, Description:Field of the Invention:
The present invention relates to solar cell and more particularly to a solar cell with enhanced quantum efficiency.
Background of the Invention:
Thin film organic solar cells (OSCs) have recently fascinated a huge number of researchers due to their potential in low-cost photovoltaic (PV) devices. Polymer thin-films have poor absorption, however, hinders real-world device development when compared with other silicon (Si) based solar cells or gallium arsenide (Ga-As) based solar cells. Polymer thin films are attributed to the low carrier mobility in the OSC’s polymer active layer, which limits the active layer thickness and, thus, the absorption rate of the solar radiation. Above a certain thickness, quantum conversion efficiency dramatically decreases since free-carrier recombination becomes significant. In addition, thicknesses of OSCs’ active layer should be less than 250 nm to ensure optimum electronic properties. This restriction results in decreased quantum power conversion efficiencies of solar power. It has become very much essential to optimize radiation-trapping and antireflection properties in order to absorb incident solar radiation as much as possible over a wide range of wavelengths and large range of incident angle to improve quantum power conversion efficiencies comparable to other existing thin-film organic solar cells.
Several approaches had been suggested in order to trap and absorb maximum incident solar radiation by thin polymer active layers. An example of increasing absorption efficiencies in thin polymer films using metallurgic nano-structures, has been already developed. They were used to scatter incident solar radiation, which excites resonant scattering, resulting in the coupling and trapping of radiation in the absorbing layer. Using metallurgic nanoparticles as optical antenna to convert incident solar radiation into localized surface plasmon resonant (LSPR) modes has also been reported. Engineering the back reflector of the OSC and integrating plasmonic nanostructures with various shapes in the active layer to excite LSPR modes can also improve solar radiation absorption through near-field enhancement effects. Improving absorption efficiencies in thin films depends on the engineering of an enhanced electromagnetic density of states. This is distinct from the concept of light localization in a suppressed electromagnetic density of states such as a photonic band gap.
Efforts to broaden the absorption enhancement bandwidth have made use of combined plasmonic gratings in thin-film OSCs. In these combined structures, front and rear gratings were integrated on the top and bottom of the active layer, respectively. Enhancement predominantly arises from excited plasmonic resonance modes around 1-D planar back gratings at wavelengths greater than 760 nm. However, this is only applicable for incident light polarized in transverse-magnetic (TM) modes, and the absorption of incident transverse-electric (TE)-polarized light was dramatically reduced. This limits solar radiation-trapping capacities of the combined gratings-based OCSs since incident solar radiation is not specifically polarized and consists of TE- and TM-polarized components. Nanotechnology-enabled fabrication of such integrated plasmonic Nano gratings in ultrathin polymer PV cells remains a challenge. Periodic 1-D planar plasmonic backside contact gratings enabled absorption enhancement of 21% in 100-nm-thick OSCs for both TE- and TM-polarization light was reported.
Summary of the Invention:
In one embodiment of the present invention, we numerically explored using perforated metallurgic masks as plasmonic resonance antennas to enhance absorption. Metallurgic masks containing narrow two-dimensional slits provide broadband total optical transmission. At a specific oblique incident angle, these metallurgicmasks enable zero reflection over a wide range of wavelengths, without necessarily relying on SPRs. It is simply based on impedance matching. This transmission is equivalent to Brewster transmission in corrugated metallurgicmasks, providing total transmission for TM polarization. The anomalous phenomenon of Brewster transmission inspires possible realization of perfectly absorbing energy harvesting devices, which provide an efficient means of trapping solar radiation over a wide range of wavelengths. Integrating these metallurgicmasks on top of 52-nm thick OSCs yielding a broadband solar radiation absorption enhancement of up to 27% relative to the bare flat cells without using metallurgic masks for both TE and TM-polarizations. This corresponds to an increase in integrated AM1.5G absorption at normal incidence from 46% to 61.5% over the wavelength range of 4200 A0 to 8350 A0. In this metallurgic mask-based OSC, light could be concentrated to enable LSPR excitation at normal incidence. At off-normal incidence, impedance matching can improve absorption in thin OSCs over a wide range of wavelengths and large range of incident angle.
