Polythiophene Composites for

Article pubs.acs.org/Macromolecules

Morphology Control in TiO2 Nanorod/Polythiophene Composites for Bulk Heterojunction Solar Cells Using Hydrogen Bonding Ying Lin, Qingshuo Wei, Gang Qian, Li Yao, and James J. Watkins* Department of Polymer Science and Engineering, University of Massachusetts Amherst, 120 Governors Drive, Amherst, Massachusetts 01003, United States S Supporting Information *

ABSTRACT: Hydrogen bond interactions between a dye adsorbed at the interface of TiO2 nanorods and functionalized P3HT was used to control nanorod dispersion, increase interfacial area, and improve efficiency in solution-processable hybrid bulk heterojunction solar cells. A series of poly(3hexylthiophene-b-ethylene glycol) (P3HT-b-PEG) copolymers were prepared by a combination of Grignard metathesis polymerization and click chemistry. The short PEG segments in P3HT-b-PEG serve as a hydrogen bond acceptor. TiO2 nanorods functionalized with N3-dye bearing multiple COOH groups function as both the electron acceptor and hydrogen bond donor. The strong preferential H-bonding interaction between TiO2 nanorods and the PEG chain limits the aggregation of the TiO2 nanorods and affords homogeneously dispersion of the nanorods within the polymer matrix to form an interpenetrating network. This structure provides large interfacial area between electron donor and acceptor and highly efficient transport pathways within the composite. Hybrid devices constructed from copolymers with 10 wt % PEG and N3-dye TiO2 nanorods exhibit power conversion efficiency ∼50% higher than that of conventional P3HT homopolymer and N3-dye TiO2 nanorods.

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INTRODUCTION Hybrid organic−inorganic bulk heterojunction (BHJ) solar cells constructing from a blend of electron-donating conjugated polymers and inorganic semiconductor nanomaterials have attracted considerable attention in recent years.1 These nanostructured hybrid devices possess many advantages attributing to conjugated polymers, such as flexibility, solution processability, roll-to-roll production, and large-area capability,2,3 in combination with the relatively high electron mobility and the physical and chemical stability of inorganic nanocrystals.4 Of the organic/inorganic hybrids, poly(3-hexylthiopene) (P3HT) is one of the most extensively studied semiconducting polymers due to its high charge carrier mobility, excellent solution processability, and environmental stability.1,5 So far, a variety of nanomaterials such as CdSe,1 CdS,6 PbS,7,8 and TiO29−14 have been explored for applications in hybrid solar cells. Although the reported best performance of P3HT/TiO2 BHJ solar cells (∼2%)15 is significantly lower compared with state-of-the art polymer/CdSe or CdS BHJs, advantages of TiO2 BHJ devices include environmentally acceptability, low toxicity, and low cost. The nanoscale morphology of the active layer plays an important role in determining the performance of hybrid devices. While phase separation of the active components is required for creating continuous charge transport pathways, micrometer-scale phase domains are unfavorable due to low interfacial area and low charge separation efficiencies.2,16−18 In hybrid systems, it remains challenging to achieve a controllable dispersion of nanocrystals within devices at the nanoscale. © 2012 American Chemical Society

Specifically due to poor compatibility, inorganic nanocrystals tend to aggregate in the presence of nonpolar conjugated polymers on micrometer scale. Although insulating organic ligands (e.g., oleic acid, tri-n-octylphosphine oxide, hexadecylamine, etc.) can facilitate the dispersion of nanocrystals in polymers, their presence severely reduces the device efficiency by impeding the charge transfer between nanocrystal and polymer as well as charge transport between adjacent nanocrystals.1,19 Therefore, the next generation of hybrid nanocrystal photovoltaics will require strategies to achieve a controllable three-dimensional interpenetrating network and a well-defined interface between nanocrystals and the polymer matrix.6,20 To this end, a number of methods have been pursued to obtain a favorable dispersion of nanocrystals in BHJ hybrid photovoltaics.6,21,22 For example, Frechet and coworkers have shown that amine end-functional P3HT enhances the performance of P3HT/CdSe solar cells by increasing the dispersion of CdSe nanorods, leading to a PCE of 1.6%.21 Very recently, controlled organic−inorganic phase separation was achieved by bonding CdS quantum dots onto crystalline P3HT nanowires through solvent-assisted grafting and ligand exchange, and an improved maximum power conversion efficiency (PCE) of 4.1% was established.6 TiO2 nanorods modified by a carboxylic acid-terminated 3-hexylthiophene oligomer can be well dispersed in P3HT, giving markedly Received: September 15, 2012 Revised: October 16, 2012 Published: October 25, 2012 8665

dx.doi.org/10.1021/ma3019393 | Macromolecules 2012, 45, 8665−8673

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Scheme 1. Hydrogen Bonding between P3HT-b-PEG and N3-Dye TiO2 and a Schematic of the Blend That Serves as the Active Layer in the Photovoltaic Device

