This invention relates to a solar cell and more particularly to solar cells with improved light absorption capacity.
Background of the invention
Though common materials used for photovoltaics (i.e., the conversion of sunlight into electrical energy) are inorganic, there has been a tremendous effort to develop organic solar cells within the last three decades. The field started by the application of small organic molecules (pigments) and since the development of semiconducting polymers, these materials were incorporated into organic solar cells resulting in remarkable improvements within the past years. The potential of semiconducting organic materials to transport electric current and to absorb light in the ultraviolet (UV)-visible part of the solar spectrum is due to the sp2-hybridization of carbon atoms. For example, in conducting polymers the electron in the pz-orbital of each sp2-hybridized carbon atom will form IT bonds with neighboring pz electrons in a linear chain of sp2-hybridized carbon atoms, which leads then to dimerization (an alternating single and double bond structure, i.e., Peierls distortion). Due to the isomeric effect, these TT electrons are of a delocalized nature, resulting in high electronic polarizability.
An important difference to inorganic solid-state semiconductors lies in the generally poor (orders of magnitudes lower) charge-carrier mobility in these materials, which has a large effect on the design and efficiency of organic semiconductor devices. However, organic semiconductors have relatively strong absorption coefficients, which partly balances low mobilities, giving high absorption in even <100 nm thin devices. Another important difference to crystalline, inorganic semiconductors is the relatively small diffusion length of primary photoexcitations (called excitons) in these rather amorphous and disordered organic materials. These excitons are an important intermediate in the
solar energy conversion process, and usually strong electric fields are required to dissociate them into free charge carriers, which are the desired final products for photovoltaic conversion. This is a consequence of exciton binding energies usually exceeding those of inorganic semiconductors. These features of organic semiconducting materials lead generally to devices with very small layer thicknesses of the order 100 nm. Most of the organic semiconductors are hole conductors and have an optical band gap around 2 eV, which is considerably higher than that of silicon and thus limits the harvesting of the solar spectrum to a great extent.
Nevertheless, the chemical flexibility for modifications on organic semiconductors via chemical synthesis methods as well as the perspective of low cost, large-scale production drives the research in this field in academia and industry.
The first generation of organic photovoltaic solar cells was based on single organic layers sandwiched between two metal electrodes of different work functions. The rectifying behavior of single layer devices was attributed to the asymmetry in the electron and hole injection into the molecular TT * and TT -orbitals, respectively, and to the formation of a Schottky-barrier, between the p-type (hole conducting). Power conversion efficiencies reported were generally poor (of the order of 10~2%), but reached remarkable 0.7% for merocyanine dyes in the early days. In this case, the organic layer was sandwiched between a metal-metal oxide and a metal electrode, thus enhancing the Schottky-barrier effect [metal-insulator-semiconductor (MIS) devices]. A typical organic solar cell is depicted in Figure 1.
So-called Schottky barrier or P-N junction photocells rely upon the fact that a built-in potential exists at the metal/semiconductor interface as in the Schottky device or at the junction between the P-type and N-type semiconductors as in the P-N junction device. Electron-hole pairs generated by the absorption of light in
the semiconductor are separated due to the built-in field at the interface, establishing an electrical potential.
Among chief materials used in the past for solar cells have been inorganic semiconductors, due to their fairly high conversion efficiencies which have been as high as 12 to 35 percent, for example, for amorphous silicon is around 11-19%, single crystalline siliocon is around 23-25%, CdTe around 15-16%. However, such devices have proven to be very expensive to construct, due to the melt and other processing techniques necessary to fabricate the semiconductor layer. As a result, such devices have had extensive practical utility only in the field of space exploration, and in terrestrial applications where subsidies from government or other sources are available for their use.
In an effort to reduce the cost of solar cells, organic photoconductors and semiconductors have been considered, due to their inexpensive formation by solvent coating and similar techniques.
U.S. Pat. No. 4,125,414 discloses elements comprising an organic photoconductive layer which includes pyrylium-type dyes together with a binder and a photoconductor. A preferred method of making such a composition features the formation of a discrete discontinuous phase is a continuous phase. A very thin nucleating layer of copper phthalocyanine can also be used with this photoconductive layer, but it does not form a rectifying junction.
Carbon nanotubes have attracted a great deal of attention in recent years due to their possibilities for use as nanoscale electronic devices, such as diodes, transistors and semiconductor circuits.
Carbon nanotubes that exhibit the behavior of a semiconductor material are typically doped using various chemical methods. In other words, different chemicals are used to create p-type (hole majority carrier) regions and n-type
(electron majority carrier) regions in the carbon nanotube. This results in a P-N junction that, when an appropriate voltage is applied, emits light (in the case of a light-emitting diode ("LED")). The chemical methods for doping a carbon nanotube, however, suffer from the problem that the p-type regions and the n-type regions are typically not well characterized, resulting in nanoscale electronic devices with reduced performance characteristics.
