Letter Cite This: ACS Appl. Nano Mater. 2018, 1, 2449−2454
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CuInS2/CdS-Heterostructured Nanotetrapods by Seeded Growth and Their Photovoltaic Properties Bomi Kim,†,‡ Kangwook Kim,†,‡ Yongju Kwon,† Woojin Lee,† Weon Ho Shin,§ Sungjee Kim,† and Jiwon Bang*,⊥ †
ACS Appl. Nano Mater. 2018.1:2449-2454. Downloaded from pubs.acs.org by UNIV OF CONNECTICUT on 10/21/18. For personal use only.
Department of Chemistry, Pohang University of Science and Technology, 77 Cheongam-ro, Namgu, Pohang 37673, Republic of Korea ⊥ Electronic Conversion Materials Division and §Energy & Environmental Division, Korea Institute of Ceramic Engineering and Technology, Jinju 52851, Republic of Korea S Supporting Information *
ABSTRACT: We investigate a seeded-growth method for the synthesis of CuInS2 (core)/CdS (arm)-heterostructured nanotetrapods (HNTs), in which wurtzite CdS arms grow on CuInS2-core quantum dot seeds with tunable lengths. The CdS arms of the nanocrystal tetrapods effectively passivate the unstable surface defects and improve the photoluminescence quantum efficiencies to above 40%. Owing to spatially separated quasi-type-II excitons, efficient electron transport along the elongated arms, and enhanced light-absorption capabilities, the CuInS2/CdS-HNT-sensitized solar-cell device exhibits a more than 3-fold increase in the power conversion efficiency compared to that of the control CuInS2@CdS (core@shell) quantum dot device. We believe that the high photoenergy conversion abilities of the quasi-type-II CuInS2/CdS HNTs make them potential candidate materials for use in cost-effective and highly efficient solar cells. KEYWORDS: copper−indium sulfide, cadmium sulfide, tetrapod, quasi-type-II heterostructure, photovoltaic device
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type-II band alignments, in which electrons are delocalized over the entire system, while holes are confined to CuInS2. The spatially separated electron and hole wave functions in the CuInS2/CdS heterostructure reduce the carrier recombination rate,7,17,20 and, consequently, they are more suited to photovoltaic and photocatalytic applications compared to bare CuInS2 NPs. Herein, we synthesized CuInS2/CdS HNTs that employ zinc-blende-structured (ZB-structured) CuInS2 QDs as their cores and examined the evolution of the optical properties of these CuInS2/CdS HNTs during CdSarm growth (Scheme 1a). Our CuInS2/CdS HNTs exhibit high photoluminescence (PL) efficiencies (in excess of 40%) with large absorptions over the entire visible spectral range. Furthermore, we evaluated the photovoltaic properties of a proof-of-concept sensitized solar cell (SSC) that exploited the CuInS2/CdS HNTs as the sensitizer (Scheme 1b). The CuInS2/CdS HNTs have several advantages over spherical CuInS2@CdS QDs, including higher sunlight-absorbing capacities and more efficient photoinduced charge separation and transport due to their unique morphologies. These combined properties led to a 340% enhancement in the
ontrolling the morphologies of colloidal semiconductor nanoparticles (NPs), such as dots, rods, platelets, and branched structures, is a topic of scientific interest that has practical applications.1,2 Specifically, tetrapod-shaped nanostructures have unique morphology-dependent optoelectrical properties that include excellent light absorption abilities originating from their relatively large volumes over conventional quantum dots (QDs), distributed surfaces with large areas, and carrier-guiding directionalities with enhanced chargecarrier-separation capabilities, which can be used in photovoltaic and photocatalytic applications.3−7 Ternary CuInS2 QDs are outstanding candidates for optoelectronic applications as light-harvesting materials in terms of their low toxicities,8 emission tunabilities