Improving the Morphology of PCDTBT:PC70BM Bulk Heterojunction

Jan 23, 2014 - crystallinity of PCDTBT and inhibited intercalated behavior. Besides the lateral phase-separated morphology, the vertical distribution ...
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Improving the Morphology of PCDTBT:PC70BM Bulk Heterojunction by Mixed-Solvent Vapor-Assisted Imprinting: Inhibiting Intercalation, Optimizing Vertical Phase Separation, and Enhancing Photon Absorption Jiangang Liu, Qiuju Liang, Haiyang Wang, Mingguang Li, Yanchun Han,* Zhiyuan Xie, and Lixiang Wang State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, People’s Republic of China S Supporting Information *

ABSTRACT: In this paper, mixed-solvent vapor-assisted imprinting annealing was proposed to improve the lateral phase separation, vertical distribution of fullerene component and the photon absorption in poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT):[6,6]-phenyl-C71 butyric acid methyl ester (PC70BM) blend system. After the mixed-solvent vapor-assisted imprinting annealing (one is good for PCDTBT, that is, carbon disulfide (CS2) and the other is good for PC70BM, that is, tetrahydrofuran (THF)) for 20 min at the vapor pressure of P = 0.9 (25 °C), both the crystallinity of PCDTBT and the aggregation of PC70BM were promoted, resulting an interpenetrating network, as confirmed by spectroscopic ellipsometry (SE) and transmission electron microscope (TEM). Furthermore, the fullerene content on the top surface was enhanced due to the reduced energy difference between the top and bottom surface of the film as well, which was demonstrated by X-ray photoelectron spectroscopy (XPS). Besides the morphology transition, surface relief grating structures were also introduced to the film surface, resulting in an increased probability of photon absorption due to light diffraction. As a consequence, the final structure of active layer was effective in enhancing photon absorption, improving the carrier mobility, and reducing the carrier recombination, which lead to a power conversion efficiency (PCE) of 7.20% under AM1.5G illumination, almost 55% higher than PCE of reference device (PCE = 4.66%). exposed surface of PCDTBT/PC70BM film due to its higher surface energy compared to fullerene, causing a serious carrier recombination. Furthermore, owing to the wide band gap (1.88 eV), PCDTBT shows an absorption onset at 660 nm, thus, could not permit more of the sun’s spectral emission to be harvested. The BHJ approach requires interpenetrating domains of donor and acceptor phases of specific dimensions and structures throughout the active layer of the device.9 However, there is sufficient space between side chains along polymer backbones in PCDTBT, permitting fullerene molecules to intercalate into the spaces then form an intercalated structure, which inhibits phase separation.10−12 As a consequence, it is hard to form continuous carrier pathways in PCDTBT/ fullerene blend system. Moreover, the crystallinity of PCDTBT is low due to weak intermolecular interaction of PCDTBT and disturbance of fullerene, which is not adequate for a high efficiency device.13,14 In recent studies, a few methods have been applied to inhibit intercalation behavior successfully, including adjusting fullerene size, and increasing the crystal-

1. INTRODUCTION Polymer solar cells (PSCs) based on bulk heterojunction (BHJ) materials comprising π-conjugated polymers and fullerene derivatives have been demonstrated promising performance, and achieved a power conversion efficiency (PCE) beyond 8.0%.1−3 As we know, the efficiency of solar cells is limited by low short-circuit current density (Jsc) and open-circuit voltage (Voc). Therefore, there have been considerable activities pursuing the development of higher PSCs efficiencies focused on alternative donor materials with a reduced energy gap and an increased ionization potential.4−7 One promising new donor polymer for PSCs applications is poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), which shows a deep low energy of the highest occupied molecular orbital (HOMO).8 When PCDTBT is blended with the fullerene acceptor [6,6]-phenyl-C71 butyric acid methyl ester (PC70BM), the device exhibits a PCE value of 6% under AM1.5G irradiance of 100 mW cm−2.5,8 Nevertheless, there are several problems underlying PCDTBT/PC70BM blend system which hamper a further improvement of device performance. First, the intercalation behavior in PCDTBT/PC70BM blend system would inhibit phase separation, which is adverse to the charge transport. In addition, PCDTBT-rich interface decorates the © 2014 American Chemical Society

Received: September 24, 2013 Revised: January 22, 2014 Published: January 23, 2014 4585

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continuous electron path and enhancing the crystallinity of PCDTBT. In addition, the vertical composition profile was optimized as well owning to the introduction of a high interfacial energy on the top of active layer, which promoted fullerene molecules to migrate to film surface and form a PC70BM-rich cap layer. Meanwhile, surface relief grating structures were introduced on film surface by the action of capillarity, which would lengthen optical paths within the active layer, thus enhancing the probability of photon absorption. The morphology transition improved the carrier mobility, especially electron mobility, and reduced the carrier recombination. With the aid of surface relief grating structures, a significant improvement of Jsc was obtained (from 10.80 to 12.84 mA/ cm2). Consequently, the optimized morphology and grating structure led to a high device performance, reaching 7.20% under AM1.5G illumination, almost 55% higher than that of reference device (PCE = 4.66%).

