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Controlling the morphology of nanofiber-P3HT:PCBM blends for organic bulk heterojunction solar cells
Organic Electronics 10 (2009) 1248–1251
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Controlling the morphology of nanofiber-P3HT:PCBM blends for organic bulk heterojunction solar cells Sabine Bertho a,*, Wibren D. Oosterbaan a, Veerle Vrindts b, Jan D’Haen a, Thomas J. Cleij a, Laurence Lutsen b, Jean Manca a,b, Dirk Vanderzande a,b a b
Hasselt University, Institute for Materials Research, Wetenschapspark 1, B-3590 Diepenbeek, Belgium IMEC vzw, IMOMEC, B-3590 Diepenbeek, Belgium
a r t i c l e
i n f o
Article history: Received 6 May 2009 Received in revised form 24 June 2009 Accepted 26 June 2009 Available online 3 July 2009 PACS: 84.60.Jt 42.70.Jk 81.05.Tp 64.75.Jk 68.37.Lp
a b s t r a c t Within the field of organic bulk heterojunction solar cells, the morphology of the active layer has a key role in obtaining high power conversion efficiencies. P3HT nanofibers, obtained in highly concentrated solutions, are able to give controlled morphologies directly upon deposition. Since the solar cell efficiency of fiber solar cells depends on the fiber content of the casting solution, it is important to control this parameter. Here, we demonstrate an easy way to control the fiber content in the casting solution, i.e. changing the solution temperature. By using solution heating, the overall molecular weight of the polymer in the blend is kept constant, fiber isolation is not needed and the use of solvent mixtures is avoided. The obtained optimal power conversion efficiency is shown to be linked to the morphology of the active layer, which is studied with Transmission Electron Microscopy (TEM). Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Organic solar cells Nanofibers Transmission electron microscopy
1. Introduction P3HT (poly(3-hexylthiophene)) has been a workhorse material for the development of organic solar cells. Its first breakthrough within the field came with the discovery of the beneficial effect of a heat-treatment of solar cells based on a P3HT:PCBM ([6-6]-phenyl C61-butyric acid methyl ester) blend [1–3]. After this, efficiencies have gradually increased towards 5–6% through a control of the nanomorphology of the polymer:PCBM blend [4,5]. Lately, P3HT nanofibers obtained in highly concentrated solutions have been shown to give optimized morphologies directly upon deposition [6–8]. This makes traditional thermal treatment superfluous. The deposition of nanofibers is
* Corresponding author. Tel.: +32 (0) 11 268887; fax: +32 (0) 11 268899. E-mail address: [email protected] (S. Bertho). 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.06.018
therefore a more appropriate method to be used with flexible substrates, such as poly(ethylene terephthalate) (PET), which have a low glass transition temperature. The solar cell efficiency of these fiber solar cells depends on the fiber content of the casting solution. Berson et al. [6] controlled the fiber content by mixing isolated fibers with well-dissolved low molecular weight (MW) P3HT; a ratio of 0.75 fiber to 0.25 well-dissolved polymer gave optimal efficiency. Li et al. [7] used the anti-solvent hexane to control the fiber content of P3HT in o-dichlorobenzene. Moulé et al. [8] added different amounts of nitrobenzene to a solvent mixture of P3HT:PCBM; this caused the formation of aggregated P3HT networks during the drying process of the devices. Here, we show that the fiber content can be easily controlled by solution temperature. In this way, the overall MW of the P3HT in the blend is kept constant. This is important, since it has been shown that MW influences the photovoltaic performance of P3HT:PCBM solar cells
S. Bertho et al. / Organic Electronics 10 (2009) 1248–1251
[9,10]. In this paper we can rule out this influence. Also fiber isolation is not needed and the use of solvent mixtures is avoided, which may allow for easier recycling of processing solvent. We found an optimal efficiency at a 42% fiber content of the casting solution and show that it can be linked to the morphology of the active layer.
