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Implementation of a submicrometer patterning technique in azopolymer films towards optimization of photovoltaic solar cells efficiency
Thin Solid Films 511 – 512 (2006) 517 – 522 www.elsevier.com/locate/tsf
Implementation of a submicrometer patterning technique in azopolymer films towards optimization of photovoltaic solar cells efficiency C. Cocoyer, L. Rocha *, C. Fiorini-Debuisschert, L. Sicot, D. Vaufrey, C. Sentein, B. Geffroy, P. Raimond CEA Saclay, DRT-LITEN\DSEN\SPVS\L2C, baˆt. 451, 91191 Gif-sur-Yvette cedex, France Available online 26 January 2006
Abstract The weak absorption of the photoactive layer appears as a one of the main factors limiting organic photovoltaic solar cells performances. In order to increase the interaction of the incident light with the photoactive materials, we investigate the effect of a periodic patterning of the solar cells surface with microstructures in the optical wavelength scale. In this aim, we present an original all optical patterning technique of polymer films. The method is based on a laser controlled mass transport in azopolymer films leading to efficient deformation of the film surface in conjunction with the incoming light interference pattern. The technique is used to pattern one-dimensional gratings on the surface of solar cells. In the work presented here, the cell photoactive material is based on the interpenetrated network of a conjugated donor polymer and a fullerene derivative. The cells investigated are illuminated in a reverse configuration through a semi-transparent top cathode. The effect of the periodic structures onto the incident light propagation has been investigated through optical characterizations. We demonstrate that a part of the incident light can be trapped inside the solar cell layers due to diffraction onto the periodic structures. D 2005 Elsevier B.V. All rights reserved. Keywords: Azo-dye polymers; Photo-induced mass transport; Surface patterning; Organic photovoltaic solar cells
1. Introduction A major limitation in the efficiency of organic photovoltaic solar cells is related to the weak absorption of the active layers, where thickness is limited by the relatively low charge mobility in organic materials. A good illustration is given when considering the interpenetrated network concept [1], which deals with the photoinduced electron transfer from a donor conjugated polymer to an acceptor molecule (a derivative of C60). Solar cells based on such concept present high charge generation efficiency due to the distribution of the exciton dissociating sites throughout the volume of the material. However, the restricted thickness, usually below 200 nm, imposed to optimize charge collection, limits the absorption and does not allow to take full benefit of this concept. In order to improve light absorption without increasing the photoactive layer thickness, a promising way consists in patterning the solar cell layers in the optical wavelength scale. * Corresponding author. Tel.: +33 1 69 08 43 17; fax: +33 1 69 08 91 75. E-mail address: [email protected] (L. Rocha). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.017
Due to diffraction effects, it is indeed possible to modify light propagation and optimize the interaction of the excitation with the photoactive material [2]. The effect of periodic structures has been investigated since the 1970s onto silicon solar cells [3– 7] and more recently in organic devices [8,9]. In the last case, a growing interest has been observed in the past decade with the development of patterning methods better adapted to organic materials than lithographic means used in silicon technology. One of the main drawbacks of lithographic techniques is that they require the use of solvents or the exposure to UV that can deteriorate the optical or electronic properties of the active layers. Among those newly developed patterning techniques, we can mention embossing [10], stamping [11] or replica moulding [12], which imprints a master by contact onto the material surface. The dimensions of the resulting structures etched at the surface of the films depend on the master features size. Such techniques have demonstrated the ability to realize structures with sub-50-nm dimensions [13]. The method presented here is an original all optical single step patterning technique in which the solid master, usually fabricated by conventional lithographic means, is replaced by a
518
C. Cocoyer et al. / Thin Solid Films 511 – 512 (2006) 517 – 522
light interference pattern. The patterning process is related to photo induced mass transport in azopolymer films [14]. This technique allows for the realization of surface relief gratings, where period and modulation amplitude can be easily controlled. We show how this technique can be used to pattern photovoltaic solar cells and present the first results towards device efficiency optimization.
CH3
CH3 C
CH2
C
n
CH22
1-n
C
C O
O
O
O
CH3
C H2C H2C N
N
N
NO2
CH3CH2
H
P
BS M
Argon Laser
2. Induction of surface relief gratings on azopolymer films The patterning method is based on a laser-controlled mass transport occurring at room temperature in polymer films containing azo-dye molecules. The irradiation of azo-dye molecules at a wavelength included in the absorption band of the chromophores leads to a molecular motion following repeated cis – trans isomerization cycles [15] (Fig. 1). This molecular process can result in a macroscopic deformation of the film surface depending on the excitation configuration [16,17]. It has been demonstrated that the irradiation of an azopolymer film with an interference pattern results in a mass transport from the high intensity regions to the low intensity regions [18]. The technique allows to control the profile and geometry of the surface deformation by the intensity profile of the exciting beam. The efficiency of the deformation process has been shown to depend strongly on the polarization state of the excitation [19 – 21]. Experimentally, we make use of films that are spin coated onto a glass substrate from a 1,1,2-tricholorethane solution of an azo-dye molecule [4-(N-(2-hydroxyethyl)-N-ethyl-) amino-4V-nitroazobenzene] (DR1) grafted as a side chain to poly(methylmethacrylate) (PMMA) with a 35% molar ratio co(DR1/MMA)(35/65) (Fig. 2). In the present study, we consider one-dimensional gratings realized with an interference pattern resulting from the superposition of two coherent light beams. The irradiation with such an interference pattern, presenting a sinusoidally modulated intensity profile, leads to a sinusoidal relief grating at the polymer film surface. The period of the grating K is equal to the fringes spacing of the interference pattern and is controlled by the incidence angle of trans form
1,8
NO2
O2N h
1,4
Optical Density
cis form
N
N
N
N
h ν , kT
1,0
N
N HO
OH
0,6
0,2 0,0 300
400
500
600
700
800
Wavelength (nm) Fig. 1. Absorption spectrum and trans – cis isomerization of the azo-dye molecule Disperse Red 1 (DR1). The molecule can be excited in the cis form with the absorption of a photon. The relaxation to the trans form can be induced optically or by thermal relaxation.