Stark and passed through a 0.45 μm PES syringe filter before spincoating. Anhydrous THF was distilled from sodium benzophenone. Anhydrous chlorobenzene was purchased from Sigma-Aldrich and used without purification. All other reagents were used as received, unless otherwise stated. Synthesis of Ethynyl-Terminated P3HT Polymer. Ethynylterminated poly(3-hexylthiophene) was prepared according to the literature procedure reported by McCullough.34 2,5-Dibromo-3hexylthiophene (0.978 g, 3.0 mmol) was introduced to a three-neck round-bottom flask under N2 and then evacuated under reduced pressure to remove any moisture and oxygen inside. Anhydrous THF (20 mL) was added via a syringe, and the solution was stirred and equilibrated at 0 °C. 2.3 mL of a solution of i-PrMgCl·LiCl in THF (1.3 M, 3.0 mmol) was added via a syringe, and the mixture was stirred at 0 °C for 2 h. The reaction mixture was then diluted to 50 mL with THF, and Ni(dppp)Cl2 (15.8 mg, 0.03 mmol) was added in one portion. The mixture was stirred for 10 min at room temperature, and then ethynylmagnesium bromide (0.5 M solution in THF, 1.0 mmol) was added via syringe. The mixture was stirred for an additional 2 min and then poured into methanol to precipitate the polymer. The polymer was filtered into an extraction thimble and then washed by Soxhlet extraction with methanol, hexanes, and chloroform. The polymer was isolated from the chloroform extraction. The 1H NMR spectrum of the material agreed with that reported (terminal ethynyl peak at 3.52 ppm) and GPC indicated Mn = 17K, PDI = 1.07. Synthesis of P3HT-b-PEG Block Copolymers. The typical synthesis procedure of diblock copolymer was as follows: one roundbottomed flask (100 mL) equipped with a three-neck stopcock was flame-dried and cooled to room temperature. A mixture of ethynylterminated P3HT (34 mg, 2.0 mmol), azide-terminated PEG (3.0 mmol), CuI catalyst (4 mmol), THF (20 mL), and (i-pr)2NEt (1 mL) were introduced to the flask under N2. Freeze−pump−thaw was applied for three cycles. The mixture was vigorously stirred under N2 at 50 °C for 2 days to complete the reaction. The reaction was quenched by adding a large excess MeOH. The crude polymer was washed successively by Soxhlet extraction using methanol, hexane, and chloroform. The solvent was removed by evaporation to give a purple solid. The chloroform fractions were analyzed by 1H NMR and GPC. Synthesis of TiO2 Nanorods with N3-Dye Ligand. Anatase TiO2 nanorods were synthesized according to the low-temperature sol−gel method developed by Cozzoli et al. in a reversed micelle solution.35 In a typical synthesis, 35 g of oleic acid was dried at 120 °C for 1 h under vigorous stirring in a 50 mL three-neck flask connected to a reflux cooler, after which it was cooled to 90 °C under nitrogen flow. 5 mmol of titanium tetraisopropoxide (1.5 mL) was then added and allowed to stir for 5 min: the solution turned from colorless to pale yellow, indicating the formation of a complex. The absence of water at this stage prevented the premature hydrolysis of the molecular precursor. 0.75 g of trimethylamine-N-oxide dihydrate solution (TMAO) in 5 mL of H2O was rapidly injected via syringe. Upon injection, the mixture appeared turbid and became more viscous as the reaction proceeded. The solution was maintained in a closed system at 90 °C and stirred under mild reflux overnight to promote further

