FeS2 nanocrystal ink as a catalytic electrode for dye-sensitized solar cells.

In the last decade, dye-sensitized solar cells (DSSCs) have attracted great interest for the fabrication of low-cost largearea photovoltaic devices as an alternative to conventional inorganic counterparts. The counter electrode (CE) is a critical component in DSSCs, where electrons are injected into the electrolyte to catalyze iodine reductions (I3 to I ). The most commonly used CE is based on indium-doped tin oxide (ITO)-coated glass loaded with platinum by sputtering. Platinum has a high catalytic activity for triiodide reduction and presents sufficient corrosion resistance. However, Pt is expensive because of its scarcity, and thus, the development of so-called Pt-free CEs for DSSCs using cheaper and abundant materials becomes technologically desirable. Recently, carbon-based materials, such as graphite, graphene, carbon nanotubes, and conducting polymers, have been used to replace Pt as electrocatalysts for triiodide reduction in DSSCs, although these devices still suffer from poor thermal stability and weak corrosion resistance. Extensive research has been performed on using inorganic compounds such as transitional metal carbides, nitrides, oxides, and sulfides as a new class of alternative catalytic materials for Pt in DSSC systems. Therefore, pursuing low-cost and stable CE materials as alternatives to expensive Pt is crucial to make DSSC systems more competitive for future commercial applications. Pyrite iron disulfide (FeS2, so-called fool s gold) is an interesting next-generation photovoltaic material candidate that is abundant in nature and is nontoxic. It is ranked as having the highest material availability among 23 existing semiconducing photovoltaic systems that could potentially lead to lower costs compared to conventional silicon solar cells. Colloidal pyrite nanocrystals (NCs) were recently synthesized and characterized, providing great potential for developing low-cost fabrications of FeS2-based photovoltaic devices using solution processes. We first demonstrated pyrite NC-based photodiode devices with a spectral response extended to near infrared (NIR) wavelengths because of its large optical absorption coefficient (> 10 cm ) and narrow band gap of 0.95 eV, which provided a crucial step toward success in producing colloidal pyrite NCs thin films as photovoltaic absorption layers. This study demonstrates an important photovoltaic application using FeS2 nanocrystal pyrite ink to fabricate a cost-effective CE in DSSCs, which has the unique advantages of earth abundance and of being solution-processable. The DSSC device with the CE using the FeS2 NC ink exhibits a promising power conversion efficiency of 7.31% comparable to that of the cell using the precious metal of Pt deposited by sputtering (7.52 %), as well as remarkable electrochemical stability of greater than 500 consecutive cycle scans. Solution-processable and semi-transparent FeS2 NC-based CEs also enable the fabrication of flexible and bifacial DSSCs. The results indicate that FeS2 NC ink is an extremely promising candidate for replacing Pt to substantially reduce the cost of DSSCs in future commercial applications and have also shed light on employing the low-cost FeS2 NC catalyst in other electrochemical cells. The FeS2 NCs were prepared using wet solution-phase chemical syntheses with a number of modifications according to our previous reports. 25] Figure 1a shows a highresolution transmission electron microscopy (HR-TEM) image of an FeS2 NC with a diameter of 15 3 nm. The clear lattice fringes of the FeS2 NCs with a lattice spacing of 0.31 nm matched the (111) plane of pyrite. The fast Fourier transform (FFT) patterns shown in Figure 1b exhibited various index facets, including {210}, {211}, and {311} on the NC, showing typical signatures of a pyrite-phased crystal structure.Figure 1 c shows a photograph of the FeS2 NCs ink. For fabricating the FeS2 NC CE, FeS2 NC ink of concentration 30 mg mL 1 was spin-coated onto an ITO glass substrate at 4000 rpm for 20 s, as shown in Figure 1d. Because as[*] Y. -C. Wang, Dr. D. -Y. Wang, H. -A. Chen, Prof. C. -W. Chen Department of Materials Science and Engineering National Taiwan University No. 1, Sec. 4, Roosevelt Rd., Taipei, 10617 (Taiwan) E-mail: chunwei@ntu.edu. tw