Abstract:
A quantum dot sensitized solar cell including an anode, a cathode and an electrolyte is provided. The anode includes a semiconductor electrode adsorbed with a plurality of quantum dots. The quantum dots have a broad light absorption range that covers the ultraviolet, visible and infrared regions. The broad absorption range increases the ability of light harvesting, and accordingly, leads to an improved conversion efficiency of the solar cell.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a solar cell and particularly to a quantum dot-sensitized solar cell (QDSSC). 
       BACKGROUND OF THE INVENTION 
       [0002]    Solar energy is a crucial technology for solving the problems of high petroleum prices and global warming. Solar energy can be harvested by various methods such as wind energy, hydroelectricity and photovoltaics. Currently, the most widely used photovoltaic devices are silicon-based solar cells, but their high cost remains a problem. Recently, dye-sensitized solar cells (DSSC) have been emerging as a low-cost alternative photovoltaic source. The key component of a DSSC is a photoanode consisting of a nanoporous TiO 2  film coated onto a transparent conductive oxide glass substrate (usually indium-doped tin oxide (ITO) or fluorine-doped tin oxide (FTO)). The TiO 2  nanoparticles are sensitized by adsorbing a monolayer of organic dye molecules onto their surface. Upon solar illumination, the photoexcited electrons of the dye molecules are injected into the conduction band (CB) of the TiO 2  nanoparticles, then injected into the FTO substrate, finally producing a photocurrent. The highest efficiency achieved to date by DSSCs has been about 11%. High efficiency is due to the three-dimensional nanoporous network of TiO 2  nanoparticles, which greatly increases the surface area for dye adsorption, in turn, enhancing light harvesting. The most commonly used organic dyes, N3 and N719, have large optical absorption coefficients in the visible range (350-700 nm), but small absorption coefficients in the infrared (IR). However, the solar spectrum covers the range of 0.3-2.5 μm, with about 70% of the photon flux being distributed beyond 700 nm. In other words, the dye wastes 70% of the solar energy. To improve efficiency in DSSCs, one needs to find new sensitizers with a broadband photoresponse, especially in the IR region. A successful option for broadband sensitizers is semiconductor (extremely thin layer) absorbers. Semiconductor quantum dots (QDs) have also been used as sensitizers. QDs have several advantages over organic dye sensitizers such as having tunable absorption bands, high extinction coefficients, and multiple electron-hole pair generation. The most extensively studied QD sensitizers are the cadmium chalcogenide systems: CdS and CdSe, which have absorption ranges of 350-700 nm. To improve efficiency, it is desirable to explore new types of QD sensitizers with broad absorption ranges extending into the IR region. 
       SUMMARY OF THE INVENTION 
       [0003]    The primary object of the present invention is to solve the problems of dye-sensitized solar cells that have small absorption coefficients in the infrared range. 
         [0004]    The present invention is directed to a quantum dot sensitized solar cell (QDSSC), which contains quantum dots as a light sensitizer. The disclosure provides a QDSSC including an anode, a cathode, and an electrolyte between the anode and the cathode. The anode includes a semiconductor electrode and a plurality of quantum dots coupled to the semiconductor electrode. The quantum dots are made of a material selected from the group consisting of Ag 2 S, Ag 2 Se, Cu x S and Cu x Se and are distributed within the semiconductor electrode layer. 
         [0005]    The QDs have a broad optical absorption range covers the UV, visible and IR of the solar spectrum, allowing enhanced absorption of the incident solar radiation. Accordingly, the power conversion efficiency of the solar cells is improved. 
         [0006]    The foregoing, as well as additional objects, features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a cross-sectional view of a quantum dot sensitized solar cell according to a first embodiment of the disclosure. 
           [0008]      FIG. 2A  is a structural cross-sectional view of an anode in the first embodiment. 
           [0009]      FIG. 2B  illustrates a quantum dot coupled to a TiO 2  particle in the first embodiment. 
           [0010]      FIG. 2C  illustrates the core-shell structure of a quantum dot. 
           [0011]      FIG. 3  is a diagram illustrating the synthesis process of quantum dots of the invention. 
           [0012]      FIG. 4  illustrates the photocurrent-voltage of QDSSCs in Experiment 2. 
           [0013]      FIG. 5  illustrates the quantum-efficiency spectra of QDSSCs with various quantum dots. 
           [0014]      FIG. 6  illustrates the photovoltaic ranges of various quantum dots over the solar spectral range. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]      FIG. 1  is a cross-sectional view of a quantum dot sensitized solar cell according to a first embodiment of the disclosure. Referring to  FIG. 1 , in the present embodiment, the QDSSC  100  consists of an anode  102 , a cathode  106 , and an electrolyte  104  between the anode  102  and a cathode  106 . An incident light  110  enters from the anode  102  side of the QDSSC  100 . 