improved photovoltaic efficiency of the resulting solar cells (PCE = 1.19%), which was more than 2 times higher than that of the device made from the physical mixture of P3HT/ pyridine-capped TiO2 nanorods (PCE = 0.54%).23 Nevertheless, much development is still needed to prepare intimate nanocomposites of conjugated polymers and semiconductor nanocrystals, specifically TiO2, in hybrid solar cells. Lessons learned from these developments can be applied to other hybrid systems. In this contribution, we have synthesized a series of P3HT-bpoly(ethylene glycol), P3HT-b-PEG, block copolymers with fixed P3HT block length via sequential Grignard metathesis polymerization and click chemistry. N3-dye bearing multiple COOH groups, which can be readily adsorbed to the TiO2 nanorods, was chosen to act as interface modifier (Scheme 1). The use of strong H-bonding interactions between the acid functionality of the TiO2 nanorods, which serves as a hydrogen bond donor, and PEG adjacent to polythiophene segments can effectively disperse the TiO2 nanocrystals to afford intimate nanocomposites with favorable morphology. Earlier reports demonstrate the hydrogen bond interactions in block copolymer/nanoparticle24 and other block copolymer/additive systems25−27 can yield well-ordered materials with high additive loadings. It has also been appreciated that hydrogen bonding encourages better molecular level ordering, enhances morphological stability, promotes interfacial electron transfer, improves charge transport, reduces charge trap sites, and extends device lifetime.28−32 Herein the use of hydrogen bonding through an interface modifier (in this case the dye) demonstrates a useful way to disperse nanocrystal domains within a polymer matrix. In this paper, we emphasize the H-bonding-related morphology and the correlation between morphology and photovoltaic behavior. We believe that this work provides a general and novel solution to the important problem of inorganic nanocrystal aggregation in composites not only for optimizing efficiency of hybrid solar cells but also for the dispersion of nanocrystals in other hybrid devices.

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EXPERIMENTAL SECTION

Materials. 2,5-Dibromo-3-alkoxythiophene was synthesized by a modified literature method.33 Azide-terminated poly(ethylene glycol) (N3-PEG) 1K, 2K, and 5K were purchased from Polymer Source; N3PEG 10K was purchased from Nanocs, Boston, MA. 2,5-Dibromo-3hexylthiophene, isopropylmagnesium chloride lithium chloride complex (i-PrMgCl·LiCl, 1.3 M solution in THF), and ethynylmagnesium bromide (0.5 M solution in THF) were purchased from SigmaAldrich. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (Baytron PVP AI 4083) was purchased from HC 8666


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Figure 1. Synthetic route to P3HT-b-PEG block copolymer. hydrolysis and crystallization of the product. Finally, TiO2 nanocrystals were precipitated upon addition of an excess of ethanol to the reaction mixture at room temperature. The resulting precipitate was isolated by centrifugation and washed twice with ethanol to remove residual surfactant. The oleic acid-coated TiO2 nanorods were then easily redispersed in solvents such as chloroform or hexane, without any further growth or irreversible aggregation. The ligand exchange process was as follows: the as-synthesized oleic acid-capped TiO2 nanorods were mixed with the N3-dye in an ∼10:1 weight ratio, dispersed in pyridine, and left stirring under N2 at 50 °C until the solution turned purple. The extraction procedures were subsequently conducted in air. TiO2 nanocrystals were readily precipitated upon addition of an excess of hexane to the reaction mixture at room temperature. The resulting precipitates were isolated by centrifugation, and these modified TiO2 nanorods were then redispersed in a mixed solvent which consisted of pyridine, chloroform, and dichloromethane in a 1:2:2 volume ratio, without any further growth or irreversible aggregation. Characterization. 1H NMR spectra were recorded on a Brükerspectrospin 300 using the residual proton resonance of the solvent as a reference point. The compositions of block copolymers were determined by comparing integrated intensities for the methylene groups directly attached to the thiophene rings, found at 2.70 for the P3HT block and 3.65 for the PEG block. Molecular weights of the polymers were estimated by gel permeation chromatography (GPC) using THF as the eluent against polystyrene standards with a refractive index detector. UV/vis absorption spectra were obtained from a Shimadzu 1601 UV spectrometer. DSC thermograms were measured using a TA Instruments Q100 DSC equipped with an RCS cooling system and nitrogen gas purge with a flow rate of 50 mL/min. All measurements were conducted in the temperature range of 20−300 °C at a constant heating and cooling rate of 10 °C/min under a nitrogen atmosphere. The reported DSC thermograms were measured during the second heating cycle. For analysis of the films by transmission electron microscopy (TEM), the solar cell device specimens were scored and immersed in water to promote dissolution of the PEDOT:PSS layer. The delaminated film was then floated onto a copper TEM grid. TEM was performed on films using a JEOL 2000 CX microscope operating at 200 kV. Atomic force microscope (AFM) measurements were performed using a Nanoscope IIIa microscope (Veeco Instruments, Santa Barbara, CA) with silicon probes in tapping mode. Transistor Device Fabrication and Characterization. Topcontact and bottom-gate geometry was used to fabricate and test fieldeffect transistors prepared using the copolymers. Heavily n-doped silicon substrates acted as gate electrode and thermally grown silicon dioxide (300 nm thick; Ci = 10.7 nF/cm2) as gate insulator. Octadecyltrichlorosilane (OTS)-modified SiO2 was prepared by soaking the substrates in a 5 mM toluene solution of OTS for 12 h in a dry N2-filled glovebox. Substrates with the structure of glass/PSS/ P3HT or modified P3HT copolymer were prepared by successive spin-coating of an aqueous solution of PSS and a 2 mg/mL chlorobenzene solution of the polymer. This polymer film was gently brought into contact with the target substrate with the polymer face down. One drop of water was placed on the edge of the two stacked substrates. The PSS layer was selectively permeated by water. After 1− 5 min, the water flowed from one side of the substrate to the other,

