Rational Design of Ternary-Phase Polymer Solar Cells by Controlling

May 2, 2014 - In this article, we report a novel route to control the ternary-phase morphology of the active layer of polymer solar cells (PSCs). Two ...
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Rational Design of Ternary-Phase Polymer Solar Cells by Controlling Polymer Phase Separation Han Yan,†,‡ Denghua Li,†,‡ Yajie Zhang,† Yanlian Yang,†,* and Zhixiang Wei†,* †

National Center for Nanoscience and Technology, Beijing 100190, P. R. China University of the Chinese Academy of Sciences, Beijing 100049, P. R. China

‡

S Supporting Information *

ABSTRACT: In this article, we report a novel route to control the ternary-phase morphology of the active layer of polymer solar cells (PSCs). Two typical polymers with complementary absorption ranges, i.e. poly(3-hexylthiophene) (P3HT) and poly[(4,4′-bis(2-ethylhexyl)dithienol[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7diyl] (PSBTBT), are selected to obtain ternary phase system by blending with (6,6)-phenyl-C71 butyric acid methyl ester (PC71BM). A more than three times increase of power conversion efficiency is observed by tuning the morphologies of ternary phase with high second polymer loading. Different from the traditional disordered intermixing morphologies, the existence of submicrometer scale domains of polymer-rich phases are observed for P3HT and PSBTBT, respectively. The measurements of photoluminescence quenching demonstrate that with the morphology varying from intermixed to hierarchical morphology, the interactions between two polymers changing from charge transfer (CT) to fluorescence resonant energy transfer (FRET); at the same time charge transfer mainly occurs at polymers and PC71BM interfaces. The photophysical process here is different from previous reports. A model named hierarchical interpenetrating networks model (HINM) is proposed to describe the optimal active layer of ternary-phase PSCs. Further Kelvin probe force microscopy (KPFM) results demonstrate the reason for our relatively low efficiency is limited by PSBTBT charge transport in blend matrix. We believe that this novel route for controlling morphology could be further optimized and would provide new thoughts and opportunities in the area of PSCs.

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and absorption intensity;24−26 thus, there exists a compromise for the overall device efficiency. The limitations of single junction device can be overcome by using tandem structures, in which two single cells are stacked together to obtain a complementary absorption.22,23,27−31 Each subcell works independently without any complex charge or energy transfer. This strategy has been proved to be effective, however promoting some new strict requirements: efficient interconnecting layer, processing issues and single device characterization.23 To connect the two subcells, an interlayer should be introduced. These interfaces have to be carefully designed and optimized, thus make the processing more complicated and reduce the transmittance light. To extend the absorption range of single junction PSC, some reports have demonstrated that the efficiency could be effectively improved by the addition of a small fraction of dye molecules or small band gap polymers into the active layer.32,33 However, the additives act only as sensitizer, lack of independent charge transporting pathway, and thus limits its further development.34 A parallel strategy may also seem feasible, blending the three

INTRODUCTION In the past decade, organic photovoltaics have made great progress and become a promising alternative to inorganic solar cells. Their unique properties, such as low cost, ease of processability and mechanical flexibility aroused tremendous efforts to improve the efficiencies aiming to a value compared to the inorganic counterparts.1−6 Various strategies have been applied to enhance the efficiency of polymer solar cells (PSCs) which include morphology optimization of the active layer7−13 and utilizing low band gap polymers.14−20 Recently, the highest conversion efficiency in lab, has exceeded 9% for single junction PSCs.21 The main limitation for further improvement is the narrow absorption range of organic materials. According to Shockley−Queisser principle, two major losses occurring in solar cells are the sub-band gap transmission and the thermalization of hot charge carriers.22 This in-efficient light absorption leads to low current density, and thus limits its overall power conversion efficiency (PCE). Therefore, intensive studies have been focused on broadening the absorption of ptype polymers. However, even for a well designed narrow band gap polymer, its absorption range only covers a small part of the solar spectrum. As a result more than 60% of the solar spectrum remains unabsorbed.23 On the other hand, further decreasing the band gap would reduce the open circuit voltage © 2014 American Chemical Society

