WO2012155293A1

WO2012155293A1 - Novel electroconductive films for quantum dot sensitized solar cells

Info

Publication number: WO2012155293A1
Application number: PCT/CN2011/000860
Authority: WO
Inventors: Linan Zhao; Li Wang; Minling Liu
Original assignee: Honeywell International Inc.
Priority date: 2011-05-17
Filing date: 2011-05-17
Publication date: 2012-11-22

Abstract

A solar cell (100) is assembled by forming a TiO₂ patterned layer containing nanotubes or nanowires (106) on a substrate electrode (102). Quantum dots (QDs) (110) are formed on the TiO₂ patterned layer. A gasket is disposed between the flexible substrate electrode (102) and a flexible counter electrode to form a sandwich. Electrolyte and sealant are injected between the substrate electrode (102) and the flexible counter electrode to form the flexible solar cell.

Description

NOVEL ELECTROCONDUCTIVE FILMS FOR

QUANTUM DOT SENSITIZED SOLAR CELLS

Field of the Invention

[0001] This invention relates to electroconductive films containing titanium dioxide (Ti0₂) and one-dimensional conductors such as nanowires and/or nanotubes on a substrate. The electroconductive films are useful in preparing quantum dot sensitized solar cells.

Background

[0002] Investigation of solar cells is important because of their advantages for transportation and photovoltaic power-supply systems equipment. New designs and applications for supplying mobile electricity for lap-top computers, mobile phones, watches, etc. are possible.

Summary

[0003] A method for preparing solar cells comprises forming a titanium dioxide (Ti0₂) patterned layer comprising nanotubes and/or nanowires contained therein on a conductive substrate electrode; forming quantum dots on the Ti0₂ patterned layer; forming a counter electrode; assembling the substrate electrode having the Ti0₂ layer comprising Ti0₂ and nanotubes and/or nanowires and quantum dots, and counter electrode into a sandwich with a gasket disposed between the surface of the substrate electrode having the Ti0₂ layer comprising nanotubes and/or nanowires and quantum dots coated thereon and the counter electrode; and injecting electrolyte and sealant (240) between the substrate electrode and counter electrode to form a solar cell. An insulating layer may be formed between the Ti0₂ layer and the quantum dots.

[0004] A solar cell comprises a substrate electrode having a patterned electroconductive Ti0₂ layer comprising conductive nanotubes and/or nanowires coated thereon. Quantum dots are disposed over the electroconductive Ti0₂ layer. A gasket is disposed between the substrate electrode and a counter electrode. An electrolyte and sealer are injected between the substrate electrode and the counter electrode. An insulating layer may be disposed between the Ti0₂ layer and the quantum dots.

Brief Description of the Drawings

[0005] FIG. 1 is a block diagram illustrating an electroconductive film containing one-dimensional conductors according to one example embodiment.

[0006] FIG. 2 is a block diagram illustrating a solar cell according to one example embodiment.

Definitions

[0007] The term CBD refers to chemical bath deposition.

[0008] The term Cd refers to the element cadmium or its cation Cd .

[0009] The term DI water refers to deionized water

[0010] The term DSSC refers to dye sensitized solar cells.

[0011] The term EC film refers to electroconductive films.

[0012] The term FF refers to the fill factor. Fill factor is defined as

FF = (VmIm)/(VocIsc), where Voc is the open-circuit voltage (when 1 = 0) and

Isc is the short-circuit current (when V = 0), and Vm and Im are the voltage and current at optimal operation when the solar cell is operated under a condition that gives the maximum output power.

[0013] The term FTO or FTO glass refers to glass coated with a layer of fluorinated tin oxide.

[0014] The term ITO or ITO glass refers to glass coated with a layer of indium tin oxide.

[0015] The term Jsc refers to short-circuit current density.

[0016] . The term magnesium acetate refers to (CH3COO-)₂ Mg²⁺

[0017] The term η% refers to the percent conversion of light power to electric power. The conversion efficiency of the solar cell h% is defined as the ratio of the generated maximum electric output power to the total power of the incident light Pin: η = (Vmlm)/Pin = VocIscFF/Pin. From this equation, high

Voc, Isc ,and FF are preferred for higher conversion efficiency

[0018] The term NH F refers to ammonium fluoride.

[0019] The term MgO refers to magnesium oxide.

[0020] The term MWCNT refere to multi- walled carbon nano tubes.

[0021] The term NTA refers to sodium aminotriacetate [N(CH₂COONa)₃].

NTA is a strong complexing agent for Cd²⁺ (and many other cations). It is also known as 2,2',2"-nitrilotriacetic acid.

