Facultad de Ciencias Exactas Y Naturales

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1 Revista NOOS Volumen 1 (2013) Pág Derechos Reservados Facultad de Ciencias Exactas Y Naturales EFFECT OF ANNEALING AND SOLVENT TYPE ON THE PROPERTIES OF THIN FILMS OF P3HT:PCBM BLENDS THIN FILMS USED AS ACTIVE LAYER IN ORGANIC SOLAR CELLS C.A. Otálora a, A. Loaiza a, E. Romero a, C. Vargas b and G. Gordillo c a, c Universidad Nacional de Colombia, Sede Bogotá, Departamento de Química, Cra 30 # b Universidad Nacional de Colombia Sede Manizales, Departamento de Física, Cra 27 # Article history: Received: 16 Abril 2013 Received in revised: 23 Abril 2013 Accepted: 01 Mayo 2013 Available online: 05 Mayo 2013 Keywords: P3HT:PCBM blend, solvent, optical and structural properties,organic solar cells ABSTRACT: This study evaluates the effect of annealing and solvent type on the optical and structural properties of thin films of P3HT: PCBM blends grown on alkaline glass substrates by spin coating. The properties were evaluat-ed through absorbance, XRD (x-ray diffraction), Raman spectroscopy and fluorescence microscopy measure-ments. The study allowed finding optimal solvent and annealing conditions to prepare PCBM:P3HT blends with adequate properties to be used as active layer in organic solar cells fabricated with bulk heterojunction type structure. Samples prepares under these conditions present high degree of crystollagraphic ordering, high absorbance in the visible region, low fluorescence and proven formation of PCBM/P3HT heterointerfac-es. It was also observed that the P3HT:PCBM films prepared using mesitylene as solvent present better spectral absorbance than those prepared using chlorobenzene, toluene and o-xylene; however the P3HT:PCBM mix-ture does not dissolve well in mesitylene, which gives raise to thin films with surface clusters. In an attempt to improve the solubility of the P3HT:PCBM blends without degrading the spectral absorbance, P3HT:PCBM blends layers were prepared using mixtures of mesitylene and chlorobenzene in various proportions. The results revealed than samples prepared using a mixture of mesitylene and chlorobenzene in a proportion of 80:20 results in the formation of cluster free layers with good homogeneity, crystalliny and spectral absorb-ance.

2 Revista NOOS Volumen 1 (2013) Pág. 57 RESUMEN: Este estudio evalúa el efecto del recocido y el tipo de solvente usado, sobre las propiedades ópticas y es-tructurales de películas delgasdas de mezcla P3HT:PCBM elaboradas sobre sustrato de vidrio alcalino mediante spin coating. Las propiedades fueron evaluadas mediante absorbancia, XRD (difracción de rayos X), espectroscopía Raman y microscopía de fluorescencia. Este estudio permitió encontrar el solvente y condiciones de recocido optimos para preparar mezcla P3HT:PCBM con adecuadas propiedades para ser usadas como capa activa en celdas solares orgánicas con estructura tipo bulk heterojunction. Las muestra preparadas bajo estas condiciones presentan alto ordenamiento cristalográfico, alta absorbancia en la región visible, baja fluorescencia y formación comprobada de heterointerfases P3HT:PCBM. También fue observado que las películas de P3HT:PCBM preparadas empleando mesitileno como solven-te, presentan mejor absorbancia espectral que las preparadas usando clorobenceno, tolueno y o-xileno; sin embargo la mezcla P3HT:PCBM no disuelve bien en mesitileno, lo que lleva a la formación de películas con aglomeración superficial. en busca de mejorar la solubilidad de la mezcla P3HT;:PCBM sin desmejorar las propiedades de absorbancia, se prepararon las películas usando mezcla de mesitileno y clorobenceno en varias proporciones. Estos resultados revelaron que las muestra preparadas usando una mezcla mesiti-leno y clorobenceno en proporción 80:20 forman dominios libres con buena homogeneidad, cristalinidad y absorbancia espectral. PALABRAS CLAVE: mezcla P3HT:PCBM, solventes, propiedades ópticas y estructuralesceldas solares orgánicas. 