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Energy Express

  • Editor: Bernard Kippelen
  • Vol. 20, Iss. S5 — Sep. 10, 2012
  • pp: A740–A753

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Analysis and optimization of surface plasmon-enhanced organic solar cells with a metallic crossed grating electrode

Eungkyu Lee and Changsoon Kim  »View Author Affiliations


Optics Express, Vol. 20, Issue S5, pp. A740-A753 (2012)
http://dx.doi.org/10.1364/OE.20.00A740


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Abstract

We perform a systematic analysis of enhanced short-circuit current density (Jsc) in organic solar cells (OSCs) where one metallic electrode is optically thick and the other consists of a two-dimensional metallic crossed grating. By examining a model device representative of such surface plasmon (SP)-enhanced OSCs by the Fourier modal and finite-element methods for electromagnetic and exciton diffusion calculations, respectively, we provide general guidelines to maximize Jsc of the SP-enhanced OSCs. Based on this study, we optimize the performance of a small-molecule OSC employing a copper phthalocyanine–fullerene donor–acceptor pair, demonstrating that the optimized SP-enhanced device has Jsc that is 75 % larger than that of the optimized device with an ITO-based conventional structure.

© 2012 OSA

1. Introduction

Organic solar cells (OSCs) have received considerable attention owing to their possibility of realizing low-cost, large-area, flexible devices [1

S. R. Forrest, “The path to ubiquitous and low-cost organic electronic appliances on plastic,” Nature 428, 911–918 (2004). [CrossRef] [PubMed]

, 2

M. R. Lee, R. D. Eckert, K. Forberich, G. Dennler, C. J. Brabec, and R. A. Gaudiana, “Solar power wires based on organic photovoltaic materials,” Science 324, 232–235 (2009). [CrossRef] [PubMed]

]. However, the highest power-conversion efficiency reported in literature is still below 10% [3

H.-Y. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu, and G. Li, “Polymer solar cells with enhanced open-circuit voltage and efficiency,” Nat. Photonics 3, 649–653 (2009). [CrossRef]

], limiting its practical deployment. One of the important device parameters determining the power-conversion efficiency is the short-circuit current density (Jsc), which is the current density passing through the illuminated device when it is short-circuited. It can be expressed as
J sc=q ∫ η ext(λ)S(λ)dλ,
(1)
where λ is the wavelength of the incident light, q is the electron charge, S is the flux density of the incoming solar photons per unit wavelength, and ηext is the external quantum efficiency defined as the number of charge carriers collected at the electrodes per incident photon. The external quantum efficiency can be broken into two terms, the light absorption efficiency (ηabs) and the internal quantum efficiency (ηint), which concern with optical and electrical aspects, respectively: ηabs is the number of photons absorbed in the active material(s) per incident photon; ηint is the number of charge carriers collected at the electrodes per absorbed photon.

Absorption of a photon in an organic semiconductor creates an exciton, a bound electron-hole pair with a binding energy ranging from ∼ 0.1 eV to ∼ 1.0 eV [4

M. Pope and C. E. Swenberg, Electronic processes in organic crystals and polymers , 2nd ed. (Oxford University Press, 1999).

, 5

I. Hill, A. Kahn, Z. G. Soos, and R. A. Pascal, “Charge-separation energy in films of π-conjugated organic molecules,” Chem. Phys. Lett. 327, 181–188 (2000). [CrossRef]

]. In OSCs, dissociation of this tightly bound exciton into an electron and a hole typically occurs at an interface between two organic semiconductors with an appropriate band offset, called a donor–acceptor (DA) interface. To achieve high ηint, most excitons generated in an active layer must diffuse to the DA interface, which means that the thickness of the active layer must be chosen to be smaller than ∼ Ld, where Ld is the exciton diffusion length in the active layer. Since Ld of organic semiconductors (∼ 10 nm [6

P. Peumans, A. Yakimov, and S. R. Forrest, “Small molecular weight organic thin-film photodetectors and solar cells,” J. Appl. Phys. 93, 3693–3723 (2003). [CrossRef]

–8

P. E. Shaw, A. Ruseckas, and I. D. W. Samuel, “Exciton diffusion measurements in poly(3-hexylthiophene),” Adv. Mater. 20, 3516–3520 (2008). [CrossRef]

]) is typically much less than the optical absorption length (= 1/α ∼ 100 nm, where α is the absorption coefficient), an active layer whose thickness is smaller than ∼ Ld is too thin to have high ηabs. In short, Jsc in OSCs is limited by a trade-off between ηint and ηabs [9

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

].

One strategy to overcome this limitation is to increase the optical absorption in thin (∼ Ld) active layers by exploiting optical resonances. In planar microcavity devices, where the active layers are sandwiched between optically thick and semitransparent metal layers, by varying the device geometry, Fabry-Perot resonances are tuned to locate the maximum of the enhanced optical field intensity near the active layers [10

C. Kim and J. Kim, “Organic photovoltaic cell in lateral-tandem configuration employing continuously-tuned microcavity sub-cells,” Opt. Express 16, 19987–19994 (2008). [CrossRef] [PubMed]

–12

H.-W. Lin, S.-W. Chiu, L.-Y. Lin, Z.-Y. Hung, Y.-H. Chen, F. Lin, and K.-T. Wong, “Device engineering for highly efficient top-illuminated organic solar cells with microcavity structures,” Adv. Mater. 24, 2269–2272 (2012). [CrossRef] [PubMed]

]. Surface plasmon (SP) resonances associated with metallic periodic structures [13

C. Kim, J.-Y. Lee, and P. Peumans, “Surface plasmon polariton assisted organic solar cells,” in Proceedings of NSTI-Nanotech 2008 1, 533–536 (2008).

–17

H. Shen and B. Maes, “Combined plasmonic gratings in organic solar cells,” Opt. Express 19, A1202–A1210 (2011). [CrossRef] [PubMed]

] or nanoparticles [18

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, 7519–7526 (2004). [CrossRef]

–21

J. Yang, J. You, C.-C. Chen, W.-C. Hsu, H.-R. Tan, X. W. Zhang, Z. Hong, and Y. Yang, “Plasmonic polymer tandem solar cell,” ACS Nano 5, 6210–6217 (2011). [CrossRef] [PubMed]

] have also been utilized to enhance the optical absorption in thin active layers. Although these demonstrations have shown improved device performance, in most cases exploiting SP resonances, a thorough analysis as to how the device geometry affects its performance has not been performed, and the trade-off between ηabs and ηint has not been fully examined in device optimizations.