lization rate of the polymer, which promote phase separation and construct the interpenetrating network structure.13,15,16 In addition, mixed-solvent vapor annealing is an effective method to optimize morphology, inducing the aggregation of fullerene to inhibit intercalation and improving the migration of polymer to enhance is crystallinity, which increases the probability of CT state dissociation, the lifetime of the carriers and also the mobility of carriers.13 Along with lateral phase separated morphology, vertical concentration profiles within the active layer is also critical.17 Since conjugated polymer surface energy is substantially less than that of fullerenes, the exposed air surface is almost exclusively composed of polymers, which blocks charge transport and increases injection barrier of electron.18 Ideally, the regular device structure, in which active layer is sandwiched between the PEDOT/PSS-coated indium tin oxide (ITO) anode and low work function metal cathode, should have a structure capped with fullerene-rich air interface and a slightly polymer-rich buried interface. Nowadays, several approaches have been proposed to modify the composition profile to achieve better device performance. For instance, modifying the surface energy of the substrate with a self-assembled monolayer or adjusting the surface energy of donor or acceptor materials, such as introducing a new fullerene derivative with a fluorocarbon chain, could switch the compositional gradient effectively.19,20 In addition, decreasing the interfacial energy difference between the bottom and top surface of active layer could also provide an uniform vertical distribution of fullerenes.21 The optimized vertical concentration profile facilitates charge transport and extraction, which gives rise to an increased Jsc and FF.22 The absorption efficiency of incoming light in PSCs is one of the major limitations toward realizing high device PCE as well. Typically, the optimum thickness of the active layer need achieve a trade-off between light absorption and charge transport efficiencies, resulting in a film thickness on the order of 100−200 nm. While such a thin layer can only lead to low light absorption, which makes it imperative to develop innovative ways to enhance absorption without increasing active layer thickness. Recently, light-trapping techniques, such as adding metallic nanoparticles23−25 and introducing nanopatterned periodic structures,26−28 have attracted great attention as approaches for increasing photon absorption without the need for a thicker film. In the case of adding metallic particles, the particle could provide surface plasmonic effect and reflect the incident light and is therefore responsible for the enhancement of photon absorption. Besides, introducing nanopatterned periodic structures could also diffract light and thereby increase the optical path length within BHJ film. With the aid of light-trapping techniques, higher Jsc could be obtained, for instance, from 10.3 to 11.2 mA/cm2 in PCDTBT/ PC70BM blend system.23 Herein, an artistic combination of mixed-solvent vapor annealing and vapor-assisted imprinting, that is, mixed-solvent vapor-assisted imprinting annealing, were performed to optimize the lateral and vertical phase separation within the active layer, as well as enhance the photon absorption. The mixed-solvent vapor is constituted by tetrahydrofuran (THF) and carbon disulfide (CS2) vapor, which optimal volume ratio is 1:1, and the stamp is PDMS film with periodic grating structures. After the mixed-solvent vapor-assisted imprinting annealing treatment, the aggregation of PC70BM was promoted, which inhibited the intercalation behavior, producing a

2. EXPERIMENTAL SECTION Materials. PCDTBT (weight averaged molecular weight, Mw = 24000; polydispersity index, PDI = 1.8) was synthesized in our laboratory. [6,6]-Phenyl-C71 butyric acid methyl ester (PC70BM, American Dye Source) and dichlorobenzene (oDCB, Sigma-Aldrich) were used as received. Poly(dimethylsiloxane) (PDMS; Sylgard 184) and its curing agent were purchased from Dow Coring. THF and CS2 were purchased from Beijing Chemical Factory, China, and used without further purification. Fabrication of PDMS Plate. The PDMS plate, prepared by mixing a 1:6 ratio of curing agent to the oligomer, was poured onto a piece of glass. After degassing until the PDMS became clear and thermal curing at 60 °C for 4 h, the PDMS plate was cooled to room temperature before being peeled off from the glass. The PDMS plate was provided with a average thickness about 0.9 mm. Fabrication of the PDMS Stamp. The PDMS stamp, prepared by mixing a 1:6 ratio of curing agent to the oligomer, was then poured onto a standard digital versatile disc (DVD), from which the protective polymer coating had been removed to expose the patterned metallic layer. After degassing until the PDMS became clear and thermal curing at 60 °C for 4 h, the PDMS stamp was cooled to room temperature before being peeled off from the DVD mater.29 The PDMS stamp with periodic gratings as well as average thickness of 0.9 mm was obtained. Mixed-Solvent Vapor Annealing. During the mixed-solvent vapor annealing process, the film was placed into a glass tube composed of THF and CS2 with a volume ratio 1:1 at bottom. Stable vapor composition along the tube could form after 6−10 h when liquid solvent location was fixed without disturbance. The vapor pressure of the sample was P = 0.81 (25 °C). (The length of tube is 42.5 cm and the liquid height is 2.5 cm. The solvent vapor pressure P is determined by P = L/L0, where L is the distance from the up edge of the setup to the specimen position and L0 is the length given by distance from the up edge of the setup to the surface of the solvent at the bottom of the tube.) The mixed-solvent vapor annealing was performed on the sample under this vapor pressure for 5 min. Mixed-Solvent Vapor Annealing Combined with PDMS Plate. The active layer was covered with a PDMS plate and fixed by a frame without air bubbles, preventing the PDMS plate swelling during the mixed-solvent annealing process. During this process, the film was placed into a glass tube 4586

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Scheme 1. Mixed-Solvent Vapor-Assisted Imprinting Annealing Process and the Morphology Transition of the Active Layer

