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  • Editor: Bernard Kippelen
  • Vol. 20, Iss. S2 — Mar. 12, 2012
  • pp: A177–A189
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Highly absorbing solar cells—a survey of plasmonic nanostructures

Ricky B. Dunbar, Thomas Pfadler, and Lukas Schmidt-Mende  »View Author Affiliations


Optics Express, Vol. 20, Issue S2, pp. A177-A189 (2012)
http://dx.doi.org/10.1364/OE.20.00A177


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Abstract

Plasmonic light trapping in thin film solar cells is investigated using full-wave electromagnetic simulations. Light absorption in the semiconductor layer with three standard plasmonic solar cell geometries is compared to absorption in a flat layer. We identify near-field absorption enhancement due to the excitation of localized surface plasmons but find that it is not necessary for strong light trapping in these configurations: significant enhancements are also found if the real metal is replaced by a perfect conductor, where scattering is the only available enhancement mechanism. The absorption in a 60 nm thick organic semiconductor film is found to be enhanced by up to 19% using dispersed silver nanoparticles, and up to 13% using a nanostructured electrode. External in-scattering nanoparticles strongly limit semiconductor absorption via back-reflection.

© 2012 OSA

1. Introduction

Solar cell technology can provide virtually limitless clean energy by converting solar irradiation into electricity. In order to enable widespread implementation of solar cells, module cost efficiency indicators such as the cost per peak power output (€/Wp) need to be improved. Low-crystallinity semiconductors such as amorphous silicon and organics are inexpensive relative to crystalline silicon, but their poor charge transport properties demand that they are used in thin films (the order of a few μm or less) so that collection paths for charge carriers are short. This leads to incomplete light absorption; much of the incident light is reflected out of the system again. Thus, thin-film solar cell manufacturers must endeavor to reach a compromise between good charge transport properties (thinner films) and high absorption (thicker films) [1

1. S. R. Forrest, “The limits to organic photovoltaic cell efficiency,” MRS Bull. 30(01), 28–32 (2005). [CrossRef]

]. It is possible to overcome this compromise by using thin films (with good charge transport) combined with light-trapping, where incident light is trapped in the semiconductor film and ultimately absorbed. In this way we simultaneously obtain low-cost, highly absorbing solar cells with good charge transport properties [2

2. J. Weickert, R. B. Dunbar, H. C. Hesse, W. Wiedemann, and L. Schmidt-Mende, “Nanostructured organic and hybrid solar cells,” Adv. Mater. (Deerfield Beach Fla.) 23(16), 1810–1828 (2011). [CrossRef] [PubMed]

]. Light-trapping structures can take many forms as outlined in recent publications [2

2. J. Weickert, R. B. Dunbar, H. C. Hesse, W. Wiedemann, and L. Schmidt-Mende, “Nanostructured organic and hybrid solar cells,” Adv. Mater. (Deerfield Beach Fla.) 23(16), 1810–1828 (2011). [CrossRef] [PubMed]

,3

3. M. Niggemann, M. Riede, A. Gombert, and K. Leo, “Light trapping in organic solar cells,” Phys. Status Solidi A 205(12), 2862–2874 (2008). [CrossRef]

]. Of particular interest is plasmonic light-trapping, where metallic (typically Ag or Au) nanostructures enable excitation of localized surface plasmons (LSPs, non-propagating excitations of conduction electrons within a metallic nanostructure) and or surface plasmon polaritons (SPPs, surface bound electromagnetic waves that propagate along metal-dielectric interfaces) which traps light at the respective metal-semiconductor interface. For a number of typical semiconductor-metal configurations, this light is preferentially absorbed in the semiconductor [4

4. H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nat. Mater. 9(3), 205–213 (2010). [CrossRef] [PubMed]

], leading to an overall enhancement in the absorption over the flat (no metallic nanostructure) case. Providing the implementation of the light-trapping structure does not modify the solar cell’s internal quantum efficiency (a measure of charge transport properties), the absorption enhancement corresponds to an enhancement in the power conversion efficiency.

A wide variety of plasmonic solar cells (PSCs) has been investigated experimentally and theoretically towards achieving performance superior to that of a flat layer solar cell (Fig. 1(a)
Fig. 1 (a) Standard (flat) solar cell architecture. The direction of incident light is indicated. (b–d) Three classes of plasmonic solar cells designed to achieve enhanced semiconductor light absorption.
). It is possible to group PSCs into three main classes: nanoparticles dispersed in the semiconductor layer (Fig. 1(b)), the nanostructured metallic electrode (Fig. 1(c)) and in-scattering nanoparticles (Fig. 1(d)) [2

2. J. Weickert, R. B. Dunbar, H. C. Hesse, W. Wiedemann, and L. Schmidt-Mende, “Nanostructured organic and hybrid solar cells,” Adv. Mater. (Deerfield Beach Fla.) 23(16), 1810–1828 (2011). [CrossRef] [PubMed]

,4

4. H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nat. Mater. 9(3), 205–213 (2010). [CrossRef] [PubMed]

]. Each PSC is fundamentally different in terms of absorption enhancement mechanisms.

Nanoparticles (NPs) dispersed in the semiconductor layer (Fig. 1(b)) can, in addition to enhancing the absorption, also improve the series resistance [5

5. K. Kim and D. L. Carroll, “Roles of Au and Ag nanoparticles in efficiency enhancement of poly(3-octylthiophene)/C-60 bulk heterojunction photovoltaic devices,” Appl. Phys. Lett. 87, 203113 (2005).

]. Absorption enhancement is typically observed in the near-field of the NP and it has recently been argued that this enhancement arises primarily due to the excitation of scattered modes at the NP (rapidly absorbed in highly absorbing materials), as opposed to excitation of LSP modes [6

6. J. Y. Lee and P. Peumans, “The origin of enhanced optical absorption in solar cells with metal nanoparticles embedded in the active layer,” Opt. Express 18(10), 10078–10087 (2010). [CrossRef] [PubMed]

,7

7. D. H. Wang, Y. Kim, K. W. Choi, J. H. Seo, S. H. Im, J. H. Park, O. O. Park, and A. J. Heeger, “Enhancement of donor-acceptor polymer bulk heterojunction solar cell power conversion efficiencies by addition of Au nanoparticles,” Angew. Chem. Int. Ed. Engl. 50(24), 5519–5523 (2011). [CrossRef] [PubMed]

]. However, Zhu et al. found that light trapping in this geometry relies heavily on plasmonic excitation, by showing a strong fall-off in enhancement for nanometer-thin NP dielectric shells [8

8. J. F. Zhu, M. Xue, H. J. Shen, Z. Wu, S. Kim, J. J. Ho, A. Hassani-Afshar, B. Q. Zeng, and K. L. Wang, “Plasmonic effects for light concentration in organic photovoltaic thin films induced by hexagonal periodic metallic nanospheres,” Appl. Phys. Lett. 98(15), 151110 (2011). [CrossRef]

]. Excitation of SPPs on the flat back-electrode is also possible in this PSC via scattering at the NPs. The relative importance of NP scattering and absorption is highly size- and shape-dependent [6

6. J. Y. Lee and P. Peumans, “The origin of enhanced optical absorption in solar cells with metal nanoparticles embedded in the active layer,” Opt. Express 18(10), 10078–10087 (2010). [CrossRef] [PubMed]

