WO2015116007A1

WO2015116007A1 - Catalytic carbon counter electrode for dye-sensitized solar cells

Info

Publication number: WO2015116007A1
Application number: PCT/TH2015/000004
Authority: WO
Inventors: Vittaya AMORNKITBAMRUNG; Samuk PIMANPANG; Pikaned UPPACHAI; Narit FAIBUT
Original assignee: The Thailand Research Fund
Priority date: 2014-01-30
Filing date: 2015-01-30
Publication date: 2015-08-06
Also published as: CN105580097B; CN105580097A

Abstract

A carbon catalyst layer for electrolyte based tri-iodide/iodine redox couples includes amorphous carbon, oxygen, silicon, tin and fluorine. The catalyst layer can be coated on a transparent conducting oxide (TCO) glass substrate for use as a counter electrode in a dye- sensitized solar cell (DSSC). By annealing the coated carbon catalyst layer at a temperature between approximately 250 - 650 °C (e.g., 300 - 600 °C) to establish an appropriate sp3 orbital fraction, the carbon catalyst layer can have low electrical resistivity, high electro-catalytic activity, and excellent photoelectric conversion efficiency, which has been measured as 98.32% of that of a DSSC having a conventional platinum catalyst based counter electrode.

Description

CATALYTIC CARBON COUNTER ELECTRODE

FOR DYE-SENSITIZED SOLAR CELLS

TECHNICAL FIELD

Aspects of the present disclosure are directed to a carbon catalyst layer that acts as a photoelectric conversion element in a dye-sensitized solar cell (DSSC).

BACKGROUND

Dye-sensitized solar cells (DSSCs) were invented in 1991 by B. O'Regan and M. Gratzel, as described in "A low-cost, high-efficiency solar cell based on dye-sensitized colloidal Ti0₂ films," B. O'Regan and M. Gratzel, Nature, 1991 , 353, 737-740. The operating principle of DSSCs is quite different from conventional p-n junction solar cells, in that a DSSC requires molecular sensitizers to generate free electrons. The typical DSSC configuration consists of a working electrode or photo-electrode; a counter electrode; and an electrolyte solution. The working electrode is a transparent conductive oxide (TCO) glass substrate coated with mesoporous Ti0₂ nanoparticles, and anchored with trimeric ruthenium complex dye sensitizer, RuL₂^-(CN)Ru(CN)L'₂)₂, 1, where L is 2,2'-bipyridine-4,4'-dicarboxylic acid and V is 2,2'- bipyridine. The liquid electrolyte typically contains tri-iodide / iodide as a redox couple, whereas the counter electrode utilizes platinum as a catalyst. Platinum is a precious metal, which means that it is quite expensive due to its rarity. Normally, DSSCs require platinum as a catalyst due to platinum's high electrochemical activity and low resistivity. Many efforts have been directed to finding a suitable less-expensive replacement for the platinum catalyst in DSSCs.

Carbon based materials are a suitable platinum catalyst replacement candidate because of properties such as high surface area, high electrical conductivity, high electrocatalytic activity, easy synthesis, and low to very low cost. Several forms of carbon based materials have been applied as a catalyst in DSSCs, including carbon nanotubes (CNTs) (W.J. Lee et al., ACS Applied Materials and Interfaces, 2009, 1(6), 1 145-1 149); carbon nanofibers (P. Joshi et al., ACS Applied Material and Interfaces, 2010, 2(12), 3572-3577); carbon black particles (N. Takurou et al., Journal of the Electrochemical Society, 2006, 153(12), A2255-A2261); graphite (G. Veerappan et al., ACS Applied Materials and Interfaces, 201 1, 3(3), 857-862); and graphene (D.W. Zhang et al., Carbon, 201 1 , 49, 5382-5388). At this point, it appears that the use of a carbon based material as a catalyst in DSSC counter electrodes could be a feasible replacement for the Platinum catalyst. Notwithstanding, prior attempts to replace the Platinum catalyst with a carbon based catalyst have been unnecessarily complex. A need exists for a very simple, reliable, easily manufacturable manner of preparing or providing an effective carbon based replacement for the Platinum catalyst in DSSCs.