scanning interval of 2θ between 5 and 30°. An incident angle (α = 0.2°) slightly above the critical angle (αc = 0.18°) was chosen, which would increase the GIXD peak intensity for investigating the crystallinity as well as the orientation throughout the film. To investigate the glass transition temperature Tg of the PCDTBT/PC70BM film, spectroscopic ellipsometry (Horiba Jobin Yvon) was adopted. During this process, the films deposited on the substrate (the substrate is Si with 25-nm-thick PEDOT/PSS) were placed on a Linkam heating/cooling stage within a cell under a N2 atmosphere. The cell possesses two transparent windows allowing transmission of the polarized incident and reflected ellipsometry beams at the same time. The films were heated from 20 to 180 °C under a N2 atmosphere and then cooled to 20 °C at a rate of 5 °C min−1. Record Ψ (the ratio of the amplitude of the incident and reflected light beams) successively during the heating and cooling cycles, employing a Cauchy model to fit Ψ, determining Tg through the sudden change in slope of Ψ versus temperature. To evaluate the extent of the phase separation of the active layers, photoluminescence (PL) spectroscopy were achieved with LabRam HR800 spectrometer (Horiba Jobin Yvon), which was equipped with an Olympus BX41 microscope in the backscattering geometry. Both the confocal hole and the slit width were fixed at 200 μm. A 632.8 nm He−Ne laser was focused on the sample with a 10× objective lens (0.75 NA). The composition surface of the active layers was measured by X-ray photoelectron spectroscopy (XPS). XPS (VG ESCALAB MK II) experiments with argon sputtering were measured at room temperature by using an Al Kα monochrom (hυ = 1486.6 eV) at 14 kV and 20 mA. The sample analysis chamber of the XPS instrument was maintained at a pressure of 8 × 10−8 Pa. The reflection spectra of the active layer was recorded by UV-vis absorption spectroscopy (reflectance mode), using a Lambda 750 spectrometer (Perkin-Elmer, Wellesley, MA). The external quantum efficiency (EQE) of the PV cells was measured with a lock-in amplifier at a chopping frequency of 280 Hz during illumination with the monochromatic light from a xenon lamp. Measurements of current density−voltage (I−V) characteristics of the PV cells were completed by a computer controlled Keithley 236 source meter under AM1.5G illumination from a calibrated solar simulator with irradiation intensity of 100 mW/ cm2.

composed of THF and CS2 with a volume ratio 1:1 at bottom. The vapor pressure of the sample was P = 0.90 (25 °C). The annealing process was performed on the sample under this vapor pressure for 20 min. Mixed-Solvent Vapor-Assisted Imprinting Annealing. The active layer contacted with a PDMS stamp and fixed by a frame without air bubbles. During the annealing process, the film was placed into a glass tube composed of THF and CS2 with a volume ratio 1:1 at bottom, and the vapor pressure of sample was P = 0.90 (25 °C). The treatment on the sample lasted 20 min under this vapor pressure . PSCs Fabrication. The fabrication of solar cells was on ITOcoated glass substrates. First, the indium tin oxide (ITO)coated glass substrates were cleaned by detergent, then ultrasonicated in water, acetone, and isopropyl alcohol, respectively, and dried by nitrogen flow subsequently. After UV ozone treatment for 15 min, a poly(ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS, Baytron P4083) layer with a thickness of 25 nm was spin-coated on this well-cleaned ITO glass substrate and dried at 140 °C in vacuum for 10 min. A solution consisting of a mixture of PCDTBT/PC70BM (2:5) in dichlorobenzene with a total concentration of 14 mg/mL was spin-casted on the top of the PEDOT/PSS layer to produce an active layer with a thickness of 190 nm in the glovebox. Mixed-solvent vapor annealing with or without PDMS modes,then dried in vacuum. Finally, after removing the PDMS modes, a bilayer structure of Ca (30 nm)/ Al (90 nm) was thermally evaporaed onto the active layer to complete the device fabrication, in a vacuum of 2 × 10−4 Pa . The cell active area was 12 mm2, which was decided by the overlapping area of the ITO and Al electrodes. Characterization. Atomic force microscopy (AFM) and transmission electron microscope (TEM) were performed to characterize the morphology of PCDTBT/PC70BM films. A SPI3800N AFM (Seiko Instruments Inc., Japan) was carried out to study the surface topography of the films with a Si tip with a spring constant of 3 N/m in tipping mode. The operation of cantilevers was slightly below their resonance frequency about 72 kHz. The acquisition of images was performed under ambient condition. TEM (JEOL JEM-1011) was utilized to probe vertical direction information of PCDTBT/PC70BM films and the images were obtained at 100 kV. Thin films were transferred onto copper grid by floating on deionized water. The samples were prepared and dried for 12 h at room temperature before TEM tests. Out-of-plane grazing incidence X-ray diffraction (GIXD) measurements and spectroscopic ellipsometry were employed to detect the crystallinity of PCDTBT/PC70BM films. Profiles were obtained by using GIXD (Bruker D8 Discover reflector) with an X-ray generation power of 40 kV tube voltage and 40 mA tube current. The measurements were conducted in a

3. RESULTS AND DISCUSSION In this work, we used a novel method, named mixed-solvent vapor-assisted imprinting annealing treatment, to modify the morphology as well as the photon absorption of PCDTBT/ PC70BM blend films, as shown in Scheme 1 (chemical structures of PCDTBT and PC70BM are shown in Figure 1). 4587

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relief gratings were introduced onto the film surface under the influence of capillarity, resulting in effective photon absorption. In order to illustrate the effects that arose from mixed-solvent vapor-assisted imprinting annealing treatment, two control experiments, including treating the blend film only with the mixd-solvent vapor annealing (the sample is labeled as M-SVA) and annealing the blend film covered with PDMS plate in the mixd-solvent vapor annealing (the sample is labeled as MSVA&P). Improved Lateral Phase Separation and Introduced Surface Relief Gratings. Vapor-assisted imprinting approach is a simple and effective tool for introducing surface relief gratings onto the active layer, which ameliorates the weak photon absorption. In addition, a nanoscale interpenetrating network structure must be constructed to ensure the effective exciton dissociation and charge transport. Herein, an interpenetrating network structure combined with surface relief grating structures were developed via mixedsolvent vapor-assisted imprinting annealing. The atomic force microscopy (AFM) and transmission electron microscope (TEM) were applied to explore the morphologies of these films without and with different annealing treatments. For the pristine film (without any treatment), no obvious textured surface can be observed in height image as displayed in Figure 2A, left. Transmission electron microscope (TEM) could provide vertical direction information by the acquisition of electrons projected through the entire film.30 As shown in Figure 2A, right, the interpenetrating networks are not well developed, and the donor−acceptor domains are difficult to distinguish. While significant variations occurred after the mixed-solvent vapor-assisted imprinting treatment: a regular surface relief grating structure on the film surface and an interpenetrating network structure through the entire film as shown in Figure 2B. In the mixed-solvent vapor-assisted imprinting process, a PDMS stamp with periodic gratings was completely conformal contacted with the blend film to serve as

Figure 1. Chemical structures of PCDTBT (left) and PC70BM (right).