[0022] The term QD refers to quantum dots.

[0023] Surlyn® is a random copolymer poly(ethylene-co-methacrylic acid) (EMAA) in which the methacrylic acid groups have been neutralized with sodium ions (Na⁺).

[0024] The term SWCNT refers to single-walled carbon nanotubes.

[0025] The term Voc is the open-circuit voltage (when the current I = 0).

[0026] The term Rs(0.8V) refers to the solar cell series resistance at 0.8 V.

[0027] The term Rs (Voc) refers to the solar cell series resistance at Voc .

[0028] The term Rsh refers to the solar cell shunting resistance.

Detailed Description

[0029] In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical, and electrical changes may be made without departing from the scope of the present invention. The following description of example embodiments is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims.

[0030] Solar cells may provide advantages for transportation and photovoltaic power-supply systems equipment. New designs and applications for supplying mobile electricity for lap-top computers, mobile phones, watches, etc. are possible. For example, replacing a rigid substrate by a flexible material allows low-cost fabrication by roll-to-roll mass production. Therefore, by applying flexible-device technologies the formation of solar cells, there exists the possibility of preparing significantly lower cost photovoltaic-generating systems.

[0031] In one embodiment, the solar cells include rigid substrate electrodes such as glass coated with indium tin oxide (ITO) or fluorinated tin oxide (FTO)

[0032] In some embodiments, the solar cells include flexible substrate electrodes, such as poly(ethylene terephthalate) coated with indium tin oxide (PET-ITO) or fluorinated tin oxide/indium tin oxide (PET/FTO), poly(ethylene naphthalene) coated with tin-doped indium oxide (PEN-ITO) or fluorinated tin oxide/indium tin oxide (PEN/FTO) or flexible titanium metal or stainless steel. A transparent flexible electrode useful as a substrate is transparent flexible PET- ITO (125 um thick, having a resistivity of 10-100 ohm/sq, and ~79%

transmission@55 Onm) .

[0033] In some embodiments a flexible metal such as titanium or stainless steel may be used as the substrate. In such embodiments, the substrate would be non-transparent and the solar cell would be illuminated through the transparent flexible counter electrode described below.

[0034] The substrate may be patterned, as for example by screen printing, to allow for the electroconductive layer to be laid down in a pattern.

[0035] The solar cells include an electron conductor (EC). The electron conductor comprises a semiconductor such as Ti0₂ or ZnO containing one- dimensional conductors such as nanotubes or nanowires. In some embodiments, the electron conductor may be formed of Ti0₂ that has been sintered.

[0036] In some embodiments, the invention includes in-situ synthesis of quantum dots onto the electroconductive Ti0₂ film containing nanotubes and/or nanowires is carried out. There are two methods: chemical bath deposition and dip coating. Both methods utilize a suitable bandgap semiconductor, for example CdSe.

[0037] Quantum dots are semiconductors whose conducting characteristics are closely related to the size and shape of the individual crystal. Generally, the smaller the size of the crystal, the larger the band gap, the greater the difference in energy between the highest valence band and the lowest conduction band becomes, therefore more energy is needed to excite the dot, and concurrently, more energy is released when the crystal returns to its resting state. For example, in fluorescent dye applications, this equates to higher frequencies of light emitted after excitation of the dot as the crystal size grows smaller, resulting in a color shift from red to blue in the light emitted. An advantage in using quantum dots is that because of the high level of control possible over the size of the crystals produced, it is possible to have very precise control over the conductive properties of the material. (See, < http://en.wikipedia.org/wiki/Quantum_dot > Accessed 09/26/2010).

[0038] Inorganic quantum dots (QDs) have potential advantages over molecular dyes:

(1) They are capable of facile tuning of effective band gaps down to the infra-red (IR) region by changing their sizes and compositions,

(2) They have a higher stability and resistance toward oxygen and water over their molecular dye counterparts,

(3) They open up new possibilities for making multilayer or hybrid

sensitizers; and

(4) They exhibit new phenomena such as multiple exciton generation and use of energy transfer-based charge collection as well as direct charge transfer schemes.

(5) They have the potential for low cost and high light to electricity

conversion efficiency.

