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Energy Express

  • Editor: Christian Seassal
  • Vol. 21, Iss. S5 — Sep. 9, 2013
  • pp: A847–A863
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Morphology-dependent light trapping in thin-film organic solar cells

Richard R. Grote, Steven J. Brown, Jeffrey B. Driscoll, Richard M. Osgood, Jr., and Jon A. Schuller  »View Author Affiliations


Optics Express, Vol. 21, Issue S5, pp. A847-A863 (2013)
http://dx.doi.org/10.1364/OE.21.00A847


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Abstract

The active layer materials used in organic photovoltaic (OPV) cells often self-assemble into highly ordered morphologies, resulting in significant optical anisotropies. However, the impact of these anisotropies on light trapping in nanophotonic OPV architectures has not been considered. In this paper, we show that optical anisotropies in a canonical OPV material, P3HT, strongly affect absorption enhancements in ultra-thin textured OPV cells. In particular we show that plasmonic and gap-mode solar cell architectures redistribute electromagnetic energy into the out-of-plane field component, independent of the active layer orientation. Using analytical and numerical calculations, we demonstrate how the absorption in these solar cell designs can be significantly increased by reorienting polymer domains such that strongly absorbing axes align with the direction of maximum field enhancement.

© 2013 OSA

1. Introduction

Due to their low materials cost, wide choice of possible substrates, and solution processability, organic photovoltaics (OPVs) are a topic of considerable interest for solar energy conversion technology [1

1. A. C. Mayer, S. R. Scully, B. E. Hardin, M. W. Rowell, and M. D. McGehee, “Polymer-based solar cells,” Mater. Today 10, 28–33 (2007). [CrossRef]

]. Despite these desirable attributes, the exciton and carrier diffusion lengths in organic materials are typically much shorter than the optical absorption length; thus, it is challenging to simultaneously absorb photons and collect carriers with high efficiency [2

2. P. Peumans, V. Bulović, and S. R. Forrest, “Efficient photon harvesting at high optical intensities in ultrathin organic double-heterostructure photovoltaic diodes,” Appl. Phys. Lett. 76, 2650–2652 (2000). [CrossRef]

]. Nanophotonic light-trapping architectures [3

3. B. O’Connor, K. H. An, K. P. Pipe, Y. Zhao, and M. Shtein, “Enhanced optical field intensity distribution in organic photovoltaic devices using external coatings,” Appl. Phys. Lett. 89, 233502 (2006). [CrossRef]

21

21. M. A. Green and S. Pillai, “Harnessing plasmonics for solar cells,” Nature Photon. 6, 130–132 (2012). [CrossRef]

] offer a potential solution to this inherent tradeoff for OPVs, enabling large optical absorption over a broad spectral range in ultra-thin active layers.

Fig. 1 (a) Surface plasmon (SP) and gap mode (GM) OPV architectures and |Ez|2 mode profiles, (b) molecular orientation of P3HT and a model material, zP3HT, which consists of P3HT molecules oriented such that the polymer chains are aligned out-of-plane, (c) anisotropic real and imaginary parts of the P3HT and zP3HT dielectric functions versus wavelength. The P3HT dielectric function is measured by spectroscopic ellipsometry, while zP3HT is modeled by switching the in-plane (||) and out-of-plane (⊥) optical coefficients of P3HT.

2. Effects of anisotropy on absorption

Under typical processing conditions, P3HT molecules align with the polymer chains parallel to the substrate surface [34

34. H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K. Bechgaard, B. M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J. Janssen, E. W. Meijer, P. Herwig, and D. M. de Leeuw, “Two-dimensional charge transport in self-organized, high-mobility conjugated polymers,” Nature 401, 685–688 (1999). [CrossRef]

,35

35. D. E. Johnston, K. G. Yager, C.-Y. Nam, B. M. Ocko, and C. T. Black, “One-volt operation of high-current vertical channel polymer semiconductor field-effect transistors,” Nano Lett. 12, 4181–4186 (2012). [CrossRef] [PubMed]

] as shown in Fig. 1(b). At optical length scales there is no preferred in-plane orientation and the optical properties are homogeneous in the x-y plane. The chains are always perpendicular to the out-of-plane (z) direction, however, defining an optical axis with distinct optical properties. Thus, P3HT is a uniaxial material with a complex dielectric function which can be described by a 2nd rank tensor,
D(ω)=ε0ε(ω)E(ω)=ε0[ε˜||(ω)000ε˜||(ω)000ε˜(ω)]E(ω)
(1)
This equation describes the displacement field D⃗ produced within the material by an electric field E⃗ at a given frequency ω, where ε0 is the permittivity of free space. Ellipsometry measurements of the in-plane (ε̃||) and out-of-plane (ε̃) complex dielectric functions of a P3HT thin-film are plotted in Fig. 1(c). We restrict our attention to a wavelength range of 300 nm to 650 nm where P3HT absorption is non-negligible and P3HT:PCBM photovoltaics exhibit appreciable internal quantum efficiencies [36

36. J. Jo, S.-I. Na, S.-S. Kim, T.-W. Lee, Y. Chung, S.-J. Kang, D. Vak, and D.-Y. Kim, “Three-dimensional bulk heterojunction morphology for achieving high internal quantum efficiency in polymer solar cells,” Adv. Funct. Mater. 19, 2398–2406 (2009). [CrossRef]

, 37

37. G. F. Burkhard, E. T. Hoke, S. R. Scully, and M. D. McGehee, “Incomplete exciton harvesting from fullerenes in bulk heterojunction solar cells,” Nano Lett. 9, 4037–4041 (2009). [CrossRef] [PubMed]

]. For a single P3HT molecule, the dominant optical excitation is a ππ* intra-chain transition with its dipole moment oriented along the polymer backbone [25

25. J. A. Schuller, S. Karaveli, T. Schiros, K. He, S. Yang, I. Kymissis, J. Shan, and R. Zia, “Orientation of luminescent excitons in layered nanomaterials,” Nature Nanotech. 8, 271–276 (2013). [CrossRef]

, 29

29. U. Zhokhavets, T. Erb, H. Hoppe, G. Gobsch, and N. S. Sariciftci, “Effect of annealing of poly(3-hexylthiophene)/fullerene bulk heterojunction composites on structural and optical properties,” Thin Solid Films 496, 679–682 (2006). [CrossRef]

, 35

35. D. E. Johnston, K. G. Yager, C.-Y. Nam, B. M. Ocko, and C. T. Black, “One-volt operation of high-current vertical channel polymer semiconductor field-effect transistors,” Nano Lett. 12, 4181–4186 (2012). [CrossRef] [PubMed]

], shown as thick colored lines in Fig 1(b). P3HT thin-films exhibit large absorption anisotropies which are congruent with the transition dipoles of in-plane oriented polymer chains; at 612 nm ε″|| is ≈ 35 times larger than ε″ as shown in Fig. 1(c).