,9

9. S. Pillai and M. A. Green, “Plasmonics for photovoltaic applications,” Sol. Energy Mater. Sol. Cells 94(9), 1481–1486 (2010). [CrossRef]

]. Dispersed NP organic PSCs using nanowires [10

10. C. H. Kim, S. H. Cha, S. C. Kim, M. Song, J. Lee, W. S. Shin, S. J. Moon, J. H. Bahng, N. A. Kotov, and S. H. Jin, “Silver nanowire embedded in P3HT:PCBM for high-efficiency hybrid photovoltaic device applications,” ACS Nano 5(4), 3319–3325 (2011). [CrossRef] [PubMed]

], truncated octahedral NPs [7

7. D. H. Wang, Y. Kim, K. W. Choi, J. H. Seo, S. H. Im, J. H. Park, O. O. Park, and A. J. Heeger, “Enhancement of donor-acceptor polymer bulk heterojunction solar cell power conversion efficiencies by addition of Au nanoparticles,” Angew. Chem. Int. Ed. Engl. 50(24), 5519–5523 (2011). [CrossRef] [PubMed]

] and NP clusters [11

11. D. H. Wang, K. H. Park, J. H. Seo, J. Seifter, J. H. Jeon, J. K. Kim, J. H. Park, O. O. Park, and A. J. Heeger, “Enhanced power conversion efficiency in PCDTBT/PC70BM bulk heterojunction photovoltaic devices with embedded silver nanoparticle clusters,” Adv. Eng. Mater. 1(5), 766–770 (2011). [CrossRef]

] have recently been reported with significant efficiency enhancements of up to 23% [7

7. D. H. Wang, Y. Kim, K. W. Choi, J. H. Seo, S. H. Im, J. H. Park, O. O. Park, and A. J. Heeger, “Enhancement of donor-acceptor polymer bulk heterojunction solar cell power conversion efficiencies by addition of Au nanoparticles,” Angew. Chem. Int. Ed. Engl. 50(24), 5519–5523 (2011). [CrossRef] [PubMed]

].

Nanostructured electrodes (Fig. 1(c)) can be easily incorporated into standard solar cell design in place of the planar metallic electrode. Design of nanostructured electrodes can draw greatly from research in metallic nanostructures such as gratings [12

12. G. Leveque and O. J. F. Martin, “Optimization of finite diffraction gratings for the excitation of surface plasmons,” J. Appl. Phys. 100(12), 124301 (2006). [CrossRef]

], void [13

13. T. A. Kelf, Y. Sugawara, R. M. Cole, J. J. Baumberg, M. E. Abdelsalam, S. Cintra, S. Mahajan, A. E. Russell, and P. N. Bartlett, “Localized and delocalized plasmons in metallic nanovoids,” Phys. Rev. B 74(24), 245415 (2006). [CrossRef]

,14

14. R. M. Cole, J. J. Baumberg, F. J. Garcia de Abajo, S. Mahajan, M. Abdelsalam, and P. N. Bartlett, “Understanding plasmons in nanoscale voids,” Nano Lett. 7(7), 2094–2100 (2007). [CrossRef]

] and hole arrays [15

15. T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, “Extraordinary optical transmission through sub-wavelength hole arrays,” Nature 391(6668), 667–669 (1998). [CrossRef]

,16

16. R. Wannemacher, “Plasmon-supported transmission of light through nanometric holes in metallic thin films,” Opt. Commun. 195(1-4), 107–118 (2001). [CrossRef]

]. It is possible to excite both LSPs on the protrusions/voids and SPPs on the intervening planar surface. The role of the nanostructured electrode is two-fold: one, to enhance absorption in the active layer and two, to collect charge carriers. A number of reports demonstrate enhanced efficiencies of up to 20% [17

17. S. I. Na, S. S. Kim, J. Jo, S. H. Oh, J. Kim, and D. Y. Kim, “Efficient polymer solar cells with surface relief gratings fabricated by simple soft lithography,” Adv. Funct. Mater. 18(24), 3956–3963 (2008). [CrossRef]

] and attribute the enhancements to scattering as well as the excitation of plasmonic modes [17

17. S. I. Na, S. S. Kim, J. Jo, S. H. Oh, J. Kim, and D. Y. Kim, “Efficient polymer solar cells with surface relief gratings fabricated by simple soft lithography,” Adv. Funct. Mater. 18(24), 3956–3963 (2008). [CrossRef]

20

20. N. N. Lal, B. F. Soares, J. K. Sinha, F. Huang, S. Mahajan, P. N. Bartlett, N. C. Greenham, and J. J. Baumberg, “Enhancing solar cells with localized plasmons in nanovoids,” Opt. Express 19(12), 11256–11263 (2011). [CrossRef] [PubMed]

].

The in-scattering NPs geometry (Fig. 1(d)) can achieve light-trapping by scattering incident light at wide angles into the active layer. This increases the light path length in the cell and can lead to total internal reflection. It is also possible for this scattered light to excite SPPs on the back interface and photonic modes in the active layer. An efficiency enhancement of 8% has been reported for amorphous silicon solar cells with Ag NPs separated from the active layer by a 20 nm intervening indium tin oxide layer [21

21. D. Derkacs, S. H. Lim, P. Matheu, W. Mar, and E. T. Yu, “Improved performance of amorphous silicon solar cells via scattering from surface plasmon polaritons in nearby metallic nanoparticles,” Appl. Phys. Lett. 89(9), 093103 (2006). [CrossRef]

]. Near-field absorption enhancement due to the NPs is also possible if their separation from the semiconductor is sufficiently small. A number of groups have recently reported efficiency enhancements by embedding metallic nanoparticles in a layer of a hole-conducting polymer, PEDOT:PSS, adjacent to an organic semiconductor film [22

22. S. S. Kim, S. I. Na, J. Jo, D. Y. Kim, and Y. C. Nah, “Plasmon enhanced performance of organic solar cells using electrodeposited Ag nanoparticles,” Appl. Phys. Lett. 93(7), 073307 (2008). [CrossRef]

24

24. A. J. Morfa, K. L. Rowlen, T. H. Reilly, M. J. Romero, and J. van de Lagemaat, “Plasmon-enhanced solar energy conversion in organic bulk heterojunction photovoltaics,” Appl. Phys. Lett. 92(1), 013504 (2008). [CrossRef]

]. For the purposes of this paper, we consider this geometry as a hybrid of the dispersed NP and in-scattering NPs PSC geometries and instead refer the interested reader to a recent report [25

25. C. J. Min, J. Li, G. Veronis, J. Y. Lee, S. H. Fan, and P. Peumans, “Enhancement of optical absorption in thin-film organic solar cells through the excitation of plasmonic modes in metallic gratings,” Appl. Phys. Lett. 96(13), 133302 (2010). [CrossRef]

].

We note that dielectric particles can also be used to excite SPPs on metal surfaces [26

26. N. Papanikolaou, “Optical properties of metallic nanoparticle arrays on a thin metallic film,” Phys. Rev. B 75(23), 235426 (2007). [CrossRef]

] and to provide scattering. However, metallic nanoparticles (mainly Au and Ag) have primarily been considered for this application, largely because of their ability to exhibit strong resonant scattering (e.g. the resonance scattering cross-section of a 20 nm Ag NP embedded in Si is 30 times larger than its geometric cross section) [9

9. S. Pillai and M. A. Green, “Plasmonics for photovoltaic applications,” Sol. Energy Mater. Sol. Cells 94(9), 1481–1486 (2010). [CrossRef]

].