[0039] Examples of specific pairs of materials for forming quantum dots (QD) include but are not limited to MgO, MgS, MgSe, MgTe, CaO, CaS, CaSe, CaTe, SrO, SrS,SrSe, SrTe, BaO, BaS, BaSe, BaTe, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, HgO, HgS, HgSe, HgTe, A1₂0₃, A1₂S₃, Al₂Se_3, Al₂Te_3> Ga₂0_3, Ga₂S_3, Ga₂Se_3, Ga₂Te_3, ln₂0_3, In₂S_3, In₂Se_3, In₂Te_3, Si0₂, Ge0_2, Sn0₂, SnS, SnSe, SnTe, PbO, Pb0₂, PbS, PbSe, PbTe, A1N, A1P, AIAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, and InSb.

[0040] The solar cell is assembled with a counter electrode and sealant.

Transparent counter electrodes (CE) may be prepared by sputter depositing a thin metal film such as a platinum film on a plastic substrate, making a pinhole at the counter electrodes. Counter electrodes may also be prepared by electrochemical deposition from chloroplatinic acid solution. The counter electrode may be rigid or flexible; transparent or opaque. If the counter electrode is transparent, illumination through the counter electrode surface may be carried out. If the counter electrode is opaque, then illumination through the photoelectrode may be carried out.

[0041] For preparation of photoelectrodes, single walled carbon nanotubes (SWNT) and/or multiwalled carbon nanotubes (MWNT) were dispersed with solvent and added into a Ti0₂ paste. A typical weight ratio of nanotubes and/or nanowires to Ti0₂ is 0.1 : 1. The nanotubes may be hollow or solid. In one embodiment the nanotubes are hollow. The length and diameter of the nanotubes and/or nanowires is not critical. In one embodiment the length of the nanotubes may be shorter than the distance between the electrodes. In one embodiment, the outer diameter of the nanotubes and/or nanowires is typically from about 10 nm to about 50 nm and the length is typically from about 1 μιτι to about 12 μιη. The nanotubes and/or nanowires and Ti0₂ paste were then mixed together using a three-roll-mill for 1 hr. A small aliquot of the resulting Ti0₂ suspension containing the one-dimensional conductor was spread onto the substrate electrodes using a doctor-blade or screen-printing. After that, the electrodes were slowly heated to 400-500°C in a nitrogen atmosphere for ca. 0.5 hr. on a hotplate or in a muffle oven.

[0042] A thin insulating layer of a metal oxide, such as Ti0₂ or MgO may be formed above the Ti0₂ layer containing the one-dimensional conductor. This insulating layer may be several molecular layers thick and is believed to inhibit charge recombination and increase current collection. It may also reduce the photo-oxidative activity of Ti0₂ increasing the stability of solar cell and efficiency.

[0043] FIG. 1 is a block diagram illustrating an electroconductive film (100) containing one-dimensional conductors according to one example embodiment. A conductive substrate (102) such as indium tin oxide (ITO) or fluorine doped tin oxide (FTO) on a substrate (not shown) is coated with Ti0₂ (104) suspension containing Ti0₂ having nanowires (106) dispersed therein. An insulating layer (108) is arranged over the Ti0₂ layer. The substrate may be flexible or rigid.

[0044] CdSe quantum dots are then deposited on Ti0₂ layer containing nanotubes or nanowires by chemical deposition. The deposition solution may be prepared by adding 0.7M potassium nitrilotriacetate [N(CH₂COOK)₃ or NTA] to 0.5M CdS0₄. Then 0.2M sodium selenosulfate (Na₂SeS0₃) in excess Na₂S0₃, prepared by stirring 0.2M Se with 0.5M Na₂S0₃ at 70° for 3-5 hr, was added, resulting in a final composition of 80 mM CdS0₄, 80mM Na₂SeS0₃ (which includes 0.12M free Na₂S0₃), and 120 mM NTA. During the deposition, the Ti0₂ film is placed in the solution, which is put in a thermostat chamber to control temperature at 10-50°C for several hours and kept in the dark.

Afterwards the samples are rinsed with water and dried in a N₂ flow.

[0045] The solar cell is then assembled into a sandwich type cell by pressing the counter electrode against the sensitized electrodes coated with quantum dots. Between the two electrodes, there is an adhesive tape, that is to say, sealed with a hotmelt gasket of 60 um thickness made of the ionomer Surlyn (DuPont). The heating temperature is about 100°C for 10 minutes. This is to control electrolyte film thickness and to avoid short-circuiting of the cell. The active area of the cells may be determined by the area of the hotmelt gasket. Surlyn® is a random copolymer poly(ethylene-co-methacrylic acid) (EMAA) in which the

methacrylic acid groups have been neutralized with sodium ions (Na⁺).

