Bringing some photonic structures for solar cells to ... - OSA Publishing

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Ludovic Escoubas,* Jean-Jacques Simon, Philippe Torchio, David Duché,. Sylvain Vedraine, Wilfried Vervisch, Judikaël Le Rouzo, François Flory,. Guillaume ...
Bringing some photonic structures for solar cells to the fore Ludovic Escoubas,* Jean-Jacques Simon, Philippe Torchio, David Duché, Sylvain Vedraine, Wilfried Vervisch, Judikaël Le Rouzo, François Flory, Guillaume Rivière, Gizachew Yeabiyo, and Hassina Derbal Aix-Marseille University, Institut Matériaux Microélectronique Nanosciences de Provence, IM2NP CNRS UMR 6242, Campus de Saint-Jérôme, Avenue Escadrille Normandie Niemen, Service 231, F-13397 Marseille Cedex 20, France *Corresponding author: ludovic.escoubas@univ‐cezanne.fr Received 23 July 2010; revised 1 December 2010; accepted 6 December 2010; posted 13 December 2010 (Doc. ID 132162); published 28 January 2011

A review on the use of photonic structures enabling a better absorption of solar radiation within solar cells is proposed. Specific geometric configurations, such as folded solar cells or fiber-based architectures, are shown to be promising solutions to reach better light absorption. Electromagnetic optimization of thin-film solar cells and the use of angular thin-film filters, proposed by several research groups, also provide solutions to better concentrate solar radiation within the active layers of solar cells. Finally, results on “photonized” solar cells comprising gratings or more advanced photonic components, such as photonic crystals or plasmonic structures, and their effects on light–matter interaction in solar cells are highlighted. © 2011 Optical Society of America OCIS codes: 040.5350, 350.6050, 310.6845, 310.1210, 250.5403, 230.5298.

1. Introduction

In photovoltaic cells, charges are generated from the absorption of photons in semiconductor materials. These materials are bulk crystalline silicon (c-Si) [1–3], thin-film microcrystalline, amorphous silicon [4–11], other alloys, such as Cu–In–Ga–Se or CdTe [12–23], or organic semiconductors in thin-film form [24–34]. Bilayer P-N junctions or bulk heterojunctions fabricated from these materials are used to separate charges and avoid electron–hole pairs (called excitons) recombination. It is obvious that optimizing the photocreation of charges in the materials composing the junction allows, with, of course, many other parameters, a better extraction of photocurrent from the solar cell. The main part (about 90%) of solar cells is made from c-Si, which is an indirect bandgap semiconductor. This intrinsic property of silicon means that only one-third of usable solar photons are 0003-6935/11/09C329-11$15.00/0 © 2011 Optical Society of America

absorbed in c-Si, thereby implying relatively thick (∼300 μm) cells. Inorganic thin-film solar cells are a potentially low-cost technology for fabricating solar cells, but, because of the small material thickness, optimizing the absorption of photons is of utmost importance. Organic plastic solar cells are also being used in scientific research in laboratories and in the industry (in companies such as Konarka or Solarmer Energy). The mechanical flexibility and low specific weight of the plastic materials open a wide field of applications; however, efficiency is limited in this case (record of 8.13% by Solarmer Energy) by the low absorptance of the donor and acceptor materials composing the bulk heterojunction in the range of the solar spectrum combined with the necessity of a small film thickness of the absorbing layer. Indeed, the maximum thickness of the organic absorbing layer (around 200 nm) is determined by the restricted mobility of the charge carriers. Thus, a significant part of the light is not absorbed. Therefore, improved light-trapping schemes are needed for thin-film inorganic and organic solar cells, 20 March 2011 / Vol. 50, No. 9 / APPLIED OPTICS

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and even for c-Si solar cells. In this paper, we discuss several possibilities for strongly enhancing the absorption of light in solar cells, from geometric optics to electromagnetic field optimization, gratings, photonic crystals, and plasmons. Examples of results recently obtained by several research groups around the world on light trapping by photonic structures is presented. 2. Geometric Optics