The effect of these anisotropies can be clearly seen by considering the steady-state power absorbed (A) within a volume (V ) of anisotropic material:
A(ω)=ωε02VEIm{ε}E*dV=ωε02Vε|||Ex|2+ε|||Ey|2+ε|Ez|2dV
(2)
The optical absorption at a given frequency depends upon the intensity of each field component within the active area as well as the imaginary part of the dielectric function experienced by that field component. Thus, in P3HT, the out-of-plane field component Ez contributes very little to the overall absorption since ε″ is very small as compared to ε″|| [Fig. 1(c)].

Untextured planar OPVs benefit from the large in-plane absorption coefficient of P3HT thin-films, which couple efficiently to normally incident sunlight. Nanophotonic light trapping architectures, however, predominantly redistribute electromagnetic energy into the out-of-plane field component, Ez. To demonstrate the effect of this redistribution, we compare light trapping in P3HT thin-films to our model morphology, zP3HT, where the polymer chains are assumed to align perpendicular to the substrate [Fig. 1(b)]. As a conservative estimate, zP3HT is modeled as a uniaxial material where the in-plane and out-of-plane optical coefficients of P3HT have been switched as shown in Fig. 1(c). This assumption likely underestimates the absorptive properties of a material comprising vertically aligned P3HT molecules (see Appendix A for further discussion). However, Ez becomes the dominant field component contributing to optical absorption, and comparisons between P3HT and zP3HT light-trapping reveal significant differences which are entirely attributable to molecular orientation. As we will show subsequently, zP3HT exhibits superior absorption to P3HT in both SP and GM light-trapping architectures.

3. Surface plasmon light trapping with anisotropy

SP modes [(Fig. 1(a)] are highly confined waveguide modes that propagate along the surface of a metal/dielectric interface, and have been studied extensively as a means to enhance light absorption in thin-film photovoltaics [10

10. D. M. Callahan, J. N. Munday, and H. A. Atwater, “Solar cell light trapping beyond the ray optic limit,” Nano Lett. 12, 214–218 (2012). [CrossRef]

, 14

14. J. N. Munday, D. M. Callahan, and H. A. Atwater, “Light trapping beyond the 4n2 limit in thin waveguides,” Appl. Phys. Lett. 100, 121121 (2012). [CrossRef]

, 16

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

, 19

19. E. A. Schiff, “Thermodynamic limit to photonic-plasmonic light-trapping in thin films on metals,” J. Appl. Phys. 110, 104501 (2011). [CrossRef]

, 20

20. X. Sheng, J. Hu, J. Michel, and L. C. Kimerling, “Light trapping limits in plasmonic solar cells: an analytical investigation,” Opt. Express 20, A496–A501 (2012). [CrossRef] [PubMed]

]. For a uniaxial dielectric cladding with a z-oriented (out-of-plane) extraordinary axis, the propagation constant of a SP mode can be written as [38

38. Z. Jacob and E. E. Narimanov, “Optical hyperspace for plasmons: Dyakonov states in metamaterials,” Appl. Phys. Lett. 93, 221109 (2008). [CrossRef]

, 39

39. M. Liscidini and J. E. Sipe, “Quasiguided surface plasmon excitations in anisotropic materials,” Phys. Rev. B 81, 115335 (2010). [CrossRef]

]
β˜SP=k0[ε˜d,ε˜m(ε˜d,||ε˜m)ε˜d,||ε˜d,ε˜m2]12
(3)
where k0 = 2π/λ0 is the free-space wavenumber, ε̃d,⊥ is the out-of-plane complex dielectric function of the top cladding, ε̃d,|| is the in-plane complex dielectric function of the top cladding, and ε̃m is the complex dielectric function of the metal (see Appendix B for derivation). The quantity in the square root is referred to as the complex effective refractive index of the SP mode, defined such that β̃SP = k0ñSP with ñSP = nSPSP. The real effective indices, nSP, for SPs at an air-Ag (blue, dashed), P3HT-Ag (black), and zP3HT-Ag (red) interface are shown in Fig. 2(a). At wavelengths above the plasma wavelength λp (Figs. 23) the dielectric function of the metal becomes large such that |ε˜m2||ε˜d,||ε˜d,| and nSP(εd,)1/2=nd,. In this wavelength regime the SP mode behaves like a z-polarized plane wave skimming along the surface of the metal, with nSP nearly equal to the out-of-plane refractive index of the dielectric, nd,, as can be seen in Fig. 2(a). As a result, Ez is the dominant field component, and SPs thus couple more efficiently to intra-chain transitions in zP3HT than P3HT. These absorptive transitions appear as a series of resonances in between 450 nm and 650 nm. While these resonances appear for both morphologies, they are significantly more pronounced in zP3HT. Similarly, SP losses κSP are an average of ≈ 3.6 times larger in zP3HT for the wavelength range of 450 nm to 650 nm, and follow closely with the out-of-plane material loss coefficient, κd,, shown in Fig. 2(b).

Fig. 2 (a) Real part of the SP effective index for P3HT (black), zP3HT (red), and air (blue, dashed), and the real part of the out-of-plane top dielectric material index for P3HT (black) and zP3HT (red). (b) Imaginary part of the SP effective index and out-of-plane dielectric index for the same materials, showing that out-of-plane absorption is the dominant contributor to SP modal loss. The dashed line indicates λp, the wavelength at which the plasma resonance εd,||εd,=εm2 occurs.