1. Introduction Organic solar cells (OSC) have emerged as a promising cost-effective alternative to siliconbased solar cells. Some of the important advantages of these cells include ease of processing, mechanical flexibility, and low cost of fabrication. A variety of solar cells with different combinations of conjugated polymers and electron acceptors has been studied; power conversion efficiencies greater than 7% has been achieved [1,2]. This efficiency is too low for practical use at present, however the device efficiency can be further increased optimizing material properties and cell architecture. In particular, the efficiency of solar cells based on P3HTand PCBM can be improved by optimizing the processing conditions, for example, solvent annealing [3] and thermal annealing [4 6]. For efficient devices, the most important parameter is the nanoscale morphology which can be controlled using appropriate solvent, layer thickness and deposition method [7,8,9]. Mixture solvent approach is a promising method to modify solar cell morphology and improve device performance. Zhang et al. found a significant enhancement in photocurrent density in polyfluorene compolymer/fullerene blends when introducing a small amount of chlorobenzene into chloroform solvent [10] and Peet et al. reported that adding alkanethiol to P3HT/PCBM in toluene can enhance device performance due to longer carrier lifetime with ordered structure in morphology [11]. Alkanethiol is also found effective to improve efficiency of low band gap polymer solar cells[12]. Chlorobenzene is the solvent most used to date for the preparation of thin films of P3HT: PCBM used in the manufacture of organic solar cells. This article aims to study the effect on optical and structural properties of thin films of P3HT: PCBM deposited by spin coating using solvents such as chlorobenzene, toluene, o-xylene, mesitylene and mixtures of

3 Revista NOOS Volumen 1 (2013) Pág. 58 solvent may lead to a significant improvement in the efficiency of the device. 2. Experimental i) ii) Fig 1: Absorption spectra of P3HT:PCBM composite thin films prepared using chlorobenzene as solvent. i) Samples annealed at: a) 80 C, b) 100 C, c) 120 C, d)140 C and e) 160 C during 30 minutes. ii) Samples annealed at 120 C during: b) 9 min., c) 15 min., c) 30min., d) 45 min., a) non annealed. these. The results were compared with those obtained using chlorobenzene as solvent. These studies were performed from UV/VIS/NIR spectroscopy, XRD, fluorescence microscopy and Raman spectroscopy measurements. The mixture of mesitylene: chlorobenzene 80:20 (v / v), used as solvent for the deposition of films of P3HT: PCBM gave rise to optical and structural properties significantly better compared to those obtained using chlorobenzene, indicating that use of this The P3HT/PCBM thin films were deposited by spin coating using a solution containing regioregular P3HT (Lumtec Corp) dissolved in different solvents ( chlorobenzene, toluene, o- xylene, mesitylene and mixtures of these, stirred at 65 C for 2 hour) and PCBM-61 (Lumtec Corp) in 1:1 weight ratio. Prior to deposition, the blend was stirred for 48 hour at room temperature. After deposition, the samples were annealed in nitrogen at temperatures ranging from 80 to 160 C, for periods of time between 0 and 45 minutes. The absorbance measurements were done using a Varian Cary 5000 spectrophotometer and the film thickness was determined using a Veeco Dektak 150 surface profiler. Further characterization involved fluorescence microscopy on a NIKON C1-Plus modell TI- Eclipsey microscope, X-ray diffraction on a Shimadzu-6000 diffractometer and Raman spectroscopy on a Horiba Jobin Yvon micro- Raman Spectrometer LabRamHR in backscattering configuration with a DPSS laser of 473 nm, 20 mw focused with a 50X objective. 3. Results 3.1. Optical Properties Fig.1 shows the absorbance spectra of P3HT: PCBM (1:1) composite thin films prepared using chlorobenzene as solvent, which were subsequently annealed at temperatures ranging from 80 to 160 C for different times. These results reveals that the P3HT/PCBM films exhibit an absorption band between 300 and 350 nm associated to the PCBM and a second absorption band between 400 and 650 nm associated to the P3HT. Annealing induces in general an increase in absorbance in the visible region, while the magnitude of the UV

