Patent Application
20080066802
The invention relates to the use of carbon nanotubes and photoactive nanoparticles, including nanoparticles of different size and composition, to form photovoltaic devices.
Increasing oil prices have heightened the importance of developing cost effective renewable energy. Significant efforts are underway around the world to develop cost effective solar cells to harvest solar energy. Current solar energy technologies can be broadly categorized as crystalline silicon and thin film technologies. More than 90% of the solar cells are made from silicon—single crystal silicon, polycrystalline silicon or amorphous silicon.
Historically, crystalline silicon (c-Si) has been used as the light-absorbing semiconductor in most solar cells, even though it is a relatively poor absorber of light and requires a considerable thickness (several hundred microns) of material. Nevertheless, it has proved convenient because it yields stable solar cells with good efficiencies (12-20%, half to two-thirds of the theoretical maximum) and uses process technology developed from the knowledge base of the microelectronics industry.
Two types of crystalline silicon are used in the industry. The first is monocrystalline, produced by slicing wafers (approximately 150 mm diameter and 350 microns thick) from a high-purity single crystal boule. The second is multicrystalline silicon, made by sawing a cast block of silicon first into bars and then wafers. The main trend in crystalline silicon cell manufacture is toward multicrystalline technology. For both mono- and multicrystalline Si, a semiconductor p-n junction is formed by diffusing phosphorus (an n-type dopant) into the top surface of the boron doped (p-type) Si wafer. Screen-printed contacts are applied to the front and rear of the cell, with the front contact pattern specially designed to allow maximum light exposure of the Si material with minimum electrical (resistive) losses in the cell.
Silicon solar cells are very expensive. Manufacturing is mature and not amenable for significant cost reduction. Silicon is not an ideal material for use in solar cells as it primarily absorbs in the visible region of the solar spectrum thereby limiting the conversion efficiency.
Second generation solar cell technology is based on thin films. Two main thin film technologies are amorphous silicon and CIGS.
Amorphous silicon (a-Si) was viewed as the “only” thin film PV material in the 1980s. But by the end of that decade, and in the early 1990s, it was dismissed by many observers for its low efficiencies and instability. However, amorphous silicon technology has made good progress toward developing a very sophisticated solution to these problems: multijunction configurations. Now, commercial, multijunction a-Si modules could be in the 7%-9% efficiency range. United Solar Systems Corporation and Kanarka plan have built 25-MW manufacturing facilities and several companies have announced plans to build manufacturing plants in Japan and Germany. BP Solar and United Solar Systems Corporation plan to build 10 MW facilities in the near future.
The key obstacles to a-Si technology are low efficiencies (about 11% stable), light-induced efficiency degradation (which requires more complicated cell designs such as multiple junctions), and process costs (fabrication methods are vacuum-based and fairly slow). All of these issues are important to the potential of manufacturing cost-effective a-Si modules.
Thin film solar cells made from Copper Indium Gallium Diselenide (CIGS) absorbers show promise in achieving high conversion efficiencies of 10-12%. The record high efficiency of CIGS solar cells (19.2% NREL) is by far the highest compared with those achieved by other thin film technologies such as Cadmium Telluride (CdTe) or amorphous Silicon (a-Si).
These record breaking small area devices have been fabricated using vacuum evaporation techniques which are capital intensive and quite costly. It is very challenging to fabricate CIGS films of uniform composition on large area substrates. This limitation also affects the process yield, which are generally quite low. Because of these limitations, implementation of evaporation techniques has not been successful for large-scale, low-cost commercial production of thin film solar cells and modules and is non-competitive with today's crystalline silicon solar modules.
