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Energy Express

Energy Express

  • Editor: Christian Seassal
  • Vol. 21, Iss. S1 — Jan. 14, 2013
  • pp: A131–A145
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An optimized surface plasmon photovoltaic structure using energy transfer between discrete nano-particles

Albert Lin, Sze-Ming Fu, Yen-Kai Chung, Shih-yun Lai, and Chi-Wei Tseng  »View Author Affiliations


Optics Express, Vol. 21, Issue S1, pp. A131-A145 (2013)
http://dx.doi.org/10.1364/OE.21.00A131


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Abstract

Surface plasmon enhancement has been proposed as a way to achieve higher absorption for thin-film photovoltaics, where surface plasmon polariton(SPP) and localized surface plasmon (LSP) are shown to provide dense near field and far field light scattering. Here it is shown that controlled far-field light scattering can be achieved using successive coupling between surface plasmonic (SP) nano-particles. Through genetic algorithm (GA) optimization, energy transfer between discrete nano-particles (ETDNP) is identified, which enhances solar cell efficiency. The optimized energy transfer structure acts like lumped-element transmission line and can properly alter the direction of photon flow. Increased in-plane component of wavevector is thus achieved and photon path length is extended. In addition, Wood-Rayleigh anomaly, at which transmission minimum occurs, is avoided through GA optimization. Optimized energy transfer structure provides 46.95% improvement over baseline planar cell. It achieves larger angular scattering capability compared to conventional surface plasmon polariton back reflector structure and index-guided structure due to SP energy transfer through mode coupling. Via SP mediated energy transfer, an alternative way to control the light flow inside thin-film is proposed, which can be more efficient than conventional index-guided mode using total internal reflection (TIR).

© 2012 OSA

1. Introduction

Energy transfer is a peculiar phenomenon existing in surface plasmon nano-particles [1

1. S. A. Maier, P. G. Kik, H. A. Atwater, S. Meltzer, E. Harel, B. E. Koel, and A. A. G. Requicha, “Local detection of electromagnetic energy transport below the diffraction limit in metal nanoparticle plasmon waveguides,” Nat. Mater. 2(4), 229–232 (2003). [CrossRef] [PubMed]

3

3. S. A. Maier, P. G. Kik, and H. A. Atwater, “Optical pulse propagation in metal nanoparticle chain waveguides,” Phys. Rev. B 67, 205402 (2003).

], where efficient electromagnetic power is coupled through successive metal particles. There has been tremendous amount on application of surface plasmon to photovoltaic cells as an emerging technique to extend photon path length. SPP & LSP have been demonstrated to enhance the light absorption by wide angle scattering by SP and strongly localized field intensity at metallic/semiconductor interface [4

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

20

20. S. Pillai, F. J. Beck, K. R. Catchpole, Z. Ouyang, and M. A. Green, “The effect of dielectric spacer thickness on surface plasmon enhanced solar cells for front and rear side depositions,” J. Appl. Phys. 109(7), 073105 (2011). [CrossRef]

]. In this paper, surface plasmon mediated energy transfer phenomenon is further tailored to increase solar cell absorbance using global optimization methods, resulting in coupling between top/bottom and adjacent metallic nano-particles which corresponds to vertical and lateral energy transfer. In addition, the Wood-Rayleigh anomaly where transmittance minimum exists is avoided [21

21. Y.-W. Jiang, L. D.-C. Tzuang, Y.-H. Ye, Y.-T. Wu, M.-W. Tsai, C.-Y. Chen, and S.-C. Lee, “Effect of Wood’s anomalies on the profile of extraordinary transmission spectra through metal periodic arrays of rectangular subwavelength holes with different aspect ratio,” Opt. Express 17(4), 2631–2637 (2009). [CrossRef] [PubMed]

]. Compared to conventional optical waveguiding with index-guided(IG) mode [22

22. V. Shah, H. Schade, M. Vanecek, J. Meier, E. Vallat-Sauvain, N. Wyrsch, U. Kroll, C. Droz, and J. Bailat, “Thin-film silicon solar cell technology,” Prog. Photovolt. Res. Appl. 12(23), 113–142 (2004). [CrossRef]

] where total internal reflection (TIR) is used to successively reflect photons back into semiconductor film, surface plasmon (SP) energy transfer(ET) provides larger scattering angle and larger in-plane wavevector component inside device cavity. As a result of genetic algorithm optimization, energy transfer between top/Bottom and adjacent nano-particles is observed. In this scenario, Ag nano-particle array essentially acts like a lumped-element transmission line, and it efficiently alters/guides the direction of photon flux flow. The in-plane component of photon wavevector is increased inside the semiconductor film, and higher absorbance is thus achieved due to the phenomenon of surface plasmonic energy transfer between discrete nano-particles (ETDNP).

