Electrocatalytic sulfur electrodes for CdS/CdSe quantum dot-sensitized solar cells†

Zusing Yang , Chia-Ying Chen , Chi-Wei Liu and Huan-Tsung Chang *
Department of Chemistry, National Taiwan University, 1, Section 4, Roosevelt Road, Taipei 106, Taiwan. E-mail: changht@ntu.edu.tw; Fax: 011-886-2-33661171; Tel: 011-886-2-33661171

First published on 30th June 2010

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Quantum dot-sensitized solar cells (QDSSCs) are interesting energy devices because of their impressive sunlight harvesting and multiple electron/hole generation, ease of fabrication, and low cost.¹ Theoretically, the conversion efficiency (η) of QDSSCs can reach 44%—considerably higher than that of dye-sensitized solar cells (DSSCs).² Most reported values of η of QDSSCs (typically below 3%) are well below DSSCs (10%), mainly because of their narrow absorption range and charge recombination at the QD–electrolyte interface.^3–6 To further increase the values of η of QDSSCs, it appears that it will be necessary to develop new types of counter electrodes. Efficient counter electrodes can resolve the severe problem of inner energy loss in QDSSCs, resulting in the improvement of fill factor (FF). Although Cu₂S⁷ and carbon based nanomaterials⁸ have been used as counter electrodes in QDSSCs, the FFs of the QDSSCs are only nearly 50%. As a result, the development of more effective counter electrodes in QDSSCs remains a challenge.

Polysulfide solutions (E_red = ca. −0.45 V vs. normal hydrogen electrode) can be used to stabilize QD-sensitized photoelectrodes against photocorrosion, thereby stabilizing the photocurrents and extending the durability of the QDSSCs.^9–11 When employing a polysulfide electrolyte, however, commonly used Pt and Au counter electrodes are only poorly active, mainly because their surface activity and conductivity are suppressed as a result of adsorption of the sulfur atoms.^10–12 Therefore, instead of using Pt and Au electrodes, metal sulfides such as cobalt sulfide (CoS), nickel sulfide (NiS), and copper sulfide (CuS) have become promising candidates as electrocatalysts for polysulfide redox reactions in photoelectrochemical cells (PECs).^10,11 In such PECs, CoS, NiS, and CuS electrodes exhibit superior electrocatalytic ability for the polysulfide redox reactions relative to that of Pt electrodes.^10,11 For example, the values of J_SC in aqueous polysulfide solutions when using CoS and Pt electrodes were 60 and 20 mA cm⁻², respectively.¹¹ Because of its high electrocatalytic ability, we suspected that CoS electrodes would also enhance the hole-recovery rate between the polysulfide solution and the CoS electrode in QDSSCs.

In this study, we used CoS as an electrocatalyst to enhance the values of η of CdS/CdSe QDSSCs. We prepared the CdS and CdSe QDs using previously reported procedures.^1,5 The band gaps, corresponding to the absorption edges, of the CdS and CdSe QDs were 2.31 (536 nm) and 1.77 (700 nm) eV, respectively, significantly higher than those reported for CdS and CdSe in the bulk (2.25 and 1.7 eV, respectively). We estimated the sizes of these CdS and CdSe QDs from the excitonic peaks in their absorption spectra (Fig. S1).^1,5 On TiO₂ films, the mean diameters of the CdS and CdSe QDs were ca. 4 and 6 nm.^1,13 Based on the relative band gaps of TiO₂ (3.2 eV), CdS QDs, and CdSe QDs, the cascade structure of TiO₂/CdS/CdSe electrodes is advantageous to the electron injection and hole recovery.¹ The energy level diagram is displayed in Fig. S2.