2. Method

The Finite Element Method (FEM) is invoked using commercially available software (COMSOL) to model light absorption in the flat SC and the three PSC geometries. The suitability of the FEM for reproducing experimental results in light trapping solar cells has been established previously [28

28. R. Dunbar, H. Hesse, D. Lembke, and L. Schmidt-Mende, “Light-trapping plasmonic nanovoid arrays,” Phys. Rev. B (accepted).

]. We numerically solve the time-harmonic wave equations in the electric field E and the magnetic field H:
××En2k02E=0
(1)
×(1n2×H)k02H=0
(2)
where n is the complex refractive index and k0 is the magnitude of the free-space wave vector. The simulated area encompasses a single cell of the periodic structure (Figs. 2(a)
Fig. 2 (a) Relevant geometry parameters for the simulations, shown here for the dispersed nanoparticle plasmonic solar cell. They can be similarly applied to the other two geometries. (b) Boundary conditions. (c–e) Absorption enhancement exhibited by plasmonic solar cells relative to a planar solar cell. The values are calculated by integrating semiconductor absorption spectra within the wavelength range 350-1000 nm with AM1.5G illumination intensity. (c) Dispersed nanoparticles. (d) Nanostructured electrode. (e) In-scattering nanoparticles.
and 2(b)). Symmetry is imposed along the direction perpendicular to the plane of incidence. This enables a 2D treatment of the problem, which compared to 3D FEM simulation, achieves numerical accuracy at a small computational expense. As a result, an investigation over a large parameter space is possible. An infinite 2D array of adjacent cells is simulated by invoking periodic boundary conditions on the sides, (Fig. 2(b)), which takes the influence of neighboring cells into account. The top and bottom boundaries of the simulation geometries are terminated with perfectly matched layers to ensure minimum artificial reflections from the boundaries.

The outcome of each simulation is the electromagnetic field profile defined for all wavelengths in the range 350–1000 nm. From this profile, the absorption A(λ) in the active layer, the (planar) electrode and the NPs can be calculated by integrating the energy dissipation density within the appropriate 2D volume according to:
A(λ)=12ωε2|E(x,y,λ)|2dV
(3)
where the integral is evaluated over the entire material volume V in the xy plane, ε2 is the imaginary part of the permittivity and ω and E are the angular frequency and electric field strength of the electromagnetic field respectively. The reflectance is calculated by directly integrating the reflected flux at the top simulation boundary. The spectrum is then scaled using the AM1.5G solar spectrum and then integrated over wavelength to obtain the total absorption in the respective layer under one sun [28

28. R. Dunbar, H. Hesse, D. Lembke, and L. Schmidt-Mende, “Light-trapping plasmonic nanovoid arrays,” Phys. Rev. B (accepted).

]. This is performed for illumination with polarization perpendicular (TM) and parallel (TE) to the nanostructure (Fig. 2(a)). In this configuration, plasmonic excitation is possible under TM illumination only. In order to quantify the absorption properties of the nanostructures under non-polarized illumination (such as sunlight), we also calculate a polarization-averaged absorption Aav, an equally-weighted sum of total absorptions (integrated over the investigated spectral range) ATM and ATE under TM- and TE-polarized illumination respectively:

Aav=ATM+ATE2
(4)

A blend of two common organic semiconductors: PCPDTBT/PC70BM (poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b‘]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]/ [6,6]-phenyl-C71-butyric acid methyl ester, is chosen as the active material. Silver is used for both the nanoparticle and electrode material. The active layer thickness is fixed at 60 nm. The NP shape is chosen to be rectangular, as much of the literature in plasmonics is based on the use of rectangular gratings [12

12. G. Leveque and O. J. F. Martin, “Optimization of finite diffraction gratings for the excitation of surface plasmons,” J. Appl. Phys. 100(12), 124301 (2006). [CrossRef]

,29

29. R. A. Pala, J. White, E. Barnard, J. Liu, and M. L. Brongersma, “Design of plasmonic thin-film solar cells with broadband absorption enhancements,” Adv. Mater. (Deerfield Beach Fla.) 21(34), 3504–3509 (2009). [CrossRef]

31

31. N. C. Panoiu and R. M. Osgood Jr., “Enhanced optical absorption for photovoltaics via excitation of waveguide and plasmon-polariton modes,” Opt. Lett. 32(19), 2825–2827 (2007). [CrossRef] [PubMed]

] and the same shape is used for all three PSC geometries to facilitate a direct comparison. The corners of the NP are rounded (2 nm radius) to eliminate inaccuracies that arise when performing FEM simulation on geometries with sharp corners [32

32. Comsol Multiphysics Users Manual. v.3.5.

]. The height of the nanostructure is fixed at 30 nm, which is comparable to the dimensions of nanoparticles commonly reported in PSCs and relevant plasmonic structures [9

9. S. Pillai and M. A. Green, “Plasmonics for photovoltaic applications,” Sol. Energy Mater. Sol. Cells 94(9), 1481–1486 (2010). [CrossRef]

,12

12. G. Leveque and O. J. F. Martin, “Optimization of finite diffraction gratings for the excitation of surface plasmons,” J. Appl. Phys. 100(12), 124301 (2006). [CrossRef]

,23

23. J. H. Lee, J. H. Park, J. S. Kim, D. Y. Lee, and K. Cho, “High efficiency polymer solar cells with wet deposited plasmonic gold nanodots,” Org. Electron. 10(3), 416–420 (2009). [CrossRef]

,31

31. N. C. Panoiu and R. M. Osgood Jr., “Enhanced optical absorption for photovoltaics via excitation of waveguide and plasmon-polariton modes,” Opt. Lett. 32(19), 2825–2827 (2007). [CrossRef] [PubMed]

]. The period of the array and the width of the NPs are varied independently, allowing an optimization of light absorption in the active layer (Fig. 2(a)). We model a simplified solar cell: a glass substrate (n = 1.4), a semiconductor/active layer and a metallic electrode and nanostructure. In order to ensure clear conclusions about the mechanisms facilitating light-trapping can be drawn, we do not consider additional layers, such as charge transport layers, in the simulation geometry. A perfect antireflection coating at the air-glass interface is assumed—we illuminate at normal incidence from within the glass (Fig. 2(b)) at a distance of 250 nm from the active layer. The three PSC geometries can be realized by modifying the flat geometry (Fig. 1(a)) by placing the NP at the appropriate location in the layer stack: centered in the active layer (Fig. 1(b)), on the back electrode (Fig. 1(c)) or embedded in the substrate above the active layer (Fig. 1(d)). The spacing between the active layer and the NP in the in-scattering geometry is fixed at 30 nm, chosen to be consistent with experimental reports of this geometry [21

21. D. Derkacs, S. H. Lim, P. Matheu, W. Mar, and E. T. Yu, “Improved performance of amorphous silicon solar cells via scattering from surface plasmon polaritons in nearby metallic nanoparticles,” Appl. Phys. Lett. 89(9), 093103 (2006). [CrossRef]

,33

33. S. Pillai, K. R. Catchpole, T. Trupke, and M. A. Green, “Surface plasmon enhanced silicon solar cells,” J. Appl. Phys. 101(9), 093105 (2007). [CrossRef]

]. Values of complex refractive indices are obtained from the literature [34

34. P. B. Johnson and R. W. Christy, “Optical-Constants of Noble-Metals,” Phys. Rev. B 6(12), 4370–4379 (1972). [CrossRef]

,35

35. G. Dennler, K. Forberich, T. Ameri, C. Waldauf, P. Denk, C. J. Brabec, K. Hingerl, and A. J. Heeger, “Design of efficient organic tandem cells: On the interplay between molecular absorption and layer sequence,” J. Appl. Phys. 102(12), 123109 (2007). [CrossRef]

].

