Electrocatalytic Zinc Composites as the Efficient Counter Electrodes of

Subscriber access provided by UOW Library

Article

Electro-catalytic Zinc Composites as the Efficient Counter Electrodes of Dye-Sensitized Solar Cells: A Study on the Electrochemical Performances and Density Functional Theory Calculations Chun-Ting Li, Hung-Yu Chang, Yu-Yan Li, Yi-June Huang, Yu-Lin Tsai, R. Vittal, Yu-Jane Sheng, and Kuo-Chuan Ho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07724 • Publication Date (Web): 24 Nov 2015 Downloaded from http://pubs.acs.org on December 15, 2015

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.

ACS Applied Materials & Interfaces 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 37

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

ACS Applied Materials & Interfaces

Electro-catalytic Zinc Composites as the Efficient Counter Electrodes of Dye-Sensitized Solar Cells: A Study on the Electrochemical Performances and Density Functional Theory Calculations Chun-Ting Li1, Hung-Yu Chang1, Yu-Yan Li1, Yi-June Huang1, Yu-Lin Tsai1, R. Vittal1, Yu-Jane Sheng1,* and Kuo-Chuan Ho1,2,* 1

Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan

2

Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan

ABSTRACT: Highly efficient zinc compounds (Zn3N2, ZnO, ZnS, and ZnSe) have been investigated as low-cost electro-catalysts for the counter electrodes (CE) of dye-sensitized solar cells (DSSCs). Among them, Zn3N2 and ZnSe are introduced for the first time in DSSCs. The zinc compounds

were

separately

mixed

with

a

conducting

binder,

poly(3,4-ethylene-

dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and thereby four composite films of Zn3N2/PEDOT:PSS, ZnO/PEDOT:PSS, ZnS/PEDOT:PSS, and ZnSe/PEDOT:PSS were coated on the tin–doped indium oxide (ITO) substrates through a simple drop-coating process. In the composite film, nanoparticles of the zinc compound form active sites for the electro-catalytic reduction of triiodide ions, and PEDOT:PSS provides a continuous conductive matrix for fast

ACS Paragon Plus Environment

1


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 37

electron transfer. By varying the weight percentage (5~20 wt%) of a zinc compound with respect to the weight of the PEDOT:PSS, the optimized concentration of a zinc compound was found to be 10 wt% in all the four cases, based on the photovoltaic performances of the corresponding DSSCs. At this concentration (10 wt%), the composites films with Zn3N2 (Zn3N2-10), ZnO (ZnO10), ZnS (ZnS-10), and ZnSe (ZnSe-10) rendered, for their DSSCs, power conversion efficiencies (η) of 8.73%, 7.54%, 7.40%, and 8.13%, respectively. The difference in the power conversion efficiency is explained based on the electro-catalytic abilities of those composite films by cyclic voltammetry (CV), Tafel polarization plots, and electrochemical impedance spectroscopy (EIS) techniques. Energy band gaps of the zinc compounds, obtained by density functional theory (DFT) calculations, were used to explain the electro-catalytic behaviors of the compounds. Among all the zinc-based composites, the one with Zn3N2-10 showed the best electro-catalytic ability and thereby rendered for its DSSC the highest η of 8.73%, which is even higher than that of the cell with the traditional Pt CE (8.50%). Therefore, Zn3N2 can be considered as a promising cheap electrocatalyst to replace the rare and expensive Pt.

KEYWORDS: Counter electrode, Density functional theory, Dye–sensitized solar cell, Electrocatalyst, Zinc nitride, Zinc oxide, Zinc sulfide, Zinc selenide


2

Page 3 of 37

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




INTRODUCTION Recently, transition metal compounds and their composites have caught a lot of attention1-4,

owing to their attractive advantages, e.g., low–cost, high conductivity, and good stability. Transition metal compounds of the following metals, titanium (Ti)5-7, vanadium (V)8-9, chromium (Cr)1, zirconium (Zr)10, niobium (Nb)11-13, molybdenum (Mo)14-17, hafnium (Hf)18, tantalum (Ta)1920

, tungsten (W)14-16, 21-22, iron (Fe)23-26, cobalt (Co)27-29, nickel (Ni)17, 28, 30, tin (Sn)31-33, copper

(Cu)34-35, bismuth (Bi)36 have been successfully used as the electro-catalytic materials for the counter electrodes (CEs) of dye–sensitized solar cells (DSSCs). Generally, the above-mentioned transition metal compounds exhibit good electro-catalytic abilities to facilitate the reduction of oxidized specie of a redox couple (e.g., I3– of I–/I3– and Co3+ of Co2+/Co3+) in the electrolyte of a DSSC37-38. For this reason, transition metal compounds have a great potential to replace the traditional noble metal, platinum (Pt)2,

39-40

. However, not many of these transition metal

compounds render high power conversion efficiencies (η) to their DSSCs. Most of them enable η’s of only 7~8% for their cells. These transition metal compounds are often synthesized via a high-cost process, e.g., high vacuum or high temperature. Some of the transition metal compounds are even rare on earth and are toxic to environment. Therefore, highly efficient, earth abundant, and non-toxic transition metal compounds are needed for the future development of DSSCs. Zinc-based binary compounds are low-cost, earth abundant, efficient, and eco-friendly opticalelectrical materials. Despite this, they were seldom applied as the electro-catalytic materials for the CEs of DSSCs. Yi et al.35 synthesized heterostructured nanorods of a composite of wurtzite copper indium disulfide and zinc sulfide (CuInS2/ZnS); two types of nanorods, i.e., burning torchlike nanorods and longer nanorods, were obtained by varying the concentration of indium precursor. The DSSCs with burning torch-like nanorods and longer nanorods of CuInS2/ZnS film


3


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 4 of 37

as the electro-catalytic CEs exhibited η’s of 7.2% and 7.5%, respectively, while the DSSC with a flame-like CuInS2 film as the electro-catalytic CE showed an η of only 6.5%. With the presence of ZnS in the composite CuInS2/ZnS CE, an obvious increase of about 1 ~ 2 mA cm-2 in the short– circuit current density (Jsc) for the cell was observed. Although the η of the cell with pure ZnS as the CE was not mentioned in this literature35, zinc sulfide can be still considered as an important component of the CuInS2/ZnS electro-catalyst to improve the Jsc of the DSSCs. Wang et al.41 prepared a CE with a composite film containing pure zinc oxide (ZnO) nanoparticles and poly(3,4ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and the pertinent DSSC showed an η of 8.17% with a high open–circuit voltage (Voc) of 0.86 mV, a high Jsc of 19.7 mA cm-2, and a poor fill factor (FF) of 0.48. The ZnO nanoparticles were reported to have a strong adsorption of I3– at the Zn site via the formation of a Zn–I bond, while a relatively weaker adsorption of I3– was found on PEDOT:PSS via the formation of two I–H bonds. Therefore, ZnO would have higher electro-catalytic activity than that of PEDOT:PSS for the reduction of I3–41. Xu et al.42 synthesized a composite film of ZnO/mesoporous carbon (ZnO/MC) using a homogeneous MC paste containing the ZnO sol, and the obtained film was annealed in ambient atmosphere. The annealing temperature was reported to be a key factor. When annealed at 300 oC, the in-situ formed and dispersed ZnO nanoparticles (NPs) functioned as bridges to bind the MC particles so as to strengthen the adhesion between the carbon ﬁlm and the substrate, providing sufficient electron transfer pathways. The cell with ZnO/MC reached an η of 6.37%. Shit et al.43 synthesized a nanocomposite film of ZnS nanoparticles/polyaniline nanotube (ZnS NP/PANI NT). With a low concentration of ZnS NPs, the doped state PANI and the conductivity of a composite film increased with an increase in ZnS concentration. The best composite film gave a cell having an η of 3.38%. An extensive literature search reveals that there were only few studies to report zinc-based binary