To overcome the limitations of the physical vapor deposition techniques that use expensive vacuum equipment, several companies have been developing high throughput vacuum processes (ex: DayStar, Global Solar) and non-vacuum processes (ex: ISET, Nanosolar) for the fabrication of CIGS solar cells. Using ink technology, very high active materials utilization can be achieved with relatively low capital equipment costs. The combined effect is a low-cost manufacturing process for thin film solar devices. CIGS can be made on flexible substrates making it possible to reduce the weight of solar cells. Cost of CIGS solar cells is expected to be lower than crystalline silicon making them competitive even at lower efficiencies. Two main problems with CIGS solar cells are: (1) there is no clear pathway to higher efficiency and (2) high processing temperatures make it difficult to use high speed roll to roll process and hence they will not be able to achieve significantly lower cost structure.
These are significant problems with the currently available technologies. Crystalline silicon solar cells which have >90% market share today are very expensive. Solar energy with c-silicon solar cells costs about 25 cents per kwh as compared to less than 10 cents per kwh for fossil fuels. In addition, the capital cost of installing solar panels is extremely high limiting its adoption rate. Crystalline solar cell technology is mature and unlikely to improve performance or cost competitiveness in near future. Amorphous silicon thin film technology is amenable to high volume manufacturing that could lead to low cost solar cells. In addition, amorphous and microcrystal silicon solar cells absorb only in the visible region.
Next generation solar cells are required to truly achieve high efficiencies with light weight and low cost. Two potential candidates are (1) polymer solar cells and (2) nanoparticle solar cells. Polymer solar cells have the potential to be low cost due to roll to roll processing at moderate temperatures (<150 C). However, polymers suffer from two main drawbacks: (1) poor efficiencies due to slow charge transport and (2) poor stability- especially to UV radiation. Hence it is unlikely that polymer solar cells will be able to achieve the required performance to become the next generation solar cell. The most promising technology for the next generation solar cell is based on quantum dot nanoparticles.
Several research groups have been conducting experimental studies on quantum dot based solar cells. Most commonly used quantum dots are made of compound semiconductors such as Group II-VI, II-IV and III-V. Some examples of these photosensitive quantum dots are CdSe, CdTe, PbSe, PbS, ZnSe.
Solar cells made from photosensitive nanoparticles as described in the art show very low efficiencies (<5%). Nanoparticles are very efficient in generating electron hole charge pairs when exposed to sunlight. The primary reason for these low efficiencies is charge recombination. To achieve high efficiencies in a solar cell the charges must be separated as soon as possible after they are generated. Charges that recombine do not produce any photocurrent and hence do not contribute towards solar cell efficiency. Charge recombination in nanoparticles is primarily due to two factors: (1) surface states on nanoparticle that facilitate charge recombination, and (2) slow charge transport. In the later case, charge recombination is generally faster compared to the charge transport rate because charges travel slowly through the electron transport and hole transport layers.
Various methods have been reported in the prior art to solve these problems of nanoparticles. Surface treatment techniques have been tried to remove surface states. (See Furis et al, MRS Proceedings, volume 784, 2004) Such techniques show improvement in photoluminescence but do not improve solar conversion efficiency as they do not impact the charge transport properties of hole transport and electron transport layers.
It is known in the art that TiO2 layers can be used to rapidly transport electrons. Dye-sensitized solar cells use TiO2 precisely for this reason. Transparent TiO2 nanotubes have been reported in the literature (Mor et al., Adv. Funct. Mater., 2005, 15, 1291-1296 (2005)). These TiO2 nanotubes have been used to prepare dye-sensitized solar cells.
Single wall carbon nanotubes (SWCNT) have been used as light absorbing material in solar cells. In addition, nanoparticles such as CdSe and CuInS have been covalently attached to carbon nanotubes. See Landi et al., Mater. Res. Symp. Proc. Vol. 836, 2005, Session L2.8 pages 1-6.
The photvoltaic devises include first and second electrodes at least one of which is transparent to solar radiation. A photoactive layer between the first and second electrodes contains photoactive nanostructures comprising carbon nanotubes (CNT) and photosensitive nanoparticles. The nanoparticles are closely associated with the carbon nanotubes and in some embodiments are covalently attached to the CNT. The photoactive layer is in electron conducting communication with the first electrode and in hole conducting communication with the second electrode. In some embodiments the photoactive layer further comprises a conducting polymer.