3. Results and discussion

3.1 Optimization study

The absorption in the active layer for the dispersed NP geometry under TM polarized illumination is shown in Fig. 2(c) for NP widths ranging from 10–120 nm and periods ranging up to 600 nm. Absorption enhancements (relative to a flat solar cell (Fig. 1(a)) exceeding 1 are observed for a large range of NP widths and periods. It is important to note that the volume of active material in this configuration is decreased due to the presence of the NP. Despite this, the total amount of light absorbed in the active layer is increased—the absorption enhancement due to the presence of the NP outweighs the reduction in absorption due to the omitted semiconductor material. A maximum absorption enhancement of 45% (enhancement factor of 1.45) is observed for a NP width of 40 nm and a period of 190 nm. At low periods the content of active material rapidly decreases (the active layer is crowded with silver) and the active absorption drops correspondingly. At large periods, where the density of features becomes small, the absorption enhancements tend towards 1, as expected. The peak of the curve is located at a value of feature width and period for which the culmination of light-trapping effects integrated across the investigated spectrum is maximized. Under TE illumination the absorption is less than that of a flat layer for all values of NP width and period (Fig. 3(a)
Fig. 3 Absorption enhancement exhibited by plasmonic solar cells relative to a planar solar cell for TE-polarized light (a,c,e). Polarization-averaged absorption enhancement (b,d,f).
). No plasmonic excitation is possible with this polarization, and scattering from the particle does not compensate for the reduction in semiconductor volume.

A maximum enhancement in absorption in the OSC of 33% is observed for the nanostructured electrode PSC with a NP width of 80 nm and a period of 190 nm (Fig. 2(d)) under TM illumination. Like the dispersed NP case, this is despite the 2D volume of semiconductor being reduced. The absorption under TE illumination is reduced for most combinations of NP width and periods, although a small absorption enhancement (1%) is observed for NP width 10 nm and period 600 nm. The higher performance of the nanostructured electrode PSC compared to the dispersed NP PSC for TE polarized light is likely due to the shadow effect present in the dispersed NP PSC.

The absorption enhancement provided by the in-scattering NP geometry with TM illumination is less than one for all simulated values of period and NP width (Fig. 2(e))—the active layer absorption of all simulated configurations are lower than that of the flat layer. The reasons for this will be discussed in the following section. However, an absorption enhancement of up to 4% is observed for TE polarization; absorption of TE-polarized light exceeds that of TM-polarized light for this PSC geometry. We identify more efficient scattering into the semiconductor layer in the absence of localized plasmonic excitation at the nanoparticles as an important contributing mechanism for this. Excitation of low-order TE waveguide modes within the semiconductor layer [29

29. R. A. Pala, J. White, E. Barnard, J. Liu, and M. L. Brongersma, “Design of plasmonic thin-film solar cells with broadband absorption enhancements,” Adv. Mater. (Deerfield Beach Fla.) 21(34), 3504–3509 (2009). [CrossRef]

] is not observed upon a rigorous inspection of the spectra – which is to be expected given the small difference in refractive index between the organic material (n~2) and glass (n = 1.4), leading to poor mode confinement within the semiconductor material.

By combining the absorption enhancements for TM- and TE- polarized light according to Eq. (4), we obtain the polarization-averaged absorption enhancement of these devices (Figs. 3(b), 3(d), 3(f)). These results apply for three-dimensional structures with an axis of symmetry as described in section 2 (c.f. line gratings). The optimized value for each PSC is shown in Table 1

Table 1. Optimized Plasmonic Solar Cell Geometries

table-icon
View This Table
.

3.2 Dependence on semiconductor layer thickness

For an active layer thickness of 60 nm, we observe an absorption enhancement of 19% as shown in Table 1. Even higher enhancements are possible—a maximum of 22% is observed for an active layer thickness of 100 nm. As the active layer thickness further increases, the PSC curve converges with that of the flat geometry. This is consistent with a filter effect: the majority of light is absorbed before it reaches the NPs. Non-negligible enhancement is observed for active layer thicknesses as large as 600 nm.

Finally we note that the active layer absorption for a 60 nm PSC exceeds that of all flat SCs with active layers thinner than 160 nm (indicated by the dashed line in Fig. 4). In other words, light trapping enables the absorption of a 60 nm thick semiconductor film in a PSC to be equivalent to that of a 160 nm flat film. By considering the area of the unit cell in each case, and accounting for the reduction in active material due to the presence of the NP, it follows that light-trapping enables an active material volume reduction of 65%.

3.3 Spectral characteristics of light-trapping

In order to focus on the role of plasmonic excitation on light trapping, we restrict our attention to TM illumination for the remainder of this paper. The absorption and reflection spectra of the flat and (TM-) optimized PSC structures are directly compared in Fig. 5
Fig. 5 Redistribution of incident light (TM polarized) for the dispersed NP (circles, NP width = 40 nm, period = 190 nm), nanostructured electrode (squares, NP width = 80 nm, period = 190 nm) and in-scattering NP (triangles, NP width = 40 nm, period = 190 nm) plasmonic solar cells and the flat solar cell (black line). (a) Absorption in organic semiconductor. (b) Reflectance. (c) Absorption in the nanoparticle. (d) Absorption in the electrode. Note for the nanostructured electrode, the absorption in the planar electrode and the (attached) nanoparticle are calculated separately.
, and corresponding spatial absorption profiles for selected wavelengths are shown in Fig. 6
Fig. 6 Spatial absorption profiles under TM polarization for selected wavelengths. (a,d): Dispersed nanoparticle solar cell (nanoparticle width = 40 nm, period = 190 nm) at wavelengths 470 nm (a) and 700 nm (d). (b,e) Nanostructured electrode (nanoparticle width = 80 nm, period = 190 nm) at wavelengths 470 nm (b) and 700 nm (e). (c) Flat solar cell at wavelength 470 nm. (f) In-scattering nanoparticle solar cell (nanoparticle width = 40 nm, period = 190 nm) at wavelength 360 nm. The color scale indicates the power dissipation density relative to the maximum in (a).
. Similar results are observed for the polarization-averaged optimized PSCs. The spectra have not been scaled by the solar spectrum. As the in-scattering geometry indicates a reduction of absorption in the active layer for all simulated values of period and NP width, there is no optimum configuration with non-zero NP width. In order to nevertheless compare the spectra with that of the other geometries to gain an insight into the loss mechanisms, we choose period = 40 nm and NP width = 190 nm (the same as those of the dispersed NP geometry).

