Patent Application
20110315201
The entire contents of Taiwan Patent Application No. 099120910, filed on Jun. 25, 2010, from which this application claims priority, are incorporated herein by reference.
1. Field of the Invention
The present invention generally relates to high efficiency solar cells and methods for fabricating their heterojunctions.
2. Description of Related Art
In the field of solar cell, the polymer/semiconductor heterojunction attracts lots of attention due to several advantages including free concern of lattice mismatch, large-area coverage, low-temperature process-capability, easy preparation, low cost, and so on. To date, researchers have demonstrated planar polymer/semiconductor heterojunction solar cells based on crystalline or amorphous silicon such as Poly-(CH3)3Si-Cyclooctatetraene/n-Si, tetraphenylporphyrin/n-Si, 4-tricyanovinyl-N,N-diethylaniline/p-Si, poly(3-hexylthiophene)/a-Si, poly(3,4-ethylenedioxy thiophene): poly(styrenesulfonate)/n-Si, polyaniline/n-Si, and phthalocyanine/n-Si, etc. However, with a planar polymer/silicon heterojunction, only the photogenerated electron-hole pairs (EHPs) near the junction will be separated and collected into electrical contacts. Other photogenerated EHPs produced in the polymer and the silicon will be mostly lost as a result of recombination. This limits the power conversion efficiency (PCE) of the polymer/silicon heterojunction solar cells.
To augment the EHPs separating and collecting, silicon nanowire (SiNW) structures are used in solar cells to enormously increase the p-n junction area and shorten the carrier diffusion distance. In addition, the nanowire structure significantly reduces the reflection and induces strong light trapping between nanowires, resulting in strong absorption. Mostly, to form SiNW p-n junction structures, a thin amorphous or nanocrystal silicon layer was deposited on the SiNW surface by chemical vapor deposition. The solar cells having so produced heterojunction show a spectrally broad incident photo-to-current conversion efficiency (IPCE) in the visible region but low IPCE in the near infrared region. High series and low shunt resistances appear to limit the power conversion efficiency (PCE) of these cells to below 1%.
Therefore, it would be advantageous to provide solar cells and their producing methods for promoting the incident photo-to-current conversion efficiency (IPCE).
An object of this invention is to provide solar cells and method for producing their heterojunctions, as well as promoting the incident photo-to-current conversion efficiency (IPCE), and reducing the material and fabricating cost.
Accordingly, one embodiment of this invention provides a method for fabricating heterojunctions of a solar cell, comprising: providing a semiconductor substrate; forming a plurality of semiconductor nanostructures on the semiconductor substrate; and adhering a conducting polymer on each semiconductor nanostructure by capillary effect, and thus forming a plurality of semiconductor nanostructure/conducting polymer heterojunctions. The heterojunctions are employed to produce a solar cell.
Accordingly, one embodiment of this invention provides a solar cell, at least comprising a plurality of heterojunctions, wherein each heterojunction comprises a semiconductor nanostructure and a conducting polymer in a form of core-sheath, and the maximum incident photo-to-current conversion efficiency (IPCE) of the solar cell is equal to or more than 30%.
In an embodiment, the hole-conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is coated on the silicon nanowires (SiNWs) to replace the conventional p-doped amorphous or nanocrystal silicon layer and thus form a solar cell having SiNW/PEDOT heterojunctions. The highest occupied molecular orbital (HOMO) energy of PEDOT is about 5.1 eV, which is similar to the valence band energy of silicon. Thus, the interface between the PEDOT layer and n-type SiNWs could possibly form good heterojunctions for electron-hole pairs (EHPs) separation.
According to the embodiment, the current-to-voltage (J-V) characteristics of the SiNW/PEDOT solar cell clearly reveal a stable rectifying diode behavior. In addition, the heterojunctions can greatly increase the exciton dissociation probability and induce light trapping effect, leading to enhanced IPCE in the near infrared region. The power conversion efficiency (PCE) is also greatly improved as compared with the solar cells without nanowire/polymer heterojunctions.
