Subscriber access provided by Eastern Michigan University | Bruce T. Halle Library
Article
Plasmonic Photovoltaic Cells with Dual-Functional Gold, Silver, and Copper Halfshell Arrays Ling Wu, Gyu Min Kim, Hiroyasu Nishi, and Tetsu Tatsuma Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02072 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 19, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 21
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
Plasmonic Photovoltaic Cells with Dual-Functional Gold, Silver, and Copper Halfshell Arrays Ling Wu, Gyu Min Kim, Hiroyasu Nishi, and Tetsu Tatsuma* Institute of Industrial Science, University of Tokyo, Meguro-ku, Tokyo 153-8505, Japan. Corresponding Author *E-mail:
[email protected].
ABSTRACT: Solid-state photovoltaic cells based on plasmon-induced charge separation (PICS) have attracted growing attention during the last decade. However, the power conversion efficiency (PCE) of the previously reported devices, which are generally loaded with dispersed metal nanoparticles as light absorbers, has not been sufficiently high. Here we report simpler plasmonic photovoltaic cells with interconnected Au, Ag, and Cu halfshell arrays deposited on SiO2@TiO2 colloidal crystals, which serve both as a plasmonic light absorber and as a current collector. The well-controlled and easily prepared plasmonic structure allows precise comparison of the PICS efficiency between different plasmonic metal species. The cell with the Ag halfshell array shows higher photovoltaic performances than the cells with Au and Cu halfshell arrays, because of high population of photogenerated energetic electrons, that gives high electron
ACS Paragon Plus Environment
1
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 21
injection efficiency and suppressed charge recombination probability, achieving the highest PCE among the solid-state PICS devices even without a hole transport layer.
INTRODUCTION When light is incident on a metallic nanostructure at a specific wavelength, it causes coherent oscillation of free electrons at the metal surface. This phenomenon is known as localized surface plasmon resonance (LSPR), by which photons are converted to surface plasmons. In a limited lifetime, these plasmons quickly decay via some different pathways. One of them is plasmoninduced charge separation (PICS),1-4 which involves energetically uphill charge transfer from the plasmonic metal nanostructure to a semiconductor (typically TiO2).1-4 The charge transfer is explained in terms of external photoelectric effect (hot electron injection) (Figure 1a,b)2,5,6 or photoinduced interfacial electron transition.2,7,8 The edge wavelength of PICS can be controled simply by tuning the energy gap between the semiconductor conduction band and the Fermi level of the metal, without band engineering.2 PICS opened up a new way for conversion of light energy, and thus it has enabled a vast array of applications2,5,6 including photocatalysis,1,9,10 photovoltaics,1,11-18 and photochromic materials,19-21 as well as other novel photofunctional materials unique to PICS.2 Among those applications, photovoltaics are important because it can be used not only for energy conversion but also for photodetection, chemical sensing, and signal transduction in microscale photonic and plasmonic circuits. Although the first PICS photovoltaic device was a wet-type cell with a liquid electrolyte,1 which could suffer from leakage, interest shifted soon to solid-state devices.11-18 An E/nSC/PL/pSC/E type cell (E = electrode, nSC = n-type semiconductor, PL = plasmonic material, and pSC = p-type semiconductor or hole transport
ACS Paragon Plus Environment
2
Page 3 of 21
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
layer) was developed first,11 but its efficiency was very low; the incident photon-to-current conversion efficiency (IPCE) was 410 nm) 2016
FTO/TiO2/PL/pSC/Au
AuNP
2.1
─
18
2017
FTO/TiO2/PL
AuNHS
0.13
0.0043 (Xe, >440 nm)
This work
AgNHS
2.92
0.112 (Xe, >440 nm)
CuNHS
0.16
0.0032 (Xe, >440 nm)
Abbreviations: ITO = indium tin oxide; FTO = fluorine-doped tin oxide; NP = nanoparticle; NR = nanorod; NHS = nanohalfshell.
Here we propose a much simpler E/nSC/PL type cell structure and report the highest PCE value of 0.112% even under >440 nm light, which does not excite TiO2. The simple structure was achieved by using a two-dimensional (2D) metal halfshell array deposited on a SiO2@TiO2 core-shell colloidal crystal (Figure 1c).22,23 This periodic and continuous nanostructured film serves both as a plasmonic light absorber and as a current collector (i.e., electrode). Although plasmonic nanostructured films are often prepared by a top-down lithographic method,24 the
ACS Paragon Plus Environment
4
Page 5 of 21
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
halfshell array can be prepared by an inexpensive, bottom-up, self-assembly technique.22,23,25 The continuous halfshell array supports both localized and propagating surface plasmons,26-28 leading to broad absorption and higher efficiency under broad-band light irradiation. The SiO2@TiO2 colloidal crystal is not only a template for the halfshells but also a lateral waveguide of light,23 which assists plasmonic light harvesting. Another advantage of the continuous halfshell arrays is their stability due to their low surface area to volume ratio, in comparison with those of nanoparticles. PICS causes electron deficiency in a plasmonic metal nanostructure as a result of electron transfer to a semiconductor, so that illuminated Ag and Cu nanoparticles on TiO2 are oxidized to ions in a solution or humid air.19,29 The surface of Au nanoparticles can also be oxidized to Au(OH)3. In order to avoid the oxidation, nanoparticles must be confined in TiO23,30 or a solid cell.11-18 Even though the present halfshell arrays are exposed to atmosphere, the continuous film structure keeps not only Au, but also less noble but cost effective Ag and Cu halfshell arrays stable. In addition, since the halfshells are prepared simply by evaporation of a metal onto the SiO2@TiO2 colloidal crystal, the morphology and the interface structure are almost the same even if different metals are used. Therefore, accurate comparison between different metals is possible in terms of PICS performances.
EXPERIMENTAL Fabrication of the Cells. A FTO-coated glass was patterned utilizing a laser marker and sonicated in Milli-Q water, acetone, and 2-propanol successively. The patterned FTO was coated with a 70-nm-thick compact TiO2 layer by a spray-pyrolysis method (0.12 MPa, 1 s, 500 °C) using a mixture of titanium diisopropoxide bis(acetylacetonate) and 2-propanol (volume ratio = 2:9).22-23 The TiO2 surface was hydrophilized by UV-irradiation for 30 min.
ACS Paragon Plus Environment
5
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 21
The size of SiO2 core has been optimized to be ~370 nm in our previous work23 in terms of the PICS efficiency. Thus, SiO2 spheres of ~370 nm diameter were synthesized by the Stöber method.31 Typically, NH3⋅H2O (28%, 3 g), Milli-Q water (5.710 g), and ethanol (99.5 vol%, 16.226 g) were mixed and stirred for 10 min at room temperature. Tetraethyl orthosilicate (1.676 mL) was then added to the solution, keeping stirring the mixture for 3 h. The resultant SiO2 spheres were collected by centrifugation at 8000 rpm and redispersed in ethanol, followed by centrifugation and redispersion in 1-butanol. The SiO2 suspension was slowly dropped into water in a Petri dish, resulting in the formation of a self-assembled monolayer of the SiO2 spheres at the air-water interface.23,25 The SiO2 colloidal crystal thus obtained was transferred to the hydrophilic TiO2/FTO substrate and coated with TiO2 (~50 nm thick) by the spray-pyrolysis method. The photovoltaic cell with metal (Au, Ag, or Cu) halfshell arrays was obtained by evaporating a 155-nm-thick metal film (0.3 nm s−1,