The dispersed NP geometry semiconductor absorption exceeds the absorption in the flat geometry at all simulated wavelengths (Fig. 5(a)). Two main peaks are seen at 570 and 750 nm. A LSP resonance is observed as a sharp increase in the NP absorption at low wavelengths (the peak lies below 350 nm) (Fig. 5(c)). We see high active absorption at the same spectral position (Fig. 5(a))—implying that light-trapping at this wavelength could be due to resonant near-field enhancement due to LSP excitation. No other LSP resonances are identified via peaks in the NP absorption spectrum. However, a spectrally broad enhancement in the semiconductor absorption is seen which cannot be solely accounted for by near-field enhancement due to (resonant) LSP excitation; scattering must play a large role. In section 3.3 we directly show that scattering alone can account for large absorption enhancements of the order observed here. We note that the small peak in the NP absorption at 600-650 nm (Fig. 5(c)) does not indicate a LSP resonance; it coincides with a dip in the absorption spectrum of the semiconductor (Fig. 5(a)), also observed in the electrode absorption (Fig. 5(d)). The absorption at 470 nm (Fig. 6(a)) is significantly more localized than in a flat SC (Fig. 6(c)), predominately occurring about the two front corners of the NP. This absorption pattern is similar with that observed for scattering into a highly absorbing material (see section 3.4). The absorption at 700 nm is preferentially localized at the back corners, which leads to intense absorption between the NP and the back electrode (Fig. 6(d)). This suggests that this structure could also be optimized to excite nanocavity modes [30

30. N. C. Lindquist, W. A. Luhman, S. H. Oh, and R. J. Holmes, “Plasmonic nanocavity arrays for enhanced efficiency in organic photovoltaic cells,” Appl. Phys. Lett. 93(12), 123308 (2008). [CrossRef]

]. The peak at 550 nm features localization (not shown) at both the front and back corners.

Like the dispersed NP geometry the active layer absorption for the nanostructured electrode geometry is significantly larger than that for the flat geometry (Fig. 5(a)) across the entire wavelength range addressed in this study. Strong absorption occurs between the NP and the top of the active layer at 470 nm (Fig. 6(b)). This is not however, due to a Fabry-Pérot resonance for this reduced active layer thickness (30 nm, between the NP and the edge of the active layer). Simulations where the thickness of the active layer for planar SCs is varied (Fig. 4), reveal no resonance for a layer thickness of 30 nm at the wavelength 470 nm (Fig. 6(b)). Therefore the NP itself must be fundamental to this absorption enhancement. As the absorption is quite delocalized and plasmonic absorption enhancement only extends into organic materials from a few to 10 nm [8

8. J. F. Zhu, M. Xue, H. J. Shen, Z. Wu, S. Kim, J. J. Ho, A. Hassani-Afshar, B. Q. Zeng, and K. L. Wang, “Plasmonic effects for light concentration in organic photovoltaic thin films induced by hexagonal periodic metallic nanospheres,” Appl. Phys. Lett. 98(15), 151110 (2011). [CrossRef]

,38

38. B. P. Rand, P. Peumans, and S. R. Forrest, “Long-range absorption enhancement in organic tandem thin-film solar cells containing silver nanoclusters,” J. Appl. Phys. 96(12), 7519–7526 (2004). [CrossRef]

], we attribute this delocalized absorption to back-scattering provided by the NP and not to near-field enhancement due to LSP excitation. The bulk of the absorption at 700 nm is similarly delocalized although localized absorption is also observed along the sides of the NP (Fig. 6(e)).

The origin of the poor performance of the in-scattering layer in this study is largely due to enhanced reflection. The back-scattering that occurs due to the NPs has the effect of dramatically increasing the reflectance for a wide wavelength range (Fig. 5(b)), leading to a reduction in absorption in the active layer (Fig. 5(a)). Given the large spacing between the NP and the active material, near-field enhancement is also unavailable. We see therefore, that the wide angle in-scattering provided by the NPs does not outweigh the detrimental effect of back-scattering. The absorption in the NPs due to LSP resonances (Fig. 5(c)) at 360 (Fig. 6(f)) and 400 nm also represents a loss, although only over a narrow wavelength range. It is expected that further simulation studies of this PSC would indeed find an enhancement if other geometries and material combinations are considered [4

4. H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nat. Mater. 9(3), 205–213 (2010). [CrossRef] [PubMed]

,9

9. S. Pillai and M. A. Green, “Plasmonics for photovoltaic applications,” Sol. Energy Mater. Sol. Cells 94(9), 1481–1486 (2010). [CrossRef]

].

3.4 Ideal vs. non-ideal conductors

3.5 Practical considerations for plasmonic solar cell fabrication

The above investigation finds excellent light-trapping properties for both dispersed nanoparticle and structured electrode organic solar cell architectures. Methods for fabricating ordered structured electrodes in solar cells have been well documented [19

19. V. E. Ferry, M. A. Verschuuren, H. B. T. Li, E. Verhagen, R. J. Walters, R. E. I. Schropp, H. A. Atwater, and A. Polman, “Light trapping in ultrathin plasmonic solar cells,” Opt. Express 18(S2Suppl 2), A237–A245 (2010). [CrossRef] [PubMed]

,39

39. K. Tvingstedt, N. Persson, O. Inganäs, A. Rahachou, and I. V. Zozoulenko, “Surface plasmon increase absorption in polymer photovoltaic cells,” Appl. Phys. Lett. 91(11), 113514 (2007). [CrossRef]

]. Conversely, the ordered deposition of metal nanoparticles such that they rest in the middle of a solution-processed organic layer has not yet, to the best of our knowledge, been experimentally demonstrated (although very conceivable for organic solar cells featuring materials that can be evaporated, such as CuPc). However, high order is not critical for strong light trapping in this configuration, as observed experimentally with nanoparticles randomly dispersed within the active layer [7

7. D. H. Wang, Y. Kim, K. W. Choi, J. H. Seo, S. H. Im, J. H. Park, O. O. Park, and A. J. Heeger, “Enhancement of donor-acceptor polymer bulk heterojunction solar cell power conversion efficiencies by addition of Au nanoparticles,” Angew. Chem. Int. Ed. Engl. 50(24), 5519–5523 (2011). [CrossRef] [PubMed]

,10

10. C. H. Kim, S. H. Cha, S. C. Kim, M. Song, J. Lee, W. S. Shin, S. J. Moon, J. H. Bahng, N. A. Kotov, and S. H. Jin, “Silver nanowire embedded in P3HT:PCBM for high-efficiency hybrid photovoltaic device applications,” ACS Nano 5(4), 3319–3325 (2011). [CrossRef] [PubMed]

,11

11. D. H. Wang, K. H. Park, J. H. Seo, J. Seifter, J. H. Jeon, J. K. Kim, J. H. Park, O. O. Park, and A. J. Heeger, “Enhanced power conversion efficiency in PCDTBT/PC70BM bulk heterojunction photovoltaic devices with embedded silver nanoparticle clusters,” Adv. Eng. Mater. 1(5), 766–770 (2011). [CrossRef]

]. We find (Fig. 3(b)) the nanoparticles in the optimized configuration have a spacing of 330 nm, which far exceeds the distance over which LSP-LSP interaction takes place in an organic semiconductor (the plasmonic near field of a small nanoparticle extends to a distance of the order of 10 nm from the nanoparticle [38

