Efficient Flexible Counter Electrode Based on Modified Graphite Paper

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Efficient Flexible Counter Electrode Basing on Modified Graphite Paper and In Situ Grown Copper Sulfide for Quantum Dot Sensitized Solar Cells Hua Zhang, Jing Tong, Wenjuan Fang, Nisheng Qian, and Qingfei Zhao ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00075 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 4, 2018

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ACS Applied Energy Materials

Efficient Flexible Counter Electrode Basing on Modified Graphite Paper and In Situ Grown Copper Sulfide for Quantum Dot Sensitized Solar Cells Hua Zhang,*a Jing Tong, a Wenjuan Fang, a Nisheng Qian, a and Qingfei Zhao*b a Shanghai Key Laboratory of Functional Materials Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China b Department of Chemistry, Shanghai Normal University, Shanghai 200234, China

ABSTRACT Flexible counter electrode (CE) plays an important role in portable quantum dot sensitized solar cells (QDSCs). However, the present power conversion efficiency (PCE) of bendable QDSC is rather limited partly due to the unsatisfactory conductivity, flexibility, catalytic activity, and fabrication technique of CE. In this work, flexible CEs composed of CuxS and graphite paper (GP) are built and fabricated through a facile successive ionic layer adsorption reaction (SILAR) method. Through designing and optimizing the surface property of GP，coverage and thickness of catalyst, excellently performed CEs are achieved with the maximal PCE of 8.70% under one full sun illumination for Zn-Cu-In-Se QDSCs. Further, fully flexible QDSCs assembled with the as-prepared CEs and plastic photoanodes show a high PCE of 2.45% under the same illumination. KEYWORDS: Flexible counter electrode; graphite paper; copper sulfide; hydrophilicity; solar cell

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INTRODUCTION As a promising candidate for new generation photovoltaic, quantum dot sensitized solar cells (QDSCs) have attracted extensive attention with the initiative to solve the energy and environmental problems.1-4 A typical QDSC comprises a glass photoanode with QD loaded TiO2 mesoporous film,5-7 a counter electrode (CE)8-10 and liquid electrolyte.11,12 Thanks to the design and adoption of new kind of QD sensitizers (light absorption range extended to near infrared window) and the interface engineering strategy to suppress charge recombination, the photovoltaic performance of QDSCs has been undergoing a rapid evolution with power conversion efficiency (PCE) increasing from less than 1% to 12.3%.13-17 At present, the CEs have been less studied in spite that CEs might be the key to further increase PCE.18-21 The CEs perform the function of withdrawing photoinduced electrons from external circuit and catalyzing them to reduce the oxidized electrolyte species. Meanwhile, superior catalytic properties of CEs, such as excellent electrical conductivity, high catalytic activity, and large specific surface area, are indispensable for optimizing device performance.22 In general, brass foil and fluorine-doped SnO2 (FTO) glass supported copper sulfide (CuxS) serve as CEs in high efficiency QDSC devices.2,10 The brass foil based CEs (Cu2S/brass) exhibit excellent catalytic activity, good electrical conductivity and sufficient mechanical flexibility, but they suffer from continuous corrosion by polysulfide electrolytes and thus bear the problem of device stability and encapsulation.23 These issues could be partly avoided by introducing FTO as substrates.24 However, FTO supported CuxS (CuxS/FTO) CE is limited by the relatively poor conductivity, leading to the reduced fill factor (FF) as well as PCE due to the larger internal series resistance (Rs).25 Meanwhile, FTO glass is not portable to serve as flexible substrate due to its rigid and fragile feature. Exactly, flexible 2


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photovoltaic devices have attracted tremendous attention due to their light weight, portability and extended application.26-28 As a result, it is worth investigating material design and facile fabrication techniques for CEs to meet the industrialization requirement of flexible substrates while maintaining high PCEs. So far, suitable candidates for flexible CEs such as polyethylene terephthalate (PET)/indium-tin-oxide (ITO) substrates and carbon fibers have been commonly used in flexible QDSCs.29-32 Among them, carbon based flexible CE has emerged as a potential mainly due to its low-cost and excellent corrosion resistance.33 Yu et al. fabricated CdSe QDSCs based on the mesoporous carbon nanofibers electrode and obtained a 4.81% efficiency.34 Particularly, some composite carbon electrodes have shown excellent electrocatalytic activity and stability, which can present higher performance than the single material.35 Meng et al. adopted Cu2S/carbon composite electrode with good electrocatalytic activity into CdS/CdSe QDSCs and achieved the PCE of 3.87%.36 Lately, the application of Ti mesh supported mesoporous carbon (MC/Ti) CEs has been believed to equilibrate catalyzing a different electron reduction reaction in polysulfide electrolyte involving intermediate with a lower equilibrium redox potential, leading to the significant enhancement of open circuit voltage as high as 800 mV.37 Further, a PCE exceeding 12% has been obtained by doping nitrogen in mesoporous carbon.4 However, all of these reported high performance QDSCs devices were generally based on the rigid photoanodes and had much complicated fabrication process. A series of methods for the synthesis of CuxS onto substrates have been explored in the previous investigations. Traditional CE fabrication techniques, such as electrochemical deposition24,38 and hydrothermal deposition,39 focused on initially depositing a layer of metal copper on mechanically rigid substrates followed by 3