2. Principle of surface plasmon mediated energy transfer

Energy transfer (ET) between metallic nano-particle has been employed to realize optical waveguide [1

1. S. A. Maier, P. G. Kik, H. A. Atwater, S. Meltzer, E. Harel, B. E. Koel, and A. A. G. Requicha, “Local detection of electromagnetic energy transport below the diffraction limit in metal nanoparticle plasmon waveguides,” Nat. Mater. 2(4), 229–232 (2003). [CrossRef] [PubMed]

3

3. S. A. Maier, P. G. Kik, and H. A. Atwater, “Optical pulse propagation in metal nanoparticle chain waveguides,” Phys. Rev. B 67, 205402 (2003).

]. In contrast to conventional index-guided mode utilizing total internal reflection (TIR), Energy transfer (ET) mediated by surface plasmon uses coupling between nano-particles to achieve waveguiding effect. This is essentially lumped-element transmission line. Since SP can condense the light below diffraction limit, the phenomenon can possibly be utilized to enhance solar cell light trapping and optical confinement to the extent that is not achievable by only index guiding. Figure 1
Fig. 1 Illustration of SP mode coupling between discrete metallic nano-particles.
illustrates the principles of surface plasmon energy transfer. The energy transfer structure under study is no back reflector to isolate the effect of SP mediated energy transfer.

3. Evolutionary structures for energy transfer

The first evolutionary structure consists of a dual SP grating on top and in the middle of silicon thin film, as illustrated in Fig. 2
Fig. 2 The first attempt of SP energy transfer structure and its parameters to be optimized.
. The coupling of re-emitted photons by Ag nano-particles is expected to further increase absorbance, compared to simple far-field enhancement. Geometrical optimization is done by adjusting each dimension, including the nano-particle heights tg1 and tg2, the fill factor of both grating FF1 and FF2, the separation between top and middle nano-particle arrays δsp, the mis-alignment between two nano-particle array ξ, and period Λ. The coupling of SP mode between top and bottom Ag arrays and coupling between adjacent Ag nano-particle is the phenomenon that is expected to enhance photon harvesting.

After GA optimization, tg1 = 0.0759μm and tg2 = 0.0497μm, the fill factor of both grating FF1 = 0.4419 and FF2 = 0.3, the separation between top and middle nano-particle arrays δsp = 0.2μm, the mis-alignment between two nano-particle array ξ = 0.081μm, period Λ = 0.25μm, and silicon thickness is kept 0.3μm throughout evolution . The spectral response for the first evolutionary structure is shown in Fig. 3
Fig. 3 Spectral response for initial attempt of optimized SP energy transfer structure.
. The baseline is planar silicon slab cell without any Ag particle on either top surface or inside silicon thin film. The short wavelength absorbance peaks for ETDNP at λ = 448.5nm and 497nm in the spectral response are likely due to Fabry-Perot type of quasi-guided mode excitation. In current structure there is no back reflector and thus the thin film absorbance is greatly affected by transmission through front surface. Since it is observed that there is not any absorptance dip in spectral resopnse, it is evident that wood-Rayleigh anomaly [21

21. Y.-W. Jiang, L. D.-C. Tzuang, Y.-H. Ye, Y.-T. Wu, M.-W. Tsai, C.-Y. Chen, and S.-C. Lee, “Effect of Wood’s anomalies on the profile of extraordinary transmission spectra through metal periodic arrays of rectangular subwavelength holes with different aspect ratio,” Opt. Express 17(4), 2631–2637 (2009). [CrossRef] [PubMed]

] is avoided as a result of GA optimization. The integrated optical absorbance, without back reflector, in the first evolutionary sturcutre is 20.46%. Further improvement can be attiained by enhancing the lateral power flow via stronger coupling into Bloch in-plane propagation mode, to increase power absorption. This can be done by using a more sophisticated evolutionary structure where inside silicon film there are two more closely spaced Ag nano-particles per period.

The second evolutionary structure also consists of a dual surface plasmon grating at the top and in the middle of silicon thin film, as illustrated in Fig. 4
Fig. 4 The second SP energy transfer structure and its parameters to be optimized.
. The difference, compared to the first evolutionary structure, is that the bottom Ag array is now with two Ag particles per period and these two Ag particles can freely move in the lateral direction during optimization. The coupling of re-emitted photon by Ag nano-particles is expected to be enhanced, especially between adjacent Ag particles, which in turn can increase in-plane propagation of solar photons and thus optical guiding and confinement. Geometrical optimization is done by adjusting each dimension, including the nano-particle heights tg1 and tg2, the fill factor FF, the separation between both nano-particle arrays δsp, the separation between two adjacent nano-particles in the bottom Ag array ξ, and period Λ. The second evolutionary structure possesses more complicated geometrical parameterization but will lead to more design flexibility. The geometry after optimization is Λ = 0.3698μm, δsp = 0.2μm, tg1 = 0.0813μm, tg2 = 0.05μm, FF = 0.4, ξ = 0.1095μm, and silicon film thickness is also kept 0.3μm.