38. B. P. Rand, P. Peumans, and S. R. Forrest, “Long-range absorption enhancement in organic tandem thin-film solar cells containing silver nanoclusters,” J. Appl. Phys. 96(12), 7519–7526 (2004). [CrossRef]

]). Therefore, light-trapping due to LSP excitation at sufficiently widely spaced nanoparticles can be considered independent of the degree of order, and only on the density of the nanoparticles. A similar argument applies for scattering. Although excitation of diffraction modes is order sensitive, random arrays of metallic nanoparticles are known to (incoherently) scatter strongly due to the large scattering coefficient of individual particles [9

9. S. Pillai and M. A. Green, “Plasmonics for photovoltaic applications,” Sol. Energy Mater. Sol. Cells 94(9), 1481–1486 (2010). [CrossRef]

,33

33. S. Pillai, K. R. Catchpole, T. Trupke, and M. A. Green, “Surface plasmon enhanced silicon solar cells,” J. Appl. Phys. 101(9), 093105 (2007). [CrossRef]

]. Therefore, the important parameter is the density of nanoparticles [8

8. J. F. Zhu, M. Xue, H. J. Shen, Z. Wu, S. Kim, J. J. Ho, A. Hassani-Afshar, B. Q. Zeng, and K. L. Wang, “Plasmonic effects for light concentration in organic photovoltaic thin films induced by hexagonal periodic metallic nanospheres,” Appl. Phys. Lett. 98(15), 151110 (2011). [CrossRef]

], implicitly defined here by the period of the nanoparticle array.

We also note that the presence of additional layers between the substrate and the organic layer such as the transparent electrode (typically indium tin oxide or aluminum-doped zinc oxide) and charge transport layers (PEDOT:PSS in non-inverted solar cells or TiO2 or ZnO in inverted solar cells) affects the distribution of absorption within the semiconductor and as a result, the parameters of the optimal metallic nanostructure will vary between material system. Absorption in indium tin oxide electrodes represents an energy loss mechanism for both plasmonic and flat solar cells.

For the nanostructured electrode geometry, intervening layers between the semiconductor and the electrode could have a large effect, depending on the material and material thickness. In non-inverted solar cells, thin layers (< 1 nm) of LiF are often used. This would not be expected to strongly inhibit near-field effects (which extends out into the organic semiconductor up to around 10 nm from the metal surface) or light-trapping due to scattering at the electrode. A thick (5-10 nm) layer of Ca, which absorbs significantly and does not support plasmonic excitation, often used to modify the work function of the back-electrode, would significantly inhibit the absorption enhancement in the semiconductor. Conversely, a 5–10 nm layer of WO3, which is optically transparent and often employed in inverted organic solar cells, may inhibit near-field enhancement but not light-trapping via scattering.

4. Conclusion

Plasmonic solar cells have been compared and optimized using finite element simulations to ascertain promising structures for high performance solar cells. The largest absorption enhancement observed for a 60 nm film of standard organic semiconductor is 19%, obtained by uniformly dispersing silver nanoparticles of width 60 nm with a periodicity of 330 nm. The absorption in a 60 nm film with this light trapping configuration is equivalent to that of a 160 nm flat film, representing a huge reduction in semiconductor consumption. Light-trapping is beneficial for all active layer thicknesses up to 600 nm. We find evidence of near-field plasmonic enhancement, but show that it is not necessary; strong absorption enhancement in the semiconductor is possible via scattering alone. This technique can be applied to a wide range of low-cost semiconductors to find the critical device thickness below which light-trapping is required. If satisfactory charge transport is possible for thin films of this thickness, then light-trapping is unnecessary. If not, thinner films (with better charge transport properties) with light-trapping are preferable.

Acknowledgments

We gratefully acknowledge the IDK (Elite Network of Bavaria), the German research foundation (DFG) (SPP1355), and the ‘Nanosystems Initiative Munich (NIM)’. We thank P. Reineck for helpful discussions.

References and links

1.

S. R. Forrest, “The limits to organic photovoltaic cell efficiency,” MRS Bull. 30(01), 28–32 (2005). [CrossRef]

2.

J. Weickert, R. B. Dunbar, H. C. Hesse, W. Wiedemann, and L. Schmidt-Mende, “Nanostructured organic and hybrid solar cells,” Adv. Mater. (Deerfield Beach Fla.) 23(16), 1810–1828 (2011). [CrossRef] [PubMed]

3.

M. Niggemann, M. Riede, A. Gombert, and K. Leo, “Light trapping in organic solar cells,” Phys. Status Solidi A 205(12), 2862–2874 (2008). [CrossRef]

4.

H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nat. Mater. 9(3), 205–213 (2010). [CrossRef] [PubMed]

5.

K. Kim and D. L. Carroll, “Roles of Au and Ag nanoparticles in efficiency enhancement of poly(3-octylthiophene)/C-60 bulk heterojunction photovoltaic devices,” Appl. Phys. Lett. 87, 203113 (2005).

6.

J. Y. Lee and P. Peumans, “The origin of enhanced optical absorption in solar cells with metal nanoparticles embedded in the active layer,” Opt. Express 18(10), 10078–10087 (2010). [CrossRef] [PubMed]

7.

D. H. Wang, Y. Kim, K. W. Choi, J. H. Seo, S. H. Im, J. H. Park, O. O. Park, and A. J. Heeger, “Enhancement of donor-acceptor polymer bulk heterojunction solar cell power conversion efficiencies by addition of Au nanoparticles,” Angew. Chem. Int. Ed. Engl. 50(24), 5519–5523 (2011). [CrossRef] [PubMed]

8.

J. F. Zhu, M. Xue, H. J. Shen, Z. Wu, S. Kim, J. J. Ho, A. Hassani-Afshar, B. Q. Zeng, and K. L. Wang, “Plasmonic effects for light concentration in organic photovoltaic thin films induced by hexagonal periodic metallic nanospheres,” Appl. Phys. Lett. 98(15), 151110 (2011). [CrossRef]

9.

S. Pillai and M. A. Green, “Plasmonics for photovoltaic applications,” Sol. Energy Mater. Sol. Cells 94(9), 1481–1486 (2010). [CrossRef]

10.

C. H. Kim, S. H. Cha, S. C. Kim, M. Song, J. Lee, W. S. Shin, S. J. Moon, J. H. Bahng, N. A. Kotov, and S. H. Jin, “Silver nanowire embedded in P3HT:PCBM for high-efficiency hybrid photovoltaic device applications,” ACS Nano 5(4), 3319–3325 (2011). [CrossRef] [PubMed]

11.

D. H. Wang, K. H. Park, J. H. Seo, J. Seifter, J. H. Jeon, J. K. Kim, J. H. Park, O. O. Park, and A. J. Heeger, “Enhanced power conversion efficiency in PCDTBT/PC70BM bulk heterojunction photovoltaic devices with embedded silver nanoparticle clusters,” Adv. Eng. Mater. 1(5), 766–770 (2011). [CrossRef]

12.

G. Leveque and O. J. F. Martin, “Optimization of finite diffraction gratings for the excitation of surface plasmons,” J. Appl. Phys. 100(12), 124301 (2006). [CrossRef]

13.