4 Revista NOOS Volumen 1 (2013) Pág. 59 absorption band remains almost constant. At low annealing temperatures (below 90 C) no significant changes in the absorbance curves are observed, indicating that at low annealing temperatures no phase separation occurs; however, when the annealing temperature rises to values greater than 100 C, the intensity of the absorbance in the visible region increases significantly. This behavior can be attributed to phase separation, which apparently starts at temperatures around 100 C and to improving of molecular ordering in the chains of P3HT. It is also observed that an increase in the annealing time leads to an increase in absorbance in the visible and a reduction of the absorbance in the UV. Increase of both, annealing time and annealing temperature gives raise to a decrease of the absorbance in the UV. This result has been attributed to the formation of crystalline fibers of P3HT, that grow strongly after thermal annealing for long periods, reaching lengths of up to a few micrometers [13] Fig.2: Absorption spectra of P3HT:PCBM composite thin films deposited by spin coating using O-xilene, mesitylene, chlorobenzene, toluene and mixture of mesitylene and chlorobenzene (80:20 v/v) as solvents. Samples annealed at 120 C, during 30 min. In Fig. 2 curves of spectral absorbance of PCBM:P3HT (1:1) blends layers prepared using o-xilene, toluene, mesitylene, chlorobenzene and mixtures of mesitylene and chlorobenzene as solvents are displayed. It is observed that the P3HT:PCBM films prepared using mesitylene as solvent has better absorbance than those prepared using the other solvents types; however the P3HT:PCBM blend does not dissolve well in mesitylene, which gives rise to the formation of surface closter. In an attempt to improve the solubility of the P3HT:PCBM blends without degrading the spectral absorbance, P3HT:PCBM blends layers were prepared using mixtures of mesitylene with chlorobenzene and toluene in various proportions. The results revealed than samples prepared using a mixture of mesitylene and chlorobenzene in a proportion of 80:20 (v/v) results in the formation of cluster free layers with good homogeneity, adherence and spectral absorbance. In the region between 300 and 400nm, where the absorption of PCBM occurs no significant changes are observed when using different solvents, while in the visible region which presents absorption of P3HT, the effect of solvents on the absorbance is significantly. In the visible region of the absorbance spectra where the absorption of P3HT occurs, three peaks around 490, 512 and 560 nm due to - * intrachain transitions are observed; a fourth peak around 600 nm apparently due to transitions between interchain states is also observed. The intensity of this peak has been associated with the molecular arrangement of the polymer since this would induce an increase in interchain interactions, which results in an increased density of electronic states of interchain nature [14]. The results shown in Fig. 2 show that use of mesitylene as a solvent to prepare films of P3HT: PCBM, favors their degree of crystallinity and hence the charge transport, since the higher crystallinity implies an increase in effective length of conjugation in the polymer; the absorption of radiation is also improved because it takes a wider region of the solar spectrum. However, the use of mesitylene results in thin films of P3HT: PCBM with high fluorescence and segregation of their

