Abstract:
Plasmonic nanocavity arrays and methods for enhanced efficiency in organic photovoltaic cells are described. Plasmonic nanocavities offer a promising and highly tunable alternative to conventional transparent conductors for photovoltaic applications using both organic and inorganic materials systems.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of U.S. Provisional Application No. 61/192,563 filed Sep. 19, 2008, the entire contents of which is incorporated herein by reference for all purposes. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to semiconductor devices. More particularly, the present invention relates to organic photovoltaic semiconductor devices. Devices of the present invention are particularly useful in solar cells, for example. 
       BACKGROUND 
       [0003]    Organic semiconductors are attractive for use in organic photovoltaic cells (OPVs) due to the potential for low-cost, high-throughput processing. While promising, limitations in the absorption efficiency must be addressed to improve OPV power conversion efficiency. In an OPV the absorption of light excites an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) as schematically illustrated in  FIG. 1 . These are analogous to the conduction band and valence band in inorganic semiconductors. This electron remains bound to the positively charged hole in the HOMO level by Coulombic forces. The bound electron-hole pair is electrically neutral and is typically referred to as an exciton. In order to extract power from the OPV this exciton is separated into its constituent electron and hole, which can then be collected at the cathode and anode respectively. 
         [0004]    Exciton dissociation in an OPV is realized using a heterojunction. A heterojunction is an interface between two organic materials with offset HOMO and LUMO levels. The material with the higher LUMO level is typically referred to as the electron donor, while the material with the lower LUMO level is typically referred to as the electron acceptor. The donor-acceptor (DA) interface will dissociate an exciton if the energy level offsets between the donor and acceptor are greater than the exciton binding energy, thus providing a path for electron relaxation through exciton dissociation. These materials are also the active optical absorbing materials in an OPV. As the excitons are created throughout the donor and acceptor layers they must diffuse to the DA interface in order to be dissociated into charge carriers. Typical exciton diffusion lengths for organic materials range from 3-40 nm. 
         [0005]    One limitation in OPVs is that the exciton diffusion length (LD˜10 nm) of the active materials is much shorter than the&#39;optical absorption length (LA˜100 nm), leading to necessarily thin absorbing layers. The optical absorption length is defined when the ratio of the transmitted light intensity over the incident light intensity is equal to exp(−1). The short LD of organic materials has lead to new design schemes for OPVs, such as using bulk heterojunctions (BHJ), planar-mixed layers and phosphorescent materials. Bulk heterojunctions and planar-mixed layers are device architectures that increase the amount of DA contact area by using interpenetrating networks, while maintaining continuous material pathways for charge collection. The interpenetrating network allows for an increased number of dissociation sites, reducing the distance an exciton must travel for dissociation. For most small molecule OPVs the interpenetrating network is achieved by thermally depositing two materials simultaneously, creating a mixture. In polymeric OPVs this morphology can be achieved by solution deposition of one blended layer that contains both the donor and acceptor materials. 
         [0006]    Phosphorescent materials typically have a longer LD than their fluorescent counterparts due to their increased exciton lifetime. Exciton lifetimes in phosphorescent materials are typically longer than fluorescent materials due to their less favorable optical transitions back to the ground state. Advances that exploit BHJs have generated OPV efficiencies exceeding 4%. 
       SUMMARY 
       [0007]    The present invention approaches the short diffusion length limitation in a different manner. Instead of modifying the device architecture to increase exciton dissociation, the present invention increases absorption in the thin active organic semiconducting layers. The absorption is increased through the use of surface plasmons and a plasmonic nanocavity, where more excitons are generated within a diffusion length of the DA interface and can contribute to photocurrent, therefore increasing device efficiency. 
         [0008]    Surface plasmons are electromagnetic surface waves confined to a metal-dielectric interface by coupling to the free electron plasma in metals. Due to their evanescent nature, SP waves are not limited by the diffraction limit, and can provide confinement of light on scales much smaller than the free-space wavelength. The possibility of subwavelength confinement and control of optical fields has generated intense interest in the rapidly developing field of plasmonics. The present invention uses the confinement of optical fields in plasmonic nanocavity arrays, thereby allowing the use of thin OPV layers without sacrificing optical absorption potential. 