The components of mixed vapor, which have been optimized in our previous work, are CS2 and THF, and its optimal ratio is 1:1. The annealing time of vapor-assisted imprinting is an important parameter; the devices were treated with different annealing times, including 15, 20, and 25 min at the same vapor pressure (P = 0.90). As shown in Figure S1, the device treated for 20 min showed the best performance. The vapor pressure for annealing treatment was also examined by changing the distance from liquid level of the solvent to the specimen position. As shown in Figure S2, the appropriate vapor pressure is P = 0.90 when the annealing time is 20 min. Thus, we would execute the mixed-solvent vapor-assisted imprinting under the optimum experimental conditions in the following text, in which the annealing time is 20 min and the vapor pressure is P = 0.90. After the mixed-solvent vapor-assisted imprinting (the sample is labeled as M-SVA&S), the lateral phase separation of blend film was optimized, thus, forming an interpenetrating network structure, which was attributed to enhanced crystallinity of PCDTBT and inhibited intercalated behavior. Besides the lateral phase-separated morphology, the vertical distribution of the components was also ameliorated. On account of higher surface energy provided by the PDMS stamp during the annealing process, PC70BM would distributed more uniformly in the vertical direction, which is beneficial to effective charge transport and extraction. Furthermore, surface

Figure 2. Tapping mode AFM images (left) and TEM (right) images of PCDTBT/PC70BM blend films without and with different vapor annealing treatment: (A) without annealing treatment; (B) M-SVA&S, sample covered with PDMS stamp and treated by mixed-solvent vapor; (C) M-SVA, sample treated by mixed-solvent vapor; (D) M-SVA&P, sample covered with PDMS plate and treated by mixed-solvent vapor; (E) the corresponding profile cut along the gray line in the AFM image of sample M-SVA&S is also drawn. 4588

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Figure 3. Tg of the films without and with different annealing treatments was determined by spectroscopic ellipsometry. The ellipsometric parameter Ψ vs temperature recorded during the first heating cycle of PCDTBT/PC70BM films (A) without annealing treatment; (B) M-SVA, sample treated by mixed-solvent vapor; (C) M-SVA&P, sample covered with PDMS plate and treated by mixed-solvent vapor; and (D) M-SVA&S, sample covered with PDMS stamp and treated by mixed-solvent vapor; (E) out of plane GIXD profile shows the crystallinity of PCDTBT/PC70BM films without and with different annealing treatment.

contrast experiment, only interpenetrating network structure was formed in the M-SVA sample as shown in Figure 2C, implying the mixed-solvent vapor annealing is main reason for the lateral morphology transition. In the M-SVA&P sample, the interpenetrating network structure could also be found, as shown in Figure 2D, which indicates the PDMS would not hinder the morphology transition during the annealing process. To confirm the crystallinity of films, spectroscopic ellipsometry (SE) was employed to characterize the glass transition temperature (Tg) of PCDTBT component in blend films. Ellipsometric parameter Ψ was recorded during the first heating process and thermal transitions are identified by the intersection of straight-line fits to the linear sections of a Ψ versus T plot, from which the possible thermal transitions can be manifested by an increase or a decrease of dΨ/dt. In all of these SE curves, as shown in Figure 3, there is one significant thermal transition appearing around 140−160 °C. According to Lidzey et al. and our precious studies,31 this increase in dΨ/dt must be ascribed to the transition from rubbery to glassy state of PCDTBT. Hence, the Tg of films treated by different process

the patterning master. Meanwhile, the PDMS stamp also acted as a solvent transport medium in the mixed-solvent vapor environment due to its gas-permeability.21,29 Because of intimate contact with the film surface, the mixed solvent in the PDMS stamp would diffuse into the film and increase its plasticity. Under the action of capillarity, the PCDTBT/ PC70BM could fill the voids between the film and the stamp, thereby generating a negative stamp as shown in Figure 2B, left. From its corresponding profile cut along the gray line as shown in Figure 2E, it is obvious that the height of the grating structure is about 100 nm and its period is about 680 nm. Moreover, the mixed-solvent vapor also modified the morphology of the blend film during the annealing process. As shown in Figure 2B, right, a fibril-like feature run across the film, and the long fibrils are tightly packed, thus, forming interpenetrating network structures. These fibrils are domains of PCDTBT crystallites, which could be attributed to the enhanced self-organized ability. In addition, the nanoscale phase separation also occurred due to the aggregation of fullerene, which inhibited the intercalated behavior.13 In the 4589