2. Experimental 2.1. Preparation of P3HT fibers P3HT was prepared via the Rieke method [11]. The regioregularity was determined at >94.5% and it had a Mn of 23.7 kg mol 1 (polydispersity index of 1.80) as determined by Analytical Size Exclusion Chromatography (SEC). The SEC was performed using a Spectra series P100 (Spectra Physics) pump equipped with two mixed-B columns (10 lm, 2 30 cm, Polymer Labs) and a Refractive Index detector (Shodex) at 60 °C. Chlorobenzene was used as the eluent at a flow rate of 1.0 mL/min. Molecular weight distributions are given relative to polystyrene standards. P3HT fibers were prepared by slow cooling of an orange coloured 0.7 wt% P3HT solution in p-xylene, as described elsewhere [11]. 2.2. Solar cell fabrication After the preparation of the P3HT fibers in the p-xylene solution, PCBM ([6-6]-phenyl C61-butyric acid methyl ester (Solenne)) was added to the solution in a 1:1 weight ratio with P3HT to act as acceptor material. The casting solution was stirred for 48 h at room temperature. The solar cells were prepared starting from indium tin oxide (ITO, 100 nm) coated glass plates that were successively cleaned in a soap solution, demineralised water and acetone, each for 10 min in an ultrasonic bath. This was followed by cleaning in boiling isopropanol for 10 min. A 25 nm thick poly(3,4-ethylenedioxythiophene–polystyrenesulfonate (PEDOT–PSS (Bayer)) layer was spin-coated on the clean glass/ITO substrates. The substrates were dried for 20 min on a hot plate at 120 °C. The active layer (P3HT:PCBM 1:1), was spin-coated at 750 rpm on top of the PEDOT–PSS layer. The solution from which the active layer was spincoated, was heated gradually to several temperatures (between 37 and 50 °C). At each temperature used for the experiment, the solution was left to stabilize for at least 30 min before the active layer was spin-coated on top of the PEDOT–PSS layer. After deposition of the active layer, the devices were left in vacuum (10 6 mbar) to get rid of the remaining solvent. Finally, a calcium layer (25 nm) and an aluminium layer (70 nm) were evaporated to form the top contact of the solar cell. Each photovoltaic device had an active area of 25 mm2. The I–V-characteristics were measured with an Oriel solar simulator equipped with a Xenon Short Arc lamp with a power of 150 W. 2.3. Morphology of the active layer studied with TEM For each solution temperature, the morphology of the active layer was studied. A thin film, identical to the active
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layer of the solar cell, was deposited on a cleaned glass substrate, using the same procedure as for preparing the complete solar cell. This film was isolated by etching with 40% hydrofluoric acid (HF) to remove the glass substrate and then put on a copper TEM-grid. TEM-measurements were performed with a Philips CM12-STEM. The SAED patterns, recorded through TEM, originally suffered from a large background that resembled inelastic scattering. To improve the signal-to-noise ratio, integration across the complete diffraction ring was done, from which a fitted background due to inelastic scattering was subtracted (under the assumption that the blend in between the fibers is quasi-amorphous [12]). 2.4. Determination of the fiber content in the solution At each solution temperature used for preparing a photovoltaic device, a small amount (<50 lL) of the casting solution was isolated just after spin-coating and diluted in 3 mL of p-xylene of the same temperature. UV–Vis measurements (with a Varian Cary 500 Scan UV–Vis–NIR spectrophotometer) were performed on this highly diluted solution after cooling to room temperature. The obtained UV–Vis spectra were fitted as a sum of previously recorded spectra of PCBM, well-dissolved P3HT and fully fibrillar P3HT, all in p-xylene. The known relation between the latter two spectra allowed for the determination of the fiber content [11].