In another embodiment of the invention, instead of adding metallurgic masks (consisting of nano-rods) on top of OSCs, they are integrated on top of the Ag back reflector. It not only results in enhancement of absorption of solar radiation of up to 42% over the wavelength range of 3100 A0 to 8150 A0for both TE & TM polarized solar radiation but enables large integrated AM1.5G absorption over a wide range of incident angles as well.
Brief Description of the Drawings:
Fig. 1 shows an Unit cell of the OSC containing Ag nanorods added on top of the
back reflector.
Fig. 2 (a) shows Integrated absorption in the active layer as function of (a, h) with R = 80 nm.
Fig. 2 (b) shows optimized Integrated absorption for various a and h with respect to R.
Fig. 3 shows Absorption characteristics in the active layer by a flat cell without nano-rods, and absorption by a cell containing optimized nano-rods of (a, R, h) = (80, 400, 50) nm.
Fig. 4 shows Electric field profiles at waveguide mode 5000A0and LSPR mode 6750A0. (a), (b) Side view and (c), (d) top view. Brighter areas represent higher electric field.
Fig. 5 shows integrated absorption characteristics in the active layer with respect to incident angle of cells with and without optimized nano-rods under TE polarized (black) and TMpolarized (red) illumination.
Detailed Description of the Invention:
A single unit cell of the metallurgic mask-based OSC structure is depicted in Fig. 1. All numerical simulations were carried out by the three dimensional finite element method. The OSC’s active layer thickness of 52 nm was chosen in all simulations in this invention since this thickness ensures that a Fabry–Perot (FP)plasmonicmodes is expected to increase absorption of solar radiationover a wide range of wavelengths. The quantum absorption efficiency was calculated by integrating the absorption spectrum over 3100 A0 to 8150 A0and weighted according to the AM1.5G simulated solar spectrum. The resulting integrated absorption efficiency (AE) is given by
AE=(?_(3100 A^0)^(8150 A^0)¦?A(?)×AM1.5G(?) d??)/(?_(3100 A^0)^(8150 A^0)¦AM1.5G(?) d?)
In order to ensure maximum-integrated absorption efficiency (AE), numerous simulations were carried out for various nanorod dimensions of (R, h) and lattice constants (a). Optimization began with a nanorod of fixed radius R = 80 nm. The highest resulting AE was for (a,h) = (400,50) nm at normal incidence, as seen in Fig. 2(a). Gradually changing R resulted in the highest optimal AE with respect to (a, h) for each R, as shown in Fig 2(b). A maximum AE of 67% was obtained for the parameters (a,R,h) = (400,80,50) nm. This corresponds to 67% of the AM1.5G solar spectrum absorbed by the thin polymer active layer over the 3100 A0 to 8150 A0range.
Absorption enhancement of up to 42% was achieved corresponding to an increase in AE from from 46% to 61.5% for flat and nanorod-containing cells, respectively. At normal incidence, the integrated light absorption and optical properties of the OSC were the same for all polarizations.
Fig. 3 illustrates absorption spectra of a flat planar cell (dashedline) and cell containing nanorods with optimized parameters of (a,R,h) = (400,80,50) nm (solid line). Broadband (FP) resonance is excited in the flat planar OSC [11]. Combining this FP mode withabsorption enhancement over the 3100 A0 to 8150 A0 range is evident.
Such enhancement is explored by analyzing electric field profiles at chosen wavelengths. Fig. 4(a) plots the electric field profile at 5000 A0. Strong field is predominantly localized inside the polymer active layer. It reveals a strong coupling of the incoming solar radiation into waveguide modes resulted in the strong light confinement in the active layer. Electric field distribution at 6750A0reveals the absorption enhancement because of the excitation of LSPR modes around the nanorod, which provides strong field confinement at the nanorod-polymer interface, as seen in Fig. 4(b). At off-plasmonic-resonance mode at 5000 A0, weak field around the nanorod is shown in Fig. 4(c).