and the PSS layer was completely dissolved. Finally, the glass substrate was easily detached from the organic layer, resulting in the transfer of the polymer film from the glass to the target substrate. Gold electrodes were evaporated onto the surface through a metal mask with a channel width (W) of 500 μm and length (L) of 50 μm. Output (Ids vs Vds) and transfer (Ids vs Vg) characteristics of the devices were measured on a Keithley 4200 semiconductor characterization system (Keithley Instruments, Cleveland, OH). Field-effect mobility was calculated from the standard equation for saturation region: Ids = μ(W/2L)Ci(Vg − Vt)2, where Ids is drain-source current, μ is field-effect mobility, W and L are the channel width and length, Ci is the capacitance per unit area of the gate insulator (Ci = 10.7 nF/cm2), Vg is the gate voltage, and Vt is the threshold voltage. Solar Cell Device Fabrication and Characterization. Blend solutions were prepared by dissolving polythiophene/TiO2 nanorods of different ratios in chlorobenzene/CHCl3/pyridine at a total concentration of 2 wt %. The solutions were stirred for 12 h before device fabrication. The photovoltaic devices were fabricated according to the following procedure: ITO-coated glass was cleaned with detergent followed by ultrasonication in water, acetone, and isopropyl alcohol and then kept at 150 °C for 30 min. After complete drying, ITO-coated glass was treated with UV-ozone to improve the wettability of PEDOT:PSS. After spin-coating 35 nm of PEDOT:PSS, the active layer was spin-coated on top of the PEDOT:PSS layer at 1500 rpm for 60 s. For thermal annealing, the uncompleted devices were placed directly onto a digitally controlled hot plate and heated to 150 °C for 5 min. Al (100 nm) was then thermally evaporated under vacuum lower than 3 × 10−6 Torr on the top of the active layer through a shadow mask. All current−voltage (I−V) characteristics of the devices were measured under simulated AM1.5G irradiation (100 mW cm−2) using a Xe lamp-based Newport 91160 300 W solar simulator. The light intensity was adjusted with an NREL-calibrated Si solar cell with a KG-5 filter. The results reported represent the median of six sets of devices.

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RESULTS AND DISCUSSION Figure 1 illustrates the synthetic approach to the P3HT-b-PEG diblock copolymer via the combination of Grignard metathesis polymerization and click chemistry. P3HT-b-PEG diblock copolymers have previously been synthesized by anionic polymerization and Grignard metathesis polymerization and used as a compatibilizer in P3HT/PCBM organic photovoltaics.36 Herein the click chemistry was chosen due to its high reactivity, quantitive yields, and mild conditions required. More specifically, the P3HT block was prepared using Ni-catalyzed quasi-living polymerization of 2,5-dibromo-3-hexylthiophene, followed by the addition of ethynyl Grignard reagent34 to produce ethynyl-capped P3HT. We utilized the Cu(I)mediated Huisgen 1,3-dipolar cycloaddition reaction popularly known as “click chemistry” to couple ethynyl-terminated P3HT and azide-bearing PEG. A series of azide-functionalized PEGs of different molecular weight were used. Comparison of the 1H NMR spectra of the ethynyl-capped P3HT to that of copolymer shows that the terminal ethynyl peak at 3.52 ppm 8667


