- Email: [email protected]
Modification of PEDOT:PSS anode buffer layer with HFA for flexible polymer solar cells
Chemical Physics Letters 572 (2013) 73–77
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Modification of PEDOT:PSS anode buffer layer with HFA for flexible polymer solar cells Natasha A.D. Yamamoto a, Lucas F. Lima a, Rodolfo E. Perdomo b, Rogério Valaski c, Vanessa L. Calil c,d, Andréia G. Macedo a,e,⇑, Marco Cremona c,d, Lucimara S. Roman a,⇑ a
Laboratory of NanoStructured Devices, Department of Physics, University Federal of Paraná, Curitiba, Paraná, Brazil Engineering and Materials Science (PIPE), University Federal of Paraná, 81531-990 Curitiba, Brazil Departament of Physics, Pontifical Catholic University of Rio de Janeiro, PUC-Rio, Rio de Janeiro, RJ, Brazil d Instituto Nacional de Metrologia, Qualidade Industrial e Tecnologia INMETRO, Duque de Caxias, Rio de Janeiro, Brazil e Department of Physics, Technological Federal University of Paraná, Curitiba, Paraná, Brazil b c
a r t i c l e
i n f o
Article history: Received 22 January 2013 In final form 11 April 2013 Available online 20 April 2013
a b s t r a c t The effect of poly(3,4-ethylenedioxythiophene)-polystyrenesulfonic acid (PEDOT:PSS) modified with an amphiphilic fluoro compound was investigated on the performance of flexible organic solar cells in bilayer geometry and using substrate based on poly(ether-imide) (PEI) with ITO films with low resistivity and high optical transmittance. The poly[9,90 -dioctyl-fluorene-co-bithiophene] (F8T2) polymer and fullerene (C60) were used as electron donor and acceptor materials, respectively. The spin-casted PEDOT:PSS films had its sheet resistance reduced from 7 MO sq1 to 40 kO sq1 after a single step treatment with hexafluoroacetone (HFA), reflecting in an enhancement of power conversion efficiency in the photovoltaic cells from 0.45% up to 1.30%. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Organic photovoltaic devices (OPVs) have been object of intense research and the production of flexible devices has increased considerably in the last years due their advantages such as the possibility of producing panels with large area and low weight [1], besides the facility of installation on non-flat areas, which is an important aspect concerning the design of roofs and facades in accordance to the modern ecological requirements [2]. Moreover, flexible devices are a feasible option to increase the efficiency of OPVs by improving the absorption of radiation at larger incident angles [3]; since the flat-glass based devices do not offer this possibility. Besides this, OPVs can be produced by printing techniques, a powerful strategy for large-scale production. However, there are still some difficulties to overcome in order to enlarge the use of flexible substrates in the development of OPVs. Among these problems are the poor flexibility of ITO [4], the poor adherence of the active layer on the flexible ITO substrate and the corrosion effects by the penetration of gas molecules as O2 and H2O in the flexible substrate [5]. In addition, the ITO sheet resistance on flexible substrate is generally higher than that on glass substrates. In order to address this issue some alternatives have been ⇑ Corresponding authors at:Laboratory of NanoStructured Devices, Department of Physics, Federal University of Paraná, Curitiba, Paraná, Brazil. E-mail addresses: [email protected] (A.G. Macedo), lsroman@fisica.ufpr.br (L.S. Roman). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.04.022
accomplished such as the development of flexible devices modified by indium zinc oxide intermediate layer [6], flexible substrates based on silver nanowires [7] and the development of flexible substrates where the ITO is replaced by gallium doped zinc oxide [8]. The development of new flexible and transparent substrates for the glass substitution also has been object of intense research. Poly(imides) (PI) are well known for its high thermal stability, besides having superior mechanical and electrical properties, and optical transparency [9], but since the PI has high insolubility their applications are limited. An important commercial example of a PI derivative is the poly(ether-imide) (PEI), that is an advanced amorphous thermoplastic presenting ether and isopropylidene groups, responsible by the solubility of this polymer in some halogenated solvents, while it retains many of the desirable features of PI such as high temperature resistance and resistance to solvents such as alcohols and acids [9–11]. For these reasons, PEI is an important thermostable polymer whose applications vary from medical to electronics areas [11]. Its glass transition temperature (217 °C) is greater than the polymers typically used as substrates for organic devices, for instance poly(ethylene terephthalate) (PET, 70 °C), this feature enables the use of PEI in industrial processes and/or annealing at higher temperatures. Recently, V.L. Calil et al. [12] developed flexible substrates based on PEI/ITO films presenting low resistivity and high optical transmittance. The performance of OLEDS based on PEI/ITO substrates confirms that PEI withstands possible high temperature processes in the device production as well high temperature operation.
74
N.A.D. Yamamoto et al. / Chemical Physics Letters 572 (2013) 73–77
Although the improvements towards the ITO-free substrates or ITO flexible substrates, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is still being used as intermediate layer due its high transmittance in visible range and in order to improve the positive charge carrier injection/collection. Recently, PEDOT:PSS has also shown promising results as transparent electrode in ITO-free flexible devices [13–15]. However, the sheet resistance of PEDOT:PSS must be decreased for a wider application, this requires modifications in the production of the PEDOT:PSS based electrodes [16]. Lower conductivity in comparison with ITO and the degradation under high radiation intensity and UV radiation are some of the main challenges that need to be solved [17]. Furthermore, poor adherence between PEDOT:PSS and the substrate, or with active layer, frequently has resulted in devices with low efficiency [18,19]. This issue becomes more complex when the PEDOT:PSS is deposited on flexible ITO [19]. Several techniques have been employed to improve the PEDOT:PSS features, for instance, post-deposition treatment or use of additives in the precursor solution as glycol and glycerol [20]. Xia et al. [21] reported the preparation of highly conductive PEDOT:PSS after treatment with hexafluoroacetone (HFA) and its use as electrode in ITO free photovoltaic devices. As cast PEDOT:PSS films are characterized by a core–shell morphology, being the conducting PEDOT mainly in the core with a shell of isolating PSS, resulting in highly resistive films. The hydrolysis of HFA with water results in a highly amphiphilic compound with the hydrophobic – CF3 and two hydrophilic –OH groups. The amphiphilic groups can preferentially interact with the hydrophobic PEDOT and hydrophilic PSS chains of PEDOT:PSS, and these interactions can give rise to conductivity enhancement as well. In this letter, we report the performance of bilayer organic solar cell [22–26] based on PEI/ITO/PEDOT:PSS anodes, with PEDOT:PSS modified by HFA. The results were compared with unmodified PEDOT:PSS based devices and the increasing in the efficiency after the HFA treatment was discussed.