4

Page 5 of 37

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


compounds (i.e., either ZnO or ZnS) as the CEs in DSSCs35, 41-43; thus, the information about the η’s of the pertinent DSSCs were very limited. In this study, we systematically investigate four efficient and earth abundant zinc-based composite films, namely, Zn3N2/PEDOT:PSS, ZnO/PEDOT:PSS, ZnS/PEDOT:PSS, and ZnSe/PEDOT:PSS, as the electro-catalytic CEs in DSSCs. Among these zinc compounds, Zn3N2 and ZnSe have been used in DSSCs for the first time. Here, the zinc-based nanoparticles form active sites to catalyze the reduction of I3– into I– (Scheme 1), while the PEDOT:PSS plays the role of a continuous conductive matrix for fast electron transfer. PEDOT:PSS is a well-known water-soluble conducting polymer, which has been reported as an electro-catalyst in the CE of DSSCs41, 44-47. When the pure PEDOT:PSS is solely used as a CE, the pertinent DSSCs usually gave poor cell efficiencies averagely < 4%, which is much less than the efficiencies of the Pt-based DSSCs. When the PEDOT:PSS is combined with other electro-catalysts to form composite CEs, the PEDOT:PSS is usually found to have a negligible electro-catalytic ability toward the I3– reduction41, 44-47. Thus, the PEDOT:PSS is generally considered as a good conductive binder, instead of a good electro-catalyst, in the CEs. Although the pure PEDOT has been used as a highly effective electro-catalyst for the CEs and rendered excellent cell performances2, 48, the existence of the non-catalytic PSS part in the PEDOT:PSS may lead to a poor electro-catalytic ability of PEDOT:PSS. The highly efficient pure PEDOT film is not used in our study, since it is normally involved an electro-polymerization process, which includes the toxic organic solvents, causes a lots of wastes in raw materials, and is hard to be realized in a large-scale production. To propose a simple, low-cost, easy-to-scale-up, and eco-friendly fabrication process, the water-soluble PEDOT:PSS is intended to work as a conductive binder so as to enhance the adhesion between a composite film and its underlying tin–doped indium oxide (ITO) substrate, and to improve the


5


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 37

linkage between the added nanoparticles (i.e., Zn3N2, ZnO, ZnS, and ZnSe), as well as to prevent the aggregation of the added nanoparticles. Among all the zinc-based composite films, the film with Zn3N2 shows the best electro-catalytic ability and gives for its DSSC the highest η of 8.73%, which is even higher than that of the cell with a standard platinum (Pt) CE (8.50%). This work demonstrates that the efficient, earth abundant, and low-cost Zn3N2 is an alternative electrocatalyst to the expensive and rare Pt in a DSSC, and pertinent film can be prepared by a simple, low-cost, low temperature, and non-vacuum drop-coating process, which is suitable for large-scale production.



EXPERIMENTAL DETAILS Preparation of the electro-catalytic films and the DSSCs. The detailed information of all the

materials and chemicals used in this study were described in the Supporting Information. Six types of electro-catalytic films were all coated on the cleaned ITO substrates as follows. (1) A standard Pt film (30 nm–thick) was obtained using a DC sputtering technique. (2) A PEDOT:PSS film (3 μm–thick) was fabricated by a simple drop–coating technique using 50 μL PEDOT:PSS mixture (ethanol/PEDOT:PSS/DMSO = 20:19:1 by volume). (3) Four Zn3N2/PEDOT:PSS composite films (Zn3N2–5, Zn3N2–10, Zn3N2–15, and Zn3N2–20) were obtained on ITO substrates by drop-coating the slurries of Zn3N2/PEDOT:PSS (each 50 μL), in which the slurries, respectively, contained 5, 10, 15, and 20 weight percent (wt%) of Zn3N2 with respect to the weight of PEDOT:PSS; a bare Zn3N2 film was also prepared using pure Zn3N2 slurry (50 μL) containing 10 wt% of Zn3N2 in ethanol. (4) Similarly, four ZnO/PEDOT:PSS composite films (ZnO–5, ZnO–10, ZnO–15, and ZnO–20) and a bare ZnO film were obtained on ITO substrates as in (3). (5) Four


6

Page 7 of 37

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


ZnS/PEDOT:PSS composite films (ZnS–5, ZnS–10, ZnS–15, and ZnS–20) and a bare ZnS film were obtained on ITO substrates as in (3). (6) Four ZnSe/PEDOT:PSS composite films (ZnSe–5, ZnSe–10, ZnSe–15, and ZnSe–20) and a bare ZnSe film were obtained on ITO substrates as in (3). To fabricate a DSSC, a standard TiO2 photoanode (15 μm–thick) adsorbed with N719 dye was prepared in accordance with our previous report49. The fabrication details of the standard TiO2 photoanode were given in the Supporting Information. The standard N719–adsorbed TiO2 photoanode was coupled to a CE containing one of the above-obtained electro-catalytic films; a 60 μm–thick Surlyn® was used as a spacer to fix the distance between a photoanode and a CE. Then an electrolyte, containing 1.2 M 1,2–dimethyl–3–propylimidazolium iodide (DMPII), 0.035 M I2, 0.1 M guanidinium thiocyanate (GuSCN), and 0.5 M 4–tert–butylpyridine (tBP) in a mixed solvent (3–methoxypropionitrile/acetonitrile (MPN/ACN) = 2/8 by volume), was injected into the cell gap by capillarity. Characterization of the electro-catalytic films and the DSSCs. For an electro-catalytic film, the surface morphology was observed by a field–emission scanning electron microscope (FE–SEM, Nova NanoSEM 230, FEI, Oregon, USA), the sheet resistance was recorded by a Keithley's instrument (Keithley 2400, Keithley Instruments Inc., USA), and the electro-catalytic ability was investigated by several electrochemical techniques, i.e., cyclic voltammetry (CV), Tafel polarization plots, and electrochemical impedance spectroscopy (EIS); the corresponding data were recorded by a potentiostat/galvanostat (PGSTAT 30, Autolab, Eco–Chemie, Utrecht, the Netherlands), equipped with an FRA2 module (for EIS). For the CV analysis, an electro-catalytic film (1 cm2), a Pt foil, and an Ag/Ag+ electrode were used as the working, counter, and reference electrodes, respectively; the electrolyte contained 10 mM LiI, 1.0 mM I2, and 0.1 M LiClO4 in ACN. For Tafel polarization plots and EIS analyses, a symmetric cell was used (containing the


7


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 8 of 37

same electro-catalytic film on both the electrodes); the electrolyte for Tafel polarization plots and EIS analyses contained 1.2 M DMPII, 0.035 M I2, 0.1 M GuSCN, and 0.5 M tBP in a mixed solvent (MPN/ACN = 2:8 by volume). The impedance spectra were obtained in the frequency range of 10 mHz to 65 kHz, using an AC amplitude of ±10 mV. The sheet resistances (Rsh), relating the electrical conductivities of the films, were measured by a four-point probe technique and were recorded by a Keithley's instrument (Keithley 2400, Keithley Instruments Inc., USA). The photovoltaic parameters and the incident photon–to–current conversion efficiency (IPCE) spectra of the DSSCs were recorded by the same potentiostat/galvanostat mentioned above. By using a class A quality solar simulator (XES–301S, AM1.5G, San–EI Electric Co., Ltd., Osaka, Japan), the photovoltaic parameters were measured under AM 1.5G with an incident light intensity of 100 mW cm–2, which was calibrated with a standard Si cell (PECSI01, Peccell Technologies, Inc., Kanagawa, Japan). By using another class-A solar simulator (PEC–L11, AM1.5G, Peccell Technologies, Inc., Kanagawa, Japan) equipped with a monochromator (model 74100, Oriel Instrument, California, USA), the IPCE spectra were obtained under a monochromatic light illumination in the wavelength region of 400 to 800 nm; the incident radiation flux (φ) was obtained by using an optical detector (model 818-SL, Newport, California, USA) and a power meter (model 1916-R, Newport, California, USA). Model and Computational method. In this study, CASTEP (Cambridge Sequential Total Energy Package)50-51 which employs the density functional theory was used to provide the molecular structures and electronic properties of various electro-catalysts, including Pt20, Zn3N25253

, ZnO54-55, ZnS56, and ZnSe57-58. The pseudopotential plane-wave method is used and the

exchange–correlation function adopts the Perdew-Burke-Ernzerhof (PBE) functional in the generalized gradient approximation (GGA)59-60. A plane-wave energy cutoff is set to be 380 eV,


8

Page 9 of 37

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


and the Monkhorst–Pack mesh with 4×4×5 k points is applied for the Brillouin zone integration. In regard to platinum, the face-centered cubic (FCC) structure belongs to the Fm-3m (225) space group, and has a lattice parameter of a = 3.97 Å (according to Joint Committee on Powder Diffraction Standards, JCPDS card No. 88-2343). As regards to zinc compounds, the cubic Zn3N2 belongs to the Ia-3 (206) space group with a lattice parameter of a = 9.77 Å (JCPDS card No. 88618); wurzite (hexagonal) ZnO and ZnS belong to the P63mc (186) space group with the lattice parameters of a = 3.81 Å and c = 6.26 Å ; zinc blende (cubic) ZnSe belongs to the F-43m (216) space group with a lattice parameter of a = 5.67 Å . Before calculating the density of states (DOS) of various electro-catalysts (Pt, Zn3N2, ZnO, ZnS, and ZnSe), self-consistent calculations were performed until the convergence criterions were fulfilled (the total energy was less than 5 x 10–6 eV atom–1, the atom internal stress was below 0.05 GPa, and the force on each atom was below 0.03 eV Å –1). The molecular structures of Zn3N2, ZnO, ZnS, and ZnSe obtained by DFT calculations are depicted in Scheme 1.