To achieve unity absorption, the SP mode must have large nSP and κSP. The local density of optical states (LDOS) is proportional to nSP; an increase in SP mode index corresponds to a larger number of SP states into which incident solar radiation can couple. The rate of absorption is proportional to κSP; an increase in this decay constant improves the likelihood that an SP is absorbed before escaping (i.e. re-radiating) via the Lambertian surface. However, some losses arise from undesired Ohmic dissipation in the metal substrate. To account for these effects we calculate the fraction of SP losses attributable to exciton generation in the polymer layer ξa (see Appendix D for derivation). As seen in Fig. 3(a), the majority of the losses take place within the organic layer, and this fraction is larger in zP3HT at all wavelengths. Taking parasitic losses into account, we can derive an upper limit for light absorption within the polymer layer:
ASP,polymer=ξaASP,total.
(5)

The results presented in Figs. 2 and 3(a) demonstrate that light trapping is significantly more effective in zP3HT than P3HT. By reorienting the polymer morphology we can increase the LDOS, absorption rate, and fraction of light absorbed within the polymer relative to the metal substrate.

Fig. 3 (a) Fraction of power absorbed in the polymer for an SP mode. The dashed line indicates λp. (b) Integrated absorption weighted by the AM1.5 solar spectrum [41] for increasing thickness, h, of the polymer thin film atop a Ag substrate (shown in inset). Single pass absorption through each material is plotted with dashed lines for comparison.

So far, we have only discussed properties of SPs at an interface with an infinitely thick organic layer. Ultimately, however, we are concerned with light trapping in very thin films. Using a transfer matrix method (TMM) for lossy, anisotropic slab waveguide modes [42

42. C. Chen, P. Berini, D. Feng, S. Tanev, and V. P. Tzolov, “Efficient and accurate numerical analysis of multilayer planar optical waveguides in lossy anisotropic media,” Opt. Express 7, 260–272 (2000). [CrossRef] [PubMed]

], we numerically evaluate nSP, κSP, ξa, and ASP, polymer for finite thickness films with the geometry illustrated in the inset of Fig. 3(b). The thickness, h, dependent integrated absorbance (defined as 300nm650nmASP,polymerBdλ0, where B is the weighted AM1.5 solar spectrum [41] normalized such that 300nm650nmBdλ0=1) for SP-coupled P3HT and zP3HT thin-films is shown in Fig. 3(b). zP3HT absorbs more incident solar radiation for all film thicknesses, reaching a maximum increase of a factor of ≈ 2. For thick films, the absorption saturates for both morphologies, reaching a higher value in zP3HT since the polymer more efficiently absorbs energy contained in SP modes as compared to the metal substrate. Normal-incidence single-pass absorption through a P3HT (black, dashed) and a zP3HT (red, dashed) film is also plotted in Fig. 3(b). The in-plane oriented polymers in P3HT couple strongly to normal-incidence light, and coupling to SPs can actually lead to a reduction in integrated absorbance for certain film thicknesses. The opposite is true in zP3HT where coupling to SPs can enhance the integrated absorbance by more than a factor of 6. Clearly, molecular orientation strongly impacts light trapping processes in organic thin-films. Properly aligning polymer and small-molecule domains to capture energy contained in the large z-fields characteristic of SP modes is essential to maximizing the potential of SP-based light-trapping architectures. In the following section, we show that such light-trapping anisotropies are not unique to SP architectures.

4. Gap mode light trapping with anisotropy

GM architectures [7

7. Z. Yu, A. Raman, and S. Fan, “Fundamental limit of nanophotonic light trapping in solar cells,” Proc. Natl. Acad. Sci. USA 107, 17491–17496 (2010). [CrossRef] [PubMed]

, 17

17. M. A. Green, “Enhanced evanescent mode light trapping in organic solar cells and other low index optoelectronic devices,” Prog. Photovolt: Res. Appl. 19, 473–477 (2011). [CrossRef]

] have recently been proposed as a method to enhance light trapping in low-index (nL) thin-films beyond the Ray Optics limit [43

43. E. Yablonovitch, “Statistical ray optics,” J. Opt. Soc. Am. 72, 899–907 (1982). [CrossRef]

] of 4nL2. A representative GM architecture [7

7. Z. Yu, A. Raman, and S. Fan, “Fundamental limit of nanophotonic light trapping in solar cells,” Proc. Natl. Acad. Sci. USA 107, 17491–17496 (2010). [CrossRef] [PubMed]

], comprising a thin polymer layer embedded between a perfect electrical conductor (PEC) and a thick high-index dielectric (nH), is illustrated in Fig. 1(a). The structure supports a hybrid TM slab waveguide mode with mode profile |Ez|2 superimposed on the figure in blue. The field intensity within the active layer exhibits large enhancement, arising entirely from the out-of-plane field component Ez. This enhancement results from distinct electromagnetic boundary conditions: the in-plane electric fields Ex and Ey and out-of-plane displacement field Dz = ε0 ε̃ Ez are each continuous across an interface. As a result, the out-of-plane electric field amplitude is enhanced by a factor of nH2/n˜L,2 and the field intensity by a factor of nH4/|n˜L,|4, resulting in large absorption enhancements. We calculate absorption enhancements in P3HT and zP3HT GM architectures using three different methods: employing analytical electrostatic (ES) approximations based on the formalism of [7

7. Z. Yu, A. Raman, and S. Fan, “Fundamental limit of nanophotonic light trapping in solar cells,” Proc. Natl. Acad. Sci. USA 107, 17491–17496 (2010). [CrossRef] [PubMed]

] and [13

13. Z. Yu, A. Raman, and S. Fan, “Thermodynamic upper bound on broadband light coupling with photonic structures,” Phys. Rev. Lett. 109, 173901 (2012). [CrossRef] [PubMed]

], numerically solving for the fundamental TM waveguide mode in a GM architecture using TMM (as done earlier with SPs), and simulating the coupling of normal-incident solar radiation into GMs with a broadband coupling grating via rigorous-coupled wave analysis (RCWA) [44

44. RSoft Design Group, Inc., http://www.rsoftdesign.com/.

].