Several studies have been performed to define geometric configurations of solar cells with the aim of a better light harvesting [35,36]. For example, Carroll et al. at Wake Forest University in North Carolina has demonstrated a fiber-based architecture for use in organic photovoltaics [37,38] (Fig. 1). This device utilizes a thin absorbing layer with indium tin oxide (ITO) and Al electrodes as the cladding. Confined radiation modes within the active layer are responsible for the high level of conversion efficiency in this architecture. By increasing the losses into the cladding through coupling at a critical angle or reducing the fiber diameter, dramatic increases in current generation as well as external efficiencies are seen (Fig. 2). A company, FiberCell (http://www.fibercellinc.com/), was created in August 2007 to fabricate and commercialize such fiber-based organic solar cells. At Stanford University, Peumans et al. studied V-shaped light-trapping configurations of thin-film solar cells allowing a substantial increase of the photocurrent generation efficiency [39] (Fig. 3). The modeled external quantum efficiency (EQE) ηEQE of a CuPc=PTCBI cell is shown in Fig. 4 for the planar configuration 2α ¼ 180° and a V-fold light trap with 2α ¼ 30°. The ηEQE of an optimized planar cell whose active bilayer structure is 150 Å CuPc=100 Å PTCBI is shown for comparison. The model predicts an increase and a broadening of the ηEQE spectrum for

Fig. 1. (Color online) Schematic of the fiber photovoltaic cell architecture and light illumination, ray diagram of light propagation, confinement of light inside the active medium through reflection from Al, and refractive index difference between the layers [37,38]. C330

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Fig. 2. (Color online) (a) Comparison of the short circuit current density (Isc) for the full circumference coated with LiF/Al and 1=3 of the circumference coated with LiF/Al on fiber and power conversion efficiency as a function of the angle of incidence with respect to the axis of the optical fiber (fiber diameter 1:5 mm). (b) Comparison of the Isc (red curve) and power conversion efficiency (blue curve) as a function of the fiber diameter for light incident along the axis and perpendicular to the plane of the cleaved surface of the fiber (only 1=3 of the fiber circumference is coated with LiF/Al electrode). Reproduced with permission from the American Institute of Physics.

the cell in the V-shaped light trap due to multiple reflections. The largest relative gains are made in spectral regions where absorption is weak. With

Fig. 3. (Color online) Geometry of the V-shaped light-trapping structure [39]. The active layer (red in the scheme) is very thin compared to the thickness of the transparent substrate. The metal foil and the transparent conductive oxide constitute the back and front electrodes, respectively.

3. Electromagnetic Field Optimization in a Thin-Film Solar Cell

In thin-film solar cells, depending on the thicknesses and optical constants of the individual layers, the interaction of a light source with the multilayer composing the cell causes distinct distributions of the electromagnetic field due to light interferences and, thus, influences the energy absorption density (Fig. 6) [40–44]. Indeed, the dissipated energy of light within the thin-film solar cell Qðz; λÞ, which depends on the vertical position in the stack z and the wavelength λ, is directly correlated to the squared modulus of the electromagnetic field by the formula Qðz; λÞ¼αðλÞni =n0 j EðzÞ=E0 j2 ; Fig. 4. Modeled ηEQE of an ITO=100 Å CuPc=30 Å PTCBI=150 Å BCP=1000 Å Ag bilayer device for the planar configuration and in a V-shaped light trap with 2α ¼ 30°. The ηEQE of an optimized planar cell of structure ITO=150 Å CuPc=100 Å PTCBI=150 Å BCP=1000 Å Ag is also shown for comparison [39]. Reproduced with permission from the American Institute of Physics.