SUMMARY

Aspects of the present disclosure are directed to a DSSC counter electrode that utilizes amorphous carbon as a catalyst, where the amorphous carbon has been treated by way of a simple annealing process that provides the amorphous carbon with low electrical resistivity and high electro-catalytic activity for an electrolyte solution containing tri-iodide and iodide redox couples, and which achieves high DSSC power conversion efficiency comparable to that for DSSCs that utilize a platinum catalyst. An amorphous carbon catalyst in accordance with an embodiment of the present disclosure is suitable as a simple, reliable, easily manufacturable, and inexpensive replacement for platinum as a DSSC catalyst.

In accordance with an aspect of the present disclosure, a carbon based catalyst layer for electrolyte based tri-iodide and iodide redox couples includes amorphous carbon, oxygen, silicon, tin and fluorine. The amorphous carbon, oxygen, silicon, tin, and fluorine are present at respective volumetric concentrations of 10.0-90.0%, 0.1-20.0%, 0.1-3.0%, 0.1-1.0% and 0.1- 1.0%). The carbon based catalyst layer has an electrical resistivity between 0.1 - 100.0 ohm/square, for instance, an electrical resistivity of less than approximately 7.0 ohms / square. The carbon based catalyst layer can include graphitic phases corresponding or similar to / resembling a fullerene-like structure or onion-like structure.

In accordance with an aspect of the present disclosure, a counter electrode structure for a dye- sensitized solar cell (DSSC) includes an annealed amorphous carbon catalyst layer having a sp³ orbital fraction between 10.0-90.0%. The annealed amorphous carbon catalyst layer can have a sp³ orbital fraction of less than approximately 50.0%), or less than approximately 25%, such as a sp³ orbital fraction of approximately 22.58%). The annealed amorphous carbon catalyst layer can include graphitic phases corresponding or similar to / resembling a fullerene-like structure or onion-like structure.

The annealed amorphous catalyst layer of the counter electrode structure can have a compositional content as set forth above; and the annealed amorphous carbon catalyst layer of the counter electrode structure can have a thickness between 0.1 - 10.0 microns.

In the counter electrode structure the amorphous carbon catalyst layer is carried by a transparent conductive oxide (TCO) substrate having a conductive layer including one of fluorine doped tin dioxide, indium doped tin dioxide, aluminum doped zinc oxide, gallium doped zinc oxide, and germanium doped indium oxide. A dye-sensitized solar cell (DSSC) includes a counter electrode structure in accordance with an embodiment of the present disclosure.

A process for manufacturing a carbon catalyst layer includes providing a substrate having a transparent conductive oxide (TCO) layer thereon; depositing an amorphous carbon layer on the TCO layer; and annealing the deposited amorphous carbon layer to have a sp³ orbital fraction between approximately 10% - 90%. Annealing the deposited amorphous carbon layer includes annealing the deposited amorphous carbon layer at a temperature between 250 - 650 °C (e.g., between approximately 300 - 600 °C), for instance, under / using helium, nitrogen, and/or argon gas at ambient or near-ambient pressure. Annealing the deposited amorphous carbon layer includes annealing the deposited amorphous carbon layer to have a sp³ orbital fraction of less than approximately 50%, or less than approximately 25%, for instance, a sp³ orbital fraction of approximately 22.58%. Annealing the deposited amorphous carbon layer can include creating graphitic phases therein corresponding or similar to / resembling a fullerene- like structure or onion-like structure (for instance, where such graphitic phases are created at a temperature of less than or significantly less than approximately 1000 °C, e.g., approximately 500 °C).

Depositing the amorphous carbon layer can include placing the substrate having the TCO layer thereon in a chamber of a radio frequency plasma-enhanced chemical vapor deposition (RF- PECVD) system; supplying methane and hydrogen gas to the chamber; and creating a plasma in the chamber. Creating the plasma within the chamber can include using RF-PECVD operating conditions of a mixture of gases of CH₄ (at a flow rate of approximately 5.0 seem) and H₂ (at a flow rate of approximately 5.5 seem), a substrate temperature of approximately 70 °C, a pressure of approximately 320 mTorr, an RF frequency of approximately 13.56 x 10⁶ Hz, and a power of approximately 60 Watts for approximately 45 minutes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a representative carbon based counter electrode structure according to an embodiment of the present disclosure.