Here, we systematically analyze a model device representative of SP-enhanced OSCs employing an optically thick metallic electrode and a two-dimensional (2D) metallic crossed grating electrode, examining both optical absorption and exciton diffusion. Based on electromagnetic and exciton-diffusion calculations using the Fourier modal and finite-element methods, respectively, we investigate how the variation in device geometry affects ηabs and ηint. The results of the investigation are then applied to a realistic device employing a copper phthalocyanine (CuPc)–fullerene (C60) heterojunction (HJ), which demonstrates general guidelines for maximizing Jsc of the SP-enhanced OSCs. The optimized CuPc–C60 device has Jsc that is 75 % larger than that of the optimized device with an ITO-based conventional structure.

This paper is organized as follows. In Sec. 2, we provide a brief analysis of short-circuit currents of planar HJ OSCs. Section 3 examines optical absorption and exciton diffusion in a model device representing SP-enhanced OSCs with a 2D metallic crossed grating electrode. In Sec. 4, we present the optimization of a SP-enhanced OSC with a CuPc–C60 heterojunction, followed by conclusion in Sec. 6.

2. Analysis of short-circuit currents of planar heterojunction organic solar cells

One strategy to overcome the trade-off between ηabs and ηint is to properly tailor the optical property of the device so that optical resonance allows for high ηabs in an active layer with a thickness t ≤∼ Ld that guarantees high ηint. A layer in a solar cell is called an active layer in this paper, if photon absorption in that layer contributes to photocurrent. A solar cell can have multiple active layers for broad spectral sensitivity. For simplicity, we assume that there is only one active layer in devices discussed in Secs. 2 and 3. For a device with multiple active layers, ηabs and ηint are the summations of the contributions from each active layer. In a planar HJ (as opposed to bulk-HJ) OSCs, ηint(λ) under short-circuit condition can be determined by
η int= ∫𝒜 J sc(r,λ)/q   d2r ∫𝒱 G exc ( r,λ)   d3r,
(2)
where Jsc(λ) is the steady-state current density of the device illuminated by monochromatic light with wavelength λ, 𝒜 is the device area, 𝒱 is the volume of the active layer, and Gexc(r, λ) is the number of photons absorbed per unit time per unit volume in the active layer. Photon absorption in the active layer generates excitons, whose steady-state profile, nexc(r, λ), can be obtained by solving
Ld2τ ∇2 n exc− n excτ+ G exc=0,
(3)
where τ is the lifetime of excitons [22

L. A. A. Pettersson, L. S. Roman, and O. Inganas, “Modeling photocurrent action spectra of photovoltaic devices based on organic thin films,” J. Appl. Phys. 86, 487–496 (1999). [CrossRef]

]. In solving Eq. (3), the following boundary conditions are commonly used: the exciton dissociation velocity at interfaces where excitons are efficiently dissociated, including the DA interface, is infinite; at other interfaces where exciton dissociation does not occur, the exciton dissociation velocity is zero [6

P. Peumans, A. Yakimov, and S. R. Forrest, “Small molecular weight organic thin-film photodetectors and solar cells,” J. Appl. Phys. 93, 3693–3723 (2003). [CrossRef]

, 23

A. S. Grove, Physics and technology of semiconductor devices (John Wiley and Sons Inc, 1967).

]. Assuming that charge carriers generated by exciton dissociation at the DA interface are collected at the electrodes with unity probability, Jsc(λ) is obtained via
J sc=q Ld2τ | en⋅∇ n exc ( r,λ)|,
(4)
where en is a unit vector normal to the DA interface.

Meanwhile, ηabs(λ) is given by
η abs= ∫𝒱 G exc ( r,λ)   d3r ∫𝒜 I0 ( r,λ)   d2r= ∫𝒱 Re { n˜(λ)}α(λ)E ( r,λ)⋅ E* ( r,λ)   d3r n0 | E0|2𝒜,
(5)
where I0 is the flux of photons incident from a medium with refractive index n0, α and ñ are, respectively, the absorption coefficient and the complex refractive index of the active layer, E denotes the complex electric field, and E0 is the amplitude of the incident plane wave. Eq. (5) indicates that the electric field intensity in the active layer must be as high as possible to maximize ηabs.

When t is sufficiently thin so that the variation in Gexc along the direction normal to the device surface can be ignored, and the characteristic dimension of the in-plane variation in Gexc is much smaller than t, the exciton diffusion can be considered a one-dimensional problem. Under this condition, the analytic solution of Eq. (3) can be used to approximate Eq. (2) as [6

P. Peumans, A. Yakimov, and S. R. Forrest, “Small molecular weight organic thin-film photodetectors and solar cells,” J. Appl. Phys. 93, 3693–3723 (2003). [CrossRef]

]:
η int≃ Ldttanh t Ld,
(6)
independent of λ. The validity of this approximation for device structures considered in this paper will be discussed in Sec. 3. Eq. (1) can then be expressed as
J sc≃q η int ∫ η abs(λ)S(λ)dλ.
(7)

Defining the solar-spectrum-weighted absorption efficiency [16

C. Min, J. Li, G. Veronis, J.-Y. Lee, S. 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, 133302 (2010). [CrossRef]

] as
〈 η abs〉= ∫ η abs(λ)S(λ)dλ ∫ S(λ)dλ,
(8)
we can express Jsc as
J sc≃q η int 〈 η abs〉 ∫ S(λ)dλ,
(9)
which clearly shows the trade-off between photon absorption and exciton dissociation: with decreasing t, ηint, as is given by Eq. (6), monotonically increases, while 〈ηabs〉 decreases.

