Synthesis and Solar Cell Application of New Alternating Donor

Article pubs.acs.org/JPCC

Synthesis and Solar Cell Application of New Alternating Donor− Acceptor Copolymers Based on Variable Units of Fluorene, Thiophene, and Phenylene Natasha A. D. Yamamoto,*,† Leah L. Lavery,‡ Bruno F. Nowacki,§ Isabel R. Grova,§ Gregory L. Whiting,‡ Brent Krusor,‡ Eduardo R. de Azevedo,∥ Leni Akcelrud,§ Ana C. Arias,‡,⊥ and Lucimara S. Roman† †

Physics Department, Federal University of Parana, Curitiba, Brazil Palo Alto Research Center, Palo Alto, California 94304, United States § Paulo Scarpa Polymer Laboratory (LaPPS), Federal University of Parana, Curitiba, Brazil ∥ Physics Institute of São Carlos, University of São Paulo, São Carlos, Brazil ‡

S Supporting Information *

ABSTRACT: A new series of donor−acceptor copolymers were synthesized via the Witting route and applied as an active layer in organic thin-films solar cells. These copolymers are composed of fluorene−thiophene and phenylene−thiophene units. The ratio between those was systematically varied, and copolymers containing 0%, 50%, and 75% of phenylene−thiophene were characterized and evaluated when used in photovoltaic devices. The copolymers' composition, photophysical, electrical, and morphological properties are addressed and correlated with device performance. The 50% copolymer ratio was found to be the best copolymer of the series, yielding a power conversion efficiency (PCE) under air mass (AM) 1.5 conditions of 2.4% in the bilayer heterojunction with the C60 molecule. Aiming at flexible electronics applications, solutions based on the heterojunction of this copolymer with PCBM (6,6-phenyl-C61-butyric acid methyl ester) were also successfully deposited using an inkjet printing method and used as an active layer in solar cells.

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INTRODUCTION The need for renewable and clean energy sources motivates the research and development of less expensive methods of production and manufacturing of novel photovoltaic structures. Among the alternative device structures that aim to meet these needs, organic photovoltaic (OPV) devices have shown promising results. Recently, power conversion efficiencies (PCE) up to 7% have been reported by several groups.1−3 In addition to increasing performance, organic photovoltaic devices offer the potential for large area, lightweight, and new form factor photovoltaics that employ lower cost methods of fabrication, such as printing.4,5 Among the most studied conjugated polymers for photovoltaic applications are the polyfluorenes, polythiophenes, polyphenylenes, and their derivatives. Copolymerization is an efficient means to obtain a set of desired optical and electronic properties from polymers since the photophysical and electronic properties of each component can be added to the system. Copolymers based on fluorene units have been largely studied in organic electronics6−9 due to their thermal and chemical stability,10,11 high fluorescence, and highly efficient blue emission, good film-forming, and hole-transporting properties,12 but their large band gap made them less suitable for applications in photovoltaic devices. On the other hand, thiophene units and their derivatives are promising materials © 2012 American Chemical Society

for photovoltaic purposes due to their low band gap and high electrical conductivity,13 thus achieving good device performance.14−17 The insertion of thiophene units between the fluorene moieties reduces their band gap, allowing for the incorporation of the fluorene unit, ideally enabling the combination of facile processing with high efficiency for photovoltaic devices.18−23 Polymers based on phenylene units are usually used in organic light emitting diodes (OLEDs). They can form highly ordered crystalline thin films, but unsubstituted phenylene units are insoluble requiring modification of the polymer backbone. Physical and electronic properties can also be altered by the inclusion of functional side groups.24,25 In this sense, the chemical linking of these monomers by means of copolymerization is a powerful tool to tune device properties.26 The correlation between optoelectronic properties with chemical structure and morphology is the key to the understanding of the physical phenomena involved, which leads to the design of improved perfomance devices. In this context, we present a systematic study of a series of donor−acceptor copolymers based on fluorene, thiophene, and phenylene units for photovoltaic applications. We report Received: June 3, 2012 Revised: August 9, 2012 Published: August 20, 2012 18641

dx.doi.org/10.1021/jp305427g | J. Phys. Chem. C 2012, 116, 18641−18648

The Journal of Physical Chemistry C

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Figure 1. Synthetic route and generic final structure of the copolymers: LaPPS35−0 is composed only by the fluorene−thiophene segment, for LaPPS35−50 is x = y = 50 (molar percentage) and for LaPPS35−75 is x = 25 and y = 75.

solar cells based on these materials in two structures: bilayer and bulk heterojunction. The copolymer composition, photophysical, electrical, and morphological properties are addressed and correlated with device efficiency.