         [0009]    Plasmonic photovoltaic structures have been examined previously in the context of dispersed metallic nanoparticles, increasing absorption in OPVs. While promising, the field enhancement from metallic nanoparticles is relatively short-range, difficult to tune, and their presence can quench the excitons. In the present invention a different approach is used that combines local SP field enhancement and strong plasmonic nanocavity modes by replacing the anode of an OPV cell with a nanostructured silver grating. The patterned anode, the OPV layers, and the cathode define a metal-insulator-metal (MIM) stack, a highly tunable structure that can offer deep subwavelength confinement and high field enhancement. To increase the free-space coupling, a periodic array of such MIM nanocavities can be used. 
         [0010]    The present invention provides OPV cells integrated with plasmonic nanocavities that combine field enhancement, tunable spectral response, and increased overall power conversion efficiency. Plasmonic nanocavity arrays for high-performance solar cells, organic or otherwise, are provided, thus allowing the use of thin active layers without sacrificing optical absorption potential. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The accompanying drawings, which are incorporated in and constitute a part of this disclosure, illustrate several aspects of the present invention and together with description of the exemplary embodiments serve to explain the principles of the present invention. A brief description of the drawings is as follows: 
           [0012]      FIG. 1  is a schematic view of device operation in an organic solar cell. Starting with light absorption (ηA), followed by exciton diffusion (ηED), charge transfer (ηCT), and charge collection (ηCC). 
           [0013]      FIG. 2  is a partial cross-sectional perspective view of an exemplary device structure in accordance with the present invention. 
           [0014]      FIG. 3  is a schematic cross-sectional view of an exemplary device structure made in accordance with the exemplary device structure shown in  FIG. 2 . 
           [0015]      FIG. 4  is a schematic view of another exemplary device structure made in accordance with the exemplary device structure shown in  FIG. 2 . 
           [0016]      FIG. 5  is a schematic view of another exemplary device structure in accordance with the present invention showing in particular a patterned anode having an array of openings. 
           [0017]      FIG. 6  is a schematic view of another exemplary device structure in accordance with the present invention showing in particular a patterned anode having an array of bumps. 
           [0018]      FIG. 7  is a graph that shows the photovoltaic response under simulated solar illumination of an exemplary device in accordance with the present invention. Agreement of the open-circuit voltage (circles) and fill factor (FF, squares) between the patterned device (filled symbols) and unpatterned device (open symbols) indicates that the patterned anode does not adversely affect transport and charge collection in the device. 
           [0019]      FIG. 8  is a graph that also shows the photovoltaic response under simulated solar illumination of an exemplary device in accordance with the present invention. The large increase in responsivity (R, diamonds) for the patterned device (filled symbols) compared to the unpatterned device (open symbols) provides a 3.2±0.4 fold increase in power conversion efficiency (triangles) at 118±12 mW/cm2 illumination. 
           [0020]      FIG. 9  is a graph that shows the polarization dependence of the photovoltaic response of exemplary devices in accordance with the present invention. Responsivity as a function of polarization is shown for devices having an anode including: unpatterned Ag (open circles), ITO (triangles), 50 μm diameter patterned Ag (circles) and 50 μm edge length patterned Ag (squares). The square pattern with a 50 μm edge has active area coverage of 32%, while the circular pattern with a 50 μm diameter has active area coverage of 25%. The larger coverage area leads to stronger polarization dependence and a larger overall increase in responsivity. The denoted error bars are representative on a fractional basis of all data points. 
           [0021]      FIG. 10  is a graph that shows simulated spectral response and internal electric field distribution of exemplary devices in accordance with the present invention. Finite-Difference-Time-Domain calculations of the photovoltaic spectral response for both patterned and unpatterned Ag anodes is shown. A distinct plasmonic nanocavity array mode is seen at 805 nm. 
           [0022]      FIG. 11  is a graph that shows simulated time averaged electric field intensity of exemplary devices in accordance with the present invention. A map of the electric field distribution at the 805 nm resonance is shown. The optical energy is tightly confined and enhanced in the organic active layers as a surface plasmon standing wave between the patterned Ag anode and the Al cathode. 
           [0023]      FIG. 12  is a graph that shows photovoltaic spectral response. External quantum efficiency (ηEQE) spectra for patterned and unpatterned devices. The increase in ηEQE at wavelengths greater than 500 nm is due to the enhanced light intensity in the device from the plasmonic nanocavity array. 