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to be established. All of the block copolymers exhibit good solubility in CHCl3, THF, and cholorobenzene; P4 with 37 wt % PEG is even readily soluble in MeOH and DMF. The optical properties of the copolymers were investigated by UV−vis spectroscopy in the solid state without any thermal treatment. The normalized absorption spectra of all copolymers (Figure 2a), except P4, exhibit three characteristic peaks at 490, 560, and 610 nm similar to P3HT homopolymer. Interestingly, the P1 and P2 show stronger absorption shoulders at 610 nm relative to that of P3HT homopolymer. Such absorption shoulders have been observed in highly ordered, regioregular P3HT films prepared by thermal annealing or slow evaporation of the solvents and is associated with an interchain absorption, the intensity of which is correlated with the degree of order in the polymer.37 Thus, the enhanced absorption shoulder indicates that the P3HT domain in P1 and P2 films is more highly ordered, even though the percentage of crystalline P3HT was reduced to 94.5% and 90% of the total polymer sample. The absorbance spectrum of P4 film exhibits a blue shift of the main absorption maximum compared to that of P3HT homopolymer. No distinct shoulder is observed on the longwavelength side of the absorption maximum, suggesting that P3HT in P4 is less ordered with the high PEG fraction. To better understand the absorption behavior, the morphology of thin films prepared using the same conditions as those used for UV−vis spectroscopy was investigated by atomic force microscopy (AFM). Figures 2b and 2c compare the AFM phase images of neat P3HT and P2. The AFM of P2 (Figure 2c) reveals a periodic nanostructure with some degree

is replaced by a PEG proton at 3.70 ppm (see Figure S1 in Supporting Information) after “click” reaction, providing evidence of the block nature of the copolymers. GPC traces of the parent homopolymers and the resulting copolymers (Figure S2) indicate that the synthesis was successful. A series of diblock copolymers with P3HT/PEG weight ratios of 94.5/5.5, 90/10, 77/23, and 63/37 were synthesized and denoted as P1, P2, P3, and P4, respectively. The compositions of the resulting diblock copolymers are shown in Table 1. For each copolymer, the P3HT segment is Table 1. P3HT-b-PEG Composition and FET Results polymer

P3HT Mn (g/mol)a

PEG Mn (g/mol)b

PEG (wt %)c

BCP PDIa

hole mobility (cm2/V·s)

P3HT P1 P2 P3 P4

17 000 17 000 17 000 17 000 17 000

0 1000 2000 5000 10000

0 5.5 10 23 37

1.07 1.11 1.24 1.21 1.35

0.15 0.11 0.25 2.75 × 10−3 7.5 × 10−4

a

Estimated by GPC using THF as the eluent against polystyrene standards. bObtained from the vendor. cEstimated from NMR.

identical with a number-average molecular weight, Mn, of ∼17 000 g/mol. The overall composition of the diblock copolymers was controlled by varying the PEG block lengths. In all cases, the polydispersity index (PDI) was low (see Table 1), indicating that these block copolymers were well-defined. The well-defined structures of the polymers enable relations between the electrical properties and the polymer block ratios

Figure 2. (a) UV−vis absorbance spectra of thin films of P3HT and P1−P4 (as-spun films, without any annealing), AFM phase image (500 nm × 500 nm) of the nanostructure of (b) the P3HT, and (c) the P2 film without annealing. 8668


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Figure 3. Transfer curves (a) and output curves (b) of the P3HT OFET device; transfer curves (c) and output curves (d) of the P2 OFET device.

backbone segments into an ordered lamellar morphology, as suggested by UV−vis and seen by AFM in Figure 2c. In the cases of P3 and P4 with high PEG content, the crystallization of the PEG segments dominates the morphology of the copolymer and frustrates organization of the domains driven by segment microphase separation or ordered P3HT crystallization. This is suggested by the crystal aggregates observed by the optical microscopy. Hole mobility in the series of copolymers was measured by using bottom gate/top contact field-effect transistors (FET). The devices were fabricated by water-assisted contact film transfer method.44 No thermal treatment was applied to the films, and the procedure for UV−vis absorption studies and AFM measurement was used. The devices exhibited typical ptype behavior as shown by the transfer and output curves of P3HT and P2 in Figure 3. A clear field effect is observed from the output characteristics, and hole mobility was calculated from the transfer characteristics in the saturation regime (VDS = −80 V). Table 1 reports the field-effect mobilities for all of the studied polymers. Interestingly, P1 and P2 exhibited a carrier mobility value comparable with P3HT homopolymer. These results suggest that despite the introduction of the insulating PEG block, P3HT-b-PEG still have effective hole transport paths in the films. The above morphology and structural analysis suggest that relatively disordered PEG segments enhance the crystallinity of the P3HT segments in the films, resulting in the formation of an ordered nanostructure. An ordered crystalline structure is favorable for charge carrier transport and high mobility of conjugated polymers.45 These properties can potentially enhance the carrier transfer properties of diblock copolymers. Similar to other previously reported P3HT-based block copolymer,38,39,46 these ordered structures are favorable for enhanced charge transport in a FET device geometry. Further increases in PEG concentration, however, are not helpful. The hole mobility of devices prepared with P3 and P4 is 2 or 3 orders of magnitude lower than that of P2. In view of the high content of the insulating PEG block and the absence of a well-ordered P3HT phase in thin films of P3 and P4, this is not unexpected. The macrophase separation at

