Light conversion efficiency-enhanced solar cell fabricated with downshifting nanomaterial

Abstract

The light conversion efficiency of a solar cell ( 10 ) is enhanced by using an optical downshifting layer ( 30 ) in cooperation with a photovoltaic material ( 22 ). The optical downshifting layer converts photons ( 50 ) having wavelengths in a supplemental light absorption spectrum into photons ( 52 ) having a wavelength in the primary light absorption spectrum of the photovoltaic material. The cost effectiveness and efficiency of solar cells platforms ( 20 ) can be increased by relaxing the range of the primary light absorption spectrum of the photovoltaic material. The optical downshifting layer can be applied as a low cost solution processed film composed of highly absorbing and emissive quantum dot heterostructure nanomaterial embedded in an inert matrix to improve the short wavelength response of the photovoltaic material. The enhanced efficiency provided by the optical downshifting layer permits advantageous modifications to the solar cell platform that enhances its efficiency as well.

1 . A solar cell, comprising:
 a photovoltaic material having a region of light conversion activity that is characterized at least in part by a primary light absorption spectrum for absorbing incident photons and generating electron and hole charge carriers in the photovoltaic material;   spaced-apart first and second electrodes for containing the photovoltaic material positioned between them, the photovoltaic material and the first and second electrodes forming respective first and second charge-separating junction interfaces for separating the electron and hole charge carriers for collection by the first and second electrodes, at least one of the first and second electrodes including an electrode material that transmits incident light wavelengths in the primary light absorption spectrum to permit incident photons in the primary light absorption spectrum to reach the photovoltaic material, the electrode material having a doping characteristic that enhances an electrical performance characteristic of the solar cell, and the doping characteristic impeding light conversion activity of the photovoltaic material in a supplemental light absorption spectrum that is substantially outside of the primary light absorption spectrum so as to avoid reduction of light conversion activity of the photovoltaic material in the primary light absorption spectrum; and   an optical downshifting layer that is transmissive to incident photons having wavelengths in the primary light absorption spectrum and includes an optically complementary nanomaterial being adapted for absorbing incident photons in the supplemental light absorption spectrum, the optically complementary nanomaterial being adapted for emitting photons in the primary light absorption spectrum of the photovoltaic material such that the optical downshifting layer transforms incident photons in the supplemental light absorption spectrum into photons in the primary light absorption spectrum of the photovoltaic material to enhance generation of electron and hole charge carriers in the photovoltaic material, the optically complementary nanomaterial increasing light conversion efficiency of the solar cell such that the light conversion efficiency increased by the optically complementary nanomaterial is an amount greater than any decrease in light conversion efficiency caused by the doping characteristic.   
     
     
         2 . The solar cell of  claim 1 , in which the photovoltaic material comprises at least one of crystalline silicon, multicrystalline silicon (mc-Si), nanocrystalline silicon, amorphous silicon, micromorphous silicon, gallium arsenide (GaAs), indium oxide (InP), indium arsenide (InAs), a III-V-based photovoltaic material, cadmium telluride (CdTe), copper indium selenide (CIS), copper indium gallium di-selenide (CIGS), an active organic photovoltaic material, a dye-sensitized solar cell (DSC) material, or an active quantum dot (QD) ensemble. 
     
     
         3 . The solar cell of  claim 1 , in which the photovoltaic material comprises a nanomaterial. 
     
     
         4 . The solar cell of  claim 1 , in which the photovoltaic material comprises an inorganic quantum dot material. 
     
     
         5 . The solar cell of  claim 1 , in which the layer of optically complementary nanomaterial comprises an electrically inactive quantum dot material. 
     
     
         6 . The solar cell of  claim 5 , in which the electrically inactive quantum dot material optically converts incident photons to provide converted photons, the incident photons having a selected wavelength that is shorter than wavelengths in the primary light absorption spectrum and the converted photons having a converted wavelength that is within the primary light absorption spectrum. 
     
     
         7 . The solar cell of  claim 1 , in which the optical downshifting layer includes an electrically inactive quantum dot material of a type that protects the photovoltaic material from thermal inefficiencies resulting from high energy photons. 
     
     
         8 . The solar cell of  claim 1 , in which the optical downshifting layer includes an electrically inactive quantum dot material of a type that randomizes directions of photons propagating within the photovoltaic material. 
     
     
         9 . The solar cell of  claim 1 , in which the primary light absorption spectrum includes wavelengths in a first wavelength range from 0.2 micron to 1.2 microns and the first supplemental light absorption spectrum includes wavelengths in a second wavelength range from 0.2 to 0.7 micron. 
     
     
         10 . The solar cell of  claim 1 , further comprising a front contact to convey electrical current from the electrode material of the first electrode to an electrical circuit, and in which the electrical performance characteristic is a reduction in resistance between the front contact and the electrode material of the first electrode. 
     
     
         11 . The solar cell of  claim 1 , in which the optical downshifting layer includes individually encapsulated nanocrystals having a quantum dot core surrounded by one or more shells. 
     
     
         12 . The solar cell of  claim 11 , in which the shell has a rod shape. 
     
     
         13 . The solar cell of  claim 1 , in which the photovoltaic material has a suboptimal light conversion efficiency. 
     
     
         14 . The solar cell of  claim 1 , in which the optical downshifting layer has an emission efficiency of greater than 90%. 
     