FIG. 2 shows a variation of electrical resistivity of carbon based counter electrodes of Examples 1 to 5, and non-carbon based counter electrodes of Comparative Examples 1 and 2.

FIG. 3A and FIG. 3B are Nyquist plots for dye-sensitized solar cells (DSSCs) that include the counter electrodes of Examples 1 to 5 and Comparative Example 2. FIG. 4 is a graph showing variations in current density versus voltage for DSSCs that have the carbon based counter electrodes of Examples 1 to 5 and the platinum based counter electrode of Comparative Example 2.

DETAILED DESCRIPTION

In the present disclosure, the depiction of a given element or object or consideration or use of a particular corresponding number in a FIG. or a reference thereto in corresponding descriptive material can encompass the same, an equivalent, or an analogous element or element number identified in another FIG. or descriptive material associated therewith. Herein, the use of "/" in text or an associated FIG. is understood to mean "and/or" unless otherwise indicated. The recitation of a particular numerical value or value range herein is understood to include or be a recitation of an approximate numerical value or value range, such as to within +/- 20%, +/- 10%, or +/- 5% of the recited value or value range. In an analogous manner, the terms "approximately" and "about" refer to approximate values or value ranges, such as to within +/- 20%, +/- 10%, or +/- 5% of a recited value or value range.

As used herein, the term "set" corresponds to or is defined as a non-empty finite organization of elements that mathematically exhibits a cardinality of at least 1 (i.e., a set as defined herein can correspond to a unit, singlet, or single element set, or a multiple element set), in accordance with known mathematical definitions (for instance, in a manner corresponding to that described in An Introduction to Mathematical Reasoning: Numbers, Sets, and Functions, "Chapter 11: Properties of Finite Sets" (e.g., as indicated on p. 140), by Peter J. Eccles, Cambridge University Press (1998)). The term "subset" as used herein correspondingly refers to a particular portion (e.g., a fractional portion) of a set having two or more elements. In general, an element of a set or subset can include or be a compound, a composition, an ingredient, a constituent, a portion of a process, a physical parameter, or a value depending upon the type of set or subset under consideration.

Representative Structural Overview

FIG. 1 illustrates a representative counter electrode structure in accordance with an embodiment of the present disclosure, which is a stacked or sandwiched structure that includes a carbon catalyst film or layer 10; a transparent conductive oxide (TCO) film or layer 20; and a transparent substrate 30, which can include or be glass or another suitable material. The carbon catalyst layer 10 can be defined as a top layer, the TCO layer 20 can be defined as a middle layer, and the transparent substrate 30 can be defined as a the bottom layer of the stacked structure.

In accordance with embodiments of the present disclosure, the carbon catalyst layer 10 includes or is formed of amorphous carbon. The amorphous carbon catalyst layer 10 contains carbon in the form of diamond and graphite structures. An individual having ordinary skill in the relevant art will understand that the diamond structure has a sp³ electron orbital configuration, in which carbon's four valence electrons are each assigned to a tetrahedrally directed sp³ electron probability distribution, results in a strong bond between adjacent atoms; and the graphite structure has a three-fold sp² electron orbital configuration, in which three of the four valence electrons are each assigned to a trigonally directed sp² electron probability distribution.

The diamond structure percentage or sp³ orbital fraction can be in the range of 10.0-90.0%. For instance, as indicated in Table 1 below, in several embodiments, the sp³ orbital fraction can be between about 20.0 - 80.0%. The thickness and electrical resistivity of the amorphous carbon catalyst layer 10 can be in the range of 0.1-10.0 microns and 0.1-100.0 ohm/square, respectively .In multiple embodiments, as also shown in Table 1 below, the carbon catalyst layer 10 includes several elements, namely carbon (C), oxygen (O), silicon (Si), tin (Sn) and Fluorine (F). The content of carbon, oxygen, silicon, tin, and fluorine are in the range of 10.0 - 90.0%,

0.1 - 20.0%, 0.1 - 3.0%, 0.1 - 1.0% and 0.1 - 1.0%, respectively.