RESULTS AND DISCUSSION Surface morphology. The FE–SEM images in Figure 1 show the surface morphologies of

different films, including Pt, PEDOT:PSS, Zn3N2-10, ZnO-10, ZnS-10, and ZnSe-10. In Figure 1a, the standard Pt film shows a very flat, continuous, and uniform surface, and is composed of plenty of Pt nanoparticles. In Figure 1b, the PEDOT:PSS film shows a very smooth and compact surface, which indicates an unfavorable surface for electrochemical reaction and for electrolyte penetration; at a higher resolution, the same smooth morphology of PEDOT:PSS can be observed (the inset of Figure 1b). In Figures 1c–f, all zinc-based composite films show a three-dimensional


9


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 10 of 37

porous structure with high roughness, indicating sufficient electro-catalytic active cites for the reduction of I3- ions. The cross-sectional FE–SEM images of various films, including Pt, PEDOT:PSS, Zn3N2-10, ZnO-10, ZnS-10, and ZnSe-10, as shown in Figure S1 in the Supporting Information, are used to estimate the film thicknesses. It can be seen from Figure S1 that the standard Pt film is about 30 nm–thick, the bare PEDOT:PSS film is about 3 μm–thick, and all zincbased composite films are about 30 μm–thick. Besides, the FE–SEM images of bare Zn3N2, bare ZnO, bare ZnS, and bare ZnSe are shown in Figure S2. Each of these films is composed of the irregular zinc-based nanoparticles, which show a severe aggregation and non-uniform distribution on the ITO substrate. The aggregated zinc-based nanoparticles rendered their films very easy to crack or detach from the ITO; this observation indicates that bare Zn3N2, bare ZnO, bare ZnS, and bare ZnSe all have poor morphologies, thus are not suitable for electrochemical applications. Owing to PEDOT:PSS as the conducting binder, all zinc-based composite films show a good adhesion to the ITO substrate, and also show good linkage among the nanoparticles, i.e., Zn3N2, ZnO, ZnS, and ZnSe. Photovoltaic performance. The photocurrent density–voltage (J–V) curves of the DSSCs with the CEs of the Zn3N2-based films (bare Zn3N2, Zn3N2-5, Zn3N2-10, Zn3N2-15, and Zn3N2-20), ZnO-based films (bare ZnO, ZnO-5, ZnO-10, ZnO-15, and ZnO-20), ZnS-based films (bare ZnS, ZnS-5, ZnS-10, ZnS-15, and ZnS-20), and ZnSe-based films (bare ZnSe, ZnSe-5, ZnSe-10, ZnSe15, and ZnSe-20) are shown in Figure S3a, S3b, S3c, and S3d, respectively. The photovoltaic parameters are summarized in Table S1. In Figure S3a, the DSSCs with the CEs of bare Zn3N2, bare Zn3N2, Zn3N2-5, Zn3N2-10, Zn3N2-15, and Zn3N2-20 films show the η’s of 2.48%, 6.12%, 8.73%, 7.43%, and 6.99%, respectively. Among the cells with Zn3N2-based CEs, the cell with bare Zn3N2 shows the least η due to its extremely low FF (0.32). This extremely low FF should be due


10

Page 11 of 37

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


to lack of good conductivity in a bare Zn3N2 film, which could happen owing to two reasons: (1) Zn3N2 is a semiconductor61 and (2) Zn3N2 nanoparticles have weak adhesion to the substrate. Compared to the cell with bare Zn3N2, all the cells with Zn3N2-based composites show much higher values of Jsc, FF, and thereby η values, apparently because the inclusion of PEDOT:PSS in the composite film provides important advantages, such as fast and continuous conducting matrix, a tight adhesion of the composite film to the ITO substrate, and a good connection among the Zn3N2 particles. With the increase of Zn3N2 content in the composite film, the increased number of electro-catalytic sites for I3– reduction causes the enhancements in the values of Jsc and FF, and thereby increases the η; however, with further increase in Zn3N2 content, the adhesion of the composite film to the substrate suffers, and thereby affects the photovoltaic parameters of the cell. Thus, among all the cells with Zn3N2-based composites, the cell with Zn3N2-10 shows the best η of 8.73%, with a Voc of 0.81 V, Jsc of 15.77 mA cm–2, and a FF of 0.69. The DSSCs with other zinc compounds exhibit similar behavior with regard to photovoltaic parameters, as can be seen in Table S1 and Figure S3. It can be noticed in Table S1 that the cells with ZnO-10, ZnS-10, and ZnSe-10 show the η’s of 7.54%, 7.40%, and 8.13%, respectively. Here, it is observed in Table S1 that the Voc, Jsc and FF values of the cells with different Zn-based CEs show the same tendency of Zn3N2-10 > ZnSe-10 > ZnO-10 > ZnS-10, and thereby result the same tendency for the values of η’s. The differences in the values of Jsc of these cells are mainly determined by the electro-catalytic abilities of their counter electrode films, while the differences in the values of FF are mainly determined by the conductivities of their counter electrode films. On the other hand, the variations in Voc values of these cells (Zn3N2-10 > ZnSe-10 > ZnO-10 > ZnS-10) are due to the variations in the electro-catalytic activities of their counter electrodes; this may be specifically explained as follows. If a CE has a very high electro-catalytic activity for triiodide ions (I3–), it fast consumes


11


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 12 of 37

the I3– at the CE/electrolyte interface. Thus, the I3– ions will not trap or recombine with the photoinduced electrons at the photoanode/electrolyte interface; the recombination reaction therefore decreases and the pertinent Voc increases. In the same way, if a CE has a very weak electro-catalytic activity for I3– ions, the recombination reaction increases and the pertinent Voc decreases. Figure 2a shows the J-V curves of the DSSCs with the CEs of Pt., PEDOT;PSS, Zn3N2-10, ZnO-10, ZnS-10, and ZnSe-10; Table 1 gives the corresponding photovoltaic parameters. The DSSC with Pt CE provides an η of 8.59%, with a Voc of 0.80 V, Jsc of 15.19 mA cm–2, and a FF of 0.71, while the cell with PEDOT:PSS CE shows an η of 2.99%, with a Voc of 0.69 V, Jsc of 12.79 mA cm–2, and a FF of 0.33. From which, the poor values of Jsc and FF of this cell is owing to the fact that PEDOT:PSS lacks sufficient electro-catalytic active sites, while the low Voc for this cell is due to the weak electro-catalytic activity of PEDOT:PSS. Compared to the cell with PEDOT:PSS, all the cells with zinc-based composites show higher values of Jsc, FF, Voc, and thereby η. This is because of the fact that a zinc-based compound provides better electro-active activity and larger surface area for its composite film for I3– reduction, and thereby enables a higher electro-catalytic activity for its CE; this higher electro-catalytic activity enables higher photovoltaic parameters for the pertinent DSSC. It can be said that the zinc-based NPs mainly function as the electro-catalysts, while the PEDOT:PSS works mainly as the conductor. Among all the DSSCs with the zinc-based composite CEs, the cell with Zn3N2-10 exhibits the highest η of 8.73%, which is even higher than that of the cell with Pt CE (8.59%). It is mainly due to that the cell with Zn3N2-10 provides a larger Jsc than that of the cell with Pt CE. The larger surface area (see Figure 1) and the higher electrocatalytic activity of Zn3N2-10 film than those of Pt film are the reasons for this higher Jsc; the higher electro-catalytic activity of Zn3N2-10 film will be proved through cyclic voltammetry analysis in further discussions (see Sec. 2.4).