θ
M
θ
S
Fig. 2. Experimental set-up for the surface relief grating formation: H: half wave plate, P: polarizer, BS: beam splitter, M: mirror, S: sample. In the upper part: azo-dye-based copolymer formula (left) and typical AFM image (right) of an induced surface relief grating. The surface grating presents a period of K = 460 nm and a modulation amplitude of Dh = 90 nm. k the beams onto the film surface: K ¼ 2nsinh , where K is the interference fringes spacing (i.e. the resulting grating period), k the excitation wavelength and n is the index of the medium in which we consider the pump beams incidence angle h. The modulation amplitude for a given period is controlled by the total energy sent to the polymer film. The polarization of the excitation is set perpendicular to the fringes in order to induce efficient mass transport from the illuminated zones to the dark ones [22]. The experimental set-up is schematized in Fig. 2. A typical surface relief grating imaged with an AFM is also represented. The grating is obtained by irradiating the film surface with the 514.5-nm line of an Argon laser at a total intensity of 300 mW cm 2 during a 15-min exposure time. The surface grating presents a period of K = 460 nm and a modulation amplitude of Dh = 90 nm.
3. Implementation of the periodic structures towards photovoltaic applications The cell investigated is based on the interpenetrated network of a conjugated polymer as donor and a fullerene derivative as acceptor materials. The patternable azopolymer film being a dielectric material, it is used as a substrate for the photovoltaic solar cell. Moreover, to avoid an absorption of the incident light by the co(DR1/MMA) substrate, the architecture of the solar cell is chosen in order to allow illumination through the top electrode. As represented in Fig. 3, the solar cell consists of seven layers deposited according to the following procedure. The patterned substrate is first covered with a thermally evaporated 100-nm-thick gold layer followed with the deposition of a 20-nm-thick ITO layer by radio-frequency magnetron sputtering. Then a film of poly(ethylene dioxythiophene) (PEDOT) is spin-coated onto the ITO. The thickness of the PEDOT layer is approximately 60 nm. The photoactive layer is spin coated from a solution of the blend poly[2methoxy-5-(3V, 7V- dimethyloctyloxy)-p-phenylenevinylene] (MDMO-PPV) and [6,6]-phenyl C60 butyric acid methyl ester
C. Cocoyer et al. / Thin Solid Films 511 – 512 (2006) 517 – 522
519
hν LiF/Al/Ag electrode (20nm)
MDMO-PPV : PCBM (100nm)
PEDOT (60nm) Au (100nm)
ITO (20nm) Co(DR1/MMA) (200nm) Glass substrate
Fig. 3. Scheme of a modulated solar cell with an illumination through the top electrode.
4. Results and discussion 4.1. Optical characterizations The effect of the periodic structures on the solar cell absorption was investigated. The planar and modulated solar cells were illuminated with a collimated beam delivered by a tungsten lamp, and the light transmitted by the samples was analyzed by a spectrometer (2.5 nm dispersion/pixel) following collection by a 2.5-cm focal length lens and propagation through an optical fiber (0.6 mm core diameter), as depicted in the insert of Fig. 4. Detection is performed using a thermoelectrically cooled charged coupled device (CCD).
For the experiment, comparable modulation amplitude gratings with periods ranging from 400 nm to 600 nm have been realized. The spectra for two solar cells modulated with periods K = 430 nm and K = 480 nm and excited in TE polarization are shown in Fig. 4. The TE polarization corresponds to a polarization parallel to the grating stripes. The spectrum transmitted by the non-modulated cell is also given. 4.1.1. Light trapping effects in patterned solar cells The spectra of the modulated cells present a localized drop in the transmission occurring at a wavelength depending on the grating period. The transmission drop appears as a ‘‘hole’’ in the spectrum and is centred at a spectral position increasing with increasing grating period. The absence of ‘‘holes’’ in the spectrum of the light transmitted by the planar cell and the spectral dependence of the phenomena with the grating period confirm the action of the periodic structures on the propagation of the incident light. The spectral dependence of the drop in the transmission can be explained according to the grating formula: kd ¼ Fki FpK
ð1Þ
with kd ¼ 2pndksinhd ; ki ¼ 2pniksinhi the projections of the wave vector of the diffracted and incident waves of wavelength k, S
5000 4500
T
4000
Transmission (arb. u.)