Table 1. Feed Composition for the Copolymerization copolymer

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LaPPS35−0 LaPPS35− 50 LaPPS35− 75

EXPERIMENTAL SECTION Materials. 2,7-Bis[(p-triphenylphosphonium)methyl]-9,9′di-n-hexylfluorene dibromide was prepared as described by ref 27, and 2,5′-thiophenedicarboxaldehyde (Aldrich), pxylylenebis(triphenylphosphonium bromide (Acros), and potassium t-butoxide (Acros, P.A.) were used as received. Methanol (Vetec, P.A.), chloroform (Vetec, P.A.), and ethanol (Synth, 99.5%) were treated according to the literature.28 Buckminsterfullerene C60 was purchased from SES Research, 99.9%, and PCBM from Nano-C. Poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was purchased from H. C. Stark. Generic Synthesis of the Copolymers. The copolymers containing fluorene, thiophene, and phenylene units were synthesized by the Witting route following the procedure reported in ref 29, originating alternate copolymers with the combination of different rates of the following structures: [(9,9dihexyl-9H-fluorene-2,7-diyl)-1,2-ethenediyl-2,5-thiophene-1,2ethenediyl] and [(phenylene-1,4-diyl)-1,2-ethenediyl-2,5-thiophene-1,2-ethenediyl]. Their final structures and synthesis route are outlined in Figure 1. The copolymers were labeled according to their composition: LaPPS35−0 contains only the fluorene−thiophene segment, LaPPS35−50 contains both components in the same ratio, and LaPPS35−75 contains 25% of fluorene−thiophene repeating units in a molar basis and, consequently, 75% of phenylene−thiophene units. The amounts of the materials used on the synthesis process for each copolymer are listed in Table 1: 2,5′-thiophenedicarboxaldehyde, depicted in Figure 1 by (1), 2,7-bis[(ptriphenylphosphonium)methyl]-9,9′-di-n-hexylfluorene dibromide (x), and p-xylylenebis(triphenylphosphonium) bromide (y) were dissolved in 30 mL of chloroform in a 100 mL flask. An amount of 30 mL of an ethanol solution of potassium tbutoxide was added dropwise at room temperature. After reacting overnight, 5 mL of hydrochloric (2% w/v) acid was added slowly, and the mixture was poured over 800 mL of cold methanol with stirring. After filtration, the collected yellow

2,5′thiophenedicarboaldehyde (1) (mmol)

fluorene monomer (x) (mmol)

phenylene monomer (y) (mmol)

1.435 1.435

1.435 0.717

-0.717

1.435

0.359

1.076

precipitate was dissolved in 10 mL of chloroform and reprecipitated in 800 mL of cold methanol. The copolymer was filtered and dried under vacuum for 48 h. Solid-state 13C NMR δ: 152.04, 141.01, 136.21, 128.33, 122.90, 54.78, 41.32, 31.08, 23.54, 14.35 ppm. Characterizations. The chemical structures and the composition of the copolymers were confirmed by solid-state 13 C NMR using a VARIAN INOVA spectrometer at 13C frequencies of 100.5 MHz. Details about the NMR experiments can be found in the Supporting Information. The molar masses of the copolymers were measured by a gel permeation chromatograph (Agilent 1100) equipped with a refractive index detector and PL gel mixed C and B columns in series, at 35 °C, using THF as solvent and monodisperse polystyrene samples as standards. The UV−vis spectra from copolymer films with the same thickness spin-coated onto quartz from chloroform solutions were taken in a Shimadzu spectrophotometer model NIR 3101. Steady-state PL spectroscopy was performed in a UV 2401 PC Shimadzu spectrophotomer, double beam. Films were supported on quartz slides, and the emission spectra were recorded using a front-face sample orientation. Slits were selected for a spectral resolution of ±1 nm in excitation and in emission. Optical bandgaps were determined from the onset of the absorption spectra. Cyclic voltammograms (CVs) were carried out on a potentiostat/galvanostat AUTOLAB PGSTAT 30 with a cell composed by three electrodes: platinum (Pt) working electrode, reference electrode of Ag/Ag+ in acetonitrile, and Pt counter electrode. The supporting electrolyte was 0.1 M tetrabutyl ammonium hexafluorophosphate in acetonitrile, and the scanning rate was 50 mV s−1. The emission spectra from the copolymer films spin-coated onto quartz from 18642