sulfidation. Yet these experiences are infeasible for the case of flexible substrates due to the incompatibility of deposited products with substrate material. Although screen printing and solvothermal method enable direct deposition of CuxS,40-43 the commonly used flexible substrates cannot survive high temperature which is indispensable in the synthesis process.44 Graphite paper (GP) is a kind of carbon material exhibiting the great properties of superior mechanical strength, low cost, and high electrical conductivity. It has been considered as a great candidate for application in fields of energy storage,45 shape memory devices,46 biosensors,47 etc. To overcome the universal problem, GP has been applied to CEs. A PCE of 2.70% has been reported from the fully flexible QDSCs employing CoS nanorod arrays/GP as CEs by Cao’s group,32 in which the CEs were prepared by two-step hydrothermal process. Solvothermal method has been applied to deposit Cu2S nanoparticles onto GP by Meng’s group and a PCE of 3.08% has been achieved basing on a rigid CdS/CdSe glass photoanode.40 However, these fabrication methods were complicated and strict when employed to the mass production. To overcome the limitations, developing simple and fast techniques for the fabrication of CEs based on flexible substrate and sulfide composites is promising. Successive ionic layer absorption and reaction (SILAR) is a simple, convenient and less-expensive synthesis method especially for preparation of chalcogenides. It has been early reported by M. Ristov and Y. F. Nicolau in mid-1980s, in which the substrate was immersed alternately into an anion and a cation precursor solution and rinsed between each.48,49 The thickness of film could easily be controlled by tuning adsorption time, number of immersion, solution concentration at mild reaction conditions including room temperature and ambient-pressure.50,51 The simple method has scarcely been exploited to prepare flexible CEs.52 4


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In this study, SILAR process was applied to grow CuxS nanoparticles on chemically modified GP to fabricate CuxS/GP composite flexible CEs, in which the GP served as the conductive substrate as well as the co-catalyst with the active CuxS nanoparticles. Under the most optimized experimental variables, the assembled Zn-Cu-In-Se QDSCs basing on the common FTO anodes have achieved a high PCE of 8.70% and improved stability. Moreover, a model of full flexible Zn-Cu-In-Se QDSC assembled with the flexible CE and ITO-PET based plastic photoanode has been built and a PCE of 2.45% was obtained. EXPERIMENTAL SECTION Preparation of flexible CuxS/GP CE. Firstly, GP (1.6 × 1.2 cm2) was ultrasonically cleaned in ethanol for 5 min followed by air drying. Subsequently, the GP was soaked into the mixed aqueous solution containing H2O2 (30%) and NH3·H2O (25%) with volume ratio of 1:1 at 50 oC for 30 min, then dried by air. For loading CuxS nanoparticles, the pre-chemically-treated GP substrate was alternately dipped into CuSO4 (0.02 ~ 0.08 M) and Na2S (0.03 ~ 0.12 M) aqueous solution for 1 min/dip and rinsed with distilled water between dips. The SILAR process was repeated for demanded cycles and the flexible CuxS/GP CE was obtained. Fabrication of rigid and flexible photoanodes. Oil-soluble Zn-Cu-In-Se QDs and the 3-mercaptopropionic acid (MPA) capped water-soluble Zn-Cu-In-Se QDs were obtained according to our previous report.13 For preparing rigid photoanodes, mesoporous TiO2 film was deposited onto FTO glass by successive screen printing and post-heat-treatment. After additionally treated with Mg2+ solution,16 the TiO2 film was immersed in water-soluble Zn-Cu-In-Se QD aqueous solution under dark condition at 50 oC for 5 h. Finally, a ZnS barrier layer was overcoated on the sensitized photoanode by alternately dipping the electrode into 0.1 M Zn(OAc)2 5



