Orientation Behavior of Bulk Heterojunction Solar Cells Based on

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J. Phys. Chem. C 2010, 114, 18001–18011

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Orientation Behavior of Bulk Heterojunction Solar Cells Based on Liquid-Crystalline Polyfluorene and Fullerene Kai Yao,† Yiwang Chen,*,† Lie Chen,*,† Daijun Zha,† Fan Li,† Jianing Pei,‡ Zhaoyang Liu,‡ and Wenjing Tian‡ Institute of Polymers/Department of Chemistry, Nanchang UniVersity, 999 Xuefu AVe., Nanchang 330031, China, and State Key Laboratory of Supramolecular Structure and Materials, Jilin UniVersity, 2699 Qianjin AVe., Changchun 130012, China ReceiVed: July 27, 2010; ReVised Manuscript ReceiVed: August 30, 2010

Novel p-type materials, liquid-crystalline polyfluorene and its copolymer containing a terphenyl mesogen pendant, namely, poly{9,9-bis[6-(4′-hexyloxy-terphenyloxy)-hexyl]-fluorene} (PFBHeT) and poly{9,9-bis[6(4′-hexyloxy-terphenyloxy)-hexyl]-fluorene-co-3-hexyl-thiophene} (PFBHeT-3HT), were designed and synthesized, respectively. The effects of the structural variation on their properties, especially the influences of the thermal treatment on the blend morphology of polymers and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), have been characterized and investigated. The spontaneous orientation of terphenyl mesogen endows the polymer and blend films with a good ordered morphology. The annealed and quenched films exhibit better liquid-crystalline properties, a lower LUMO, and better ordered domains compared with the untreated one, and the annealed films are favorable. It indicates that the spontaneous assembly of the liquid-crystalline molecules pushes PCBM clusters to form an oriented nanodispersing structure with highly oriented channel layers upon heating at liquid-crystalline states, and the annealing process offers enough stress relaxation time for the molecules to pack into a well-ordered stacking. Furthermore, the bulk heterojunction devices based on the PFBHeT-3HT:PCBM (1:2) active layer have been constructed. Without extensive optimization, annealing the devices yields a Voc of 0.63 V and a power conversion efficiency of 0.25%, showing a significantly increased Jsc and FF with respect to its untreated counterpart. Introduction The increasing demands for inexpensive renewable energy sources have stimulated extensive research activities to develop low-cost and highly efficient photovoltaic (PV) devices, particularly polymer solar cells (PSCs). To maximize the donoracceptor heterojunction interfacial area for efficient exciton dissociation, mainstream PSC devices adopt the concept of a bulk heterojunction (BHJ), where an active layer contains a p-type donor and an n-type acceptor to form an interpenetrating nanoscale network, and the conjugated polymer and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend BHJ solar cell is the most promising candidate. However, the power conversion efficiency (PCE) is still limited by the space charge effects inherent in the BHJ structure due to the imbalance between electron and hole mobility and the unfavorable morphology.1,2 In a BHJ, the ordering microphase separation is crucial to ensuring optimum charge-carrier photogeneration, extraction, and the transfer to the electrodes. Most conjugated polymers used for PSCs, such as poly(3-hexylthiophene) (P3HT) and poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene)] (MDMO-PPV), are semicrystalline or amorphous, exhibiting a significant amount of kinetically trapped disorders in the solid-state film.3,4 Besides, the bad aggregation of the PCBM further deteriorates the situation. It is necessary to control the nanoscale morphology of the active layer and lead to an * To whom correspondence should be addressed. Tel: +86 791 3969562. Fax: +86 791 3969561. E-mail: [email protected] (Y.C.), chenlienc@ 163.com (L.C.). † Nanchang University. ‡ Jilin University.