Received: November 3, 2013 Revised: April 26, 2014 Published: May 2, 2014 10552

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Figure 1. (a) Energy level diagram of P3HT/PSBTBT/PC71BM and their chemical structures. (b) UV−vis absorption spectroscopy of binary- (1:1 by weight ratio) and ternary-phase (1:1:2 by weight ratio) films prepared from CB and p-x. (c) Schematic illustration of the PSC device structure. (d) The J−V curves correspond to binary and ternary devices prepared at the same condition with the absorption spectroscopy measurements.

1a.44 From the energy level alignment, one can deduce that there may exist charge transfer between each polymer and fullerene derivative as well as between two polymers. On the other hand, there also exist energy donor-P3HT and energy acceptor-PSBTBT, indicating a possible fluorescence resonant energy transfer (FRET) from P3HT to PSBTBT. To study these processes, a polar solvent chlorobenzene (CB) or a nonpolar solvent p-xylene (p-x) was chosen as the solvent. The absorption spectra of binary- and ternary- phase blend films prepared in different conditions are shown in Figure 1b. The thicknesses of binary-phase blend films are about 60 nm, and they are controlled to be about 120 nm for ternaryphase systems by spin-coating speed (Table 1). The absorption

components together in one layer: two polymer donors and a fullerene derivative acceptor. Several reports have focused on this issue. The device performance and photophysical processes have been studied.33,35−46 On the basis of the photophysical processes between two donors, this kind of polymer solar cells can be separated into three categories: cascade charge transfer, energy transfer and parallel-like charge transfer/transport.47,48 And, based on the intermixed morphology of the active layer, charge transfer type polymer combination can only load a limited second polymer, thus suppressing the increase of absorption intensity.48 Furthermore, due to the complexity of polymer phase behaviors concerning polymer/polymer mixtures combining with polymer/fullerene mixtures, only a few reports have been focused on the phase behavior of active layer.49−51 In this article, we report a detailed study to control the ternary-blend morphology of the active layers for PSCs. We selected two typical polymers: poly(3-hexylthiophene) (P3HT) andpoly[(4,4′-bis(2-ethylhexyl)dithienol[3,2-b:2′,3′-d]silole)2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl](PSBTBT), which have complementary absorption ranges. Fullerene derivatives ((6,6)-phenyl-C71 butyric acid methyl ester, PC71BM) was used as acceptor in all cases. A three times PCE enhancement is achieved with high second polymer loading. Combining with photoluminescence quenching spectroscopy, atomic force microscopy (AFM), and Kelvin probe force microscopy (KPFM), the morphology-related optoelectronic behaviors are systematically studied in this complicated system. A new model called hierarchical interpenetrating networks model (HINM) is proposed to describe the optimal active layer of ternary-phase PSCs.

Table 1. Performance of PSCs Fabricated with Different Solvents materials P3HTPC71BM PSBTBTPC71BM ternary system ternary system

solvent

film thickness (nm)

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

p-x

59

0.63

5.8

60

2.6

p-x

57

0.66

4.1

34

1.2

CB

117

0.66

3.8

25

0.73

p-x

115

0.65

7.2

43

2.4

spectroscopy of ternary-phase blend films follows a simple superimposition of the binary films and no additional features are identified. However, their absorption range from 300 to 800 nm is much broader than any single component polymer. The absorption of UV−vis spectra gave a first indication that ternary system might be effective in the polymer solar cells. For a detailed comparison, the single-, binary- and ternary-phase absorption of solutions and films are listed in the Supporting

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RESULTS AND DISCUSSION The corresponding chemical structure of the investigated materials and their energy level diagram are presented in Figure 10553

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Figure 2. (a, c, and e) Fluorescence quenching spectroscopy of films prepared from different solvents: (a) P3HT and binary-phase blend films under 550 nm excitation; (c) P3HT based binary- and ternary-phase blend films under 550 nm excitation; (e) PSBTBT based binary- and ternary-phase blend films under 670 nm excitation. (b, d and f) Schemes of corresponding exciton processes according to the PL spectra.