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can be identified. As shown in Figure 3A, a thermal transition is apparent at 142 °C for the pristine film. While after the mixedsolvent vapor annealing treatment, Tg of the film increases to 152 °C, as shown in Figure 3B. The elevated Tg indicates the average degree of intermolecular interaction of PCDTBT was enhanced at the present of mixed-solvent vapor, which is coincident with the appearance of nanofibrils in AFM and TEM images as shown in Figure 2. Surprisingly, the self-organized ability was further enhanced when the PCDTBT/PC70BM film was covered with PDMS. As shown in Figure 2C,D, the Tg of M-SVA&P and M-SVA&S is 157 and 156 °C, respectively. From Figure 2E, the out of plane GIXD profile also shows the crystallinity of PCDTBT in blend films without and with different annealing treatment. The peaks centered at 2θ = 18.3° is ascribed to the distance between coplanar π-stacked polymer backbones, and its intensities can be used for deducing the crystallinity of PCDTBT in the films. Judging from intensities of these peaks without and with different treated process, it is also obvious that the crystallinity of PCDTBT was enhanced after the vapor treatment, and further improved in the MSVA&P and M-SVA&S process. This phenomenon could be explained from two aspects: a reduced influence of fullerenes on the PCDTBT molecular interaction and a prolonged time of molecular migration. As is well-known, it is difficult for conjugated polymers to self-organize in a polymer/fullerene blend system, which could be ascribed to the fact that fullerene was finely dispersed on a molecular basis between polymer chains, thus, preventing its crystallizing.14,32 Yang et al. have already demonstrated that the crystallinity of polymer was enhanced as decreasing the loading of PCBM in P3HT/PCBM blend system.32 In our experiment, more fullerene molecules would diffuse to the top of blend film when the film was covered with PDMS (this phenomenon would be clarified in detail in the following text). In other words, the PC70BM content in the inner of sample M-SVA&P and M-SVA&S is lower compared to the one in sample M-SVA. Because of the reduced disturbances generating from fullerenes, the crystallinity of PCDTBT was raised. In addition, the post annealing cooling rate, which corresponds to the evaporation rate of residual solvent in the film in our experiment, also played an important role in the crystallization of PCDTBT. In the end of mixed-solvent vapor annealing process, the sample M-SVA was taken out directly from the vapor environment. Due to the low vapor pressure of CS2 and THF, the solvent molecules would diffuse out of the film quickly, reducing the free volume of PCDTBT. Therefore the PCDTBT was “frozen in” within a few seconds, which is far from equilibrium. As expected, when the film was covered with PDMS, PDMS stamp or PDMS plate would depress the volatization of CS2 and THF in the blend film. As a result, the PCDTBT chains would continue to adjust its conformation and reach the final state that closes to equilibrium, which is the same as decreasing the rate of solvent drying.33−35 To assess the extent of phase separation, photoluminescence (PL) spectroscopy, as shown in Figure 4, has been performed. For the pristine film, PL appears to be significantly quenched, which suggests that most of excitons were diffused to the interface between the donor and acceptor. It is an evidence for the fullerene being mixed with PCDTBT at the molecular level thus forming intercalated structure.15,36 While the observed PL quenching of M-SVA is not as significant, it indicates an enhanced phase separation after being treated by mixed-solvent vapor annealing.11,37 Even adding PDMS onto the top of film,

Figure 4. PL spectra measured by Raman spectra shows the extent of phase separation of the PCDTBT/PC70BM films without and with different annealing treatments.

the phase separation of films were also enhanced. It could be confirmed by the PL intensities of M-SVA&P and M-SVA&S, which are comparable to the one of sample M-SVA, as shown in Figure 4. The enhanced phase separation could be ascribed to the effect of mixed-solvent vapor. During the annealing process, the migration of polymers and fullerenes were increased due to its good solubility in CS2. Furthermore, THF content could induce fullerene to diffuse out of PCDTBT side chains, thus, forming large aggregates, which inhibited the intercalation behavior and induced phase separation.13 Improved Vertical Phase Separation Induced by Interfacial Energy. As with the improvement of lateral phase separated morphology, the vertical distribution of the components in the blend film was also modified via mixedsolvent vapor-assisted imprinting process. As is well-known, the vertical phase separation is crucial to device performance, which partly dominated charge transport and extraction. In conventional devices, acceptor materials should be aggregated on the top of active layer, in contrast, donor materials should assemble on the bottom of active layer. However, polymers usually migrate to the exposed surface and fullerenes enrich at the anode interface in polymer/fullerene blend system due to the surface energy difference between polymer and fullerene (the surface energy of polymers is lower than that of fullerenes, which promotes polymer aggregates on film surface to reduce the overall free energy of blend system).21 In PCDTBT/PC70BM blend system, it also has a nonideal composition profile. X-ray photoelectron spectroscopy (XPS) offers a useful tool for determining the composition at the sample surfaces.17 In our experiment, the PC70BM to PCDTBT weight ratios at the top surface of films are evaluated using C/N atomic ratios obtained from the XPS measurement.19 The peaks at about 285 eV and 399 eV are caused by the C 1s and N 1s, respectively, and C/N atomic ratios are obtained from the peak area of corresponding elements, as shown in Figure 5A. The weight ratio of PC70BM to PCDTBT at film surface can be determined by using the following formula: mPC70BM mCDTBT

=

⎞ ⎛N × ⎜ C − 14⎟ × 82 ⎝ NN ⎠

M PC70BM MCDTBT

(1)

where NC and NN are the number of carbon atoms and nitrogen atoms, MPC70BM and MCDTBT are the molar mass of PC70BM and repeat unit of PCDTBT, 14 comes from the mole 4590

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treatment.39 Within our expectation, the content of fullerene on the top further increased when PDMS plate or PDMS stamp was covered onto the blend film, since the weight ratio of PC70BM to PCDTBT is 0.51 for sample M-SVA&P and is 0.55 for sample M-SVA&S. As we know, the PDMS film could provide a higher surface energy than that of the air surface, so the thermodynamically favored fullerene diffused to the top surface.21,40 Moreover, the contact area between the PDMS stamp and active layer was larger than the one between the PDMS plate and active layer regarding of the grating structures. As a consequence, the driving force for diffusion of fullerene in sample M-SVA&S must be stronger than that in sample MSVA&P, which resulted in higher fullerene content on the top surface of sample M-SVA&S. Enhanced Photon Absorption by Light Diffraction. The absorption efficiency of incident light in PSCs is one of major parameters toward achieving high device PCE. The exploitation of surface relief grating structure that increase the optical path length within the active layer leads to an enhanced light absorption.41,42 After the mixed-solvent vapor-assisted imprinting treatment, a regular surface relief grating structure on the film surface was obtained due to the action of capillarity. As shown in Figure 2B, left, the period of grating structure is about 680 nm, which is small enough to diffract the light into the solar cell active layer. As shown in Figure 6A, incident lights reaching the grating structures can be diffracted backward, and the increasement in optical path can be estimated by the (Ld − Li)/Li = 1/cos θd − 1, and θd can be determined according to the following equation: Figure 5. (A) XPS spectra of N 1s region and C 1s region from top surface of the PCDTBT/PC70BM films without and with different annealing treatments. (B) The columnar section shows the weight ratio of PC70BM to PCDTBT at film top surface without and with different annealing treatments.