optimum segregated D/A morphology.5 Various independent approaches, including thermal annealing,6 postfabrication annealing at high temperature,7 additives, and slow film growth by controlling the solvent evaporation rate of the active layer (so-called “solvent annealing”), have been demonstrated to develop a morphology with an optimum phase segregation with crystalline domains of different compositions, leading to the PCE improvement of the blends. Polyfluorenes have recently received much attention because of their high charge mobility, good processability, high absorption coefficients, high photoluminescence efficency, low-lying HOMO levels, and exceptional chemical stability.8-10 Currently, PCEs approaching 3.5-5.5% have been reported by several groups for organic PSCs based on blend films of polyfluorene copolymers and PCBM, indicating that polyfluorenes are good candidates for converting solar energy into electricity.11-16 The fluorene-bithiophene copolymer, poly[9,9′-dioctyl-fluorene-cobithiophene] reveals good hole-transporting properties17 and excellent thermotropic liquid crystallinity.18-20 However, the annealing and slow growth effects on the morphology of the polymer based on a fluorene copolymer is still not clear, and it also has been recently reported that the functional groups attached to the donor polymer lead to the formation of films with uniform and stable nanophase morphologies.21 In our previous work,22,23 we have synthesized a series of liquid-crystalline conjugated polymers (LCCPs) containing terphenyl mesogens and found that the polymer is not only soluble in organic solvents but also easily aligned by spontaneous orientation of the liquid-crystalline (LC) group. In view of this, incorporating a calamitic and mesomorphic terphenyl core24 as a side chain onto the polyfluorene probably can improve the

10.1021/jp1070314  2010 American Chemical Society Published on Web 09/27/2010

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ordering arrangement of the polymer and promote the diffusion of PCBM in the entire active layer, which forms bicontinuous pathways and enhances efficient charge separation and transport. Besides, liquid-crystalline polyfluorene also could be macroscopically aligned by an external perturbation, such as heating, shear stress, and an electric or magnetic force field, to optimize the morphology of the blends. In this context, a novel liquidcrystalline polyfluorene derivative containing terphenyl pendants, poly{9,9-bis[6-(4′-hexyloxy-terphenyloxy)-hexyl]-fluorene} (PFBHeT), is synthesized, and its corresponding copolymer, poly{9,9-bis[6-(4′-hexyloxy-terphenyloxy)-hexyl]-fluorene-co3-hexyl-thiophene} (PFBHeT-3HT) with 3-hexyl thiophene (3HT) units, is also designed, by virtue of the exceptional properties of P3HT used for PSC.25,26 The effect of spontaneous orientation of the terphenyl mesogens on the morphology of the polymers and the polymer/PCBM active layer are studied systematically. We also investigate how the different thermal treatments influence the morphology of the polymers and the device performance. Experimental Section Materials. Trimethyl borate, n-butyllithium, 2,7-dibromo9,9-bis(6-bromohexyl)-fluorene, 4-(4-bromophenyl)phenol, 3-hexyl-thiophene, bis(1,5-cyclooctadiene)nickel, 4-bromophenol, and tetrakis(triphenylphosphine)palladium were purchased from Alfa Aesar and used as received without any further purification. Tetrahydrofuran (THF) was dried over sodium. Other chemicals were obtained from Shanghai Reagent Co., Ltd. and used as received. Indium-tin oxide (ITO) glass was purchased from Delta Technologies Limited, whereas PEDOT:PSS (Baytron PAl4083) was obtained from Bayer Inc. Characterizations. The nuclear magnetic resonance (NMR) spectra were collected on a Bruker ARX 400 NMR spectrometer with deuterated chloroform as the solvent and with tetramethylsilane (δ ) 0) as the internal standard. The ultraviolet-visible (UV-vis) spectra of the samples were recorded on a Hitachi UV-3010 spectrophotometer. Fluorescence measurement for photoluminescence (PL) of the polymers was carried out on a Shimadzu RF-5301 PC spectrofluorophotometer with a xenon lamp as the light source. Gel permeation chromatography (GPC), so-called size-exclusion chromatography (SEC) analysis, was conducted with a Breeze Waters system equipped with a Rheodyne injector, a 1515 Isocratic pump, and a Waters 2414 differential refractometer using polystyrenes as the standard and tetrahydrofuran (THF) as the eluent at a flow rate of 1.0 mL/ min and 40 °C through a Styragel column set, Styragel HT3 and HT4 (19 mm × 300 mm, 103 + 104 Å), to separate the molecular weight (MW) ranging from 102 to 106. Thermogravimetric analysis (TGA) was performed on a PerkinElmer TGA 7 for thermogravimetry at a heating rate of 20 °C/min under nitrogen with a sample size of 8-10 mg. Differential scanning calorimetry (DSC) was used to determine phase-transition temperatures on a Shimadzu DSC-60 differential scanning calorimeter with a constant heating/cooling rate of 10 °C/min. Texture observations by polarizing optical microscopy (POM) were made with a Nikon E600POL polarizing optical microscope equipped with an Instec HS 400 heating and cooling stage. The X-ray diffraction (XRD) study of the samples was carried out on a Bruker D8 Focus X-ray diffractometer operating at 30 kV and 20 mA with a copper target (λ ) 1.54 Å) and at a scanning rate of 1°/min. The cyclic voltammetry was performed on a CHI660C potentiostat, in an acetonitrile solution of 0.1 mol/L of [Bu4N]PF6 at a potential sweep rate of 0.01 V/s at room temperature under the protection of dry N2. A glassy