           [0024]      FIG. 13  is a graph that shows the ratio of patterned versus unpatterned ηEQE and simulated power enhancement inside an exemplary device in accordance with the present invention. The spectral features of the experimental data and simulation show close agreement with the simulation data having a slight red-shift for the large enhancement at 805 nm. The simulation enhancement at 805 nm is larger than the experimental data for reasons discussed below. 
           [0025]      FIG. 14  is a graph that shows simulated spectral response for exemplary devices in accordance with the present invention having varying nanocavity width. 
           [0026]      FIG. 15  is a graph that shows the time-averaged intensity of the plasmonic field within a nanocavity in accordance with the present invention. 
           [0027]      FIG. 16  is a graph that shows intensity of the plasmonic field of an exemplary device in accordance with the present invention. 
           [0028]      FIG. 17  is a graph that shows external quantum efficiency enhancement of an exemplary device in accordance with the present invention. 
           [0029]      FIG. 18  is a graph showing the increase in responsivity and total power conversion efficiency for an exemplary device in accordance with the present invention. 
           [0030]      FIG. 19  is a graph showing the open circuit voltage and fill factor for devices with an ITO, unpatterned silver, and patterned silver anode. The pattern consists of a two dimensional hole array created by a self assembly method. 
           [0031]      FIG. 20  is a graph showing device responsivity and efficiency for ITO, unpatterned silver, and patterned silver anode devices. The pattern consists of a two dimensional hole array created by a self assembly method. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    The exemplary embodiments of the present invention described herein are not intended to be exhaustive or to limit the present invention to the precise forms disclosed in the following detailed description. Rather the exemplary embodiments described herein are chosen and described so those skilled in the art can appreciate and understand the principles and practices of the present invention. 
         [0033]    Organic semiconductors are attractive as active media in photovoltaic cells due to the potential for high-throughput processing and compatibility with inexpensive, flexible substrates. While progress in organic photovoltaic cell (OPV) development has been significant, fundamental limitations relating to device absorption efficiency remain to be addressed to realize significant gains in power conversion efficiency. 
         [0034]    Optical absorption in an organic semiconductor results in the creation of a tightly bound exciton. The exciton diffuses to an electron donor-acceptor heterojunction where the exciton is dissociated to generate a photocurrent. Typically, OPVs comprise thin active layers because the exciton diffusion length (LD˜10 nm) is much shorter than the optical absorption length (LA˜100 nm). Several device architectures have been exploited to overcome the short LD, including bulk and planar-mixed heterojunctions, and the use of long LD phosphorescent materials. 
         [0035]    The present invention overcomes the short LD by incorporating the OPV into a plasmonic nanocavity array. Surface plasmons are electromagnetic surface waves coupled to the free electron plasma in metals and confined to a metal-dielectric interface. The plasmonic nanocavity exploits the coupling between surface plasmons in one or both of a nanopatterned metallic anode and a metallic cathode. 
         [0036]    Devices in accordance with the present invention trap and manipulate incident light, leading to the excitation of surface plasmon resonances and high absorption. The use of a plasmonic nanocavity in accordance with the present invention in an exemplary OPV can provide a significant increase in device power conversion efficiency compared to a similar device having an unpatterned metallic anode. For example, in one exemplary embodiment, a 3.2±0.4 fold increase in device power conversion efficiency compared to a similar device having an unpatterned metallic anode under AM1.5G simulated solar illumination can be provided. 
         [0037]    Plasmonic waveguides and cavities, such as metal-insulator-metal (MIM) stacks or metal-semiconductor-metal (MSM) gratings can provide significant optical field confinement and resonant absorption. Although any subwavelength metallic structure can generate surface plasmons, one preferred technique includes surface plasmon-mediated enhanced light transmission through periodic subwavelength aperture arrays. The periodic array acts as a grating coupler providing the necessary momentum matching between incident photons and the surface plasmon modes, resonantly enhancing the surface plasmon field, and the overall transmission. Similarly, periodically arranged plasmonic MIM nanocavities (a plasmonic nanocavity array), are able to both resonantly generate and tightly confine surface plasmon waves. 
         [0038]    The application of surface plasmon enhancement in semiconductor devices has been previously examined in the context of metal nanoparticles and thin metallic interfacial layers. Enhancement of the optical field in the vicinity of metallic nanoparticles has been shown to increase absorption in both OPVs and silicon based photovoltaic cells. The field enhancement from metallic nanoparticles, however, is localized to within 5-10 nm and is difficult to control and optimize. Similar approaches have involved the use of energy transfer via surface plasmons created by an interfacial Ag film, with the surface plasmons acting as an antenna through which to excite a photoactive organic semiconductor. As is the case with metallic nanoparticles, the spectral response of this architecture is not easily tuned. 