     
         15 . The solar cell of  claim 1 , in which the optical downshifting layer has a minimum emission wavelength and a maximum absorption wavelength, and the minimum emission wavelength is at least 50 nm greater than the maximum absorption wavelength. 
     
     
         16 . The solar cell of  claim 1 , further comprising first and second front grid contacts having a minimum spacing of 3 mm. 
     
     
         17 . The solar cell of  claim 1 , further comprising front grid contacts that shade the solar cell by less than 5%. 
     
     
         18 . The solar cell of  claim 1 , in which the electrode material has a thickness of greater than 400 nm. 
     
     
         19 . The solar cell of  claim 1 , in which the electrode material has an upper surface, a thickness of greater 500 nm, and doping concentration of greater than 1 e 20 /cm 3 at the upper surface. 
     
     
         20 . A method of supplementing light conversion efficiency of a photovoltaic material of a wafer-based solar cell platform, the photovoltaic material having a region of light conversion activity that is characterized at least in part by a primary light absorption spectrum for absorbing incident photons and generating electron and hole charge carriers in the photovoltaic material, the wafer-based solar cell platform including spaced-apart first and second electrodes, and the wafer material and the first and second electrodes forming respective first and second charge-separating junction interfaces for separating electron and hole charge carriers for collection by the first and second electrodes, comprising:
 providing at least one of the first and second spaced-apart electrodes with an emitter electrode material that transmits incident light wavelengths in the primary light absorption spectrum to permit incident photons in the primary light absorption spectrum to reach the photovoltaic material, the emitter electrode material having a doping characteristic that enhances an electrical performance characteristic of the wafer-based solar cell platform, and the doping characteristic impeding light conversion activity of the photovoltaic material in a supplemental light absorption spectrum that is substantially outside of the primary light absorption spectrum so as to avoid reduction of light conversion activity of the photovoltaic material in the primary light absorption spectrum; and   forming a downshifting layer including an optically complementary nanomaterial-containing layer for absorbing incident photons in the supplemental light absorption spectrum, the optically complementary nanomaterial being adapted for emitting photons in the primary light absorption spectrum of the photovoltaic material such that the optical downshifting layer transforms incident photons in the supplemental light absorption spectrum into photons in the primary light absorption spectrum of the photovoltaic material to enhance generation of electron and hole charge carriers in the photovoltaic material, the optically complementary nanomaterial increasing light conversion efficiency of the solar cell such that the light conversion efficiency increased by the optically complementary nanomaterial is an amount greater than any decrease in light conversion efficiency caused by the doping characteristic.   
     
     
         21 . The method of  claim 20 , further comprising a front contact to convey electrical current from the electrode material of the first electrode to an electrical circuit, and in which the electrical performance characteristic is a reduction in resistance between the front contact and the electrode material of the first electrode. 
     
     
         22 . A solar cell, comprising:
 a photovoltaic material having a region of light conversion activity that is characterized at least in part by a primary light absorption spectrum for absorbing incident photons and generating electron and hole charge carriers in the photovoltaic material, the primary light absorption spectrum including wavelengths in a first wavelength range from 0.2 micron to 1.2 microns;   spaced-apart first and second electrodes for containing the photovoltaic material positioned between them, the photovoltaic material and the first and second electrodes forming respective first and second charge-separating junction interfaces for separating the electron and hole charge carriers for collection by the first and second electrodes, at least one of the first and second electrodes including an electrode material that transmits incident light wavelengths in the primary light absorption spectrum to permit incident photons in the primary light absorption spectrum to reach the photovoltaic material; and   an optical downshifting layer that is transmissive to a majority of incident photons having wavelengths in the primary light absorption spectrum and includes individually encapsulated non-spherical nanocrystals having a quantum dot core surrounded by a non-spherical shell, the non-spherical nanocrystals being adapted for absorbing incident photons in a supplemental light absorption spectrum, the supplemental light absorption spectrum including wavelengths in a second wavelength range from 0.2 to 0.7 microns, the non-spherical nanocrystals being adapted for emitting photons in the primary light absorption spectrum of the photovoltaic material such that the optical downshifting layer transforms incident photons in the supplemental light absorption spectrum into photons in the primary light absorption spectrum of the photovoltaic material to enhance generation of electron and hole charge carriers in the photovoltaic material, the non-spherical nanocrystals increasing light conversion efficiency of the solar cell.   
     
     
         23 . The solar cell of  claim 22 , in which the non-spherical nanocrystals comprise quantum dot heterostructures having a shape resembling a nanorod, a nanotetrapod, or a nanosheet. 
     
     
         24 . The solar cell of  claim 22 , in which the non-spherical nanocrystals comprise quantum dot heterostructures having a CdSe quantum dot core and a rod-shaped CdS shell, the quantum dot heterostructures being encapsulated in a silica encapsulating material. 
     
     
         25 . The solar cell of  claim 24 , in which the non-spherical nanocrystals comprise quantum dot heterostructures that exhibit maximum absorption at a wavelength shorter than 500 nm and maximum emission at a wavelength between 500 nm and 700 nm. 
     
     
         26 . The solar cell of  claim 24 , in which the optical downshifting layer includes more than one type of encapsulated non-spherical quantum dot heterostructure. 
     
     
         27 . The solar cell of  claim 24 , in which at least some of the non-spherical nanocrystals have a quantum dot core surrounded by more than one shell.