of order, confirming the formation of highly ordered structures of lamellar π-stacked aggregates as suggested by UV−vis spectroscopy. While P3HT homopolymer of the same molecular weight of that of the P3HT block in P3HT-b-PEG does show some morphology at 20 nm length scale (Figure 2b), no periodic order in the morphology is observed. We note that this is an as-spun film and that no thermal or solvent annealing was applied. This result suggests that formation of highly ordered self-organized P3HT segments can be enhanced by the more flexible PEG segments present in in the copolymer during the spin-coating process. For P3 and P4 with higher PEG molecular weight, attempts to acquire high quality AFM images at nanoscale failed, likely due to rough surface morphologies arising from PEG crystallites. DSC curves in Figure S3 show that there are melting peaks at around 60 °C attributed to PEG crystallites in P3 and P4 and an absence of such peaks for P1 and P2. Figure S4 shows optical microscopy images of the different copolymer samples cast on Si wafers. In the case of P3 and P4, PEG segments grew to form dendritic crystals, tens or hundreds of micrometer in length, but no evidence of this is observed for P1 and P2. In principle, a block copolymer containing a rigid P3HT block and a flexible PEG coil has the potential to generate various morphologies (e.g., spherical, cylindrical, lamellar phase domains) resulting from microphase separation. This selfassembly behavior was not observed for these samples as in other P3HT block copolymer systems.28,38−43 We propose that several factors contribute to the organization of the nanostructures in these systems: phase separation, crystallization of the P3HT segment, and crystallization of the PEG segment. The morphology is due to the interplay among these factors with contributions from both thermodynamics and kinetics. In the case of P1 and P2 with low PEG content, the volume fraction of PEG is about 5% and 10%, respectively; microphase segregation may not be expected due to the steep slope for critical χN (microphase separation strength) in the phase diagram. Thus, the crystallization of P3HT segments plays a dominant role. The amorphous PEG coil increases segment mobility, allowing π−π stacking of thiophene 8669


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Figure 4. (a) TEM image and (b) XRD of as-synthesized TiO2 nanorods. The scale bar is 20 nm.

micrometer scale, resulting from PEG crystal aggregates (Figure S4), likely also hinders charge transportation and contributes to low hole mobility. Anatase TiO2 nanorods were prepared according to the lowtemperature sol−gel method developed by Cozzoli et al. in a reverse micelle solution.35 The dimensions of TiO2 nanorods were ∼20−30 nm in length and 3−4 nm in diameter. A representative TEM image is shown in Figure 4a. Nanorods instead of nanoparticles were chosen because of its advantage of precluding the need for electron hopping in nanoparticle-based photovoltaics and facilitating carrier transport owing to their intrinsic structural anisotropy.47 The XRD pattern (Figure 4b) shows that the patterns of the as-prepared samples match well with the standard patterns of anatase TiO2 (JCPDS 30-0820, a = 2.80, c = 4.45 Å). No peaks for other types of TiO2 were observed. The as-synthesized TiO2 nanorods are passivated with ∼2.5 nm long insulating oleic acid ligands, which act as a barrier impeding efficient charge transport between nanocrystal and polymer, as well as the transport of electrons between adjacent nanocrystals. Therefore, in hybrid solar cells, such ligands have to be removed to ensure intimate electrical contact. The surface of the TiO2 nanorods was modified by employing ligand exchange to replace the long alkyl chain ligands with a more effective interfacial molecule. The N3-dye was chosen as the ligand based on the following two characteristics. First, it can provide an appropriate band alignment, facilitate charge separation, and prevent back recombination at the interfaces of P3HT/TiO2 nanorod hybrids.15 Second, the excess COOH groups enable H-bonding with the matrix, specifically with the PEG side chains of the P3HT copolymer. We employed FTIR to verify the interaction between N3-dye and PEG side chain segments because of this method’s sensitivity to hydrogen bond formation. Figure 5 shows infrared spectra for pure N3-dye TiO2, P3HT-b-PEG, and their blends. The monomodal peak centered at 1719 cm−1, which is characteristic of the CO stretching mode of the carboxylic acid dimer in N3-dye, is clearly split into two bands centered at 1735 and 1719 cm−1 in the blend. The shoulder in the vicinity of 1735 cm−1 is attributable to the COOH bonded to ether, indicating the carboxylic acid−ether oxygen interaction.28,48 To determine if strong interactions between the block copolymer and N3-dye facilitate the formation of a well-