         [0039]    The present invention provides plasmonic OPVs where the spatial extent of the optical field can be tightly localized and the optical energy enhanced within a thin donor-acceptor OPV. The combination of local field enhancement near the patterned metallic anode and a strong field enhancement due to the plasmonic nanocavity provides integration of plasmonic nanostructures in semiconductor devices. The spectral response of this technique is highly tunable and broadly applicable in enhancing the optical absorption and overall power conversion efficiency (η P ) in photovoltaic systems in accordance with the present invention. 
         [0040]    Referring to  FIGS. 2-4 , an exemplary device structure is illustrated. In particular,  FIG. 2  is a partial cross-sectional perspective view of an exemplary device structure in accordance with the present invention.  FIG. 3  is a schematic cross-sectional view of an exemplary device structure made in accordance with the exemplary device structure shown in  FIG. 2 .  FIG. 4  is a schematic view of another exemplary device structure made in accordance with the exemplary device structure shown in  FIG. 2 . 
         [0041]    With reference to  FIG. 2  initially, device structure  10  comprises substrate  12 , patterned metallic anode  14 , first organic layer  16 , second organic layer  18 , third organic layer  20 , and metallic cathode  22 . Patterned metallic anode  14  preferably comprises plasmonic nanocavities  24  as described in greater detail below. In  FIG. 3 , exemplary device structure  26  is consistent with exemplary device structure  10  of  FIG. 2  and includes an exemplary scale in microns along the x-axis and the z-axis. Additionally, exemplary device structure  28  of  FIG. 4  comprises a structure consistent with exemplary device structures  10  and  28 . Device structure  28  includes exemplary organic layers comprising first organic layer  30  comprising copper phthalocyanine (CuPc), second organic layer  32  comprising C 60 , and third organic layer  34  comprising bathocuproine. 
         [0042]    An exemplary device structure in accordance with the present invention preferably comprises a glass substrate coated with a 30-nm-thick film of metal such as Ag. Subwavelength slits are preferably patterned into the Ag via focused ion beam (FIB) milling, thus defining the edges of the plasmonic nanocavities. In an exemplary embodiment such pattern includes a 50 μm diameter array of periodic slits with a period of 415 nm and width of 120 nm. The resulting exemplary plasmonic nanocavities thus have a width of 295 nm. The periodic array generates and sustains surface plasmon waves, which are channeled into the photoactive layers. In an exemplary embodiment organic semiconductor layers are preferably sequentially deposited onto the Ag film, and may comprise, for example, a 20-nm-thick film of copper phthalocyanine (CuPc) and a 40-nm-thick film of C 60 . An optional 10-nm-thick film of bathocuproine (BCP) can be deposited on C 60  and preferably functions as an exciton blocking layer. In an exemplary embodiment, organic layers and the cathode are preferably deposited through a metal shadow mask that defines devices having a diameter of 115 μm. The structure is then preferably capped with a metallic cathode such as a 50-nm-thick Al layer, for example. 
         [0043]    Device structures in accordance with the present invention are not limited to the exemplary materials listed. Anode and cathodes can include alternative metals. The organic layers can include any organic semiconducting materials that function as electron donors (ex. CuPc) and electron acceptors (ex. C 60 ). In addition to BCP as an exciton blocking layer, any material that functions to block excitons and still conduct charge can be used. Also, alternative substrates such as plastics and foils can be used be used instead of or in addition to glass substrates. 
         [0044]    Exemplary structures described herein comprise planar photovoltaic devices, however the structures of the present invention can be used with bulk and mixed heterojunction devices. Other devices using nanocrystalline materials, semiconducting polymers, and polymer/nanoparticle combinations can also be incorporated. 
         [0045]    Although certain dimensions, geometries, and periodicities are described herein, such as for the nanocavities, such parameters are exemplary. Devices including patterned contacts that cover any desired portion (including the complete active area) of the device can be made. The metallic contact that is patterned is not limited to the anode and the metallic cathode can also be patterned. 
         [0046]    In  FIG. 5  another exemplary device structure  36  in accordance with the present invention is schematically shown. Device structure  36  preferably comprises a patterned anode  38  having an array of openings  40 . The array is preferably periodic but not required to be so. Any desired spacing can be used for the array. The openings are preferably circular but not required to be so. Any desired geometry can be used. 