3. Results and discussion Fig. 1 shows the photovoltaic (PV) parameters of solar cells with a nanofiber-P3HT:PCBM 1:1 active layer as a function of casting solution temperature together with the fiber content of the solution at each temperature. The graph shows that the fiber content of the solution is very sensitive to temperature. When raising the temperature from 37 to 50 °C, the fiber content of the solution decreases from 60% to 14%. The solar cell efficiency displays a quadratic dependence on temperature. An optimal efficiency of 3.2% is found for a solution temperature of 45 °C. At this temperature, the fiber content was 42%. When comparing the different PV-parameters, it seems that the behavior of the efficiency is strongly related to the behavior of the short circuit current Jsc. The open circuit voltage Voc of the solar cells has an increasing trend. The fill factor follows a rather rough course. The origin of the dependence of the photovoltaic performance on the temperature of the casting solution can be found in the active layer morphology. Fig. 2 shows Bright Field Transmission Electron Microscopy (BFTEM) images of the active layers. The active layer spin-coated from a solution at a relatively low temperature (37 °C) shows large (up to 50 lm) needles (Fig. 2(a, left)). Spots in the Selected Area Electron Diffraction (SAED) patterns of the needles confirm that they are groups of PCBM single crystals [13]. In between the needles, fibers of P3HT are visible (Fig. 2(a, right)). They appear dark against a light background. SAED also gives information about this part of the active layer. In Fig. 3, the residual intensities of the
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S. Bertho et al. / Organic Electronics 10 (2009) 1248–1251
Fig. 1. Fiber content of the casting solution, efficiency g, short circuit current Jsc, open circuit voltage Voc and fill factor of nanofiber-P3HT:PCBM (1:1) solar cells as a function of solution temperature.
SAED patterns of the homogeneous areas of the active layers are shown (the area in between the PCBM crystals). These curves were obtained after integration across the diffraction ring followed by subtraction of a fitted background due to inelastic scattering (cf. Experimental). For the film spin-coated at 37 °C, the residual intensity SAED pattern shows a clear peak at 0.38 nm (Fig. 3). This peak can be attributed to the superposition of the (0 0 2) and (0 2 0) reflections from the crystal planes of semi-crystalline P3HT [14]. The remainder of the SAED pattern appears featureless and no peaks corresponding to PCBM can be distinguished. This indicates that the major part of PCBM is assembled in the needles. By slightly increasing the solution temperature to 40 °C, a change in morphology is immediately observed. The PCBM needles have grown larger (Fig. 2b). We believe that the PCBM needles were formed in the casting solution during the long (>48 h) stirring at room temperature and elevated temperatures and not during spin-coating of the active layer. The spin-coating takes <120 s, which does not leave much time for the formation of such large needles. The long time available during stirring together with the perfect crystal shape and dependence of the crystal size on the temperature of the casting solution, all indicate that the crystals were formed during stirring of the casting solution by an Ost-
Fig. 2. Bright Field TEM images of P3HT:PCBM 1:1 spin-coated from a solution at 37.1 °C (a), 40.5 °C (b), 41.9 °C (c), 44.9 °C (d) and 49.9 °C (e), at low temperatures PCBM assembles in needle-like structures, P3HT fibers appear as dark or light areas depending on the presence of PCBM within the polymer matrix. The insets show the SAED patterns of either the PCBM needles (left) or the homogeneous P3HT:PCBM regions (right).
Fig. 3. Residual intensities of the SAED patterns of the polymer:PCBM matrix for several solution temperatures.
wald ripening process [15]. Higher temperatures accelerate the Ostwald ripening; therefore less but larger PCBM needles are visible at the solution temperature of 40 °C. These needles have grown at the expense of the needles
S. Bertho et al. / Organic Electronics 10 (2009) 1248–1251