In [7

7. Z. Yu, A. Raman, and S. Fan, “Fundamental limit of nanophotonic light trapping in solar cells,” Proc. Natl. Acad. Sci. USA 107, 17491–17496 (2010). [CrossRef] [PubMed]

] the authors derive an ES approximation for the absorption enhancement in a thin low-index inclusion embedded between two thick high-index dielectric slabs:
FES=4nHnL(13+13+nH43nL4).
(6)
The first two terms in Eq. (6) correspond to the in-plane Ex and Ey fields, and are associated with modest enhancement. The third term corresponds to the out-of-plane Ez field and the enhancement factor can be quite large, since it scales with the fourth power of the index contrast. Adapting this analysis for an anisotropic low-index layer, and taking into account the second law of thermodynamics [13

13. Z. Yu, A. Raman, and S. Fan, “Thermodynamic upper bound on broadband light coupling with photonic structures,” Phys. Rev. Lett. 109, 173901 (2012). [CrossRef] [PubMed]

], we calculate the fraction of absorbed light assuming Lambertian in-coupling (see Appendix E):
AES=αL,||hαL,||h+[4nHnL,||(23+13εL,εL,||nH4|n˜L,|4)]1.
(7)
The results for P3HT (black) and zP3HT (red) layers coupled to transparent dielectrics of index nH = 3.6 are shown in the left panel of Fig. 4(b) (solid lines). As expected, the fraction of absorbed light is always larger in zP3HT, which is better suited to leverage the large intensity enhancement within the gap.

Fig. 4 (a) Effective indices of a gap mode with P3HT and zP3HT in the gap as a function of gap height and wavelength, (b) integrated absorbance of the gap-mode structure for both P3HT (black) and zP3HT (red) as a function of gap height using the following methods: transfer matrix method (TMM, left panel, dashed-dotted lines), electrostatic limit (ES, left panel, solid lines), rigorous coupled wave analysis (RCWA, right panel, solid lines), and single pass absorption (right panel, dashed lines). (inset) Out-of-plane field profile at 550 nm in the active layer beneath the numerically optimized coupling grating from [7]. (c) Side view of the grating structure used for RCWA calculations and indices used for TMM calculations where the scattering layer has been replaced with an average index of ns = 1.87.

The analysis described above assumes a sufficiently thin active layer, such that the fields within can be described by ES approximations (e.g. Ez is constant across the gap), and that the high index layer is optically thick (see Appendix E for further discussion). These assumptions overestimate the total fraction of absorbed light and do not accurately capture what happens as active layer thickness is increased. Using the TMM method [42

42. C. Chen, P. Berini, D. Feng, S. Tanev, and V. P. Tzolov, “Efficient and accurate numerical analysis of multilayer planar optical waveguides in lossy anisotropic media,” Opt. Express 7, 260–272 (2000). [CrossRef] [PubMed]

] discussed earlier in the paper, we calculate nGM and κGM for the lowest order GM waveguide mode supported by the structure in Fig. 4(c). Results are shown in Fig. 4(a) and reveal a significantly larger for zP3HT especially above 450 nm where κGM is up to 18 times larger than in P3HT for h < 10 nm. Plugging these values into Eq. (4) (Fig. 4(b), left panel), we see that the integrated absorbance for a zP3HT GM is ≈ a factor of 2 larger than that for a P3HT GM (dashed-dotted lines). We note that a similar treatment for an isotropic organic layer is performed in [10

10. D. M. Callahan, J. N. Munday, and H. A. Atwater, “Solar cell light trapping beyond the ray optic limit,” Nano Lett. 12, 214–218 (2012). [CrossRef]

] and [14

14. J. N. Munday, D. M. Callahan, and H. A. Atwater, “Light trapping beyond the 4n2 limit in thin waveguides,” Appl. Phys. Lett. 100, 121121 (2012). [CrossRef]

].

Lastly, we move beyond the assumption of Lambertian coupling and use RCWA to numerically model how a broadband grating couples to anisotropic GMs. The grating considered here (identical to the design in [7

7. Z. Yu, A. Raman, and S. Fan, “Fundamental limit of nanophotonic light trapping in solar cells,” Proc. Natl. Acad. Sci. USA 107, 17491–17496 (2010). [CrossRef] [PubMed]

]) is shown in Fig. 4(c) and the inset of Fig. 4(b). The RCWA results are shown in the right panel of Fig. 4(b) (solid lines). Similar to TMM the results are less than those predicted by ES approximations; however, zP3HT still exhibits superior integrated absorbance properties. For all thicknesses and methods of analysis considered here, zP3HT provides significantly larger integrated absorption than P3HT–reaching a maximum of 113% larger absorption in the ES limit, 150% larger absorption in the single mode TMM calculated limit, and ≈ 63% in the RCWA grating structure. All cases far exceed single-pass absorption for both orientations (dashed lines, Fig. 4(b), right panel).

5. Discussion

The results presented in Sections 3 and 4 illustrate the need to align the strongly absorbing axis of P3HT with the anisotropic field enhancement of these light trapping architectures. Similar anisotropies can be expected in other light trapping architectures that share traits common to SPs and GMs. For instance, SPs and GMs are both TM waveguide modes characterized by in-plane Ex and out-of-plane Ez components. Because of the distinct continuity conditions for these fields, the out-of-plane Ez fields can exhibit much larger field enhancements and also need not go to zero in active layers that are adjacent to a metal substrate with large optical conductivity. Additionally, both light-trapping architectures couple to modes which are evanescent within the organic active layer (neff > n). Such modes have purely out-of-plane oriented electric fields when neff = n, and only exhibit an appreciable in-plane Ex component if neffn.

In general, these results illustrate the necessity for the explicit modeling of anisotropies in the active layer materials used in nanophotonic OPV architectures. It can be seen from Eq. (2) that the absorption for a uniaxial material is maximized when the energy contained in the field component pointing in the direction of strongly absorbing axis is maximum. For P3HT, where the strongly absorbing axis is in-plane, architectures with strong in-plane field enhancements such as hyperbolic metamaterials [45

45. T. U. Tumkur, L. Gu, J. K. Kitur, E. E. Narimanov, and M. A. Noginov, “Control of absorption with hyperbolic metamaterials,” Appl. Phys. Lett. 100, 161103 (2012). [CrossRef]

] could prove to be very beneficial. This concept can be extended to architectures where the localized field enhancements are not as anisotropic as the cases explored here. For example, Bloch modes, dielectric resonances and localized SP resonances have all been demonstrated for OPV nanophotonic light trapping. In these architectures the field enhancements must be designed with the morphology of the active layer in mind. Alternatively, through materials engineering, the morphology of the active layer can be tailored to maximize the benefit of the light trapping strategy being used. This may be beneficial for carrier collection considerations as well.

