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CuInS/CdS-Heterostructured Nanotetrapods by Seeded Growth and their Photovoltaic Properties Bomi Kim, Kangwook Kim, Yongju Kwon, Woo Jin Lee, Weon Ho Shin, Sungjee Kim, and Jiwon Bang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00250 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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ACS Applied Nano Materials

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* ,‡ †

Department of Chemistry, Pohang University of Science and Technology (POSTECH), 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 52852, Republic of Korea.

ABSTRACT We investigate a seeded-growth method for the synthesis of CuInS2 (core)/CdS (arm) tetrapod hetero-nanostructures (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 (QYs) to above 40%. Owing to spatially separated quasi-type II excitons, efficient electrontransport along the elongated arms, and enhanced light-absorption capabilities, the CuInS2/CdSHNT-sensitized solar-cell device exhibits a more than three-fold increase in power conversion efficiency compared to that of the control CuInS2@CdS (core@shell) quantum dot device. We

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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

Main Text Controlling 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(QD), distributed surfaces with large areas, and carrier-guiding directionalities with enhanced charge-carrier-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, CdS17-19) to form core@shell architectures. Sakamoto and co-workers7 recently reported the synthesis of CuInS2/CdS


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heterotetrapod nanostructures and the characterization of their light-stimulated carrier dynamics through transient absorption spectroscopy. CuInS2/CdS-NP heterostructures exhibit a quasi-type II band alignments, in which electrons are delocalized over the entire system, while the holes are confined to the CuInS2. The spatially separated electron and hole wavefunctions 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-heterojunction nanotetrapods (HNTs) that employ zincblende-structured (ZB-structured) CuInS2 QDs as their cores, and examined the evolution of the optical properties of these CuInS2/CdS HNTs during CdS-arm 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-separations and transport due to their unique morphologies. These combined properties led to a 340% enhancement in photovoltaic powerconversion efficiency (PCE) compared to spherical CuInS2@CdS QDs, which can be widely applied to photo-energy conversion systems.


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Scheme 1. Schematic representation of the present work. (a) Luminescent CuInS2/CdS HNTs that employ CdS zinc-blende-structured (ZB-structured) CuInS2 QDs as junctions for the growth of wurtzite (WZ) CdS arms. (b) Schematic illustration of the CuInS2/CdS-HNTsensitized solar cell 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-structure of CuInS2 and the bulk WZ-structure of 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.


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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 (Fig. 1a).22 CuInS2/CdS HNTs were synthesized using a seeded-growth method with the ZB-CuInS2 QDs as seeds.7 Briefly, a mixture of cadmium oxide and octadecylphosphonic acid (ODPA) was heated to 320 °C in trioctylphosphine oxide (TOPO) 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). Fig. 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, Fig. 1b). The corresponding HRTEM image (Fig. 1c) reveals lattice fringe spacings in the CdS arm of 0.31 nm that matches the (002) plane of the WZ structure, confirming CdS-arm growth in the [0001] direction.3,7 Elemental analyses by energy-dispersive X-ray spectroscopy (EDS) at the center of the structure and an elongated arm also indicated the CuInS2/CdS (core/arm) structure (Fig. S1). The CuInS2/CdS HNTs were determined by powder XRD to have condensed into WZ-CdS structures (Fig. 1a). The c-axis-oriented WZ-CdS arms are epitaxially grown on the four {111}


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facets of the ZB-CuInS2 seeds, which is typical for the growth of II-VI tetrapods and is consistent with previous reports.3,23,24

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, and (d) 60 min (scale bar: 50 nm). (e)

Absorption (black) and PL (red) spectra, and (f) the corresponding PL QYs of the CuInS2 QD and CuInS2/CdS HNTs.


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ODPA functions as a surface ligand, while the synthesis of the WZ-CdS arm led to growth in the [0001] direction. It is well known that alkylphosphonic acids play key roles in directing 0) growth along the c-axis of the WZ structure of cadmium chalcogenides by selective (11 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 (Fig. 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 Fig. 2. The bare CuInS2 QDs exhibit a weak PL emission at 890 nm (Fig. 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 (Fig. 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 defect-related DAP recombination predominantly occurs over the CB-related 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-CuInS2 seeds to form branched seeds (Fig. 2b). We observed that CdS overgrowth leads to a blue shift in the PL-peak components (Figs. 2e and S3), which is explained by cation exchange in which Cu+ or In3+ ions on the CuInS2-QD surface are exchanged for Cd2+; this results in a smaller effective CuInS2-QDs 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 the CuInS2 (Fig. 2f). In addition, the probability of CB-toacceptor recombination is sharply increased, compared to DAP recombination, during CdS arm