hand, an obvious Jsc enhancement could be observed comparing to the binary-phase devices for the device fabricated from p-x. The Jsc enhancement could ascribe to the usage of a broader solar spectrum in ternary-phase devices comparing to that in binary-phase devices. Unfortunately, even for the best ternaryphase device performance from p-x, the PCE value is still lower than the superimposition of the two binary-phase devices. This implies that the equivalent circuit of the active layer is not simply in series or in parallel. To understand the PCE variations when casting from different solvents, we use steady-state photoluminescence spectrum (PL) to study the photophysics of excitons in corresponding active layers. For traditional binary-phase blend films, the main photophysical process for excitons is CT. However, for ternary-blend solar cell, the process is much more complicated. As analyzed in Figure 1a, introducing another polymer in this system may bring in a CT or a FRET process between two polymers. P3HT plays the roles of both electrondonor and energy-donor. The two main processes of CT and FRET could be distinguished by the measurements of fluorescence quenching. Both CT and FRET could induce

Information in details (Figure S1), which further confirm their superimposing behavior on the absorption spectra in ternaryphase blend films. To study the photovoltaic performance based on ternaryphase system, devices were fabricated with standard sandwichtype structures (shown in the inset of Figure 1c). To analyze the roles of each polymer in this complex matrix, devices based on binary-phase systems were also fabricated with the same device architecture. Details on the device optimization were shown as Tables S1−S3 in Supporting Information. Figure 1d shows the current−voltage (J−V) curves of the optimized PSCs. The photovoltaic performance data, including the film thickness, open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and power conversion efficiency (PCE) values, are summarized in Table 1. The device performance differences between the ternary-blend PSCs fabricated from CB and p-x could be clearly observed. The device J−V curve obtained from CB presents an “S” shape; this phenomenon is normally ascribed to geminate or bimolecule recombination in the active layer.6 This would greatly decrease the Jsc, FF, and the overall device performance. On the other 10554

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fluorescence quenching, while the red-shifted fluorescence is enhanced at the same time for FRET. We first study the two component interaction. (Figure 2a) When excited at 550 nm, we could confirm the exciton process of P3HT. For single P3HT, the film prepared from p-x would induce a significant fluorescence quenching, this is consistent with previous reports attributing to the self-assembly. When blended with PSBTBT, a large difference appeared. If the film was prepared from CB, the P3HT photoluminescence peak quenched a lot and no red-shift peak appeared. We confirm the main photophysical process is CT, electron transfer from P3HT to PSBTBT. However, when the film was prepared from p-x, obvious quenching effect was also observed while new peak at 830 nm appeared with almost the same intensity of P3HT emission peak, ascribing to the PSBTBT photoluminescence. Energy transfer occurs in this condition. P3HT can play both the roles of electron donor or energy donor by adjusting the film fabricating processes. The different photophysical processes are summarized in Figure 2b. Besides the exciton generating, the photogenerated current is actually dominated by the exciton separation efficiency at the polymer/PC71BM interfaces. The blend films based on P3HT were first studied (Figure 2c). When excited by 550 nm light, it was observed that no matter P3HT blended with PC71BM or PSBTBT/PC71BM, more than 95% fluorescence quenched in blend films compared to pure P3HT film. This demonstrates that efficient charge transfer could occur at the interfaces of blend films (Figure 2d). Furthermore, 670 nm light source was used to investigate the optoelectronic process for blend films based on PSBTBT. As shown in Figure 2e, only one peak at 820 nm was observed. The quenching extent was almost the same for binary- and ternary-blend films, and no other redshifted peaks are observed. Thus, we suggest CT is the major process in ternary phase system (Figure 2f). Considering the same composition and ratio of the two samples spin-coating from CB and p-x, the main reason induced the performance variation may originate from their mesoscopic morphology. And the most common tool for characterizing polymer blends for PSCs is AFM. This method enables imaging of surface topography with high spatial resolution less than 10 nm, combining with phase images, regions with different composition can be distinguished. The surface morphologies of all binary and ternary phases are determined by tapping mode atomic force microscopy (tpAFM) (Figure 3), and their corresponding height images are shown as Figure S2. The self-assembled structures of pure P3HT could be clearly seen in Figure 3a. And more importantly, the aggregation behaviors for P3HT and PSBTBT are different in their blends with PC71BM. P3HT tended to form nanorods (Figure 3a), while PSBTBT tended to form nanoparticles (Figure 3b). According to their self-assembly characteristics, we could accurately distinguish the two distinct phases in their ternary phase films. The ternary-phase phase images prepared form CB and p-x are shown in Figure 3c and d, respectively. When spin-coating from CB, a rather uniform intermixing morphology is presented. Small PSBTBT particles dispersed uniformly between P3HT naorods, and no distinct larger phase separation domains could be observed. When preparing from p-x, however, a hierarchical structure was observed with micrometer scale P3HT-rich and PSBTBT-rich domains (Figure 3d). The difference on the morphology of ternary blend film can be explained by free energy of blends. Here, p-x is a good solvent for PSBTBT, while a poor solvent for P3HT. When