regard to polymer solar cells, P3HT:PCBM cells of different thicknesses (70, 110, and 170 nm) were configured as large area devices (cell area ¼ 2:4–3:2 mm2 ) that occupy the complete V-shaped area. J SC versus 2α measured under 32 mW=cm2 AM1:5 G illumination is shown in Fig. 5(a). Compared to the planar configuration, J SC increases by 68%, 57%, and 43% for the 170, 110, and 70 nm thick cells, respectively, for 2α ¼ 35°. The decrease in J SC for 2α < 35° is attributed to anisotropy of the optical constants of the P3HT:PCBM films. The polymer chain alignment is likely stronger for the thinner devices and may also explain the reduced benefit of the V-shaped light trap for the thinner P3HT:PCBM cells. The power conversion efficiency ηP and open circuit voltage V OC are plotted in Fig. 5(b). The 170 nm thick cell achieves ηP ¼ 3:5% at 2α ¼ 35° and a 52% increase over ηP ¼ 2:2% for the planar configuration. V OC systematically decreases as 2α decreases due to an increase in dark current proportional to the area of the cell.

Fig. 5. (a) J SC of ITO=500 Å PEDOT : PSS=P3HT : PCBM=1000 Å Al cells as a function of the V-shape opening angle 2α [39]. The active layer thicknesses are 70 (square), 110 (circle), and 170 nm (triangle). (b) V OC (filled symbols) and ηP (open symbols) of the same cells [39]. The lines are guides for the eye. Reproduced with permission from the American Institute of Physics.

ð1Þ

where αðλÞ is the spectral extinction coefficient, ni is the refractive index of the film ni , n0 is the refractive index of the incident medium, E0 is the amplitude of the incident electromagnetic field, and EðzÞ is the amplitude of the electromagnetic field at the vertical position z. Consequently, the electromagnetic modeling or even the thickness optimization of both organic [45,46] and inorganic thin-film solar cells is mandatory in order to gain optimal energy conversion. At the Technical University of Ilmenau (Germany), Hoppe et al. demonstrated an optimization method for organic solar cells, exploiting optical interference effects for maximizing the short circuit photocurrent [47]. Counterintuitively, the short circuit photocurrent is raised when lowering the absorption layer thickness. Furthermore, the fill factor also benefits from a thickness reduction, and, thus, the power conversion efficiency could be significantly raised. Lederer et al. at the Institut für Festkorpertheorie und Optik in Jena have studied the design of a rugate filter applied on top of a 10 μm thick monocrystalline silicon solar cell having at least one diffusing surface

Fig. 6. (Color online) Scheme of the electromagnetic field in a thin-film solar cell. The film thicknesses have to be optimized so that the maximal electromagnetic field value is localized in the active layer. Thus, the maximal light absorption occurs in the active layer. 20 March 2011 / Vol. 50, No. 9 / APPLIED OPTICS

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[48] (Fig. 7). This coating acts as an angle and wavelength selective filter, which increases the average path length and absorptance at long wavelengths without altering the solar cell performance at short wavelengths. The filter design is based on a continuous variation of the refractive index in order to minimize undesired reflection losses. More precisely, the direct sunlight impinges normally onto the solar cell, passes the angle selective filter, and is scattered isotropically by the diffusor. The Lambertian scatterer is assumed to be impedance matched to the following absorbing layer and, therefore, does not cause additional reflections at the diffusor–absorber interface. Electron–hole pairs are generated inside the absorbing layer, which was exemplarily chosen to be a 10 μm thick Si layer. Outgoing solar cell radiation, consisting of reemitted and unabsorbed light that already passed through the Lambertian scatterer, is trapped inside the cell according to the angle- and wavelength-dependent filter reflectance. At first glance, limiting the acceptance cone to the aperture of the circumsolar disk independent of the wavelengths should lead to the highest possible efficiency. The ideal filter [Fig. 8(a)] shows perfect transmission up to a wavelength of ∼870 nm. For longer wavelengths (up to ∼1300 nm), the filter should be perfectly reflecting only for angles of incidence larger than 2:5°. The optical characteristics of the filter above ∼1300 nm will not affect the conversion efficiency, as the energy of the light is too small to permit for absorption by Si. The exact threshold wavelength of the transition between perfect transmission and perfect reflection depends on the thickness, the wavelength-dependent absorption coefficient, and the spectra of the direct sunlight and the diffuse scattered light from the sky. For the 10 μm thick Si layer considered in this paper, the threshold wavelength was determined to be around 870 nm. Figure 8(b) shows the computed optimized rugate filter refractive index profile and Fig. 8(c) shows the angleresolved transmission spectrum of the rugate filter. The structure represented in Fig. 7 theoretically allows a 30.1% efficiency to be reached, whereas only a