methanol solution and 0.1 M Na2S aqueous solution for six cycles. The detailed experiments are available in supporting information (SI). For preparing flexible photoanodes, the precipitate of oil-soluble Zn-Cu-In-Se QDs was first milled together with 0.25 g of P25 and 0.5 mL of home-made binder23 in an agate mortar to obtain slurry paste. The as-prepared pastes were subsequently coated onto the cleaned ITO-PET plastics via doctor blade followed by maintaining at 120 oC for 60 min. The flexible photoanode was alternately dipped into 0.1 M Zn(OAc)2 methanol solution and 0.1 M Na2S aqueous solution for 30 s/dip, followed by being alternately rinsed with distilled water and ethanol between dips. This process was repeated for four times. Fabrication of QDSCs. Zn-Cu-In-Se QDSCs were prepared by assembling CuxS/GP CEs and QD-sensitized rigid photoanodes using a 60 µm-thick Scotch spacer with a binder clip. Poly(2-vinylpyridine) (PVP) modified polysulfide electrolyte was obtained by adding 2.0 g PVP (Mr = 8000 g/mol) into 10 mL polysulfide electrolyte (2.0 M Na2S, 2.0 M S, and 0.2 M KCl) under stirring at room temperature for ten mins, and finally drilled into the cells. The full flexible solar cells were assembled with optimal CuxS/GP CEs and Zn-Cu-In-Se QD sensitized ITO-PET photoanodes. Then the PVP modified polysulfide electrolyte was injected through a pre-drilled hole. Characterization. The contact angle of water/GP interface was carried out through a JC2000D Contact Angle Meter and obtained via a five-point fitting method. X-ray diffraction (XRD) was measured on a Siemens D5005 X-ray powder diffractometer equipped with graphite-monochromatized Cu-Kα radiation (λ = 1.5406 Å). To avoid the strong interference of carbon signal, the XRD sample was obtained by scraping 6


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the CuxS nanoparticles from substrates. The structure and morphology of samples were investigated by a field emission scanning electron microscope (FESEM, FEI NOVA Nano SEM 450). Photovoltaic performances extracted from J−V curves of solar cells were recorded by a Keithley 2400 source meter illuminated under AM 1.5 G solar simulator (Oriel, Model No. 94022A). The light intensity was calibrated to 100 mW/cm2 by using a NREL Si solar reference cell. The photoactive area was 0.236 cm2 defined by a mask. The incident photon-to-current conversion efficiency (IPCE) curves were obtained by a Keithley 2000 multimeter under the illumination of 300 W tungsten lamp. Electrochemical impedance spectroscopy (EIS) measurements were performed under dark condition on an impedance analyzer (Zahner, Germany) with a frequency ranging from 0.1 Hz to 100 kHz and a perturbation of 10 mV. Tafel polarization curves were recorded on a CHI660d electrochemical workstation at a scan rate of 10 mVs-1. The CE samples measured for EIS and Tafel were identical. Cyclic voltammetry (CV) measurements were carried out on an electrochemical workstation by using a Pt wire as auxiliary electrode, an Hg2Cl2/Hg electrode as reference electrode and a prepared CE as working electrode in a polysulfide electrolyte containing 1.0 M Na2S, 1.0 M S, and 0.1 M KCl at a scan rate of 10 mVs-1. RESULTS AND DISCUSSION Fabrication and structure characterization of flexible CuxS/GP CE. In our previous work,10,41 pastes containing CuxS or Cu were first prepared and then screen printed onto FTO followed by sulfidation for the latter. Finally CEs were obtained after sintered at high temperature. The synthesis process was complicated and the organic residuals decreased the conductivity. Meanwhile, the conditions and printing are unsuitable for flexible GP substrate. Herein, the flexible CuxS/GP CE was simply fabricated via SILAR method as illustrated in Figure 1. CuxS nanoparticles were in 7