Yao et al. carbon electrode coated with a thin film of polymers was used as the working electrode. A Pt wire and an Ag wire were used as the counter electrode and quasireference electrode, respectively. The electrochemical potential was calibrated against Fc/ Fc+. Atomic force microscopic (AFM) images were measured on a Nanoscope III A (Digital Instruments) scanning probe microscope using the tapping mode. For comparison, the optical and photoluminescence properties of the polymers were investigated in thin solid films. Annealing of some films was conducted by heating in the mesophase for 1 h, followed by cooling to room temperature at a cooling speed of 1°/min, whereas in the case of quenched ones, they were quenched with liquid nitrogen from their liquid-crystalline states after heating for 1 h. The cross-sectional morphology was observed with scanning electron microscopy (SEM, Quanta 200F) after gold vapor deposition onto the samples in an Edwards Auto 306. Device Fabrication and Characterization. The polymer photovoltaic cells (PVCs) were fabricated with the active layer consisting of the copolymer PFBHeT-3HT:PCBM in a 1:2 wt/ wt ratio. The ITO glass substrates were precleaned by detergent and acetone and boiled in H2O2. Highly conducting poly(3,4ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS, Baytron) was spin-cast (3000 rpm) in a thickness of ∼40 nm from aqueous solution (after passing through a 0.25 µm filter). The substrate was annealed at 120 °C for 15 min on a hot plate. The copolymers were dissolved in chlorobenzene to make 25 mg · mL-1 solutions, followed by blending with PCBM (purchased from Lumtec. Corp) in 80 wt %. The active layers were obtained by spin-coating the blend solutions at 1000 rpm for 30 s, and the thickness of the films was ∼90 nm. Subsequently, LiF (0.6 nm) and Al (100 nm) electrodes were deposited via thermal evaporation in vacuum (0.1 eV) between the electrochemical band gap and the optical band gap for PFBHeT and PFBHeT3HT is similar to other (such as P3HT) large differences between their electrochemical band gaps and optical band gaps.37 Using similar methods, the HOMO and LUMO levels and electrochemical band gaps for the polymers after annealing and quenching from the liquid-crystalline state are calculated and listed in Table 3. The HOMO and LUMO energy levels of the PFBHeT-3HT is in good agreement with the optimal levels for obtaining good performance in photovoltaic cells using PC61BM (LUMO ) -4.2 eV) as an acceptor. The deeper-lying HOMO levels of these polymers will provide a higher open-circuit voltage (Voc), according to the theoretical prediction.38 To overcome the exciton binding energy of the polymer and ensure efficient electron transfer from donor to acceptor, the LUMO energy level of the donor (polymer) must be positioned above the LUMO energy level of the acceptor (PCBM) by at least 0.3 eV. It is worthwhile to compare the energy level diagram of PFBHeT-3HT with P3HT against PCBM, as shown in Figure 8. The LUMO of the nontreated PFBHeT-3HT is 1.29 eV higher than that of PCBM, well above the required energy for efficient charge separation. The similar cases happen to the condition of polymer PFBHeT-3HT after being annealed or quenched from the liquid-crystalline state. The LUMO levels significantly decrease after thermal treatments; in contrast, the HOMO levels experience little change, keeping the high opencircuit voltage. For the copolymer, it is worth noting that the annealed film can lower the LUMO levels more than the quenching one. In comparison to P3HT, the extra beneficial point associated with the lower HOMO of the annealed PFBHeT-3HT film is the advantage of the higher open-circuit voltage (Voc) of photovoltaic devices (40% enhancement). Film Topography. A number of studies have demonstrated that the configuration of the polymer backbone, as well as the relative orientation of the polymer chain, is of crucial importance for the conduction of holes. These parameters are largely dependent on the deposition method of the polymer layer as well as on the regioregularity and the annealing process. Thermal annealing of PV based on P3HT:PCBM active layers has been

Figure 8. (a) Energy-level diagram showing the HOMO and LUMO energy levels from CV data of P3HT, PFBHeT-3HT before and after annealing and quenching from the liquid crystallinity, and PCBM.