that were present in solution at 37 °C. For the solution temperature of 40 °C, a small increase in the diffraction intensity at 0.46 nm can be seen in the SAED pattern of the area in between the needles (Fig. 3). This small peak indicates that some amount of nano-crystalline PCBM [16] is now present in between the fibers. Apparently a small part of the PCBM has dissolved in the p-xylene at this temperature; the PCBM is no longer completely grouped in the needles. This gives a higher yield of exciton dissociation at the polymer:PCBM interface and thus a higher Jsc and better solar cell performance (cf. Fig. 1). As opposed to the film made from a solution at 37 °C, the P3HT fibers are now visible as light areas in the BFTEM image (Fig. 2). This is caused by the presence of PCBM with a higher density in between the polymer matrix [2]. A further increase of the solution temperature leads to the disappearance of the PCBM needles (Fig. 2). For solution temperatures of 42 °C, 45 °C (Fig. 3) and 48 °C (not shown) strong peaks corresponding to both P3HT (at 0.38 nm) and PCBM (at 0.2, 0.31 and 0.46 nm [16]) are present. These temperatures also correspond to the best photovoltaic performances (Fig. 1). Good charge transport due to crystalline P3HT, in combination with a large interfacial area between polymer and PCBM render good solar cells. At a solution temperature of 50 °C, there is a slight drop in solar cell efficiency. At this point the fiber content has decreased strongly, which will likely result in less efficient hole transport. 4. Conclusion We have demonstrated that the fiber content of the P3HT-fiber:PCBM casting solution can be easily controlled by temperature. Meanwhile, the dissolution of PCBM varies with temperature. The combined effect can be used to control the active layer morphology in photovoltaic devices and thus also their performance. An optimal solar cell efficiency was found at a solution temperature of 45 °C when the fiber content was 42%. BFTEM images of the active layer morphology combined with SAED patterns revealed that this optimal photovoltaic efficiency occurred when the PCBM was dissolved completely at the highest possible fiber content. The initial increase in efficiency with temperature of the casting solution could be attributed to an increased solubility of PCBM in the casting solu-
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tion. At temperatures lower than 41 °C, PCBM formed large (>50 lm) needles in the casting solution by Ostwald ripening, resulting in poorly mixed active layers and low solar cell efficiencies. At temperatures of 45 °C and higher, the PCBM was dissolved well and, after reaching its maximum, the solar cell efficiency started to decrease again with decreasing fiber content (due to the increasing solution temperature). Acknowledgements The research was carried out in the framework of the SBO-project 030220 ‘‘PolySpec”, the FWO-project R-1226 and the interregional project OLED+. The work, as part of the project ‘‘SOHYDs” within the European Science Foundation EUROCORES Programme was also supported from funds by the FWO (G.0685.06) and the EC Sixth Framework Programme, under Contract No. ERAS-CT-2003-980409. Sabine Bertho is research assistant of the Fund for Scientific Research, Flanders (Belgium) (FWO). References [1] F. Padinger, R.S. Rittberger, N.S. Sariciftci, Adv. Funct. Mater. 13 (2003) 85. [2] X. Yang, J. Loos, S.C. Veenstra, W.J.H. Verhees, M.M. Wien, J.M. Kroon, M.A.J. Michels, R.A.J. Janssen, Nano Lett. 5 (2005) 579. [3] Y. Kim, S.A. Choulis, J. Nelson, D.D.C. Bradley, Appl. Phys. Lett. 86 (2005) 063502. [4] X. Yang, G. Lu, L. Li, E. Zhou, Small 3 (2007) 611. [5] W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Adv. Funct. Mater. 15 (2005) 1617. [6] S. Berson, R. De Bettignies, S. Bailly, S. Guillerez, Adv. Funct. Mater. 17 (2007) 1377. [7] L. Li, G. Lu, X. Yang, J. Mater. Chem. 18 (2008) 1984. [8] A.J. Moulé, K. Meerholz, Adv. Mater. 20 (2008) 240. [9] W. Ma, J.Y. Kim, K. Lee, A.J. Heeger, Macromol. Rapid Commun. 28 (2007) 1776. [10] P. Schilinsky, U. Asawapirom, U. Scherf, M. Biele, C.J. Brabec, Chem. Mater. 17 (2005) 2175. [11] W.D. Oosterbaan, V. Vrindts, S. Berson, S. Guillerez, O. Douhéret, B. Ruttens, J. D’Haen, P. Adriaensens, J. Manca, L. Lutsen, D. Vanderzande, J. Mater. Chem., in press. doi:10.1039/b900670b. [12] L. Reimer, Transmission Electron Microscopy, Springer-Verlag, Berlin, 1989. [13] M.T. Rispens, A. Meetsma, R. Rittberger, C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Chem. Commun. (2003) 2116. [14] S. Hugger, R. Thomann, T. Heinzel, T. Thurn-Albrecht, Colloid Polym. Sci. 282 (2004) 932. [15] R. Boistelle, J.P. Astier, J. Cryst. Growth 90 (1988) 14. [16] X. Yang, J.K.J. van Duren, M.T. Rispens, J.C. Hummelen, R.A.J. Janssen, M.A.J. Michels, J. Loos, Adv. Mater. 16 (2004) 802.
