Abstract:
A photovoltaic (PV) device having a quantum dot sensitized interface includes a first conductor layer and a second conductor layer. At least one of the conductor layers is transparent to solar radiation. A quantum dot (nanoparticle) sensitized photo-harvesting interface comprises a photo-absorber layer, a quantum dot layer and a buffer layer, placed between the two conductors. The absorber layer is a p-type material and the buffer layer is an n-type material. The quantum dot layer has a tunable bandgap to cover infrared (IR), visible light and ultraviolet (UV) bands of solar spectrum.

Description:
FIELD OF THE INVENTION 
     The present invention relates to semiconductor-based photovoltaic energy converters, also known as “solar cells,” and to the design and fabrication of the same. 
     BACKGROUND OF THE INVENTION 
     With appropriate electrical loading, photovoltaic solid state semiconductor devices, commonly known as solar cells, convert sunlight into electrical power by generating both a current and a voltage upon illumination. The current source in a solar cell is the charge carriers that are created by the absorption of photons. 
     CIGS-Based Solar Cells 
     Copper ternary chalcogenide compounds and alloys are efficient light-absorbing materials for solar cell applications. Their efficiency is due to their direct and tunable energy band gaps, very high optical absorption coefficients in the visible to near-infrared (IR) spectrum range and high tolerance to defects and impurities. Copper indium-gallium-selenium/sulfur (CIGS) thin film solar cells provide the advantages of low-cost, high-efficiency, long-term stability, superior performance under weak illumination, and desirable resistance to radiation. For useful background material, refer, for example, to H. W. Schock et al., “CIGS-based Solar Cells for the Next Millennium,”  Prog. Photovolt. Res. Appl.  8, 151-160 (2000). 
     Unlike the basic silicon solar cell, which can be described as a simple p-n junction device, CIGS based solar cells comprise a more complex heterojunction system. Solar cells based on CIGS have achieved the highest efficiency of existing thin film solar cells. 
     A cross-sectional view of an exemplary CIGS device in accordance with a prior art embodiment is shown in  FIG. 1 . The various layers of the solar cell are deposited on a substrate  110 . Solar cells based on p-type CIGS absorbers are typically fabricated on glass, polymer, stainless steel or other substrates  110  using various deposition techniques known in the art. Incident sunlight  180  is partially blocked by the metallic grid shown as contact elements  170 , which covers approximately 5% of the surface of the device. These contact elements  170  can comprise Ni/Al fingers or other appropriate contact elements. The incident sunlight  180  is partially reflected by the surface of the transparent conducting-oxide (TCO) layer  150  and  160 , shown as a i-ZnO/ZnO:Al layer, due to the difference in the index of refraction. Some short-wavelength photons are absorbed in the n-CdS layer  140 . Most of the sunlight, however, enters the semiconductor and is absorbed in the CIGS absorber layer  130 . The CIGS absorber layer  130  is shown disposed on molybdenum (Mo) layer  120 , which is disposed on a stainless steel substrate  110  as shown. Other substrates known in the art are expressly contemplated, including glass and polymers. 
     The front metal contact fingers (Ni/Al)  170  are optional and are not required for operation of the photovoltaic device. The ZnO layers  150 &amp; 160  and CdS layer  140  typically comprise n-type material, and the CIGS  130  layer typically comprises p-type material. The semiconducting junction is formed at or proximate to the CdS-CIGS (n-p) interface. Electrons that are generated within the junction-field region or within about one diffusion length of the n-p junction will generally be collected. 
     According to standard prior art CIGS thin film solar cells, a highest efficiency of 19.9% has been achieved with an effective area of 0.42 cm 2  prepared by the so-called three-stage co-evaporation process. For useful background material on efficiencies of CIGS solar cells, refer, for example, to I. Repins et. al., “19.9% efficient ZnO/CdS/CuInGaSe 2  solar cell with 81.2% fill factor”, Prog. Photovolt: Res. Appl. 2008; 16:235-239. 