12

Page 13 of 37

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


Incident photon–to–current conversion efficiency (IPCE) spectra of the DSSCs with various electro-catalytic films as the CEs were obtained at short-circuit condition from 400-800 nm (Figure 2b). A comparative glance at Figure 2b and Table 1 clearly shows that the IPCE values are in consistency with the Jsc values, i.e., the higher the IPCE value, the higher the Jsc value. Except for the cell with PEDOT:PSS, other DSSCs (including the cells with Pt, Zn3N2-10, ZnO10, ZnS-10, and ZnSe-10) show good IPCE values around 75~90% in the wavelength region of 400 to 600 nm. These good IPCE values indicate that these films have reduced the triiodide ions rapidly to produce iodide ions, the iodide ions have rapidly regenerated the oxidized dye, and the oxidized dye has injected light-induced electrons rapidly into the conduction bands of these films. With the same rationale, the low IPCE value of PEDOT:PSS-based DSSC can be explained, i.e., its low IPCE value is because of low electro-catalytic ability of its PEDOT:PSS film. Besides, each IPCE curve is further integrated to get the corresponding short-circuit current density (Jsc-IPCE) value. As summarized in Table 1, the Jsc-IPCE values agree well with the Jsc values obtained from J–V curves. In brief, the Zn3N2-10 electro-catalytic film shows a great potential to replace the expensive Pt. Cyclic voltammetry. All the zinc-based composite films in Figure 1 indicate larger surface areas than that of the PEDOT:PSS film. This is vindicated in Figure 3 by their larger electrocatalytic abilities. Figure 3 shows cyclic voltammograms of various electro-catalytic films, i.e., Pt, PEDOT:PSS, Zn3N2-10, ZnO-10, ZnS-10, and ZnSe-10. Cyclic voltammetry (CV) was applied to investigate the redox kinetics of iodide/triiodide (I–/I3–) at the surfaces of different electrocatalytic films. Since I3– ions are strongly electron deficient, their just-in-time consumption is very important for the prevention of recombination reactions. The electro-catalytic film of a CE facilitates the reduction of I3- at its surface with the electrolyte, as shown in Eqn. (1)40.


13


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

I3- +2e - →3I -

Page 14 of 37

(1)

In Figure S4 of the “Supporting Information”, the CV curves of Figure 3 are shown separately, along with their background curves. For each CV curve in Figure S4, the net peak current density from the cathodic current peak to the background curve is defined as the cathodic peak current density (Jpc), while the potential difference between the anodic and cathodic current peaks is defined as the peak potential separation (ΔE)28, 40, 62. The values of Jpc and ΔE for various films, including Pt, PEDOT:PSS, Zn3N2-10, ZnO-10, ZnS-10, and ZnSe-10, are listed in Table 2. The ΔE of a film is inversely proportional to the intrinsic kinetic redox capability for I–/I3–; the smaller ΔE of a film, the lower overpotential is required to trigger I–/I3– redox reaction, the better is its intrinsic kinetic redox capability28, 62. The ΔE values of Pt, PEDOT:PSS, Zn3N2-10, ZnO-10, ZnS10, and ZnSe-10 were evaluated to be 0.40, 0.64, 0.41, 0.43, 0.52, and 0.42 V, respectively. All zinc-based composite films show smaller ΔE values than that of the PEDOT:PSS film; it can be deduced that that an improved intrinsic kinetic redox capability is provided via incorporating a zinc-based NP (Zn3N2, ZnO, ZnS, or ZnSe) into the PEDOT:PSS polymer matrix. The ΔE values of the zinc-based composite films show a tendency of Zn3N2-10 < ZnSe-10 < ZnO-10 < ZnS-10; this infers that the intrinsic kinetic redox capability of these zinc-based NPs follows an order of Zn3N2 < ZnSe < ZnO < ZnS. On the other hand, the Jpc of a film indicates its overall electro-catalytic ability for I3– reduction; the larger Jpc of a film, the larger is its overall electro-catalytic ability. The Jpc values of Pt, PEDOT:PSS, Zn3N2-10, ZnO-10, ZnS-10, and ZnSe-10 were estimated to be 1.39, 0.40, 1.43, 1.13, 0.87, and 1.30 mA cm-2, respectively; these values are separately obtained from their net peak current densities, as calculated from the difference between their cathodic current peaks and the pertinent background curves (Figure S4). Clearly, all zinc-based composite films show higher Jpc


14

Page 15 of 37

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


values than that of the PEDOT:PSS film; it indicates that the electro-catalytic abilities of these zinc-based composite films are essentially determined by their zinc-based NPs rather than by their polymer component. Those zinc-based NPs are therefore assumed to render to their composite films both large surface area and good electro-catalytic ability for I3– reduction (Scheme 1). The Jpc values of the zinc-based composite films show a tendency of Zn3N2-10 > ZnSe-10 > ZnO-10 > ZnS-10, which agrees well with the tendency of both Jsc and η values of the pertinent DSSCs (see Table 1). Among all the composite films, Zn3N2-10 film shows the highest Jpc value of 1.43 mA cm-2, which is even higher than that of Pt (1.39 mA cm-2). Needless to say that the Zn3N2-10 film has the highest electro-catalytic ability than any other film in this work. Tafel polarization curves. Tafel polarization analysis was further used to explore the electrocatalytic abilities of different films for I3– reduction in a practical electrolyte used in the DSSCs (with a high I–/I3– concentration). For an electro-catalytic film, a linear sweep voltammetry (LSV) curve was obtained, at a low scan rate of 50 mV s-1, by using a symmetric cell composed of the same film on both the electrodes (two-electrode cell). Generally, a Tafel curve can be divided into three zones: (1) the polarization zone (|V| < 120 mV), (2) the Tafel zone (120 mV< |V| < 400 mV), and (3) the diffusion zone (|V| > 400 mV).63 In the Tafel zone, the exchange current density (J0) of an electro-catalytic film can be obtained by extrapolating the anodic and cathodic curves and reading the cross point at 0 V. A higher value of J0 indicates a better electro-catalytic ability of the film. According to the Tafel polarization curves as shown in Figure 4, the J0 values for various electro-catalytic films are obtained and summarized in Table 2. The tendency of J0 values of the films shows a perfect consistency with that of their Jpc values; the tendency for these parameters for the films are as follows: Zn3N2-10 > Pt >ZnSe-10 > ZnO-10 > ZnS-10 > PEDOT:PSS. Thus,


15


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 16 of 37

the electro-catalytic abilities of the films are validated not only by CV analysis (with lower I–/I3– concentration) but also by Tafel curves (with higher I–/I3– concentration). Moreover, the J0 value of an electro-catalytic film can be used to calculate the charge transfer resistance (Rct-Tafel) corresponding to the film/electrolyte interface, via Eqn. (2)40, 63.

J0 

RT nFRct -Tafel

(2)

where R is the ideal gas constant, T is the absolute temperature, F is Faraday constant, and n is the number of electrons transferred for I3– reduction. In general, a smaller Rct-Tafel value refers to a larger amount of electrons transferring through the electro-catalytic film/electrolyte (same as the CE/electrolyte) interface, and thereby implies a faster electron transfer capability of the film. In Table 2, all zinc-based composite films show much lower Rct-Tafel values, compared to that of the PEDOT:PSS film, because the pertinent zinc-based NPs render two key properties to their composite film: one is enhanced electro-catalytic nature and the other is enlarged surface area for I3– reduction. Consequently, much higher values of Jsc and FF are obtained for the DSSCs with zinc-based composite films. The best Zn3N2-10 film shows the highest J0 of 12.30 mA cm-2 and the lowest Rct-Tafel of 1.04 Ω cm2, indicating its outstanding electro-catalytic ability, which is even better than that of Pt (J0=7.31 mA cm-2 and Rct-Tafel=1.75 Ω cm2). Electrochemical impedance spectroscopy. Electrochemical impedance spectroscopy (EIS) was used for investigating the interfacial resistances, namely series resistance (Rs) and charge transfer resistance (Rct-EIS) at the electro-catalytic film/electrolyte (same as the CE/electrolyte) interface in a DSSC. A simplified symmetric cell with the same film on both the electrodes was used for this purpose. Figure 5 shows the EIS spectra of such cells with various electro-catalytic