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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).

growth, resulting in the two distinct PL features observed for CuInS2/CdS (Fig. S3). After reacting 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 (Fig. 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 HNT 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 CuInS2/CdS HNT samples with CdS-shell growth times of 30 and 60 min are 43% and 31%, respectively (Fig. 2f). We expect that the notable optical characteristics, such as strong light-absorbing abilities and bright emissions with large Stokes shifts, exhibited by our CuInS2/CdS HNTs make these materials promising as single-particle-imaging probes on bio-molecules,30 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 NCs were


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produced. However, increasing the Cd:S molar ratio produced predominantly tetrapod-shaped CuInS2/CdS NPs with longer CdS arms (Fig. 3). The average CdS-arm 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 (0001) facet of the CdS arms,3,31 resulting in anisotropic NC growth along the unique c-axis;32 however, further analysis is required to clarify this hypothesis. The effect of reaction temperature on CuInS2/CdS-NP morphology was also investigated (Fig. S4). A high temperature (340 °C), without any other changes, accelerates the CdS-arm growth rate presumably due to the rapid diffusion of monomers toward the growth facet;32 however the CdSarm lengths and diameters of the resulting HNTs were not noticeably different to 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 due to 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

Table 1. Photovoltaic characteristics of CuInS2/CdS SSCs. The branched quasi-type II CuInS2/CdS-heterostructured NPs possess many features that are favorable for photovoltaic applications. Firstly, the CuInS2/CdS NPs efficiently absorb light from the ultraviolet to near-infrared regions of the spectrum (Fig. 2e). Furthermore, loosely bound


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carriers originating from partially separated electron and hole wavefunctions, 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.

Figure 4. (a) Schematic illustration of the energy levels of a CuInS2/CdS SSC. (b) J-V curves for a CuInS2@CdS-QD SSC (black) and CuInS2/CdS-HNT SSC (red).

To evaluate the photovoltaic performance of our quasi-type II CuInS2/CdS HTNs, we fabricated SSCs using the CuInS2/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 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


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strongly than CuInS2@CdS QD-TiO2, especially at wavelengths below 500 nm (Fig. S5b) due to 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 (< ~30 nm)35 despite the spatial incompatibility between the mp-TiO2 layer and the elongated CdS arms of the CuInS2/CdS-HNT structure. After deposition of the NPs on the mp-TiO2 anode, Cu2S was employed as the counter electrode with a sulfide/polysulfide redox couple. Referring to the potential diagram depicting the SSC components (Fig. 4a), the photoexcited electrons are injected from the CdS arms of the CuInS2/CdS HNTs into the TiO2 anode, while holes in the CuInS2 core are transferred to the electrolyte. Current-voltage (I-V) behavior was investigated for the SSCs fabricated from the CuInS2/CdS-HNTs and control CuInS2@CdS QDs. The CuInS2/CdS-HNT device shows synergistic improvements in short-circuit current (Jsc) with slight increase in open-circuit voltage (Voc) and fill factor (FF), leading to a three-fold increase in overall PCE compared to that of the CuInS2@CdS-QD control device (Fig. 4b and Table 1). The photocurrent response of the CuInS2/CdS-HNT-sensitized device to incident light was characterized by external quantum efficiency (EQE) spectroscopy. (Fig. S7) The photoresponse EQE spectrum is well matched to the absorption spectrum of the CuInS2/CdS-HNT sensitizer (Fig. 2e), and the high EQE signal in at wavelengths below ~500 nm indicates light absorption by the CdS arms followed by possible hole transfer to the CuInS2 core. Jsc is determined by the collected photogenerated carriers, which depends on the carrier-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 the following equation (1):

G = ∙ 1 − 10 ,

(1)