situ grown onto GP at room temperature without any sintering. No organic residuals existed. To modify the hydrophilicity of GP substrate and to increase the loading amount of CuxS catalyst, GP was first chemically-treated with an aqueous mixture of H2O2 and NH3·H2O. To evaluate the important effect of the pre-treatment, the contact angle of water/GP interface was obtained with results shown in Figure 2. It can be seen that GP without chemical treatment (referred to GP) has the contact angle of 96.05o. After treatment (referred to mGP) the mGP has the contact angle of 37.52o, dramatically smaller than that of un-treatment GP. Obviously, it is clear that the chemical treatment could effectively decrease the contact angle, leading to the dramatically increased hydrophilicity of GP substrate due to the introduction of OHfunctional groups. Meanwhile, the surface roughness has also been enlarged, which could be confirmed from SEM images as shown in Figure 3. The surface of GP is clean and smooth (Figure 3a), while the layered structure and enlarged roughness are visible at mGP surface (Figure 3b), indicating that the larger specific surface area has been realized after hydrophilic modification. It is anticipated that the surface modification will be beneficial to the effective penetrating of water soluble electrolyte and to increase the loading amount of CuxS catalysts which dominantly affect the catalytic activity. After SILAR ten cycles, CuxS nanoparticles were grown on mGP surface as shown in Figure 3c. It could be observed that the deposited CuxS nanoparticles are well-dispersed on the surface of mGP. The crystal structure of resultant products was characterized by XRD as shown in Figure 3d. The diffraction peaks could well be indexed to rhombohedral Cu1.8S (JCPDS NO 47-1748), with the typical crystal facets of (0015), (1010) and (0120) presented in the pattern.

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Figure 1. Schematic illustration of the flexible CuxS/mGP CEs via SILAR procedure.

Figure 2. Measurements of contact angle at (a) water/untreated GP interface and (b) water/modified GP interface.

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Figure 3. Top view SEM images of the untreated GP (a), modified GP (b) and CuxS/mGP CEs (c). The insets are digital photos of the three films. XRD pattern of the prepared CuxS sample (d).

Performance of QDSCs with different CEs. To investigate the synergistic effect of CuxS and GP, CEs composed of bare GP and CuxS/GP composite have been fabricated. Meanwhile, chemically modified mGP has been used to evaluate the influence of surface property and hydrophilicity on the performance of CE. Further, the commonly used CuxS/FTO CE has also been fabricated via SILAR method to investigate the effect of different substrates. Figure 4a shows the J-V curves of QDSCs assembled with different CEs and the corresponding photovoltaic parameters are summarized in Table 1. It could be seen that the QDSC assembled with GP CE showed a PCE of 2.73% (Voc = 0.601 V, Jsc = 16.11 mA/cm2, FF = 0.254), suggesting the great potential of GP serving as CE 10


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substrates as well as catalysts. Compared with GP, the PCE of mGP as CE was 3.22% with about 17.9% increase, the Jsc dramatically increased to 19.12 mA/cm2, exhibiting that the surface chemical modification of GP could improve the performance of QDSC. It can be attributed to the enhancement of roughness and hydrophilicity of surface. When CuxS nanoparticles were grown on the GP substrate, the PCE of the assembled QDSC increased to 6.23% with 128.2% and 93.5% increases compared with bare GP and mGP, respectively. It suggested that the addition of CuxS nanoparticles could dramatically improve the PCE mainly attributed to the increase of Jsc and FF, respectively. When CuxS nanoparticles were grown on the mGP substrate, the PCE with CuxS/mGP CE further increased to the value of 8.70% with Jsc of 27.32 mA/cm2 and FF of 0.516. The 39.6% increase in PCE compared with CuxS/GP CE could be ascribed to the rougher surface of mGP with abundant active sites for the catalytic materials. It could be concluded that the combination of CuxS nanoparticles with both GP and mGP could dramatically enhance the performance of the corresponding QDSC. The driving force should come from the synergistic effect of CuxS and GP as discussed below in Figure 4d. In addition, when rigid FTO substrate was used instead of mGP, the resultant CuxS/FTO CE exhibited a lower PCE of 5.27% with 2.6%, 14.1%, 27.5% and 39.4% decline in Voc, Jsc, FF and PCE, respectively. Comparing the CE structures of CuxS/FTO and CuxS/mGP, we know that the replacement of FTO with mGP could significantly improve the performance of QDSCs mainly because of the better conductivity of mGP substrate. Four-probe measurements showed that the sheet resistance of mGP was only 0.0517 Ω/□, and it was much smaller than that of FTO with 7.8555 Ω/□ resistance.

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Figure 4. (a) J-V curves of Zn-Cu-In-Se QDSCs with different CEs under optimal conditions. (b) Nyquist plots and (c) Tafel polarization of different CEs tested using symmetric cells. (d) Description of co-catalysis effect in CuxS/GP CE.