Figure 5
Fig. 5 Spectral response for the Optimized second evolutionary structure with more closely spaced bottom Ag grating
is the spectral response for the second evolutionary structure with modified, more closely spaced Ag particles inside silicon film. The baseline is planar silicon slab cell without any Ag particle on either top surface or inside silicon thin film. The peak for ETDNP at λ = 448.5nm and λ = 509.1nm is due to Fabry-Perot like quasi-guided mode. The peak at λ = 642.4nm is likely due to the initiation of energy transfer between Ag nano-particles which will be more evident if looking at field profile plot in the following paragraph (Fig. 7& Fig. 8). In this scenario the Ag nano-particles essentially act as SP waveguide and power is efficiently coupled between top/bottom Ag array and adjacent Ag particles. Wood-Rayleigh anomaly is again avoided by GA [21

21. Y.-W. Jiang, L. D.-C. Tzuang, Y.-H. Ye, Y.-T. Wu, M.-W. Tsai, C.-Y. Chen, and S.-C. Lee, “Effect of Wood’s anomalies on the profile of extraordinary transmission spectra through metal periodic arrays of rectangular subwavelength holes with different aspect ratio,” Opt. Express 17(4), 2631–2637 (2009). [CrossRef] [PubMed]

]. The integrated optical absorbance of optimized structure is 21.47%, compared to 20.46% for optimized first evolutionary structure, and 15.58% for optimal baseline cell. In contrast to conventional index-guided mode where TIR is used to achieve waveguiding, energy transfer using mode coupling between Ag particles can potentially achieve higher light trapping capability, due to the surface plasmon propagation, field condensation, photon re-emission, far field light scattering and coupling to adjacent nano-particles, and combined mechanism of these. For index guided mode, diffraction is primarily the underlying phenomenon for light trapping while surface plasmon effect is the beyond-diffraction-limit phenomenon.

Figure 6
Fig. 6 Genetic algorithm statistics for the second evolutionary structure.
shows the details of GA evolution. The values of integrated optical absorbance within a generation are plotted as scattered points and the mean within one generation is shown in thick black line. It is observed that there is initial increase in absorbance and saturation after enough number of generations. This is typical GA running procedure. In this study, 20 individuals are employed in each generation, the generation gap is 0.9, and the recombination rate is 0.7. Compared the non-optimized population mean at generation 0 to the mean at saturation, there is 22% increase in absorbance, demonstrating the effectiveness of GA for locating integrated absorbance maximal.

4. Launching of surface plasmon mode

Figure 7
Fig. 7 Electric field profile of optimized energy transfer structure at λ = 642.4nm (a) for scattering problem (b) for corresponding eigen mode (c) successive mode coupling between adjacent nano-particles.
is the field profile at λ = 642.4nm and corresponding eigen mode profile. It is obvious that the similarity between two field profiles is the direct evidence of eigen mode excitation. The photon re-emission from metallic Ag particle is pronounced since in the field profiles for both scattering and eigen mode problems, strong upward field emission especially from middle Ag grating inside silicon thin-film is observed. The field intensity is strong at bottom Ag particle, as is evident from Fig. 7(a). This is the result of coupling of SP mode between adjacent Ag particles, which enhances the lateral power flow and in-plane wave propagation. Photon life time and light trapping can therefore be promoted. Localized surface plasmon (LSP) is observed especially at corner of Ag, though the effect is not very pronounced. The LSP might subject to carrier interface recombination. Surface plasmon polariton(SPP) is not observed since no metallic back reflector exist and no evanescent wave can propagate along metal dielectric interface.

Figure 8(a)
Fig. 8 (a) Vector field plot of Poynting vector for optimized energy transfer structure at λ = 642.4nm. (b) lateral component of Poynting vector at λ = 642.4nm.
plots the time-averaged Poynting vector in SP solar cell. It is observed that the coupling between top/ bottom metallic particle is very pronounced. It should be pointed out that since this is the field profile of vertical scattering problem, the direction of power flow is still primarily in negative y direction as is evident from Fig. 8(a). Therefore, the vertical coupling between adjacent Ag particles is easier to observed, while the lateral coupling between nano-particles is blurred by the strong field profile of incident wave. In order to observe the lateral coupling, the x-compoent of Poynting vector Px is plotted in Fig. 8(b):

Ppoynting,avg=Ppoynting,avg,xa^x+Ppoynting,avg,ya^y=12Re{Ey(r)Hz*(r)}a^x12Re{Ex(r)Hz*(r)}a^y
(2)

From Fig. 8(b) it can be seen there exists field pattern with two types of alternating localized field spots, i.e. blue and orange in the profile of Fig. 8(b). The direction of Px in these two spots are in reversed direction and consecutive lateral power flow can be identified by the black arrows as drawn. The lateral power transfer starts from the surface of Ag particle and then it will go through successive blue (or orange) spots, and then settles at the surface of adjacent Ag particles. This type of energy transfer using SP metal particle as lumped-element transmission line is the phenomenon utilized here to increase light-trapping in thin-film photovoltaics due to its capability to increase in-plane propagation, therefore resulting in large scattering angle and extended photon path length.