over wide wavelength ranges, from the ultraviolet to the near-infrared, 9 and high absorption coefficients.10 Accordingly, CuInS2 QDs have been successfully applied to photovoltaic devices,11−14 and an efficiency in excess of 5% has been achieved.13 The optical and electrical properties of CuInS2 QDs can be adjusted for various applications through the introduction of inorganic shell overcoatings (ZnS15,16 and CdS17−19) to form core@shell architectures. Sakamoto and co-workers7 recently reported the synthesis of CuInS2/CdS-heterostructured nanotetrapods (HNTs) and the characterization of their lightstimulated carrier dynamics through transient absorption spectroscopy. CuInS2/CdS-NP heterostructures exhibit quasi© 2018 American Chemical Society
Received: February 14, 2018 Accepted: May 16, 2018 Published: May 16, 2018 2449
DOI: 10.1021/acsanm.8b00250 ACS Appl. Nano Mater. 2018, 1, 2449−2454
Letter
ACS Applied Nano Materials Scheme 1. Schematic Representation of the Present Worka
a (a) Luminescent CuInS2/CdS HNTs that employ CdS ZB-structured CuInS2 QDs as junctions for the growth of WZ) CdS arms. (b) Schematic illustration of the CuInS2/CdS HNT SSC and the proposed photocarrier-transfer mechanism in the sensitized solar cell.
Figure 1. (a) XRD patterns of CuInS2 seed QDs (black) and CuInS2/CdS HNTs (red). The lower and upper bars indicate the patterns of the bulk ZB-structured CuInS2 and the bulk WZ-structured CdS, respectively. (b) TEM image of the CuInS2/CdS HNTs with an HAADF-STEM image of a CuInS2/CdS HNT in the inset. (c) HRTEM image of a CuInS2/CdS HNT. The inset shows lattice fringe spacings of ∼3.10 Å in a CdS arm under magnification.
indicated the CuInS2/CdS (core/arm) structure (Figure S1). The CuInS2/CdS HNTs were determined by powder XRD to have condensed into WZ-CdS structures (Figure 1a). The caxis-oriented WZ-structured CdS arms are epitaxially grown on the four {111} facets of the ZB-structured CuInS2 seeds, which is typical for the growth of II−VI tetrapods and is consistent with previous reports.3,23,24 ODPA functions as a surface ligand, while the synthesis of the WZ-structured CdS arm led to growth in the [0001]̅ direction. It is well-known that alkylphosphonic acids play key roles in directing growth along the c axis of the WZ structure of cadmium chalcogenides by selective (112̅0) passivation parallel to the WZ c axis.25 Spherically shaped CuInS2/CdS (core/ shell) structures were obtained when oleic acid (OA) was used instead of ODPA during CdS-shell growth (Figure S2). Clearly, the ODPA ligands tend to induce anisotropic CdS-arm growth on the CuInS2 seed. The evolution of the morphologies and the corresponding absorption and PL spectra of the CuInS2/CdS HNTs during CdS-arm growth at 300 °C are shown in Figure 2. The bare CuInS2 QDs exhibit a weak PL emission at 890 nm (Figure 2e) with a large global Stokes shift of ∼130 nm. The PL band of the CuInS2 QDs exhibits asymmetry that is characterized by two distinct Gaussian curves (Figure S3). The higher-energy and lower-energy PL bands are presumably attributed to conduction-band (CB)-to-acceptor and defect-related donor− acceptor pair (DAP) recombination channels, respectively.26,27 The PL feature of the CuInS2 core QDs indicates that defectrelated DAP recombination predominantly occurs over the CBrelated recombination, which is ascribable to the high density of surface defects that act as donor or acceptor levels. Short CdS arms grow on the {111} facets of the ZB-structured CuInS2 seeds to form branched seeds (Figure 2b). We observed that CdS overgrowth leads to a blue shift in the PL peak components (Figures 2e and S3), which is explained by cation