Thus, what is needed are a method and associated structure for forming an electrostatically-doped carbon nanotube device having well characterized p-type regions and n-type regions, allowing for the creation of nanoscale electronic devices, such as LEDs and the like, with enhanced performance characteristics.
US Patent 6890780 provides a method and associated structure for forming an electrostatically-doped carbon nanotube device having well characterized p-type regions and n-type regions, allowing for the creation of nanoscale electronic devices, such as light-emitting diodes ("LEDs") and the like, with enhanced performance characteristics. More specifically, this invention provides for the use of a plurality of doping electrodes that are decoupled from a plurality of bias electrodes. Thus, the doping of a carbon nanotube may be finely tuned by varying the bias of each of the plurality of bias electrodes.
"Carbon nano-tube p-n junction diodes", J.U. Lee et. Al., Applied physics Letters 85, July 2004 and "Photovoltaic effect in ideal carbon nano-tube diodes", Ji Ung Lee, Applied Physics Letters 87, August 2005 provide photovoltaic cells with carbon nanotube. Figure 2 depict a photovoltaic cell as per these prior art.
However, the carbon nanotube used in the above patent and published papers is a single tube, which has less charge carrying capacity. The substrate used in these prior art is not conductive and so the charges created by the light absorption cannot be transmitted to the electrodes from the bottom of the solar cell.
"Organic solar cell Architecture", PhD Thesis, Klaus Petritsch, Cambridge and Graz, July 2000 and "Organic photovoltaic films", Jenny Nelson, Current Opinion in Solid State and Materials Science 6, January 2002 disclose about photovoltaic cells comprising a transparent substrate, a layer of ITO superimposed on said transparent substrate, an electron transport layer, a hole transport layer with semiconductors and two metal electrodes.
However, the layer of ITO reduces absorption of light by the solar cell. The metal electrodes are on the way of transmission of light, which reduces the absorption capacity of light of the existing solar cell. Absorption of light from both side of the solar cell is not possible.
Object and summary of the invention
To obviate the aforesaid drawbacks the object of the instant invention is to provide a solar cell with improved light absorption capacity.
It is another object of the present invention is to provide a cost effective solar cell.
It is still another object of the present invention is to provide a solar cell with improved efficiency of generation of power.
To meet the above-mentioned objectives and overcome the limitation of the existing solar cell as explained in the background section, the present invention provides a solar cell comprising:
- a transparent substrate;
- a light absorbing semiconductor layer placed over said transparent
substrate;
- a carbon nano-tube film placed over said light absorbing semiconductor
layer;
- said carbon nano-tube film has at least two electrodes;
- a patterned insulating layer having at least two electrodes biased with
opposite polarity by a first and second bias disposed on said carbon nano-
tube film;
- the arrangement being such that when the light is transmitted through the
transparent substrate, photovoltaic effect will be observed between said
electrodes placed on the carbon nano-tube film.
Brief description of the accompanying drawings:
These and other aspects of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings in which like designations are used to designate substantially identical elements.
Figure 1 illustrates a typical organic solar cell.
Figures 2 illustrate a carbon nano-tube solar cell according to the prior art.
Figure 3 depicts a solar cell according to the present invention.
Figures 4 to 8 depict the alternate embodiments of the solar cell according to the present invention.
Detailed description of the accompanying drawings
Figure 3 represents a solar cell according to the present invention. The solar cell uses a transparent substrate (100) at the bottom surface. The transparent substrate is a glass, plastic or transparent conducting oxides such as Indium Tin
Oxide. A light absorbing semiconductor material layer (101) that absorbs incident light of the appropriate wavelength is placed on the top of the transparent substrate. The semiconductor material of said layer comprises of organic or inorganic semiconductor. The semiconductor material is MEHPPV, CNPPV, P3HT, P30T, fullerenes and its derivatives or amorphous silicon. On the top of the semiconductor layer, a thin film of carbon nano-tube film (102) is deposited. The carbon-nano tube is in an arranged or scattered form. Due to the presence of a different organic semi-conducting film next to the carbon nano-tube film, wider spectrum of light will be absorbed by appropriately choosing the organic film material. There is no need for a conducting transparent layer like ITO or TiO2 (unlike traditional organic solar cells) as contact are made directly to the carbon nano-tube film. Low resistively paths are provided by the CNT film for the carriers generated. Two metal electrodes (103) are placed at the two end of said carbon-nano tube film. The electrodes comprise a metal selected from the group consisting of Au, Al, Cu, W or any conducting material that can be deposited and patterned. A thin layer of insulator (105) is deposited on top of the CNT film. The insulating layer comprises a dielectric material which may be transparent, translucent or opaque that can be deposited and patterned easily, such as, silicon dioxide, silicon nitride, AI2C>3, HfO2, or PMMA. The insulating layer may also be replaced by organic semiconductor material. The insulating layer is transparent to light for absorbing light from the top of the solar cell. Two metal electrodes (104a and 104b) are placed at the two ends of the insulating layer. The electrodes comprise a metal selected from the group consisting of Au, Al, Cu, Wor any conducting material that can be deposited and patterned.