         [0047]    In  FIG. 6  another exemplary device structure  42  in accordance with the present invention is schematically shown. Device structure  42  preferably comprises a patterned anode  44  having an array of bumps  46 . The array is preferably periodic but not required to be so. Any desired spacing can be used for the array. Any desired geometry for bumps  46  can be used. 
         [0048]    Device performance for both patterned and unpatterned exemplary devices in accordance with the present invention under simulated AM1.5G solar illumination are shown in  FIGS. 7 and 8 . The dependence of the open-circuit voltage (VOC) and fill factor (FF) on illumination intensity ( FIG. 7 ) are similar for both patterned and unpatterned devices. This indicates that the patterning of the anode does not detrimentally impact the collection of dissociated charge carriers in the device. Device dark currents were also noted to be independent of patterning. The η P  increases with patterning by a factor of 3.2±0.4 under 118±12 mW/cm 2  illumination. This large increase is due to a similar increase in device responsivity ( FIG. 8 ) as a result of the enhancement of the internal optical field, and the device absorption efficiency. The enhancement is independent of illumination intensity up to 500 mW/cm 2 . 
         [0049]    To confirm the role of the plasmonic nanocavities in enhancing η P  in accordance with the present invention, the responsivity of exemplary devices was measured as a function of incident light polarization and can be seen in  FIG. 9 . For unpatterned devices, very little modulation with incident polarization was observed, with the in-phase to out-of-phase responsivities differing by 3.2%. For comparison, similar devices deposited on a glass slide pre-coated with indium tin oxide (ITO) as an anode also showed a weak modulation of 8.6% with incident light polarization. 
         [0050]    Patterned substrates exhibited a very strong dependence on polarization, because the enhancement is greatest when the incident electric field is polarized perpendicular to the slit array (TM polarized). The modulation with polarization was observed to increase as the patterned device area was increased. Two exemplary configurations were examined: the first pattern had a diameter of 50 gm, while the second pattern was square with an edge length of 50 μm. In both cases, the device active area had a diameter of 115 μm. The observed responsivity enhancement with polarization for the square pattern (56.4%) was more than twice as large as that of the circular pattern (26.2%), consistent with the increase in patterned device from 25% to 32%. 
         [0051]    Devices in accordance with an embodiment of the present invention preferably overcome the strong polarization dependence by utilizing a two-dimensional array of subwavelength apertures that couple to surface plasmon modes without exhibiting the intrinsic polarization-dependence of slits. Furthermore, certain geometries, such as plasmonic MIM cavities and void plasmon structures have omnidirectional absorption resonances that are independent of the incident angle and can be used with devices in accordance with the present invention. 
         [0052]    The increase in optical field intensity within the OPV layers as a result of the plasmonic nanocavity array was quantified using finite-difference-time-domain (FDTD) simulations. Incident light from the glass side was TM polarized, to maximize the cavity response. To give a reasonable comparison to the efficiency enhancement seen in the experiments, only the optical energy that contributes to the photocurrent generation was numerically simulated, that is, the energy confined to roughly within a diffusion length of the CuPc/C 60  interface. (L D,CuPc =10 nm and L D,C60 =40 nm). 
         [0053]      FIG. 10  shows the simulated spectral response for exemplary patterned and unpatterned devices. The strongest optical mode of the structure is at a wavelength of 805 nm.  FIG. 11  maps the strong confinement and enhancement of the optical field within the organic layers at this resonance in the form of surface plasmon standing waves. As the incident light feeds the surface plasmon nanocavity mode, only 5% is reflected. A smaller resonance is observed at a wavelength of 570 nm. By patterning the metallic anode film and forming the plasmonic nanocavity array, a maximum enhancement of the optical intensity of around 50 is observed. Over the rest of the spectrum, a broadband enhancement between 2 to 4 times is observed, consistent with the attenuating nature of the unpatterned metallic anode, and the slits serving to open a fraction of the anode area. 
         [0054]    Because the absorption losses are relatively low at the maximum surface plasmon nanocavity peak at 805 nm, a very well defined surface plasmon nanocavity mode is observed. This configuration was chosen in order to resolve the novel plasmonic nanocavity enhancement at 805 nm. Matching the resonances of the absorption to a strong surface plasmon nanocavity mode can maximize the efficiency enhancement, which can be partly achieved by tuning the periodicity of the patterned structure. 
