Letter pubs.acs.org/NanoLett
High Responsivity Photoconductors Based on Iron Pyrite Nanowires Using Sulfurization of Anodized Iron Oxide Nanotubes Jiang Wu,†,‡,§ Lihui Liu,† Shenting Liu,† Peng Yu,† Zerui Zheng,† Muhammad Shafa,† Zhihua Zhou,†,‡ Handong Li,†,‡ Haining Ji,†,‡ and Zhiming M. Wang*,†,‡,∥ †
Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, P.R. China ‡ State Key Laboratory of Electronic Thin Film and Integrated Devices, School of Microelectronics and Solid-State Electronics, University of Electronic Science and Technology of China, Chengdu 610054, China § Department of Electronic and Electrical Engineering, University College London, Torrington Place, London WC1E 7JE, United Kingdom ∥ Engineering Research Center for Semiconductor Integrated Technology, Institute of Semiconductors, Chinese Academy of Science, Beijing 100083, P. R. China S Supporting Information *
ABSTRACT: Iron pyrite (FeS2) nanostructures are of considerable interest for photovoltaic applications due to improved material quality compared to their bulk counterpart. As an abundant and nontoxic semiconductor, FeS2 nanomaterials offer great opportunities for low-cost and green photovoltaic technology. This paper describes the fabrication of FeS2 nanowire arrays via sulfurization of iron oxide nanotubes at relatively low temperatures. A facile synthesis of ordered iron oxide nanotubes was achieved through anodization of iron foils. Characterization of the iron sulfide nanowires indicates that pyrite structures were formed. A prototype FeS2 nanowire photoconductor demonstrates very high responsivity (>3.0 A/W). The presented method can be further explored to fabricate various FeS2 nanostructures, such as nanoparticles, nanoflowers, and nanoplates. KEYWORDS: Iron pyrite, sulfurization, anodization, self-assembly, nanowire
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Pyrite iron persulfide (β-FeS2, fool’s gold) is one of the most attractive materials among these materials in terms of cost, availability, and environment compatibility.8 Not only does FeS2 have a huge material availability, but FeS2 also has favorable physical properties as a promising PV material. FeS2 has a suitable bandgap (0.95 eV, indirect; 1.03 eV, direct), and according to the Shockley−Queisser model, a theoretical power conversion efficiency of 28% can be expected.9 Furthermore, FeS2 has a high photon absorption coefficient (α > 6 × 105 cm−1 for hν > 1.3 eV), which is over 2 orders of magnitude higher than that of crystalline silicon in the visible region.10,11 Single crystalline FeS2 also can reach a high carrier mobility of 360 cm2V−1s−1 and a long minority carrier diffusion length (0.1−1.0 μm).9,12 Due to the high absorption coefficient and long minority carrier diffusion length, only a thin FeS2 absorber layer is required for solar cells without compromising the cell efficiency, and thus, material cost can be further reduced. The interest in FeS2 as a PV material can be traced back to 1984, when Ennaoui and Tributsch demonstrated the first
n the last decades, new classes of PV technologies have been developed in various materials, including amorphous silicon (a-Si), CIGS [CuInxGa1−xSe(S)2], CdTe, InAs, organic polymer, and PbSe(S) aiming for low-cost PV electricity generation.1−7 Solar cells made from low-cost materials, such as a-Si, CIGS, CdTe, and organic polymer, have demonstrated plausible efficiency to meet this goal. Yet, these solar cells are still far away from being competitive against fossil fuels in terms of cost. Moreover, the toxicity and low conservation of some materials, such as cadmium, have limited their application as the sustainable energy materials. Earth-abundant binary materials, such as pyrite FeS2, Cu2S, Cu2O, and Zn3P2, recently gain increasing attention in PV applications. According to a model comparing the least cost per Watt and high theoretical performance of solar cells, these earth-abundant materials would be good alternatives to traditional solar grade silicon wafers.8 The extraction costs of Cu2O, PbS, CdS, Cu2S, CuO, Zn3P2, a-Si, and FeS2 are 1 order of magnitude lower than that of crystalline Si, and apparently, the solar cell cost can be reduced by using these materials. However, in order to meet low-cost requirement without sacrificing PV electricity generation capacity, a PV material must also have potential in obtaining high power conversion efficiency. © 2014 American Chemical Society
Received: August 8, 2014 Revised: September 6, 2014 Published: September 18, 2014 6002
dx.doi.org/10.1021/nl503059t | Nano Lett. 2014, 14, 6002−6009
Nano Letters
Letter
Figure 1. SEM images of the anodized iron films in the ethylene glycol electrolyte at different anodization conditions: (a) anodizing of 30 V and water content of 1.0 M; (b) anodizing voltage of 40 V and water content of 0.5 M; (c−e) anodizing voltage of 40 V and water content is 1.0, 1.5, and 2.0 M, respectively; (f, g) anodizing voltage of 40 and 50 V, respectively, and water content of 1.0 M. The anodizing time for all samples is 30 min. The scale bars are 1 μm. Insetted is the illustration of the chemical dissolution model of the chemical dissolution of anodic iron oxide nanotube arrays with increasing water content and anodizing voltage, respectively.
pyrite photoelectrochemical and Schottky solar cells.13 Since then, a number of studies on FeS2 based solar cells have been reported, among which FeS2 solar cells demonstrated high quantum efficiencies (>90%) and photocurrents (>40 mA/ cm2).14−17 However, the FeS2 solar cells generally show very poor open circuit voltages (