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measured properties were accounted for in this parameter. They are also within the limits of similar polymer structures reported.26 Since the reactivity of the comonomers may vary, the copolymer composition may differ from that of the feed composition used in the polymerization reaction medium. Therefore, the quantitative composition of each copolymer was characterized by NMR spectroscopy using the same methodology as described in ref 29 (see Supporting Information for details on the NMR experiment). The experimental x/y ratio is in good agreement with the theoretical x/y ratio as shown in Table 2, showing that the actual values of the x/y ratios are very close to that expected from the feed composition.

chloroform solutions were taken in a Shimadzu spectrofluorophotometer model 5301 PC. Fabrication and Characterization of Photovoltaic Devices. The bilayer heterojunction devices were fabricated by the deposition of PEDOT:PSS at 5000 rpm and annealed at 100 °C for 15 min onto precleaned patterned tin oxide doped with fluorine (FTO) substrates (10−20 Ω per square).30 Each copolymer film was spin coated in air from 5 mg mL−1 chloroform solution, giving a thickness around 30 nm on top of the electrodes FTO/PEDOT:PSS with no subsequent thermal annealing. Then, 30 nm of buckminsterfullerene C60 and 100 nm of Al were thermally evaporated through a shadow mask at a vacuum pressure of 6 × 10−6 Torr. The active area was 3 mm2. Subsequently, the FTO/PEDOT:PSS/active layer/ C60/Al devices were encapsulated under a nitrogen atmosphere using glass and epoxy. The bulk heterojunction solar cells were fabricated by the deposition of PEDOT:PSS at 5000 rpm and annealed at 100 °C for 15 min onto precleaned patterned indium tin oxide (ITO) coated glass substrates. The photoactive layer of each copolymer blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in a 1:1 weight rate of copolymer:PCBM was spin coated under a nitrogen atmosphere from 20 mg mL−1 chlorobenzene solution giving a thickness around 100−130 nm. The substrates were then put in a thermal evaporation chamber to evaporate 70 nm of the Al layer under high vacuum (6 × 10−6 Torr) through a shadow mask. The final active area was 8 mm2. Then, the ITO/PEDOT:PSS/active layer:PCBM/Al devices were encapsulated under a nitrogen atmosphere by a thin spin-coated layer of the commercial polymer Cytop.31 The inkjet printed PFT−PPV 50−50:PCBM active layer was also deposited under a nitrogen atmosphere from a 20 mg mL−1 chlorobenzene solution using the printer system located at the Palo Alto Research Center. It consists of a piezoelectric print head, translation stages, substrate holder, and alignment camera, as described by Street et al.32 The copolymer charge transport properties were investigated by the current density−voltage (J−V) curves taken in the dark of devices with the following structure: glass/FTO/PEDOT:PSS/copolymer/MoO3/Al. All the reported solar cells were characterized by their spectral response and current density versus voltage curves. The spectral response is expressed by the external quantum efficiency (EQE) which is the ratio of the photocurrent to incoming photons flux: EQE = 1240 Jph/λI0, where Jph is the photocurrent density (A cm−2); I0 is the light intensity (W m−2); and λ is the wavelength (nm). All the photovoltaic characterization was performed in a Keithley picoammeter with power supply (model 6487) and a monochromator/spectrometer (1/4 m Oriel) using a homemade software (SICADI) in Basic programming language to control the equipment. The solar simulation was made using an air mass (AM 1.5) filter with a power illumination of 100 mW cm−2 from a 150 W Oriel Xenon lamp. The morphology of the film was investigated by atomic force microscopy Shimadzu SPM-9500 J3 operating in dynamic mode, and thickness measurements were performed with a Dektak 150 profilometer (Veeco Instruments).