T. A. Kelf, Y. Sugawara, R. M. Cole, J. J. Baumberg, M. E. Abdelsalam, S. Cintra, S. Mahajan, A. E. Russell, and P. N. Bartlett, “Localized and delocalized plasmons in metallic nanovoids,” Phys. Rev. B 74(24), 245415 (2006). [CrossRef]

14.

R. M. Cole, J. J. Baumberg, F. J. Garcia de Abajo, S. Mahajan, M. Abdelsalam, and P. N. Bartlett, “Understanding plasmons in nanoscale voids,” Nano Lett. 7(7), 2094–2100 (2007). [CrossRef]

15.

T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, “Extraordinary optical transmission through sub-wavelength hole arrays,” Nature 391(6668), 667–669 (1998). [CrossRef]

16.

R. Wannemacher, “Plasmon-supported transmission of light through nanometric holes in metallic thin films,” Opt. Commun. 195(1-4), 107–118 (2001). [CrossRef]

17.

S. I. Na, S. S. Kim, J. Jo, S. H. Oh, J. Kim, and D. Y. Kim, “Efficient polymer solar cells with surface relief gratings fabricated by simple soft lithography,” Adv. Funct. Mater. 18(24), 3956–3963 (2008). [CrossRef]

18.

C. Cocoyer, L. Rocha, L. Sicot, B. Geffroy, R. de Bettignies, C. Sentein, C. Fiorini-Debuisschert, and P. Raimond, “Implementation of submicrometric periodic surface structures toward improvement of organic-solar-cell performances,” Appl. Phys. Lett. 88(13), 133108 (2006). [CrossRef]

19.

V. E. Ferry, M. A. Verschuuren, H. B. T. Li, E. Verhagen, R. J. Walters, R. E. I. Schropp, H. A. Atwater, and A. Polman, “Light trapping in ultrathin plasmonic solar cells,” Opt. Express 18(S2Suppl 2), A237–A245 (2010). [CrossRef] [PubMed]

20.

N. N. Lal, B. F. Soares, J. K. Sinha, F. Huang, S. Mahajan, P. N. Bartlett, N. C. Greenham, and J. J. Baumberg, “Enhancing solar cells with localized plasmons in nanovoids,” Opt. Express 19(12), 11256–11263 (2011). [CrossRef] [PubMed]

21.

D. Derkacs, S. H. Lim, P. Matheu, W. Mar, and E. T. Yu, “Improved performance of amorphous silicon solar cells via scattering from surface plasmon polaritons in nearby metallic nanoparticles,” Appl. Phys. Lett. 89(9), 093103 (2006). [CrossRef]

22.

S. S. Kim, S. I. Na, J. Jo, D. Y. Kim, and Y. C. Nah, “Plasmon enhanced performance of organic solar cells using electrodeposited Ag nanoparticles,” Appl. Phys. Lett. 93(7), 073307 (2008). [CrossRef]

23.

J. H. Lee, J. H. Park, J. S. Kim, D. Y. Lee, and K. Cho, “High efficiency polymer solar cells with wet deposited plasmonic gold nanodots,” Org. Electron. 10(3), 416–420 (2009). [CrossRef]

24.

A. J. Morfa, K. L. Rowlen, T. H. Reilly, M. J. Romero, and J. van de Lagemaat, “Plasmon-enhanced solar energy conversion in organic bulk heterojunction photovoltaics,” Appl. Phys. Lett. 92(1), 013504 (2008). [CrossRef]

25.

C. J. Min, J. Li, G. Veronis, J. Y. Lee, S. H. Fan, and P. Peumans, “Enhancement of optical absorption in thin-film organic solar cells through the excitation of plasmonic modes in metallic gratings,” Appl. Phys. Lett. 96(13), 133302 (2010). [CrossRef]

26.

N. Papanikolaou, “Optical properties of metallic nanoparticle arrays on a thin metallic film,” Phys. Rev. B 75(23), 235426 (2007). [CrossRef]

27.

M. A. Sefunc, A. K. Okyay, and H. V. Demir, “Plasmonic backcontact grating for P3HT:PCBM organic solar cells enabling strong optical absorption increased in all polarizations,” Opt. Express 19(15), 14200–14209 (2011). [CrossRef] [PubMed]

28.

R. Dunbar, H. Hesse, D. Lembke, and L. Schmidt-Mende, “Light-trapping plasmonic nanovoid arrays,” Phys. Rev. B (accepted).

29.

R. A. Pala, J. White, E. Barnard, J. Liu, and M. L. Brongersma, “Design of plasmonic thin-film solar cells with broadband absorption enhancements,” Adv. Mater. (Deerfield Beach Fla.) 21(34), 3504–3509 (2009). [CrossRef]

30.

N. C. Lindquist, W. A. Luhman, S. H. Oh, and R. J. Holmes, “Plasmonic nanocavity arrays for enhanced efficiency in organic photovoltaic cells,” Appl. Phys. Lett. 93(12), 123308 (2008). [CrossRef]

31.

N. C. Panoiu and R. M. Osgood Jr., “Enhanced optical absorption for photovoltaics via excitation of waveguide and plasmon-polariton modes,” Opt. Lett. 32(19), 2825–2827 (2007). [CrossRef] [PubMed]

32.

Comsol Multiphysics Users Manual. v.3.5.

33.

S. Pillai, K. R. Catchpole, T. Trupke, and M. A. Green, “Surface plasmon enhanced silicon solar cells,” J. Appl. Phys. 101(9), 093105 (2007). [CrossRef]

34.

P. B. Johnson and R. W. Christy, “Optical-Constants of Noble-Metals,” Phys. Rev. B 6(12), 4370–4379 (1972). [CrossRef]

35.

G. Dennler, K. Forberich, T. Ameri, C. Waldauf, P. Denk, C. J. Brabec, K. Hingerl, and A. J. Heeger, “Design of efficient organic tandem cells: On the interplay between molecular absorption and layer sequence,” J. Appl. Phys. 102(12), 123109 (2007). [CrossRef]

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A. J. Moulé and K. Meerholz, “Minimizing optical losses in bulk heterojunction polymer solar cells,” Appl. Phys. B: Lasers Opt. 86(4), 721–727 (2007). [CrossRef]

37.

W. Wang, S. M. Wu, K. Reinhardt, Y. L. Lu, and S. C. Chen, “Broadband light absorption enhancement in thin-film silicon solar cells,” Nano Lett. 10(6), 2012–2018 (2010). [CrossRef] [PubMed]

38.

B. P. Rand, P. Peumans, and S. R. Forrest, “Long-range absorption enhancement in organic tandem thin-film solar cells containing silver nanoclusters,” J. Appl. Phys. 96(12), 7519–7526 (2004). [CrossRef]

39.