28.7% efficiency limit was predicted by Kerr et al. [49] for a configuration without the rugate filter. More recently, at the University of California, Heeger et al. demonstrated, using a combination of optical modeling and device experiments, that a TiOx layer can act as an optical spacer between the bulk heterojunction and the metal contact in an organic tandem solar cell, thus providing an increase of both the short-circuit current and the fill factor [50]. Figure 9 shows the structure of the multilayer polymer tandem solar cell together with the chemical structure of its components. The tandem solar cell, in which two solar cells with different absorption characteristics are linked to use a wider range of the solar spectrum, was fabricated with each layer processed from solution with the use of bulk heterojunction materials comprising semiconducting polymers and fullerene derivatives. The charge-separation layer for the bottom cell is a bulk heterojunction composite of PCPDTBT and PCBM. The charge-separation layer for the top cell is a bulk heterojunction composite of P3HT and PC70 BM. The two polymer–fullerene junctions are separated by a transparent TiOx layer and a highly conductive hole transport layer, PEDOT:PSS. Electrons from the first cell combine with holes from the second cell at the TiOx − PEDOT : PSS interface. The TiOx layer serves five separate functions. First, hydrophilic TiOx separates the PEDOT:PSS that is cast on it from the aqueous solution, from the underlying hydrophobic PCPDTBT:PCBM chargeseparating layer of the front cell; the hydrophobic TiOx precursor becomes hydrophilic after conversion to TiOx . Second, the TiOx layer breaks the symmetry in the front cell, thereby creating the open circuit voltage. Third, the TiOx functions as an electron transport layer. Fourth, the TiOx functions as a hole-blocking layer because the top of the valence band of TiOx is sufficiently electronegative (8:1 eV below the vacuum) to block holes. Finally, the top TiOx layer (i.e., between the P3HT : PC70 BM chargeseparating layer of the back cell and the aluminum electrode) acts as an optical spacer that redistributes the light intensity to optimize the efficiency of the

Fig. 7. (Color online) Scheme of a thin-film solar cell comprising an angle selective filter (rugate filter) and a Lambertian scatterer. The active layer of the cell is a 10 μm thick Si film. C332

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Fig. 9. (Color online) Device structure of the polymer tandem solar cell (ITO). The absorption band of the P3HT complements the absorption band of PCPDTBT in visible range.

Fig. 8. (Color online) (a) Transmission of unpolarized light of the ideal wavelength and an angle selective filter [48]. (b) Optimized refractive index profile of the rugate filter (the z axis corresponds to the light propagation direction) [48]. (c) Angle-resolved transmission spectrum of the rugate filter [48]. Reproduced with permission from the Optical Society of America.

back cell. Power-conversion efficiencies of more than 6% were achieved at an illumination of 200 mW=cm2. 4. Gratings