     QD-Enhanced Solar Cells 
     The efficiency of a solar cell can also be enhanced when a quantum dot (QD) effect is applied to a solar cell, thereby significantly improving the energy conversion rate thereof. Quantum dots have bandgaps that are tunable across a wide range of energy levels by changing the size (i.e. diameter) of the quantum dots. The quantum dot effect achieved by a quantum dot solar cell generally relates to an impact ionization effect and an Auger recombination (AR) effect. 
     The impact ionization effect occurs in semiconductor material, when energy of two bandgaps is provided from external, excited electrons can exist in form of hot electrons. When the hot electrons are transited form high energy level to low energy level excitation state, the released energy can excite another electron from a valence band to a conduction band, and such phenomenon is referred to as the impact ionization effect. According to the impact ionization effect, one high energy photon can excite two or more hot electrons. 
     The other effect is an Auger recombination (AR) effect relative to the impact ionization effect. The AR effect refers to the energy released in the semiconductor material due to recombination of hot electrons and holes can excite another hot electron to transit to a higher energy level, thereby prolonging a lifetime of the hot electron in the conduction band. 
     When the semiconductor material displays a quantum dot size, the continuous conduction band is gradually split into small energy levels, so that the cooling speed of the electrons is slowed down, and therefore the impact ionization effect and the Auger recombination effect can be effectively utilized. According to theoretical calculations, the tradition single junction solar cell only can achieve 31% energy conversion efficiency, and if combining with the impact ionization and Auger recombination effects, the maximum theoretical efficiency of the solar cell can be 66%, which confirms the potential ability to use the quantum dots in the solar cell. 
     Solar cells made from photosensitive nanoparticles show very low efficiencies. Nanoparticles are very efficient in generating electron-hole charge pairs when exposed to sunlight. The primary reason for the low efficiencies is charge recombination. To achieve high efficiencies in a solar cell the charges are desirably 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 efficiencies. Charge recombination in nanoparticles is primarily due to two factors: (a) Surface states on nanoparticles that facilitate charges recombination and (b) Slow charge transport. In nanoparticles/QD solar cells, the charge recombination is faster as compared to charge transport. 
     Electricity produced by a solar cell is expensive due to high solar cell module cost. In order to significantly reduce the cost of solar electricity, it is desirable to both increase cell efficiency and to significantly reduce the costs of photovoltaic (PV) module fabrication. A thin film form of cell reduces the fabrication cost, but yields relatively lower efficiency compared to a single crystalline wafer based cell. Thus, it is desirable to provide a system and method to enhance efficiency of the solar cell. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the disadvantages of the prior art by providing a photovoltaic (PV) device having a quantum dot sensitized interface and an absorber layer. In an illustrative embodiment, the photovoltaic device includes a first conductor layer and a second conductor layer, in which at least one of which is transparent to solar radiation. The quantum dot (nanoparticle) sensitized photo-harvesting interface comprises a photo-absorber layer, a quantum dot layer and a buffer layer, placed between the two conductors. 