3. Electromagnetic and exciton-diffusion analyses of model devices

Figure 1 schematically shows a structure of a planar HJ OSC employing a 2D Ag grating electrode supporting SP resonances. The device consists of, from the bottom layer in Fig. 1, 100 nm Ag electrode / organic multilayer / 2D Ag grating electrode / glass. The 2D grating electrode has a square lattice symmetry with a period of ΛG, and the linewidth and the thickness of the grating are 0.25ΛG and 20 nm, respectively. The organic multilayer with total thickness torg consists of lower transport layer (LTL) / active layer / upper transport layer (UTL), where the LTL and UTL are optically inactive and transport photo-generated holes and electrons, respectively, from the active layer to the respective electrode. The LTL and UTL thicknesses are tl and tu, respectively. For efficient collection of holes, square regions between the grating lines are filled with the same material chosen for the UTL. The active layer with thickness ta is composed of an absorbing material with Re(ñ) = 1.75 and α = 1.5 × 105 cm−1 throughout the spectral region that we consider, 300 nm ≤ λ ≤ 800 nm. For the UTL and LTL, ñ = 1.75, and for glass, ñ = 1.5. For Ag, we use the values for ñ reported in literature [24

E. D. Palik, Handbook of optical constants of solids , 1st ed. (Academic Press, 1997).

]. The exciton diffusion length in the active layer is Ld, and the exciton dissociation velocities at the LTL– and UTL–active layer interfaces are infinite and zero, respectively. We also assume that the glass substrate occupies the infinite lower (z < 0) half-space, ignoring small reflection at the air-glass interface, which in practice is minimized by an anti-reflection coating. With these assumptions, the device shown in Fig. 1 serves as a model device, of which we perform electromagnetic and exciton-diffusion simulations to investigate how ηabs and ηint vary with the device geometry in the broad spectral region.

Fig. 1 Schematic of a model device representative of a surface plasmon-enhanced OSC. The two-dimensional Ag grating electrode has a square lattice with a period of ΛG, whose unit cell is drawn in red-dotted lines. The grating linewidth is 0.25ΛG. The orientation of the Cartesian coordinates is shown, with O on the grating surface denoting the origin. Also shown are the directions of the wave vector (k) and electric field (E) of an incident plane wave.

Figure 2(a) shows calculated ηabs, as a function of λ and ΛG, of the 10-nm-thick active layer in the model device with tu = 30 nm and tl = 30 nm, when it is illuminated by a x-polarized (E || x̂) plane wave with wavelength λ and a wave vector normal to the device surface. The active layer thickness is chosen to be similar to Ld’s of organic semiconductors commonly employed in OSCs [6

P. Peumans, A. Yakimov, and S. R. Forrest, “Small molecular weight organic thin-film photodetectors and solar cells,” J. Appl. Phys. 93, 3693–3723 (2003). [CrossRef]

–8

P. E. Shaw, A. Ruseckas, and I. D. W. Samuel, “Exciton diffusion measurements in poly(3-hexylthiophene),” Adv. Mater. 20, 3516–3520 (2008). [CrossRef]

]. The calculation was performed using the Fourier modal method [25

M. G. Moharam and T. K. Gaylord, “Rigorous coupled-wave analysis of metallic surface-relief gratings,” J. Opt. Soc. Am. A 3, 1780–1787 (1986). [CrossRef]

, 26

H. Kim, J. Park, and B. Lee, Fourier modal method and its applications in computational nanophotonics , 1st ed. (CRC Press, 2012).

]. We applied Li’s Fourier factorization rules [27

L. Li, “New formulation of the Fourier modal method for crossed surface-relief gratings,” J. Opt. Soc. Am. A. 14, 2758–2767 (1997). [CrossRef]

] and the enhanced transmittance matrix method [28

M. G. Moharam, D. A. Pommet, E. B. Grann, and T. K. Gaylord, “Stable implementation of the rigorous coupled-wave analysis for surface-relief gratings: enhanced transmittance matrix approach,” J. Opt. Soc. Am. A 12, 1077–1086 (1995). [CrossRef]

], and exploited the two-fold mirror-reflection symmetry [29

C. Zhou and L. Li, “Formulation of the Fourier modal method for symmetric crossed gratings in symmetric mountings,” J. Opt. A-Pure Appl. Op. 6, 43–50 (2004). [CrossRef]

] to obtain E(r, λ), from which ηabs was obtained from Eq. (5). The high-ηabs region including point marked ‘a’ is attributed to resonant excitation of a SP mode. The z-component of the electric field (Ez) spreads throughout the organic multilayer, with induced charges localized at the LTL–Ag interface and the top and bottom surfaces of the grating, as shown in Fig. 2(b). There exists another SP mode that can be resonantly excited, whose Ez profile corresponding to point marked ‘b’ in Fig. 2(a) is shown in Fig. 2(d). In this case, the field is more concentrated near the grating compared with the profile in Fig. 2(b), displaying the field profile characteristic of the localized dipolar SP resonance associated with a metallic nanoparticle. The profiles of the electric field intensity in the horizontal planes bisecting the active layer are shown in Figs. 2(c) and 2(e). The intensity profile for point ‘a’ is distributed throughout the active layer regions below the grating holes, with some degree of inter-hole coupling through the grating lines parallel to the x axis (Fig. 2(c)). In contrast, for the case of point ‘b’, the electric field intensity is localized along the regions below the edges of the grating lines parallel to the y axis, with negligible inter-hole coupling. Noting the contrasts in electric field intensity distribution in the active layer (Fig. 2(c) and 2(e)), as well as in Ez profile throughout the device (Fig. 2(b) and 2(d)), we refer to the mode corresponding to point ‘a’ (or ‘b’) as the distributed-SP (or localized-SP) mode. Although the intensity maximum in the active layer for the localized-SP (LSP) mode is higher that that for the distributed-SP (DSP) mode, the high-intensity region for the LSP mode is limited in its extent, resulting in lower ηabs compared with the DSP case.