In other embodiments, the photovoltaic device further includes a hole conducting layer between the first electrode and the photoactive layer that facilitates hole transfer to the first electrode. In a preferred embodiment, the hole conducting layer contains p-type CNTs.
In the same or other embodiments, an electron conducting layer is positioned between the second electrode and the photoactive layer to facilitate electron transfer to the second electrode. In a preferred embodiment, the electron conducting layer contains n-type CNTs.
The carbon nanotube is preferably a single wall carbon nanotube (SWCNT). The SWCNT is preferably functionalized so as to be chemically reactive with the photosensitive nanoparticles of photosensitive nanoparticles that have been modified to contain functional groups that are reactive with the CNT/SWCNT or a moiety used to link the CNT/SWCNT photosensitive nanoparticle.
The photosensitive nanoparticles can be quantum dots, nanorods, nanobipods, nanotripods, nanomultipods or nanowires. Preferred photosensitive nanoparticles include CdSe, ZnSe, PbSe, InP, PbS, ZnS, Si, Ge, SiGe, CdTe, CdHgTe, or Group II-VI, II-IV or III-V materials. In some embodiments first and second nanoparticle that adsorb radiation from different portions of the solar spectrum are used in the photovoltaic device. The first and second nanoparticles can differ in composition, size or a combination of size and composition and absorb in different portions of the solar spectrum. The first and second can be nanoparticles contained or the same or different CNTs. For example two different photosensitive nanoparticles can each be associated with a single CNT. Alternatively, a first nanoparticle can be associated with a first CNT and a second nanoparticle with a second CNT. In either case a single photoactive layer can be made for such photoactive nanostructures.
The components used in the photovoltaic device are chosen so that appropriate band alignment exists between the photoactive nanostructure and the electrodes. When a conducting polymer is used in the photoactive layer, the HOMO and LUMO levels the conducting polymer are such that charge transfer is facilitated from the nanostructure to the conducting polymer and from conducting polymer to the electrode. Similarly, appropriate band alignment should exist between the photoactive layer and any electron or hole conducting layer used in the devices to facilitate charge extraction and charge transfer.
In another embodiment, a second photoactive layer is used that contains second photoactive nanostructures made of carbon nanotubes and nanoparticles that absorb radiation from different portions of the solar spectrum as compared to the nanoparticles of the first photoactive layer. The nanoparticles in the first and said second photoactive layer can differ in composition, size or a combination of size and composition.
In some embodiments, the hole conducting layer is a hole conducting polymer such as a p-type semiconducting polymer. Examples of p-type semiconducting polymers include P3HT, P3OT, MEH-PPV or PEDOT. In most embodiments, PVK is not used as a hole conducting polymer. In other embodiments, the hole conducting layer is a p-type semiconductor. Examples of p-type semiconductor include p-doped Si, p-doped Ge or p-doped SiGe. In the case of Si the p-type semiconductor can be p-doped amorphous silicon, p-doped microcrystalline silicon or p-doped nanocrystalline silicon. In some cases the hole conducting layer is made of two or more layers of p-type semiconductor. The p-type semiconductor layers can be a p-doped silicon layer, a p-doped germanium layer and/or a p-doped SiGe layer.
In a preferred embodiment the hole conducting layer contains CNTs, preferably SWCNTs. For example, SWCNTs can be combined with p-type P3HT and used as a hole conducting layer.
In some embodiments, the electron conducting layer is an electron conducting material such as aluminum quinolate (AlQ3) and/or n-type SWCNTs made by doping SWCNTs with Cl2, Br2 or Cs.
An embodiment of the photovoltaic device disclosed herein is made from two electrodes and a photoactive layer comprising photoactive nanostructures. The photoactive nanostructures contain at least two components: (1) CNTs and/or SWCNTs and (2) photosensitive nanoparticles. The nanoparticles associate with the surface of the CNT by self assembly and cover at least 10% of the CNT's exterior surface although lighter particle densities, such as 50%, 70% or 90%, can be used. In preferred embodiments, the nanoparticles form a monolayer covering most of the CNT surface.