Chemical bath deposition was applied to prepare the CoS electrodes.¹⁴ Briefly, a conductive F-doped tin oxide substrate (FTO; sheet resistance: 9 Ω/□) was immersed in an aqueous solution (3 mL) containing Co²⁺ and S²⁻ ions (both 0.5 M) at ambient temperature for 30 s to deposit micro-sized CoS as a thin film on the surface of the FTO substrate. The absorption spectrum of the as-prepared CoS electrode reveals (Fig. 1A) that it absorbed light over the wavelength 350–800 nm. Scanning electron microscopy (SEM) images revealed (Fig. 1B) that the sizes of CoS particles ranged from 8 to 80 μm. We estimated the covering percentage of CoS (gray area) on the FTO surface to be about 60%. The thicknesses of the CoS film were in the range 93–225 nm, as determined from cross-sectional SEM images (Fig. 1C). The as-prepared CdS/CdSe QDSSC featuring a CoS electrode exhibited a value of η (0.4%) that was close to the corresponding cell incorporating a Pt electrode (0.5%; Fig. S3) because of the similar values of V_OC (0.310 vs. 0.355 V), J_SC (3.00 vs. 3.78 mA cm⁻²), and FF (40.9 vs. 40.4%). To investigate the influence of each electrode on the value of η of the QDSSCs, we measured its reflectance and incident photon-to-current conversion efficiency (IPCE). The reflectance of the CoS electrode was greater than that of the Pt electrode (Fig. S4A); as a result, the incident light reflected more efficiently on the TiO₂ layer and, thus, sunlight was absorbed more effectively.¹⁵ The IPCE of the QDSSC featuring a CoS electrode was similar to that of the cell incorporating a Pt electrode (Fig. S4B).

To improve the values of V_OC, J_SC, and FF of CdS/CdSe QDSSCs featuring a CoS counter electrode, we evaluated the effect of using polysulfide solutions—comprising Na₂S (0.5–2.0 M), 2.0 M S, and 0.2 M KCl in methanol–water (7:3, v/v)—as electrolytes.⁵ The photocurrent density–voltage (J–V) curves reveal that the Na₂S concentration had a significant influence on the performance of the QDSSCs (Fig. S5). At 0.5, 1.0, 2.0, and 2.5 M Na₂S, the apparent values of η of the QDSSCs were 0.4, 1.0, 3.4 and 2.6%, respectively. At a high Na₂S concentration (<2.0 M), the oxidation of polysulfides [S²⁻ + 2h⁺ → S; S + S²⁻ → S_x²⁻ (x = 2–5)] was accelerated at the photoelectrode (CdS/CdSe)–electrolyte interface, leading to faster hole recovery between the CdS/CdSe photoelectrodes and the electrolyte.^5,16 However, corrosion of the electrode occurred easily at high Na₂S concentration (>2.0 M). To support our reasoning, we further measured the redox potentials (E_redox) of these polysulfide electrolytes.^9,17 From the potentiostatic current–potential (I–V) data (Fig. S6), we determined the values of E_redox at 0.5, 1.0, and 2.0 M Na₂S to be −0.533, −0.682, and −0.714 V, respectively. As E_redox became more negative, the hole recovery rate from the photoelectrode to the electrolyte increased, leading to increases in the values of J_SC and FF.^1,5 In addition, the value of V_OC increased when the value of E_redox of the electrolyte became more negative.¹⁸ Our results reveal that the Na₂S concentration in the electrolyte played an important role in determining the values of η of the QDSSCs.

Counter electrodes play two roles in solar cells: transferring the electrons arriving from the external circuit back to the redox electrolytes and catalyzing the reduction of the electrolytes. To exploit the ability of the CoS electrodes to enhance the value of η of the QDSSCs, we measured the sheet resistances (R_sh),¹⁵ charge-transfer resistances (R_ct),¹⁹ and electrocatalytic activities of CoS and Pt electrodes using a polysulfide solution of 2.0 M Na₂S, 2.0 M S, and 0.2 M KCl in methanol–water (7:3, v/v) as the electrolyte.^10,11 The value of R_sh of a counter electrode is an important factor determining the efficiency of a DSSC, especially for the FF. A higher value of R_sh of an electrode provides a lower FF for a DSSC and, as a result, a lower value of η.¹⁵ Using a four-point probe, we determined the values of R_sh of the CoS and Pt electrodes to be 14.72 and 18.07 Ω/□, respectively. Because these values are similar, we conclude that the value of R_sh was not responsible for the greater value of η of the QDSSCs. To compare the rates of reduction of the polysulfide solutions on the CoS and Pt electrode surfaces, we used conducting electrochemical impedance spectroscopy (EIS) to determine values of R_ct at the electrode–electrolyte interfaces of 154.2 and 618.2 Ω cm², respectively, (Fig. S7).¹⁹ These values reveal that, of the two systems, less inner energy loss occurred at the CoS electrode–electrolyte interface, leading to a higher FF and, thus, a greater value of η.¹⁵