6. Conclusion

In summary, we have shown that light-trapping in SP and GM OPV architectures can be strongly influenced by morphology-dependent optical anisotropies of the organic active layer. Using P3HT as a representative system, we have shown that orienting organic domains such that the strongly absorbing axes are aligned perpendicular to the substrate leads to significant absorption enhancements. This orientation-dependent coupling arises from the significant anisotropic field enhancements intrinsic to these light trapping architectures. More generally, these results highlight the importance of considering optical anisotropies when modeling light absorption in any OPV architecture comprising well-ordered morphologies.

Appendix A: Estimation of absorptive properties for out-of-plane aligned P3HT molecules

We compare typical P3HT morphologies, wherein polymer chains lie parallel to a substrate, with a model morphology, zP3HT, in which the polymer chains are oriented perpendicular to the substrate (see reference [33

33. J. Liu, Y. Sun, X. Gao, R. Xing, L. Zheng, S. Wu, Y. Geng, and Y. Han, “Oriented poly(3-hexylthiophene) nanofibril with the π- π stacking growth direction by solvent directional evaporation,” Langmuir 27, 4212–4219 (2011). [CrossRef] [PubMed]

] of the manuscript for a recent experimental demonstration). A typical P3HT film, exhibits a uniaxial dielectric tensor with the optical axis oriented out-of-plane along the z-direction:
εP3HT=[ε˜||000ε˜||000ε˜]=[ε˜SA000ε˜SA000ε˜WA]
(8)
Here, ε̃SA and ε̃WA refer to the relative permittivities in the strongly absorbing and weakly absorbing directions, respectively. The simplest model for zP3HT is to rotate the P3HT dielectric tensor:
εP3HT(rotated)=[ε˜WA000ε˜SA000ε˜SA].
(9)
Physically, this dielectric tensor describes a morphology in which polymer backbones are randomly oriented within the y-z plane, defining an optical axis along the x-direction. Although such a morphology could be realized using an ultramicrotome, for example, typical fabrication techniques exhibit in-plane (x-y plane) symmetry at optical length scales; the substrate has no preferred in-plane direction. In the manuscript we model this more physically realistic morphology with a conservative assumption: that both in-plane permittivities are equal to ε̃WA,
εzP3HT=[ε˜WA000ε˜WA000ε˜SA].
(10)

It is instructive to compare the results of our calculations for these two optical models of a zP3HT morphology. In the manuscript, we assume the dielectric function in Eq. (10). The SP effective indices are identical along all in-plane directions and are plotted in Figs. 5(a) and 5(b) (red curves). In the rotated model Eq. (9), SPs propagating along the x-axis exhibit identical dispersion as the red curves in Fig. 5. SPs propagating along the y-axis experience an identical out-of-plane dielectric function but a different in-plane dielectric function and consequently have different properties (blue curves). Despite these physical differences, the SP properties are very similar, confirming that SPs are largely insensitive to in-plane permittivities. However, both of the optical models described above underestimate the dielectric function expected of a P3HT morphology in which the polymer chains are predominantly aligned out-of-plane.

Fig. 5 (a) Real part of the SP effective index for P3HT, zP3HT, P3HT(rotated, y-direction), and zP3HT(max). (b) Imaginary part of the SP effective index for the same set of materials showing the theoretical bounds of losses with reorientation.

In a typical P3HT morphology, polymer chains align parallel to the substrate, but are randomly oriented within this plane. The in-plane permittivities thus arise from a mixture of transitions along the polymer backbone and between polymer chains. Although spectroscopic signatures of inter-molecular excitations in P3HT have been identified [46

46. P. J. Brown, D. S. Thomas, A. Köhler, J. S. Wilson, J.-S. Kim, C. M. Ramsdale, H. Sirringhaus, and R. H. Friend, “Effect of interchain interactions on the absorption and emission of poly(3-hexylthiophene),” Phys. Rev. B 67, 064203 (2003). [CrossRef]

], these transitions are relatively weak and the dielectric function is dominated by intra-molecular excitations aligned along the polymer backbone [29

29. U. Zhokhavets, T. Erb, H. Hoppe, G. Gobsch, and N. S. Sariciftci, “Effect of annealing of poly(3-hexylthiophene)/fullerene bulk heterojunction composites on structural and optical properties,” Thin Solid Films 496, 679–682 (2006). [CrossRef]

]. Assuming purely intra-chain dipole transitions with susceptibility χ, the effective in-plane susceptibility (χeff) for P3HT is:
χeff=χ0πcos(θ)cos(θ)dθ0πdθ=χ2.
(11)
The strongly absorbing dielectric function is then:
ε˜SA=1+χeff=1+χ2.
(12)
Assuming instead that all polymer chains are aligned out-of-plane, the z-oriented dielectric function equals:
ε˜zP3HT(max),=1+χ=2ε˜SA1.
(13)
Unlike the typical P3HT morphology, all chains are aligned along a single axis and the dielectric tensor for this assumption equals:
εzP3HT(max)=[100010002ε˜SA1].
(14)
The resulting real and imaginary (SP) effective indices are also plotted in Fig. 5 as zP3HT(max) (orange curve). The real (imaginary) SP effective index is 20% (50%) larger assuming the dielectric function in Eq. (14) compared to the values of Eq. (10), used in the manuscript, when averaged over wavelengths from 450 nm to 650 nm. Thus, the calculations used in the manuscript likely underestimate the light-trapping benefits arising from a reorientation of polymer constituents. Regardless, we chose the most conservative assumption possible in order to highlight differences in light-trapping which arise solely from reorienting the dielectric functions of a material. Further studies are needed to accurately quantify the dielectric functions of morphologies similar to zP3HT.