16

Page 17 of 37

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


films. An EIS spectrum usually shows two semicircles in the frequency range of 10 mHz to 65 kHz2, 40. According to the equivalent circuit2, 40 shown in the inset of Figure 5, a series resistance (Rs) value equals to the onset point of the first semicircle (left-hand side) on the x-axis in the high frequency region. A lower Rs reflects a better ohmic contact between the substrate and the electrocatalytic film. The charge transfer resistance (Rct-EIS) is obtained from the radius of the first semicircle in the middle frequency region; a lower Rct-EIS refers to a larger amount of charge passing through the CE/electrolyte interface. The second semicircle (right-hand side) is owing to the Warburg diffusion resistance of the electrolyte, measured in the low frequency region. As summarized in Table 2, the Rs values of the cells with PEDOT:PSS and with all zinc-based composite films lie between 15~18 Ω cm2, implying that these films all adhere well to their FTO substrates. All zinc-based composite films show slightly higher Rs values, compared to the film of PEDOT:PSS; this is because of the fact that the presence of a zinc-based semiconductor in a composite film causes a slight decrease in the conductivity of the composite film; this phenomenon will be discussed in Sec. 3.6 and Sec. 3.7 next. The Pt film shows a slightly lesser Rs value (14.25Ω cm2), with reference to those of other films, because the ultra-thin Pt adheres very well to the ITO substrate. The Rct-EIS values of the films show a tendency of Zn3N2-10 < Pt < ZnSe-10 < ZnO-10 < ZnS-10 < PEDOT:PSS, which is consistent with the tendency of Rct-Tafel values obtained from Tafel polarization curves. It is notable that all the Rct values, irrespective of their measurement technique (Rct-Tafel or Rct-EIS), show a perfect consistency with the values of Jpc and J0. The Rct-EIS value of Zn3N2-10 (2.37 Ω cm2) is even lower than that of Pt (2.98 Ω cm2). In brief, according to CV, Tafel, and EIS analyses, the electro-catalytic abilities of the films are in the order of Zn3N210 > Pt >ZnSe-10 > ZnO-10 > ZnS-10 > PEDOT:PSS. Among all electro-catalytic films, the


17


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 18 of 37

composite film with Zn3N2-10 exhibits the best electro-catalytic ability for I3– reduction, and shows a great potential to replace the expensive Pt. Density functional theory calculations. Density functional theory (DFT) calculations of Pt and zinc-based materials (Zn3N2, ZnO, ZnS, and ZnSe) were made to investigate their total electronic density of states (DOS); DOS is reported as an important index to influence the electro-catalytic ability of a material20, 64. Figure 6 shows DOS’s of Pt and other zinc compounds. In the case of Pt, it can be seen that the DOS is uniformly distributed without an energy band gap (Eg), suggesting that Pt can easily conduct charges as a general metal material20. However, the DOS’s of all zinc compounds are quite different from that of Pt; they all show an Eg, indicating that they are indeed semiconductors61, 65-67. It can be deduced that the charge transport within a zinc-based material is slower than that within the Pt. For this reason, the Jsc, FF and Voc values of the DSSCs with bare Zn3N2, ZnO, ZnS, and ZnSe CEs are all much less than these in the case of the Pt-based DSSC. However, when Zn3N2, ZnO, ZnS, and ZnSe NPs were incorporated in their composite films with the conducting binder, PEDOT:PSS, the charge transfer are facilitated in the composite film or at its interface with the electrolyte. Thus, a composite film provides the better electro-catalytic ability as well as the higher DSSC performance (Table S1) than those of its zinc-based component. The calculated values (Eg) of Zn3N2, ZnO, ZnS, and ZnSe are 1.05, 3.06, 3.58, 2.27 eV, respectively; these values agree well with the reported Eg values of these materials61, 65-67. A narrower band gap indicates an easier charge transfer from the valence band to the conduction band of a semiconductor under an applied voltage (i.e., the photo-induced voltage in a DSSC); in other words, the narrower Eg reflects the higher electrical conductivity (lower resistance) of the semiconductor. Therefore, the interesting trend of Eg values of the zinc compounds (Eg of Zn3N2 < ZnSe < ZnO < ZnS) shows a great consistency with the tendency of FF and Jsc values of the compounds (FF and


18

Page 19 of 37

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


Jsc of Zn3N2 > ZnSe > ZnO > ZnS). It can be concluded that Zn3N2 possesses the lowest Eg and superior electro-catalytic ability; therefore, the corresponding DSSC shows the highest η among the cells with zinc-based CEs. Four-point probe measurement. The sheet resistances (Rsh), referring to electrical conductivities, of different films were obtained by a four-point probe method (Table 2). The Rsh values of bare Zn3N2, ZnO, ZnS, and ZnSe films (with an average thickness of 30 μm, recorded by FE-SEM) were measured to be 44.91, 193.70, 250.35, and 128.47 Ω sq.–1, respectively. These values are much larger than that of the Pt film (3.02 Ω sq.–1, with an average thickness of 30 nm, recorded by FE-SEM). The tendency of Rsh (Pt < Zn3N2 < ZnSe < ZnO < ZnS) agrees well with the results obtained from the DOS calculations. However, when Zn3N2, ZnO, ZnS, and ZnSe NPs were incorporated in the conducting binder PEDOT:PSS (7.27 Ω sq.–1, with an average film thickness of 3 μm, recorded by FE-SEM), the composite films of Zn3N2-10, ZnSe-10, ZnO-10, ZnS-10 show greatly reduced Rsh values of 9.88, 17.17, 21.50, 13.91 Ω sq.–1, respectively (with an average thickness of 30 μm, recorded by FE-SEM). Therefore, it can be said that the improvement in the Jsc, FF and Voc values of the cells with zinc-based composite films, compared to these values of the cells with the corresponding bare zinc compounds, are mainly due to the decreases in the Rsh values of the films (in other words, due to the increases in the conductivity values of the films). Moreover, all zinc-based composite films possess larger Rsh values than that of the bare PEDOT:PSS film. It can be said that the addition of one type of zinc-based NP (Zn3N2, ZnO, ZnS, or ZnSe) into the PEDOT:PSS actually caused the decrease in the conductivity but the increase in the electro-catalytic ability (verified by the increased Jpc, see Table 2) of a zinc-based composite film. Since this increased electro-catalytic ability can be attributed to the increase of the electrocatalytic active sites or the increase of the intrinsic electro-catalytic property, a Al2O3/PEDOT:PSS


19


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 20 of 37

composite film consisted of 10 wt% of non-catalytic aluminum oxide (Al2O3) nanoparticles and PEDOT:PSS (denoted as Al2O3-10) was prepared in contrast to those zinc-based composite films. For this Al2O3-10 composite film, its FE–SEM image, CV curve, and the pertinent J–V curve are shown in Figure S5 in the Supporting Information. It can be found in Figure S5a that this Al2O310 composite film possesses more porous morphology, suggesting the larger electro-catalytic active sites than that of the bare PEDOT:PSS (Figure 1b). However, Al2O3-10 composite film gave a negligible Jpc value (Figure S5b) and rendered a poor cell efficiency of 2.15% to its DSSC (Figure S5c); these data indicated that the Al2O3-10 composite film has poor electro-catalytic ability toward I3– reduction. Although the increased electro-catalytic active sites were formed by adding the Al2O3 nanoparticles into PEDOT:PSS, the non-catalytic Al2O3 nanoparticles would cause a severe decrease in the intrinsic electro-catalytic property and thereby rendered a poor electro-catalytic ability to its composite film. All Zn-based composite films (Zn3N2-10, ZnO-10, ZnS-10, and ZnSe-10) were found simultaneously to give more porous morphologies (Figure 1), better electro-catalytic abilities (Table 2), and higher cell efficiencies (Table 1), compared to those of the bare PEDOT:PSS film. Thus, the Zn-based nanoparticles (Zn3N2, ZnO, ZnS, and ZnSe) are thought to provide not only the large electro-catalytic active sites but also the good intrinsic electro-catalytic property to their composite film. In brief, in the zinc-based composite films, the zinc-based NPs (Zn3N2, ZnO, ZnS, and ZnSe) function as the electro-catalysts, while PEDOT:PSS acts as the conductor.