Fig. 2 (a) Calculated absorption efficiency (ηabs) of the model device with torg = 70 nm, tu = 30 nm, ta = 10 nm, and tl = 30 nm, as a function of the grating period (ΛG) and the wavelength (λ) of the incident light. The maximum of ηabs is 0.71. (b) The z-component of the electric field corresponding to point ‘a’ (λ = 680 nm, ΛG = 380 nm) in (a). (c) The electric field intensity (|E|2 = E · E*) corresponding to point ‘a’. (d) The z-component of the electric field corresponding to point ‘b’ (λ = 760 nm, ΛG = 320 nm). (e) The electric field intensity corresponding to point ‘b’. E0 denotes the amplitude of the incident electric field. In (b) to (e), solid lines represent materials boundaries, and grating boundaries that do not lie on the planes shown are drawn as dotted lines.

The field profile shown in Fig. 2(d) suggests that if torg is decreased, the overlap between the high-intensity region of the LSP mode and the active layer may increase, allowing one to utilize both DSP and LSP modes for the absorption enhancement. Figure 3(a) shows ηabs(λ, ΛG) of the active layer in a model device, where torg is decreased from 70 nm to 50 nm by decreasing tu from 30 nm to 10 nm. The other geometrical parameters and materials properties are not changed. The Ez and the intensity profiles of the DSP mode (point ‘a’ in Fig. 3(a)), shown in Figs. 3(b) and 3(c), respectively, resemble the corresponding profiles shown in Fig. 2, resulting in the enhancement in ηabs similar to that obtained in the model device with torg = 70 nm. More notable is that another high-ηabs region, including point ‘b’, associated with the LSP modes is now clearly visible. As shown in Fig. 3(d), the decrease in torg leads to stronger coupling between SPs localized in the grating region and at the LTL–Ag interface. As a result, compared with the previous case, the electric field intensity is more uniform along the z direction. This, along with the fact that the active layer is located closer to the grating–UTL interface, results in a much higher field intensity in the active layer (Fig. 3(e)), thereby leading to high ηabs. Consequently, unlike the previous case with torg = 70 nm, both DSP and LSP modes contribute to the absorption enhancement, which is obtained over a broad spectral region.

Fig. 3 (a) Calculated absorption efficiency (ηabs) of the model device with torg = 50 nm, tu = 10 nm, ta = 10 nm, and tl = 30 nm, as a function of the grating period (ΛG) and the wavelength (λ) of the incident light. The maximum of ηabs is 0.76. (b) The z-component of the electric field corresponding to point ‘a’ (λ = 660 nm, ΛG = 380 nm) in (a). (c) The electric field intensity (|E|2 = E · E*) corresponding to point ‘a’. (d) The z-component of the electric field corresponding to point ‘b’ (λ = 760 nm, ΛG = 320 nm). (e) The electric field intensity corresponding to point ‘b’.

The linewidth of the grating is another factor that affects ηabs. By calculating ηabs(λ, ΛG) of the model device with torg = 50 nm for different grating fill factors (F), defined as the ratio of the grating linewidth to ΛG, we find the following. When ∼ 0.2 < F <∼ 0.3, the magnitudes and the extents of the high-ηabs regions in the (λ, ΛG)-plane associated with the DSP and LSP modes remain similar to those in Fig. 3(a). Also, with increasing F (in ∼ 0.2 < F <∼ 0.3), (i) the absorption enhancement associated with the LSP modes occurs at longer wavelengths, since the width of the metallic “particles” increases, and (ii) the location of that associated with the DSP modes in the (λ, ΛG)-plane remains relatively unchanged, since the distributed nature of the DSP modes makes them sensitive primarily to ΛG. When F deviates farther from 0.25, ηabs decreases due to decreased resonance features (for small F) or increased reflection (for large F). In optimizing a practical SPP-enhanced OSC, the optimum F in ∼ 0.2 < F <∼ 0.3 is chosen considering the overlap between the absorption spectra of active materials used and the high-ηabs regions.

Next, we examine the dependence of ηint of the model device with (torg, tu, ΛG) = (50 nm, 10 nm, 320 nm) on Ld, ta, and λ, by solving Eq. (3) using the finite element method [30

COMSOL Multiphysics®, http://www.comsol.com.

], where the exciton generation term, Gexc, is obtained by the electromagnetic simulations based on the Fourier modal method. For the illumination wavelengths, we choose λ = 400, 580, and 760 nm, corresponding to off-, DSP-, and LSP-resonance cases, respectively, for the model device with ta = 10 nm. The results plotted versus γ ≡ ta/Ld show that all data points closely follow those calculated using Eq. (6) (Fig. 4(a)). Since ta is sufficiently small, Gexc does not vary significantly along the z-direction in a thin active layer. This means that the exciton diffusion in the z-direction is mostly driven by the boundary conditions at the LTL– and UTL–active layer interfaces. The exciton diffusion in the xy-directions, however, is driven by the horizontal variation in Gexc, whose characteristic dimension is much larger than ta. Consequently, the exciton diffusion in the active layer is well described by the analytical solution of the one-dimensional case with constant Gexc [Eq. (3)], as Fig. 4(a) clearly shows. Deviation from the analytical solution increases with ta, since the variation in Gexc along the z-direction can no longer be ignored as ta increases. We note that the dependence of ηint on λ is very weak, and therefore Eq. (9) is valid. Figure 4(b) shows the steady-state exciton density profile in the active layer when ta = 3.5 nm, Ld = 5 nm, and λ = 760 nm.

Fig. 4 (a) Internal quantum efficiency (ηint) of the model device with torg = 50 nm, tu = 10 nm, and ΛG = 320 nm versus γ ≡ ta/Ld, calculated for different values of Ld and λ. The black solid line shows ηint calculated using Eq. (6). (b) Steady-state exciton density profile (nexc) in the active layer when ta = 3.5 nm, Ld = 5 nm, and λ = 760 nm. The top (z = 30 nm) and bottom (z = 33.5 nm) faces correspond to the UTL– and LTL–active layer interfaces, respectively, where at the former (or latter) interface the exciton dissociation velocity is zero (or infinite).