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Table 1 Photovoltaic parameters extracted from the J-V curves for cells with different CEs (10 SILAR cycles, 0.06 M CuSO4, 0.09 M Na2S) Voc

Jsc

CEs

GP

mGP

CuxS/FTO

CuxS/GP

CuxS/mGP

a

PCE

Rs

2R1

2Rct

FF (V)

(mA/cm2)

0.601

16.11

0.254

2.73

(0.594)

(16.09)

(0.249)

(2.65±0.13)a

0.608

19.12

0.277

3.22

(0.607)

(18.44)

(0.270)

(3.02±0.16)a

0.601

23.47

0.374

5.27

(0.602)

(23.40)

(0.366)

(5.16±0.08)a

0.613

25.21

0.405

6.23

(0.611)

(25.02)

(0.405)

(6.19±0.05)a

0.617

27.32

0.516

8.70

(0.609)

(27.11)

(0.520)

(8.61±0.11)a

(%)

(Ω cm2) (Ω cm2) (Ω cm2)

2.19

4.08

32.93

5.63

2.52

12.94

10.45

5.79

24.25

2.46

2.83

9.44

2.31

1.76

5.34

The average parameters and standard deviation of over three cells in parallel are inside the

parentheses, and the outside are values for champion cells in each group. The detailed data are shown in Table S1 (SI).

EIS measurements were performed to verify the electrochemical characteristics of different CEs as illustrated in Figure 4b and the extracted parameters are shown in Table 1 (the equivalent circuit is shown in Figure S1). Rs accounts for series resistance, R1 represents the charge transfer resistance at catalyst/substrate interface, Rct represents the charge transfer resistance at CE/electrolyte interface. It could be seen that the Rs (unit is Ω cm2) values of different CEs were 2.19, 5.63, 10.45, 2.46, and 2.31 for GP, mGP, CuxS/FTO, CuxS/GP, and CuxS/mGP, respectively. The order of CuxS/mGP < GP < CuxS/GP < mGP < CuxS/FTO suggested that the conductivity 13



was CuxS/mGP > GP > CuxS/GP > mGP > CuxS/FTO. From the results, we can draw some conclusions: 1) the chemical surface modification of bare GP can weaken the conductivity though it has enhanced the roughness and active sites; 2) the effect of CuxS nanoparticles growth on the conductivity of un-treated GP could be ignored, while the growth of CuxS on mGP could decrease the Rs value to less than half of that for mGP; 3) among them, CuxS/mGP shows the highest conductivity, while the commonly used CuxS/FTO shows the lowest one. People have generally evaluated the contact degree of conducting substrate with catalyst from the value 2R1 and the catalytic activity from the value 2Rct. Both of the two factors are very important to the total performance of solar devices. When comparing mGP with GP, we found that 2R1 and 2Rct of mGP decreased from 4.08 to 2.52 Ω cm2 and from 32.93 to 12.94 Ω cm2, suggesting the improved contact degree and dramatically enhanced catalytic activity. It could be contributed to the better hydrophilicity and more active cites of mGP. When CuxS nanoparticles were grown on three types of substrate, both 2R1 and 2Rct values were in the order of CuxS/mGP < CuxS/GP < CuxS/FTO, exhibiting the contact degree and catalytic activity in order of CuxS/mGP > CuxS/GP > CuxS/FTO. In detail, CuxS/mGP showed the lowest 2R1 value (1.76 Ω cm2) and 2Rct value (5.34 Ω cm2), all about 60% of that for CuxS/GP, demonstrating the excellent electron transfer rate at CuxS/mGP interface and catalytic activity at CE/electrolyte interface. Furthermore, the EIS parameters from CuxS/mGP were only 22 ~ 30% of that from common CuxS/FTO. It should be noted that although the EIS parameters from CuxS/FTO were twice as large as that from mGP, the PCE with CuxS/FTO CE was 1.6 fold as that of mGP. This could demonstrate that CuxS nanoparticles as catalysts have played critical role in improving the efficiency of cells, 14