5. Angular distribution of plasmon dipole emission

It is illustrative to study the angular characteristic of proposed Energy Transfer between Discrete Nano-Particle (ETDNP) enhancement, and compare it to widely studied surface plasmon polariton(SPP) enhancement and conventional index-guided (IG) enhancement. At steady state the time-averaged electromagnetic energy store in a finite volume is constant:

ΕEMW,avgt=0
(3)

Therefore, the Poynting theorem can be written as:
VPLoss,avgdV+SPpoynting,avgdS=0
(4)
where PLoss,avg, and Ppoynting,avg is the time-averaged power loss per unit volume and time-averaged Poynting vector in unit of watt per unit area. Notice that dV represents volume integration and dS represents surface integration. The time-average evaluation is necessary since the field is time-harmonic. Equation (4) can be expanded as:
12VRe{E(r)J*(r)}dv+12SRe{E(r)×H*(r)}dS=12VE(r)σ(λ)E*(r)dv+12SRe{E(r)×H*(r)}dS=0
(5)
where E and H is electric and magnetic field intensity, J is electromagnetic wave induced current density, and σ is material conductivity at optical frequency. From Eq. (4) and Eq. (5), it is known that at every point on computing grid there is a Poynting vector indicating the power flow direction. For efficient waveguiding of solar photons, the power flow should be parallel to the film (perpendicular to the film growth direction) to ensure the longest optical path length. In order to characterize the angular distribution of electromagnetic wave inside silicon thin-film, the angle of Poynting vector is evaluated throughout entire silicon thin-film:

θ=tan1Ppoynting,avg,xPpoynting,avg,y=tan1Re{Ey(r)Hz*(r)}Re{Ex(r)Hz*(r)}
(6)

The result is then plotted in polar plane, and the angular feature can thus be observed and compared. The averaged angle, θavg is calculated by first modulus the angle by 180° and then minus it by 90°. Afterward absolute value of the result is taken. Averaging over entire silicon thin-film region weighted by the magnitude of Poynting vector is performed to arrive the final answer for θavg.

θavg=θ=02π|modulus(θ,π)π2|×Ppoynting,avg(θ)θ=02πPpoynting,avg(θ)=θ=0π|θπ2|×Ppoynting,avg(θ)+θ=π2π|θ3π2|×Ppoynting,avg(θ)θ=02πPpoynting,avg(θ)
(7)

The averaged angle, θavg, is used to evaluate the deviation of power flow from the in-plane Bloch wavevector direction, which is x-direction in our simulation. The second line in Eq. (7) better reflects this point. The second line of Eq. (7) can be derived from the first line of Eq. (7) by simple algebraic manipulation. The justification for using Eq. (7) to calculate θavg is that from the viewpoint of deviation from in-plane parallel Bloch wavevector direction(x-axis), there is mirror symmetry about both x- and y-axis since the deviation from Bloch wavevector direction(x-axis) is the same for 90° + θ and 90°- θ, or 180° + θ and 180°- θ. The Bloch wavevector is the direction where waveguiding effect is most significant since photons propagate in parallel to the film. From the viewpoint of escape cone, symmetry about y-axis also exists where 90° + θ = 90°- θ, since critical angle is no left-incidence or right-incidence preference. In addition, there is also mirror symmetry about x-axis where 180° + θ = 180°- θ, due to the fact that large angle downward power flow is likely to result in large angle upward power flow. This is true because for the case of planar back reflector this certainly holds because incident angle equals reflection angle. For the case of corrugated back reflector, this is also a reasonable assumption since scattering component is strongest in its specular direction. That is the reason that in all cases large angle light scattering is desirable.

The angular distribution of Poynting vector is very crucial for thin-film solar cell since it determines the effectiveness of total internal reflection(TIR) and photon path length inside the film. The stronger the coupling of incident power into Bloch in-palne propagation mode is, the more efficient the light trapping can be achieved. If all of the incident wave power is coupled into the in-plane propagation mode, the film-thin solar cell is essentially acting as perfect waveguide. Mathematically, this concept is expressed as Bloch theorem:

E(r)=u(r)exp(ikInPlaner)
(8)

The kInPlane is the in-plane component of incident wavevector. It can be seen from Eq. (8) that if efficient coupling into Bloch propagation mode is achieved, the photon actually propagates in solar cell as plane wave, until it is fully absorbed by thin-film.

5.1 Energy transfer between discrete nano-particles (ETDNP)

The Red arrows in the filed profile is the Poynting vector and the field profile itself plots electric field norm. The first two peaks in the spectral response (Fig. 9(a)) are due to Fabry- Perot type resonance where standing wave between top and bottom interface is formed and absorbance enhancement is thus achieved, and the corresponding field profiles are not included for paper length consideration. In order to observe the enhancement due to energy transfer between discrete nano-particles, the third absorbance peak is chosen. The energy transfer phenomenon is more pronounced in long wavelength portion because of weak absorption. Light trapping is also the most important in this long wavelength regime of solar spectrum. From the spectral response and the angular emission plot, it is seen that wide angle emission is achieved by energy transfer between nano-particles. The angular plot shows an averaged angle of 48.29°, which is significantly higher than the averaged angle for SPP(44.42°) and IG(34.47°) enhancement at absorption peak. Therefore, large in-plane Bloch type propagation can be realized by SP nano-particle mode coupling.