In contrast, at on-plasmonic-resonance mode at 6750A0, strong field localized around the nanorod is evident in Fig. 4(d) for the same field magnitude scale. An important aspect of PV cell performance is the ability to absorb all available incoming solar radiation at off-normal incidence, including TE- and TM-polarizations over a broad angular range. For TE modes, the electric field vector is perpendicular to theplane of incidence whereas for TM modes, the electric field vector is parallel to the plane of incidence. Calculated angular dependences of AE on TE- and TM-polarizations are shownin Fig. 5. Dashed lines represent AE for the reference flat cell, while solid lines correspond to the cell containing Ag nano rods with optimized parameters of (a,R,h) = (400,80,50) nm. The absorption spectrum of the latter for each polarization surpassed that of the reference flat cell, at least up to 60°. No substantial degradation in the AE of the nanorod-containing cell happens up to this angle for both polarizations. Interestingly, the TM-polarized light yields AE as much as that of TE over a wide angular range. Beyond this angle, the TM-polarized light absorption still exceeds that of the cell without nanorods. However, the enhancement is lost for the TE-polarized light in the presence of the nanorods. Other previous methods using plasmonic nanostructures to enhance optical absorption are mainly applicable for TM-polarized light. Even if adding cylindrical antennas on the same OSC can enhance absorption for both polarizations, the enhancement is just 22%, which is still smaller than 42% of the present OSC.
In this invention, integrating nano-rods on top of the metallurgical back contact of thin-film OSCs resulted in an absorption enhancement of up to 42% have been reported. They enabled quantum power conversion efficiency enhancement over a wide range of wavelengths and large range of incident angle for both TE & TM polarization. These results suggest that the proper positioning of plasmonic nano-rods may provide a route to the development of high-efficiency solar energy-harvesting devices based on polymer materials on flexible substrates.

Claims:I Claim:
1. A photovoltaic solar cell with enhanced quantum efficiency comprising of :
Metallurgical mask containing narrow two-dimensional slits to provide broadband total optical transmission wherein at a specific oblique incident angle, the said metallurgical masks enable zero reflection over a wide range of wavelengths characterized in that the said metallurgical masks are nano-rods which are integrated on top of Organic Solar Cell (OSCs) yielding a broadband solar radiation absorption enhancement of up to 27%.
2. A photovoltaic solar cell as claimed in claim 1 wherein the active layer thickness of the Organic Solar Cells is 52 nm.
3. A photovoltaic solar cell as claimed in claimed in claims 1 and 2 above wherein the maximum integrated absorption efficiency (AE) of 67% is achieved when the radius of Nano-Rod [R] is 80nm, height of the Nano-Rod [h] is 50nm and lattice constant [a] is 400 nm.
4. A photovoltaic solar cell as claimed in claim 1, 2 and 3 wherein the photovoltaic solar cell optionally uses Surface Plasmon resonant.
5. A photovoltaic solar cell as claimed in claim 1, 2, 3 and 4 wherein light is concentrated to enable localized Surface Plasmon Resonant excitation at normal incidence.
6. A photovoltaic solar cell as claimed in any of the claims above wherein there is an increase in integrated AM1.5G absorption at normal incidence from 46% to 61.5% over the wavelength range of 4200 A0 to 8350 A0.
7. A photovoltaic solar cell with enhanced quantum efficiency comprising of
Metallurgical mask, the said metallurgical masks being made up of nano-rods characterized in that the said metallurgical masks are integrated on the top of the Ag back reflector thereby enhancing the broadband solar radiation absorption up to 42%.
8. A photovoltaic solar cell as claimed in claim 6 wherein the active layer thickness of the Organic Solar Cells is 52 nm.
9. A photovoltaic solar cell as claimed in claim 6 or 7 wherein there is an increase in integrated AM1.5G absorption at normal incidence up to 42% over the wavelength range of 3100 A0 to 8150 A0 for both TE & TM polarized solar radiation.
10. A photovoltaic solar cell as claimed in claimed in claims 6,7 and 8 above wherein the maximum integrated absorption efficiency (AE) of 67% is achieved when the radius of Nano-Rod [R] is 80nm, height of the Nano-Rod [h] is 50nm and lattice constant [a] is 400 nm.
11. A photovoltaic solar cell as herein described with reference to the accompanying drawings.