CONCLUSIONS


20

Page 21 of 37

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


Zinc-based composites, including Zn3N2/PEDOT:PSS, ZnO/PEDOT:PSS, ZnS/PEDOT:PSS, and ZnSe/PEDOT:PSS were systematically investigated as the electro-catalytic materials for counter electrodes of DSSCs; Zn3N2 and ZnSe are introduced for the first time in DSSCs. FE– SEM images reveal that all zinc-based composite films have sufficient electro-catalytic active cites for the reduction of I3– ions. The DSSCs with Zn3N2-10, ZnO-10, ZnS-10, and ZnSe-10 show cell efficiencies of 8.73, 7.54, 7.40 and 8.13%, respectively, where the number 10 indicates the optimized weight percentage of the zinc compound in the composite film. The η of the DSSC with Zn3N2-10 (8.73%) is found to be even higher than that of the cell with the Pt CE (8.50%). The IPCE values are in consistency with the Jsc values of the cells, i.e., the higher the IPCE value, the higher the Jsc value. The Jpc values obtained from the CV analysis and the J0 values obtained from the Tafel analysis are found to be consistent with the Jsc values obtained from J-V curve, and they (Jpc and J0 values) clearly reveal that the overall electro-catalytic abilities of the films show a tendency of Zn3N2-10 > ZnSe-10 > ZnO-10 > ZnS-10 > PEDOT:PSS. Thus, we conclude that the electro-catalytic abilities of the composite films are mainly determined by the zinc-based nanoparticles. Tafel and EIS analyses both establish that the charge transfer resistances of the zincbased composites films are much smaller than that of PEDOT:PSS film; this indicates: (1) zincbased nanoparticles promote the I3– reduction at CE/electrolyte interface and (2) the PEDOT:PSS matrices provides a good conductive path to facilitate the charge transfer between the electrocatalytic film and the FTO substrate. Density functional theory (DFT) has also been used to calculate the total electronic density of states (DOS) of those zinc-based electro-catalysts. The values of Eg (Zn3N2 < ZnSe < ZnO < ZnS) calculated from DOS infer the trend of FF and Jsc values (Zn3N2-10 > ZnSe-10 > ZnO-10 > ZnS-10) of the cells with zinc-based composite CEs. The Rsh values (Zn3N2-10 < ZnSe-10 < ZnO-10 < ZnS-10) obtained from four-point probe measurement


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

Page 22 of 37

agrees well with the results obtained from the DOS calculations. Among all electro-catalytic films, the composite film with Zn3N2-10 (precisely, Zn3N2/PEDOT:PSS containing 10 wt% Zn3N2 nanoparticles) exhibits the best electro-catalytic ability for I3– reduction, and shows a great potential to replace the expensive Pt. Last but not least, the earth abundant Zn3N2 benefits the lowcost production of DSSCs.



ASSOCIATED CONTENT

Supporting Information. The detailed information of all the materials/chemicals used in this study and the fabrication details of the TiO2 photoanode were given. This material is available free of charge via the Internet at http://pubs.acs.org.”



AUTHOR INFORMATION

Corresponding Author *E–mail: [email protected] (Kuo-Chuan Ho) *E–mail: [email protected] (Yu-Jane Sheng)



ACKNOWLEDGMENT

This work was supported by the Ministry of Science and Technology (MOST) of Taiwan, under grant numbers 102-2221-E-002-186-MY3 and 103-2119-M-007-012.


22

Page 23 of 37

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


 1.

REFERENCES Wu, M.; Lin, X.; Wang, Y.; Wang, L.; Guo, W.; Qi, D.; Peng, X.; Hagfeldt, A.; Gratzel, M.; Ma, T., Economical Pt-Free Catalysts for Counter Electrodes of Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2012, 134, 3419-3428.

2.

Yun, S.; Hagfeldt, A.; Ma, T., Pt-Free Counter Electrode for Dye-Sensitized Solar Cells with High Efficiency. Adv. Mater. 2014, 26, 6210-6237.

3.

Kong, C.; Min, S.; Lu, G., Dye-Sensitized NiSx Catalyst Decorated on Graphene for Highly Efficient Reduction of Water to Hydrogen under Visible Light Irradiation. ACS Catal. 2014, 4, 2763-2769.

4.

Mori, K.; Kakudo, H.; Yamashita, H., Creation of Nickel-Based Active Species within a Macroreticular Acidic Resin: A Noble-Metal-Free Heterogeneous Catalyst for VisibleLight-Driven H2 Evolution from Water. ACS Catal. 2014, 4, 4129-4135.

5.

Jiang, Q. W.; Li, G. R.; Gao, X. P., Highly Ordered TiN Nanotube Arrays as Counter Electrodes for Dye-Sensitized Solar Cells. Chem. Commun. 2009, 6720-6722.

6.

Wang, Y.; Wu, M.; Lin, X.; Hagfeldt, A.; Ma, T., Optimization of the Performance of DyeSensitized Solar Cells Based on Pt-Like TiC Counter Electrodes. Eur. J. Inorg. Chem. 2012, 2012, 3557-3561.

7.

Li, C. T.; Lee, C. P.; Li, Y. Y.; Yeh, M. H.; Ho, K. C., A Composite Film of TiS2/PEDOT:PSS as the Electrocatalyst for the Counter Electrode in Dye-Sensitized Solar Cells. J. Mater. Chem. A. 2013, 1, 14888-14896.

8.

Wang, Y.; Wu, M.; Lin, X.; Shi, Z.; Hagfeldt, A.; Ma, T., Several Highly Efficient Catalysts for Pt-Free and FTO-Free Counter Electrodes of Dye-Sensitized Solar Cells. J. Mater. Chem. 2012, 22, 4009-4014.


23


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

9.

Page 24 of 37

Wang, Y.; Zhao, C.; Wu, M.; Liu, W.; Ma, T., Highly Efficient and Low Cost Pt-based Binary and Ternary Composite Catalysts as Counter Electrode for Dye-Sensitized Solar Cells. Electrochim. Acta. 2013, 105, 671-676.

10.

Wei, W.; Wang, H.; Hu, Y. H., Unusual Particle-Size-Induced Promoter-to-Poison Transition of ZrN in Counter Electrodes for Dye-Sensitized Solar Cells. J. Mater. Chem. A. 2013, 1, 14350-14357.

11.

Lin, X.; Wu, M.; Wang, Y.; Hagfeldt, A.; Ma, T., Novel Counter Electrode Catalysts of Niobium Oxides Supersede Pt for Dye-Sensitized Solar Cells. Chem. Commun. 2011, 47, 11489-11491.

12.

Guo, J.; Shi, Y.; Zhu, C.; Wang, L.; Wang, N.; Ma, T., Cost-Effective and MorphologyControllable Niobium Diselenides for Highly Efficient Counter Electrodes of DyeSensitized Solar Cells. J. Mater. Chem. A. 2013, 1, 11874-11879.

13.

Wang, L.; Diau, E. W.; Wu, M.; Lu, H. P.; Ma, T., Highly Efficient Catalysts for Co(II/III) Redox Couples in Dye-Sensitized Solar Cells. Chem. Commun. 2012, 48, 2600-2602.

14.

Wu, M.; Lin, X.; Hagfeldt, A.; Ma, T., Low-Cost Molybdenum Carbide and Tungsten Carbide Counter Electrodes for Dye-Sensitized Solar Cells. Angew. Chem. Int. Ed. 2011, 50, 3520-3524.

15.

Wu, M.; Zhang, Q.; Xiao, J.; Ma, C.; Lin, X.; Miao, C.; He, Y.; Gao, Y.; Hagfeldt, A.; Ma, T., Two Flexible Counter Electrodes Based on Molybdenum and Tungsten Nitrides for DyeSensitized Solar Cells. J. Mater. Chem. 2011, 21, 10761-10766.

16.

Wu, M.; Wang, Y.; Lin, X.; Yu, N.; Wang, L.; Hagfeldt, A.; Ma, T., Economical and Effective Sulfide Catalysts for Dye-Sensitized Solar Cells as Counter Electrodes. Phys. Chem. Chem. Phys. 2011, 13, 19298-192301.


24

Page 25 of 37

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


17.

Zheng, X.; Guo, J.; Shi, Y.; Xiong, F.; Zhang, W. H.; Ma, T.; Li, C., Low-Cost and HighPerformance CoMoS4 and NiMoS4 Counter Electrodes for Dye-Sensitized Solar Cells. Chem. Commun. 2013, 49, 9645-9647.

18.

Yun, S.; Zhou, H.; Wang, L.; Zhang, H.; Ma, T., Economical Hafnium Oxygen Nitride Binary/Ternary Nanocomposite Counter Electrode Catalysts for High-Efficiency DyeSensitized Solar Cells. J. Mater. Chem. A. 2013, 1, 1341-1348.

19.

Yun, S.; Wang, L.; Guo, W.; Ma, T., Non-Pt Counter Electrode Catalysts Using Tantalum Oxide for Low-Cost Dye-Sensitized Solar Cells. Electrochem. Commun. 2012, 24, 69-73.