Fig. 2A displays the performance of the QDSSCs featuring Pt and CoS electrodes, with values of η of 2.1 and 3.4%, respectively, V_OC of 0.400 and 0.454 V, respectively, J_SC of 12.75 and 14.95 mA cm⁻², respectively, and FF of 40.3 and 50.5%, respectively. The increases in the values of J_SC and FF suggest that the more rapid rate of hole recovery at the CoS electrode–electrolyte interface, relative to that for the Pt-containing system, was an important factor toward the improved value of η.⁵ Next, we further investigated the efficiency of the electrocatalytic activity of the Pt and CoS electrodes (Fig. 2B).^10,11 Relative to the Pt electrode, the CoS electrode provided a 10-fold increase in current density at −0.80 V in the cathodic direction, revealing the superior electrocatalytic activity of the CoS electrodes.^10,11 During regeneration in the QDSSCs, the oxidized species, S_x²⁻, had to be converted back (reduced) to S²⁻ on the counter electrode (S_x²⁻ + 2e⁻ → S_x−1²⁻ + S²⁻).⁵ The electrolyte reduction rate was higher on the surface of the CoS electrode that provided the higher value of J_SC.¹⁵ Our results reveal that CoS electrodes promoted the reduction of polysulfide at the CoS electrode–electrolyte interface and provided a lower value of R_ct, leading to enhanced values of η of their corresponding QDSSCs. We further conducted IPCE measurements of the QDSSCs featuring the CoS electrodes to confirm the J_SC obtained from J–V measurement (Fig. S8).²⁰ The η values of QDSSCs with and without zinc sulfide coating were 3.4 and 1.4%, respectively, mainly because zinc sulfide effectively reduced the charge recombination between QDs and electrolyte (Fig. S9).^1,6

For comparison, we also investigated the performance of QDSSCs featuring CuS and NiS electrodes instead of CoS electrodes (Fig. 2). Under the optimal conditions, the QDSSCs incorporating CuS and NiS electrodes provided values of η of 2.5 and 0.4%, respectively. From the potentiostatic I–V and EIS data, the electrocatalytic activities for the sulfide electrodes followed the order CuS > CoS > NiS. The values of R_ct for the CuS, CoS and NiS electrodes were 25.2, 154.2, and 1057.9 Ω cm², respectively. One drawback when using the CuS electrode is that it is quite unstable in the polysulfide electrolyte, limiting its practicality.¹¹ Indeed, the current decayed from 1.66 to 0.42 to 0.13 mA after 1, 5, and 10 scan cycles, respectively, as a result of strong corrosion of the polysulfide electrolyte on the CuS electrode (Fig. S10).¹¹ In contrast, the currents of the CoS and NiS electrodes remained almost constant after 30 scan cycles. Relative to the QDSSCs featuring the CoS electrode, the cell incorporating the CuS electrode provided a lower stability. Although the NiS electrode had higher electrocatalytic activity relative to that of the Pt electrode, it provided a higher value of R_ct (1057.9 vs. 618.2 Ω cm²) and, thus, lower values of J_SC and η for its QDSSCs.

In conclusion, we have employed low-cost CoS counter electrodes in the fabrication of highly efficient CdS/CdSe QDSSCs. These CoS electrodes promote the reduction of polysulfide at the CoS electrode–electrolyte interface and exhibit great electrocatalytic activity, leading to increased values of η for their corresponding QDSSCs. Indeed, the value of η of the CdS/CdSe QDSSC incorporating a CoS electrode and using a polysulfide electrolyte [2.0 M Na₂S, 2.0 M S, and 0.2 M KCl in methanol–water (7:3, v/v)] was as high as 3.4%. Not only do QDSSCs featuring CoS electrodes provide high efficiency, they are also easy to prepare at low cost.

We thank the National Science Council, Taiwan, for financial support (NSC 98-2113-M-002-011-MY3, 98-2627-M-002-013, 98-2627-M-002-014). Z.Y. thanks National Taiwan University for a postdoctoral fellowship in the Department of Chemistry.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details on the preparation and characterization of counter electrodes and CdS/CdSe QDSSCs. See DOI: 10.1039/c0cc00642d

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