5.2 Surface plasmon polariton (SPP)

Figure 10 shows the spectral response, angular distribution of Poynting vector, and field profile together with Poynting vector arrow plot for surface plasmon polariton (SPP) enhancement. The only pronounced absorbance peak at long wavelength is at λ = 812nm. The long wavelength absorbance peak is used to investigate angular scattering behavior since the light trapping is essential in weak absorption regime. The averaged angle is 44.42°. SPP is observed at back reflector grating at Ag/Si interface where the field intensity exponentially decays from the interface. Here again the short wavelength absorption peak is of Fabry-Perot type so it is not chosen. The surface plasmon polariton is the coupling between surface plasmon and photon, and the result of SPP formation is the propagation along metal/dielectric interface. After solving for Helmholtz equation and matching the boundary condition of electric and magnetic field, it can be shown that the wavevector in propagation direction (x direction, parallel to the interface) and decaying direction (z direction, normal to the interface) are:

kx=k'x+ik"x=ωc(εmεdεm+εd)1/2
(9a)
kz,m=k'z,m+ik"z,m=ωc(εm2εm+εd)1/2
(9b)

In Eq. (9a) and Eq. (9b), kx is likely to be real and kz,m is likely to be imaginary due to metal dielectric constant εm is negative, and this results in propagating wave parallel to the interface and evanescent wave normal to the interface. Although SPP or localized surface plasmon(LSP) type of enhancement can lead to strong localized electric field at interface, carrier interface recombination might degrade the actual photocurrent gain.

5.3 Index guided (IG) mode

Here smaller Poynting vector value at angular distribution plot results from the fact that the chosen absorption peak for IG configuration is at shorter wavelength, and higher absorption coefficient leads to smaller power flow value especially deeper into the film. The exact value of Poyntng vector is nonetheless not of much important here since the relative intensity among different angles is of more interest.

6. Metallic absorption loss and interface recombination

The silicon absorbance and metallic absorbance for various schemes are included in Table 1

Table 1. Comparison of Silicon Absorbance and Metallic Loss for Various Schemes

table-icon
View This Table
. For only top Ag grating structure, no embedded bottom Ag grating in Si, and the particle height and FF of top Ag grating is optimized w.r.t. silicon integrated absorbance. For only bottom Ag grating in Si, no top Ag grating on the top of Si surface, and the geometry of embedded bottom Ag in Si is the same as ETDNP. The integrated metal absorbance for SPP is 0.2807, and adding dielectric spacer(IG) reduces it to 0.1568. This means 28.07%(or 15.68%) of solar photon, in unit of #cm−2s−1, will be absorbed by metal. Since SPP and IG structure here is quite typical (Si/Ag or Si/ZnO/Ag), this amount of metallic loss is common in these conventional thin-film solar cell. Interested reader can repeat the calculation to verify result. Currently very few literature reports integrated metallic loss and mostly found is absorption cross-section along Mie scattering. For ETDNP, metallic loss is 0.2492, slightly lower than SPP. Although 15%-30% metal absorbance looks like high value, metallic absorption is primarily at long wavelength where silicon absorption is weak and photon can reach metal. Thus these long wavelength photons, absorbed by metal, are unlikely to be absorbed by silicon even in the absence of metal, while SP effect enhances Si absorption at long wavelength. This is evident from the fact that IG has significantly lower metal absorption but its silicon absorption also lower than SPP (26.91% for IG and 28.59% for SPP). The metallic loss for only Ag grating on top of silicon without back reflector is much lower, but in reality back reflector is necessary, possibly with the exception of organic semiconductor where absorption is high. When back reflector is added the loss will be the same. The potential reduction for metallic absorption can be wrapping Ag with dielectric [20

20. S. Pillai, F. J. Beck, K. R. Catchpole, Z. Ouyang, and M. A. Green, “The effect of dielectric spacer thickness on surface plasmon enhanced solar cells for front and rear side depositions,” J. Appl. Phys. 109(7), 073105 (2011). [CrossRef]

] to have balance between SP scattering and metal absorption, or using new metal materials to reduce metallic absorption loss such as titanium nitride [33

33. G. V. Naik, J. L. Schroeder, X. Ni, A. V. Kildishev, T. D. Sands, and A. Boltasseva, “Titanium nitride as a plasmonic material for visible and near-infrared wavelengths,” Opt. Mater. Express 2(4), 478–489 (2012). [CrossRef]

]. It is also worth to point out the back reflector consideration for future ETDNP structure. Since the current ETDNP structure is no back reflector to isolate the effect of energy transfer, adding a back reflector and optimizing full structure will further increase efficiency. Since the surface plasmon effect has been realized in energy transfer arrays, the back reflector can be non-plasmonic structure where the metal absorption can be lower or eliminated. Dielectric mirror such as composite distributed Bragg reflector [34

34. M. Y. Kuo, J. Y. Hsing, T. T. Chiu, C. N. Li, W. T. Kuo, T. S. Lay, and M. H. Shih, “Quantum efficiency enhancement in selectively transparent silicon thin film solar cells by distributed Bragg reflectors,” Opt. Express 20(S6Suppl 6), A828–A835 (2012). [CrossRef] [PubMed]

] or high index contrast(HCG) grating can be potential candidates. The metallic nano-particle size in Si can be reduced when back reflector is added due to the light trapping is now not only by energy transfer but also by conventional waveguiding due to air/Si/metal stack. Therefore, the metallic loss for ETDNP can be further reduced.