4. Optimization of SP-enhanced organic solar cells based on CuPc and C60

Building upon the analyses in Sec. 3, we optimize small-molecule organic solar cells based CuPc–C60 DA heterojunction. The device structure is, from the bottom layer in Fig. 5(b), Ag / 4, 7-diphenyl-1, 10-phenanthroline (BPhen) / C60 / CuPc / poly(3, 4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) / 2D Ag grating electrode / glass. In comparison to the structure of the model device discussed in Sec. 3, the bilayer of CuPc and C60 is the active layer; the BPhen and the PEDOT:PSS are the LTL and the UTL, respectively, since they are optically inactive in the spectral region where CuPc and C60 are sensitive, and possess good charge transport properties [6

P. Peumans, A. Yakimov, and S. R. Forrest, “Small molecular weight organic thin-film photodetectors and solar cells,” J. Appl. Phys. 93, 3693–3723 (2003). [CrossRef]

, 11

J. Meiss, M. Furno, S. Pfuetzner, K. Leo, and M. Riede, “Selective absorption enhancement in organic solar cells using light incoupling layers,” J. Appl. Phys. 107, 053117 (2010). [CrossRef]

, 31

L. A. A. Pettersson, S. Ghosh, and O. Inganas, “Optical anisotropy in thin films of poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate),” Org. Electron. 3, 143–148 (2002). [CrossRef]

, 32

M. Y. Chan, S. L. Lai, K. M. Lau, C. S. Lee, and S. T. Lee, “Application of metal-doped organic layer both as exciton blocker and optical spacer for organic photovoltaic devices,” Appl. Phys. Lett. 89, 163515 (2006). [CrossRef]

]. Depending on its thickness, the BPhen layer may need to be doped with, for example, ytterbium, where the absorption coefficient of the doped film is almost identical to that of the pristine BPhen layer [32

M. Y. Chan, S. L. Lai, K. M. Lau, C. S. Lee, and S. T. Lee, “Application of metal-doped organic layer both as exciton blocker and optical spacer for organic photovoltaic devices,” Appl. Phys. Lett. 89, 163515 (2006). [CrossRef]

].

Fig. 5 (a) (Top) Absorption coefficient (α) of CuPc (red) and C60 (blue). (Bottom) Absorption efficiency (ηabs), as a function of the grating period (ΛG) and the wavelength (λ), of the active layer in the organic solar cell based on the CuPc and C60 DA heterojunction. The maximum of ηabs is 0.74. (b) The z-component of the electric field corresponding to point ‘a’ (λ = 600 nm, ΛG = 320 nm) in (a). (c) The z-component of the electric field corresponding to point ‘b’ (λ = 620 nm, ΛG = 200 nm).

Since when γ ≪ (or ≫)1.0, ηext is limited by too small ηabs (or ηint), we first set γ = tCuPc/LCuPc = tC60/L C60 = 1.2, where tCuPc (or tC60) and LCuPc (or LC60) are the thickness and the exciton diffusion length of the CuPc (or C60) layer, respectively. Assuming that LCuPc = 7.4 nm and LC60 = 14.4 nm [33

J. Lee, S.-Y. Kim, C. Kim, and J.-J. Kim, “Enhancement of the short circuit current in organic photovoltaic devices with microcavity structures,” Appl. Phys. Lett. 97, 083306 (2010). [CrossRef]

], the thicknesses of the CuPc and the C60 are then determined as: tCuPc = 8.9 nm, tC60 = 17.3 nm (ta = tCuPc + tC60 = 26.2 nm). Under this condition, ηint = 0.694 from Eq. (6). Figure 5(a) shows ηabs(λ, ΛG) of the CuPc–C60 active layer in the SP-enhanced solar cell, where tu = 5 nm, ta = 26.2 nm, tl = 18.8 nm (torg = 50 nm), and the grating geometry is the same as that in the model devices. As in the case of the model device in Fig. 3, the active layer is located close to the UTL–Ag grating interface to maximize its optical absorption. The values for ñ of all materials are measured using spectroscopic ellipsometry. The enhancement in ηabs is mainly attributed to the enhanced absorption in the CuPc layer, since the region in the λ–ΛG plane where resonant excitation of the DSP or LSP mode occur overlaps mostly with the absorption spectrum of CuPc; the high-ηabs region in Fig. 5(a) approximately coincides with that in Fig. 3(a) overlapped with α(λ) of CuPc (the top portion of Fig. 5(a)). The electric field profiles for points ‘a’ and ‘b’, shown in Figs. 5(b) and 5(c), resembles those for the corresponding points in Fig. 3(a), confirming that the enhanced absorption is due to the LSP and DSP resonances.

The maximization of Jsc of the SP-enhanced device is performed as the following. From ηabs(λ, ΛG) shown in Fig. 5(a), we first choose ΛG that maximizes 〈ηabs〉: ΛG = 250 nm, 〈ηabs〉 = 0.164, resulting in Jsc = 8.78 mA/cm2. Recalling that this result is obtained for γ = 1.2 (tCuPc = 8.9 nm and tC60 = 17.3 nm), Jsc can be further increased, if an increase (or decrease) in the active layer thickness leads to a gain in 〈ηabs〉 (or ηint) that more than compensates a concomitant loss in ηint (or 〈ηabs〉). To investigate this possibility, we vary γ with the values of torg and tu fixed, and calculate the maximum value of 〈ηabs〉 at different γ. Since the maximum Jsc is expected to be obtained not too far away from γ = 1.2, we assume that ηabs(λ, ΛG) does not change with γ, meaning that 〈ηabs〉 is still maximized at ΛG = 250 nm. For more rigorous optimization, ηabs(λ, ΛG) can be re-calculated to find ΛG maximizing 〈ηabs〉 for each γ. Figure 6(a) shows, as functions of γ, the maximum 〈ηabs〉, ηint given by Eq. (6), and resulting Jsc obtained using Eq. (9), where the trade-off between ηint and ηabs is clearly seen. Furthermore, it shows that our initial guess of γ = 1.2 is not the optimum choice: by decreasing the thickness of the active layer, Jsc can be increased owing to the increase in ηint, until γ becomes 1.0, where Jsc is maximized at Jsc = 8.88 mA/cm2.