In one embodiment of the invention the solar cell uses a transparent substrate (100) at the bottom surface. A light absorbing semiconductor material layer (101) that absorbs incident light of the appropriate wavelength is placed on the top of the transparent substrate. On the top of the semiconductor layer, a thin film of carbon nano-tube film (102) is deposited. A thin layer of insulator (105) is deposited on top of the carbon nano-tube film. Two metal electrodes (103) are
placed below the carbon nano-tube film and on top of said light absorbing semiconductor material layer (101). Said electrodes (103) are placed at the two ends of the semiconductor material layer. Figure 4 depicts a solar cell according to this embodiment.
In another embodiment of the invention the solar cell uses a transparent substrate (100) at the bottom surface. A light absorbing semiconductor material layer (101) that absorbs incident light of the appropriate wavelength is placed on the top of the transparent substrate. On the top of the semiconductor layer, a thin film of carbon nano-tube film (102) is deposited. A thin layer of insulator (105) is deposited on top of the carbon nano-tube film. The patterned insulating layer (105) consists of two separate insulation layers and each layer having a separate electrode (104a and 104b). A solar cell with this arrangement is illustrated in Figure 5.
Figure 6 represents a solar cell according to another embodiment of the present invention. In this embodiment the solar cell uses a transparent substrate (100) at the bottom surface. A light absorbing semiconductor material layer (101) that absorbs incident light of the appropriate wavelength is placed on the top of the transparent substrate. On the top of the semiconductor layer, a thin film of carbon nano-tube film (102) is deposited. A thin layer of insulator (105) is deposited on top of the carbon nano-tube film. Two electrodes (103) are placed below the carbon nano-tube film and on top of said light absorbing semiconductor material layer. Said electrodes (103) are placed at the two end of the semiconductor material layer. The patterned insulating layer (105) consists of two separate insulation layers and each layer having a separate electrode (104a and 104b).
In another embodiment of the invention the solar cell uses a transparent substrate (100) at the bottom surface. A light absorbing semiconductor material layer (101) that absorbs incident light of the appropriate wavelength is placed on the top of the transparent substrate. On the top of the semiconductor layer, a thin
film of carbon nano-tube film (102) is deposited. A thin layer of insulator (105) is deposited on top of the carbon nano-tube film. The patterned insulating layer (105) consists of two separate insulation layers and each layer having a separate electrode (104a and 104b). The two separate insulation layers of the insulating layer are placed on the top of the carbon nano-tube film (102) and at the two end of said carbon nano-tube film. Two more metal electrodes (103) are placed in between said insulation layers. The metal electrodes (103) are placed at a predetermined distance from each other. Said metal electrodes (103) are placed at the top of said carbon nano-tube film. A solar cell with this arrangement is illustrated in Figure 7.
In another embodiment of the invention the patterned insulating layer (105) consists of two separate insulation layers and each layer having a separate electrode (104a and 104b). The two separate insulation layers of the insulating layer are placed on the top of the carbon nano-tube film (102) and at the two end of said carbon nano-tube film. Two more metal electrodes (103) are placed between said insulation layer of the insulating layer. The metal electrodes (103) are placed at a predetermined distance from each other. Said electrodes (103) are placed below the carbon nano-tube film and on top of said light absorbing semiconductor material layer. Figure 8 depicts a solar cell according to this embodiment.
Voltages biases of different polarities Vpos (say +10V) and Vneg (say -10V) are applied to the metal electrodes (104a) and the metal electrode (104b) respectively. This would create two distinct regions side-by-side in the carbon nano-tube film (102) beneath the insulating layer (105). One region attracts holes and the other region attracts electrons. The position of the p-n junction between these two regions will depend upon the actual values of the voltage applied. When the light is incident on the active layer through the transparent substrate, electron-hole pairs or excitons are generated. These excitons move towards the carbon nano-tube layer by diffusion where they can get separated from free
electron and holes. Moreover, electron-hole pairs may also be generated in the carbon nano-tube (CNT) film itself due to absorption of light in the CNT. Because of the lateral electric field built in the film due to the biases applied, the 'free electron and holes formed get separated. The separated charge carriers can reach the electrodes by moving through the respective CNTs. The carriers can be collected from the electrodes by external circuit, allowing a current to flow through the device. The device thus behaves as a photo voltaic cell. High probability of exciton separation is possible in the solar cell due to the proximity of the exciton formation and the organic filrn-CNT interface.