2. Experimental The poly(ether-imide) (Ultem 1000) used in this study was kindly supplied by Saudi Basic Industries Corporation (SABIC). Before use, PEI pellets were dried in oven at 60 °C–100 °C for several days to remove any water content. The casting solution was prepared by dissolution of PEI in n-methyl pyrrolidone (NMP) solvent by stirring for one day at room temperature in the proportion of 10 wt.% PEI. The solution was then poured onto a silicon wafer and the polymeric film was obtained by inversion phase by solvent evaporation in a vacuum oven at 70 °C. The formed film was treated at 200 °C in vacuum oven to remove any solvent residue and to obtain the final dimensions. The obtained PEI films were transparent (over 80% between 460 nm and 900 nm) with an average thickness of 0.1 mm. ITO thin films were deposited in Amod Series Angstrom Engineering Coating Systems by r.f. magnetron sputtering using a 3 inch diameter ceramic target 90 wt.% In2O3 and 10 wt.% SnO2 from Kurt J. Lesker. The r.f. power (13.56 MHz) was supplied by an r.f. generator matched to the target by a tuning network, both from RF VII Inc. ITO films were produced with a thickness of 350 nm and 10 O/sq onto PEI substrates. Before the ITO deposition, the vacuum chamber was evacuated until 5 107 Torr and filled with Ar for three times successively. The Ar work pressure was kept at 1.5 103 Torr during the ITO deposition. Devices were fabricated as follows: poly(3,4-ethylenedioxythiophene)-polystyrenesulfonic acid (PEDOT:PSS) was filtered in a 0.45 lm filter, then a film with thickness of 40 nm was spin-coated at 6000 rpm onto the cleaned ITO patterned PEI substrate and
annealed at 100 °C for 24 h in vacuum. The HFA treatment was performed by dropping 100 lL pure HFA3H2O (Aldrich) on a PEDOT:PSS film on a hot plate at 140 °C (in nitrogen atmosphere). The films dried after about 5 min. They were cooled down to room temperature, and then were rinsed with DI water and dried at 140 °C again. These films were characterized by four probe technique. The polymer film was spin-coated at 900 rpm (in nitrogen atmosphere) from chlorobenzene solution and thermally treated at 100 °C for 15 min (in vacuum). Then, 30 nm of C60 and 100 nm of Al were thermally evaporated through a shadow mask at vacuum pressure of 4.5 106 Torr, respectively. Subsequently, the PEI/ITO/PEDOT:PSS/polymer/C60/Al devices received an additional annealing at 70 °C for 15 min (in vacuum). The thickness was determined in a Dektak 3 profilometer. Power conversion efficiency (g) was determined by the comparison between JxV graphics under AM 1.5 G illumination through the PEI/ITO electrode. The g is calculated through the expression:
g ¼ FF
J SC :V OC I
ð1Þ
where I the radiation intensity (1000 W/m2), Jsc is the short-circuit current density, Voc the open circuit voltage and FF is the fill factor [27] . The JxV graphics were obtained under environmental conditions. 3. Results and discussion PEI substrate is a suitable solution for the degradation arising from the UV radiation since it presents no transmittance under 380 nm, as it is shown at Figure 1a. The spin-casted PEDOT:PSS films had its sheet resistance reduced from 7 MO sq1 to 40 kO sq1 after a single step treatment with HFA. This reduction is caused mainly by the PSS induced segregation, which is an insulating barrier between the conducting PEDOT rich shells. Figure 1b shows the absorbance spectra of as cast and HFA treated PEDOT:PSS films, the bands with maxima at 193 nm and 225 nm and a broader shoulder at slightly longer wavelengths (250–280 nm) can be attributed to the p–p transitions of the benzene rings of PSS. Although these spectra are very similar, there is a reduction in the absorbance intensity in the HFA treated PEDOT:PSS films when compared with the spectrum of as cast sample, which can be explained by the reduction of PSSH in the former film after reaction with HFA [21]. Additionally, a slight increase of the full-width at half maximum (FWHM) occurs in the bands of the HFA treated PEDOT:PSS film, as can be observed in inset of Figure 1b, where the partial normalized spectra are plotted for comparison. The increase in the FWHM may be related with the expanded core–shell structure of PEDOT:PSS and the segregation of PSS after the treatment. A schematic representation of the initial structure of PEDOT:PSS, hydrolysis of HFA and the effect on the reorganization of the polymeric chains are showed at Figure 2. AFM images of PEDOT:PSS before and after the treatment with HFA are presented at Figure 3. Both films present similar morphology, although the root mean square roughness (Rrms) increases from 1.6 nm up to 2.2 nm after reaction with HFA. This roughness increasing was also observed in PEDOT:PSS films based on precursor solution modified by ethylene glycol addition and by dimethyl sulfoxide (DMSO), and the increase of conductivity was majoritarily attributed to the segregation of PSS than to the increase of surface roughness [28]. This is coherent with the phase images showed at Figure 3, where two different phases in the film treated with HFA can be distinguished, these two phases can be the result of PSS segregation, that increases the film conductivity because the PSS works as an energy barrier between PEDOT grains with the electronic conduction taking place between PEDOT grains [29,30]. PSS segregation causes an increasing in the PEDOT domain grain with a consequent
75
N.A.D. Yamamoto et al. / Chemical Physics Letters 572 (2013) 73–77
(a)
0.6 PEI Absorbance (arb. units)
100
Transmittance (%)
80 PEI+ITO 60 40 20 0 300
400 500 600 Wavelength (nm)
700
800
(b)
0.4
220 240 260 280 300
0.2
Wavelength (nm)
0.0 200
240 280 320 Wavelength (nm)
360
Figure 1. (a) Transmittance of PEI (dotted line) and PEI + ITO (solid line) and (b) Absorbance spectra of as-cast (solid line) and HFA treated (dotted line) PEDOT:PSS. Inset: partial normalized absorbance spectra.