K. Tvingstedt, N. Persson, O. Inganäs, A. Rahachou, and I. V. Zozoulenko, “Surface plasmon increase absorption in polymer photovoltaic cells,” Appl. Phys. Lett. 91(11), 113514 (2007). [CrossRef]

OCIS Codes
(040.5350) Detectors : Photovoltaic
(240.6680) Optics at surfaces : Surface plasmons
(310.6860) Thin films : Thin films, optical properties

ToC Category:
Photovoltaics

History
Original Manuscript: November 11, 2011
Revised Manuscript: December 23, 2011
Manuscript Accepted: December 29, 2011
Published: January 11, 2012

Citation
Ricky B. Dunbar, Thomas Pfadler, and Lukas Schmidt-Mende, "Highly absorbing solar cells—a survey of plasmonic nanostructures," Opt. Express 20, A177-A189 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-S2-A177


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References

  1. S. R. Forrest, “The limits to organic photovoltaic cell efficiency,” MRS Bull.30(01), 28–32 (2005). [CrossRef]
  2. J. Weickert, R. B. Dunbar, H. C. Hesse, W. Wiedemann, and L. Schmidt-Mende, “Nanostructured organic and hybrid solar cells,” Adv. Mater. (Deerfield Beach Fla.)23(16), 1810–1828 (2011). [CrossRef] [PubMed]
  3. M. Niggemann, M. Riede, A. Gombert, and K. Leo, “Light trapping in organic solar cells,” Phys. Status Solidi A205(12), 2862–2874 (2008). [CrossRef]
  4. H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nat. Mater.9(3), 205–213 (2010). [CrossRef] [PubMed]
  5. K. Kim and D. L. Carroll, “Roles of Au and Ag nanoparticles in efficiency enhancement of poly(3-octylthiophene)/C-60 bulk heterojunction photovoltaic devices,” Appl. Phys. Lett.87, 203113 (2005).
  6. J. Y. Lee and P. Peumans, “The origin of enhanced optical absorption in solar cells with metal nanoparticles embedded in the active layer,” Opt. Express18(10), 10078–10087 (2010). [CrossRef] [PubMed]
  7. D. H. Wang, Y. Kim, K. W. Choi, J. H. Seo, S. H. Im, J. H. Park, O. O. Park, and A. J. Heeger, “Enhancement of donor-acceptor polymer bulk heterojunction solar cell power conversion efficiencies by addition of Au nanoparticles,” Angew. Chem. Int. Ed. Engl.50(24), 5519–5523 (2011). [CrossRef] [PubMed]
  8. J. F. Zhu, M. Xue, H. J. Shen, Z. Wu, S. Kim, J. J. Ho, A. Hassani-Afshar, B. Q. Zeng, and K. L. Wang, “Plasmonic effects for light concentration in organic photovoltaic thin films induced by hexagonal periodic metallic nanospheres,” Appl. Phys. Lett.98(15), 151110 (2011). [CrossRef]
  9. S. Pillai and M. A. Green, “Plasmonics for photovoltaic applications,” Sol. Energy Mater. Sol. Cells94(9), 1481–1486 (2010). [CrossRef]
  10. C. H. Kim, S. H. Cha, S. C. Kim, M. Song, J. Lee, W. S. Shin, S. J. Moon, J. H. Bahng, N. A. Kotov, and S. H. Jin, “Silver nanowire embedded in P3HT:PCBM for high-efficiency hybrid photovoltaic device applications,” ACS Nano5(4), 3319–3325 (2011). [CrossRef] [PubMed]
  11. D. H. Wang, K. H. Park, J. H. Seo, J. Seifter, J. H. Jeon, J. K. Kim, J. H. Park, O. O. Park, and A. J. Heeger, “Enhanced power conversion efficiency in PCDTBT/PC70BM bulk heterojunction photovoltaic devices with embedded silver nanoparticle clusters,” Adv. Eng. Mater.1(5), 766–770 (2011). [CrossRef]
  12. G. Leveque and O. J. F. Martin, “Optimization of finite diffraction gratings for the excitation of surface plasmons,” J. Appl. Phys.100(12), 124301 (2006). [CrossRef]
  13. T. A. Kelf, Y. Sugawara, R. M. Cole, J. J. Baumberg, M. E. Abdelsalam, S. Cintra, S. Mahajan, A. E. Russell, and P. N. Bartlett, “Localized and delocalized plasmons in metallic nanovoids,” Phys. Rev. B74(24), 245415 (2006). [CrossRef]
  14. R. M. Cole, J. J. Baumberg, F. J. Garcia de Abajo, S. Mahajan, M. Abdelsalam, and P. N. Bartlett, “Understanding plasmons in nanoscale voids,” Nano Lett.7(7), 2094–2100 (2007). [CrossRef]
  15. T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, “Extraordinary optical transmission through sub-wavelength hole arrays,” Nature391(6668), 667–669 (1998). [CrossRef]
  16. R. Wannemacher, “Plasmon-supported transmission of light through nanometric holes in metallic thin films,” Opt. Commun.195(1-4), 107–118 (2001). [CrossRef]
  17. S. I. Na, S. S. Kim, J. Jo, S. H. Oh, J. Kim, and D. Y. Kim, “Efficient polymer solar cells with surface relief gratings fabricated by simple soft lithography,” Adv. Funct. Mater.18(24), 3956–3963 (2008). [CrossRef]
  18. C. Cocoyer, L. Rocha, L. Sicot, B. Geffroy, R. de Bettignies, C. Sentein, C. Fiorini-Debuisschert, and P. Raimond, “Implementation of submicrometric periodic surface structures toward improvement of organic-solar-cell performances,” Appl. Phys. Lett.88(13), 133108 (2006). [CrossRef]
  19. V. E. Ferry, M. A. Verschuuren, H. B. T. Li, E. Verhagen, R. J. Walters, R. E. I. Schropp, H. A. Atwater, and A. Polman, “Light trapping in ultrathin plasmonic solar cells,” Opt. Express18(S2Suppl 2), A237–A245 (2010). [CrossRef] [PubMed]
  20. N. N. Lal, B. F. Soares, J. K. Sinha, F. Huang, S. Mahajan, P. N. Bartlett, N. C. Greenham, and J. J. Baumberg, “Enhancing solar cells with localized plasmons in nanovoids,” Opt. Express19(12), 11256–11263 (2011). [CrossRef] [PubMed]
  21. D. Derkacs, S. H. Lim, P. Matheu, W. Mar, and E. T. Yu, “Improved performance of amorphous silicon solar cells via scattering from surface plasmon polaritons in nearby metallic nanoparticles,” Appl. Phys. Lett.89(9), 093103 (2006). [CrossRef]
  22. S. S. Kim, S. I. Na, J. Jo, D. Y. Kim, and Y. C. Nah, “Plasmon enhanced performance of organic solar cells using electrodeposited Ag nanoparticles,” Appl. Phys. Lett.93(7), 073307 (2008). [CrossRef]
  23. J. H. Lee, J. H. Park, J. S. Kim, D. Y. Lee, and K. Cho, “High efficiency polymer solar cells with wet deposited plasmonic gold nanodots,” Org. Electron.10(3), 416–420 (2009). [CrossRef]
  24. A. J. Morfa, K. L. Rowlen, T. H. Reilly, M. J. Romero, and J. van de Lagemaat, “Plasmon-enhanced solar energy conversion in organic bulk heterojunction photovoltaics,” Appl. Phys. Lett.92(1), 013504 (2008). [CrossRef]
  25. C. J. Min, J. Li, G. Veronis, J. Y. Lee, S. H. Fan, and P. Peumans, “Enhancement of optical absorption in thin-film organic solar cells through the excitation of plasmonic modes in metallic gratings,” Appl. Phys. Lett.96(13), 133302 (2010). [CrossRef]
  26. N. Papanikolaou, “Optical properties of metallic nanoparticle arrays on a thin metallic film,” Phys. Rev. B75(23), 235426 (2007). [CrossRef]
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