5 Revista NOOS Volumen 1 (2013) Pág. 60 components in large domains; further, the thickness is difficult to control due to the high viscosity of the mesitylene based solution. To avoid these drawbacks, films of P3HT: PCBM were grown using mixtures of mesitylene and chlorobenzene at different ratios and temperatures. No samples were deposited using mixtures with o-xylene and toluene because the o-xylene have no good optical properties and toluene does not allow good homogeneity in the thickness of the films. The results showed that films of P3HT: PCBM deposited using mixture of mesitylene: chlorobenzene 80:20 (v / v) have good absorbance in a wide spectral region and low fluorescence (see section 3.3). 3.2 Structural Properties The effect of annealing and solvent type on the crystallographic properties of PCBM:P3HT blends was studied though XRD and Raman spectroscopy measurements. Fig. 3 shows XRD patterns corresponding to P3HT:PCBM blends, before and after annealing at temperatures ranging from 80 to 160 C. it was found that in nonannealed P3HT:PCBM blends, peaks at 2 =5,45, 2 =13,4 and 2 =16,2 are present. The peak at 2 =5.45 corresponds to reflections in planes associated with the stacking laminar of P3HT and the peaks at 2 = 3.4 and 2 =16.2 correspond to reflections of the second and third order for the same stacking, which has associated an interplanar distance of about 16.1 Å. In samples annealed at temperatures ranging between 80 and 160 C only one peak at 2 =5,45 is observed, indicating that the annealing improves the degree of molecular ordering of P3HT chains. It is also observed that annealing of P3HT:PCBM samples at temperatures greater than 120 C gives raise to a shift of the 2 =5,45 peak toward lower values of 2. This result shows that annealing at high temperatures induces an increase in the interchain distance d. Fig. 3: XRD pattern of P3HT:PCBM blends showing the effect of annealing temperature. Figure 4 shows diffractograms of P3HT:PCBM thin films prepared from solutions containing as solvent mesitylene, ortho-xylene, toluene, chlorobenzene and mixing 80:20 v / v of mesitylene: chlorobenzene, subjected to annealing at 120 C during 30 min; in table 1 are listed the corresponding values of FWHM(full width at half maximum) and intensity of the peak around 2 = 5,45 Fig. 4: Diffractograms of P3HT: PCBM thin films prepared from solutions containing as solvent mesitylene, ortho-xylene, toluene, chlorobenzene and mixing 80:20 v / v of mesitylene: chlorobenzene. Table 1: Values of FWHM and intensity of the peak around 2 =5,45 corresponding to the diffractograms shown in Fig. 4.

6 Revista NOOS Volumen 1 (2013) Pág. 61 Type of solvent 2 FWHM Intensity Toluene 5,38 0, ,2 o-xilene 5,31 0, ,7 Chlorobenzene 5,43 0, ,6 Mesitylene 5,48 0, ,1 M/Cl 80/20 5,33 0, ,4 It is observed that the intensity of the reflexion at 2 =5.45 for samples deposited using a solution containing as solvent mesitylene, chlorobenzene, and mixture of chlorobenzene and mesitylene is significantly greater than that of the samples prepared from solutions containing toluene and o-xylene, apparently because the former have a higher degree of crystallinity. Moreover, samples prepared using as solvent mesitylene and mixture mesitylene/chlorobenzene presents FWHM values higher than those of the other samples, indicating that the first type of samples present domains smaller than samples prepared from toluene, chlorobenzene and o-xilene. Besides X-ray analysis, Raman spectroscopy study is useful to investigate the molecular arrangement of P3HT in the blend. Fig. 5 shows room temperature Raman spectra of P3HT:PCBM thin films prepared from solutions containing as solvent mesitylene, ortho-xylene, toluene, chlorobenzene and mixing 80:20v/v of mesitylene: chlorobenzene. Fig. 5: Raman spectra of P3HT:PCBM thin films grown from solutions containing as solvent mesitylene, o-xylene, toluene, chlorobenzene and mixing 80:20v/v of mesitylene: chlorobenzene. The Raman spectra plotted in Fig. 5 show two peaks. The first at 1380 cm -1 corresponding to the C-C stretching within the ring of P3HT and the second around 