[0046] The electrolyte and sealant are then injected through the pinhole. In one embodiment the electrolyte comprises a solution of a sulfide salt, sulfur, and an ionic conductor in in a mixture of water and an alcohol. A typical electrolyte solution comprises a solution of 1M Na₂S, 0.1M S, 0.2M KC1, in a mixture of pure water and methanol (volume ratio: 1 : 1). A drop of the electrolyte put on the hole in the back of the counter electrode. The electrolyte is introduced into the cell via vacuum backfilling. The hole may be sealed with a Surlyn layer. In an embodiment, the gasket has two or more holes. The electrolyte is introduced through one hole and allowed to flow through the cell until electrolyte begins to flow out the other hole(s). The holes may then be sealed with a Surlyn layer. [0047] FIG. 2 is an expanded block cross section diagram of a solar cell (200) formed in the above manner, using a Ti0₂ photoelectrode containing a one- dimensional conductor. A top counter electrode layer (210) is shown with a hole (215). Gasket (220) is shown between the top counter electrode layer (210) and a bottom substrate photoelectrode that includes an electroconductive Ti0₂ layer containing one-dimensional conductor (235) with quantum dots. The

electroconductive Ti0₂ layer contains the one-dimensional conductor such as nanowires, Ti0₂ nanotubes, multi-walled carbon nanotubes, single-walled nanotubes, or mixtures thereof. The electroconductive layer is coated onto a conductive substrate (230). Finally, electrolyte and sealer are represented at (240), and may be injected through hole (215) in one embodiment.

Example - Preparation of Solar Cells Containing Electroconductive Layers

One-Dimensional Conductors

[0048] Samples were prepared using Ti0₂ with and without single walled carbon nanotubes and with single walled carbon nanotubes.

[0049] Samples containing multi-walled carbon nanotubes (MWCNT) were prepared by dispersing multi-walled carbon nanotubes into a Ti0₂ paste at a weight ratio of 0.1 : 1. The paste was then mixed using a three-roll-mill for one hour. The carbon nanotubes were hollow and had an outer diameter of 15 ± 5 nm and a length of 1 to 5 μιη.

[0050] The Ti0₂ layers were prepared by screen printing onto a fluorinated tin oxide glass substrate and heated on a hotplate in a nitrogen atmosphere for 30 minutes at 450°C. A sample was also prepared having a magnesium oxide layer coated above the Ti0₂ layer containing the carbon nanotubes.

[0051] The Ti0₂ electrode samples were immersed in an ethanolic solution of 120 mmol/L of magnesium acetate (CH₃COO^")₂ Mg²⁺ in ethanol for 1 minute at 70°C. They were then heated on a hotplate in a nitrogen atmosphere for 30 minutes at 450°C to form a magnesium oxide insulating layer above the Ti0₂ layer. TABLE I shows the composition of the three Ti0₂ layers. TABLE I

CdSe Quantum Dot

Sample Electroconductive Components

Deposition Time

1 Ti0₂ 260 min

Ti0₂+ multi-walled carbon nanotubes

2 260 min

(MWCNT)

Ti0₂+ multi-walled carbon nanotubes

3 260 min

(MWCNT) + MgO layer

[0052] The mesoscopic Ti0₂ film on the conductive substrate electrode was immersed in NH₄F (1 M) (ammonium fluoride) solution for about 3 minutes, removed and washed with deionized (DI) water.

[0053] A chemical bath deposition layer was prepared. A solution of potassium nitrilotriacetate/cadmium (NTA/Cd) in a ratio of 1.475 : 1 was prepared using CdS0₄ with the concentration of Cd²⁺ at 0.2 M (other soluble cadmium salts such as cadmium chloride (CdCl₂) or Cd(N0₃)₂ can be used if desired.) Approximately 2 mL the thus prepared NTA/Cd solution and 2 mL of sodium selenosulphate solution (such Na₂SeS0₃ solution, 0.2 M) in 1 1 mL deionized water (DI water) were placed in a bottle. Thus, 26.7mM Cd²⁺ solution was obtained. The pH was adjusted to 10.5. The NH₄F-treated electro- conductive film was placed in the bottle, and the bottle was placed in a thermostated water bath. The water bath was at a temperature of 30°C for the chemical bath deposition solution to synthesize and deposit CdSe-quantum dots onto the Ti0₂ films and thereby sensitize it. After a sufficient time for deposition of the CdSe quantum dot (ca. 260 min), the CdSe-quantum dot sensitized Ti0₂ films were removed from the deposition solution, and washed with deionized water.

[0054] The samples were again immersed into NH₄F (1 M) solution three times for approximately 3 minutes each time. The samples were removed from the, NH₄F solution, washed with deionized water, and allowed to dry.