The TCO layer 20 serves to transfer electrons from an external load to the carbon catalyst layer 10. The TCO layer 20 can include or be formed of fluorine-doped tin dioxide (FTO), indium doped-tin dioxide (ITO), aluminum-doped zinc oxide (AZO), gallium-doped zinc oxide (GZO), germanium-doped indium oxide (GIO), or another suitable material.

Representative Manufacturing Process

Examples 1 - 5

In an embodiment, a process for preparing, providing, or manufacturing a carbon based counter electrode or counter electrode structure in accordance with the present disclosure is described hereafter. More particularly, the following process was used to prepare amorphous carbon counter electrode structures for Examples 1 - 5 considered herein.

1. Glass substrates 30 each carrying a TCO layer 20 were cleaned using a series of acetone, methanol, and deionized (DI) water in an ultrasonic cleaner for 30 minutes for each of the acetone, methanol, and DI water.

2. The substrates 30 were then placed on the ground electrode of a radio frequency plasma- enhanced chemical vapor deposition (RF-PECVD) system, and the vacuum chamber thereof was evacuated down to a pressure of approximately 1.5xl0^~5 Torr.

3. The surfaces of the TCO layer 20 carried by the substrates 30 were cleaned in the RF- PECVD system using an argon (Ar) plasma for 15 minutes, a frequency of 13.56 MHz, a power of 50 Watts, and a pressure of approximately 350 mTorr. 4. An amorphous carbon layer 10 was deposited on the TCO layer 20 by way of feeding a mixture of gases including CH₄ (5.0 seem) and H₂ (5.5 seem) into the RF-PECVD system; the use of a substrate temperature of 70 °C, a frequency of 13.56 MHz, a power of 60 Watts, and a pressure of approximately 325 mTorr during deposition; and a deposition time of 45 minutes. An individual having ordinary skill in the relevant art will understand that the deposited carbon layer 10 had hydrogen incorporated therein as a result of such deposition parameters / conditions. 5. A set of amorphous carbon counter electrode structures, each of which now carries the TCO layer 20 and an amorphous carbon layer 10 deposited thereon, were set aside to serve as non- annealed as-manufactured or as-prepared samples of Example 1.

6. Other substrates 30, each of which now carries the TCO layer 20 and an amorphous carbon layer 10 deposited thereon, were annealed under an argon ambient atmosphere at different temperatures A, B, C, and D, with temperature increments therebetween of 100 °C, where A is a lowest and D is a highest temperature considered to respectively form the carbon based counter electrode structures of Example 2, Example 3, Example 4, and Example 5 considered herein. Temperatures A, B, C, and D were in the range of 300-600 °C. For instance, temperatures, A, B, C, and D can be 300, 400, 500, and 600 +/- 50 °C, respectively. Each of temperatures A, B, C, and D can individually have a temperature range of +/- 50 °C. Thus, temperature A can be 300 +/- 50 °C; temperature B can be 400 +/- 50 °C; temperature C can be 500+/- 50 °C; and temperature D can be 600 +/- 50 °C. The annealing temperatures were ramped up at a rate of 2 °C/min, and a total annealing time of 2 hours in the argon atmosphere was used. As an alternative to an argon atmosphere, a nitrogen or helium atmosphere, or an atmosphere including a combination of argon, nitrogen, and helium gases, can be used.

Comparative Examples

Comparative Example 1 was a bare TCO layer 20 coated on a transparent substrate 30.