In a preferred embodiment, the nanoparticle is covalently attached to the CNT. This can be achieved by modifying the CNT and/or nanoparticles to contain a moiety/moieties that provide reactive sites for covalent linkage. In some instances (discussed below) a linker molecule is used to covalently attach the nanoparticle to the CNT.
As used herein, the term “nanoparticle” or “photosensitive nanoparticle” refers to photosensitive materials that generate electron hole pairs when exposed to solar radiation. Photosensitive nanoparticles are generally nanocrystals such as quantum dots, nanorods, nanobipods, nanotripods, nanomultipods, or nanowires.
Photosensitive nanoparticles can be made from compound semiconductors which include Group II-VI, II-IV and III-V materials. Some examples of photosensitive nanoparticles are CdSe, ZnSe, PbSe, InP, PbS, ZnS, CdTe Si, Ge, SiGe, CdTe, CdHgTe, and Group II-VI, II-IV and III-V materials. Photosensitive nanoparticles can be core type or core-shell type. In a core shell nanoparticle, the core and shell are made from different materials. Both core and shell can be made from compound semiconductors.
Quantum dots are a preferred nanoparticle. As in known in the art, quantum dots having the same composition but having different diameters absorb and emit radiation at different wave lengths.
The photoactive nanoparticle can be modified to contain a linker Xa—Rn—Yb where X and Y can be reactive moieties such as carboxylic acid groups, phosphonic acid groups, sulfonic acid groups, amine containing groups etc., a and b are independently 0 or 1 where at least one of a and b is 1, R is a carbon, nitrogen, sulfur and/or oxygen containing group such as —CH2, —NH—, —S— and/or —O—, and n is 0-10. One reactive moiety can react with the nanoparticle while the other can react with the CNT. The linkers also passivate the nanoparticles and increase their stability, light absorption and photoluminescence. They can also improve the nanoparticle solubility or suspension in common organic solvents.
Functionalized nanoparticles are reacted with suitable reactive groups such as hydroxyl or others on the CNTs to deposit a monolayer of dense continuous nanoparticles by a molecular self assembly process. By adjusting the components of Xa—Rn—Yb, the distance between the surface of the CNT and nanoparticle can be adjusted to minimize the effect of surface states in facilitating charge recombination. The distance between these surfaces is typically 10 Angstroms or less preferably 5 Angstroms or less. This distance is maintained so that electrons tunnel through this gap from the nanoparticles to the highly conducting CNTs. This facile electron transport helps in reducing charge recombination and results in efficient charge separation which leads to efficient solar energy conversion.
As used herein a “hole conducting layer” is a layer that preferentially conducts holes. Hole transporting layers can be made from (1) inorganic molecules including p-doped semiconducting materials such as p-type amorphous or microcrystalline silicon or germanium; (2) organic molecules such as metal-thalocyanines, aryl amines etc.; (3) conducting polymers such as polyethylenethioxythiophene (PEDOT), P3HT, P3OT and MEH-PPV; and (4) p-type CNTs or p-type SWCNTs.
As used herein an “electron conducting layer” is a layer that preferentially conducts electrons. Electron transporting layers can be made from aluminum quinolate (AlQ3) and/or n-type CNTs or n-type SWCNTs.