Fig. 6 (a) Short-circuit current density (Jsc) (red), solar-spectrum-weighted absorption efficiency (〈ηabs〉) (blue), and internal quantum efficiency (ηint) (black) versus γ = tCuPc/LCuPc = tC60/LC60 of the surface plasmon (SP)-enhanced CuPc–C60 solar cell with torg = 50 nm, and tu = 5 nm. (b) External quantum efficiency (ηext) of the optimized SP-enhanced device (red, γ = 1.0), compared with that of the optimized ITO-based device (black) consisting of: glass / 150 nm ITO / 5 nm PEDOT:PSS / 13 nm CuPc / 21 nm C60 / 29 nm BPhen / 100 nm Ag.

Figure 6(b) shows the ηext spectrum of the SP-enhanced CuPc–C60 device with γ = 1.0, compared with that of the optimized ITO-based conventional device based on the same materials. The structure of the ITO-based device is glass / 150 nm ITO / 5 nm PEDOT:PSS / 13 nm CuPc / 21 nm C60 / 29 nm BPhen / 100 nm Ag, where the thicknesses of the CuPc, C60, and BPhen layers are chosen so as to maximize Jsc. Compared with the ITO-based device, the SP-enhanced device has greater ηext in the entire spectral region shown in Fig. 6(b), with the enhancement in the CuPc-active region is more pronounced than that in the C60-active region. As a result, the SPP-enhanced device has Jsc (= 8.88 mA/cm2) that is 75 % higher than that of the ITO-based device (Jsc = 5.07 mA/cm2).

5. Discussion

The type of OSCs that our analysis and optimization approach are directly applicable is multilayer planar HJ OSCs. In essence, what we have discussed in this paper is how to design a plasmonic structure shown in Fig. 1 so that an active layer whose thickness is much smaller than its optical absorption length can absorb most incident photons. In other words, to overcome the trade-off between ηint and ηabs, we have begun with a device configuration with high ηint, and increased ηabs by exploiting SP resonances. Organic solar cells based on a bulk-heterojunction (BHJ) [34

C. J. Brabec, S. Gowrisanker, J. J. M. Halls, D. Laird, S. Jia, and S. P. Williams, “Polymer-fullerene bulk-heterojunction solar cells,” Adv. Mater. 22, 3839–3856 (2010). [CrossRef] [PubMed]

, 35

G. Li, R. Zhu, and Y. Yang, “Polymer solar cells,” Nat. Photonics 6, 153–161 (2012). [CrossRef]

], which refers to an extended region of interpenetrating networks of donor and acceptor, can be viewed as an alternative approach to overcome the ηint–ηabs trade-off; owing to the interpenetrating networks, the active layer thickness of a BHJ device can be increased much beyond the exciton diffusion lengths of donor and acceptor materials to achieve high ηabs without sacrificing ηint. Therefore, the optimization methodology presented in this paper, as well as absorption enhancement by SP resonances in general, is less effective in enhancing Jsc of BHJ OSCs than in enhancing that of planar HJ OSCs with thin active layers. Nevertheless, various device structures with metallic grating electrodes have been proposed to enhance optical absorption in BHJ OSCs, since a decrease in the active layer thickness typically leads to the decreased recombination of photo-generated carriers and series resistance [15

W. Bai, Q. Gan, G. Song, L. Chen, Z. Kafafi, and F. Bartoli, “Broadband short-range surface plasmon structures for absorption enhancement in organic photovoltaics,” Opt. Express 18, A620–A630 (2010). [CrossRef] [PubMed]

, 17

H. Shen and B. Maes, “Combined plasmonic gratings in organic solar cells,” Opt. Express 19, A1202–A1210 (2011). [CrossRef] [PubMed]

, 36

Z. Ye, S. Chaudhary, P. Kuang, and K.-M. Ho, “Broadband light absorption enhancement in polymer photovoltaics using metal nanowall gratings as transparent electrodes,” Opt. Express 20, 12213–12221 (2012). [CrossRef] [PubMed]

].

In contrast to SP-enhanced OSCs with one-dimensional metallic grating, where only p-polarized (electric field perpendicular to grating lines) illumination can excite SP resonances [13

C. Kim, J.-Y. Lee, and P. Peumans, “Surface plasmon polariton assisted organic solar cells,” in Proceedings of NSTI-Nanotech 2008 1, 533–536 (2008).

, 16

C. Min, J. Li, G. Veronis, J.-Y. Lee, S. 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, 133302 (2010). [CrossRef]

, 37

P. Zilio, D. Sammito, G. Zacco, M. Mazzeo, G. Gigli, and F. Romanato, “Light absorption enhancement in heterostructure organic solar cells through the integration of 1-D plasmonic gratings,” Opt. Express 20, A476–A488 (2012). [CrossRef] [PubMed]

], the four-fold rotational symmetry of the 2D grating electrode in our device leads to polarization-insensitive absorption enhancement for normal-incidence illumination. For practical implementation, the variation of Jsc with respect to the incident angle of solar illumination is important. In Fig. 7, Jsc of the optimized SPP-enhanced device discussed in Sec. 4 is shown as a function of incident angle, and compared with that of the ITO-based device. The calculation was performed for different azimuthal (ϕ) and polar angles (θ), where θ refers to an angle between the wave vector k and the z-axis in air, and the reflection at the air-glass interface is ignored for both SPP-enhanced and ITO-based devices. We also examine the dependency of Jsc on polarizations. The s-polarized (or p-polarized) light refers to a plane wave whose electric (or magnetic) field is perpendicular to the plane of incidence drawn as green dotted lines in Fig. 7(a). For normal incidence (θ = 0), owing to the four-fold rotational symmetry of the grating electrode, the values of Jsc are identical for both polarizations at all ϕ. Although this no longer holds for off-normal cases, our calculation shows that the angle and polarization dependencies of Jsc are weak: data points in Fig. 7(c) closely follow the three black lines, which are Jsc(θ = 0°)cosθ with the cosine factor representing the decrease in the solar power flux as θ deviates from 0°.