ratio of carbon atoms and nitrogen atoms in PCDTBT, and 82 is the number of carbon atoms in one PC70BM molecule. The weight ratios of PC70BM to PCDTBT are summarized in Table 1, and the obvious changing trend can be found in Figure 5B. Table 1. C/N Atomic Ratios and the Weight Ratio of PC70BM to PCDTBT at Film Surface Treated by Different Annealing Processes process

without annealing

M-SVA

M-SVA&P

M-SVA&S

NC/NN mPC70BM/mPCDTBT

27.81 0.25

32.57 0.33

42.67 0.51

44.61 0.55

Although the weight ratio of PC70BM to PCDTBT in the blend film is 5:2, the weight ratio of PC70BM to PCDTBT of pristine film measured on the top surface were only 0.25, which indicates a large accumulation of PCDTBT on the top. After the mixed-solvent vapor annealing treatment, the weight ratio of PC70BM to PCDTBT increased to 0.33, indicating parts of fullerene molecules migrated onto the film surface during the annealing process.35 This can be explained by the nucleation of fullerenes: THF in mixed-solvent vapor could induce fullerenes to nucleation and then form aggregates.38 Resulting from the smaller spatial confinement of the film surface (compared with the one of inner film), fullerene molecules inclined to diffuse onto the top surface and nucleate during the annealing

Figure 6. (A) Schematic representation of the effect of the surface relief grating structure on increasing the optical path length. (B) Reflection spectra of PCDTBT/PC70BM films without and with different annealing treatments. 4591

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Table 2. Parameters of the Device Performance without and with Different Vapor Annealing Treatments under 100 mW cm−2 Simulated AM1.5G Illumination pristine M-SVA M-SVA&P M-SVA&S

voc (V)

JSC (mA/cm2)

FF

PCE (%; avg/max)

Rsh (Ω·cm2)

Rs (Ω·cm2)

0.88 0.88 0.89 0.89

10.80 11.92 12.03 12.84

0.49 0.62 0.64 0.63

4.66 (4.51/4.68) 6.50(6.38/6.53) 6.85 (6.82/6.91) 7.20(7.05/7.20)

1060 1653 2322 2120

15.98 12.01 7.66 6.98

mλ = nP(sin θi + sin θd)

(2)

where Li and Ld are the optical path of incident light and diffracted light, respectively, and θi and θd are the incidence and diffraction angles, respectively, m is the diffraction order, λ is the wavelength of incident light, n is the refractive index of the active layer, and P is the grating period.41,42 Herein, a constant n was considered as 2 in absorption range (about between 350 and 650 nm) of the active layer. In our experiment, only zeroth, first, and second order reflection occurred. When the incidence light (λ = 350 nm) impinged onto the grating structures, the first and second diffraction was bent by 14.9 and 30.98°, respectively. As the wavelength of incidence light increased to 650 nm, the corresponding first and second diffraction angle increased to 28.6 and 72.91°, respectively. The optical path increasement relative to the film without grating structure is obvious and the light-trapping effect was also experimentally evaluated from reflection spectra as shown in Figure 6B. The reflection spectra for four samples show a similar reflection peaks, but a decreased reflection intensity in the overall absorption range of sample M-SVA&S. Because the prolonged optical path which generated from the light diffraction raise the probality of photon absorption. Increased Device Performance by Enhanced Photon Absorption, Increased Carrier Mobility, And Decreased Charge Recombination. After mixed-solvent vapor-assisted imprinting process, both lateral phase-separated morphology and vertical distribution of the components in the blend film are improved, as well as a surface relief grating structure was introduced onto the active layer. These morphology transitions have profound influences on device performance. The lateral phase separation resulted in an increased carrier mobility, the vertical phase separation reduced the probability of carrier recombination, and the grating structure enhanced photon absorption of active layer. As a result, the device M-SVA&S showed a PCE of 7.20% under AM1.5G illumination, almost 55% higher than that without an annealing treatment. Photovoltaic devices based on PCDTBT/PC70BM blend films with different annealing treatments were made according to the standard ITO/PEDOT:PSS/PCDTBT:PC70BM/Ca/Al architecture. The performances of these devices are outlined in Table 2 and the current density−voltage (J−V) curves are shown in Figure 7A. For the device based on a pristine PCDTBT/PC70BM film, the low Jsc and FF of 10.80 mA/cm2 and 49%, respectively, contributing to a PCE of only 4.66%. After a mixed-solvent vapor-assisted imprinting process, a substantial increase in Jsc (12.84 mA/cm2) and FF (0.63) eventually led to an improved PCE of 7.20%, which resulted from morphology transitions and the introduction of surface relief grating structures. In order to illustrate the effect of active layer structure on device performance, the devices M-SVA and M-SVA&P were also fabricated. After the mixed-solvent vapor annealing treatment, a substantial increase in Jsc and FF eventually contributed to an improved PCE of 6.50% for device

Figure 7. (A) J−V curves for devices processed from PCDTBT/ PC70BM films without and with different annealing treatments. (B) EQE spectra with the AM1.5G solar spectrum of devices without and with different annealing treatments.