         [0016]    Referring to  FIG. 2A , the anode  200  includes a semiconductor electrode layer  212  coated on a transparent conductive oxide (TCO) substrate  204 . The transparent conductive oxide  204  can be made of materials of indium-doped tin oxide (ITO) or fluorine-doped tin oxide (FTO). The semiconductor electrode layer  212  comprises semiconductor electrodes  206 , and a plurality of quantum dots  208  distributed within the semiconductor electrode layer  212 , in other words, quantum dots  208  are deposited on the surface of the semiconductor electrode  206 . A particle diameter of the quantum dots  208  is smaller than 20 nm. The materials of the semiconductor electrode layer  212  may be TiO 2 , N-doped TiO 2  and ZnO. The shapes of the semiconductor materials may be nanoparticles, nanorods or nanotubes. In this embodiment, the quantum dots  208  could be Ag 2 S, Ag 2 Se, Cu x S or Cu x Se. 
         [0017]    Referring to  FIG. 2B , quantum dots  208  can be coupled directly to the surface of the TiO 2  particle of the semiconductor electrode  206 . Alternatively, quantum dots  208  can be coupled to the semiconductor electrode  206  particle using a ligand linker  210 . 
         [0018]    Referring to  FIG. 2C , a quantum dot  208  can have a core-shell or an inverse core-shell structure. In the core-shell structure the core material  214  can be Ag 2 S, Ag 2 Se, Cu x S or Cu x Se. The shell material  216  can be CdS, CdSe, CdTe, In 2 S 3 , In 2 Se 3 , In 2 Te 3 , PbS, PbSe, PbTe, SnS, SnSe, SnTe, Sb 2 S 3 , Sb 2 Se 3 , AlN, AlP, AlAs, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, Si or Ge. 
         [0019]    In the inverse core-shell structure the shell material  216  can be Ag 2 S, Ag 2 Se, Cu x S or Cu x Se. The core material  214  can be CdS, CdSe, CdTe, In 2 S 3 , In 2 Se 3 , In 2 Te 3 , PbS, PbSe, PbTe, SnS, SnSe, SnTe, Sb 2 S 3 , Sb 2 Se 3 , AlN, AlP, AlAs, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, Si or Ge. 
         [0020]    Also referring to  FIG. 3 , the present embodiment provides a series of processes for fabricating the QDSSC. First, step  300  is performed to fabricate a TiO 2  semiconductor electrode  206  on transparent conducting oxide glass. In step  302 , quantum dots  208  are coated on the semiconductor electrode  206  using a sequential ion layer adsorption reaction (SILAR) process. In step  304 , the quantum dot coated semiconductor electrode layer  212  is assembled with a cathode  106  into a solar cell. In step  306 , an electrolyte  104  is injected into the assembled solar cell through two predrilled holes on the cathode  106 . In step  308 , measurements are carried out to study the photovoltaic performance, including photocurrent, voltage and power conversion efficiency, of the fabricated QDSSC  100 . 
         [0021]    Referring to  FIG. 1 , the cathode (or counterelectrode)  106  can be a TCO substrate coated with a thin layer of Pt film. The deposition of the Pt film can be performed with physical vapor deposition, magnetron sputtering deposition, or SILAR. The thickness of the Pt film is 2-4 nm. The electrolyte  104  can be a liquid-state electrolyte such as I − /I 3   −  polyiodide, S −2 /S x   −  polysulfide, or polycobolt liquid electrolyte. The electrolyte  104  can also be a solid-state electrolyte such as spirobifuorene. 
         [0022]    Quantum dots  208  can be prepared using a chemical method such as the sequential ion layer adsorption reaction (SILAR) process. A precursor supplies the positive ions and a second precursor supplies the negative ions. A semiconductor electrode is sequentially dipped into the positive and negative ions. Repeated dipping produces quantum dots  208  on the semiconductor electrode  206 . 
       Experiment 1 
     Fabrication of a Quantum Dot Sensitized Solar Cell 
       [0023]    The steps are described as follows: 
         [0024]    Step 1: Preparation of the TiO 2  electrode: An FTO glass substrate of resistivity 15Ω/□ is used as the substrate. A layer of TiO 2  of thickness about 12 μm is coated on the FTO glass using the doctor blade technique. 
         [0025]    Step 2: The TiO 2  coated substrate is placed in a furnace and then heated at 500° C. for 50 min. 
         [0026]    Step 3: Quantum dots are deposited onto the surface of the TiO 2  electrode using the SILAR process. The successive ionic layer adsorption and reaction deposition (SILAR) process for the growth of Ag 2 S QDs is described as follows. First, a TiO 2  electrode was dipped into an AgNO 3  solution, washed with ethanol to obtain Ag +  ions. The electrode is subsequently dipped into a Na 2 S solution to obtain S 2− . The procedure produces Ag 2 S QDs on the surface of the TiO 2  nanoparticles. The diameter of the QDs can be controlled by varying the number of the SILAR cycles. QDs with diameters in the range of 3-10 nm can be obtained after the reaction. 
         [0027]    Step 4: A counterelectrode is prepared by coating a thin layer of Pt film on FTO glass. 
         [0028]    Step 5: A solar cell is assembled by sandwiching the QD-coated electrode with the Pt counterelectrode using a surlyn spacer. 
         [0029]    Step 6: An electrolyte is injected into predrilled holes on the counterelectrode. The holes are finally sealed with an epoxy. This finishes the fabrication of the QDSSC. 
       Experiment 2 
     Photovoltaic Measurements 
       [0030]    1. Photovoltaic performance:  FIG. 4  shows the photocurrent-voltage curves of QDSSCs sensitized with Ag 2 S, Ag 2 Se and Cu x S QDs. The photovoltaic parameters are listed in Table 1. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 J sc  (mA 
                   