(PCBM) in chlorobenzene with a weight ratio of 1:4. The thickness of the resulting photoactive layer is 100 nm. Finally, a three-layer cathode is evaporated on the photoactive layer. This cathode is realized by the successive thermal evaporation of an approximately 1-nm-thick lithium fluoride (LiF), a 5-nmthick aluminum layer and a 15-nm-thick silver layer. Cathode evaporation is performed through a shadow mask with opening diameters of 0.28 cm2 allowing the realization of two cells onto the same substrate for direct comparison between a modulated and a planar cell. In the structure presented, the thin ITO layer is used for the holes extraction, whereas the 100-nm-thick gold film compensates for the quite low conductivity of the thin ITO layer. For the cathode, we chose the widely employed combination of LiF/Al [23,24] as an interfacial layer to allow efficient electron collection, completed by a 15-nm Ag layer to increase the conductivity of the cathode while limiting the drop of the cathode transparency. In this configuration, the cell is irradiated through the semi-transparent cathode. The structure is shown in Fig. 3. The integration of the periodic structures inside the photovoltaic cell is provided by the property of the thermal evaporation deposition technique to transfer the surface state of the patterned co(DR1/MMA) substrate to the evaporated layer surface. AFM measurements of the samples after deposition of the 100-nm-thick gold layer have shown that the co(DR1/MMA) film surface modulation is transferred to the gold surface. More precisely, the gold surface relief grating presents the same period K than the co(DR1/ MMA) substrate with a 10% modulation amplitude attenuation. On the other hand, the spin coating deposition technique tends to smooth the modulations. After deposition of all the solar cell layers, the remaining modulation amplitudes were about 40% of the initial amplitudes.
P
C
L
F
3500 3000 2500
θd
2000 1500 1000 500 0 400
500
600
700
800
900
Wavelength (nm) Fig. 4. Transmission spectra for a cell modulated with a period K = 430 nm (full symbols) and a cell modulated with a period K = 480 nm (empty symbols). The spectrum transmitted by a planar cell is also given (solid line). In the inset: experimental measurement set-up scheme. T: tungsten lamp, P: polarizer, C: solar cell, L: lens, F: fibre, S: spectrometer. The lower part of the figure schematizes the diffraction process: a wave diffracted with an angle h d lying outside the escape cone (dashed lines) is coupled in the cell layers.
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and K ¼ 2p K the grating vector. The integer p corresponds to the diffraction order, n i is the index of the incident medium and n d is the index of the medium where the angle h d of the diffracted wave is considered. If the incident wave is diffracted with an angle h d lying outside the escape cone (see the inset of Fig. 4), the diffracted wave will undergo total internal reflections at the interfaces of the solar cell and will then remain trapped into the layers leading to the decrease of the transmission at the wavelengths guided in the cell layers. In the experimental configuration used here, with an incident wave propagating in a direction normal to the film plane (sinh i = 0), the diffraction angle is given by the following formula: nd sinhd ¼ Fp
k K
In the two cases considered in Fig. 4, the coupling condition can be satisfied only for a diffraction in the first order p = T 1. Indeed for a diffraction in higher orders, in the spectral range considered, the values of the estimated effective indices n eff = n d sinh d are higher than the index of the highest optical index layer involved in the solar cell structure (n ITO å 2). For higher diffraction orders and a TE polarization, any propagating wave can be coupled in the cell structure. For the multilayer used in our experiments, the solar cell supports only the fundamental TE and TM propagating modes. As a consequence, for a given wavelength, only one propagating mode of each polarization can be excited in the solar cell, which is in good agreement with the presence of only one ‘‘hole’’ in the transmission spectra of Fig. 4 for each grating period. Eq. (2) also evidences, for the spectral range investigated here, that an increase of the grating period must be followed by an increase of the coupled wavelength confirming the spectral behaviour observed in Fig. 4. Indeed, a propagating wave of lower wavelength will present a higher effective index due to a better confinement in the layers and to an increase of the optical indices related to the index dispersion when the wavelength draw closer to the absorption band. As well, a
ð2Þ
For a given grating period K, coupling in the waveguide will be observed for a light with wavelength k if the effective index n eff = n d sinh d of the resulting propagation mode at this wavelength k verifies Eq. (2).
θi=0°
4000 3500
θi=8°
3000
transmission (arb. u.)
transmission (arb. u.)
λ1 λ2
3500
3000 2500 2000 1500 1000 500
2500 2000 1500 1000 500
0 -500
0 300
400
500
600
700
800
900
1000 1100
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700
800
θi=12°
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λ0
λ 2 > λ1
λ2 λ1
2500
transmission (arb. u.)
λ0
900
wavelength (nm)
2000 1500 1000 500 0 400
500
600
700
800
900
wavelength (nm) Fig. 5. Effect of the illumination incidence angle on the light trapping in the solar cell. Down left is schematized trapping effect as a function of the illumination angle.