Figure 2. Schematic representation of the initial core–shell structure of PEDOT:PSS, after hydrolysis of HFA the hydrophobic part (–CF3) interacts with PEDOT chains, while the hydrophilic groups (–OH) interacts with PSS chains reducing the amount of PSSH in the former film.
increasing in the film roughness. However, this roughness modification is not enough to explain the considerable reduction of the sheet resistance in an order of 103 after HFA treatment as it was probed by four-probe technique. Thus, the effect of the HFA treatment is not only limited to the changes in morphology. A. Dkhissi et al. [31] have claimed that the addition of diethyl glycol in the precursor solution leads to a better PEDOT chains ordering in suspension, improving the film conductivity. The authors stressed that the morphological conformation of PEDOT:PSS chain presents a strong dependence on the electrostatic interaction between PSS and polarized PEDOT segments. Furthermore, the PSS structure depends on the interactions between the phenyl-SO3 groups. Petrosino et al. [32] have suggested that PEDOT:PSS is characterized by the presence of permanent dipoles and that the polar behavior of PSS affects the polar component of the surface energy. The hydrolyzed HFA can interact with phenyl-SO3 groups in PSS and can also produces an improvement in the organization of the PEDOT chains due to the high electron affinity of fluorine in HFA and, consequently, it enhances the transport properties of the junction PEDOT:PSS/active layer. Figure 4 shows the JxV curves acquired under AM1.5 illumination for PEI/ITO/PEDOT:PSS/F8T2/C60/Al devices, with pristine and HFA treated PEDOT:PSS. Table 1 lists the photovoltaic characteristics achieved from these devices. HFA treatment improved all the device features, decreasing the series resistance (Rs) and increasing the shunt resistance (Rsh), which led to a considerable enhancement
in the overall device efficiency. The Rs and Rsh are concepts imported from the semiconductors theory for photovoltaics where the device is characterized by an equivalent circuit composed of simple components as resistors and diodes. The Rs is related with the charge carrier transport phenomena and depends on the bulk active layer conductance and interface/contact resistances whereas the Rsh takes into account leak currents and short-circuits [33]. Nevertheless, the conventional theory cannot be applied to the organic photovoltaic devices without considering the differences in the charge generation and charge transport phenomena in these two classes of devices [34]. In conventional devices the Rsh is higher than in the organic ones and the transport phenomena and consequently the Jsc, is dominated by Rs and Voc by the Rsh. On the other hand, in the organic devices the Jsc can be affected by the Rsh [35] that carries out to the Jsc dependence on both Rsh and Rs. However, when the difference between the Rsh and Rs values is large (103 typically), the conventional theory can be used as a way to compare the performance of different organic devices. In this situation, the Voc is dominated by Rsh with weak dependence on Rs changes [36]. As the Rsh depends mainly from the interface electrode/active layer morphology an interface modification which produces a roughness decreasing causes a Rsh to increase and a consequent improvement of Voc [37]. However, the devices with HFA treated PEDOT:PSS presented an opposite behavior. The increase in PEDOT:PSS roughness did not decrease the Rsh and Voc values. The possible changes in the PEDOT:PSS distribution after the HFA treatment and may have
76
N.A.D. Yamamoto et al. / Chemical Physics Letters 572 (2013) 73–77
Figure 3. Topography and phase AFM images of (a and b) as-cast and (c and d) HFA treated PEDOT:PSS films, scan size 2 2 lm2.
level of donor and the lowest unoccupied molecular orbital level of acceptor as well as the work function difference between two electrodes or light intensity. As the thickness of the active electron layer and C60 of both devices are the same and as no changes in the energy levels of PEDOT:PSS are expected after the HFA treatment the decrease of Voc value in flexible devices when compared with those fabricated onto glass can be the result of the higher resistivity and higher roughness of the ITO on the PEI substrate. By testing a similar device onto glass substrate the efficiency was increased from 2.1% up to 2.8% after the use of HFA.
0
2
J (mA/cm )
-1 -2 -3 -4 -5
4. Conclusion 0.0
0.1
0.2 0.3 Voltage (V)
0.4
0.5
Figure 4. JxV characteristics under AM1.5 illumination (100 mW/cm2) from devices having as cast (h) or HFA treated PEDOT:PSS buffer layer (j).
Table 1 Photovoltaic characteristics of PEI/ITO/PEDOT:PSS/F8T2/C60/Al.