Comparative Example 2 was a counter electrode structure having a conventional platinum catalyst layer. In the preparation of Comparative Example 2, a glass substrate 30 carrying a TCO layer 20 as FTO was provided. A platinum film was coated thereon by spin coating 20 mM of chloroplatinic acid, i.e., H₂PtCl₆H₂0 (Sigma Aldrich) and 0.01 g of ethylcellulose (Sigma Aldrich) in ethanol, and annealing at 500 °C for 1 hour in an ambient environment. Chemical Composition Characterization of Carbon Catalyst Layers of Examples 1 to 5

The sp³ fraction and elements in the amorphous carbon catalyst layer 10 of the counter electrode structures of Examples 1 to 5 were analyzed by using X-ray photoelectron emission spectroscopy (XPS), the results of which are shown in Table 1 hereafter:

Table 1 : XPS Results for Amorphous Carbon Layers in Examples 1 - 5

It can be seen from Table 1 that the sp³ content exhibits a decreasing trend for Examples 1, 2, 3 and 4. The amorphous carbon catalyst layer 10 of Example 4 has a lowest sp³ fraction value of 22.58 %, meaning that the amorphous structure of the carbon film had transformed into more of the graphite phase. In contrast, compared to Example 4, the sp³ content increased to 51.1 1 % for Example 5. Thus, the sp³ content exhibits a decreasing trend with increasing annealing temperature, until the annealing temperature reaches a certain transition temperature, at or beyond which the sp³ content increases (e.g., corresponding to an inflection point with respect to sp content versus annealing temperature behavior).

As also shown in Table 1, the content of carbon, oxygen, silicon, tin and fluorine elements in the amorphous carbon layer 10 according to Examples 1 to 5 are in the range of 10-90, 0.1-20, 0.1-3, 0.1-1 and 0.1-1%, respectively. The impurity atoms which are oxygen, silicon, tin and fluorine may be diffused from the substrate during the film deposition and annealing processes.

Electrical Characterization

FIG. 2 provides a graph showing measured sheet resistances of the Comparative Example 1 (corresponding to a bare TCO substrate), Comparative Example 2 (corresponding to a conventional platinum catalyst) and each Example 1 to 5. It can be seen that the electrical resistivity values for Examples 1 to 5 exhibit a decreasing trend with increasing annealing temperature, until a transition temperature is reached, beyond which electrical resistivity increases (e.g., corresponding to an inflection point with respect to electrical resistivity versus annealing temperature behavior). The amorphous carbon layer 10 of Example 4 has a lowest electrical resistivity value of 6.61 ohm/square. This can be explained by the structural transformation of diamond-like carbon into graphite-like carbon as the sp³ content or fraction decreases, as shown in Table 1. Moreover, it was found that the carbon layer 10 according to Example 4 has carbon, oxygen, silicon, tin and fluorine content of 78.72, 18.89, 1.91, 0.30 and 0.17%, respectively, which may be an improved or optimized compositional content for achieving the lowest electrical resistivity of the Examples considered herein.

It should be noted that some of the impurity elements in the carbon layers 10 may act as dopants in a manner seen in or analogous to that for n-type semiconductor materials, and some of impurity elements can also act as structural transformation catalysts for changing amorphous-like to graphite-like carbon at reduced, significantly reduced, or low(er) temperature. Surprisingly, transmission electron microscopy (TEM) studies performed by P. Uppachai, an .inventor named on this application, and his collaborators (manuscript in preparation) found that the transformation of amorphous-like carbon to graphite-like or graphitic phase carbon in carbon layers 10 produced in accordance with embodiments of the present disclosure occurs at 500 °C; and as a result of an annealing process in accordance with embodiments of the present disclosure, a certain amount of the graphitic phases appear to correspond or be similar to a fullerene-like structure or onion-like structure. Thus, carbon layers 10 in accordance with some embodiments of the present disclosure can include graphitic phases corresponding or similar to / resembling a fullerene-like or onion-like structure. Prior published references directed to the synthesis of onion-like carbon indicate that such synthesis requires a high temperature of approximately 1000 °C and a directly supplied metal catalyst (Co and/or Fe). The formation of fullerene-like structures or onion-like structures, and/or structures similar thereto, in graphitic phases of carbon layers 10 produced in accordance with embodiments of the present disclosure indicates that impurity atoms from the substrate can diffuse into the film (e.g., during the plasma bombardment and/or annealing process), and can act as catalysts for producing graphitic phases that are similar to fullerene-like carbon structures or onion-like carbon structures at a significantly lower or very significantly reduced temperature than previously reported.