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Controlling the morphology of nanofiber-P3HT:PCBM blends for organic bulk heterojunction solar cells Sabine Bertho a,*, Wibren D. Oosterbaan a, Veerle Vrindts b, Jan D’Haen a, Thomas J. Cleij a, Laurence Lutsen b, Jean Manca a,b, Dirk Vanderzande a,b a b
Hasselt University, Institute for Materials Research, Wetenschapspark 1, B-3590 Diepenbeek, Belgium IMEC vzw, IMOMEC, B-3590 Diepenbeek, Belgium
a r t i c l e
i n f o
Article history: Received 6 May 2009 Received in revised form 24 June 2009 Accepted 26 June 2009 Available online 3 July 2009 PACS: 84.60.Jt 42.70.Jk 81.05.Tp 64.75.Jk 68.37.Lp
a b s t r a c t Within the field of organic bulk heterojunction solar cells, the morphology of the active layer has a key role in obtaining high power conversion efficiencies. P3HT nanofibers, obtained in highly concentrated solutions, are able to give controlled morphologies directly upon deposition. Since the solar cell efficiency of fiber solar cells depends on the fiber content of the casting solution, it is important to control this parameter. Here, we demonstrate an easy way to control the fiber content in the casting solution, i.e. changing the solution temperature. By using solution heating, the overall molecular weight of the polymer in the blend is kept constant, fiber isolation is not needed and the use of solvent mixtures is avoided. The obtained optimal power conversion efficiency is shown to be linked to the morphology of the active layer, which is studied with Transmission Electron Microscopy (TEM). Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Organic solar cells Nanofibers Transmission electron microscopy
1. Introduction P3HT (poly(3-hexylthiophene)) has been a workhorse material for the development of organic solar cells. Its first breakthrough within the field came with the discovery of the beneficial effect of a heat-treatment of solar cells based on a P3HT:PCBM ([6-6]-phenyl C61-butyric acid methyl ester) blend [1–3]. After this, efficiencies have gradually increased towards 5–6% through a control of the nanomorphology of the polymer:PCBM blend [4,5]. Lately, P3HT nanofibers obtained in highly concentrated solutions have been shown to give optimized morphologies directly upon deposition [6–8]. This makes traditional thermal treatment superfluous. The deposition of nanofibers is
* Corresponding author. Tel.: +32 (0) 11 268887; fax: +32 (0) 11 268899. E-mail address: [email protected] (S. Bertho). 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.06.018
therefore a more appropriate method to be used with flexible substrates, such as poly(ethylene terephthalate) (PET), which have a low glass transition temperature. The solar cell efficiency of these fiber solar cells depends on the fiber content of the casting solution. Berson et al. [6] controlled the fiber content by mixing isolated fibers with well-dissolved low molecular weight (MW) P3HT; a ratio of 0.75 fiber to 0.25 well-dissolved polymer gave optimal efficiency. Li et al. [7] used the anti-solvent hexane to control the fiber content of P3HT in o-dichlorobenzene. Moulé et al. [8] added different amounts of nitrobenzene to a solvent mixture of P3HT:PCBM; this caused the formation of aggregated P3HT networks during the drying process of the devices. Here, we show that the fiber content can be easily controlled by solution temperature. In this way, the overall MW of the P3HT in the blend is kept constant. This is important, since it has been shown that MW influences the photovoltaic performance of P3HT:PCBM solar cells
S. Bertho et al. / Organic Electronics 10 (2009) 1248–1251
[9,10]. In this paper we can rule out this influence. Also fiber isolation is not needed and the use of solvent mixtures is avoided, which may allow for easier recycling of processing solvent. We found an optimal efficiency at a 42% fiber content of the casting solution and show that it can be linked to the morphology of the active layer.