1445 cm -1 corresponding to the symmetric stretching C=C inside the ring of P3HT. These results show that the different solvents used in the preparation of the P3HT: PCBM films affect the FWHM as well as the ratio of the intensities of the C-C and C=C signals. A shift of the signal associated to the stretching C=C is also observed when changing solvent. Table 2 lists values of FWHM, the ratio of the intensities of the signals C-C and C=C and shift of the maximum of the signal associated to the C=C stretching, obtained from the Raman spectra plotted in Fig.5. Table 2: Values of FWHM, C-C/C=C ratio and wavenumber of the signal associated to the C=C stretching, obtained from the Raman spectra plotted in Fig.5. Type solvent of Wavenumber: C=C signal (cm -1 ) FWHM: C=C signal C- C/C=C o-xylene 1455,3 38,8 0,1960 Toluene 1454,7 40,2 0,1834 Mesitylene 1452,8 31,9 0,2616 Chlorobenzene 1454,3 36,6 0,2526 M/Cl 80/ ,9 33,5 0,2526 Considering that the shift of the signal associated to the C=C stretching toward lower values of wavenumber and the increase of the C-C/C=C ratio have been associated to a better molecular arrangement [15] and that increasing the value of the FWHM is associated to dispersion generated by the presence of both crystalline and noncrystalline domains, we can conclude that the samples

7 Revista NOOS Volumen 1 (2013) Pág. 62 prepared using solutions based on mesitylene and the mixture mesitylene: chlorobenzene 80:20 (v / v) exhibit crystallinity better than that of the other samples. 3.3 Fluorescence microscopy study The fluorescence generated by excitation of PCBM:P3HT blends with radiation of 543 nm was observed with fluorescence microscopy. The photogenerated excitons ussually decay radiatively to the ground state; however they can also decay non radiatively via electron transfer, yielding an electron on a PCBM molecule and a hole on a polymer chain. Therefore luminescence quenching is regarded as indication for the occurrence of electron transfer. across the PCBM/p3HT interface. Fig. 6 display curves of the luminescence emitted by PCBM:P3HT blends prepared using chlorobenzene based solution as a function of the annealing time at temperatures varying between 80 and 160 C. The fluorescence intensity was normalized with the absorbance measured at 543 nm. It is observed that samples annealed at temperatures below 100 C during shorter annealing times exhibit high luminescence apparently because under these conditions no phase separation occur. Increasing the annealing temperature to values around 120 degrees occur a pronounced decrease of the intensity of luminescence, indicating a phase separation in the PCBM:P3HT blend. An increase of annealing time to values of about 30 minutes, results in an additional decrease of the intensity of luminescence, probably due to an enlargement in the size of the domains of PCBM and P3HT. Samples annealed a temperatures above 140 C exhibit a strong increase in the intensity of the luminescence, apparently due to an exaggerated enlargement in the size of the domains of PCBM and P3HT. Fluorescence images taken to PCBM:P3HT blends annealed at temperatures above 120 degrees revealed the formation of microfibers in the form of needles, which increase in number and size with increasing annealing temperature and time. Fig. 6: Annealing time dependence of the fluorecencia of thin films of P3HT: PCBM prepared using chlorobenzene based solution and subsequent annealing at temperatures ranging between 80 C and 160 C. Fluorescence measurements performed to thin films of P3HT: PCBM prepared from solutions based on chlorobenzene, mesitylene, toluene, ortho-xylene and mixture of mesitylene: chlorobenzene 80:20 with subsequent annealing at 120 C during 30 minutes revealed that the samples prepared from solutions based on chlorobenzene and mixture of mesitylene and chlorobenzene (80:20) have much lower intensity fluorescence than the emitted by the samples prepared using the other types of solvents. 4. Conclussions The effect of annealing and solvent type on the crystalline structure, spectral absorbance and molecular structure of PCBM:P3HT blends deposited by spin coating was evaluated using techniques such as XRD, UV-VIS absorbance fluorescence microscopy and Raman spectroscopy. The study allowed us to find conditions to prepare P3HT:PCBM blends with clear evidence of phase separation as well as to optimize the spectral absorbance and degree of