According to an embodiment of this invention, a same semiconductor substrate can be repeatedly used to form nanostructures, greatly reducing the material cost. In addition, the fabricating steps may preferably use solution process to reduce the fabricating cost.
a)-2(d) show the scanning electron microscopy (SEM) images and transmission electron microscopy (TEM) images of the SiNWs array and the SiNW/PEDOT heterojunctions made by the embodiment shown in
a) and
Reference will now be made in detail to specific embodiments of the invention. Examples of these embodiments are illustrated in accompanying drawings. While the invention will be described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to these embodiments. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. The present invention may be practiced without some or all of these specific details. In other instances, well-known process operations and components are not described in detail in order not to unnecessarily obscure the present invention. While drawings are illustrated in detail, it is appreciated that the quantity of the disclosed components may be greater or less than that disclosed, except where expressly restricting the amount of the components. Wherever possible, the same or similar reference numbers are used in drawings and the description to refer to the same or like parts.
This example employs a metal-assisted chemical etching method (an electroless etching method) to produce silicon nanowires (SiNWs). Detail of the metal-assisted chemical etching method is described in U.S. application Ser. No. 12/713,094, entitled “Silicon substrate having nanostructures and method for producing the same and application thereof,” the entire contents of which are incorporated herein by reference.
First, an n-type 1-10 ohm-cm Si (100) wafer is provided to produce a SiNWs array in an aqueous solution of silver nitrate (AgNO3) and hydrofluoric acid (HF) at room temperature. The concentrations of AgNO3 and HF in solution are 0.023 and 5.6 molL−1, respectively. After etching, the SiNWs array is immersed in a bath containing concentrated nitric acid to remove all Ag dendritic structures from the nanowire surfaces. Finally, the SiNWs array is immersed in a buffer oxidation etchant (BOE) to remove the oxide layer on the SiNWs and form H-terminated bonds (hydrogen bonds) on the silicon surface. The SiNWs array prepared by this method is aligned vertically over the area up to the wafer size, and its density may be equal to or more than 20 wires/μm2, i.e., there are equal to or more than 20 silicon nanowires (or pieces) distributed per square of micrometer. Further, an aluminum layer is evaporated on the back side (the side without SiNWs) of the silicon substrate as the electrical contact. Besides, silicon nanowires could also be produced by a dry etching method.
In this example, poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate), referred to as “PEDOT” or “PEDOT:PSS” in this text, is used to form a SiNW/PEDOT heterojunctions solar cell.
After the SiNWs array is formed in the foregoing example, take PEDOT gel particles with a mean diameter of 80 nm to disperse it in water, and a PEDOT aqueous solution is formed. Because the surface of SiNWs with H-terminated bonds is hydrophobic after being immersed in the buffer oxidation etchant (BOE), it should be modified to hydrophilic, such that PEDOT can adhere to the SiNW surface and form SiNW/PEDOT heterojunction structures. To form a hydrophilic surface, the SiNWs array is placed in an environment with controlled relative humility of 60% and temperature of 25° C. for 2 hours, to form a very thin native oxide layer on the SiNW surface. By doing so, the thin native oxide layer is hydrophilic and has a contact angle below 20°. Then, instead of being directly spin-coated on the SiNWs array, the PEDOT aqueous solution is spin-coated on the ITO-coated glass and form a wet PEDOT film on it. The thickness of the wet PEDOT film is about 9 μm (the thickness of the dry PEDOT film is about 200 nm). Before the PEDOT film dried, the top portion of the SiNWs array is immersed in the wet PEDOT thin film.
a)-2(d) show the scanning electron microscopy (SEM) images and transmission electron microscopy (TEM) images of the SiNWs array and the SiNW/PEDOT heterojunctions made by the foregoing example.
Where,
The increase of the short-circuit current density (Jsc) is due to two major reasons. First, the diffusion distance of carriers from the core of SiNW to SiNW/PEDOT heterojunction is only tens of nanometers or below. In contrast, the distance may be several micrometers for the planar solar cell. Thus, the electron-hole pairs (EHP) separation and collection efficiency is greatly enhanced in the SiNW/PEDOT solar cell. Second, the reflectance of the SiNW arrays reduces to below 5% over the spectral range of 400-1100 nm as compared with the planar silicon surface having reflectance over 30%. In addition, light trapping by the SiNW arrays increases the light absorption of SiNWs. These lead to the enhancement of photocurrent.