Appendix B: Derivation of the anisotropic surface plasmon dispersion

Given the topology shown in Fig. 6, we derive the dispersion relation for an anisotropic SP mode. The SP is a TM mode with a complex magnetic field amplitude that decays evanescently into both media,
Hy=H0{exp(jβ˜x+jk˜zIz)RegionIexp(jβ˜xjk˜zIz)RegionII.
(15)
Region I is described by a complex dielectric tensor,
εd=(ε˜d,||000ε˜d,||000ε˜d,),
(16)
and region II is described by the complex dielectric function of Ag, ε̃m. Applying Ampère’s law ∇ × H⃗ = − jωε0 ε⃡ · E⃗, the electric field components of the mode are obtained,
Ex={k˜zIωε0ε˜d,||HyRegionIk˜zIIωε0ε˜mHyRegionII
(17)
Ez={β˜ωε0ε˜d,HyRegionIβ˜ωε0ε˜mHyRegionII.
(18)
Applying Gausss law in region I,
εd(ω)E(ω)=xε˜d,||Ex+zε˜d,EzI=0,
(19)
as well as for region II, we can now relate the z-field components in both media to the x-field component:
Ex=k˜zIε˜d,β˜ε˜d,||EzI=k˜zIIβ˜EzII
(20)
Applying the continuity of the displacement fields at the interface along with Gausss law provides a relation between the out-of-plane k-vectors in each medium,
k˜zII=ε˜mε˜d,||k˜zI,
(21)
while Faradays law ∇ × E⃗ = − 0ωH⃗(ω) can be combined with the above substitutions:
k˜zI2ε˜d,||+β˜2ε˜d,=k02
(22)
εmk˜zI2ε˜d,||+β˜2ε˜m=k02.
(23)
Solving Eqs. (22) and (23) simultaneously results in the dispersion relation given in Eq. (3) of the main text, and is consistent with [38

38. Z. Jacob and E. E. Narimanov, “Optical hyperspace for plasmons: Dyakonov states in metamaterials,” Appl. Phys. Lett. 93, 221109 (2008). [CrossRef]

, 39

39. M. Liscidini and J. E. Sipe, “Quasiguided surface plasmon excitations in anisotropic materials,” Phys. Rev. B 81, 115335 (2010). [CrossRef]

].

Fig. 6 Geometry and propagation vector definitions for anisotropic SP dispersion derivation.

Appendix C: Upper limit of absorption for finite thickness surface plasmons and gap modes: a waveguide perspective

Appendix D: Fraction of loss in organic layer for surface plasmon modes

For the slab geometry given Fig. 6 we can define the power absorption per unit area in x and y from Eq. (2):
A=ωε02[0εd,|||Ex|2+εd,|Ez|2dz+εm0|Ex|2+|Ez|2dz].
(32)
Integrating Eq. (32) and using the E-field relations from Eqs. (17) and (18), the absorption per unit length in regions I and II can be expressed as,
AI=14ωε0[εd,|||k˜zI|2|εd,|||2k˜zI+εd,|β˜|2|εd,|2k˜zI]H02exp(2βx),
(33)
AII=14ωε0[εm|k˜zII|2|εm|2k˜zII+εm|β˜|2|εm|2k˜zII]H02exp(2βx).
(34)
Equations (33) and (34) can be combined to define the fraction of power absorbed in the polymer region,
ξa=AIAI=AII=εd,|||kzI|2|ε˜d,|||2kzI+εd,|β˜|2|ε˜d,|2kzIεd,|||kzI|2|ε˜d,|||2kzI+εd,|β˜|2|ε˜d,|2kzI+εm|kzII|2|ε˜m|2kzII+εm|β˜|2|ε˜m|2kzII,
(35)
which is consistent with the definition provided in [20

20. X. Sheng, J. Hu, J. Michel, and L. C. Kimerling, “Light trapping limits in plasmonic solar cells: an analytical investigation,” Opt. Express 20, A496–A501 (2012). [CrossRef] [PubMed]

].

Appendix E: Upper limit of absorption for a gap mode in the electrostatic limit

Appendix F: RCWA simulation details

The RCWA simulations were run with 29 harmonics in both the x and y directions, and the calculated absorption values were found to converge within < 3.5% when integrated over the solar spectrum. The absorption, A, as a function of λ0 and h are plotted in Fig. 7(a) for P3HT and Fig. 7(c) for zP3HT, where it can be seen that zP3HT has a very large absorption band ranging from 450 nm to 650 nm. This large absorption band corresponds to the large κeff values in this wavelength range, shown in Fig. 4(a) of the main text. Field components and absorption density Ua [defined as the integrand of Eq. (2)] are shown at λ0 = 550 nm and h = 5 nm [indicated as white circles in Figs 7(a) and 7(c)] in Fig. 7(b) for P3HT and Fig. 7(d) for zP3HT. All quantities are normalized relative to the peak of the Ex component for a P3HT active medium. These field plots clearly illustrate that the out-of-plane Ez component is the dominant contributor to Ua regardless of active material, and that Ua is significantly larger for a zP3HT active medium as compared to P3HT.

Fig. 7 (a),(c) RCWA calculated absorption versus wavelength and active layer thickness, (b),(d) field intensities and absorption density inside the active area at λ0 = 550 nm and h = 5 nm for P3HT and zP3HT, indicated by white circles in (a), (c).

Acknowledgments

The authors acknowledge L. Richter for P3HT ellipsometry data, A. Raman and S. Fan for discussions on numerical modeling of gap mode structures and article preprints. Research on plasmonic light-trapping was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, as part of the Center for Energy Efficient Materials Energy Frontier Research Center (award # DE-SC0001009). Research on dielectric gap-mode light-trapping was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, as part of the Center for Re-Defining Photovoltaic Efficiency through Molecule Scale Control Energy Frontier Research Center (award # DE-SC0001085).

References and links

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P. Peumans, V. Bulović, and S. R. Forrest, “Efficient photon harvesting at high optical intensities in ultrathin organic double-heterostructure photovoltaic diodes,” Appl. Phys. Lett. 76, 2650–2652 (2000). [CrossRef]

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D.-H. Ko, J. R. Tumbleston, A. Gadisa, M. Aryal, Y. Liu, R. Lopez, and E. T. Samulski, “Light-trapping nano-structures in organic photovoltaic cells,” J. Mater. Chem. 21, 16293–16303 (2011). [CrossRef]

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K. S. Nalwa, J.-M. Park, K.-M. Ho, and S. Chaudhary, “On realizing higher efficiency polymer solar cells using a textured substrate platform,” Adv. Mater. 23, 112–116 (2011). [CrossRef]

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D. M. Callahan, J. N. Munday, and H. A. Atwater, “Solar cell light trapping beyond the ray optic limit,” Nano Lett. 12, 214–218 (2012). [CrossRef]

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T. Z. Oo, N. Mathews, G. Xing, B. Wu, B. Xing, L. H. Wong, T. C. Sum, and S. G. Mhaisalkar, “Ultrafine gold nanowire networks as plasmonic antennae in organic photovoltaics,” J. Phys. Chem. C 116, 6453–6458 (2012). [CrossRef]