C. Cocoyer et al. / Thin Solid Films 511 – 512 (2006) 517 – 522
propagating wave of higher wavelength will be less confined in the same structure leading to a lower effective index. Then, for the two gratings considered, the coupling condition for the grating of higher period will be satisfied for a higher wavelength. 4.1.2. Effect of the incidence angle of the illumination To confirm the ‘‘trapping’’ effect of the grating, additional measurements were performed. In the following experiment, the transmission spectra were recorded as a function of the incidence angle of the light onto the solar cells. The spectra obtained for three different incidence angles are shown in Fig. 5 in the case of a cell modulated with a period K = 460 nm. The spectra evidence a splitting of the absorbed band resulting in two ‘‘holes’’, where spectral separation increases with the incidence angle. One of the bands shifts towards the lower wavelengths, whereas the other one shifts to the higher wavelengths. Indeed, in the case of an incidence with an angle h i m 0, the projection k i of the incident wave vector on the cell plane differs from 0 resulting in a coupling condition that can be verified for two different waves: kF ¼
ðneff Fsinhi Þ K p
ð4Þ
We show then the existence of two coupled wavelengths for a grating of given period K confirming the behaviour observed: when h i increases, k decreases and k + increases. The phenomenon described by Eq. (4) is roughly illustrated in Fig. 5. It must be noted that a coupling can also occur in the modulated co(DR1/MMA) layer for the transmitted light which has not been trapped inside the solar cell layers. However, for the grating periods investigated here, an estimation of the effective indices in the co(DR1/MMA) film led, according to formula (1), to guided modes propagating at wavelengths higher than the spectral range investigated here (k > 700 nm). Such guided modes are then not observable in the present experimental configuration. 4.2. First electrical characterizations Electrical characterizations have been performed in both planar and modulated solar cells under 1sun (100 mW cm 2, AM1.5) solar simulator illumination in inert atmosphere of N2. The photovoltaic efficiencies that were measured could reach no more than 0.5%, which is low in comparison with the yields published for solar cells based on the same photoactive material. Such a value is not surprising here since the cathode has not been optimized yet for the application investigated here. The realization of efficient solar cells based on such a reversed configuration will need the development of top electrodes presenting a good transparency in the spectral region of the active material absorption. In this aim, the studies led in the case of organic light emitting diodes towards the integration with silicon based driver technology should benefit to photovoltaic devices.
521
The first results have not allowed to evidence an effect of the periodic gratings onto the photovoltaic conversion yields. This can be explained by the use of gratings which coupling properties lead to trapped modes at wavelengths that are not in the photoactive MDMO-PPV/PCBM material absorption band. The realization of gratings with lower periods is then required to improve the interaction of the light with the active material at the wavelengths of interest. In order to optimize the solar cell structure and the grating geometry, a modelling of the electromagnetic field in the different layers is also needed. In this respect, rigorous coupled wave analysis [25] has shown to be a suitable computational method for the calculation of the optical near field in micropatterned organic solar cells [26]. 5. Conclusion We have demonstrated the potential of a single step all optical technique for the periodical patterning of organic solar cells in view of the optimization of the photovoltaic conversion efficiency. First experiments have evidenced the possibility to increase the interaction of the incident light with the active material through diffraction effects. Although the periodic structures investigated were not optimized for the photoactive material absorption, optical characterizations have shown that a part of the incident light could be trapped inside the layers of the solar cells. The wavelengths at which the trapping occurs as well as the efficiency of the process can be controlled by the grating parameters. Further improvements of the process will need the calculation of the optical field distribution in the patterned solar cell layers stacking. The experiments were performed in a reverse configuration with an illumination through the cathode. Such a configuration should provide a better control of the diffracted waves in the solar cell structure by improving the confinement of the resulting guided modes inside the organic active layer. Moreover, an inverse configuration presents a potential interest regarding the use of substrates of different conformations and materials such as tiles. In this respect, an important issue will be the optimization of the optical and electrical properties of the top electrode in order to improve its transmission while allowing for efficient charge extraction. References [1] N.S. Sariciftci, D. Braun, C. Zhang, V.I. Srdanov, A.J. Heeger, G. Stucky, F. Wudl, Appl. Phys. Lett. 62 (1993) 585. [2] M.L. Dakss, L. Kuhn, P.F. Heidrick, B.A. Scott, Appl. Phys. Lett. 16 (1970) 523. [3] D. Redfield, Appl. Phys. Lett. 25 (1974) 647. [4] P. Campbell, M.A. Green, J. Appl. Phys. 62 (1987) 243. [5] C. Heine, R.H. Morf, Appl. Opt. 34 (1995) 2476. [6] C. Eisele, C.E. Nebel, M. Stutzmann, J. Appl. Phys. 89 (2001) 7722. [7] N. Senoussaoui, M. Krause, J. Muˆller, E. Bunte, T. Brammer, H. Stiebig, Thin Solid Films 451 – 452 (2004) 397. [8] L.S. Roman, O. Ingana¨s, T. Granlund, T. Nyberg, M. Svensson, M.R. Andersson, J.C. Hummelen, Adv. Mater. 12 (2000) 189. [9] M. Niggemann, M. Glatthaar, A. Gombert, A. Hinsch, V. Wittwer, Thin Solid Films 451 – 452 (2004) 619. [10] S.Y. Chou, P.R. Krauss, P.J. Renstrom, Appl. Phys. Lett. 67 (1995) 3114.
522 [11] [12] [13] [14] [15] [16] [17] [18] [19]
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A. Kumar, G.M. Whitesides, Appl. Phys. Lett. 63 (1993) 2002. E. Kim, Y. Xia, G.M. Whitesides, Nature 376 (1995) 581. S.Y. Chou, P.R. Krauss, P.J. Renstrom, Appl. Phys. Lett. 67 (1995) 3114. P. Rochon, E. Batalla, A. Natansohn, Appl. Phys. Lett. 66 (1995) 136. T. Todorov, L. Nikolova, N. Tomova, Appl. Opt. 23 (1984) 4309. C.J. Barrett, A. Natansohn, P.L. Rochon, J. Phys. Chem. 100 (1998) 1505. P. Lefin, C. Fiorini, J.M. Nunzi, Pure Appl. Opt. 7 (1998) 71. N.K. Viswanathan, et al., J. Mater. Chem. 9 (1999) 1941. X.L. Jiang, L. Li, J. Kumar, D.Y. Kim, V. Shivshankar, S.K. Tripathy, Appl. Phys. Lett. 68 (1996) 2618. [20] P. Lefin, C. Fiorini, J.-M. Nunzi, Pure Appl. Opt. 7 (1997) 71.