2. Experimental 2.1. Preparation of P3HT fibers P3HT was prepared via the Rieke method [11]. The regioregularity was determined at >94.5% and it had a Mn of 23.7 kg mol 1 (polydispersity index of 1.80) as determined by Analytical Size Exclusion Chromatography (SEC). The SEC was performed using a Spectra series P100 (Spectra Physics) pump equipped with two mixed-B columns (10 lm, 2 30 cm, Polymer Labs) and a Refractive Index detector (Shodex) at 60 °C. Chlorobenzene was used as the eluent at a flow rate of 1.0 mL/min. Molecular weight distributions are given relative to polystyrene standards. P3HT fibers were prepared by slow cooling of an orange coloured 0.7 wt% P3HT solution in p-xylene, as described elsewhere [11]. 2.2. Solar cell fabrication After the preparation of the P3HT fibers in the p-xylene solution, PCBM ([6-6]-phenyl C61-butyric acid methyl ester (Solenne)) was added to the solution in a 1:1 weight ratio with P3HT to act as acceptor material. The casting solution was stirred for 48 h at room temperature. The solar cells were prepared starting from indium tin oxide (ITO, 100 nm) coated glass plates that were successively cleaned in a soap solution, demineralised water and acetone, each for 10 min in an ultrasonic bath. This was followed by cleaning in boiling isopropanol for 10 min. A 25 nm thick poly(3,4-ethylenedioxythiophene–polystyrenesulfonate (PEDOT–PSS (Bayer)) layer was spin-coated on the clean glass/ITO substrates. The substrates were dried for 20 min on a hot plate at 120 °C. The active layer (P3HT:PCBM 1:1), was spin-coated at 750 rpm on top of the PEDOT–PSS layer. The solution from which the active layer was spincoated, was heated gradually to several temperatures (between 37 and 50 °C). At each temperature used for the experiment, the solution was left to stabilize for at least 30 min before the active layer was spin-coated on top of the PEDOT–PSS layer. After deposition of the active layer, the devices were left in vacuum (10 6 mbar) to get rid of the remaining solvent. Finally, a calcium layer (25 nm) and an aluminium layer (70 nm) were evaporated to form the top contact of the solar cell. Each photovoltaic device had an active area of 25 mm2. The I–V-characteristics were measured with an Oriel solar simulator equipped with a Xenon Short Arc lamp with a power of 150 W. 2.3. Morphology of the active layer studied with TEM For each solution temperature, the morphology of the active layer was studied. A thin film, identical to the active
1249
layer of the solar cell, was deposited on a cleaned glass substrate, using the same procedure as for preparing the complete solar cell. This film was isolated by etching with 40% hydrofluoric acid (HF) to remove the glass substrate and then put on a copper TEM-grid. TEM-measurements were performed with a Philips CM12-STEM. The SAED patterns, recorded through TEM, originally suffered from a large background that resembled inelastic scattering. To improve the signal-to-noise ratio, integration across the complete diffraction ring was done, from which a fitted background due to inelastic scattering was subtracted (under the assumption that the blend in between the fibers is quasi-amorphous [12]). 2.4. Determination of the fiber content in the solution At each solution temperature used for preparing a photovoltaic device, a small amount (<50 lL) of the casting solution was isolated just after spin-coating and diluted in 3 mL of p-xylene of the same temperature. UV–Vis measurements (with a Varian Cary 500 Scan UV–Vis–NIR spectrophotometer) were performed on this highly diluted solution after cooling to room temperature. The obtained UV–Vis spectra were fitted as a sum of previously recorded spectra of PCBM, well-dissolved P3HT and fully fibrillar P3HT, all in p-xylene. The known relation between the latter two spectra allowed for the determination of the fiber content [11].