Another strategy to improve light coupling in solar cells is the use of gratings either to diffract the light at high angles to trap it in the material or to couple

the light in the form of a guided wave [51–59]. The choice of the grating period with respect to the wavelength allows selecting diffraction or guided wave configurations. Submicrometric periodic patterning of an organic solar cell surface has been investigated at the CEA Saclay (France) in order to optimize the photovoltaic conversion efficiency of the device [60]. Patterning was achieved using a single-step all-optical technique based on photoinduced mass transport in azopolymer films. Control of the surface patterns geometry can be directly achieved through the polarization state and the intensity profile of the exciting beam. A periodic structure is inscribed onto the film surface using the interference pattern resulting from the superposition of two coherent light beams. The periodic structure presents a sinusoidal profile with a period equal to the interference fringes spacing. The period is easily controlled by the incidence angle of the beams onto the film surface while the modulation amplitude for a given period is directly dependent on the total energy received by the polymer film. The device structure is shown in Fig. 10(a). The patterned co(DR1/MMA) film, being a dielectric material, is used as a substrate for the photovoltaic cells. The chosen cell structure is based on the classical copper phthalocyanine (CuPc) and C60 heterojunction [labeled “active layer” in Fig. 10(a)]. In order to avoid light absorption by the co(DR1/MMA) film, the cell geometry is adapted to enable illumination through the top electrode. EQE measurements have been performed [Fig. 10(b)]. The optical reflection spectrum of a cell patterned with a period of 510 nm is compared to the reflection spectrum of a planar cell. The patterned cell absorption curve evidences a localized additional absorption band centered at the wavelength 555 nm, appearing as a “hole” in the reflected spectrum. In parallel, the EQE spectrum also shows an additional band at the same spectral 20 March 2011 / Vol. 50, No. 9 / APPLIED OPTICS

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5. Photonic Crystals

As in atomic crystal where the periodic position of atoms governs the electronic properties and the socalled electronic Bloch modes, the refractive index periodicity in one, two, or three dimensions in photonic crystals allows one to tailor the light propagation in the form of optical Bloch modes. Thus, several possibilities are offered to the photonic designer by photonic crystals for improving light trapping in solar cells: diffracting light as with gratings or guiding light through the excitation of resonances or using photonic band diagrams and dispersion curves to design specific photonic structures allowing an improved light–matter interaction [64–67]. The area of photonic-engineered solar cells is wide open. For example, at the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts, Kimerling et al. studied top-contacted crystalline Si solar cells combining reflection grating and onedimensional (1D) photonic crystal (distributed back reflector) [68]. The device structure is shown in Fig. 11(a). The textured photonic crystal acts as a backside reflector. The solar cell power conversion efficiency was measured using a sun simulator under AM1.5 conditions. The devices demonstrated significantly improved EQE and an overall increase of power

Fig. 10. (Color online) (a) Device structure of the solar cell comprising the light imprinted grating in the azopolymer film. The active layer is a classical CuPc and C60 heterojunction. (b) Reflection and EQE spectra for a cell patterned with a grating period of 510 nm. The corresponding spectra for a planar cell are also given for comparison [60]. Reproduced with permission from the American Institute of Physics.

position, showing that the increased absorption results in an increase of the current photogenerated in the solar cell. This feature originates directly from the coupling of a part of the incident light as a guided mode inside the layers through diffraction onto the patterned surface. Periodic submicrometer structures have also been investigated by Kim et al. at the Heeger Center for Advanced Materials in Gwangju (Korea) to improve the performance of organic solar cells [61]. A soft lithographic approach that uses photoresponsive azo polymer films as masters and poly(dimethylsiloxane) as stamps was used to form surface relief gratings on the active layers. An overall increase in power conversion efficiencies resulting from the enhancement of the short circuit current density demonstrates that periodic structures induce further photon absorption in the active film. Several strategies aiming at improving light absorption in solar cells using gratings have also been proposed by Gombert et al. at the Fraunhofer ISE [62] and Ingana ̈ s et al. at Linköping University (Sweden) [63]. C334

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Fig. 11. (Color online) (a) Device structure of the back reflector combining reflection grating and DBR. (b) J-V characteristics for solar cells with different back structures. Reproduced with permission from the American Institute of Physics.