     The quantum dot sensitized interface PV device is fabricated or otherwise synthesized according to an illustrative procedure that commences by providing a substrate. A bottom electrode layer is then deposited on the substrate, and an optical absorber layer is deposited on the bottom electrode. A quantum dot layer is formed on the optical absorber layer. A buffer layer is then deposited on the quantum dot layer and a quantum dot layer is thereby formed at the interface of the optical absorber and the buffer layer. The procedure then deposits top light transparent electrode layers on the buffer layer to complete the PV device having a quantum dot sensitized interface. The resulting PV device provides improved efficiency over the prior art CIGS-based and/or QD-enhanced solar cells. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention description below refers to the accompanying drawings, of which: 
         FIG. 1A , already described, is a schematic cross-sectional view of a copper indium-gallium-selenium/sulphur (CIGS) thin film solar cell device according to a prior art illustrative embodiment; 
         FIG. 1B , already described, is a band diagram of the CIGS thin film solar cell of  FIG. 1A , according to the prior art illustrative embodiment; 
         FIG. 2  is a schematic cross-sectional view of a quantum dot sensitized thin film solar cell having a copper-based absorber layer, according to an illustrative embodiment; 
         FIG. 3  is a band diagram of a PN junction of the quantum dot sensitized thin film solar cell of  FIG. 2 , showing the impact ionization effect, according to the illustrative embodiment; and 
         FIG. 4  is a flow diagram of a procedure for fabricating a quantum dot sensitized thin film solar cell according to the illustrative embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A schematic cross-sectional view of a quantum dot (QD) sensitized thin film solar cell having a copper-based absorber layer is shown in  FIG. 2 , with the corresponding band-diagram shown in  FIG. 3 , according to an illustrative embodiment. With reference to  FIG. 2 , the QD sensitized thin film solar cell includes a bottom electrode layer  220 , optical absorption layer  230 , buffer layer  250  and top light transparent electrode layers  260  &amp;  270  sequentially deposited on a substrate  210 . A layer containing a plurality of quantum dots  240  is formed at the interface of the optical absorber layer  230  and the buffer layer  250 . According to the illustrative embodiment, the QD layer comprises photosensitive nanoparticles. The absorption layer  230  comprises a p-type semiconductor layer and is in electrical communication with the bottom electrode layer  220 . The photoactive QD layer  240  comprises photosensitive nanoparticles proximate the p-type absorber payer  230 . The buffer layer  250  comprises an n-type layer, and is in contact with the QD layer  240  and top transparent electrode layers  260 ,  270 . The top transparent electrode layers are in electrical communication with a top metal contact  280 . 
     The presence of QD layers at the PN interface  310  (see  FIG. 3 ) and the strong electric field created at the depletion region facilitate effective separation of electron-hole pairs generated in the QDs (see break-out detail  320  of  FIG. 3 ). In addition, the electric field drives the separated charges to their respective electrodes and minimizes recombination of photo-generated carriers. Thus, as shown at  320 , quantum size induced generation of multiple electron-hole pairs for a single photon  325 , through the impact ionization and the Auger recombination effect, can be effectively separated and collected at the respective electrodes. Accordingly, this discloses a yield of higher photocurrent generation, resulting in solar cell efficiency greater than the efficiency observed employing prior art devices. 
     Quantum Dots (i.e. the nanoparticles) form a schottky junction solar cell that does not yield high efficiency by itself. Typically, according to the prior art, the nanoparticles are embedded in other semiconductor materials, for improved utilization of QDs. For example, InAs nanoparticles are inserted in GaAs host material thin film. Current CIGS solar cell gives its maximum efficiency of approximately 20% and its theoretical efficiency limit is approximately 31%. Introduction of QD&#39;s proximate the CIGS layer increases its theoretical efficiency limit to approximately 66%. The CIGS-based solar cells can create one electron-hole (charge) pairs for a photon. With the introduction of QDs, the solar cell can create multiple electron-hole pairs for a photon and it can extend the life-time of the generated charges (as shown in detail  320  of  FIG. 3 ). The resulting solar cell has the ability to enhance the spectrum of light that used for energy conversion. 
     Referring back to  FIG. 2 , the optical absorber layer  230  can be formed according to a physical vapor deposition process (for example co-evaporation, sputtering and selenization, etc.), solution based process (such as electro-deposition, nanoparticle ink coating, etc.), and other techniques known in the art. Given that the buffer layer  250  is typically formed by chemical bath deposition, the optical absorber layer  230  can be coated together with the QD layer  240  by chemical bath deposition. 
     The optical absorber layer  230  is a copper-based substance selected from a group consisting of copper indium gallium diselenide (CIGS), copper indium diselenide (CIS), copper gallium diselenide (CGS), copper gallium ditelluride (CGT), and copper indium aluminum diselenide (CIAS). The n-type buffer layer is a substance selected from a group consisting of cadmium sulfide (CdS), zinc sulfide (ZnS), indium sulfide (In 2 S 3 ), and other similar materials known to those skilled in the art. 