– HFA
Jsc (mA cm2)
Voc (V)
FF (%)
Rs (X cm2)
Rsh (X cm2)
g (%)
2.30 4.44
0.44 0.49
44 58
67 17
1128 1194
0.45 1.30
improved the adhesion of the active layer, reducing the Rs and improving the internal electrical field that caused the increase on the Voc. However, the Voc presented by our PEI based flexible devices are lower than the typical value of F8T2 bilayer solar cells prepared in glass/ITO substrate (0.65–0.91 V) [38], it should be noted that after treatment with HFA the FF and Voc values increased 24% and 10%, respectively. It is well known that Voc is influenced mainly by the difference between the highest occupied molecular orbital
PEI substrates were effectively used as flexible substrates in OPVs. By using this substrate it is possible to apply thermal annealing at temperatures until approximately 200 °C. Lower Voc observed in devices with PEI/ITO substrates, when compared with glass/ITO substrates, pointed out the requirement of decreasing the PEI/ITO resistivity. Modification of PEDOT:PSS with HFA resulted in a reduction of its sheet resistance from 7 MO sq1 to 40 kO sq1 after a single step treatment. The effect of HFA is not only to produce PSS segregation but also changes the morphology and transport characteristics of PEDOT:PSS. Bilayer devices prepared with this HFA modified PEI/ITO/PEDOT:PSS anodes and having F8T2 as active layer and C60 as electron accepting material resulted in devices with power conversion efficiency of 1.3%.
Acknowledgments The authors would like to thank CNPq and INEO for financial support.
N.A.D. Yamamoto et al. / Chemical Physics Letters 572 (2013) 73–77
References [1] X. Matheu, J.P. Enriquez, A. Romeo, A.N. Tiwari, Sol. Energy 77 (2004) 831. [2] A. Romeo, G. Khrypunov, F. Kurdesau, M. Arnold, D.L. Bätzner, H. Zogg, A.N. Tiwari, Sol. Energy Mater. Sol. Cells 90 (2006) 3407. [3] S.Y. Chuang, C.C. Yu, H.L. Chen, W.F. Su, C.W. Chen, Sol. Energy Mater. Sol. Cells 95 (2011) 2141. [4] W. Cao, Y. Zheng, Z. Li, E. Wrzesniewski, W.T. Hammond, J. Xue, Org. Electron. 13 (2012) 2221. [5] S. Logothetidis, Mater. Sci. Eng., B 152 (2008) 96. [6] B.J. Lee, H.J. Kim, W. Jeong, J.J. Kim, Sol. Energy Mater. Sol. Cells 94 (2010) 542. [7] J. Ajuria, I. Ugarte, W. Cambarau, I. Etxebarria, R.T. Zaera, R. Pacios, Sol. Energy Mater. Sol. Cells 102 (2012) 148. [8] L. Wang et al., Org. Electron. 11 (2010) 1555. [9] C.A. Harper, E.M. Petrie, Plastics Materials and Processes: A Concise Encyclopedia, Hoboken: John Wiley & Sons Inc., 2003. p. 427–434. [10] J.A. Brydson, Plastic Materials, 7th ed., Butterworth-Heinemann, Oxford, 1999. p. 920. [11]
77
[17] S.R. Dupont, M. Oliver, F.C. Krebs, R.H. Dauskardt, Sol. Energy Mater. Sol. Cells 97 (2012) 171. [18] L. Chen, P. Wang, F. Li, S. Yu, Y. Chen, Sol. Energy Mater. Sol. Cells 102 (2012) 66. [19] Y.T. Cheng et al., Appl. Surf. Sci. 256 (2010) 7606. [20] W. Cai, X. Gong, Y. Cao, Sol. Energy Mater. Sol. Cells 94 (2010) 114. [21] Y. Xia, K. Sun, J. Ouyang, Energy Environ. Sci. 5 (2012) 5325. [22] M. Koehler, L.S. Roman, O. Inganäs, M.G.E. da Luz, J. Appl. Phys. 96 (2004) 40. [23] A.G. Macedo, C.F.N. Marchiori, I.R. Grova, L. Akcelrud, M. Koehler, L.S. Roman, Appl. Phys. Lett. 98 (2011) 253501. [24] C.D. Canestraro, P.C. Rodrigues, C.F.N. Marchiori, C.B. Schneider, L. Akcelrud, M. Koehler, L.S. Roman, Sol. Energy Mater. Sol. Cells 95 (2011) 2287. [25] C.F.N. Marchiori et al., Org. Electron. 13 (2012) 2716. [26] N.A.D. Yamamoto et al., J. Phys. Chem. C 10 (2006) 24. [27] R. Valaski, F. Muchenski, R.M.Q. Mello, L. Micaroni, L.S. Roman, I.A. Hümmelgen, J. Solid State Electron. 10 (2006) 24. [28] Z. Hu, J. Zhang, Z. Hao, Y. Zhao, Sol. Energy Mater. Sol. Cells 95 (2011) 2763. [29] P. Wilson, C. Lekakou, J.F. Watts, Org. Electron. 12 (2012) 409. [30] G. Wang, S.I. Na, T.W. Kim, Y. Kim, S. Park, T. Lee, Org. Electron. 13 (2012) 771. [31] A. Dkhissi, D. Beljonne, R. Lazzaroni, Synth. Metals 159 (2009) 546. [32] M. Petrosino, A. Rubino, Synth. Metals 161 (2012) 2714. [33] S. Yoo, B. Domercq, B. Kippelen, J. Appl. Phys. 97 (2005) 103706. [34] B. Mazhari, Sol. Energy Mater. Sol. Cells 90 (2006) 1021. [35] A. Moliton, J.M. Nunzi, Polym. Int. 55 (2006) 583. [36] A. Guerrero, T.R. Sanchis, P.P. Boix, G.G. Belmonte, Org. Electron. 13 (2012) 2326. [37] J.D. Servaites, S. Yeganeh, T. Marks, M.A. Ratner, Adv. Funct. Mater. 20 (2010) 97. [38] N.A.D. Yamamoto, A.G. Macedo, L.S. Roman, J. Nanotechnol. 1 (2012).