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J. N. Munday, D. M. Callahan, and H. A. Atwater, “Light trapping beyond the 4n2 limit in thin waveguides,” Appl. Phys. Lett. 100, 121121 (2012). [CrossRef]

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M. A. Green, “Enhanced evanescent mode light trapping in organic solar cells and other low index optoelectronic devices,” Prog. Photovolt: Res. Appl. 19, 473–477 (2011). [CrossRef]

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E. A. Schiff, “Thermodynamic limit to photonic-plasmonic light-trapping in thin films on metals,” J. Appl. Phys. 110, 104501 (2011). [CrossRef]

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X. Sheng, J. Hu, J. Michel, and L. C. Kimerling, “Light trapping limits in plasmonic solar cells: an analytical investigation,” Opt. Express 20, A496–A501 (2012). [CrossRef] [PubMed]

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M. A. Green and S. Pillai, “Harnessing plasmonics for solar cells,” Nature Photon. 6, 130–132 (2012). [CrossRef]

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M. K. Debe, “Variable angle spectroscopic ellipsometry studies of oriented phthalocyanine films. II. copper phthalocyanine,” J. Vac. Sci. Technol., A 10, 2816–2821 (1992). [CrossRef]

23.

O. D. Gordan, M. Friedrich, and D. R. T. Zahn, “The anisotropic dielectric function for copper phthalocyanine thin films,” Org. Electron. 5, 291–297 (2004). [CrossRef]

24.

M. Dressel, B. Gompf, D. Faltermeier, A. K. Tripathi, J. Pflaum, and M. Schubert, “Kramers-kronig-consistent optical functions of anisotropic crystals: generalized spectroscopic ellipsometry on pentacene,” Opt. Express 16, 19770–19778 (2008). [CrossRef] [PubMed]

25.

J. A. Schuller, S. Karaveli, T. Schiros, K. He, S. Yang, I. Kymissis, J. Shan, and R. Zia, “Orientation of luminescent excitons in layered nanomaterials,” Nature Nanotech. 8, 271–276 (2013). [CrossRef]

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M. Campoy-Quiles, P. G. Etchegoin, and D. D. C. Bradley, “On the optical anisotropy of conjugated polymer thin films,” Phys. Rev. B 72, 045209 (2005). [CrossRef]

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S.-Y. Chuang, H.-L. Chen, W.-H. Lee, Y.-C. Huang, W.-F. Su, W.-M. Jen, and C.-W. Chen, “Regioregularity effects in the chain orientation and optical anisotropy of composite polymer/fullerene films for high-efficiency, large-area organic solar cells,” J. Mater. Chem. 19, 5554–5560 (2009). [CrossRef]

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Z. Jacob and E. E. Narimanov, “Optical hyperspace for plasmons: Dyakonov states in metamaterials,” Appl. Phys. Lett. 93, 221109 (2008). [CrossRef]

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P. J. Brown, D. S. Thomas, A. Köhler, J. S. Wilson, J.-S. Kim, C. M. Ramsdale, H. Sirringhaus, and R. H. Friend, “Effect of interchain interactions on the absorption and emission of poly(3-hexylthiophene),” Phys. Rev. B 67, 064203 (2003). [CrossRef]

OCIS Codes
(040.5350) Detectors : Photovoltaic
(160.4890) Materials : Organic materials
(240.6680) Optics at surfaces : Surface plasmons
(350.6050) Other areas of optics : Solar energy
(350.4238) Other areas of optics : Nanophotonics and photonic crystals
(250.5403) Optoelectronics : Plasmonics

ToC Category:
Light Trapping for Photovoltaics

History
Original Manuscript: June 18, 2013
Revised Manuscript: July 26, 2013
Manuscript Accepted: July 27, 2013
Published: August 15, 2013

Citation
Richard R. Grote, Steven J. Brown, Jeffrey B. Driscoll, Richard M. Osgood, and Jon A. Schuller, "Morphology-dependent light trapping in thin-film organic solar cells," Opt. Express 21, A847-A863 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-S5-A847