The electrical resistivity slightly increased to 6.98 ohm/square for Example 5. This may be due to thermal degradation causing diamond-like carbon decomposition and a corresponding increase in the sp³ fraction to 51.11%.

Electrochemical and Cell Performance Testing

Electrochemical Impedance Spectroscopy (EIS) and solar cell efficiency testing were conducted to compare the performance of the amorphous carbon counter electrode of Examples 1 to 5 against that of Comparative Example 2. The amorphous carbon counter electrodes of Examples 1 to 5 and the conventional platinum counter electrode of Comparative Example 2 were assembled with Ti0₂ working electrode structures to fabricate a DSSC. Manufacture of the TiO? Working Electrode

The Ti0₂ working electrode was prepared using a conventional screen printing method. Briefly, transparent and scattering Ti0₂ films were fabricated using commercial Ti0₂ pastes, PST-18NR and PST-400C, respectively (CCIC-JGC, Japan).Ti0₂ films were sintered at 500 °C for 1.5 hours, and treated with UV radiation for 10 minutes. The Ti0₂ films were immersed in 0.3 mM of c -bis-(isothiocyanato)-bis-(2,2'-bipyridyl-4,4'-dicarboxylato)-ruthenium(II)-bis- tetrabutylammonium dye (N719, Solaronix, Switzerland) solution for 24 hrs. Dye residuals were removed by rinsing in ethanol. A tri -iodide/iodide electrolyte solution consisting of 0.05 M of iodine (I₂), 0.10 M of lithium iodide (Lil), 0.0025 M of lithium carbonate (Li₂C0₃), 0.50 M of 4-tert-butylpyridine (TBP) and 0.60 M of l-methyl-3-propylimidazolium iodide (MPI) in acetonitrile was used as the electrolyte.

Electrochemical Impedance Spectroscopy of the Cell with carbon counter electrode

The impedances of the DSSCs using the counter electrodes of Examples 1 to 5 and Comparative Example 2 were measured using electrochemical impedance spectroscopy (EIS, Gamry REF 3000, USA), varying frequency from 0.1 Hz to 100,000 Hz and an AC amplitude of 10 mV. FIG. 3 shows Nyquist plots of these DSSCs, which have identical types of working electrodes, but different counter electrode structures. Y and X axes represent reactance (Zj_m) and resistance (Z_re) values, respectively, in a manner readily understood by an individual having ordinary skill in the relevant art. Both of such values are proportional to charge transfer resistance (RCT) and capacitance (C) of electrons moving through the interface between two materials. The Nyquist plot is normally presented in semicircle form, with the semicircle radius depending on the overall impedance. According to FIG. 3A and 3B, the semicircle radii of the DSSCs obtained from the electrodes of Examples 1 to 4 exhibit a decreasing impedance trend. The DSSC obtained from the electrode of Example 4 has the smallest semicircle of the Examples considered herein, which is close to that for the electrode of Comparative Example 2, i.e., the electrode having the conventional platinum catalyst. This means that carbon catalyst film 10 of Example 4 exhibits low electrical resistance and high catalytic activity for an electrolyte solution containing tri-iodide and iodide redox couples. As can be seen from FIG. 3B, the size of semicircle slightly increased for the counter electrode obtained the Example 5. This result should corresponding to the increased electrical resistivity of the counter electrode structure of Example 5, as shown in FIG. 2.

Performance testing

The following test was used to evaluate the performance of DSSCs with counter electrodes obtained from Examples 1 to 5, and Comparative Example 2 as a reference. A J-V curve measurement was performed by using standard conditions including a light source from a solar simulator with an intensity of 100 mW/cm² and environmental temperature of 25 °C. The increasing of voltage was performed with step rate of 2 mV/sec.