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Modification of PEDOT:PSS anode buffer layer with HFA for flexible polymer solar cells Natasha A.D. Yamamoto a, Lucas F. Lima a, Rodolfo E. Perdomo b, Rogério Valaski c, Vanessa L. Calil c,d, Andréia G. Macedo a,e,⇑, Marco Cremona c,d, Lucimara S. Roman a,⇑ a
Laboratory of NanoStructured Devices, Department of Physics, University Federal of Paraná, Curitiba, Paraná, Brazil Engineering and Materials Science (PIPE), University Federal of Paraná, 81531-990 Curitiba, Brazil Departament of Physics, Pontifical Catholic University of Rio de Janeiro, PUC-Rio, Rio de Janeiro, RJ, Brazil d Instituto Nacional de Metrologia, Qualidade Industrial e Tecnologia INMETRO, Duque de Caxias, Rio de Janeiro, Brazil e Department of Physics, Technological Federal University of Paraná, Curitiba, Paraná, Brazil b c
a r t i c l e
i n f o
Article history: Received 22 January 2013 In final form 11 April 2013 Available online 20 April 2013
a b s t r a c t The effect of poly(3,4-ethylenedioxythiophene)-polystyrenesulfonic acid (PEDOT:PSS) modified with an amphiphilic fluoro compound was investigated on the performance of flexible organic solar cells in bilayer geometry and using substrate based on poly(ether-imide) (PEI) with ITO films with low resistivity and high optical transmittance. The poly[9,90 -dioctyl-fluorene-co-bithiophene] (F8T2) polymer and fullerene (C60) were used as electron donor and acceptor materials, respectively. The spin-casted PEDOT:PSS films had its sheet resistance reduced from 7 MO sq1 to 40 kO sq1 after a single step treatment with hexafluoroacetone (HFA), reflecting in an enhancement of power conversion efficiency in the photovoltaic cells from 0.45% up to 1.30%. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Organic photovoltaic devices (OPVs) have been object of intense research and the production of flexible devices has increased considerably in the last years due their advantages such as the possibility of producing panels with large area and low weight [1], besides the facility of installation on non-flat areas, which is an important aspect concerning the design of roofs and facades in accordance to the modern ecological requirements [2]. Moreover, flexible devices are a feasible option to increase the efficiency of OPVs by improving the absorption of radiation at larger incident angles [3]; since the flat-glass based devices do not offer this possibility. Besides this, OPVs can be produced by printing techniques, a powerful strategy for large-scale production. However, there are still some difficulties to overcome in order to enlarge the use of flexible substrates in the development of OPVs. Among these problems are the poor flexibility of ITO [4], the poor adherence of the active layer on the flexible ITO substrate and the corrosion effects by the penetration of gas molecules as O2 and H2O in the flexible substrate [5]. In addition, the ITO sheet resistance on flexible substrate is generally higher than that on glass substrates. In order to address this issue some alternatives have been ⇑ Corresponding authors at:Laboratory of NanoStructured Devices, Department of Physics, Federal University of Paraná, Curitiba, Paraná, Brazil. E-mail addresses: [email protected] (A.G. Macedo), lsroman@fisica.ufpr.br (L.S. Roman). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.04.022
accomplished such as the development of flexible devices modified by indium zinc oxide intermediate layer [6], flexible substrates based on silver nanowires [7] and the development of flexible substrates where the ITO is replaced by gallium doped zinc oxide [8]. The development of new flexible and transparent substrates for the glass substitution also has been object of intense research. Poly(imides) (PI) are well known for its high thermal stability, besides having superior mechanical and electrical properties, and optical transparency [9], but since the PI has high insolubility their applications are limited. An important commercial example of a PI derivative is the poly(ether-imide) (PEI), that is an advanced amorphous thermoplastic presenting ether and isopropylidene groups, responsible by the solubility of this polymer in some halogenated solvents, while it retains many of the desirable features of PI such as high temperature resistance and resistance to solvents such as alcohols and acids [9–11]. For these reasons, PEI is an important thermostable polymer whose applications vary from medical to electronics areas [11]. Its glass transition temperature (217 °C) is greater than the polymers typically used as substrates for organic devices, for instance poly(ethylene terephthalate) (PET, 70 °C), this feature enables the use of PEI in industrial processes and/or annealing at higher temperatures. Recently, V.L. Calil et al. [12] developed flexible substrates based on PEI/ITO films presenting low resistivity and high optical transmittance. The performance of OLEDS based on PEI/ITO substrates confirms that PEI withstands possible high temperature processes in the device production as well high temperature operation.
74
N.A.D. Yamamoto et al. / Chemical Physics Letters 572 (2013) 73–77
Although the improvements towards the ITO-free substrates or ITO flexible substrates, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is still being used as intermediate layer due its high transmittance in visible range and in order to improve the positive charge carrier injection/collection. Recently, PEDOT:PSS has also shown promising results as transparent electrode in ITO-free flexible devices [13–15]. However, the sheet resistance of PEDOT:PSS must be decreased for a wider application, this requires modifications in the production of the PEDOT:PSS based electrodes [16]. Lower conductivity in comparison with ITO and the degradation under high radiation intensity and UV radiation are some of the main challenges that need to be solved [17]. Furthermore, poor adherence between PEDOT:PSS and the substrate, or with active layer, frequently has resulted in devices with low efficiency [18,19]. This issue becomes more complex when the PEDOT:PSS is deposited on flexible ITO [19]. Several techniques have been employed to improve the PEDOT:PSS features, for instance, post-deposition treatment or use of additives in the precursor solution as glycol and glycerol [20]. Xia et al. [21] reported the preparation of highly conductive PEDOT:PSS after treatment with hexafluoroacetone (HFA) and its use as electrode in ITO free photovoltaic devices. As cast PEDOT:PSS films are characterized by a core–shell morphology, being the conducting PEDOT mainly in the core with a shell of isolating PSS, resulting in highly resistive films. The hydrolysis of HFA with water results in a highly amphiphilic compound with the hydrophobic – CF3 and two hydrophilic –OH groups. The amphiphilic groups can preferentially interact with the hydrophobic PEDOT and hydrophilic PSS chains of PEDOT:PSS, and these interactions can give rise to conductivity enhancement as well. In this letter, we report the performance of bilayer organic solar cell [22–26] based on PEI/ITO/PEDOT:PSS anodes, with PEDOT:PSS modified by HFA. The results were compared with unmodified PEDOT:PSS based devices and the increasing in the efficiency after the HFA treatment was discussed.