         [0055]    Simulations of the spectral response of the structure were compared to external quantum efficiency (ηEQE) spectra for the exemplary plasmonic nanocavity devices in accordance with the present invention.  FIG. 12  shows the η EQE  for both patterned and unpatterned devices, with patterned devices exhibiting a broadband enhancement in spectral response with particular emphasis at larger wavelengths. Because the η EQE  depends proportionally on the absorption efficiency of the OPV, an enhancement in the internal field leads to an enhancement in the observed spectral response. Thus, the ratio of the η EQE  between patterned and unpatterned devices is a measure of the relative increase in absorption efficiency, and the internal optical field. This ratio of patterned to unpatterned η EQE  is shown in  FIG. 13 , and is compared to the FDTD simulation of the optical field enhancement in this nanostructure. The experimental spectral response is reproduced well in the simulation, with the most striking difference being the difference in the magnitude of the observed and predicted field enhancement. This difference arises because the simulation assumes that the entire device area is patterned, and that the incident light polarization is aligned for maximum response. Experimentally, the patterned area is only a fraction of the total device area, and the incident light is randomly polarized. 
         [0056]    In an exemplary embodiment of the present invention anodes can be patterned through a self-assembly technique. In this exemplary process polymer beads such as those comprising polystyrene or the like are deposited onto a substrate such as glass, for example and organized in a uniform pattern through self-assembly. The beads are then reduced in size such as by etching, for example. A layer of metal, such as silver, for example, is then preferably deposited over the beads, after which the beads are removed to create a two-dimensional array of openings (typically holes) in the metal anode. 
         [0057]    Devices in accordance with the present invention can be fabricated or otherwise provided on top of these substrates with performance of exemplary devices shown in  FIGS. 19 and 20 .  FIG. 19  shows that devices comprising unpatterned and patterned silver have similar open circuit voltages. This demonstrates that anode patterning does not adversely affect device performance. Also shown in  FIG. 19  is the improvement in fill factor of an exemplary device with a patterned anode.  FIG. 20  demonstrates slight increases in device responsivity and efficiency for devices including a patterned anode. Both  FIGS. 19 and 20  show that device performance with either silver anode is slightly lower but comparable to the performance of devices having an ITO anode. 
         [0058]    In an embodiment of the present invention device efficiency can be increased by a factor of 3.2±0.4 by incorporating a plasmonic nanocavity array into the device architecture in accordance with the present invention. The use of a plasmonic nanocavity allows for thin organic semiconducting layers to efficiently harvest light without sacrificing absorption potential. With optimization of pattern design and materials selection further enhancements in efficiency are provided. 
       Optical Modeling 
       [0059]    Finite-difference-time-domain (FullWAVE™, RSoft Design Group) simulations were performed using an adaptive two-dimensional mesh, with a width of 400 nm and a length of 3000 nm. The mesh was periodic in its width, defining the periodicity of the plasmonic nanocavities (400 nm). Each nanocavity was 267 nm wide, leaving 133 nm wide slits. The adaptive mesh had a nominal grid spacing of 5 nm, a material interface grid spacing of 1 nm, and incorporated at least 15 grid points between each material interface. The complex dielectric functions of the exemplary Ag and Al films were modeled as having a Drude-Lorentz response. A non-linear least-squares fit of multiple Lorentzian oscillators was used to model the complex dielectric functions 
         [0060]    Finite-difference time-domain (FDTD) simulations were also used to aid in device design, optimization, and characterization. To offer a reasonable comparison to experimental results, only the optical power within a diffusion length of the CuPc/C 60  interface was numerically simulated (10 nm for CuPc, 40 nm for C 60 ). Material parameters were modeled with a non-linear least-squares fit of multiple Lorentzian oscillators to measured optical constants. Metals were simulated with a Lorentz-Drude model.  FIG. 14  shows simulated spectral responses where the nanocavity width is changed from 199 nm to 359 nm, showing an easily tunable response near 800 nm. The nanocavity periodicity was fixed at 409 nm. The spectral response is also tunable (not shown) by changing the periodicity of the nanocavity arrays. The spectra are normalized to the response of an unpatterned device, showing significant power enhancement. The blackened spectrum has the cavity width of the fabricated device. On resonance, only about 7% of the incident light is reflected. A much less pronounced enhancement resonance, which occurs mainly on the Ag-glass interface, is seen near 575 nm.  FIG. 15  shows the spatial field distribution of the nanocavity in the form of SP standing waves. Taking a vertical slice (dash), enhancement is seen through the thickness of the device layers, shown in  FIG. 16 . 