FIG. 4 illustrates measured current density versus voltage for the DSSCs corresponding to Examples 1 to 5 and the DSSC corresponding to Comparative Example 2. The data are provided in Table 2, where η is the overall efficiency calculated based on the following equation:

where P_m is the input power of the solar simulator. Counter

Jsc (mA/cm²) Voc (Volts) FF

Electrode

Comparative

15.24 0.74 0.69 7.74 Example 2 Platinum

Carbon/un-

6.94 0.98 0.04 0.24

Example 1 annealed

Carbon/annealed

10.52 0.78 0.20 1.68

Example 2 at Temp. A

Carbon/annealed

13.77 0.75 0.43 4.44

Example 3 at Temp. B

Carbon/annealed

15.70 0.74 0.66 7.61

Example 4 at Temp. C

Carbon/annealed

14.93 0.75 0.51 5.76

Example 5 at Temp. D

Table 2: Measured DSSC Performance

The photoelectric conversion efficiency characteristics of DSSCs incorporating / using counter electrodes or counter electrode structures in accordance with embodiments of the present disclosure are provided in Table 2. From Table 2 and FIG. 4, it can be seen that the DSSC having the counter electrode of Example 1 has low Jsc and fill factor (FF) values. However, as the annealing temperature increases such as in DSSCs corresponding to Examples 2, 3 and 4, the Jsc and FF values increase (i.e., each of Jsc and FF increase). The increase in both of such values, (i.e, each of Jsc and FF values) as a result of the annealing process can be associated with the inclusion of hydrogen in the deposited carbon layers 10. The DSSC having the counter electrode of Example 4 had the highest Jsc and FF values of the Examples considered herein, which compare quite favorably to the DSSC corresponding to Comparative Example 2. Furthermore, the DSSC having the counter electrode of Example 4 exhibited the highest efficiency of 7.61%, which is 98.32% of the efficiency of the DSSC corresponding to Comparative Example 2. This result can be due to the decrease of internal resistance as a result of material structural changes in response to annealing to have more graphite like carbon structures exhibiting high catalytic activity. Therefore, the catalytic effect should be influenced by the annealing temperature, such that a higher annealing temperature gives rise to an enhanced catalytic effect. However, the efficiency of the DSSC having the counter electrode of Example 5 was slightly reduced relative to that of the DSSC having the counter electrode of Example 4. This results from the lower Jsc and FF values, causing increased electrical resistivity of the carbon film 10 as shown in FIG. 2. Thus, in a manner consistent with other measurements described above, the catalytic effect increases with increasing annealing temperature, until reaching a transition temperature beyond which the catalytic effect decreases or degrades (e.g., corresponding to an inflection point with respect to catalytic effect versus annealing temperature behavior).

In view of the foregoing, embodiments in accordance with the present disclosure can provide a carbon based counter electrode or counter electrode structure that is suitable as a simple, reliable, easily manufacturable, and inexpensive replacement for platinum as a DSSC catalyst. Aspects of particular embodiments of the present disclosure address at least one aspect, problem, limitation, and/or disadvantage associated with conventional platinum counter electrode structures and prior attempts to develop a carbon based replacement for therefor. While features, aspects, and/or advantages associated with certain embodiments have been described in the disclosure, other embodiments may also exhibit such features, aspects, and/or advantages, and not all embodiments need necessarily exhibit such features, aspects, and/or advantages to fall within the scope of the disclosure. It will be appreciated by a person of ordinary skill in the art that several of the above-disclosed systems, apparatuses, components, processes, or alternatives thereof, may be desirably combined into other different systems, apparatuses, components, processes, and/or applications. In addition, various modifications, alterations, and/or improvements may be made to various embodiments that are disclosed by a person of ordinary skill in the art within the scope of the present disclosure.

Claims

I . A carbon based catalyst layer for electrolyte based tri-iodide and iodide redox couples, where the carbon based catalyst layer comprises amorphous carbon, oxygen, silicon, tin, and fluorine.

2. The carbon based catalyst layer of claim 1, wherein the amorphous carbon, oxygen, silicon, tin, and fluorine are present at respective volumetric concentrations of 10.0-90.0%, 0.1-20.0%, 0.1-3.0%, 0.1-1.0% and 0.1-1.0%.

3. The carbon based catalyst layer of claim 1, wherein the carbon based catalyst layer has an electrical resistivity between 0.1 - 100.0 ohm/square.