2. Experimental The poly(ether-imide) (Ultem 1000) used in this study was kindly supplied by Saudi Basic Industries Corporation (SABIC). Before use, PEI pellets were dried in oven at 60 °C–100 °C for several days to remove any water content. The casting solution was prepared by dissolution of PEI in n-methyl pyrrolidone (NMP) solvent by stirring for one day at room temperature in the proportion of 10 wt.% PEI. The solution was then poured onto a silicon wafer and the polymeric film was obtained by inversion phase by solvent evaporation in a vacuum oven at 70 °C. The formed film was treated at 200 °C in vacuum oven to remove any solvent residue and to obtain the final dimensions. The obtained PEI films were transparent (over 80% between 460 nm and 900 nm) with an average thickness of 0.1 mm. ITO thin films were deposited in Amod Series Angstrom Engineering Coating Systems by r.f. magnetron sputtering using a 3 inch diameter ceramic target 90 wt.% In2O3 and 10 wt.% SnO2 from Kurt J. Lesker. The r.f. power (13.56 MHz) was supplied by an r.f. generator matched to the target by a tuning network, both from RF VII Inc. ITO films were produced with a thickness of 350 nm and 10 O/sq onto PEI substrates. Before the ITO deposition, the vacuum chamber was evacuated until 5 107 Torr and filled with Ar for three times successively. The Ar work pressure was kept at 1.5 103 Torr during the ITO deposition. Devices were fabricated as follows: poly(3,4-ethylenedioxythiophene)-polystyrenesulfonic acid (PEDOT:PSS) was filtered in a 0.45 lm filter, then a film with thickness of 40 nm was spin-coated at 6000 rpm onto the cleaned ITO patterned PEI substrate and
annealed at 100 °C for 24 h in vacuum. The HFA treatment was performed by dropping 100 lL pure HFA3H2O (Aldrich) on a PEDOT:PSS film on a hot plate at 140 °C (in nitrogen atmosphere). The films dried after about 5 min. They were cooled down to room temperature, and then were rinsed with DI water and dried at 140 °C again. These films were characterized by four probe technique. The polymer film was spin-coated at 900 rpm (in nitrogen atmosphere) from chlorobenzene solution and thermally treated at 100 °C for 15 min (in vacuum). Then, 30 nm of C60 and 100 nm of Al were thermally evaporated through a shadow mask at vacuum pressure of 4.5 106 Torr, respectively. Subsequently, the PEI/ITO/PEDOT:PSS/polymer/C60/Al devices received an additional annealing at 70 °C for 15 min (in vacuum). The thickness was determined in a Dektak 3 profilometer. Power conversion efficiency (g) was determined by the comparison between JxV graphics under AM 1.5 G illumination through the PEI/ITO electrode. The g is calculated through the expression:
g ¼ FF
J SC :V OC I
ð1Þ
where I the radiation intensity (1000 W/m2), Jsc is the short-circuit current density, Voc the open circuit voltage and FF is the fill factor [27] . The JxV graphics were obtained under environmental conditions. 3. Results and discussion PEI substrate is a suitable solution for the degradation arising from the UV radiation since it presents no transmittance under 380 nm, as it is shown at Figure 1a. The spin-casted PEDOT:PSS films had its sheet resistance reduced from 7 MO sq1 to 40 kO sq1 after a single step treatment with HFA. This reduction is caused mainly by the PSS induced segregation, which is an insulating barrier between the conducting PEDOT rich shells. Figure 1b shows the absorbance spectra of as cast and HFA treated PEDOT:PSS films, the bands with maxima at 193 nm and 225 nm and a broader shoulder at slightly longer wavelengths (250–280 nm) can be attributed to the p–p transitions of the benzene rings of PSS. Although these spectra are very similar, there is a reduction in the absorbance intensity in the HFA treated PEDOT:PSS films when compared with the spectrum of as cast sample, which can be explained by the reduction of PSSH in the former film after reaction with HFA [21]. Additionally, a slight increase of the full-width at half maximum (FWHM) occurs in the bands of the HFA treated PEDOT:PSS film, as can be observed in inset of Figure 1b, where the partial normalized spectra are plotted for comparison. The increase in the FWHM may be related with the expanded core–shell structure of PEDOT:PSS and the segregation of PSS after the treatment. A schematic representation of the initial structure of PEDOT:PSS, hydrolysis of HFA and the effect on the reorganization of the polymeric chains are showed at Figure 2. AFM images of PEDOT:PSS before and after the treatment with HFA are presented at Figure 3. Both films present similar morphology, although the root mean square roughness (Rrms) increases from 1.6 nm up to 2.2 nm after reaction with HFA. This roughness increasing was also observed in PEDOT:PSS films based on precursor solution modified by ethylene glycol addition and by dimethyl sulfoxide (DMSO), and the increase of conductivity was majoritarily attributed to the segregation of PSS than to the increase of surface roughness [28]. This is coherent with the phase images showed at Figure 3, where two different phases in the film treated with HFA can be distinguished, these two phases can be the result of PSS segregation, that increases the film conductivity because the PSS works as an energy barrier between PEDOT grains with the electronic conduction taking place between PEDOT grains [29,30]. PSS segregation causes an increasing in the PEDOT domain grain with a consequent
75
N.A.D. Yamamoto et al. / Chemical Physics Letters 572 (2013) 73–77
(a)
0.6 PEI Absorbance (arb. units)
100
Transmittance (%)
80 PEI+ITO 60 40 20 0 300
400 500 600 Wavelength (nm)
700
800
(b)
0.4
220 240 260 280 300
0.2
Wavelength (nm)
0.0 200
240 280 320 Wavelength (nm)
360
Figure 1. (a) Transmittance of PEI (dotted line) and PEI + ITO (solid line) and (b) Absorbance spectra of as-cast (solid line) and HFA treated (dotted line) PEDOT:PSS. Inset: partial normalized absorbance spectra.