[0055] The samples were then dipped in to Zn(Ac)₂ (zinc acetate) solution and Na₂S (sodium sulfide) solution respectively for 1 min. This procedure was repeated twice to form a ZnS-shell (zinc sulfide-shell). Washing with deionized water and drying afforded the photovoltaic electrode.

[0056] Counter electrodes (CE) were prepared by sputter depositing a 100 nm conductive platinum film onto a glass substrate. The counter electrode contained two pinholes.

[0057] The electrodes were assembled with a counter electrode as the cathode and filled with electrolyte to provide a quantum dot sensitized solar cell.

[0058] The counter electrode was pressed against the sensitized electrodes coated with quantum dots. A hotmelt gasket of 60 um thickness made of the ionomer Surlyn (DuPont) was placed between the two electrodes. The assembled solar cell was sealed by heating at about 100°C for 10 minutes. The seal controls the electrolyte film thickness and avoids short-circuiting of the cell. The active area of the cells was determined by the area of the hotmelt gasket.

[0059] The electrolyte and sealant were then injected. The electrolyte comprises a solution of a sulfide salt, sulfur, and an ionic conductor in in a mixture of water and an alcohol. The electrolyte solution comprised a solution of 1M Na₂S, 0.1M S, 0.2M KC1, in a mixture of pure water and methanol (volume ratio: 1 : 1 ). The electrolyte was introduced via a capillary using two holes in the back of the counter electrode. The holes were sealed with an epoxy.

[0060] The results, shown below in TABLE II demonstrate the increase in short-circuit current density (Jsc) due to the addition of multi walled carbon nanotube one-dimensional conductors to the Ti0₂ layer. The results, further demonstrate the increase in FF and Rsh due to the thin MgO layer that may inhibit the recombination of electrons and holes at the quantum dot/Ti0₂-MCNT interface.

TABLE II

[0061] The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention.

Claims

We Claim:

1. A method comprising:

forming a Ti0₂ patterned layer comprising Ti0₂ (104) and nanotubes and/or nanowires (106) on a conductive substrate electrode (102, 230);

forming quantum dots (1 10) on the Ti0₂ patterned layer;

forming a counter electrode (210);

assembling the substrate electrode having the Ti0₂ layer comprising TiO₂ and nanotubes and/or nanowires and quantum dots (235), and counter electrode into a sandwich with a gasket (220) disposed between the surface of the substrate electrode having the Ti0₂ layer comprising nanotubes and/or nanowires and quantum dots coated thereon and the counter electrode; and

injecting electrolyte and sealant (240) between the substrate electrode and counter electrode to form a solar cell.

2. The method of claim 1 or 2 wherein the conductive substrate electrode and counter electrodes are flexible electrodes.

3. The method of claims 1 or 2 wherein the nanotubes and/or nanowires are hollow carbon nanotubes and have an outer diameter of from about 10 ran to about 50 nm and a length of from about 1 μηι to about 12 μηι.

4. The method of any of claims 1 to 3 wherein an insulating layer (108) is formed between the the Ti0₂ layer comprising Ti0₂ and nanotubes and/or nanowires and the quantum dot layer.

5. A solar cell comprising:

a conductive substrate electrode (102, 230) having coated thereon, a patterned electroconductive Ti0₂ layer comprising Ti0₂ (104) and nanotubes and/or nanowires (106)

quantum dots (1 10) disposed on the surface of the Ti0₂ layer;

a counter electrode (210); a gasket (220) disposed between the surface of the substrate electrode having the Ti0₂ layer comprising nanotubes and/or nanowires and quantum dots (235) and the counter electrode (210); and

an electrolyte and sealer (240) disposed between the substrate electrode and the counter electrode.

6. The solar cell of claim 5 wherein the substrate electrode and counter electrode are formed of a flexible polymer.

7. The solar cell of claim 5 or 6 wherein the nanotubes and/or nanowires are hollow carbon nanotubes and have an outer outer diameter of from about 10 nm to about 50 nm and a length of from about 1 μιη to about 12 μι^η.

8. The solar cell of any of claims 5 to 7 comprising an insulating layer (108) between the nanotube or nanowire containing Ti0₂ layer and the quantum dot layer.

9. An electroconductive layer comprising a conductive substrate electrode (102, 230) having a patterned Ti0₂ layer comprising Ti0₂ (104) and nanotubes and/or nanowires (106).

10. The electroconductive layer of claim 9 further comprising an insulating layer (108) positioned above the Ti0₂ layer comprising Ti0₂ and nanotubes and/or nanowires.