Table 2. Quantifications of the x/y Ratio of the Copolymers from the 13C CPMAS Solid-State NMR Spectra and Energy Levels

copolymer LaPPS35−0 LaPPS35− 50 LaPPS35− 75 a

fluorene/ phenylene

fluorene/ phenylene

x/y theoreticala

x/y experimentalb

HOMO (eV)

LUMO (eV)

Eg (eV)

100/0 50/50

100/0 43/57

−5.4 −5.5

−3.1 −3.1

2.3 2.4

25/75

23/77

−5.4

−3.2

2.2

Molar composition in reaction feed. bCopolymer molar composition.

The energy levels of the copolymers are estimated with a combination of absorption measurements and cyclic voltammetry (CV) and are displayed in Table 2. The highest occupied molecular orbital (HOMO) is measured at the onset oxidation potentials in the voltammograms. As the reduction potentials are not clearly seen in CV, the lowest unoccupied molecular orbital (LUMO) energy was calculated using the optical band gap energy (Ep) and HOMO values. The Ep was estimated from the absorption spectra (Figure 2(a)) using Tauc’s equation, which relates the absorption coefficient (α) with the optical γ 33 band gap energy: ℏωα ∼ (ℏω − Eopt These energy levels g ) . are similar, and there is no trend correlating the band gap energy with the increase of the phenylene−thiophene content in the chemical structure. The photophysical properties such as absorption and emission spectra for the copolymer film with the same thickness (30 nm) onto quartz substrates are displayed in Figure 2(a). The absorption spectrum of the copolymer containing only fluorene−thiophene units peaks at 450 nm. The maximum peak for LaPPS35−50 is 417 nm and for LaPPS35−75 is 430 nm. Furthermore, the absorption coefficient of the 50−50 copolymer is the highest within the visible region as compared to the other compositions. The emission spectra of the copolymers present a red shift with the increase of the phenylene−thiophene content. A similar behavior was already found for other systems with increasing concentration of thiophene groups in the copolymer chain.26,34 Copolymer hole mobilities were evaluated with a diode configuration of FTO/PEDOT:PSS/copolymer/MoO3/Al by taking the current density−voltage (J−V) curves in the dark and using the space charge limited current model J = 9ε0εμV2/ 8L3, where ε0 is the permittivity of free space, ε the polymer dielectric constant, μ the hole mobility, V the voltage drop across the device, and L the polymer thickness. Figure 2(b)

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RESULTS AND DISCUSSION The values of Mw for LaPPS35−0, LaPPS35−50, and LaPPS35−75 were 7700, 7700, and 7500 g mol−1, respectively. Since these values are in the same range, no differences in the 18643



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Figure 2. (a) Absorption coefficient (left axis) and normalized emission spectra (right axis) for LaPPS35−0, LaPPS35−50, and LaPPS35−75 copolymers. Films were spin-coated onto quartz substrates with the same thickness (30 nm). (b) (J−V) characteristics of FTO/PEDOT:PSS/copolymer/MoO3/Al devices in the dark.