20.

Yun, S.; Wu, M.; Wang, Y.; Shi, J.; Lin, X.; Hagfeldt, A.; Ma, T., Pt-Like Behavior of HighPerformance Counter Electrodes Prepared from Binary Tantalum Compounds Showing High Electrocatalytic Activity for Dye-Sensitized Solar Cells. ChemSusChem. 2013, 6, 411416.

21.

Jang, J. S.; Ham, D. J.; Ramasamy, E.; Lee, J.; Lee, J. S., Platinum-Free Tungsten Carbides as an Efficient Counter Electrode for Dye Sensitized Solar Cells. Chem. Commun. 2010, 46, 8600-8602.

22.

Wu, M.; Mu, L.; Wang, Y.; Lin, Y.-n.; Guo, H.; Ma, T., One-Step Synthesis of Nano-Scaled Tungsten Oxides and Carbides for Dye-Sensitized Solar Cells as Counter Electrode Catalysts. J. Mater. Chem. A. 2013, 1, 7519-7524.

23.

Wang, L.; Shi, Y.; Wang, Y.; Zhang, H.; Zhou, H.; Wei, Y.; Tao, S.; Ma, T., Composite Catalyst of Rosin Carbon/Fe3O4: Highly Efficient Counter Electrode for Dye-Sensitized Solar Cells. Chem. Commun. 2014, 50, 1701-1703.


25


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

24.

Page 26 of 37

Li, G. R.; Song, J.; Pan, G. L.; Gao, X. P., Highly Pt-Like Electrocatalytic Activity of Transition Metal Nitrides for Dye-Sensitized Solar Cells. Energy Environ. Sci. 2011, 4, 1680-1683.

25.

Wang, W.; Pan, X.; Liu, W.; Zhang, B.; Chen, H.; Fang, X.; Yao, J.; Dai, S., FeSe 2 Films with Controllable Morphologies as Efficient Counter Electrodes for Dye-Sensitized Solar Cells. Chem. Commun. 2014, 50, 2618-2620.

26.

Wang, Y. C.; Wang, D. Y.; Jiang, Y. T.; Chen, H. A.; Chen, C. C.; Ho, K. C.; Chou, H. L.; Chen, C. W., FeS2 Nanocrystal Ink as a Catalytic Electrode for Dye-Sensitized Solar Cells. Angew. Chem. Int. Ed. 2013, 52, 6694-6698.

27.

Sun, H.; Zhang, L.; Wang, Z. S., Single-Crystal CoSe2 Nanorods as an Efficient Electrocatalyst for Dye-Sensitized Solar Cells. J. Mater. Chem. A. 2014, 2, 16023-16029.

28.

Gong, F.; Wang, H.; Xu, X.; Zhou, G.; Wang, Z. S., In Situ Growth of Co0.85Se and Ni0.85Se on Conductive Substrates as High-Performance Counter Electrodes for Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2012, 134, 10953-10958.

29.

Kung, C. W.; Chen, H. W.; Lin, C. Y.; Huang, K. C.; Vittal, R.; Ho, K. C., CoS Acicular Nanorod Arrays for the Counter Electrode of an Efficient Dye-Sensitized Solar Cell. ACS Nano. 2012, 6, 7016-7025.

30.

Sun, X.; Dou, J.; Xie, F.; Li, Y.; Wei, M., One-Step Preparation of Mirror-Like NiS Nanosheets on ITO for the Eﬃcient Counter Electrode of Dye-Sensitized Solar Cells. Chem. Commun. 2014, 50, 9869-9871.

31.

Wu, M.; Lin, X.; Guo, W.; Wang, Y.; Chu, L.; Ma, T.; Wu, K., Great Improvement of Catalytic Activity of Oxide Counter Electrodes Fabricated in N2 Atmosphere for DyeSensitized Solar Cells. Chem. Commun. 2013, 49, 1058-1060.


26

Page 27 of 37

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


32.

Chen, X.; Hou, Y.; Zhang, B.; Yang, X. H.; Yang, H. G., Low-Cost SnSx Counter Electrodes for Dye-Sensitized Solar Cells. Chem. Commun. 2013, 49, 5793-5795.

33.

Yang, B.; Zuo, X.; Xiao, H.; Zhou, L.; Yang, X.; Li, G.; Wu, M.; Ma, Y.; Jin, S.; Chen, X., SnS2 as Low-Cost Counter-Electrode Materials for Dye-Sensitized Solar Cells. Mater. Lett. 2014, 133, 197-199.

34.

Liu, Y.; Huang, F.; Xie, Y.; Cui, H.; Zhao, W.; Yang, C.; Dai, N., Controllable Synthesis of Cu2In2ZnS5 Nano/Microcrystals and Hierarchical Films and Applications in Dye-Sensitized Solar Cells. J. Phys. Chem. C. 2013, 117, 10296-10301.

35.

Yi, L.; Liu, Y.; Yang, N.; Tang, Z.; Zhao, H.; Ma, G.; Su, Z.; Wang, D., One Dimensional CuInS2–ZnS Heterostructured Nanomaterials as Low-Cost and High-Performance Counter Electrodes of Dye-Sensitized Solar Cells. Energy Environ. Sci. 2013, 6, 835-840.

36.

Zhang, H.; Yang, L.; Liu, Z.; Ge, M.; Zhou, Z.; Chen, W.; Li, Q.; Liu, L., Facet-Dependent Activity of Bismuth Sulfide as Low-Cost Counter-Electrode Materials for Dye-Sensitized Solar Cells. J. Mater. Chem. 2012, 22, 18572-18577.

37.

Ke, W.; Fang, G.; Tao, H.; Qin, P.; Wang, J.; Lei, H.; Liu, Q.; Zhao, X., In Situ Synthesis of NiS Nanowall Networks on Ni Foam as a TCO-Free Counter Electrode for Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces. 2014, 6, 5525-5530.

38.

Li, Y.; Wang, H.; Feng, Q.; Zhou, G.; Wang, Z. S., Reduced Graphene Oxide-TaON Composite as a High-Performance Counter Electrode for Co(bpy)33+/2+-Mediated DyeSensitized Solar Cells. ACS Appl. Mater. Interfaces. 2013, 5, 8217-8224.

39.

Wu, M.; Ma, T., Recent Progress of Counter Electrode Catalysts in Dye-Sensitized Solar Cells. J. Phys. Chem. C. 2014, 118, 16727-16742.


27


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

40.

Page 28 of 37

Wu, M.; Ma, T., Platinum-Free Catalysts as Counter Electrodes in Dye-Sensitized Solar Cells. ChemSusChem. 2012, 5, 1343-1357.

41.

Wang, H.; Wei, W.; Hu, Y. H., Efficient ZnO-based Counter Electrodes for Dye-Sensitized Solar Cells. J. Mater. Chem. A. 2013, 1, 6622-6628.

42.

Xu, S.; Luo, Y.; Xiao, Z.; Zhong, W.; Luo, Y., In-Situ Formation of Dispersed ZnO Nanoparticles in Mesoporous Carbon Counter Electrode for Efficient Dye-Sensitized Solar Cells. Electrochim. Acta. 2013, 114, 574-581.

43.

Shit, A.; Chatterjee, S.; Nandi, A. K., Dye-Sensitized Solar Cell from Polyaniline-ZnS Nanotubes and Its Characterization Through Impedance Spectroscopy. Phys. Chem. Chem. Phys. 2014, 16, 20079-20088.

44.

Maiaugree, W.; Pimanpang, S.; Towannang, M.; Saekow, S.; Jarernboon, W.; Amornkitbamrung, V., Optimization of TiO2 Nanoparticle Mixed PEDOT–PSS Counter Electrodes for High Efficiency Dye Sensitized Solar Cell. J. Non-Cryst. Solids. 2012, 358, 2489-2495.

45.

Song, D.; Li, M.; Wang, T.; Fu, P.; Li, Y.; Jiang, B.; Jiang, Y.; Zhao, X., Dye-Sensitized Solar Cells Using Nanomaterial/PEDOT–PSS Composite Counter Electrodes: Effect of the Electronic and Structural Properties of Nanomaterials. J. Photochem. Photobiol., A. 2014, 293, 26-31.

46.