M-SVA. The improvement is attributed to the enhanced crystallinity of PCDTBT and the inhibited intercalated behavior in PCDTBT/PC70BM blends, which led to a reduced charge transfer (CT) state recombination, an increased lifetime of the mobile carrier and carrier mobility as we have reported.13 Accordingly, from the J−V curve of an illuminated solar cell, Rs and Rsh can be calculated and the values are shown in Table 2. Comparing to the pristine device, the device M-SVA shows an increased Rsh and decreased Rs, which is also consistent with better organization of the pathway for charge flow.43 The device M-SVA&P exhibits a higher Jsc (12.03 mA/cm2) and FF (0.64), which could be ascribed to the vertical distribution of the components on the base of interpenetrating network. As confirmed by the XPS measurement, the content of fullerene at the top increased when a PDMS was covered. The introduced PC70BM-rich superficial region would link isolated PC70BM 4592

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domains beneath, in other words, electrons trapped in isolated fullerene domains are released owing to the connection of buried conducting paths to the cathode (i.e., decreasing Rs). Meanwhile, the fullerene-rich layer also minimized the electron−hole recombination leakage at the cathode interface by providing a high energy barrier for hole transport toward Al cathode (i.e., increasing Rsh). Fortunately, the performance of device M-SVA&S was further improved after the introduction of surface relief grating structures. The improvement is mainly attributed to the enhanced photon absorption compared with device M-SVA&P, which resulted in a higher Jsc (12.84 mA/ cm2). As a consequence, the optimized morphology and grating structure led to a higher device performance, reaching up to 7.20% under AM1.5G illumination. The external quantum efficiency (EQE) curves of devices without and with different annealing treatments are shown in Figure 7B. The device without treatment show photoconversion efficiency in the range between 375 and 600 nm with EQE values about 60%. While devices including device M-SVA, device M-SVA&P and device M-SVA&S show an EQE values about 70%. Especially, the maximum EQE of device M-SVA&S is over 70%, which indicated efficient photoconversion efficiency after the mixedsolvent vapor-assisted imprinting process. In addition, the J−V measurements are rather accurate as further examined by integrating the EQE data with the AM1.5G solar spectrum. The calculated Jsc is 12.76 mAcm−2 in the M-SVA&S device, comparing to the measured Jsc of 12.84 mAcm−2, it represents an error of 0.6%. To further confirm the effect of morphology on device performance, we investigated carrier mobility via so-called single carrier devices as well (the hole mobility of the device has a structure of ITO/PEDOT:PSS/PCDTBT:PC70BM/MoO3/ Al, and the electron mobility of the device has a structure of Al/ PCDTBT:PC70BM/Ca/Al). We calculated the mobility was calculated by Child’s Law from Figure 8, the hole and electron mobilities of four devices based on different experimental conditions are also listed in Table 3. Owing to the intercalated behavior, the carrier mobility of device without annealing treatment is low (the hole mobility is 5.21 × 10−5 cm2/V s and the electron mobility is 4.62 × 10−5 cm2/V s), which implies a discontinuous pathway for carriers. After the mixed-solvent vapor treatment, the hole mobility of the device M-SVA increased to 7.46 × 10−5 cm2/V s, which was attributed to the improved crystallinity of PCDTBT and the good connectivity of crystalline nanofibers. Furthermore, the electron mobility was also significantly improved (3.06 × 10−4 cm2/V s), which resulted from the aggregation of PC70BM, thus induced phase separation and formed continuous electron pathways.13 The vertical distribution of components has an important influence on carrier mobility as well. The increased fullerene content on surface of sample M-SVA&P and M-SVA&S led to an enhancement for both hole and electron mobility (the hole mobility is above 9.5 × 10−5 cm2/V s and the electron mobility is above 4.5 × 10−4 cm2/V s). The increased hole mobility must result from the increased crystallinity as confirmed by SE measurement. Moreover, the increased electron mobility can be ascribed to the PC70BM-rich superficial region on top of the films, which reduced the probability of charge recombination and provided a continuous pathway for electrons to transport to cathode. The improved electron and hole mobility enhance charge transport and retard charge recombination and are common features of high photovoltaic performance blend films.

Figure 8. J−V curves of hole-only devices (A) and electron-only devices (B) based on films without and different annealing treatments.

4. CONCLUSIONS In summary, a novel method named mixed-solvent vaporassisted imprinting annealing was employed to control the morphology and surface structure of PCDTBT/PC70BM blend films. The components of the mixed solvent are THF and CS2, and their appropriate ratio is 1:1. After treating the films with the method mentioned above for 20 min under the vapor pressure P = 0.9 (25 °C), an optimized film structure was obtained: an interpenetrating network structure with a nanoscale phase separation, increased fullerene content on the top surface, and an introduced surface relief grating structure. During the annealing process, the mixed-solvent vapor induced PC70BM to formed aggregates, which inhibited the intercalated behavior and enhanced the crystallinity of PCDTBT, resulting in an optimized lateral phase separation. In addition, a PDMS stamp was covered onto the blend film during the annealing process, which provided a higher surface energy than that of the air surface. As a consequence, the thermodynamically favored fullerene diffused to the top surface, and the weight ratio of PC70BM to PCDTBT increased from 0.25 to 0.55. Furthermore, surface relief grating structures were obtained due to the action of capillarity, and the grating structures could diffract the light into the active layer, thus, enhancing photon absorption. With the aid of optimized morphology and surface relief grating structures, photon absorption was enhanced, carrier mobility was improved, and the carrier recombination was also reduced, improving the PCE to a great extent, reaching up to 7.20% under AM1.5G illumination, almost 55% higher than PCE of reference device (PCE = 4.66%). 4593

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Table 3. Hole and Electron Mobility Characteristics of Four Devices Based on Different Experimental Conditions

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mobility

without annealing

M-SVA

M-SVA&P

M-SVA&S

μh (cm2/V s) μe (cm2/V s)