Figure 5. FTIR spectra of N3-dye TiO2, P3HT-b-PEG, and their blend with the ratio of P3HT-b-PEG/N3-dye TiO2 (1/1) respectively (from top to bottom).

dispersed polymer/TiO2 nanohybrid, TEM was carried out to study the morphology. Neat P3HT that does not have strong interaction with the TiO2 was blended with N3-dye TiO2 at a 1/1 ratio as a control experiment. Figure 6a shows a typical TEM image of a TiO2/P3HT composite film. It exhibits significant macrophase segregation, suggesting poor dispersion of nanocrystals in the polymer. In contrast, it is apparent that the modified-P3HT/TiO2 system (Figure 6b,c) exhibits uniform dispersion of TiO2. A high degree of homogeneity is apparent, and no aggregation of TiO2 can be seen. This illustrates the importance of the favorable interaction through hydrogen bonding between TiO2 and modified P3HT. Blends of P3HT-b-PEG with N3-dye TiO2 were incorporated into BHJ OPVs as active layers with a device architecture of ITO/PEDOT:PSS/active layer/Al on glass substrates (Figure 7a). Figure 7b shows the current−voltage characteristics of the hybrid solar devices using the series of block copolymers, and Table 2 summarizes the short circuit current density (JSC), open circuit voltage (VOC) data, fill factor (FF), and PCE. We note that the control system P3HT/N3-dye TiO2 blends shows a maximum value of 1.22%, less efficient than reported maximum efficiencies of 2.20% in the literature.15 This is mainly because the difference of device configuration. An additional thin layer of TiO2 nanorods, sandwiched between 8670


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Figure 6. TEM images of blends of N3-dye TiO2 with (a) P3HT homopolymer, (b) P2, and (c) P4 at 50/50 wt. The films for TEM imaging were prepared directly from measured devices.

Figure 7. (a) Schematic representation of the OPV device architecture. (b) Current−voltage characteristics of the OPV devices using different polymers under AM 1.5 (100 mW/cm2) illumination.

gives the highest PCE of 1.71% among P1−P4. The fill factor of P2 is lower than that of P3HT and P1. One possibility is that the better dispersion of the TiO2 nanorods in P2 relative to P3HT and P1 reduces the fill factor due to fewer points of contact for the TiO2 nanorods that would promote continuous and efficient charge transport pathways. A higher fill factor in well-dispersed systems may be achievable by employing TiO2 nanorods with higher aspect ratios. P3 and P4 systems showed decreased power conversion efficiency (Table 2, entries 4 and 5) even though TiO2 is well dispersed in the modified P3HT. This is because although PEG can facilitate the dispersion of nanocrystals in polymers, their excess presence reduces the device efficiency by impeding the transfer of charge between nanocrystal and polymer as well as the transport of electrons between adjacent nanocrystals. To investigate the impact of this morphology difference on the photovoltaic device performance in detail, BHJ devices composed of P2/N3-dye TiO2 and P3HT/N3-dye TiO2 with different component ratios were fabricated side-by-side. Again, the films for TEM imaging were obtained directly from measured devices to ensure provide accurate information on device morphology. The solar cell device specimens were scored and immersed in water to promote dissolution of the PEDOT:PSS layer. The delaminated film was then floated onto a copper TEM grid. Figure 8 compares their morphology as a function of TiO2 weight fraction. The nanorods are more homogeneously distributed within the P2 matrix than in neat P3HT. At 50 wt % concentration, TiO2 nanorods form an interpenetrating network, which provides both extremely large interfacial area between electron donor and acceptor and highly efficient conductive channels through out the whole composite.

