Efficient p-type dye-sensitized solar cells with all-nano-electrodes: NiCo2S4 mesoporous nanosheet counter electrodes directly converted from NiCo2O4 photocathodes
© Shi et al.; licensee Springer. 2014
Received: 10 August 2014
Accepted: 23 September 2014
Published: 11 November 2014
We report the successful growth of NiCo2S4 nanosheet films converted from NiCo2O4 nanosheet films on fluorine-doped tin oxide substrates by a low-temperature solution process. Low-cost NiCo2S4 and NiCo2O4 nanosheet films were directly used for replacing conventional Pt and NiO as counter electrodes and photocathodes, respectively, to construct all-nano p-type dye-sensitized solar cells (p-DSSCs) with high performance. Compared to Pt, NiCo2S4 showed higher catalytic activity towards the I-/I3- redox in electrolyte, resulting in an improved photocurrent density up to 2.989 mA/cm2, which is the highest value in reported p-DSSCs. Present p-DSSCs demonstrated a cell efficiency of 0.248 % that is also comparable with typical NiO-based p-DSSCs.
Dye-sensitized solar cells (DSSCs) are typically composed of dye-sensitized nanocrystalline semiconductors, an iodide/triiodide (I-/I3-) redox electrolyte, and a counter electrode (CE) [1–3]. One research direction to improve conversion efficiency of DSSCs is to construct a tandem DSSC in a sandwich configuration, which combines a photoanode from an n-type DSSC (n-DSSC) and a photocathode from a p-type DSSC (p-DSSC) [4, 5]. In a tandem cell, the overall photovoltage is the sum of two parts, while the photocurrent is limited by the photoelectrode with a smaller current. Recently, the conversion efficiency of n-DSSCs reaches as high as 12% , but the p-DSSCs still suffers from low efficiencies and thus limit the overall efficiency of the tandem DSSCs. Although much attention has been paid to search ideal p-type semiconductor materials and efficient dyes [7–10], the best p-DSSC exhibits efficiency of only 1.3% by utilizing [Co(en)3]2+/3+ redox couple and PMI-6 T-TPA dye . It is worthy to note that few results are devoted to the study of CEs for p-DSSCs, ignoring a feasible approach to further improve the conversion efficiency. The function of CEs is to transfer the electrons (holes) arriving from the external circuit to the redox electrolyte to catalyze the reduction (oxidization) of the redox couple. Generally, platinum (Pt) is the preferred CE material due to its superior electrocatalytic activity, high electrical conductivity, and excellent chemical stability. However, as a noble metal, Pt is one of the most expensive materials and has low abundance in the earth, preventing it from being used for large-scale manufacture of DSSCs.
Since Grätzel's group found that cobalt sulfide (CoS) has excellent catalytic activity for the iodine-based redox couples , great efforts have been made to exploit abundant low-cost substitutes for Pt, including metal sulfides, nitrides, and carbides [13–16] Unfortunately, to date, these reported catalysts can hardly compete with Pt in the performance of DSSCs. As an important class of chalcogenides, semiconducting sulfides have drawn intensive attention due to their distinctive electronic properties, interesting chemical behaviors, and a variety of applications. Particularly, binary metal sulfides (NiS and CoS) have exhibited almost the same conversion efficiency as Pt in DSSCs [17–19]. Compared with binary NiS and CoS, ternary sulfide NiCo2S4 is expected to offer richer redox reactions due to the contributions from both nickel and cobalt ions. For example, NiCo2S4 has demonstrated enhanced catalytic activity for oxygen evolution and polysulfide redox couple [20, 21]. Recent results also showed that NiCo2S4 can be used as CEs of n-type DSSCs, but its efficiencies are lower than those of the Pt-based cells [22, 23].
In this paper, we reported low-cost NiCo2S4 nanosheet (NS) films grown on a fluorine-doped tin oxide (FTO) substrate as a high-performance CE for p-DSSCs composed of p-type NiCo2O4 semiconductor photocathode. The NiCo2S4 NS film was synthesized via a facile two-step process including the synthesis of NiCo2O4 NS film on a FTO substrate and then an anion ion exchange process under hydrothermal reaction. When applying the NiCo2S4 as a CE and NiCo2O4 as a photocathode, novel all-nano-p-DSSCs achieved an impressive photocurrent of 2.989 mA/cm2 and cell efficiency of 0.248% versus 1.824 mA/cm2 and 0.158% for Pt under the same conditions. To the best of our knowledge, this efficiency is comparable and even higher than that of NiO-based p-DSSCs with an I-/I3- redox couple.
Synthesis of NiCo2S4 nanosheet films
A two-step hydrothermal process was used to synthesize NiCo2S4 nanosheet films on FTO substrates. In the first step, NiCo2O4 nanosheet arrays were synthesized by a modified low-temperature hydrothermal method [24–28]. Typically, 30 mmol of urea and 8 mmol of NH4F were dissolved completely in 30 mL deionized water, followed by the addition of 1 mmol of Ni(NO3)2 · 6H2O and 2 mmol of Co(NO3)2 · 6H2O. The mixture was transferred to a capped bottle with a FTO growth substrate facing down in the precursor at 85°C for 6 h. Once the reaction was finished, the samples were rinsed with deionized water and treated in air at 350°C for 3 h, and the NiCo2O4 nanosheet films were obtained. In the second step, to synthesize NiCo2S4 nanosheet films, the NiCo2O4 nanosheet films were put into Na2S · 9H2O solution (2 mol/L) and reacted in an autoclave at 160°C for 10 h. After the reaction, the samples were rinsed thoroughly with deionized water and dried at 60°C for 5 h in a vacuum oven.