         [0061]    The external quantum efficiency (η EQE ) depends proportionally on optical absorption efficiency. The ratio of η EQE  for a patterned versus an unpatterned device is thus a measure of the relative internal optical field and efficiency enhancement.  FIG. 17  shows the η EQE  enhancement, agreeing well with  FIG. 14 , with peaks near 800 nm and 575 nm. This increase corresponds to a 2.75 fold increase in responsivity R and total power conversion efficiency η P , shown in  FIG. 18 , overall efficiency increased by 3.2. Only 16% of the active area was patterned due to FIB milling limitations, however it is contemplated that the enhancement could be higher. Electrical properties such as open-circuit voltage, fill factor, and dark current magnitude were relatively unchanged, indicating that patterning did not adversely affect collection of dissociated charge carriers, and that the enhancement was an optical effect. 
       Device Fabrication 
       [0062]    Exemplary devices were grown on glass substrates coated with a 30-nm-thick layer of Ag deposited by thermal evaporation at 10 −7  Torr. Subwavelength periodic nanostructures were patterned into the Ag layer by focused ion beam milling. Prior to deposition of organic thin films, the Ag coated substrate was degreased with solvents. A shadow mask used to define the device active area was aligned on top of the patterned substrate using an optical microscope. The organic layers and Al cathode were deposited in direct succession without vacuum break by thermal evaporation at 3×10 −7  Torr. All organic materials were used as received. Devices were also grown on glass substrates pre-coated with a 140-nm-thick layer of indium tin oxide (ITO) having a sheet resistance of ˜15Ω/□ (Colorado Concept Coatings). These substrates were degreased with solvents and then cleaned with UV-ozone exposure prior to device growth. Although focused ion beam milling was used here to make the exemplary patterned nanocavities, any method to generate a patterned metallic contact can be used. 
         [0063]    Exemplary nanocavities were fabricated having a width of 289 nm, a periodicity of 409 nm, and covering an area of 1661 μm 2 . In an embodiment of the present invention an exemplary device comprised a total area of 10381 μm 2 . Simulated solar illumination (AM1.5G) incident from the glass side excited surface plasmons on the slits, which were resonantly fed into the nanocavities, increasing absorption and overall power conversion efficiency. The plasmonic enhancement was quantified by making reference devices where the metallic anode was not patterned. 
       Device Characterization 
       [0064]    Exemplary devices were tested under broadband optical excitation using a solar simulator (Newport Model 96000) with an Air Mass 1.5 Global filter. Incident light was focused onto a testing plate having a 1.5 mm diameter aperture. The incident light intensity was measured using a Newport 818P-010-12 High-Power Detector. Current-voltage sweeps were performed with an Agilent Technologies 4155C Semiconductor Parameter Analyzer. The external quantum efficiency was measured using a 100 W Xenon arc lamp (Newport Model 6257) and an Oriel cornerstone 130 1/8m monochromator. The incident light intensity was measured as a function of wavelength using a Newport 818-UV Low Power detector. To improve the photocurrent signal-to-noise ratio, the incident light was chopped with a Stanford Research Systems (SRS) SR540 Optical Chopper at a frequency of 200 Hz and the current was input into an SRS SR570 low-noise current pre-amplifier and measured using an SRS SR810 DSP lock-in amplifier. In all Figures containing device characterization data, error bars are calculated based on the statistical variation in device-to-device performance. 
       REFERENCES 
       [0065]    The following references are each entirely incorporated by reference herein in for all purposes.
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         [0096]    The present invention has now been described with reference to several exemplary embodiments thereof. The entire disclosure of any patent or patent application identified herein is hereby incorporated by reference for all purposes. The foregoing disclosure has been provided for clarity of understanding by those skilled in the art. No unnecessary limitations should be taken from the foregoing disclosure. It will be apparent to those skilled in the art that changes can be made in the exemplary embodiments described herein without departing from the scope of the present invention. Thus, the scope of the present invention should not be limited to the exemplary structures and methods described herein, but only by the structures and methods described by the language of the claims and the equivalents of those claimed structures and methods.