                   
                 Efficiency 
               
               
                   
                 Sample 
                 cm −2 ) 
                 V oc  (V) 
                 FF (%) 
                 (%) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Ag 2 S 
                 7.26 
                 0.33 
                 40.8 
                 0.98 
               
               
                   
                 Ag 2 Se 
                 28.5 
                 0.27 
                 23.8 
                 1.76 
               
               
                   
                 Cu x S 
                 28.1 
                 0.17 
                 18.9 
                 0.90 
               
               
                   
                   
               
             
          
         
       
     
         [0031]    2. Quantum efficiency:  FIG. 5  illustrates the quantum-efficiency (QE) spectra of QDSSCs sensitized with Ag 2 S, Ag 2 Se and CIO QDs. The Ag 2 S and Cu x S spectra cover the spectra range of 350-1100 nm. The Ag 2 Se spectrum covers the spectral range of 350-2500 nm. The quantum efficiency spectra are further supported by the absorption spectra in  FIG. 6 . 
         [0032]    3.  FIG. 6  displays the absorption spectra of various QDs. The solar power spectrum is also shown for comparison. It can be seen that the Ag 2 S and Cu x S spectra covers the range of 350-1100 nm, i.e., UV, visible and IR. The cutoff of the QE spectra is at the wavelength about 1100 nm, which is equal to the wavelength of an optimal solar absorber. This indicates that Ag 2 S and Cu x S QDs can be ideal high-efficiency absorbers for solar cells. The Ag 2 Se QE spectrum exhibits an intriguing feature-it covers the full solar spectral range of 350-2500 nm, indicating that Ag 2 Se can utilize the full solar power for energy conversion. 
         [0033]    In summary, the Ag 2 S and Cu x S QDs have broad photovoltaic ranges that cover the UV, visible and IR ranges. In addition, the QE spectra have a cutoff wavelength close to that of an optimal solar absorber. The Ag 2 Se QDs have a photovoltaic range that covers the full solar spectrum of 350-2500 nm. A broad photovoltaic range means that the solar cell can convert a broader range of the incident solar power into electrical current, which results in a large photocurrent and high power conversion efficiency.