0.5

Voltage (V vs. SCE)

450

c)

0

MC/Ti

300

50 150 0 -150 -300 -1.5

-1.0

-0.5

0.0

0.5

Voltage (V vs. SCE)

Figure 4. Electrochemical stability of carbon based CEs relative to conventional Cu2S based CE. Consecutive CV measurement at a scan rate of 100 mV/s at Sn2-/S2- redox electrolyte media: (a) 10 cycles for Cu2S/FTO CEs; (b) and (c) 50 cycles for AC/Ti and MC/Ti CEs, respectively. Prior work demonstrates that the relatively poor device stability of QDSCs is mainly derived from the adoption of chemical unstable Cu2S CE catalyst.22,60,61 Therefore the electrochemical stability of our carbon based CEs was compared with the conventional Cu2S/FTO CEs by repeated CV measurements in Sn2-/S2- redox electrolyte media. The relative stability of a studied CE can be evaluated on the basis of degradation degree of the CV curves. Meanwhile, the relative catalytic activity of the tested CEs can also be reflected by the intensity of current density peak in the CV curves.24 Figure 4 shows the evolution of CV curves under reduplicate measurement at a scan rate of 100 mV s-1. The conventional Cu2S/FTO CEs show a pronounced irreversibility in the continuous CV scan with current densities decrease to only 51% of the initial value in a course of 5 cycles of consecutive measurements (Figure 4a). In contrast, the peak positions and current densities hardly changed over a course of 50 cycles for both AC and MC based CEs (Figure 4b, c). This demonstrates that the carbon based electrode has highly chemical stability and superior corrosion resistance in comparison with that of conventional Cu2S based CEs. Furthermore, the intensities of the peak initial current density follows the order of AC/Ti ≈ MC/Ti > Cu2S/FTO. This indicates the same sequence of electro-catalytic performance of the CEs and is in accordance with the photovoltaic performance of the resultant cell devices. In summary, an ideal CE for QDSCs with combined advantages of large specific surface

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area for effective catalytic reduction of polysulfide electrolyte and fast electrolyte mass transfer kinetics was realized in MC/Ti CEs with 3D interconnected mesoporous framework. The confined area defined by the grid of the Ti mesh substrate not only allows for the deposition of robust and stable film with thickness up to 0.3 mm, but also provides an efficient 3D electrical tunnel for fast electron transport. As a result, the main obstacles for realizing high performance QDSCs, low FF and Voc, have been overcome. Thus this novel MC/Ti CE approach decreases contribution to internal series resistance of the cell improving FF by a factor larger than 12%. By other side, we demonstrate that the MC/Ti CE is capable of downshifting the redox potential of polysulfide, and therefore improve the Voc (~ 0.1 V). The combination of these two effects boosts PCE of QDSCs beyond 11%. This result opens a new strategy to further improve the efficiency of QDSCs solar cells using our MC/Ti CE or the similar CE concept.

EXPERIMENTAL SECTION Preparation of Carbon Counter Electrodes: The MC materials used in this work were synthesized according to standard literature methods,38 with detailed procedure available in the Supplemental Information. Activated carbon (AC) were purchased from Aldrich. The carbon paste was made by mixing 0.1 g carbon powder and 1.0 mL binder solution (obtained by mixing 8.0 mL of terpineol with 0.2 g of ethyl cellulose and 0.5 mL titanium isopropoxide) and ultrasonically dispersed for 30 min. Then the carbon paste was coated onto Ti mesh for preparing carbon/Ti CEs or onto FTO glass for carbon/FTO electrodes via the successive screen printing followed by gradual drying at 120 °C for 7 min each time to get an appropriate thickness. Then the film was finally heated at 450 °C for 30 min in Ar atmosphere. The Cu2S/FTO CEs were prepared according to our previously reported method.39 Sensitizing Photoanode and Assembling QDSCs: TiO2 mesoporous film electrodes composed of a 9.0-µm transparent layer and a 6.0-µm scattering layer were prepared according to literature method.45 5.2 nm sized CdSe0.65Te0.35 QDs with an absorption onset at ~800 nm were synthesized according to literature method,7,12 and used as model QD sensitizers. The pre-prepared CdSeTe QDs was firstly deposited onto the TiO2 films via capping ligand-induced self-assembly approach,9, 62 and then the sensitized electrodes were overcoated with thin layer of amorphous TiO2, ZnS, and SiO2 according to literature

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method.12,15 The sandwich-type cells were assembled by sealing the sensitized photoanode and the investigated CEs using a Surlyn thermoplastic spacer (60 µm) and polysulfide electrolyte (2.0 M Na2S, 2.0 M S aqueous solution) was then filled. For each kind of QDSCs, five cells with the same CE were prepared and evaluated in parallel. Photoelectrochemical Characterization: J–V curves were measured on a Keithley 2400 source meter for QDSCs illuminated by a solar simulator (AM 1.5 G, Oriel, Model No. 91160) with a 150 W xenon lamp. Light intensity was calibrated to 100 mW/cm2 with the use of a NREL standard Si solar cell. Photoactive area of 0.236 cm2 was defined by a black metal mask. EIS measurements were performed on an impedance analyzer (Zahner, Zennium) in dark conditions using sandwiched cells composed of two identical CEs placed face to face. The scan frequency for EIS measurements ranged from 100 mHz to 100 kHz, and the amplitude was set to 10 mV. The active area of the studied CEs is 0.36 cm2 and the distance between the two CEs is 60 µm, which is defined by a 3M tape. Electrolyte consists of 2.0 M Na2S and 2.0 M S aqueous solution. CV was performed on the same instrument as EIS with a three-electrode configuration (Pt wire as an auxiliary electrode, SCE as a reference electrode, and the studied CE as a working electrode with an active area of 0.36 cm2) in an aqueous solution containing 0.5 M Na2S and 0.5 M S.

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ASSOCIATED CONTENT Supporting Information Experimental procedures and additional photovoltaic measurement results. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions #

These authors contributed equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This work is supported by the Natural Science Foundation of China (Nos. 91433106, 21421004), the Programme of Introducing Talents of Discipline to Universities (B16017), the Fundamental Research Funds for the Central Universities in China, and the global frontier R&D program on center for multiscale energy system (NRF 2011-0031571) in Korea. The work at INAM-UJI was supported by Generalitat Valenciana project PROMETEO/2014/020.

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