shows the typical J−V for each one of the copolymers, and the values of hole mobilities (μ) were found to be 1.68 × 10−7, 2.34 × 10−5, and 3.12 × 10−7 cm2 V−1 s−1 for LaPPS35−0, LaPPS35−50, and LaPPS35−75, respectively. The LaPPS35− 50 hole mobility is higher by 2 orders of magnitude when compared to the other copolymers, suggesting that this material presents better intermolecular π−π stacking and thus higher film organization.14,35 The surface morphology of the copolymer films coated on glass/FTO/PEDOT:PSS substrates is shown in Figure 3. The values of root-mean-square (rms) roughness are 15 ± 2, 35 ± 4, and 18 ± 3 nm for LaPPS35−0, LaPPS35−50, and LaPPS35− 75, respectively. The LaPPS35−50 film presents a higher rms roughness value when compared to the other copolymers. LaPPS35−0 and LaPPS35−75 present similar values of rms roughness despite varying compositions. The chains of the copolymers can self-organize into different structures depending primarily on the relative lengths of each unit, which will affect the film morphology, thus affecting the roughness.36 The roughness is affected by several parameters such as polymer content, solvent solubility, and deposition method. In our study, the latter was kept constant, and the solvent solubility was similar for all the materials, suggesting that the film morphology is mainly affected by the relative lengths of each copolymer unit. Bilayer devices are good device architectures to study photovoltaic conjugated polymers, as the optoelectronic properties can be investigated in separated layers with a welldefined active region (copolymer/C60 interface) without the influence of materials miscibility, as in the case of bulk heterostructures. The photovoltaic properties of each copoly-

Figure 3. AFM images of each copolymer coated on glass/FTO/ PEDOT:PSS substrates: (a) LaPPS35−0, (b) LaPPS35−50, and (c) LaPPS35−75. Scan size is 10 μm × 10 μm and Z-height range is 0− 220 nm.

mer were investigated in bilayer heterojunction solar cells using the following configuration: glass/fluorine-doped tin oxide (FTO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/copolymer layer/buckminsterfullerene C60/Al, with no thermal annealing of the copolymer layer. Due to the high electron affinity value of C60, fast charge transfer from the copolymer to the small molecule should be possible as the C60 improves the exciton dissociation in the bilayer geometry because most of the available excitons near the interface polymer/C60 contribute to the photocurrent as reported elsewhere.37,38 18644



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efficiency when compared to the devices based only on fluorene−thiophene units. The highest photovoltaic performance was found for devices fabricated with LaPPS35−50 as the photoactive layer, with power conversion efficiency (PCE) of 2.40%, open circuit voltage (Voc) of 0.75 V, short current density (Jsc) of 8.30 mA cm−2, and fill factor (FF) of 38%. Devices based on LaPPS35−0 exhibited a Jsc of 5.10 mA cm−2, Voc of 0.90 V, FF of 24%, and PCE of 1.09%. For the LaPPS35−75 copolymer we have found a Jsc of 6.15 mA cm−2, Voc of 0.80 V, FF of 28%, and PCE of 1.40%. Despite the lower value of Voc, LaPPS35−50 presented a higher Jsc which is related to the higher hole mobility found for this copolymer. The increase in PCE in this device can also be attributed to the higher roughness of the film providing larger interfacial contact area between the copolymer and the molecule C60. This effect can also be obtained by thermal annealing of the polymer/C60 device improving charge collection as the donor/acceptor layers intermix.39 Our bilayer devices were not annealed, and as the LaPPS35−50 roughness is on the order of the film thickness (∼30 nm) when C60 is deposited, the interface formed is more like an ideal bulk heterojunction with interpenetrating network of the two phases. The results obtained for the LaPPS35−50 copolymer bilayer devices motivated the fabrication of the copolymers in bulk heterojunction structures. Bulk heterojunction (BHJ) organic photovoltaic devices present the intrinsic advantage of having the photoactive layers composed by electron donors and acceptors deposited from one solution. It has been shown that the photogeneration efficiency and ambipolar transport of charges in BHJ devices are highly connected to the miscibility of the electron acceptor, thus affecting the morphology of the film.40,41 The photovoltaic properties of the copolymers was further investigated using the following configuration: glass/ indium tin oxide (ITO)/(PEDOT:PSS)/active layer/Al. The active layer was composed of spin-coated blends of each copolymer with fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in a 1:1 weight ratio of copolymer and PCBM. Self-forming heterojunctions present opportunities for printing-based production of solar cells.4,5 The highest reported efficiency of a inkjet printed device is 3.5% for a solar cell based on poly(3-hexylthiophene):PCBM.42 To explore printing feasibility, we have also fabricated devices with the copolymer LaPPS35−50 where the active layer was deposited by inkjet printing instead of spin coating. The spectral response of the BHJ devices is presented in Figure 5(a). The comparison of the action spectra from the three copolymers deposited by spin coating shows that the blend LaPPS35−50:PCBM presented the highest photocurrent conversion with maximum EQE value around 33% at λ = 450 nm. EQE values of 20% and 12% were measured for the LaPPS35−0:PCBM and LaPPS35−75:PCBM blends, respectively. Figure 5(b) shows the current density−voltage (J−V) curves for the BHJ devices, and the photovoltaic parameters obtained from these curves are summarized in Table 4. The device based on the spin-coated LaPPS35−50 presented the highest photovoltaic performance with PCE of 0.70%, Voc of 0.55 V, Jsc of 2.72 mA cm−2, and FF of 47%. Although the inkjet (IJ) printed device of LaPPS35− 50:PCBM presented a promising value of EQE (29% at λ = 450 nm) as shown in Figure 5(a), the device had poorer photovoltaic performance (Figure 5(b)). The fill factor for the IJ device is low (25%), indicating that the series resistance is