3. Results and discussion Fig. 1 shows the photovoltaic (PV) parameters of solar cells with a nanofiber-P3HT:PCBM 1:1 active layer as a function of casting solution temperature together with the fiber content of the solution at each temperature. The graph shows that the fiber content of the solution is very sensitive to temperature. When raising the temperature from 37 to 50 °C, the fiber content of the solution decreases from 60% to 14%. The solar cell efficiency displays a quadratic dependence on temperature. An optimal efficiency of 3.2% is found for a solution temperature of 45 °C. At this temperature, the fiber content was 42%. When comparing the different PV-parameters, it seems that the behavior of the efficiency is strongly related to the behavior of the short circuit current Jsc. The open circuit voltage Voc of the solar cells has an increasing trend. The fill factor follows a rather rough course. The origin of the dependence of the photovoltaic performance on the temperature of the casting solution can be found in the active layer morphology. Fig. 2 shows Bright Field Transmission Electron Microscopy (BFTEM) images of the active layers. The active layer spin-coated from a solution at a relatively low temperature (37 °C) shows large (up to 50 lm) needles (Fig. 2(a, left)). Spots in the Selected Area Electron Diffraction (SAED) patterns of the needles confirm that they are groups of PCBM single crystals [13]. In between the needles, fibers of P3HT are visible (Fig. 2(a, right)). They appear dark against a light background. SAED also gives information about this part of the active layer. In Fig. 3, the residual intensities of the
1250
S. Bertho et al. / Organic Electronics 10 (2009) 1248–1251
Fig. 1. Fiber content of the casting solution, efficiency g, short circuit current Jsc, open circuit voltage Voc and fill factor of nanofiber-P3HT:PCBM (1:1) solar cells as a function of solution temperature.
SAED patterns of the homogeneous areas of the active layers are shown (the area in between the PCBM crystals). These curves were obtained after integration across the diffraction ring followed by subtraction of a fitted background due to inelastic scattering (cf. Experimental). For the film spin-coated at 37 °C, the residual intensity SAED pattern shows a clear peak at 0.38 nm (Fig. 3). This peak can be attributed to the superposition of the (0 0 2) and (0 2 0) reflections from the crystal planes of semi-crystalline P3HT [14]. The remainder of the SAED pattern appears featureless and no peaks corresponding to PCBM can be distinguished. This indicates that the major part of PCBM is assembled in the needles. By slightly increasing the solution temperature to 40 °C, a change in morphology is immediately observed. The PCBM needles have grown larger (Fig. 2b). We believe that the PCBM needles were formed in the casting solution during the long (>48 h) stirring at room temperature and elevated temperatures and not during spin-coating of the active layer. The spin-coating takes <120 s, which does not leave much time for the formation of such large needles. The long time available during stirring together with the perfect crystal shape and dependence of the crystal size on the temperature of the casting solution, all indicate that the crystals were formed during stirring of the casting solution by an Ost-
Fig. 2. Bright Field TEM images of P3HT:PCBM 1:1 spin-coated from a solution at 37.1 °C (a), 40.5 °C (b), 41.9 °C (c), 44.9 °C (d) and 49.9 °C (e), at low temperatures PCBM assembles in needle-like structures, P3HT fibers appear as dark or light areas depending on the presence of PCBM within the polymer matrix. The insets show the SAED patterns of either the PCBM needles (left) or the homogeneous P3HT:PCBM regions (right).
Fig. 3. Residual intensities of the SAED patterns of the polymer:PCBM matrix for several solution temperatures.
wald ripening process [15]. Higher temperatures accelerate the Ostwald ripening; therefore less but larger PCBM needles are visible at the solution temperature of 40 °C. These needles have grown at the expense of the needles
S. Bertho et al. / Organic Electronics 10 (2009) 1248–1251