     A bandgap of the quantum dots  240  can cover an infrared (IR), a visible light and an ultraviolet (UV) bands of solar spectrum. Various types of QD materials can be simultaneously used to increase a photon absorption range. For example, if the bandgap of the QD is in the IR range, the material of the quantum dots is one or a plurality of substance(s) selected from a substance group consisting of PbS, GaSb, InSb, InAs and CIS, and other similar materials known to those skilled in the art. 
     If the bandgap of the quantum dots is in the visible light rage, the material of the quantum dots is one or a plurality of the substance selected from a substance group consists of InP and CdSe etc. If the bandgap of the quantum dots is in the UV range, the material of the quantum dots is one or a plurality of substance selected from a substance group consisting of TiO 2 , ZnO and SnO 2  etc. Selection of the materials of the quantum dots is highly variable and determines the conduction feasibility of the conduction band energy levels of the optical absorber layer. The different types of materials can be used simultaneously to provide the desired coverage of solar spectrum. 
     Reference is now made to  FIG. 4  showing an illustrative procedure  400  for fabricating a PV device having a quantum dot sensitized interface with an optical absorber layer as shown and described herein in accordance with the illustrative embodiment. The steps of the procedure  400  correspond to the various steps in fabricating a QD sensitized thin film solar cell as shown in  FIG. 2  and described herein. Accordingly, the layers comprise similar materials as described and are synthesized using techniques as described herein and known in the art. It is expressly contemplated that the steps can be performed in any order within ordinary skill to achieve the overall PV structure in accordance with the illustrative embodiments herein. 
     As shown, the procedure  400  commences at step  410  by providing a substrate. The substrate can comprise glass, polymer, stainless steel, or other substrates known in the art for use in solar cell applications and devices. According to the illustrative procedure, a bottom electrode layer is deposited on the substrate at step  412 . The bottom electrode layer can comprise Mo (Molybdenum) or another appropriate layer that is deposited on the substrate. Then at step  414  an optical absorber layer is deposited on the bottom electrode layer. The optical absorber layer can comprise CIGS or any other appropriate optical absorber as described herein. At step  416 , a QD layer is formed on the optical absorber. The QD layer can be coated by chemical bath deposition or another technique known in the art. A buffer layer is deposited on the optical absorber layer at step  418 . The buffer layer can comprise a CdS layer, which allows some of the short-wavelength photons to be absorbed therein, according to illustrative embodiments. The buffer layer is typically formed by chemical bath deposition. Other techniques within ordinary skill can also be employed to achieve the overall PV cell structure. Finally, at step  420 , top light transparent electrode layers are deposited on the buffer layer. The resulting thin film solar cell, for example as shown in  FIG. 4 , provides a higher yield of photocurrent generation, resulting in solar cell efficiency greater than the efficiency observed by prior art devices. 
     The foregoing has been a detailed description of illustrative embodiments of the invention. Various modifications and additions can be made without departing from the spirit and scope of this invention. Each of the various embodiments described above may be combined with other described embodiments in order to provide multiple features. Furthermore, while the foregoing describes a number of separate embodiments of the apparatus and method of the present invention, what has been described herein is merely illustrative of the application of the principles of the present invention. For example, the illustrative embodiments can include additional layers to perform further functions or enhance existing, described functions. Likewise, while not shown, the electrical connectivity of the cell structure with other cells in an array and/or external conduit is expressly contemplated and highly variable within ordinary skill. More generally, while some ranges of layer thickness and illustrative materials are described herein, it is expressly contemplated that additional layers, layers having differing thicknesses and/or material choices can be provided to achieve the functional advantages described herein. In addition, directional and locational terms such as “top,” “bottom,” “center,” “front,” “back,” “above,” and “below” should be taken as relative conventions only, and not as absolute. Furthermore, it is expressly contemplated that various semiconductor and thin films fabrication techniques can be employed to form the structures described herein. Accordingly, this description is meant to be taken only by way of example, and not to otherwise limit the scope of this invention.