The spectral response of the bilayer devices is presented in Figure 4(a). The external quantum efficiency spectra of the

Figure 4. (a) External quantum efficiency for bilayer devices (FTO/ PEDOT:PSS/copolymer layer/C60 /Al). (b) J−V curves under AM 1.5 illumination of 100 mW cm−2 for the same devices in (a). Power conversion efficiency (PCE) for each copolymer: LaPPS35−0 (1.09%), LaPPS35−50 (2.40%), and LaPPS35−75 (1.40%).

devices follows the absorption spectra of the copolymers and the C60 reflecting the contribution of the copolymers and the fullerene to the photocurrent. Comparison of the action spectra from the three copolymers shows that the most efficient device of the series was prepared with the LaPPS35−50 copolymer. Its maximum EQE value was around 52% under illumination at λ = 450 nm. Bilayer devices based on copolymers LaPPS35−0 and LaPPS35−75 presented a maximum EQE value around 35% and 45%, respectively. This result agrees with the absorption coefficient presented in Figure 2(a). The higher absorption coefficient exhibited by the LaPPS35−50 copolymer indicates that once illuminated more excitons are available for dissociation. The J−V curves are shown in Figure 4(b), and the photovoltaic parameters obtained from these curves are summarized in Table 3. The devices based on copolymers containing phenylene−thiophene units presented higher Table 3. Device Characteristics of the Solar Cells Based on the Bilayer Heterojunction with the C60 Molecule Tested under AM 1.5 Conditions copolymer LaPPS35−0 LaPPS35− 50 LaPPS35− 75

Jsc (mA cm−2)

Voc (V)

FF (%)

PCE (%)

active layer thickness (nm)

5.10 8.30

0.90 0.75

24 38

1.09 2.40

35 30

6.15

0.80

28

1.40

30

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Figure 6. AFM height and simultaneously acquired phase (inset) images of each 1:1 blend by weight of copolymer and PCBM on glass/ ITO/PEDOT:PSS substrates: (a) LaPPS35−0, (b) LaPPS35−50, (c) LaPPS35−75, and (d) LaPPS35−50 deposited by inkjet printing. Scan size is 10 μm × 10 μm.

with the formation of PCBM-rich domains and polymer-rich domains which improve the charge transport properties by creating electron acceptor−donor interfaces for exciton dissociation if the phases are interpenetrating (e.g., interdigitated fingers).44,45 In our case, the domains at the surface are segregated which could be altered by optimizing the printing conditions such as temperature or solvent to further improve the electrical properties. Finer phase separation achieved by inkjet printing over spin coating from micrometer-sized to nanometer-sized domains has been reported in polyfluorene BHJ cells to further enhance photovoltaic performance.46,47 The improved miscibility of copolymer LaPPS35−50 with the PCBM molecules in spin-coated devices and the absence of coarse-phase separation in the AFM images (Figure 6(b)) led to better device performance when compared to the other copolymers. The poorest performing BHJ spin-coated device is based on LaPPS35−75, also corroborating with the morphology observed by AFM where it is possible to see different phases for the materials in microscaling (Figure 6(c)). When compared to the bilayer devices the BHJ devices presented low values of power conversion efficiency indicating that future investigations are needed to improve device efficiency: optimal blend ratio (copolymer:PCBM) is necessary to create phase separation and fullerene intercalation for efficient BHJ devices.48