5.21 × 10−5 4.62 × 10−5

7.46 × 10−5 3.06 × 10−4

9.54 × 10−5 4.67 × 10−4

9.71 × 10−5 5.15 × 10−4

(12) Miller, N. C.; Gysel, R.; Miller, C. E.; Verploegen, E.; Beiley, Z.; Heeney, M.; McCulloch, I.; Bao, Z.; Toney, M. F.; McGehee, M. D. The Phase Behavior of a Polymer-Fullerene Bulk Heterojunction System That Contains Bimolecular Crystals. J. Polym. Sci., Part B: Polym. Phys. 2011, 49, 499−503. (13) Liu, J.; Chen, L.; Gao, B.; Cao, X.; Han, Y.; Xie, Z.; Wang, L. Constructing the Nanointerpenetrating Structure of PCDTBT:PC70BM Bulk Heterojunction Solar Cells Induced by Aggregation of PC70BM via Mixed-Solvent Vapor Annealing. J. Mater. Chem. A 2013, 1, 6216−6225. (14) Liu, J.; Shao, S.; Wang, H.; Zhao, K.; Xue, L.; Gao, X.; Xie, Z.; Han, Y. The Mechanisms for Introduction of N-Dodecylthiol to Modify the P3HT/PCBM Morphology. Org. Electron. 2010, 11, 775− 783. (15) Miller, N. C.; Sweetnam, S.; Hoke, E. T.; Gysel, R.; Miller, C. E.; Bartelt, J. A.; Xie, X.; Toney, M. F.; McGehee, M. D. Molecular Packing and Solar Cell Performance in Blends of Polymers with a Bisadduct Fullerene. Nano Lett. 2012, 12, 1566−1570. (16) Xin, H.; Guo, X.; Ren, G.; Watson, M. D.; Jenekhe, S. A. Efficient Phthalimide Copolymer-Based Bulk Heterojunction Solar Cells: How the Processing Additive Influences Nanoscale Morphology and Photovoltaic Properties. Adv. Energy Mater. 2012, 2, 575−582. (17) Yang, B. W.; Tsai, M. Y.; Cheng, W. H.; Chen, J. S.; Hsu, S. L. C.; Chou, W. Y. Synergistic Amplification of Short Circuit Current for Organic Solar Cells via Modulation of P3HT:PCBM Spatial Distribution with Solvent Treatment. J. Phys. Chem. C 2012, 14, 8313−8318. (18) Kokubu, R.; Yang, Y. Vertical Phase Separation of Conjugated Polymer and Fullerene Bulk Heterojunction Films Induced by High Pressure Carbon Dioxide Treatment at Ambient Temperature. Phys. Chem. Chem. Phys. 2012, 14, 8313−8318. (19) Xu, Z.; Chen, L. M.; Yang, G. W.; Huang, C. H.; Hou, J. H.; Wu, Y.; Li, G.; Hsu, C. S.; Yang, Y. Vertical Phase Separation in Poly(3hexylthiophene): Fullerene Derivative Blends and its Advantage for Inverted Structure Solar Cells. Adv. Funct. Mater. 2009, 19, 1227− 1234. (20) Germack, D. S.; Chan, C. K.; Kline, R. J.; Fischer, D. A.; Gundlach, D. J.; Toney, M. F.; Richter, L. J.; DeLongchamp, D. M. Interfacial Segregation in Polymer/Fullerene Blend Films for Photovoltaic Devices. Macromolecules 2010, 43, 3828−3836. (21) Park, H. J.; Kang, M.-G.; Ahn, S. H.; Guo, L. J. A Facile Route to Polymer Solar Cells with Optimum Morphology Readily Applicable to a Roll-to-Roll Process without Sacrificing High Device Performance. Adv. Mater. 2010, 22, E247−253. (22) Xue, B. F.; Vaughan, B.; Poh, C. H.; Burke, K. B.; Thomsen, L.; Andrew Stapleton, X. Z.; Bryant, G. W.; Belcher, W.; Dastoor, P. C. Vertical Stratification and Interfacial Structure in P3HT:PCBM Organic Solar Cells. J. Phys. Chem. C 2010, 114, 15797−15805. (23) Wang, D. H.; Kim, D. Y.; Choi, K. W.; Seo, J. H.; Im, S. H.; Park, J. H.; Park, O. O.; Heeger, A. J. Enhancement of Donor-Acceptor Polymer Bulk Heterojunction Solar Cell Power Conversion Efficiencies by Addition of Au Nanoparticles. Angew. Chem., Int. Ed. 2011, 50, 5519−5523. (24) Wu, J. L.; Chen, F. C.; Hsiao, Y. S.; Chien, F. C.; Chen, P.; Kuo, C. H.; Huang, M. H.; Hsu, C. S. Surface Plasmonic Effects of Metallic Nanoparticles on the Performance of Polymer Bulk Heterojunction Solar Cells. ACS Nano 2011, 5, 959−967. (25) Fan, G. Q.; Zhuo, Q. Q.; Zhu, J. J.; Xu, Z. Q.; Cheng, P. P.; Li, Y. Q.; Sun, X. H.; Lee, S. T.; Tang, J. X. Plasmonic-Enhanced Polymer Solar cells Incorporating Solution-Processable Au NanoparticleAdhered Graphene Oxide. J. Mater. Chem. 2012, 22, 15614−15619. (26) Li, K.; Zhen, H.; Huang, Z.; Li, G.; Liu, X. Embedded Surface Reef Gratings by a Simple Method to Improve Absorption and

ASSOCIATED CONTENT

S Supporting Information *

The performance of devices treated with different annealing times (15, 20, and 25 min) and the performance of devices treated under different vapor pressures (P = 0.81, 0.90, and 0.97). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21334006, 51273191, 51303177), the Ministry of Science and Technology of China (2014CB643505), and Scientific Development Program of Jilin Province (20130101007JC).

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REFERENCES

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