[21] C. Hubert, C. Fiorini-Debuisschert, P. Raimond, J.M. Nunzi, Adv. Mater. 14 (2002) 729. [22] X.L. Jiang, L. Li, J. Kumar, D.Y. Kim, V. Shivshankar, S.K. Tripathy, Appl. Phys. Lett. 68 (1996) 2618. [23] L.S. Hung, C.W. Tang, M.G. Mason, Appl. Phys. Lett. 70 (1997) 152. [24] G.E. Jabbour, B. Kippelen, N.R. Amsrtong, N. Pyghambarian, Appl. Phys. Lett. 73 (1998) 1185. [25] M.G. Moharam, T.K. Gaylord, J. Opt. Soc. Am. 72 (1982) 1385. [26] M. Niggemann, B. Bla¨si, A. Gombert, A. Hinsch, H. Hoppe, P. Lalanne, D. Meissner, V. Wittwer, 17th European Photovoltaic Solar Energy Conference Proceedings, 2002, pp. 284 – 287.
Implementation of a submicrometer patterning technique in azopolymer films towards optimization of photovoltaic solar cells efficiency C. Cocoyer, L. Rocha *, C. Fiorini-Debuisschert, L. Sicot, D. Vaufrey, C. Sentein, B. Geffroy, P. Raimond CEA Saclay, DRT-LITEN\DSEN\SPVS\L2C, baˆt. 451, 91191 Gif-sur-Yvette cedex, France Available online 26 January 2006
Abstract The weak absorption of the photoactive layer appears as a one of the main factors limiting organic photovoltaic solar cells performances. In order to increase the interaction of the incident light with the photoactive materials, we investigate the effect of a periodic patterning of the solar cells surface with microstructures in the optical wavelength scale. In this aim, we present an original all optical patterning technique of polymer films. The method is based on a laser controlled mass transport in azopolymer films leading to efficient deformation of the film surface in conjunction with the incoming light interference pattern. The technique is used to pattern one-dimensional gratings on the surface of solar cells. In the work presented here, the cell photoactive material is based on the interpenetrated network of a conjugated donor polymer and a fullerene derivative. The cells investigated are illuminated in a reverse configuration through a semi-transparent top cathode. The effect of the periodic structures onto the incident light propagation has been investigated through optical characterizations. We demonstrate that a part of the incident light can be trapped inside the solar cell layers due to diffraction onto the periodic structures. D 2005 Elsevier B.V. All rights reserved. Keywords: Azo-dye polymers; Photo-induced mass transport; Surface patterning; Organic photovoltaic solar cells
1. Introduction A major limitation in the efficiency of organic photovoltaic solar cells is related to the weak absorption of the active layers, where thickness is limited by the relatively low charge mobility in organic materials. A good illustration is given when considering the interpenetrated network concept [1], which deals with the photoinduced electron transfer from a donor conjugated polymer to an acceptor molecule (a derivative of C60). Solar cells based on such concept present high charge generation efficiency due to the distribution of the exciton dissociating sites throughout the volume of the material. However, the restricted thickness, usually below 200 nm, imposed to optimize charge collection, limits the absorption and does not allow to take full benefit of this concept. In order to improve light absorption without increasing the photoactive layer thickness, a promising way consists in patterning the solar cell layers in the optical wavelength scale. * Corresponding author. Tel.: +33 1 69 08 43 17; fax: +33 1 69 08 91 75. E-mail address: [email protected] (L. Rocha). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.017
Due to diffraction effects, it is indeed possible to modify light propagation and optimize the interaction of the excitation with the photoactive material [2]. The effect of periodic structures has been investigated since the 1970s onto silicon solar cells [3– 7] and more recently in organic devices [8,9]. In the last case, a growing interest has been observed in the past decade with the development of patterning methods better adapted to organic materials than lithographic means used in silicon technology. One of the main drawbacks of lithographic techniques is that they require the use of solvents or the exposure to UV that can deteriorate the optical or electronic properties of the active layers. Among those newly developed patterning techniques, we can mention embossing [10], stamping [11] or replica moulding [12], which imprints a master by contact onto the material surface. The dimensions of the resulting structures etched at the surface of the films depend on the master features size. Such techniques have demonstrated the ability to realize structures with sub-50-nm dimensions [13]. The method presented here is an original all optical single step patterning technique in which the solid master, usually fabricated by conventional lithographic means, is replaced by a
518
C. Cocoyer et al. / Thin Solid Films 511 – 512 (2006) 517 – 522
light interference pattern. The patterning process is related to photo induced mass transport in azopolymer films [14]. This technique allows for the realization of surface relief gratings, where period and modulation amplitude can be easily controlled. We show how this technique can be used to pattern photovoltaic solar cells and present the first results towards device efficiency optimization.