By using p-i-n diode, which is common in thin-film silicon solar cell, strong built-in electric field in intrinsic region where SP nano-particles are embedded, ensures charge collection and eliminates interfacial recombination loss. Carriers are swept out of depleted i region by electric field before recombination. Surface passivation is also important to reduce recombination loss in ETDNP structure. Placing metallic nano-particles in junction has been employed in organic solar cell such as organic bulk-heterojunction or tandem cell [35

35. 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]

], and thus the proposed ETDNP can be applied well to organic photovoltaics. For plasmonic material selection, Ag provides strongest SP effect at visible range compared to Au, Cu, Al. New plasmonic material such titanium nitride or zirconium nitride provide smaller real permittivity and the tunability of dielectric response and thus is very promising to be employed in ETDNP structure to lead to lower metallic absorption loss.

7. Conclusion

Genetic algorithm and global optimization is demonstrated here to be highly effective for SP energy transfer type photovoltaic cell optimization. Result shows the avoidance of Wood-Rayleigh anomaly and excitation of SP mode with energy coupling between top/bottom and adjacent Ag nano-particles. This enables guiding the photon flow inside photovoltaic thin-film using lumped-element transmission line like structures, which provides more design versatility, compared to conventional index-guided mode or SPP back reflector. The mode coupling also increases in-plane Bloch wavevector component and leads to longer photon path length and higher solar cell absorbance. Specifically compared to SPP, LSP, or IG enhancement, Energy Transfer between Discrete Nano-Particles(ETDNP) methodology provides larger scattering angle, evident from the angular distribution of Poynting vector in silicon thin-film. Based on this work, photonic design for solar cell using successive mode coupling between surface plasmonic nano-paritcles is made possible, which is a new way to achieve proper control of light flow inside photovoltaic thin-film.

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A. Meyer and H. Ade, “The effect of angle of incidence on the optical field distribution within thin film organic solar cells,” J. Appl. Phys. 106(11), 113101 (2009). [CrossRef]

14.

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(13), 133302 (2010). [CrossRef]

15.

J. N. Munday and H. A. Atwater, “Large Integrated Absorption Enhancement in Plasmonic Solar Cells by Combining Metallic Gratings and Antireflection Coatings,” Nano Lett. 11(6), 2195–2201 (2011). [CrossRef] [PubMed]

16.

U. W. Paetzold, E. Moulin, D. Michaelis, W. Bottler, C. Wächter, V. Hagemann, M. Meier, R. Carius, and U. Rau, “Plasmonic reflection grating back contacts for microcrystalline silicon solar cells,” Appl. Phys. Lett. 99(18), 181105 (2011). [CrossRef]

17.

U. W. Paetzold, E. Moulin, B. E. Pieters, R. Carius, and U. Rau, “Design of nanostructured plasmonic back contacts for thin-film silicon solar cells,” Opt. Express 19(S6Suppl 6), A1219–A1230 (2011). [CrossRef] [PubMed]

18.

W. E. I. Sha, W. C. H. Choy, and W. C. Chew, “Angular response of thin-film organic solar cells with periodic metal back nanostrips,” Opt. Lett. 36(4), 478–480 (2011). [CrossRef] [PubMed]

19.

H.-Y. Lin, Y. Kuo, C.-Y. Liao, C. C. Yang, and Y.-W. Kiang, “Surface plasmon effects in the absorption enhancements of amorphous silicon solar cells with periodical metal nanowall and nanopillar structures,” Opt. Express 20(S1), A104–A118 (2012). [CrossRef] [PubMed]

20.

S. Pillai, F. J. Beck, K. R. Catchpole, Z. Ouyang, and M. A. Green, “The effect of dielectric spacer thickness on surface plasmon enhanced solar cells for front and rear side depositions,” J. Appl. Phys. 109(7), 073105 (2011). [CrossRef]

21.

Y.-W. Jiang, L. D.-C. Tzuang, Y.-H. Ye, Y.-T. Wu, M.-W. Tsai, C.-Y. Chen, and S.-C. Lee, “Effect of Wood’s anomalies on the profile of extraordinary transmission spectra through metal periodic arrays of rectangular subwavelength holes with different aspect ratio,” Opt. Express 17(4), 2631–2637 (2009). [CrossRef] [PubMed]

22.

V. Shah, H. Schade, M. Vanecek, J. Meier, E. Vallat-Sauvain, N. Wyrsch, U. Kroll, C. Droz, and J. Bailat, “Thin-film silicon solar cell technology,” Prog. Photovolt. Res. Appl. 12(23), 113–142 (2004). [CrossRef]

23.