Table 2. Effect of Block Ratio in the P3HT-b-PEG Block Copolymers on Device Propertiesa P3HT/TiO2 P1/TiO2 P2/TiO2 P3/TiO2 P4/TiO2

VOC (V)

JSC (mA/cm2)

FF (%)

0.71 0.73 0.79 0.60 0.63

3.43 3.81 5.56 2.96 1.52

43.1 47.8 39.0 35.7 35.0

PCE (PCEmax) (%) 1.05 1.33 1.71 0.64 0.34

(1.22) (1.51) (2.02) (0.75) (0.40)

a

P3HT-b-PEG/TiO2 = 1/1 (by weight). Film thickness is about 100 nm. Measurement was conducted under irradiation of AM1.5 (100 mW/cm2). The results reported represent the median of six sets of devices, and the numbers in parentheses are the maximum value.

the active layer and the aluminum electrode, functioning as a hole blocking layer and also as an optical spacer, would contribute to increased efficiency. We omitted this layer in our cell design such that we could unambiguously image nanorod dispersion in the active layer of our devices without the possibility of contamination of nanorods from the hole blocking layer. Using our device design the P1 and P2/N3-dye TiO2 systems exhibit a higher PCE in comparison to the control systems. The TEM imaging of the active layers in the devices allow the improvements to be attributed to improved dispersion. For the P3HT/N3-dye TiO2 system, large domains of N3-dye TiO2 (up to 200 nm) reduce the interfacial area available for charge separation and result in lower-efficiency devices. The ability to disperse N3-dye TiO2 in a polymer matrix by H-bonding is shown to be advantageous in this instance. We note that the P2 system shows an average JSC of 5.56 mA/cm2, a VOC of 0.79 V, and a fill factor of 39.0% and 8671


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Figure 8. TEM images of blends of P3HT homopolymer with (a) 20, (b) 30, (c) 40, and (d) 50 wt % N3-dye TiO2; TEM images of blends of P2 copolymer with (e) 20, (f) 30, (g) 40, and (h) 50 wt % N3-dye TiO2, respectively. The scale bar represents 100 nm. The films for TEM imaging were prepared directly from measured devices.

functionalized with N3 dye and the PEG chain allows for the homogeneous dispersion of TiO2 nanorods within the polymer matrix. The resulting interpenetrating network of TiO2 provides both extremely large interfacial area between electron donor and acceptor and highly efficient transport pathways throughout the whole composite. Overall, the incorporation of tailored hydrogen bond interactions in the photoactive layer has led to improved photovoltaic performance as a result of enhanced morphological control of the BHJ. This work provides a general approach that employs hydrogen bond mediated interfacial interactions to control morphology and optimize efficiency not only in polymer/nanorod photovoltaic devices but also in other hybrid devices.

Figure 9 compares the PCE of these two systems as a function of the weight fraction of TiO2. A plot of the AM 1.5 power

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

* Supporting Information S

Experimental details, additional figures showing the 1H NMR, GPC, optical microscopy, DSC results. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 9. PCE as a function of TiO2 concentration in the composites. The SD is calculated by the average number of 6 devices.

conversion efficiency versus the weight fraction of TiO2 in the active layer for each type of device is shown in Figure 9. When their weight ratios of nanorods in the active layer were the same, devices made using P2 exhibited significant increases in power efficiency when compared to devices made using P3HT homopolymer. The enhancement in PCE can be attributed to both the increased charge separation efficiency in the photoactive films, enabled by the increased interfacial area and the enhanced charge collection efficiency enabled by the interpenetrating nanomorphology in hybrid solar devices that can provide more effective transport pathways for both electrons and holes.

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding from the NSF Center for Hierarchical Manufacturing (CMMI-0531171) supported studies on the morphology of blends with strongly interacting components. Funding from the Department of Energy supported Energy Frontier Research Center at the University of Massachusetts (DOE DE-PS0208ER15944) supported the photovoltaic device fabrication, characterization, and testing.

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CONCLUSIONS A series of P3HT-b-PEG were synthesized by a combination of Grignard metathesis polymerization and click chemistry. UV− vis absorption or AFM data suggest a highly ordered crystalline structure of P3HT segments within copolymers P1 and P2. The strong H-bonding interaction between TiO2 nanorods

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REFERENCES

(1) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. 8672


Macromolecules

Article

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