reported for annealing temperatures ranging between 75 and 230 °C,39-41 and power conversion efficiencies up to 5% have been reported through the optimization of annealing temperature and time, achieved by annealing at 150 °C for 60 min. Thus, the active layer morphology is a key factor for the efficiency of BHJ solar cells. To investigate in detail the microstructure of the polymer: PCBM films at different thermal annealing temperatures or after quenching, we conducted scanning electron microscopy of the polymer: PCBM films spin-coated on the ITO/PEDOT substrate (Figure 9). From the figure, we can see that the terphenyl mesogens endow the polymers with the ordered microphology and the homopolymer blend can achieve a better morphology than the copolymer due to thiophene units disturbing the packing alignment of the molecules. Furthermore, the thermal treatments dramatically favor the dispersion of PCBM in the films. In the case of the annealing at temperatures below the liquid-crystal state treatment, the composite shows the large-scale PCBM clusters, whereas the quenched film at the LC state exhibits the nanoscale uniform phase separation, and the annealing one from the LC state is likely to enhance the tendency, in accordance with the luminescent results and AFM analysis (see below). The effect of different thermal annealing temperatures can be further confirmed by atomic force microscopy (AFM). Figure 10 presents the AFM height images of the surface of the ascast film and those after thermal treatments. The as-cast film reveals a most coarse surface with the root-mean-square (rms) roughness of 18.56 nm, as shown in Figure 10a at 10 um × 10 um scan sizes, and obvious PCBM grain aggregation with the size distribution above 200 nm existing in the PFBHeT-3HT matrix, which may result in a large-scale phase separation, decreased diffusional escape probability for mobile charge carriers, and hence increased recombination. This is fully consistent with the relative lowest short-circuit densities obtained for the photovoltaic cell, which was investigated in the following. However, after undergoing annealing at 100 °C, quenching at 132 °C (LC state), and annealing at 132 °C, the rms roughness at 2 um × 2 um decreased to 5.72, 2.34, and 1.69 nm, respectively (Figure 10b-d). The roughness is considered to be a signal of polymer self-organization and phase separation.42 The decrease of the composite film roughness is because the PFBHeT-3HT chains self-organize into a more ordered structure

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Figure 9. SEM topographs (500 nm) of the polymer:PCBM (1:2 wt %) blend thin films spin-coated onto the ITO/PEDOT substrate for various thermal treatments. Top panels: PFBHeT (a) 120 °C annealing, (b) quenching from 168 °C, (c) 168 °C annealing. Bottom panels: PFBHeT-3HT (d) 100 °C annealing, (e) quenching from 132 °C, (f) 132 °C annealing.

induced by mesogens after annealing treatment, which may push PCBM clusters to form an oriented nanodispersing structure. The AFM phase image further demonstrates the pack structures of the composite films (see the inset pictures of Figure 11).43 The obvious molecular orientation is observed in the AFM phase image, and the proposed micromorphology of liquid-crystalline polymer PFBHeT-3HT and PCBM nanodomains in thin films for various thermal treatments are represented in Figure 11. Photovoltaic Properties. The current-voltage characteristics under illumination for PV devices that have undergone various thermal annealings are shown in Figure 12, and the device parameters (PCE, Voc, Jsc, and FF) are summarized in Table 4. Under 1.5 G illumination (100 mW/cm2), the cell based on nontreated PFBHeT-3HT:PCBM film as the active layer has a short-circuit current density (Jsc) of 0.18 mA/cm2, an open-circuit voltage (Voc) of 0.62 V, a calculated fill factor (FF) of 0.19, and a power conversion efficiency (PCE) of 0.021%. The low Jsc of the cell may be caused by the poor match with the solar absorption spectrum and the bad aggregated configuration in the film. The relative low molecular of the copolymer also negatively affects the power conversion efficiency.44 However, after liquid-crystalline temperature annealing, the Voc of the ITO/ PEDOT:PSS/PFBHeT-3HT:PCBM (1:2)/LiF/Al solar cell remained unchanged, whereas its Jsc and the FF significantly increased from 0.18 and 0.19 to 0.95 and 0.41 upon annealing, respectively. As a result, the PCE increased from 0.021% to 0.25%, which is 3 times higher than the case of lowertemperature annealing (100 °C). Liquid-crystalline phase temperature annealing of the devices favors activating PCBM molecules to diffuse and aggregate for better electron transport, and annealing can stimultaneously enhance the packing arrangement of PFBHeT-3HT induced by terphenyl mosogens and reduce the density of defects at the interface in order to facilitate the hole transport.45,46 It is well recognized that the performance of solar cells is related to the nanoscale morphology of polymer:fullerene blends influenced by parameters, including the solvent, the processing temperature, the solution concentration, the relative ratio in composition between the polymer and the fullerene, the chemical