An alternative embodiment is to use a CNT film with n and p-type region defined by doping the CNT with dopants instead of the voltage bias.
All documents cited in the description are incorporated herein by reference. The present invention is not to be limited in scope by the specific embodiments and examples which are intended as illustrations of a number of aspects of the invention and any embodiments which are functionally equivalent are within the scope of this invention. Those skilled in the art will know, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. These and all other equivalents are intended to be encompassed by the following claims.

We claim:
1. A solar cell comprising:
- a transparent substrate (100);
- a light absorbing semiconductor layer (101) placed over said transparent
substrate (100);
- a carbon nano-tube film (102) placed over said light absorbing
semiconductor layer (101);
- said carbon nano-tube film has at least two electrodes (103);
- a patterned insulating layer (105) having at least two electrodes (104a,
104b) biased with opposite polarity by a first and second bias disposed on
said carbon nano-tube film;
- the arrangement being such that when the light is transmitted through the
transparent substrate, photovoltaic effect will be observed between said
electrodes placed on the carbon nano-tube film.

2. A solar cell as in claim 1, wherein the electrodes (103) are placed below
the carbon nano-tube film (102) and on top of said light absorbing
semiconductor layer (101).
3. A solar cell as in claim 1, wherein the patterned insulating layer (105)
consists of two separate insulation layer and each layer having separate
electrodes (104a, 104b).
4. A solar cell as in claim 1 to 3, wherein the patterned insulating layer (105)
consists of two separate insulation layers and each layer having separate
electrodes (104a, 104b); and
the electrodes (103) are placed below the carbon nano-tube film (102) and on top of said light absorbing semiconductor layer (101).
5. A solar cell as in claim 1 to 3, wherein the patterned insulating layer (105)
consists of two separate insulation layers and are placed on the top of the
carbon nano-tube film (102) and at the two end of said carbon nano-tube
film;
each of the insulation layer having separate electrodes (104a, 104b); and the electrodes (103) are placed at the top of said carbon nano-tube film and between said insulation layers.
6. A solar cell as in claim 1 to 5, wherein the patterned insulating layer (105)
consists of two separate insulation layers and are placed on the top of the
carbon nano-tube film (102) and at the two end of said carbon nano-tube
film;
each of the insulation layer having separate electrodes (104a, 104b);
the electrodes (103) are placed below the carbon nano-tube film (102) and
on top of said light absorbing semiconductor layer (101); and
said electrodes (103) are placed between said insulation layers and
separated by a predetermined distance.
7. A solar cell as claimed in claim 1, wherein the transparent substrate (100)
is a glass, plastic, or transparent conducting oxides such as Indium Tin
Oxide (ITO).
8. A solar cell as claimed in claim 1, wherein the light absorbing
semiconductor layer (101) comprising organic or inorganic semiconductor.
9. A solar cell as claimed in claim 8, wherein the semiconductor is MEHPPV,
CNPPV, P3HT, P3OT, fullerenes and its derivatives or amorphous silicon.
10. A solar cell as claimed in claim 1, wherein the carbon nano-tube (102) is in an arranged or scattered form.
11. A solar cell as claimed in claim 1, wherein the insulating layer comprises a
dielectric material which may be transparent, translucent or opaque that
can be deposited and patterned easily, such as, silicon dioxide, silicon
nitride, AI2O3, HfO2, or PMMA.
12. A solar cell as claimed in claim 1, wherein the dielectric material is
replaced by organic semiconductor materials.
13. A solar cell as claimed in claim 1, wherein electrodes (104a, 104b) placed
on the insulating layer (105) comprise a metal selected from the group
consisting of Au, Al, Cu, Wor any conducting material that can deposited
and patterned.
14. A solar cell as claimed in claim 1, wherein electrodes placed on the
carbon nano-tube film (102) comprise a metal selected from the group
consisting of Au, Al, Cu, W or any conducting material that can deposited
and patterned.
15. A solar cell as claimed in claim 1, wherein the insulating layer (105) is
transparent to light for absorbing light from the top of the solar cell.
16. A solar cell as claimed in claim 1, wherein the carbon nano-tube film (102)
having a n-type and p-type region by doping said carbon nano-tube with
dopants.
17. A solar cell as claimed in claim 1 and 16, wherein lateral electric field is
created by formation of n-type region and p-type region or by application
of bias in said solar cell for charge separation and transport of said
charges through the carbon nano-tube film.
18. A solar cell as claimed in claim 1, wherein the first bias applied to metal
contact (104a) is operable for making one end of the carbon nano-tube
film a n-type semiconductor.
19. A solar cell as claimed in claim 1, wherein the second bias applied to
metal contact (104b) is operable for making the other end of the carbon
nano-tube film an p-type semiconductor.
20. A solar cell, substantially as herein described with reference to the
accompanying drawings.