photovoltaic power conversion efficiency (PCE) compared to that of spherical CuInS2@CdS QDs, which can be widely applied to photoenergy conversion systems. The crystal structures of the CuInS2/CdS HNTs were determined by high-resolution transmission electron microscopy (HRTEM) and powder X-ray diffraction (XRD), which revealed the growth of wurtzite (WZ)-structured CdS arms on the CuInS2 QDs. The lengths of the tetrapod arms can be tuned from 8 to 22 nm by modifying the reaction conditions, as discussed in detail below. ZB-structured CuInS2 QD seeds were prepared using previously reported pyrolysis methods.15,21 The average diameter of the as-synthesized CuInS2 QDs was 4.2 nm. The CuInS2 QD crystal structure was determined by powder XRD, which revealed a ZB structure (Figure 1a).22 CuInS2/CdS HNTs were synthesized using a seeded-growth method with the ZB-structured CuInS2 QDs as seeds.7 Briefly, a mixture of cadmium oxide and octadecylphosphonic acid (ODPA) was heated to 320 °C in trioctylphosphine oxide as the solvent, after which the CuInS2 QDs and a sulfur/ trioctylphosphine solution were sequentially injected into the growth solution. The CdS arms were grown on the CuInS2 seeds at 300−340 °C for about 1 h (detailed synthetic protocols are given in the Supporting Information). Figure 1b displays a TEM image of the as-prepared CuInS2/CdS HNTs, which reveals multiple tetrapod structures. The high-angle annular-dark-field scanning TEM (HAADF-STEM) image shows a three-armed CuInS2/CdS NP with much higher contrast in the central region of the structure, which is attributed to the fourth arm extending upward, toward the viewer (inset, Figure 1b). The corresponding HRTEM image (Figure 1c) reveals lattice fringe spacings in the CdS arm of 0.31 nm that match the (002) plane of the WZ structure, confirming CdS-arm growth in the [0001̅] direction.3,7 Elemental analyses by energy-dispersive X-ray spectroscopy (EDS) at the center of the structure and an elongated arm also 2450
DOI: 10.1021/acsanm.8b00250 ACS Appl. Nano Mater. 2018, 1, 2449−2454
Letter
ACS Applied Nano Materials
orientation.3,29 The photoexcited electron−hole pair in the CuInS2/CdS HNT is spatially separated, and the hole mostly resides in the CuInS2 core, whereas the electron is spread over the entire tetrapod structure, resulting in slight red shifts in the PL peaks and slight decreases in the PL QEs of the CuInS2/ CdS HNTs with longer CdS arms. The PL QEs of the CuInS2/ CdS HNT samples with CdS-shell growth times of 30 and 60 min are 43% and 31%, respectively (Figure 2f). We expect that the notable optical characteristics, such as strong lightabsorbing abilities and bright emissions with large Stokes shifts, exhibited by our CuInS2/CdS HNTs make these materials promising as single-particle-imaging probes on biomolecules30 or luminescent solar concentrators.19 The CuInS2/CdS HNT arm lengths can be tuned by adjusting the molar ratio of the Cd and S precursors. When the Cd:S molar ratio was 1:2, bipod-, tripod-, and tetrapod-shaped nanocrystals were produced. However, increasing the Cd:S molar ratio produced predominantly tetrapod-shaped CuInS2/ CdS NPs with longer CdS arms (Figure 3). The average CdSarm lengths were 8.2, 14.5, and 22.0 nm at Cd:S molar ratios of 1:2, 1:1, and 2:1, respectively. The Cd precursor should be used in excess in order to promote anisotropic CdS-arm growth under our experimental conditions. We assume that high monomer flux, especially of the Cd precursor, drives kinetic growth on the S-terminated (0001̅) facet of the CdS arms,3,31 resulting in anisotropic nanocrystal growth along the unique c axis;32 however, further analysis is required to clarify this hypothesis. The effect of the reaction temperature on CuInS2/ CdS NP morphology was also investigated (Figure S4). A high temperature (340 °C), without any other changes, accelerates the CdS-arm growth rate presumably because of the rapid diffusion of monomers toward the growth facet;32 however, the CdS-arm lengths