Maiaugree, W.; Towannang, M.; Thiangkaew, A.; Harnchana, V.; Jarernboon, W.; Pimanpang, S.; Amornkitbamrung, V., Composited NiSO4 and PEDOT:PSS Counter Electrode for Efficient Dye-Sensitized Solar Cell Based on Organic T2/T− Electrolyte. Mater. Lett. 2013, 111, 197-200.


28

Page 29 of 37

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


47.

Wei, W.; Wang, H.; Hu, Y. H., A Review on PEDOT-Based Counter Electrodes for DyeSensitized Solar Cells. Int. J. Energy Res. 2014, 38, 1099-1111.

48.

Ellis, H.; Vlachopoulos, N.; Häggman, L.; Perruchot, C.; Jouini, M.; Boschloo, G.; Hagfeldt, A., PEDOT Counter Electrodes for Dye-Sensitized Solar Cells Prepared by Aqueous Micellar Electrodeposition. Electrochim. Acta. 2013, 107, 45-51.

49.

Li, C. T.; Lee, C. P.; Fan, M. S.; Chen, P. Y.; Vittal, R.; Ho, K. C., Ionic Liquid-Doped Poly(3,4-ethylenedioxythiophene) Counter Electrodes for Dye-Sensitized Solar Cells: Cationic and Anionic Effects on the Photovoltaic Performance. Nano Energy. 2014, 9, 1-14.

50.

Segall, M. D.; Lindan, P. J. D.; Probert, M. J.; Pickard, C. J.; Hasnip, P. J.; Clark, S. J.; Payne, M. C., First-Principles Simulation: Ideas, Illustrations and the CASTEP Code. J. Phys.: Condens. Matter. 2002 14, 2717-2744.

51.

Materials Studio 7.0; Accelrys, Inc. San Diego, CA. www.accelrys.com.

52.

Long, R.; Dai, Y.; Yu, L.; Guo, M.; Huang, B., Structural, Electronic, and Optical Properties of Oxygen Defects in Zn3N2. J. Phys. Chem. B. 2007, 111, 3379-3383.

53.

Long, R.; Dai, Y.; Yu, L.; Huang, B.; Han, S., Atomic Geometry and Electronic Structure of Defects in Zn3N2. Thin Solid Films. 2008, 516, 1297-1301.

54.

Zheng, S. W.; Fan, G. H.; He, M.; Zhang, T., Electronic Structures and Energy Band Properties of Be- and S-Doped Wurtzite ZnO. Chin. Phys. B. 2014, 23, 066301(1)-(7).

55.

Saoud, F. S.; Plenet, J. C.; Henini, M., Band Gap and Partial Density of States for ZnO: Under High Pressure. J. Alloys Compd. 2015, 619, 812-819.

56.

Wan, H.; Xu, L.; Huang, W. Q.; Huang, G. F.; He, C. N.; Zhou, J. H.; Peng, P., Band Engineering of ZnS by Codoping for Visible-Light Photocatalysis. Appl. Phys. A: Mater. Sci. Process. 2014, 116, 741-750.


29


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

57.

Page 30 of 37

Rashid, M.; Hussain, F.; Imran, M.; Ahmad, S. A.; Noor, N. A.; Sohaib, M. U.; Alay-eAbbas, S. M., Structural, Electronic, and Optical Properties of ZnO1−xSex Alloys Using FirstPrinciples Calculations. Chin. Phys. B. 2013, 22, 087301(1)-(4).

58.

Adetunji, B. I.; Adebambo, P. O.; Adebayo, G. A., First Principles Studies of Band Structure and Electronic Properties of ZnSe. J. Alloys Compd. 2012, 513, 294-299.

59.

Vanderbilt, D., Soft Self-Consistent Pseudopotentials in a Generalized Eigenvalue Formalism. Phys. Rev. B. 1990, 41, 7892-7895.

60.

Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868.

61.

Yoo, S. H.; Walsh, A.; Scanlon, D. O.; Soon, A., Electronic Structure and Band Alignment of Zinc Nitride, Zn3N2. RSC Adv. 2014, 4, 3306-3311.

62.

Lin, J. Y.; Chan, C. Y.; Chou, S. W., Electrophoretic Deposition of Transparent MoS 2Graphene Nanosheet Composite Films as Counter Electrodes in Dye-Sensitized Solar Cells. Chem. Commun. 2013, 49, 1440-1442.

63.

Bard, A. J.; Faulkner, L. R., Electrochemical Methods: Fundamentals and Applications. 2nd ed.; John Wiley & Sons, Inc.: 2001.

64.

Zhang, H.; Ge, M.; Yang, L.; Zhou, Z.; Chen, W.; Li, Q.; Liu, L., Synthesis and Catalytic Properties of Sb2S3 Nanowire Bundles as Counter Electrodes for Dye-Sensitized Solar Cells. J. Phys. Chem. C. 2013, 117, 10285-10290.

65.

Chowdhury, A.; Biswas, B.; Bera, R. N.; Mallik, B., Nanostructured Organic–Inorganic Heterojunction Diodes as Gas Sensors. RSC Adv. 2012, 2, 10968-10976.

66.

Donega, C. d. M., Synthesis and Properties of Colloidal Heteronanocrystals. Chem. Soc. Rev. 2011, 40, 1512-1546.


30

Page 31 of 37

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


67.

Chen, Y. H.; Li, W. S.; Liu, C. Y.; Wang, C. Y.; Chang, Y. C.; Chen, L. J., ThreeDimensional Heterostructured ZnSe Nanoparticles/Si Wire Arrays With Enhanced Photodetection and Photocatalytic Performances. J. Mater. Chem. C. 2013, 1, 1345-1351.


31


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 32 of 37

Table 1. Photovoltaic parameters of the DSSCs with various CEs, measured at 100 mW cm–2 (AM 1.5G) light intensity. η

Voc

CE

Jsc

Jsc-IPCE FF

(%)

(V)

(mA cm-2)

(mA cm-2)

Pt

8.59

0.80

15.19

0.71

13.09

PEDOT:PSS

2.99

0.69

12.79

0.33

10.28

Zn3N2-10

8.73

0.81

15.77

0.69

13.95

ZnO-10

7.54

0.74

15.35

0.66

13.13

ZnS-10

7.40

0.76

14.99

0.65

12.73

ZnSe-10

8.13

0.77

15.72

0.68

13.34

Table 2. Electrochemical parameters of various CEs obtained from CV, Tafel plots, EIS, and fourpoint probe analyses. Jpc CE

ΔE

J0

Rct-Tafel

Rs

Rct-EIS

Rsh

(mA cm-2) (V)

(mA cm-2) (Ω cm2)

(Ω cm2)

(Ω cm2)

(Ω sq.-1)

1.39

0.40

7.31

1.75

14.25

2.98

3.02

PEDOT:PSS 0.40

0.64

0.22

57.51

15.78

N.A

7.27

Zn3N2-10

1.43

0.41

12.30

1.04

16.96

2.37

9.88

ZnO-10

1.13

0.43

4.48

2.86

17.87

3.78

17.17

ZnS-10

0.87

0.52

3.63

3.54

18.36

4.55

21.50

ZnSe-10

1.30

0.42

5.42

2.34

17.34

3.21

13.91

Pt


32

Page 33 of 37

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


Scheme 1. Triiodide reduction at a counter electrode with a zinc-based composite film. Among a zinc-based composite film, zinc-based nanoparticles mainly works as the electro-catalysts for I3– reduction, while the PEDOT:PSS matrix provides a good conductive path to facilitate the charge transfer between the electro-catalytic film and the ITO substrate.

Figure 1. FE–SEM images of (a) Pt, (b) PEDOT:PSS, (c) Zn3N2-10, (d) ZnO-10, (e) ZnS-10, and (f) ZnSe-10; the inset in (b) is obtained at a higher resolution.


33


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 34 of 37

Figure 2. (a) Photocurrent density–voltage curves and (b) incident photon–to–current conversion efficiency curves of the DSSCs with various electro-catalytic films.

Figure 3. Cyclic voltammograms of various electro-catalytic films.


34

Page 35 of 37

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


Figure 4. Tafel polarization plot of the symmetric cells with various electro-catalytic films.

Figure 5. Electrochemical impedance spectra of the symmetric cells with various electrocatalytic films.


35


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 36 of 37

Figure 6. Total density of state of Pt, Zn3N2, ZnO, ZnS, and ZnSe.


36

Page 37 of 37

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


Table of Content


37

Electrocatalytic Zinc Composites as the Efficient Counter Electrodes of

Recommend Documents