Figure 2. Schematic representation of the initial core–shell structure of PEDOT:PSS, after hydrolysis of HFA the hydrophobic part (–CF3) interacts with PEDOT chains, while the hydrophilic groups (–OH) interacts with PSS chains reducing the amount of PSSH in the former film.
increasing in the film roughness. However, this roughness modification is not enough to explain the considerable reduction of the sheet resistance in an order of 103 after HFA treatment as it was probed by four-probe technique. Thus, the effect of the HFA treatment is not only limited to the changes in morphology. A. Dkhissi et al. [31] have claimed that the addition of diethyl glycol in the precursor solution leads to a better PEDOT chains ordering in suspension, improving the film conductivity. The authors stressed that the morphological conformation of PEDOT:PSS chain presents a strong dependence on the electrostatic interaction between PSS and polarized PEDOT segments. Furthermore, the PSS structure depends on the interactions between the phenyl-SO3 groups. Petrosino et al. [32] have suggested that PEDOT:PSS is characterized by the presence of permanent dipoles and that the polar behavior of PSS affects the polar component of the surface energy. The hydrolyzed HFA can interact with phenyl-SO3 groups in PSS and can also produces an improvement in the organization of the PEDOT chains due to the high electron affinity of fluorine in HFA and, consequently, it enhances the transport properties of the junction PEDOT:PSS/active layer. Figure 4 shows the JxV curves acquired under AM1.5 illumination for PEI/ITO/PEDOT:PSS/F8T2/C60/Al devices, with pristine and HFA treated PEDOT:PSS. Table 1 lists the photovoltaic characteristics achieved from these devices. HFA treatment improved all the device features, decreasing the series resistance (Rs) and increasing the shunt resistance (Rsh), which led to a considerable enhancement
in the overall device efficiency. The Rs and Rsh are concepts imported from the semiconductors theory for photovoltaics where the device is characterized by an equivalent circuit composed of simple components as resistors and diodes. The Rs is related with the charge carrier transport phenomena and depends on the bulk active layer conductance and interface/contact resistances whereas the Rsh takes into account leak currents and short-circuits [33]. Nevertheless, the conventional theory cannot be applied to the organic photovoltaic devices without considering the differences in the charge generation and charge transport phenomena in these two classes of devices [34]. In conventional devices the Rsh is higher than in the organic ones and the transport phenomena and consequently the Jsc, is dominated by Rs and Voc by the Rsh. On the other hand, in the organic devices the Jsc can be affected by the Rsh [35] that carries out to the Jsc dependence on both Rsh and Rs. However, when the difference between the Rsh and Rs values is large (103 typically), the conventional theory can be used as a way to compare the performance of different organic devices. In this situation, the Voc is dominated by Rsh with weak dependence on Rs changes [36]. As the Rsh depends mainly from the interface electrode/active layer morphology an interface modification which produces a roughness decreasing causes a Rsh to increase and a consequent improvement of Voc [37]. However, the devices with HFA treated PEDOT:PSS presented an opposite behavior. The increase in PEDOT:PSS roughness did not decrease the Rsh and Voc values. The possible changes in the PEDOT:PSS distribution after the HFA treatment and may have
76
N.A.D. Yamamoto et al. / Chemical Physics Letters 572 (2013) 73–77
Figure 3. Topography and phase AFM images of (a and b) as-cast and (c and d) HFA treated PEDOT:PSS films, scan size 2 2 lm2.
level of donor and the lowest unoccupied molecular orbital level of acceptor as well as the work function difference between two electrodes or light intensity. As the thickness of the active electron layer and C60 of both devices are the same and as no changes in the energy levels of PEDOT:PSS are expected after the HFA treatment the decrease of Voc value in flexible devices when compared with those fabricated onto glass can be the result of the higher resistivity and higher roughness of the ITO on the PEI substrate. By testing a similar device onto glass substrate the efficiency was increased from 2.1% up to 2.8% after the use of HFA.
0
2
J (mA/cm )
-1 -2 -3 -4 -5
4. Conclusion 0.0
0.1
0.2 0.3 Voltage (V)
0.4
0.5
Figure 4. JxV characteristics under AM1.5 illumination (100 mW/cm2) from devices having as cast (h) or HFA treated PEDOT:PSS buffer layer (j).
Table 1 Photovoltaic characteristics of PEI/ITO/PEDOT:PSS/F8T2/C60/Al.