conversion efficiency. Indeed, all the back structures improve the cell efficiency appreciably, as evidenced by the J–V characteristics of cells with different back structures in Fig. 11(b). Although all cells have a similar V oc of about 620 mV and a fill factor around 77%, the short circuit current density J sc increases from 23:3 mA=cm2 for the cell without back structure to 27:5 mA=cm2 for the one with Si3 N4 =Si DBR plus grating. In another paper [69], Joannopoulos et al., also at MIT, studied a solar cell made of a 2 μm thin film of cSi and a six-bilayer distributed Bragg reflector (DBR) in the back. In this paper, it is shown that the power generation can be enhanced by a relative amount of 24.0% by adding a 1D grating, 26.3% by replacing the DBR with a six-period triangular photonic crystal made of airholes in silicon, 31.3% by a DBR plus two-dimensional grating, and 26.5% by replacing it with an eight-period inverse opal photonic crystal. At the University of North Carolina, Lopez et al. reported organic solar cells with a photonic crystal nanostructure embossed in the photoactive bulk heterojunction layer. A scanning electronic microscope (SEM) image of the device is shown in Fig. 12(a). Light enters the photonic crystal and may be trapped as a quasi-guided mode in the photoactive region, leading to sharp enhancements in absorption, as shown near the band edge of Fig. 12(b). A threefold enhancement of the absorption in specific regions of

Fig. 12. (Color online) (a) Scanning electron micrograph of a hexagonal array of solar cell bulk organic heterojunction. Reproduced with permission from the American Chemical Society. (b) Theoretical spectral absorption at normal incidence for 400 nm 1D photonic crystal device compared to planar control cell for both s- and p-polarized light. Reproduced with permission from the American Institute of Physics.

the solar spectrum is demonstrated experimentally in part through multiple excitation resonances [70–72]. A strong efficiency improvement is obtained and results not only from greater absorption, but also from electrical enhancements. The use of a periodic nanostructuration of an organic solar cell made of the standard P3HT–PCBM organic semiconductor materials to allow slow Bloch modes coupling (group velocity close to zero) has also been reported [73]. Slow Bloch mode coupling is possible at the extrema of flat dispersion curves, as shown on the photonic band diagram of Fig. 13(a). This structure enables a strong light–matter interaction and, thus, a 35.6% increase of absorption in the 600–700 nm spectral range [Fig. 13(b)]. We can also

Fig. 13. (Color online) (a) Photonic band diagram of a photonic crystal. The dispersion curves are drawn versus the high symmetry directions of the first Brillouin zone. Slow optical Bloch modes can be coupled at the extrema of flat dispersion curves, as shown on the graph (orange circle). (b) Computed P3HT:PCBM blend bulk heterostructure absorption compared to P3HT:PCBM photonic crystal-ordered heterostructure absorption. A strong absorption enhancement is demonstrated in the case of the P3HT: PCBM photonic crystal. Reproduced with permission from the American Institute of Physics. 20 March 2011 / Vol. 50, No. 9 / APPLIED OPTICS

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Fig. 14. (a) SOI with 1:25 μm active Si. (b) Wafer-based 300 μm planar Si cell. Reproduced with permission from the American Institute of Physics.

notice the work of Seassal et al., which demonstrated the interest of patterning an absorbing amorphous Si (a-Si) layer as a planar photonic crystal. They showed that the absorption of a thick a-Si layer may significantly be increased by 35% over the whole 300–750 nm spectral range, these performances being relatively independent on the incident light polarization [74].

in Fig. 14. The samples used were 1:25 μm silicon-oninsulator (SOI) test cells [Fig. 14(a)] and planar passivated emitter rear locally diffused Si cells [Fig. 14(b)]. Figure 15 shows SEM images of the silver particles corresponding to mass thicknesses of 14, 16, 18, and 27 nm of silver deposited onto the SOI. The samples were annealed in nitrogen for 50 min. Image analysis of these SEM pictures shows the particle sizes to vary from 120 to 350 nm for the 14 and 27 nm cases, respectively. At wavelengths close to the bandgap of Si, a sevenfold enhancement was observed for wafer-based cells at λ ¼ 1200 nm