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which could also be confirmed by the addition of CuxS to GP and mGP. The difference in catalytic activity of various CEs could also be verified by Tafel polarization plots with results shown in Figure 4c. The exchange current density (J0) arranging in the order of CuxS/mGP > CuxS/GP > CuxS/FTO > mGP > GP was observed in accordance with the results from J-V curves. This further verified the superior catalytic property of CuxS/GP and CuxS/mGP, especially for the latter. Both EIS and Tafel polarization plots were satisfactorily consistent with J-V curves for cells assembled with different CEs. Therefore, the replacement of rigid FTO substrate with flexible GP as substrate could effectively and dramatically improve the electrical conductivity and catalytic activity, leading to the increased conversion efficiency. Meanwhile, after surface hydrophilic modification, the efficiency of cells could be further improved. In addition, when CuxS nanoparticles were combined with substrates, the assembled cells showed superior PCEs mainly associated with the significantly increased Jsc and FF. When the composite CE composed of CuxS and GP or mGP was used, the dramatic improvement in efficiency could be attributed to “co-catalysis effect”. The effect can be inferred from the mechanism of electron transfer as shown in Figure 4d in which CuxS/GP is used as a model. For pure GP or mGP as CE, electrons transfer in the mode of route I (blue) as marked in Figure 4d. In this configuration, the reduction reaction takes place on GP or mGP after electrons are collected by substrate. Here, GP or mGP serves not only as conducting substrate but also as catalyst. Unfortunately, the catalytic activity of GP or mGP is so poor to obtain highly efficient cells. But it is very satisfactory that the electrical conductivity of them are superior, even better than the common used FTO. In the configuration of CuxS/GP and CuxS/mGP, the substrate GP or mGP collects electrons and transfers them to CuxS nanoparticles where the 15



reductions of electrolyte occur. This electron transfer mode has been denoted as route II (yellow) in Figure 4d. In this structure, flexible GP or mGP dominantly serves as electron transporter with small series resistance, and CuxS nanoparticles serve as highly efficient catalyst with the assistance of GP or mGP. So it could be considered that the electrolyte reduction is completed under the co-catalysis effect of CuxS and GP or mGP. Furthermore, the surface modification of GP to form mGP has provided higher surface area and enhanced hydrophilicity for depositing more CuxS and facilitating electron transfer. Thus, upon design and optimization of the structure CuxS/mGP, the superior electrical conductivity as well as high catalytic activity of CEs are realized, leading to the excellent performance of the assembled QDSCs. Effect of precursor concentration on flexible CuxS/mGP CEs performance. The effect of precursor solution concentration on the photovoltaic performance of QDSCs based on CuxS/mGP CEs was investigated. The basic precursor solution concentration, denoted as cB, was 0.02 M for CuSO4 and 0.03 M for Na2S, respectively. A series of higher concentrations as 2, 3, and 4 times as cB , denoted as 2cB, 3cB, and 4cB, respectively, were adopted to fabricate CuxS/mGP CEs. The corresponding J-V curves and summarized photovoltaic parameters are illustrated in Figure 5a, Table 2 and Table S2 (SI), respectively. It could be seen that when the concentration was increased from cB to 2cB and 3cB, most photovoltaic parameters continuously grew, and the maximal PCE reached to 8.70% at 3cB with about 17.3% increase over that of cB. The persuasive reason for the continuous enhancement of performance is that with the concentration increasing, more CuxS catalysts can deposit on mGP substrate, thus resulting in the better catalytic activity, higher Voc and Jsc. The change tendency of Jsc was consistent with the results of IPCE curves as shown in Figure 5b. The integrated Jsc values from the available data of IPCE curves was calculated in Table S3 (SI), 16


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which is close to the Jsc values in J-V measurements. 2R1 and 2Rct in Table 2 from EIS curves in Figure 5c changed synchronously in reversed mode and they decreased continuously along with the increase of concentration. They respectively declined to ~ 34% and 45% of that for cB, demonstrating the decreased electron transfer resistances in the interfaces between substrate, catalyst and electrolyte. When the concentration was further increased to 4cB, Voc and Jsc began to decline with FF nearly unchanged, leading to the decreased PCE of 8.05%. The electron transfer resistances reflected from 2R1 and 2Rct also grew, mainly contributing to the declined PCE. By the way, the lowest concentration of cB was favor for the highest conductivity but with the lowest cell efficiency.

Figure 5. (a) J-V and (b) IPCE curves of Zn-Cu-In-Se QDSCs based on CuxS/mGP CEs prepared under different solution concentrations. (c) Nyquist impedance plots and (d) Tafel curves of CuxS/mGP CEs prepared under different solution concentrations using symmetric cells for test. 17



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Table 2 Photovoltaic parameters extracted from the J-V curves for champion cells with CuxS/mGP CEs prepared under different concentrations. Voc

Jsc

(V)

(mA/cm2)

cB

0.569

25.48

0.518

7.42

1.81

2.68

9.72

2cB

0.606

26.57

0.503

8.09

2.88

2.67

8.24

3cB

0.617

27.32

0.516

8.70

2.31

1.76

5.34

4cB

0.594

25.84

0.525

8.05

2.19

2.05

6.26

PCE

conc.