8 Revista NOOS Volumen 1 (2013) Pág. 63 ordering of polymer chains. Evidence was also found that annealing at temperatures over 120 C induces an increase in the polymer interchain distance. It was found that the preparation of P3HT/PCBM thin films using mesitylene as a solvent in the precursor solution is obtained a higher molecular arrangement of P3HT and high absorption in a wider region of the solar spectrum; however due to the difficulty to grow homogeneous films with low fluorescence using this solvent, it was proposed the use of mixture of mesitylene / chlorobenzene 80/20 (v / v) as solvent of the precursor solution. The use of this mixture as a solvent improves the homogeneity and crystallinity of the films, enhances the absorption in a wide range of the solar spectrum and results in low fluorescence samples, which is expected to improve properties such as carrier mobility and thus the performance of organic solar cells including the preparation of the active layer using this solvent. 5. References [1] Liang Y, Xu Z, Xia J, Tsai ST, Wu Y, Li G, Ray C, Yu L., For the bright future bulk heterojunction polymer solar cells with power conversion efficiency of 7.4%. Advanced Materials 2010; 22: E135 E138. [2]. [accessed on July 2012]. [3] G. Li, Y. Yao, H. Yang, V. Shrotriya, G. Yang, Y. Yang, Solvent annealing effect in polymer solar cells based on poly(3-hexylthiophene) and methanofullerenes, Adv.Funct.Mater.17(2007) [4] F. Padinger, R.S. Rittberger, N.S. Sariciftci, Effects of postproduction treatment on plastic solar cells, Adv. Funct. Mater.13(2003) [5] X.N. Yang, J.K.J. Van Duren, M.T. Rispens, J.C. Hummelen, R.A.J. Janssen, M.A.J. Michels, J. Loos, Crystalline organization o a methanofullerene as used for plastic solar-cell applications, Adv. Mater.16 (2004) [6] N. Camaioni, G. Ridolfi, G. Casalbore-Miceli, G. Possamai, M. Maggini, The effect of a mild thermal treatment on the performance of poly(3- alkylthio- phene)/fullerene solar cells, Adv. Mater. 14 (2002) [7] S.E. Shaheen, C.J. Brabec, N.S. Sariciftci, F. Padinger, T. Fromhert, J.C. Hummelen, 2.5% efficient organic plastic solar cells. Applied Physics Letters 2001; 78: [8] J.J.M. Halls, A.C Arias, J. D. Mackenzie, W.Wu,M. Inbasekaran, P. Ed Woo, R.H. Friend, Photodiodes based on polyfluorene composites: influence of morphology. Advanced Materials 2000; 12: [9] P.M. Buschbaum, J.S. Gutmann, M. Stamm, Influence of blend composition on phase separation and dewetting of thin polymer blend films. Macromolecules 2000; 33: [10] F. Zhang, K. G. Jespersen, C. Bjorstrom, M. Svensson, M. R. Andersson, V. Sundstrom, K. Magnusson, E. Moons, A. Yartsev, O. Inganas, Influence of Solvent Mixing on the Morphology and Performance of Solar Cells Based on Polyfluorene Copolymer/Fullerene Blends Adv. Funct. Mater. 2006, 16, 667. [11] J. Peet, C. Soci, R. C. Coffin, T. Q. Nguyen, A. Mikhailovsky, D. Moses, G. C. Bazan, Method for increasing the photoconductive response in conjugated polymer/fullerene composites Appl. Phys. Lett. 2006, 89, [12] J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger, G. C. Bazan, Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane dithiols, Nat. Mater. 2007, 6, 497. [13] J. W. Grzgorczyk, T. J. Savenije, M. Heeney, S. Thierney, I. McCulloch, S. van Bavel and L.D.A. Siebbeeles, J. Phys. Chem. C 2008, 112, [14] P. Kumar, S. Chand, Recent progress and future aspects of organic solar cells, Progress in Photovoltaics: Research and Appl., 2012, 20, [15] U. Zhokhavets, T. Erb, et al., Thin Solid Films, 2006, 496, 2, pp