J. M. Khoshman and M. E. Kordesch, “Optical constants and band edge of amorphous zinc oxide thin films,” Thin Solid Films 515(18), 7393–7399 (2007). [CrossRef]

24.

S. J. Kang and Y. H. Joung, “Influence of substrate temperature on the optical and piezoelectric properties of ZnO thin films deposited by rf magnetron sputtering,” Appl. Surf. Sci. 253(17), 7330–7335 (2007). [CrossRef]

25.

C. Munuera, J. Zuniga-Perez, J. F. Rommeluere, V. Sallet, R. Triboulet, F. Soria, V. Munoz-Sanjose, and C. Ocal, “Morphology of ZnO grown by MOCVD on sapphire substrates,” J. Cryst. Growth 264(1-3), 70–78 (2004). [CrossRef]

26.

A. S. Ferlauto, G. M. Ferreira, J. M. Pearce, C. R. Wronski, R. W. Collins, X. Deng, and G. Ganguly, “Analytical model for the optical functions of amorphous semiconductors and its applications for thin film solar cells,” Thin Solid Films 455–456, 388–392 (2004).

27.

H. Kim, A. Pique, J. S. Horwitz, H. Murata, Z. H. Kafafi, C. M. Gilmore, and D. B. Chrisey, “Effect of aluminum doping on zinc oxide thin films grown by pulsed laser deposition for organic light-emitting devices,” Thin Solid Films 377–378, 798–802 (2000). [CrossRef]

28.

E. D. Palik, Handbook of optical constants of solids (Academic Press, 1985).

29.

A. Lin and J. D. Phillips, “Optimization of random diffraction gratings in thin-film solar cells using genetic algorithms,” Sol. Energy Mater. Sol. Cells 92(12), 1689–1696 (2008). [CrossRef]

30.

P. Bhattacharya, Semiconductor optoelectronic devices, 2nd ed. (Prentice-Hall, 2006).

31.

C. AB, Comsol multiphysics RF module user guide V 3.3 (2006).

32.

Synopsys, “Sentaurus device EMW user manual V. X-2005.10,” (2005), pp. 78–79.

33.

G. V. Naik, J. L. Schroeder, X. Ni, A. V. Kildishev, T. D. Sands, and A. Boltasseva, “Titanium nitride as a plasmonic material for visible and near-infrared wavelengths,” Opt. Mater. Express 2(4), 478–489 (2012). [CrossRef]

34.

M. Y. Kuo, J. Y. Hsing, T. T. Chiu, C. N. Li, W. T. Kuo, T. S. Lay, and M. H. Shih, “Quantum efficiency enhancement in selectively transparent silicon thin film solar cells by distributed Bragg reflectors,” Opt. Express 20(S6Suppl 6), A828–A835 (2012). [CrossRef] [PubMed]

35.

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]

OCIS Codes
(040.5350) Detectors : Photovoltaic
(310.6845) Thin films : Thin film devices and applications

ToC Category:
Plasmonics

History
Original Manuscript: October 8, 2012
Revised Manuscript: November 24, 2012
Manuscript Accepted: December 3, 2012
Published: December 13, 2012

Citation
Albert Lin, Sze-Ming Fu, Yen-Kai Chung, Shih-yun Lai, and Chi-Wei Tseng, "An optimized surface plasmon photovoltaic structure using energy transfer between discrete nano-particles," Opt. Express 21, A131-A145 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-S1-A131