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where N, A, and λ are the number of solar photons per unit area, the absorbance of the photoactive layer, and the photon wavelength, respectively. Only the visible region of the solar spectrum (400–800 nm) was considered because the solar spectrum below 400 nm contributes less than 3% to the overall solar photon flux at the Earth’s surface, and the CuInS2/CdS photoanode cannot absorb light with wavelengths above 800 nm. The SSC composed of HNTs exhibits only a ~1.5-fold increase in the exciton-generation rate over that of the SSC composed of QDs. However, the observation that HNT-SSC has a Jsc value more than three times that of the QD-SSC indicates that the photogenerated carriers in the HNT-SSC device are about two times more efficiently collected at the electrode than those in the QD-SSC. The spherical core@shell-QD structure cannot sufficiently suppress electron-hole recombination due to their three-dimensional confined carrier wavefunctions, whereas the quasi-type II band offset of the CuInS2/CdS heterostructure reduces the electron-hole overlap integral. The geometric characteristics of the CuInS2/CdS-HNTs, such as arm length and aspect ratio, also influence photovoltaic performance. For example, the aspect ratio of the arms can affect the photogenerated-carrier separation and transport abilities of the HNT; increasing the CdS-arm aspect ratio can enhance the separation of the photogenerated electrons and holes by increasing the delocalization of the electron wavefunction.36, 37 Hence, we expected that a CuInS2/CdS-HNT sensitizer with a high aspect ratio will collect charge carriers more efficiently in SSCs. Accordingly, the PCEs of CuInS2/CdS-HNTs with similar CdS-arm lengths increase as the aspect ratio is increased from 1.6 to 2.9, even though both CuInS2/CdS-HNT photoanodes have similar incident-light absorbing powers (Figs. S5 and S6). Our proof-of-concept study demonstrated that the elongated CdS arms of the CuInS2/CdS HNT structures are advantageous for separating photoexcited electron-hole pairs that facilitate electron transfer from the HNTs to


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TiO2. Even though our CuInS2/CdS-HNT SSC exhibited a low energy-conversion efficiency compared to other reports,11-14 the PCE of CuInS2/CdS-HNT solar cells should be further improved through optimization of the CdS-arm length and the mp-TiO2 pore size to enhance the QD-coverage density on the TiO2 particles.38 In summary, we describe a seeded-growth procedure for the synthesis of monodispersed tetrapod-shaped CuInS2/CdS heterostructures. The branched structures of the CuInS2/CdS HNTs facilitate enhanced absorption in the visible region of the spectrum and brighter PL than bare CuInS2 QDs by covering traps on the core surface. The HNTs show potential for applications to optoelectronic devices; we demonstrated their use in a QD-SSC consisting of a TiO2/CuInS2CdS-tetrapod/ZnS structure, a polysulfide electrolyte, and a CuS counter electrode. Consequently, an enhanced photocurrent is generated from the CuInS2-CdS-tetrapod structure compared to a QD-SSC using isotropic CuInS2@CdS QDs as sensitizers. We believe that the high photoenergy-conversion ability of the CuInS2/CdS HNTs is a development with enormous potential for cost-effective and highly efficient solar cells.

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Procedures for the synthesis of the ZB-CuInS2 QDs, CuInS2/CdS HNTs, and photovoltaic device fabrication. EDS point data for a CuInS2/CdS HNT (Figure S1). Additional TEM images of CuInS2 and CuInS2/CdS QDs (Figure S2). Fitted PL spectra of CuInS2 QDs and CuInS2/CdS HNTs, (Figure S3). TEM images and UV-vis absorption spectra of CuInS2 and CuInS2/CdS


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HNT sensitizer samples (Figures S4 and S5). J-V curves and photovoltaic parameters (Figures S5 and S6). EQE spectrum of CuInS2/CdS HNT-SSC (Figure S7).

AUTHOR INFORMATION Corresponding Author *Jiwon Bang. Tel: +82-55-792-2677, Fax: 82-55-792-2492, E-mail: [email protected] Author Contributions B. K., S. K., and J. B. initiated and designed the research project. B. K., K. K. and Y. K. synthesized the nanocrystals, B. K., K. K., W. H. S., and W. L. fabricated devices and analyzed data. B. K., S. K., K. K., and J. B. co-wrote the manuscript, and all authors provided valuable feedback. $These authors contributed equally: Bomi Kim and Kangwook Kim. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was supported by the Basic Science Research Program and the Pioneer Research Center Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2016R1E1A1A01941427, NRF2015M3C1A3056411, and NRF-2016R1C1B1007099). This research was also partially supported by the Pohang Iron and Steel Company (POSCO).

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BRIEFS

We report a seeded-growth method for the synthesis of tetrapod-shaped CuInS2/CdS heterostructures for enhanced photocurrents in sensitized solar cells.

Graphical Abstract


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CdS-Heterostructured ... - ACS Publications

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