CH3
CH3 C
CH2
C
n
CH22
1-n
C
C O
O
O
O
CH3
C H2C H2C N
N
N
NO2
CH3CH2
H
P
BS M
Argon Laser
2. Induction of surface relief gratings on azopolymer films The patterning method is based on a laser-controlled mass transport occurring at room temperature in polymer films containing azo-dye molecules. The irradiation of azo-dye molecules at a wavelength included in the absorption band of the chromophores leads to a molecular motion following repeated cis – trans isomerization cycles [15] (Fig. 1). This molecular process can result in a macroscopic deformation of the film surface depending on the excitation configuration [16,17]. It has been demonstrated that the irradiation of an azopolymer film with an interference pattern results in a mass transport from the high intensity regions to the low intensity regions [18]. The technique allows to control the profile and geometry of the surface deformation by the intensity profile of the exciting beam. The efficiency of the deformation process has been shown to depend strongly on the polarization state of the excitation [19 – 21]. Experimentally, we make use of films that are spin coated onto a glass substrate from a 1,1,2-tricholorethane solution of an azo-dye molecule [4-(N-(2-hydroxyethyl)-N-ethyl-) amino-4V-nitroazobenzene] (DR1) grafted as a side chain to poly(methylmethacrylate) (PMMA) with a 35% molar ratio co(DR1/MMA)(35/65) (Fig. 2). In the present study, we consider one-dimensional gratings realized with an interference pattern resulting from the superposition of two coherent light beams. The irradiation with such an interference pattern, presenting a sinusoidally modulated intensity profile, leads to a sinusoidal relief grating at the polymer film surface. The period of the grating K is equal to the fringes spacing of the interference pattern and is controlled by the incidence angle of trans form
1,8
NO2
O2N h
1,4
Optical Density
cis form
N
N
N
N
h ν , kT
1,0
N
N HO
OH
0,6
0,2 0,0 300
400
500
600
700
800
Wavelength (nm) Fig. 1. Absorption spectrum and trans – cis isomerization of the azo-dye molecule Disperse Red 1 (DR1). The molecule can be excited in the cis form with the absorption of a photon. The relaxation to the trans form can be induced optically or by thermal relaxation.
θ
M
θ
S
Fig. 2. Experimental set-up for the surface relief grating formation: H: half wave plate, P: polarizer, BS: beam splitter, M: mirror, S: sample. In the upper part: azo-dye-based copolymer formula (left) and typical AFM image (right) of an induced surface relief grating. The surface grating presents a period of K = 460 nm and a modulation amplitude of Dh = 90 nm. k the beams onto the film surface: K ¼ 2nsinh , where K is the interference fringes spacing (i.e. the resulting grating period), k the excitation wavelength and n is the index of the medium in which we consider the pump beams incidence angle h. The modulation amplitude for a given period is controlled by the total energy sent to the polymer film. The polarization of the excitation is set perpendicular to the fringes in order to induce efficient mass transport from the illuminated zones to the dark ones [22]. The experimental set-up is schematized in Fig. 2. A typical surface relief grating imaged with an AFM is also represented. The grating is obtained by irradiating the film surface with the 514.5-nm line of an Argon laser at a total intensity of 300 mW cm 2 during a 15-min exposure time. The surface grating presents a period of K = 460 nm and a modulation amplitude of Dh = 90 nm.
3. Implementation of the periodic structures towards photovoltaic applications The cell investigated is based on the interpenetrated network of a conjugated polymer as donor and a fullerene derivative as acceptor materials. The patternable azopolymer film being a dielectric material, it is used as a substrate for the photovoltaic solar cell. Moreover, to avoid an absorption of the incident light by the co(DR1/MMA) substrate, the architecture of the solar cell is chosen in order to allow illumination through the top electrode. As represented in Fig. 3, the solar cell consists of seven layers deposited according to the following procedure. The patterned substrate is first covered with a thermally evaporated 100-nm-thick gold layer followed with the deposition of a 20-nm-thick ITO layer by radio-frequency magnetron sputtering. Then a film of poly(ethylene dioxythiophene) (PEDOT) is spin-coated onto the ITO. The thickness of the PEDOT layer is approximately 60 nm. The photoactive layer is spin coated from a solution of the blend poly[2methoxy-5-(3V, 7V- dimethyloctyloxy)-p-phenylenevinylene] (MDMO-PPV) and [6,6]-phenyl C60 butyric acid methyl ester
C. Cocoyer et al. / Thin Solid Films 511 – 512 (2006) 517 – 522
519
hν LiF/Al/Ag electrode (20nm)
MDMO-PPV : PCBM (100nm)
PEDOT (60nm) Au (100nm)
ITO (20nm) Co(DR1/MMA) (200nm) Glass substrate
Fig. 3. Scheme of a modulated solar cell with an illumination through the top electrode.
4. Results and discussion 4.1. Optical characterizations The effect of the periodic structures on the solar cell absorption was investigated. The planar and modulated solar cells were illuminated with a collimated beam delivered by a tungsten lamp, and the light transmitted by the samples was analyzed by a spectrometer (2.5 nm dispersion/pixel) following collection by a 2.5-cm focal length lens and propagation through an optical fiber (0.6 mm core diameter), as depicted in the insert of Fig. 4. Detection is performed using a thermoelectrically cooled charged coupled device (CCD).