Fabrication of p-DSSC devices
The photocathode was prepared by immersing a NiCo2O4 sample in an ethanol solution containing 0.5 mM of N719 dye (cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylic acid)ruthenium(II)) (Solaronix SA, Aubonne, Switzerland) for 3 h, followed by rinsing in ethanol to remove dye absorbed physically, and drying in air. The Pt counter electrode was prepared by spin coating 1 mM of chloroplatinic acid (H2PtCl6 · 6H2O, Aldrich, 99.9%) in 2-propanol (Sigma-Aldrich, St. Louis, MO, USA; 99.7%) onto a FTO substrate and then heating at 350°C for 30 min. The as-prepared NiCo2S4 nanosheet films were used directly as counter electrodes. The dye-coated photocathodes were sealed against Pt or NiCo2S4 counter electrodes with hot melt plastic spacers (Solaronix, 60-μm thick). The electrolyte (0.1 M LiI, 0.03 M I2, 0.5 M tetrabutylammonium iodide, and 0.5 M 4-tert-butylpyridine in acetonitrile) was introduced into the gap between two electrodes by a syringe. The active area of DSSCs was 0.2 cm2.
The morphology was characterized by field emission scanning electron microscope (FESEM; Hitachi SU8010, Hitachi Ltd., Tokyo, Japan). The microstructure was analyzed by high-resolution transmission electron microscopy (HRTEM) with selected area electron diffraction (SAED) (FEI Tecnai G2 F20 S-TWIN TMP, FEI, Hillsboro, OR, USA). The phase of products was checked by an X-ray diffractometer (XRD).
Photocurrent-voltage (J-V) characteristics were performed using a Keithley 2400 SourceMeter (Keithley Instruments Inc., Cleveland, OH, USA) under simulated AM 1.5G illumination (100 mW/cm2) provided by a solar light simulator (94043A, Newport Corp., Irvine, CA, USA). Cyclic voltammetry (CV) and the electrochemical impedance spectroscopy (EIS) were measured with an Autolab electrochemical workstation (PGSTAT 302 N, Metrohm AG, Utrecht, The Netherlands). CV was carried out in a three-electrode system with different counter electrodes as working electrodes, a Pt foil as counter electrode, and a Ag/Ag+ electrode as reference electrode at a scan rate of 50 mV/s. The electrodes were immersed into an anhydrous acetonitrile solution containing 0.1 M LiClO4, 10 mM LiI, and 1 mM I2. EIS was actualized with a symmetric cell assembled with two identical counter electrodes at open-circuit voltage (Voc) bias under dark condition. The measured frequency ranged from 10 mHz to 1 MHz and the magnitude of the alternative signal was 10 mV.
Results and discussion
In the CV curves, the peak-to-peak separation (Epp) and the peak current density are two important parameters for comparing catalytic performance of different CEs . The anodic current density is generally related with the rate of reaction for I- oxidation in p-DSSCs. Compared to Pt CE, the NiCo2S4 NS CE possesses higher anodic and cathodic current densities, indicating that NiCo2S4 NS has higher catalytic activity than Pt. In addition, the Epp value is inversely correlated with the standard electrochemical rate constant of a redox reaction. The Epp of the NiCo2S4 NS CE is 0.420 V, which is almost similar with that (0.408 V) of the Pt CE. Enhanced current density and similar Epp imply that the performance of NiCo2S4 NS CE is comparable to and even better than that of Pt CE.
Detailed photovoltaic and EIS parameters of the DSSCs with counter electrodes composed of NiCo 2 S 4 nanosheets and Pt
where k is the Boltzmann constant, T is the absolute temperature, n is the number of holes involved in the electrochemical oxidation of I- at the electrode, e0 is the elementary charge, c is the concentration of I-, A is the electrode area, ω is the angular frequency, D is the diffusion coefficient of I-, and δ is the thickness of the diffusion layer. The ZN values for NiCo2S4 and Pt CEs are 3.58 and 4.44 Ω cm2, respectively, indicating that the D of I- in the NiCo2S4 cell is larger than that in the Pt cell. For the same electrolyte, a larger D means that the electrode has higher electrocatalytic activity, because faster oxidation of the I- on the surface of catalysts can accelerate the diffusion of I- ions in electrolyte.
Low-cost NiCo2S4 NS films with high catalytic activity have been utilized as CEs of p-DSSCs based on NiCo2O4 NS photocathodes. The mesoporous nanosheets provide a large catalytically active area and facilitate the transport of I-/I3- redox in electrolyte. The DSSCs with NiCo2S4 as a CE produce a higher Jsc and η (2.989 mA/cm2 and 0.248%, respectively) than those (1.824 mA/cm2 and 0.158%, respectively) of the cell with a Pt CE. This Jsc can almost match the performance of p-DSSCs, but the Voc is still low. In the future, improving the Voc by doping materials and replacing electrolyte types is an important route. The use of cost-effective NiCo2S4 as an alternative to noble Pt, in combination with a facile fabrication method, may pave the way for low-cost, scalable, and high-efficiency DSSCs.
We acknowledge the support from the National Natural Science Foundation (51422206, 51372159, 11304217), 1000 Youth Talents Plan, and Jiangsu Shuangchuang Plan, a project supported by Jiangsu Scientific and Technology Committee for Distinguished Young Scholars (BK20140009) and funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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