Fig. 7 (a) Schematic diagram showing the orientation and polarization of an incident plane wave. An s-polarized (or p-polarized) plane wave whose electric field is drawn as a red (or blue) arrow has the electric (or magnetic) field vector perpendicular to the plane of incidence drawn as green dotted lines. (b) Schematic of a device considered to calculate the ergodic limit shown in (c), where an active layer is sandwiched between a perfect back reflector and an ideal front Lambertian surface with an ideal anti-reflection coating. (c) Short-circuit current density (Jsc) of the optimized SP-enhanced device in Sec. 4 versus incident polar angle (θ) for three azimuthal angles, ϕ = 0° (red), 22.5° (blue), and 45° (green). For comparison, Jsc of the optimized ITO-based device in Sec. 4 (brown) and the device shown in (b) (Ergodic limit, black open squares) are also shown. ‘+’ and ‘×’ symbols refer to s- and p-polarizations, respectively. The three black lines are J sc maxcosθ, where J sc max is Jsc(θ = 0°) for each device.

Also shown in Fig. 7 is the comparison of the absorption enhancement in the SPP-enhanced device with the so-called ergodic limit, which represents the maximum possible absorption enhancement that can be obtained in the geometrical optics regime [38

E. Yablonovitch and G. D. Cody, “Intensity enhancement in textured optical sheets for solar cells,” IEEE Trans. Electron. Devices 29, 300–305 (1982). [CrossRef]

]. For this comparison, we consider a device shown in Fig. 7(b) consisting of an active layer sandwiched between a perfect reflector and a Lambertian front surface with random texturing coated with an ideal anti-reflection coating. For simplicity, we assume that the active layer in this device is composed of an effective single material, whose thickness is ta (= tCuPc + tC60) of the optimized SPP-enhanced device, with its real part of the refractive index (ñeff) and absorption coefficients (αeff) given by, respectively,

Re ( n˜ eff)= Re ( n˜ CuPc) t CuPc+Re ( n˜ C 60) t C 60 ta     and
(10)
α eff= α CuPc t CuPc+ α C 60 t C 60 ta.
(11)

Here, tCuPc = 7.4 nm, tC60 = 14.4 nm, and ta = 21.8 nm, as in the optimized SPP-enhanced device. The absorption efficiency of this device is then obtained by the summation of a geometric series, with each term in the series is the light absorption in a single round-trip between the reflector and the front surface [39

S. B. Mallick, M. Agrawal, and P. Peumans, “Optical light trapping in ultra-thin photonic crystal crystalline silicon solar cells,” Opt. Express 18, 5691–5706 (2010). [CrossRef] [PubMed]

, 40

H. W. Deckman, C. B. Roxlo, and E. Yablonovitch, “Maximum statistical increase of optical absorption in textured semiconductor films,” Opt. Lett. 8(9), 491–493 (1983). [CrossRef] [PubMed]

]:
η abs= 1− e −4 α eff ta 1− e −4 α eff ta+ e −4 α eff ta ( Re ( n˜ eff)) −2.
(12)
The values of Jsc calculated using Eqs. (9) and (12) are shown as open squares in Fig. 7(c). Although resonant excitation of a SP mode under a certain condition allows ηabs to surpass the ergodic limit (for example, when ϕ = θ = 0°, and λ = 570 nm, ηabs for the SPP-enhanced device is 0.69, while that of the device shown in Fig. 7(b) is 0.59), considering all wavelengths, Jsc of the SPP-enhanced device is approximately 77 % of what can be achieved in the ergodic limit. We note that the ray-optics approach, which is strictly applicable to a film of an absorbing medium whose thickness is much larger than λ, is less effective in our case, since the active layer thickness is only 21.8 nm and an ideal Lambertian front surface is difficult to realize in this sub-wavelength regime.

As an alternative method for absorption enhancement, a dielectric grating structure has also been applied to thin film solar cells [41

M. B. Duhring, N. A. Mortensen, and O. Sigmund, “Plasmonic versus dielectric enhancement in thin-film solar cells,” Appl. Phys. Lett. 100, 211914 (2012). [CrossRef]

]. Coherent light scattering by a dielectric grating can enhance the electric field intensity in the active layer, as in the plasmonic cases. Since optical loss by metal absorption can be avoided, this approach can be advantageous over the plasmonic approach in certain cases, especially when the absorption coefficient of the active layer is not very large. In the SPP-enhanced device based on the CuPc–C60 active layer discussed in Sec. 4, optical loss due to absorption in the grating layer is quite limited: at λ = 580 nm, absorption in the grating and back metal electrodes, and reflection are 17.4 %, 5.3 %, and 5.8 %, respectively, and ηabs of the active layer is 0.702. Furthermore, in addition to creating surface plasmon resonances, the 2D metallic grating in our device functions as a transparent electrode by replacing a brittle and expensive ITO or Al-doped zinc oxide, being suitable for flexible solar cells.

6. Conclusion

We have performed a systematic analysis of a model device representative of SP-enhanced planar HJ OSCs where one electrode is optically thick and the other consists of 2D metallic grating, taking into account both optical absorption and exciton diffusion. Our analysis has provided the following guidelines by which Jsc of such SP-enhanced OSCs can be maximized: (i) to maximize optical absorption in a thin active layer, the device thickness needs to be decreased until the SPs associated with the grating electrode are coupled with those localized at the interface between the organic and thick metal layers, with the active layer located near the grating electrode; (ii) for high ηint (> 0.7), the thickness of the active layer should be smaller than ∼ Ld, in which case ηint is closely approximated by Eq. (6), independent of wavelengths where active materials are sensitive; (iii) device optimization is achieved by determining the grating period maximizing 〈ηabs〉, followed by fine-tuning the active layer thickness to fully maximize Jsc. As an example, we have optimized the performance of a small-molecule organic solar cell employing the CuPc–C60 donor–acceptor pair, demonstrating that the optimized device has Jsc that is 75 % larger than that of the optimized device with an ITO-based conventional structure.