For the experiment, comparable modulation amplitude gratings with periods ranging from 400 nm to 600 nm have been realized. The spectra for two solar cells modulated with periods K = 430 nm and K = 480 nm and excited in TE polarization are shown in Fig. 4. The TE polarization corresponds to a polarization parallel to the grating stripes. The spectrum transmitted by the non-modulated cell is also given. 4.1.1. Light trapping effects in patterned solar cells The spectra of the modulated cells present a localized drop in the transmission occurring at a wavelength depending on the grating period. The transmission drop appears as a ‘‘hole’’ in the spectrum and is centred at a spectral position increasing with increasing grating period. The absence of ‘‘holes’’ in the spectrum of the light transmitted by the planar cell and the spectral dependence of the phenomena with the grating period confirm the action of the periodic structures on the propagation of the incident light. The spectral dependence of the drop in the transmission can be explained according to the grating formula: kd ¼ Fki FpK
ð1Þ
with kd ¼ 2pndksinhd ; ki ¼ 2pniksinhi the projections of the wave vector of the diffracted and incident waves of wavelength k, S
5000 4500
T
4000
Transmission (arb. u.)
(PCBM) in chlorobenzene with a weight ratio of 1:4. The thickness of the resulting photoactive layer is 100 nm. Finally, a three-layer cathode is evaporated on the photoactive layer. This cathode is realized by the successive thermal evaporation of an approximately 1-nm-thick lithium fluoride (LiF), a 5-nmthick aluminum layer and a 15-nm-thick silver layer. Cathode evaporation is performed through a shadow mask with opening diameters of 0.28 cm2 allowing the realization of two cells onto the same substrate for direct comparison between a modulated and a planar cell. In the structure presented, the thin ITO layer is used for the holes extraction, whereas the 100-nm-thick gold film compensates for the quite low conductivity of the thin ITO layer. For the cathode, we chose the widely employed combination of LiF/Al [23,24] as an interfacial layer to allow efficient electron collection, completed by a 15-nm Ag layer to increase the conductivity of the cathode while limiting the drop of the cathode transparency. In this configuration, the cell is irradiated through the semi-transparent cathode. The structure is shown in Fig. 3. The integration of the periodic structures inside the photovoltaic cell is provided by the property of the thermal evaporation deposition technique to transfer the surface state of the patterned co(DR1/MMA) substrate to the evaporated layer surface. AFM measurements of the samples after deposition of the 100-nm-thick gold layer have shown that the co(DR1/MMA) film surface modulation is transferred to the gold surface. More precisely, the gold surface relief grating presents the same period K than the co(DR1/ MMA) substrate with a 10% modulation amplitude attenuation. On the other hand, the spin coating deposition technique tends to smooth the modulations. After deposition of all the solar cell layers, the remaining modulation amplitudes were about 40% of the initial amplitudes.
P
C
L
F
3500 3000 2500
θd
2000 1500 1000 500 0 400
500
600
700
800
900
Wavelength (nm) Fig. 4. Transmission spectra for a cell modulated with a period K = 430 nm (full symbols) and a cell modulated with a period K = 480 nm (empty symbols). The spectrum transmitted by a planar cell is also given (solid line). In the inset: experimental measurement set-up scheme. T: tungsten lamp, P: polarizer, C: solar cell, L: lens, F: fibre, S: spectrometer. The lower part of the figure schematizes the diffraction process: a wave diffracted with an angle h d lying outside the escape cone (dashed lines) is coupled in the cell layers.
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and K ¼ 2p K the grating vector. The integer p corresponds to the diffraction order, n i is the index of the incident medium and n d is the index of the medium where the angle h d of the diffracted wave is considered. If the incident wave is diffracted with an angle h d lying outside the escape cone (see the inset of Fig. 4), the diffracted wave will undergo total internal reflections at the interfaces of the solar cell and will then remain trapped into the layers leading to the decrease of the transmission at the wavelengths guided in the cell layers. In the experimental configuration used here, with an incident wave propagating in a direction normal to the film plane (sinh i = 0), the diffraction angle is given by the following formula: nd sinhd ¼ Fp
k K
In the two cases considered in Fig. 4, the coupling condition can be satisfied only for a diffraction in the first order p = T 1. Indeed for a diffraction in higher orders, in the spectral range considered, the values of the estimated effective indices n eff = n d sinh d are higher than the index of the highest optical index layer involved in the solar cell structure (n ITO å 2). For higher diffraction orders and a TE polarization, any propagating wave can be coupled in the cell structure. For the multilayer used in our experiments, the solar cell supports only the fundamental TE and TM propagating modes. As a consequence, for a given wavelength, only one propagating mode of each polarization can be excited in the solar cell, which is in good agreement with the presence of only one ‘‘hole’’ in the transmission spectra of Fig. 4 for each grating period. Eq. (2) also evidences, for the spectral range investigated here, that an increase of the grating period must be followed by an increase of the coupled wavelength confirming the spectral behaviour observed in Fig. 4. Indeed, a propagating wave of lower wavelength will present a higher effective index due to a better confinement in the layers and to an increase of the optical indices related to the index dispersion when the wavelength draw closer to the absorption band. As well, a
ð2Þ
For a given grating period K, coupling in the waveguide will be observed for a light with wavelength k if the effective index n eff = n d sinh d of the resulting propagation mode at this wavelength k verifies Eq. (2).
θi=0°
4000 3500
θi=8°
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transmission (arb. u.)
transmission (arb. u.)
λ1 λ2
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3000 2500 2000 1500 1000 500
2500 2000 1500 1000 500
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0 300
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λ0
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λ2 λ1
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λ0
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wavelength (nm)
2000 1500 1000 500 0 400
500
600
700
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wavelength (nm) Fig. 5. Effect of the illumination incidence angle on the light trapping in the solar cell. Down left is schematized trapping effect as a function of the illumination angle.