In the inset of
The architecture of the SiNW/PEDOT solar cell takes advantage of charge carrier transport in the nanowire structures. In the foregoing example of this invention, a planar ITO thin film is used to contact the top of the SiNWs via PEDOT as the front contact(s) instead of conventional metal fingers. In conventional planar solar cells, charge carriers in the emitter will transport near the front surface to the metal fingers. Therefore, if the front surface of the solar cells forms nanostructures, the charge carriers in the emitter will transport a much longer distance than that in the planar solar cells. This may increase the series resistance. The SiNW/PEDOT solar cell architecture proposed by the foregoing example means that the charge carriers separated in the SiNWs could be immediately collected to the ITO electrode instead of diffusing a long distance through SiNW arrays as shown in
a) and
The SiNW/PEDOT solar cell harvests photons from 400 nm to 1100 nm, and gives a maximum IPCE of 32% at 700 nm. In contrast, for planar cell, the maximum IPCE is only about 1.94% occurring at 682 nm, as shown in the inset of
A person skilled in the art recognizes that modifications and alternatives may be made in the foregoing examples of this invention. Another embodiment of this invention provides a method of producing a solar cell with heterojunctions. The method comprises the steps of: (1) provide a semiconductor substrate; (2) form a plurality of semiconductor nanostructures on the semiconductor substrate; (3) adhere a conducting polymer on each semiconductor nanostructure by capillary effect, and thus form a plurality of semiconductor nanostructure/conducting polymer heterojunctions; and (4) employ the semiconductor nanostructure/conducting polymer heterojunctions to produce a solar cell.
The above-mentioned semiconductor nanostructures may include silicon nanowires, germanium nanowires, III-V compound nanowires, II-VI compound nanowires, and so on. The foregoing metal-assisted chemical etching method may be employed to etch the semiconductor substrate and thus form the semiconductor nanostructures. Alternatively, other known vapor-phase epitaxy or liquid-phase epitaxy methods may be used to produce the semiconductor nanostructures. The conducting polymer may include [poly(3,4-ethylenedioxy thiophene): poly(styrene sulfonate)]; (PEDOT:PSS), (Poly(3-hexylthiophene); P3HT), (6,6-phenyl-C61-butyric acid methyl ester; PCBM), polyaniline, phthalocyanine, Poly-(CH3)3Si-Cyclooctatetraene, tetraphenylporphyrin, 4-tricyanovinyl-N,N-diethylaniline, and the like.
In addition, method to adhere the conducting polymer to the semiconductor nanostructures is exemplarily summarized as follows. First, dissolve the conducting polymer in a solvent that is water or an organic solvent, forming a conducting polymer solution. If the solvent is the organic solvent such as acetone, methanol, or isopropanol, then modify the surface of the semiconductor nanostructures to be hydrophobic; if the solvent is water, then modify the surface of the semiconductor nanostructures to be hydrophilic. Then, the top portions of the semiconductor nanostructures are inserted into the conducting polymer solution, which then adhere to the surface of the semiconductor nanostructures by capillary effect. For example, the conducting polymer solution is coated on a transparent conducting substrate or a transparent electrode of a transparent substrate, and then the top portions of the semiconductor nanostructure are inserted into the conducting polymer solution, which is wet and moveable at that time. Afterward, heat the conducting polymer solution to make it solid, and thus the conductivity of which is promoted.
The foregoing transparent conducting substrate or transparent electrode may comprise indium tin oxide (ITO), and the transparent substrate may be glass substrate, plastic substrate, quartz substrate, and so on. Further, spin coating or dip coating method may be employed for coating the conducting polymer solution on the transparent conducting substrate or the transparent electrode.
According to embodiments of this invention, the diffusion distance of charge carriers is significantly reduced, reducing the probability of lattice colliding with the carriers and thus increasing the hot carrier effect. Therefore, the output voltage, i.e., the open circuit voltage (Voc), of the semiconductor nanostructure/conducting polymer heterojunctions solar cell will be higher than planar cells, and gives a maximum IPCE of 32% at 700 nm. In addition, if the semiconductor nanostructures are detached from the semiconductor substrate by further employing the side-etching and transferring method described above and then employ it to produce a solar cell, not only the material cost but also the thickness of light absorbing layer can be greatly reduced. In addition to employ the above advantageous processes, an embodiment of this invention made a hot carrier solar cell having a maximum IPCE equal to or more than 40% or 50%, by further properly selecting the difference between the work function of the two electrical contacts (electrodes) and cooperating with other suitable materials.
Although specific embodiments have been illustrated and described, it will be appreciated by those skilled in the art that various modifications may be made without departing from the scope of the present invention, which is intended to be limited solely by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 099120910 | Jun 2010 | TW | national |