Successful realization of the SPP-enhanced OSCs depends critically on the capability to accurately create metallic nanostructures over large area at low cost. Metal patterning by imprint [42

M.-G. Kang, M.-S. Kim, J. Kim, and L. J. Guo, “Organic solar cells using nanoimprinted transparent metal electrodes,” Adv. Mater. 20, 4408–4413 (2008). [CrossRef]

] or layer-transfer techniques [43

C. Kim, M. Shtein, and S. Forrest, “Nanolithography based on patterned metal transfer and its application to organic electronic devices,” Appl. Phys. Lett. 80, 4051–4053 (2002). [CrossRef]

], or methods using metal nanowires [44

M.-G. Kang, T. Xu, H. J. Park, X. Luo, and L. J. Guo, “Efficiency enhancement of organic solar cells using transparent plasmonic Ag nanowire electrode,” Adv. Mater. 22, 4378–4383 (2010). [CrossRef] [PubMed]

] may be utilized to achieve this goal.

Acknowledgments

This work was supported by the Basic Science Research program under Grant No. 2011-0026517, the Converging Research Center program under Grant No. 2011K000586, and the Engineering Research Center program under Grant No. 2009-0093428, all through the National Research Foundation of Korea, funded by the Ministry of Education, Science and Technology. The authors thank Hwi Kim and Mukul Agrawal for helpful discussion.

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J. Meiss, M. Furno, S. Pfuetzner, K. Leo, and M. Riede, “Selective absorption enhancement in organic solar cells using light incoupling layers,” J. Appl. Phys. 107, 053117 (2010). [CrossRef]

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34.

C. J. Brabec, S. Gowrisanker, J. J. M. Halls, D. Laird, S. Jia, and S. P. Williams, “Polymer-fullerene bulk-heterojunction solar cells,” Adv. Mater. 22, 3839–3856 (2010). [CrossRef] [PubMed]

35.

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36.

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37.

P. Zilio, D. Sammito, G. Zacco, M. Mazzeo, G. Gigli, and F. Romanato, “Light absorption enhancement in heterostructure organic solar cells through the integration of 1-D plasmonic gratings,” Opt. Express 20, A476–A488 (2012). [CrossRef] [PubMed]

38.

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39.

S. B. Mallick, M. Agrawal, and P. Peumans, “Optical light trapping in ultra-thin photonic crystal crystalline silicon solar cells,” Opt. Express 18, 5691–5706 (2010). [CrossRef] [PubMed]

40.

H. W. Deckman, C. B. Roxlo, and E. Yablonovitch, “Maximum statistical increase of optical absorption in textured semiconductor films,” Opt. Lett. 8(9), 491–493 (1983). [CrossRef] [PubMed]

41.

M. B. Duhring, N. A. Mortensen, and O. Sigmund, “Plasmonic versus dielectric enhancement in thin-film solar cells,” Appl. Phys. Lett. 100, 211914 (2012). [CrossRef]

42.

M.-G. Kang, M.-S. Kim, J. Kim, and L. J. Guo, “Organic solar cells using nanoimprinted transparent metal electrodes,” Adv. Mater. 20, 4408–4413 (2008). [CrossRef]

43.

C. Kim, M. Shtein, and S. Forrest, “Nanolithography based on patterned metal transfer and its application to organic electronic devices,” Appl. Phys. Lett. 80, 4051–4053 (2002). [CrossRef]

44.

M.-G. Kang, T. Xu, H. J. Park, X. Luo, and L. J. Guo, “Efficiency enhancement of organic solar cells using transparent plasmonic Ag nanowire electrode,” Adv. Mater. 22, 4378–4383 (2010). [CrossRef] [PubMed]

OCIS Codes
(040.5350) Detectors : Photovoltaic
(240.6680) Optics at surfaces : Surface plasmons
(260.5740) Physical optics : Resonance
(250.5403) Optoelectronics : Plasmonics
(310.6845) Thin films : Thin film devices and applications

ToC Category:
Photovoltaics

History
Original Manuscript: July 19, 2012
Revised Manuscript: August 23, 2012
Manuscript Accepted: August 24, 2012
Published: August 29, 2012

Citation
Eungkyu Lee and Changsoon Kim, "Analysis and optimization of surface plasmon-enhanced organic solar cells with a metallic crossed grating electrode," Opt. Express 20, A740-A753 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-S5-A740


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References

  1. S. R. Forrest, “The path to ubiquitous and low-cost organic electronic appliances on plastic,” Nature428, 911–918 (2004). [CrossRef] [PubMed]
  2. M. R. Lee, R. D. Eckert, K. Forberich, G. Dennler, C. J. Brabec, and R. A. Gaudiana, “Solar power wires based on organic photovoltaic materials,” Science324, 232–235 (2009). [CrossRef] [PubMed]
  3. H.-Y. Chen, J. Hou, S. Zhang, Y. Liang, G. Yang, Y. Yang, L. Yu, Y. Wu, and G. Li, “Polymer solar cells with enhanced open-circuit voltage and efficiency,” Nat. Photonics3, 649–653 (2009). [CrossRef]
  4. M. Pope and C. E. Swenberg, Electronic processes in organic crystals and polymers, 2nd ed. (Oxford University Press, 1999).
  5. I. Hill, A. Kahn, Z. G. Soos, and R. A. Pascal, “Charge-separation energy in films of π-conjugated organic molecules,” Chem. Phys. Lett.327, 181–188 (2000). [CrossRef]
  6. P. Peumans, A. Yakimov, and S. R. Forrest, “Small molecular weight organic thin-film photodetectors and solar cells,” J. Appl. Phys.93, 3693–3723 (2003). [CrossRef]
  7. S. Cook, A. Furube, R. Katoh, and L. Han, “Estimate of singlet diffusion lengths in PCBM films by time-resolved emission studies,” Chem. Phys. Lett.478, 33–36 (2009). [CrossRef]
  8. P. E. Shaw, A. Ruseckas, and I. D. W. Samuel, “Exciton diffusion measurements in poly(3-hexylthiophene),” Adv. Mater.20, 3516–3520 (2008). [CrossRef]
  9. S. R. Forrest, “The limits to organic photovoltaic cell efficiency,” MRS. Bull.30, 28–32 (2005). [CrossRef]
  10. C. Kim and J. Kim, “Organic photovoltaic cell in lateral-tandem configuration employing continuously-tuned microcavity sub-cells,” Opt. Express16, 19987–19994 (2008). [CrossRef] [PubMed]
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