In some embodiments, the solar cell is a broadband solar cell that is capable of absorbing solar radiation at different wave lengths. Photosensitive nanoparticles generate electron-hole pairs when exposed to light of a specific wave length. The band gap of the photosensitive nanoparticles can be adjusted by varying the particle size or the composition of the nanoparticles. By combining a range of nanoparticle sizes and a range of the nanomaterials used to make the nanoparticles, broadband absorption over portions of or the entire solar spectrum can be achieved. Thus, in one embodiment, a mixture of photosensitive nanoparticles having a different size and/or composition can be layered on to the same or different CNTS to make broadband solar devices such as that set forth in
Photosensitive nanoparticles can be made from Group IV, II-IV, II-VI, III-V materials. Examples of photosensitive nanoparticles include Si, Ge, CdSe, PbSe, ZnSe, CdTe, CdS, PbS. Nanoparticle sizes can be varied (for example: 2-10 nm) to obtain a range of bandgaps. These nanoparticles can be prepared by following the methods well known in the art. Nanoparticles can also be functionalized by following the methods well known in the art. Functional groups can include carboxylic (—COOH), amine (—NH2), Phosphonate (—PO4), Sulfonate (—HSO3), Aminoethanethiol, etc. Carbon nanotubes can be prepared by following methods well known in the art. See, e.g., Landi et al., supra. They can also be purchased from Cheap Tubes Battleboro, Vt. or Aldrich. Carbon nanotubes are preferably single wall carbon nanotubes
Carbon nanotubes can be functionalized by following the methods well known in the art. See, e.g., Landi et al., supra. And Cho et al., Advanced Materials, 19, 232-236 (2007). Functionalized carbon nanotubes are soluble in common organic solvents such as chloroform. Functionalized carbon nanotubes can be reacted with functionalized photosensitive nanoparticles with appropriate functional groups dissolved in suitable solvent to prepare photosensitive nanoparticle sensitized carbon nanotubes. The density of the nanoparticle layer can be adjusted by varying the reaction conditions and by varying functional groups. Ideally a carbon nanotube densely decorated with photosensitive nanoparticles is desired. A layer of photosensitive nanoparticle sensitized carbon nanotubes can be deposited on ITO coated glass substrate by spin coating or other well known molecular self assembly techniques. This layer can be one monolayer or multiple monolayers. A solar cell built according this embodiment is expected to have high efficiency. In this device electron hole pairs are generated when sunlight is absorbed by the nanoparticles and the resulting electrons are rapidly transported by the carbon nanotubes to the cathode for collection. This rapid removal of electrons from the electron-hole pairs generated by the nanoparticles reduces the probability of electron-hole recombination commonly observed in nanoparticle based solar cell devices.
Another embodiment is shown in
EXAMPLE 3
Another embodiment using photosensitive nanoparticle sensitized single wall carbon nanotubes (SWCNT) is shown in
In a version of this embodiment shown in
In another embodiment, shown in
The SWCNT used for layers 1130 and 1150 can be optionally functionalized to enable its dissolution in suitable organic solvents and to enhance its adhesion to the other layers. SWCNT and nanoparticle deposition can be done by spin coating or other molecular self assembly methods well known in the art. The SWCNT layers used in this embodiment are expected to improve efficiency. SWCNT layer 1130 can be made from a p-type SWCNT. SWCNT layer 1150 can be made from an n-type SWCNT.
In a version of this embodiment shown in
In another embodiment shown in
In a version of this embodiment shown in
In another embodiment, shown in
Photosensitive nanoparticles can be made from Group IV, II-IV, II-VI, III-V materials. Photosensitive nanoparticles include Si, Ge, CdSe, PbSe, ZnSe, Cdje, CdS, PbS. One or more of these materials can be used to make the nanoparticles. Photosensitive nanoparticle sizes can range from 2-10 nm to obtain a range of bandgaps. Functionalized nanoparticles and functionalized SWCNT can be made using the methods described in Example 1.
For example, functionalized SWCNTs can be reacted with an appropriate mixture of functionalized photosensitive nanoparticles dissolved in suitable solvent to prepare photoactive nanostructures containing SWCNTs with multiple different photosensitive nanoparticles 1560, 1570 and 1580 attached as shown in
The solar cell shown in
In another version of this embodiment shown in
In another embodiment shown in
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The solar cell shown in
In
In another version of this embodiment shown in
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/785,651, filed on Mar. 23, 2006, under 35 U.S.C. §119(e), which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60785651 | Mar 2006 | US |