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References

  1. A. C. Mayer, S. R. Scully, B. E. Hardin, M. W. Rowell, and M. D. McGehee, “Polymer-based solar cells,” Mater. Today10, 28–33 (2007). [CrossRef]
  2. P. Peumans, V. Bulović, and S. R. Forrest, “Efficient photon harvesting at high optical intensities in ultrathin organic double-heterostructure photovoltaic diodes,” Appl. Phys. Lett.76, 2650–2652 (2000). [CrossRef]
  3. B. O’Connor, K. H. An, K. P. Pipe, Y. Zhao, and M. Shtein, “Enhanced optical field intensity distribution in organic photovoltaic devices using external coatings,” Appl. Phys. Lett.89, 233502 (2006). [CrossRef]
  4. F.-C. Chen, J.-L. Wu, C.-L. Lee, Y. Hong, C.-H. Kuo, and M. H. Huang, “Plasmonic-enhanced polymer photovoltaic devices incorporating solution-processable metal nanoparticles,” Appl. Phys. Lett.95, 013305 (2009). [CrossRef]
  5. D. M. O’Carroll, C. E. Hofmann, and H. A. Atwater, “Conjugated polymer/metal nanowire heterostructure plasmonic antennas,” Adv. Mater.22, 1223–1227 (2010). [CrossRef]
  6. H. A. Atwater and A. Polman, “Plasmonics for improved photovoltaic devices,” Nature Mater.9, 205–213 (2010). [CrossRef]
  7. Z. Yu, A. Raman, and S. Fan, “Fundamental limit of nanophotonic light trapping in solar cells,” Proc. Natl. Acad. Sci. USA107, 17491–17496 (2010). [CrossRef] [PubMed]
  8. D.-H. Ko, J. R. Tumbleston, A. Gadisa, M. Aryal, Y. Liu, R. Lopez, and E. T. Samulski, “Light-trapping nano-structures in organic photovoltaic cells,” J. Mater. Chem.21, 16293–16303 (2011). [CrossRef]
  9. K. S. Nalwa, J.-M. Park, K.-M. Ho, and S. Chaudhary, “On realizing higher efficiency polymer solar cells using a textured substrate platform,” Adv. Mater.23, 112–116 (2011). [CrossRef]
  10. D. M. Callahan, J. N. Munday, and H. A. Atwater, “Solar cell light trapping beyond the ray optic limit,” Nano Lett.12, 214–218 (2012). [CrossRef]
  11. D. M. O’Carroll, J. S. Fakonas, D. M. Callahan, M. Schierhorn, and H. A. Atwater, “Metal-polymer-metal split-dipole nanoantennas,” Adv. Mater.24, OP136–OP142 (2012). [CrossRef]
  12. T. Z. Oo, N. Mathews, G. Xing, B. Wu, B. Xing, L. H. Wong, T. C. Sum, and S. G. Mhaisalkar, “Ultrafine gold nanowire networks as plasmonic antennae in organic photovoltaics,” J. Phys. Chem. C116, 6453–6458 (2012). [CrossRef]
  13. Z. Yu, A. Raman, and S. Fan, “Thermodynamic upper bound on broadband light coupling with photonic structures,” Phys. Rev. Lett.109, 173901 (2012). [CrossRef] [PubMed]
  14. J. N. Munday, D. M. Callahan, and H. A. Atwater, “Light trapping beyond the 4n2 limit in thin waveguides,” Appl. Phys. Lett.100, 121121 (2012). [CrossRef]
  15. S. A. Mann, R. R. Grote, R. M. Osgood, and J. A. Schuller, “Dielectric particle and void resonators for thin film solar cell textures,” Opt. Express19, 25729–25740 (2011). [CrossRef]
  16. N. C. Panoiu and R. M. Osgood, “Enhanced optical absorption for photovoltaics via excitation of waveguide and plasmon-polariton modes,” Opt. Lett.32, 2825–2827 (2007). [CrossRef] [PubMed]
  17. M. A. Green, “Enhanced evanescent mode light trapping in organic solar cells and other low index optoelectronic devices,” Prog. Photovolt: Res. Appl.19, 473–477 (2011). [CrossRef]
  18. S. B. Mallick, N. P. Sergeant, M. Agrawal, J.-Y. Lee, and P. Peumans, “Coherent light trapping in thin-film photovoltaics,” MRS Bulletin36, 453–460 (2011). [CrossRef]
  19. E. A. Schiff, “Thermodynamic limit to photonic-plasmonic light-trapping in thin films on metals,” J. Appl. Phys.110, 104501 (2011). [CrossRef]
  20. X. Sheng, J. Hu, J. Michel, and L. C. Kimerling, “Light trapping limits in plasmonic solar cells: an analytical investigation,” Opt. Express20, A496–A501 (2012). [CrossRef] [PubMed]
  21. M. A. Green and S. Pillai, “Harnessing plasmonics for solar cells,” Nature Photon.6, 130–132 (2012). [CrossRef]
  22. M. K. Debe, “Variable angle spectroscopic ellipsometry studies of oriented phthalocyanine films. II. copper phthalocyanine,” J. Vac. Sci. Technol., A10, 2816–2821 (1992). [CrossRef]
  23. O. D. Gordan, M. Friedrich, and D. R. T. Zahn, “The anisotropic dielectric function for copper phthalocyanine thin films,” Org. Electron.5, 291–297 (2004). [CrossRef]
  24. M. Dressel, B. Gompf, D. Faltermeier, A. K. Tripathi, J. Pflaum, and M. Schubert, “Kramers-kronig-consistent optical functions of anisotropic crystals: generalized spectroscopic ellipsometry on pentacene,” Opt. Express16, 19770–19778 (2008). [CrossRef] [PubMed]
  25. J. A. Schuller, S. Karaveli, T. Schiros, K. He, S. Yang, I. Kymissis, J. Shan, and R. Zia, “Orientation of luminescent excitons in layered nanomaterials,” Nature Nanotech.8, 271–276 (2013). [CrossRef]
  26. M. Campoy-Quiles, P. G. Etchegoin, and D. D. C. Bradley, “On the optical anisotropy of conjugated polymer thin films,” Phys. Rev. B72, 045209 (2005). [CrossRef]
  27. D. M. DeLongchamp, R. J. Kline, E. K. Lin, D. A. Fischer, L. J. Richter, L. A. Lucas, M. Heeney, I. McCulloch, and J. E. Northrup, “High carrier mobility polythiophene thin films: structure determination by experiment and theory,” Adv. Mater.19, 833–837 (2007). [CrossRef]
  28. M. C. Gurau, D. M. Delongchamp, B. M. Vogel, E. K. Lin, D. A. Fischer, S. Sambasivan, and L. J. Richter, “Measuring molecular order in poly(3-alkylthiophene) thin films with polarizing spectroscopies,” Langmuir23, 834–842 (2007). [CrossRef] [PubMed]
  29. U. Zhokhavets, T. Erb, H. Hoppe, G. Gobsch, and N. S. Sariciftci, “Effect of annealing of poly(3-hexylthiophene)/fullerene bulk heterojunction composites on structural and optical properties,” Thin Solid Films496, 679–682 (2006). [CrossRef]
  30. S.-Y. Chuang, H.-L. Chen, W.-H. Lee, Y.-C. Huang, W.-F. Su, W.-M. Jen, and C.-W. Chen, “Regioregularity effects in the chain orientation and optical anisotropy of composite polymer/fullerene films for high-efficiency, large-area organic solar cells,” J. Mater. Chem.19, 5554–5560 (2009). [CrossRef]
  31. D. S. Germack, C. K. Chan, R. J. Kline, D. A. Fischer, D. J. Gundlach, M. F. Toney, L. J. Richter, and D. M. De-Longchamp, “Interfacial segregation in polymer/fullerene blend films for photovoltaic devices,” Macromolecules43, 3828–3836 (2010). [CrossRef]
  32. M. R. Hammond, R. J. Kline, A. A. Herzing, L. J. Richter, D. S. Germack, H.-W. Ro, C. L. Soles, D. A. Fischer, T. Xu, L. Yu, M. F. Toney, and D. M. DeLongchamp, “Molecular order in high-efficiency polymer/fullerene bulk heterojunction solar cells,” ACS Nano5, 8248–8257 (2011). [CrossRef] [PubMed]
  33. J. Liu, Y. Sun, X. Gao, R. Xing, L. Zheng, S. Wu, Y. Geng, and Y. Han, “Oriented poly(3-hexylthiophene) nanofibril with the π- π stacking growth direction by solvent directional evaporation,” Langmuir27, 4212–4219 (2011). [CrossRef] [PubMed]
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