C. Cocoyer et al. / Thin Solid Films 511 – 512 (2006) 517 – 522
propagating wave of higher wavelength will be less confined in the same structure leading to a lower effective index. Then, for the two gratings considered, the coupling condition for the grating of higher period will be satisfied for a higher wavelength. 4.1.2. Effect of the incidence angle of the illumination To confirm the ‘‘trapping’’ effect of the grating, additional measurements were performed. In the following experiment, the transmission spectra were recorded as a function of the incidence angle of the light onto the solar cells. The spectra obtained for three different incidence angles are shown in Fig. 5 in the case of a cell modulated with a period K = 460 nm. The spectra evidence a splitting of the absorbed band resulting in two ‘‘holes’’, where spectral separation increases with the incidence angle. One of the bands shifts towards the lower wavelengths, whereas the other one shifts to the higher wavelengths. Indeed, in the case of an incidence with an angle h i m 0, the projection k i of the incident wave vector on the cell plane differs from 0 resulting in a coupling condition that can be verified for two different waves: kF ¼
ðneff Fsinhi Þ K p
ð4Þ
We show then the existence of two coupled wavelengths for a grating of given period K confirming the behaviour observed: when h i increases, k decreases and k + increases. The phenomenon described by Eq. (4) is roughly illustrated in Fig. 5. It must be noted that a coupling can also occur in the modulated co(DR1/MMA) layer for the transmitted light which has not been trapped inside the solar cell layers. However, for the grating periods investigated here, an estimation of the effective indices in the co(DR1/MMA) film led, according to formula (1), to guided modes propagating at wavelengths higher than the spectral range investigated here (k > 700 nm). Such guided modes are then not observable in the present experimental configuration. 4.2. First electrical characterizations Electrical characterizations have been performed in both planar and modulated solar cells under 1sun (100 mW cm 2, AM1.5) solar simulator illumination in inert atmosphere of N2. The photovoltaic efficiencies that were measured could reach no more than 0.5%, which is low in comparison with the yields published for solar cells based on the same photoactive material. Such a value is not surprising here since the cathode has not been optimized yet for the application investigated here. The realization of efficient solar cells based on such a reversed configuration will need the development of top electrodes presenting a good transparency in the spectral region of the active material absorption. In this aim, the studies led in the case of organic light emitting diodes towards the integration with silicon based driver technology should benefit to photovoltaic devices.
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The first results have not allowed to evidence an effect of the periodic gratings onto the photovoltaic conversion yields. This can be explained by the use of gratings which coupling properties lead to trapped modes at wavelengths that are not in the photoactive MDMO-PPV/PCBM material absorption band. The realization of gratings with lower periods is then required to improve the interaction of the light with the active material at the wavelengths of interest. In order to optimize the solar cell structure and the grating geometry, a modelling of the electromagnetic field in the different layers is also needed. In this respect, rigorous coupled wave analysis [25] has shown to be a suitable computational method for the calculation of the optical near field in micropatterned organic solar cells [26]. 5. Conclusion We have demonstrated the potential of a single step all optical technique for the periodical patterning of organic solar cells in view of the optimization of the photovoltaic conversion efficiency. First experiments have evidenced the possibility to increase the interaction of the incident light with the active material through diffraction effects. Although the periodic structures investigated were not optimized for the photoactive material absorption, optical characterizations have shown that a part of the incident light could be trapped inside the layers of the solar cells. The wavelengths at which the trapping occurs as well as the efficiency of the process can be controlled by the grating parameters. Further improvements of the process will need the calculation of the optical field distribution in the patterned solar cell layers stacking. The experiments were performed in a reverse configuration with an illumination through the cathode. Such a configuration should provide a better control of the diffracted waves in the solar cell structure by improving the confinement of the resulting guided modes inside the organic active layer. Moreover, an inverse configuration presents a potential interest regarding the use of substrates of different conformations and materials such as tiles. In this respect, an important issue will be the optimization of the optical and electrical properties of the top electrode in order to improve its transmission while allowing for efficient charge extraction. References [1] N.S. Sariciftci, D. Braun, C. Zhang, V.I. Srdanov, A.J. Heeger, G. Stucky, F. Wudl, Appl. Phys. Lett. 62 (1993) 585. [2] M.L. Dakss, L. Kuhn, P.F. Heidrick, B.A. Scott, Appl. Phys. Lett. 16 (1970) 523. [3] D. Redfield, Appl. Phys. Lett. 25 (1974) 647. [4] P. Campbell, M.A. Green, J. Appl. Phys. 62 (1987) 243. [5] C. Heine, R.H. Morf, Appl. Opt. 34 (1995) 2476. [6] C. Eisele, C.E. Nebel, M. Stutzmann, J. Appl. Phys. 89 (2001) 7722. [7] N. Senoussaoui, M. Krause, J. Muˆller, E. Bunte, T. Brammer, H. Stiebig, Thin Solid Films 451 – 452 (2004) 397. [8] L.S. Roman, O. Ingana¨s, T. Granlund, T. Nyberg, M. Svensson, M.R. Andersson, J.C. Hummelen, Adv. Mater. 12 (2000) 189. [9] M. Niggemann, M. Glatthaar, A. Gombert, A. Hinsch, V. Wittwer, Thin Solid Films 451 – 452 (2004) 619. [10] S.Y. Chou, P.R. Krauss, P.J. Renstrom, Appl. Phys. Lett. 67 (1995) 3114.
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