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References

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  2. J. R. Krenn, “Nanoparticle waveguides: Watching energy transfer,” Nat. Mater.2(4), 210–211 (2003). [CrossRef] [PubMed]
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  9. D. Cheyns, B. P. Rand, B. Verreet, J. Genoe, J. Poortmans, and P. Heremans, “The angular response of ultrathin film organic solar cells,” Appl. Phys. Lett.92(24), 243310 (2008). [CrossRef]
  10. 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]
  11. N. Lagos, M. M. Sigalas, and E. Lidorikis, “Theory of plasmonic near-field enhanced absorption in solar cells,” Appl. Phys. Lett.99(6), 063304 (2011). [CrossRef]
  12. K. Q. Le, A. Abass, B. Maes, P. Bienstman, and A. Alù, “Comparing plasmonic and dielectric gratings for absorption enhancement in thin-film organic solar cells,” Opt. Express20(S1), A39–A50 (2012). [CrossRef] [PubMed]
  13. A. Meyer and H. Ade, “The effect of angle of incidence on the optical field distribution within thin film organic solar cells,” J. Appl. Phys.106(11), 113101 (2009). [CrossRef]
  14. 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(13), 133302 (2010). [CrossRef]
  15. J. N. Munday and H. A. Atwater, “Large Integrated Absorption Enhancement in Plasmonic Solar Cells by Combining Metallic Gratings and Antireflection Coatings,” Nano Lett.11(6), 2195–2201 (2011). [CrossRef] [PubMed]
  16. U. W. Paetzold, E. Moulin, D. Michaelis, W. Bottler, C. Wächter, V. Hagemann, M. Meier, R. Carius, and U. Rau, “Plasmonic reflection grating back contacts for microcrystalline silicon solar cells,” Appl. Phys. Lett.99(18), 181105 (2011). [CrossRef]
  17. U. W. Paetzold, E. Moulin, B. E. Pieters, R. Carius, and U. Rau, “Design of nanostructured plasmonic back contacts for thin-film silicon solar cells,” Opt. Express19(S6Suppl 6), A1219–A1230 (2011). [CrossRef] [PubMed]
  18. W. E. I. Sha, W. C. H. Choy, and W. C. Chew, “Angular response of thin-film organic solar cells with periodic metal back nanostrips,” Opt. Lett.36(4), 478–480 (2011). [CrossRef] [PubMed]
  19. H.-Y. Lin, Y. Kuo, C.-Y. Liao, C. C. Yang, and Y.-W. Kiang, “Surface plasmon effects in the absorption enhancements of amorphous silicon solar cells with periodical metal nanowall and nanopillar structures,” Opt. Express20(S1), A104–A118 (2012). [CrossRef] [PubMed]
  20. S. Pillai, F. J. Beck, K. R. Catchpole, Z. Ouyang, and M. A. Green, “The effect of dielectric spacer thickness on surface plasmon enhanced solar cells for front and rear side depositions,” J. Appl. Phys.109(7), 073105 (2011). [CrossRef]
  21. Y.-W. Jiang, L. D.-C. Tzuang, Y.-H. Ye, Y.-T. Wu, M.-W. Tsai, C.-Y. Chen, and S.-C. Lee, “Effect of Wood’s anomalies on the profile of extraordinary transmission spectra through metal periodic arrays of rectangular subwavelength holes with different aspect ratio,” Opt. Express17(4), 2631–2637 (2009). [CrossRef] [PubMed]
  22. V. Shah, H. Schade, M. Vanecek, J. Meier, E. Vallat-Sauvain, N. Wyrsch, U. Kroll, C. Droz, and J. Bailat, “Thin-film silicon solar cell technology,” Prog. Photovolt. Res. Appl.12(23), 113–142 (2004). [CrossRef]
  23. J. M. Khoshman and M. E. Kordesch, “Optical constants and band edge of amorphous zinc oxide thin films,” Thin Solid Films515(18), 7393–7399 (2007). [CrossRef]
  24. S. J. Kang and Y. H. Joung, “Influence of substrate temperature on the optical and piezoelectric properties of ZnO thin films deposited by rf magnetron sputtering,” Appl. Surf. Sci.253(17), 7330–7335 (2007). [CrossRef]
  25. C. Munuera, J. Zuniga-Perez, J. F. Rommeluere, V. Sallet, R. Triboulet, F. Soria, V. Munoz-Sanjose, and C. Ocal, “Morphology of ZnO grown by MOCVD on sapphire substrates,” J. Cryst. Growth264(1-3), 70–78 (2004). [CrossRef]
  26. A. S. Ferlauto, G. M. Ferreira, J. M. Pearce, C. R. Wronski, R. W. Collins, X. Deng, and G. Ganguly, “Analytical model for the optical functions of amorphous semiconductors and its applications for thin film solar cells,” Thin Solid Films455–456, 388–392 (2004).
  27. H. Kim, A. Pique, J. S. Horwitz, H. Murata, Z. H. Kafafi, C. M. Gilmore, and D. B. Chrisey, “Effect of aluminum doping on zinc oxide thin films grown by pulsed laser deposition for organic light-emitting devices,” Thin Solid Films377–378, 798–802 (2000). [CrossRef]
  28. E. D. Palik, Handbook of optical constants of solids (Academic Press, 1985).
  29. A. Lin and J. D. Phillips, “Optimization of random diffraction gratings in thin-film solar cells using genetic algorithms,” Sol. Energy Mater. Sol. Cells92(12), 1689–1696 (2008). [CrossRef]
  30. P. Bhattacharya, Semiconductor optoelectronic devices, 2nd ed. (Prentice-Hall, 2006).
  31. C. AB, Comsol multiphysics RF module user guide V 3.3 (2006).
  32. Synopsys, “Sentaurus device EMW user manual V. X-2005.10,” (2005), pp. 78–79.
  33. G. V. Naik, J. L. Schroeder, X. Ni, A. V. Kildishev, T. D. Sands, and A. Boltasseva, “Titanium nitride as a plasmonic material for visible and near-infrared wavelengths,” Opt. Mater. Express2(4), 478–489 (2012). [CrossRef]
  34. M. Y. Kuo, J. Y. Hsing, T. T. Chiu, C. N. Li, W. T. Kuo, T. S. Lay, and M. H. Shih, “Quantum efficiency enhancement in selectively transparent silicon thin film solar cells by distributed Bragg reflectors,” Opt. Express20(S6Suppl 6), A828–A835 (2012). [CrossRef] [PubMed]
  35. 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]

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