structure of the polymer, etc. Because we have not yet optimized the conditions for the device fabrication of all the polymers, the data shown in Table 4 do not represent the best performance for the copolymer. The comparison here is merely based on one specific device fabrication condition. Conclusions In conclusion, we have developed a series of novel, highly stable, and high-performing liquid-crystalline polyfluorenes, homopolymer PFBHeT and copolymer PFBHeT-3HT, by intramolecularly incorporating mesogenic terphenyl as a side chain linked in 9,9-dialkyl-fluorene. The spontaneous orientation of terphenyl mesogen endows the polymer and polymer:PCBM blend films with a better ordered morphology and enhanced properties. The incorporation of 3HT units into the polyfluorene can effectively improve solar light harvesting. Compared with the untreated one, the annealed and quenched films from the liquid-crystalline state exhibit a red shifted absorption (even to 45 nm). The thermal treatment, especially from the liquidcrystalline state, favors the more ordered nanoscale morphology. Besides, the annealing process offers an enhanced behavior than the quenching one, thanks to the better diffusion of PCBM in the films and the formed well-aligning bicontinuous phases resulting from enough stress relaxation time for the molecules to pack better. At the same time, the HOMO energy levels of these polymers remain low in different heat processing, whereas the copolymer after annealing and quenching can get a narrower band gap, by decreasing the LUMO 0.15 and 0.11 eV, respectively. Without extended optimization, a power conversion efficiency of 0.25% and a high Voc of 0.63 V were achieved by incorporating the PFBHeT-3HT:PC61BM (1:2) blend and annealing from the liquid-crystalline state as the active layer, which is superior to that of the copolymer cells as-prepared (0.021%) and cells annealing from low temperature (0.070%) under the same experimental conditions. It is observed that the thermal annealing of the PFBHeT-3HT:PCBM film, especially in liquid crystalline, leads to an overall enhanced photovoltaic perfor-

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Figure 12. J-V curves of the copolymer photovoltaic cells based on PFBHeT-3HT:PCBM (1:2 wt %) as-prepared and after different annealing temperatures under the illumination of AM 1.5, 100 mW/ cm2 white light. The inset shows a schematic device configuration of the solar cell.

TABLE 4: Photovoltaic Properties of Bulk Heterojunction Solar Cells Based on PFBHeT-3HT:PCBM before and after Various Annealings compositions of active layer PFBHeT-3HT:PCBM (1:2 wt %) PFBHeT-3HT:PCBM (1:2 wt %), 100 °C annealed PFBHeT-3HT:PCBM (1:2 wt %), 132 °C annealed

Figure 10. Tapping-mode AFM topography height images of the blend films spin-coated from chlorobenzene for PFBHeT-3HT:PCBM (1:2 wt %) at various thermal treatments, (a) before annealing, (b) after 100 °C annealing, (c) quenching from 132 °C, and (d) after 132 °C annealing, and the corresponding topographical roughness on the ITO/ PEDOT substrates.

mance, including Jsc, FF, and PCE. Thus, in the molecular design of controlling the molecular organization of polymers, introducing a mesogenic side chain onto the conjugated backbone is found to be a promising method to develop highly oriented

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

0.62

0.18

0.19

0.021

0.64

0.42

0.26

0.070

0.63

0.95

0.41

0.25

ordered supramolecular systems for the active layer in solar cells. In this system, the investigations are mainly focused on the electronically active liquid-crystalline polymer to understand the relationship between the nanostructure compositions and the electronic properties, which is beneficial to designing new photovoltaic materials and controlling device performance. Acknowledgment. This work was supported by the National Natural Science Foundation of China (51073076, 50902067 and 51003045), the Natural Science Foundation of Jiangxi Province (2007GZC1727 and 2008GQH0046), and Foundation of Jiangxi Provincial Department of Education (GJJ10012 and GJJ10035).

Figure 11. Schematic representation of liquid-crystalline domains in PFBHeT-3HT:PCBM (1:2 wt %) thin films spin-coated onto the ITO/PEDOT substrate for various thermal treatment temperatures, where qz is the surface normal direction. The insets show the AFM phase pictures (1 um × 1 um) of the corresponding film.

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