and diameters of the resulting HNTs were not noticeably different from those of the HNTs grown at 300 °C. In contrast, CdS-shell growth at 270 °C resulted in irregularly branched structures with thick, short CdS arms, which is possibly because of 1D-to-2D ripening and the rearrangement of the CdS layer resulting from the insufficient supply of the active precursor at the lower temperature.33 The branched quasi-type-II CuInS2/CdS-heterostructured NPs possess many features that are favorable for photovoltaic applications. First, the CuInS2/CdS NPs efficiently absorb light from the ultraviolet-to-near-infrared regions of the spectrum (Figure 2e). Furthermore, loosely bound carriers originating from partially separated electron and hole wave functions, a result of the quasi-type-II band alignment, facilitate efficient carrier separation.7,17,18,34 In addition, the anisotropic-branched morphology not only provides more efficient charge transport through the elongated arm structure but more readily achieves a large interface area for charge transfer than separated rods. To evaluate the photovoltaic performance of our quasi-typeII CuInS2/CdS HTNs, we fabricated SSCs using the CuInS2/
Figure 2. TEM images of (a) CuInS2 QDs and (b−d) CuInS2/CdS HNTs with varying CdS-arm growth times: (b) 10 min; (c) 30 min; (d) 60 min (scale bar: 50 nm). (e) Absorption (black) and PL (red) spectra. (f) Corresponding PL QEs of the CuInS2 QD and CuInS2/ CdS HNTs.
exchange in which Cu+ or In3+ ions on the CuInS2 QD surface are exchanged for Cd2+; this results in a smaller effective CuInS2 QD size, which is consistent with previous observations.17,20,28 The CuInS2/CdS core/branched shell structure (growth time 10 min) enhances the PL quantum efficiency (QE; up to 52%) because the CdS layers effectively passivate the unstable surface sites of CuInS2 (Figure 2f). In addition, the probability of CB-to-acceptor recombination is sharply increased, compared to DAP recombination, during CdS-arm growth, resulting in the two distinct PL features observed for CuInS2/CdS (Figure S3). After reaction for about 30 min, the asymmetric growth of the CdS shells on the branched seeds led to tetrapods in which absorption from the CdS arms dominated at wavelengths below 480 nm (Figure 2c−e). The tetrahedral symmetry of the four CdS arms of the HNTs contributes to the enormous absorption cross section that effectively captures photons of any polarization irrespective of the HNT
Figure 3. TEM images of CuInS2/CdS HNTs grown using different initial Cd:S precursor ratios of (a) 1:2, (b) 1:1, and (c) 2:1 (scale bar: 50 nm). 2451
DOI: 10.1021/acsanm.8b00250 ACS Appl. Nano Mater. 2018, 1, 2449−2454
Letter
ACS Applied Nano Materials
generation rate in the photoactive layers and the rate of their extraction from the cell. We simply calculated the rate of exciton generation in the SSC films from the optical-absorption spectrum of the photoactive layer and the AM 1.5 G solar spectrum through eq 1
CdS HTNs as light-harvesting sensitizers. Photoactive SSC anodes were prepared by the electrodeposition of CuInS2/CdS HNTs, in which the average dimensions of the CdS arms were 13.1 ± 2.0 nm (length) and 4.5 ± 0.7 nm (width) on mesoporous TiO2 (mp-TiO2) films (see the Supporting Information).11 Spherically shaped CuInS2@CdS (core/shell structured) QDs were also prepared for comparison, and all photoanodes were finally twice coated with ZnS by successive ionic layer adsorption and reaction to reduce recombination processes at the TiO2/NP/electrolyte interface.35 The CuInS2/ CdS-HNT-sensitized electrode absorbed more strongly than CuInS2@CdS QD-TiO2, especially at wavelengths below 500 nm (Figure S5b) because of the large volumes of their CdS arms. This also indicates that the HNTs are effectively deposited on an mp-TiO2 layer of limited pore size (