Figure 5. (a) External quantum efficiency for bulk heterojunction devices with the PCBM molecule in a 1:1 weight ratio of copolymer:PCBM. The solid line represents the IPCE curve for the LaPPS35−50 active layer deposited by inkjet printing. (b) J−V curves under AM 1.5 illumination of 100 mW cm−2 for the same devices in (a). Power conversion efficiency (PCE) for each copolymer: LaPPS35−0 (0.50%), LaPPS35−50 (0.70%), LaPPS35−75 (0.13%), and LaPPS35−50 inkjet printed (0.12%).

Table 4. Device Characteristics of the Solar Cells Based on Bulk Heterojunction with the PCBM Molecule in Weight Proportion of 1:1 (Copolymer:PCBM) Tested under AM 1.5 Conditions copolymer LaPPS35−0 LaPPS35−50 LaPPS35−75 inkjet printed LaPPS35−50

Jsc (mA cm−2)

Voc (V)

FF (%)

PCE (%)

active layer thickness (nm)

1.87 2.72 1.45 1.00

0.65 0.55 0.25 0.50

41 47 21 25

0.50 0.70 0.13 0.12

130 100 100 700

large.43 The active layer thickness of the IJ device was 700 nm. Devices based on thick active layers have the efficiency compromised by the transport due to higher resistance of the layer. It was necessary to adopt this thickness to ensure reproducibility in our fabrication process. However, it is clear that printing conditions need to be further optimized to achieve better device characteristics. The surface morphology of LaPPS35−50:PCBM active layers deposited by inkjet printing and spin coating was investigated by AFM and is shown in Figure 6. The inkjet printed film presented quite different surface morphology (Figure 6(d)) when compared to the smoother surface of the spin-coated sample (Figure 6(b)). The inkjet surface has small circular features with a height of about 10 nm and width ranging from 0.5 to 0.7 μm, which may be domains of different materials. Phase separation is typically observed in BHJ devices

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CONCLUSION A new series of donor−acceptor copolymers based on the combination of thiophene, phenylene, and fluorene units was synthesized and investigated as an active layer in bulk heterojunction and bilayer device architectures. The copolymer contents were confirmed by NMR spectroscopy, and the photophysical, electrical, and morphological properties were correlated with device performance. The film morphology investigation showed high roughness for the LaPPS35−50 film indicating that in fact the bilayer device is more like a heterojunction structure obtained by the C60 evaporation. Moreover, it was found that for both device structures the LaPPS35−50 presented the best photovoltaic performance 18646



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which is highly related to the film morphology and also corroborating with absorption coefficient and hole mobility value. A PCE of 2.4% has been obtained for the LaPPS35−50 bilayer device which is a quite good result considering this simple device architecture. For the bulk heterojunction structure it was possible to inkjet print LaPPS35−50:PCBM films indicating that this copolymer could be applied in flexible electronics. Despite the promising monochromatic efficiency (EQE), the device performance and morphology indicate that the printing conditions must be improved to achieve higher power conversion efficiency.

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ASSOCIATED CONTENT

S Supporting Information *

Solid-state 13C NMR spectra of the copolymers LaPPS35−0, LaPPS35−50, and LaPPS35−75. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +55 41 3361-3661. Fax: +55 41 3369-5055. E-mail: nady04@fisica.ufpr.br. Present Address ⊥

Department of Electrical Engineering and Computer Science, University of California, Berkeley, United States of America Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank CAPES, CNPq, the Palo Alto Research Center (PARC), and INEO for financial support. ERdA also thanks FAPESP (proc. 2009/18354-8).

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