that were present in solution at 37 °C. For the solution temperature of 40 °C, a small increase in the diffraction intensity at 0.46 nm can be seen in the SAED pattern of the area in between the needles (Fig. 3). This small peak indicates that some amount of nano-crystalline PCBM [16] is now present in between the fibers. Apparently a small part of the PCBM has dissolved in the p-xylene at this temperature; the PCBM is no longer completely grouped in the needles. This gives a higher yield of exciton dissociation at the polymer:PCBM interface and thus a higher Jsc and better solar cell performance (cf. Fig. 1). As opposed to the film made from a solution at 37 °C, the P3HT fibers are now visible as light areas in the BFTEM image (Fig. 2). This is caused by the presence of PCBM with a higher density in between the polymer matrix [2]. A further increase of the solution temperature leads to the disappearance of the PCBM needles (Fig. 2). For solution temperatures of 42 °C, 45 °C (Fig. 3) and 48 °C (not shown) strong peaks corresponding to both P3HT (at 0.38 nm) and PCBM (at 0.2, 0.31 and 0.46 nm [16]) are present. These temperatures also correspond to the best photovoltaic performances (Fig. 1). Good charge transport due to crystalline P3HT, in combination with a large interfacial area between polymer and PCBM render good solar cells. At a solution temperature of 50 °C, there is a slight drop in solar cell efficiency. At this point the fiber content has decreased strongly, which will likely result in less efficient hole transport. 4. Conclusion We have demonstrated that the fiber content of the P3HT-fiber:PCBM casting solution can be easily controlled by temperature. Meanwhile, the dissolution of PCBM varies with temperature. The combined effect can be used to control the active layer morphology in photovoltaic devices and thus also their performance. An optimal solar cell efficiency was found at a solution temperature of 45 °C when the fiber content was 42%. BFTEM images of the active layer morphology combined with SAED patterns revealed that this optimal photovoltaic efficiency occurred when the PCBM was dissolved completely at the highest possible fiber content. The initial increase in efficiency with temperature of the casting solution could be attributed to an increased solubility of PCBM in the casting solu-
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tion. At temperatures lower than 41 °C, PCBM formed large (>50 lm) needles in the casting solution by Ostwald ripening, resulting in poorly mixed active layers and low solar cell efficiencies. At temperatures of 45 °C and higher, the PCBM was dissolved well and, after reaching its maximum, the solar cell efficiency started to decrease again with decreasing fiber content (due to the increasing solution temperature). Acknowledgements The research was carried out in the framework of the SBO-project 030220 ‘‘PolySpec”, the FWO-project R-1226 and the interregional project OLED+. The work, as part of the project ‘‘SOHYDs” within the European Science Foundation EUROCORES Programme was also supported from funds by the FWO (G.0685.06) and the EC Sixth Framework Programme, under Contract No. ERAS-CT-2003-980409. Sabine Bertho is research assistant of the Fund for Scientific Research, Flanders (Belgium) (FWO). References [1] F. Padinger, R.S. Rittberger, N.S. Sariciftci, Adv. Funct. Mater. 13 (2003) 85. [2] X. Yang, J. Loos, S.C. Veenstra, W.J.H. Verhees, M.M. Wien, J.M. Kroon, M.A.J. Michels, R.A.J. Janssen, Nano Lett. 5 (2005) 579. [3] Y. Kim, S.A. Choulis, J. Nelson, D.D.C. Bradley, Appl. Phys. Lett. 86 (2005) 063502. [4] X. Yang, G. Lu, L. Li, E. Zhou, Small 3 (2007) 611. [5] W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Adv. Funct. Mater. 15 (2005) 1617. [6] S. Berson, R. De Bettignies, S. Bailly, S. Guillerez, Adv. Funct. Mater. 17 (2007) 1377. [7] L. Li, G. Lu, X. Yang, J. Mater. Chem. 18 (2008) 1984. [8] A.J. Moulé, K. Meerholz, Adv. Mater. 20 (2008) 240. [9] W. Ma, J.Y. Kim, K. Lee, A.J. Heeger, Macromol. Rapid Commun. 28 (2007) 1776. [10] P. Schilinsky, U. Asawapirom, U. Scherf, M. Biele, C.J. Brabec, Chem. Mater. 17 (2005) 2175. [11] W.D. Oosterbaan, V. Vrindts, S. Berson, S. Guillerez, O. Douhéret, B. Ruttens, J. D’Haen, P. Adriaensens, J. Manca, L. Lutsen, D. Vanderzande, J. Mater. Chem., in press. doi:10.1039/b900670b. [12] L. Reimer, Transmission Electron Microscopy, Springer-Verlag, Berlin, 1989. [13] M.T. Rispens, A. Meetsma, R. Rittberger, C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Chem. Commun. (2003) 2116. [14] S. Hugger, R. Thomann, T. Heinzel, T. Thurn-Albrecht, Colloid Polym. Sci. 282 (2004) 932. [15] R. Boistelle, J.P. Astier, J. Cryst. Growth 90 (1988) 14. [16] X. Yang, J.K.J. van Duren, M.T. Rispens, J.C. Hummelen, R.A.J. Janssen, M.A.J. Michels, J. Loos, Adv. Mater. 16 (2004) 802.