Rs

2R1

2Rct

FF (%)

(Ω cm2) (Ω cm2) (Ω cm2)

Figure 6 shows the SEM images of CuxS/mGP prepared under different ion concentrations from cB to 4cB. It could be observed that the coverage of CuxS catalyst on the surface of mGP increased along with the enlarged concentration from cB to 3cB, implying that the amount of CuxS catalyst increased. This possibly could explain the mechanism of the continuously enhanced PCE when concentration changed from cB to 2cB and then to 3cB. When the concentration was increased to 4cB, the quantity of catalyst was further increased that has been confirmed from Figure 6d. But the nanoparticles seriously aggregated which will reduce the surface area and prevent electrolyte from penetrating, thus leading to the decreased PCE. The changes in catalytic activity dependent of ion concentrations could also be investigated by Tafel curves in Figure 5d and CV results in Figure S2 (SI). Both of the results were in accordance with that of J-V, EIS and SEM.

18


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Figure 6. SEM images from CuxS/mGP CEs prepared under different concentrations corresponding to (a) cB, (b) 2cB, (c) 3cB and (d) 4cB respectively.

Effect of SILAR cycle on flexible CuxS/mGP CEs performance. Ion concentration can influence the quantity and dispersion of CuxS on mGP surface, both of which are closely associated with the devices performance. Similarly, the active layer thickness dominantly determining the quantity of CuxS catalyst via SILAR method could also affect the devices performance. Here, we controlled the thickness of CuxS by changing the SILAR cycles (4, 8, 10, 12 and 14 cycles). The resultant photovoltaic parameters are presented in Table 3 and the corresponding curves are illustrated in Figure 7. It can be visually observed that with the SILAR cycles increased from 4 to 10, the Jsc and Voc values continuously increased. And the maximal PCE of 8.70% (Jsc = 27.32 mA/cm2, Voc = 0.617 V and FF = 0.516) was obtained with the SILAR cycles of 10. However, when SILAR cycles was further increased from 12 to 14, the solar cell performance was deteriorated. During the thickness of catalyst was controlled, the 19



Page 20 of 32

parameters of Voc and FF fluctuated within a certain limit. In EIS results (curves in Figure S3, SI), the lowest R1 and Rct were obtained from 10 cycles though with the slightly higher Rs than the others. It could be concluded that the thickness of 10 cycles is favor for obtaining highly efficient cell, which is mainly attributed to the enhanced Jsc due to the increased catalyst quantity and catalytic activity. The influence and mechanism are similar to that of ion concentrations below 3cB. The J-V curves for cells with CEs of different SILAR cycles and the corresponding photovoltaic parameters are available in Figure S3 and Table S4 (SI). Table 3 Photovoltaic parameters extracted from the J-V curves for champion cells with CuxS/mGP CEs prepared under different SILAR cycles Voc

Jsc

cycles

PCE

Rs

2R1

2Rct

(%)

(Ω cm2)

(Ω cm2)

(Ω cm2)

FF (V)

(mA/cm2)

4

0.637

25.65

0.494

8.10

1.79

2.79

17.17

8

0.632

26.72

0.500

8.50

1.75

2.87

9.75

10

0.617

27.32

0.516

8.70

2.31

1.76

5.34

12

0.628

26.21

0.492

8.48

1.77

2.85

10.12

14

0.632

26.75

0.485

8.41

2.25

3.16

11.55

20


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Figure 7. Average Voc and Jsc (a), PCE and FF (b) dependent on SILAR cycles.

Performance of fully flexible QDSCs. It is very clear that the optimal CuxS/mGP CEs have been easily fabricated and they could potentially be utilized to fully flexible QDSCs. As a model, fully flexible Zn-Cu-In-Se QDSC has been assembled as shown in Figure 8a, in which the characteristic of flexibility could be clearly seen. Figure 8b shows the corresponding J-V curve and parameters with maximal PCE of 2.45%. According to our previous work, the mechanical stability of the fully flexible QDSC was further tested by repeatedly bending the device within a bending radius of ~ 15 mm.23 The main parameters after bending different times are shown in Figure 8c. PCE of the measured cell showed original value of 1.99% and got slightly increased before 90 cycles, while other parameters fluctuated. However, after bending 120 cycles, all the parameters began to decrease which might be ascribed to the generated microcracks or flaking-off of photoanode. After 150 cycles, PCE decreased to 1.51%. The results exhibit that the flexible solar cell has the good tolerance to the bending. The optimization of flexible photoanodes will be further investigated in the future.