– HFA
Jsc (mA cm2)
Voc (V)
FF (%)
Rs (X cm2)
Rsh (X cm2)
g (%)
2.30 4.44
0.44 0.49
44 58
67 17
1128 1194
0.45 1.30
improved the adhesion of the active layer, reducing the Rs and improving the internal electrical field that caused the increase on the Voc. However, the Voc presented by our PEI based flexible devices are lower than the typical value of F8T2 bilayer solar cells prepared in glass/ITO substrate (0.65–0.91 V) [38], it should be noted that after treatment with HFA the FF and Voc values increased 24% and 10%, respectively. It is well known that Voc is influenced mainly by the difference between the highest occupied molecular orbital
PEI substrates were effectively used as flexible substrates in OPVs. By using this substrate it is possible to apply thermal annealing at temperatures until approximately 200 °C. Lower Voc observed in devices with PEI/ITO substrates, when compared with glass/ITO substrates, pointed out the requirement of decreasing the PEI/ITO resistivity. Modification of PEDOT:PSS with HFA resulted in a reduction of its sheet resistance from 7 MO sq1 to 40 kO sq1 after a single step treatment. The effect of HFA is not only to produce PSS segregation but also changes the morphology and transport characteristics of PEDOT:PSS. Bilayer devices prepared with this HFA modified PEI/ITO/PEDOT:PSS anodes and having F8T2 as active layer and C60 as electron accepting material resulted in devices with power conversion efficiency of 1.3%.
Acknowledgments The authors would like to thank CNPq and INEO for financial support.
N.A.D. Yamamoto et al. / Chemical Physics Letters 572 (2013) 73–77
References [1] X. Matheu, J.P. Enriquez, A. Romeo, A.N. Tiwari, Sol. Energy 77 (2004) 831. [2] A. Romeo, G. Khrypunov, F. Kurdesau, M. Arnold, D.L. Bätzner, H. Zogg, A.N. Tiwari, Sol. Energy Mater. Sol. Cells 90 (2006) 3407. [3] S.Y. Chuang, C.C. Yu, H.L. Chen, W.F. Su, C.W. Chen, Sol. Energy Mater. Sol. Cells 95 (2011) 2141. [4] W. Cao, Y. Zheng, Z. Li, E. Wrzesniewski, W.T. Hammond, J. Xue, Org. Electron. 13 (2012) 2221. [5] S. Logothetidis, Mater. Sci. Eng., B 152 (2008) 96. [6] B.J. Lee, H.J. Kim, W. Jeong, J.J. Kim, Sol. Energy Mater. Sol. Cells 94 (2010) 542. [7] J. Ajuria, I. Ugarte, W. Cambarau, I. Etxebarria, R.T. Zaera, R. Pacios, Sol. Energy Mater. Sol. Cells 102 (2012) 148. [8] L. Wang et al., Org. Electron. 11 (2010) 1555. [9] C.A. Harper, E.M. Petrie, Plastics Materials and Processes: A Concise Encyclopedia, Hoboken: John Wiley & Sons Inc., 2003. p. 427–434. [10] J.A. Brydson, Plastic Materials, 7th ed., Butterworth-Heinemann, Oxford, 1999. p. 920. [11]
77
[17] S.R. Dupont, M. Oliver, F.C. Krebs, R.H. Dauskardt, Sol. Energy Mater. Sol. Cells 97 (2012) 171. [18] L. Chen, P. Wang, F. Li, S. Yu, Y. Chen, Sol. Energy Mater. Sol. Cells 102 (2012) 66. [19] Y.T. Cheng et al., Appl. Surf. Sci. 256 (2010) 7606. [20] W. Cai, X. Gong, Y. Cao, Sol. Energy Mater. Sol. Cells 94 (2010) 114. [21] Y. Xia, K. Sun, J. Ouyang, Energy Environ. Sci. 5 (2012) 5325. [22] M. Koehler, L.S. Roman, O. Inganäs, M.G.E. da Luz, J. Appl. Phys. 96 (2004) 40. [23] A.G. Macedo, C.F.N. Marchiori, I.R. Grova, L. Akcelrud, M. Koehler, L.S. Roman, Appl. Phys. Lett. 98 (2011) 253501. [24] C.D. Canestraro, P.C. Rodrigues, C.F.N. Marchiori, C.B. Schneider, L. Akcelrud, M. Koehler, L.S. Roman, Sol. Energy Mater. Sol. Cells 95 (2011) 2287. [25] C.F.N. Marchiori et al., Org. Electron. 13 (2012) 2716. [26] N.A.D. Yamamoto et al., J. Phys. Chem. C 10 (2006) 24. [27] R. Valaski, F. Muchenski, R.M.Q. Mello, L. Micaroni, L.S. Roman, I.A. Hümmelgen, J. Solid State Electron. 10 (2006) 24. [28] Z. Hu, J. Zhang, Z. Hao, Y. Zhao, Sol. Energy Mater. Sol. Cells 95 (2011) 2763. [29] P. Wilson, C. Lekakou, J.F. Watts, Org. Electron. 12 (2012) 409. [30] G. Wang, S.I. Na, T.W. Kim, Y. Kim, S. Park, T. Lee, Org. Electron. 13 (2012) 771. [31] A. Dkhissi, D. Beljonne, R. Lazzaroni, Synth. Metals 159 (2009) 546. [32] M. Petrosino, A. Rubino, Synth. Metals 161 (2012) 2714. [33] S. Yoo, B. Domercq, B. Kippelen, J. Appl. Phys. 97 (2005) 103706. [34] B. Mazhari, Sol. Energy Mater. Sol. Cells 90 (2006) 1021. [35] A. Moliton, J.M. Nunzi, Polym. Int. 55 (2006) 583. [36] A. Guerrero, T.R. Sanchis, P.P. Boix, G.G. Belmonte, Org. Electron. 13 (2012) 2326. [37] J.D. Servaites, S. Yeganeh, T. Marks, M.A. Ratner, Adv. Funct. Mater. 20 (2010) 97. [38] N.A.D. Yamamoto, A.G. Macedo, L.S. Roman, J. Nanotechnol. 1 (2012).