6. Plasmons

The scattering from metal nanoparticles near their localized plasmon resonance is also a promising way of increasing the light absorption in thin-film solar cells [75,76]. Enhancements in photocurrent have been observed for a wide range of semiconductors and solar cell configurations. For example, at the University of New South Wales (Australia), Green et al. investigated the suitability of localized surface plasmons on silver nanoparticles for enhancing the absorbance of silicon solar cells [77]. The device structures comprising silver nanoparticules for localized plasmon coupling are shown

Fig. 15. SEM pictures showing silver metal particles corresponding to a mass thickness of (a) 14 nm, (b) 16 nm, (c) 18 nm, and (d) 27 nm of silver [77]. Reproduced with permission from the American Institute of Physics. C336

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Fig. 16. (Color online) (a) Device fabricated with a thin silver film deposited onto ITO on a glass substrate. PEDOT:PSS was spun onto the silver layer followed by P3HT:PCBM and a barium/aluminum back electrode. (b) FESEM micrograph of a representative 2 nm silver film on ITO. (c) IPCE spectra of devices containing 1, 2, 3, and 4 nm silver films demonstrate the increased photocurrent at wavelengths over 500 nm. The strong absorption of the silver films on the IPCE is also seen as a dip centered near 450 nm. Reproduced with permission from the American Institute of Physics.

and up to a 16-fold enhancement was observed at λ ¼ 1050 nm for 1:25 μm thin SOI cells. Van De Lagemaat et al. at the National Renewable Energy Laboratory (Golden, Colorado) included plasmon-active silver nanoparticle layers in solutionprocessed bulk heterojunction solar cells [78], as shown in Fig. 16(a). Figure 16(b) shows a field emission (FE) SEM micrograph of a representative ITO substrate with a 2 nm (by mass) discontinuous silver film. The island nature of this film is apparent as white dots on a gray background. The average island diameter for 2 and 4 nm thick films is 11.6 and 13:0 nm, respectively. The incident photon to current conversion efficiency (IPCE) spectra are shown in Fig. 16(c). Two significant differences with respect to the reference are observed. The silver samples exhibit a decrease in quantum efficiency at 450 nm. This feature coincides with the dip observed in the transmission spectra owing to the surface plasmon resonance of the silver nanoparticles. At longer wavelengths (>500 nm), a strong increase in EQE is observed in all silver samples, except that with a nominal thickness of 4 nm. This increase is the source of the increased short circuit current and, thus, of efficiency. The resulting solar energy conversion efficiency for a bulk heterojunction photovoltaic device of P3HT:PCBM was found to increase from 1.3% to 2.2% for devices employing thin plasmonactive layers. Beyond about 2 nm of silver thickness, the efficiency decreases again. The V oc decreases slightly with increasing silver coverage. This may be due to a decrease in work function of the transparent electrode. Enhanced absorption of light up to 50% by plasmonic structures in organic solar cells has also been obtained by the OPTO-PV group in Marseille (France) in a 50 nm thick blend layer, including spin-coated silver nanospheres [79]. Spatial distributions of electromagnetic field power density in the structures, computed by finite difference time domain, show confinement of the power at the interface or in the vicinity of the nanoparticles, depending on the wavelength and the preferential directions. 7. Conclusion

Photonic engineering of solar cells, either inorganic or organic, composed of bulk or thin-film materials, is of high interest as, by simply optimizing structures such as gratings or photonic crystals or using plasmonic effects, a much better photon harvesting can be obtained. Higher photocurrent leading to a better photovoltaic efficiency of the devices has been demonstrated by several groups. Still, the optimization of photonic structures needs a perfect knowledge of the optical constants (wavelength dependency of refractive index and extinction coefficient) of materials in thin-film forms, either inorganic or organic. Furthermore, modelization software programs allowing both electromagnetic field and electric properties computations in solar cells comprising thin films, gratings, photonic crystals, or plasmons are highly

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