21



Figure 8. (a) Digital photograph and (b) J-V curve of the fully flexible QDSC. (c) Photovoltaic parameters including Jsc, Voc, FF, and PCE measured from the flexible QDSC after a series of bending cycles.

CONCLUSIONS In conclusion, we have combined graphite paper, i.e., GP, possessing excellent conductivity with CuxS possessing superior catalytic activity to construct flexible CE of CuxS/GP. Meanwhile, a simple SILAR method and surface property control were employed to fabricate and improve the flexible composite CE. The chemically modified graphite paper, i.e., mGP, was verified to improve the performance of QDSC due to the enhanced roughness and hydrophilicity after surface hydrophilic modification. The experimental conditions including reaction cycles of SILAR and precursor solution concentration were investigated and optimized. The champion conversion efficiency of 8.70% has been achieved for Zn-Cu-In-Se QDSCs based on the conventional rigid photoanode assembled with CuxS/mGP CE, with larger enhancement than that of the pure graphite paper CE. On the one hand, because of the 22


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better conductivity of modified graphite paper substrate, the replacement of FTO glass could significantly improve the performance of QDSCs. On the other hand, due to the “co-catalysis effect” between CuxS and modified graphite paper, the CuxS/mGP CE could dramatically improve the performance under the optimal reaction conditions. In addition, a conversion efficiency of 2.45% can be achieved from the constructed flexible device based on CuxS/mGP CE. Furthermore, in this paper, a simple and facile way to fabricate flexible CEs was proposed. The CuxS/mGP CE possesses good flexibility, lightweight, low cost as well as excellent stability. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: The synthesis of Zn-Cu-In-Se sensitizers, the assembly procedure of Zn-Cu-In-Se QDSCs, equivalent circuit used for fitting EIS, average photovoltaic parameters of Zn-Cu-In-Se QDSCs based on different CEs, including different substrates, different concentrations (0.02 M, 0.04 M, 0.06 M, 0.08 M) of CuSO4 solution and (0.03 M, 0.06 M, 0.09 M, 0.12 M) of Na2S, current densities (Jsc) from IPCE for CEs with different concentration treatment, J-V for CEs with different SILAR cycles and the related electrochemical measurements for different CEs. AUTHOR INFORMATION Corresponding Author

* Phone: +86 21 6425 3681. Email: [email protected]; [email protected] ORCID Hua Zhang: 0000-0002-4065-8179 23



Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge the National Natural Science Foundation of China (Nos 91433106, 24121004), the Programme of Introducing Talents of Discipline to Universities (B16017). REFERENCES (1) Kamat, P. V. Boosting the Efficiency of Quantum Dot Sensitized Solar Cells through Modulation of Interfacial Charge Transfer. Acc. Chem. Res. 2012, 45, 1906-1915. (2) Pan, Z. X.; Mora-Seró, I.; Shen, Q.; Zhang, H.; Li, Y.; Zhao, K.; Wang, J.; Zhong, X. H.; Bisquert, J. High-Efficiency “Green” Quantum Dot Solar Cells. J. Am. Chem. Soc. 2014, 136, 9203-9210. (3) Zhang, H.; Wang, C.; Peng, W.; Yang, C.; Zhong, X. H. Quantum Dot Sensitized Solar Cells with Efficiency Up to 8.7% Based on Heavily Copper-Deficient Copper Selenide Counter Electrode. Nano Energy 2016, 23, 60-69. (4) Jiao, S.; Du, J.; Du, Z. L.; Jiang, W. Y.; Pan, Z. X.; Li, Y.; Zhong, X. H. Nitrogen-Doped Mesoporous Carbons as Counter Electrodes in Quantum Dot Sensitized Solar Cells with a Conversion Efficiency Exceeding 12%. J. Phys. Chem. Lett. 2017, 8, 559-564. (5) Du, Z. L.; Zhang, H.; Bao, H. L.; Zhong, X. H. Optimization of TiO2 Photoanode Films for Highly Efficient Quantum Dot-Sensitized Solar Cells. J. Mater. Chem. 24


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Efficient Flexible Counter Electrode Based on Modified Graphite Paper

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