4. The carbon based catalyst layer of claim 3, wherein the carbon based catalyst layer has an electrical resistivity of less than approximately 7.0 ohms / square.

5. The carbon based catalyst layer of claim 4, wherein the carbon based catalyst layer includes graphitic phases similar to a fullerene-like structure or onion-like structure.

6. A counter electrode structure for a dye-sensitized solar cell (DSSC) comprising an annealed amorphous carbon catalyst layer having a sp³ orbital fraction between 10.0-90.0%.

7. The counter electrode structure of claim 6, wherein the annealed amorphous carbon catalyst layer has a sp orbital fraction of less than approximately 50.0%.

8. The counter electrode structure of claim 7, wherein the annealed amorphous carbon catalyst layer has a sp³ orbital fraction of less than approximately 25%.

9. The counter electrode structure of claim 8, wherein the annealed amorphous carbon catalyst layer has a sp³ orbital fraction of approximately 22.58%.

10. The counter electrode structure of any one of claims 6 to 9, wherein the annealed amorphous catalyst layer has a compositional content of carbon oxygen, silicon, tin, and fluorine present at respective volumetric concentrations of 10.0-90.0%, 0.1-20.0%, 0.1-3.0%, 0.1-1.0% and 0.1-1.0%.

I I . The counter electrode structure of claim 10, wherein the annealed amorphous catalyst layer includes graphitic phases similar a fullerene-like structure or onion-like structure.

12. The counter electrode structure of any one of claims 6 to 9, wherein the annealed amorphous catalyst layer has a thickness between approximately 0.1 - 10.0 microns.

13. The counter electrode structure of any one of claims 6 to 9, wherein the amorphous carbon catalyst layer is carried by a transparent conductive oxide (TCO) substrate having a conductive layer comprising one of fluorine doped tin dioxide, indium doped tin dioxide, aluminum doped zinc oxide, gallium doped zinc oxide, and germanium doped indium oxide.

14. A dye-sensitized solar cell (DSSC) comprising a counter electrode structure of any one of claims 6 to 13.

15. A method of manufacturing a carbon catalyst layer, the method comprising: providing a substrate having a transparent conductive oxide (TCO) layer thereon; depositing an amorphous carbon layer on the TCO layer; annealing the deposited amorphous carbon layer to have a sp³ orbital fraction between approximately 10% - 90%.

16. The method of claim 15, wherein annealing the deposited amorphous carbon layer comprises annealing the deposited amorphous carbon layer at a temperature between 250-650

°C.

17. The method of claim 16, wherein annealing the deposited amorphous carbon layer comprises annealing the deposited amorphous carbon layer using argon gas at ambient pressure.

18. The method of any one of claims 15 - 17, wherein annealing the deposited amorphous carbon layer comprises annealing the deposited amorphous carbon layer to have a sp³ orbital fraction of less than approximately 50%.

19. The method of claim 18, wherein annealing the deposited amorphous carbon layer comprises annealing the deposited amorphous carbon layer to have a sp³ orbital fraction of less than approximately 25%.

20. The method of claim 19, wherein annealing the deposited amorphous carbon layer comprises annealing the deposited amorphous carbon layer to have a sp³ orbital fraction of approximately 22.58%.

21. The method of any one of claims 18 - 20, wherein annealing the deposited amorphous carbon layer includes creating graphitic phases in the amorphous carbon layer similar a fullerene-like structure or onion-like structure.

22. The method of claim 15, wherein depositing the amorphous carbon layer comprises:

placing the substrate having the TCO layer thereon in a chamber of a radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) system;

supplying methane and hydrogen gas to the chamber; and

creating a plasma in the chamber.

23. The method of claim 22, wherein creating the plasma within the chamber comprises using RF-PECVD operating conditions with a mixture of gasses of CH₄ at a flow rate of approximately 5.0 seem and H₂ at approximately 5.5 seem, a substrate temperature of approximately 70 °C, a pressure of approximately 320 mTorr, an RF frequency of approximately 13.56 x 10⁶ Hz, and a power of approximately 60 Watts for approximately 45 minutes.