Figure 3. Tapping-mode AFM (2 μm × 2 μm) phase images: (a) pure P3HT film prepared from CB; (b) pure PSBTBT film prepared from p-x; (c) ternary-phase blend film prepared from CB; (d) ternary-phase blend film prepared from p-x.

casting from p-x, there exists preassembly for P3HT. This could be confirmed by XRD results shown in Figure S3. The peak intensity is higher for samples casted from p-x, indicating higher crystallinity. As previously reported, the assembly of P3HT is an enthalpy-driven process.52 Thus, it could effectively reduce the enthalpy by forming the P3HT nanorods. Micrometer scale phase separation could be further induced due to the aggregation of preself-assembled P3HT nanorods when casted from p-x. The micrometer scale phase separation of the two polymers avoids CT from P3HT to PSBTBT, and induces FRET from P3HT to PSBTBT. The differences of device performances and photophysical processes obtained from different solvent could be attributed to their various morphologies. As discussed above, a new model is proposed to control the active layers of ternary-phase PSCs. Different from the traditional one-step route for the formation of bulk heterojuctions, this process in a ternary phase system could be divided into a two-step route. This novel route is shown in the ternary phase diagram (Figure 4a). First, the polymer solution in p-x was heated, and then cooled to the room temperature. As p-x is good solvent for PSBTBT but a poor solvent for P3HT, P3HT is preassembled in this process. When we mixed this suspension with PC71BM, we change the state of ternary-phase from P1 to separated P3 and P4 state. When forming films from P1 point (in the solution of CB), the morphology will intermixed as modeled in Figure 4b. From P2 point, P3HT and PSBTBT will form microscale phase separation without interference of PC71BM (Figure 4c). The preassembly of P3HT equals to changing from P1 to the mixture of P3 and P4; thus, the hierarchical interpenetrating networks were obtained. This novel model is named as “hierarchical interpenetrating network model” (HINM) as shown in Figure 4d. The HINM consists of a hierarchical architecture composed of interpenetrating networks, and the two scale phase separation plays its own role. First, there exists a micrometer scale phase 10555

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According to the energy level showed in Figure 1a, the energy level of P3HT is higher than PSBTBT, thus the bright spots represent P3HT-rich domains and the dark spots represent PSBTBT-rich domains. To detect the optoelectronic properties of P3HT-rich and PSBTBT-rich domains separately, the surface potential is tested under the illumination at 532 and 650 nm using a 50 mW laser source. The surface potential images were recorded over the same area of the same sample for Figure 5a and Figure S5. The statistical contrast data are listed in Figure 5b, from which we could clearly see profound differences. The contrast shifts with and without illuminations are referred to the variation between their central values indicated by the dashed lines. A left-shift contrast was observed when 532 nm laser was incident, while right-shift under the illumination of 650 nm laser. The decreased or increased contrasts were ascribed to charge transfer between polymers and PC71BM under excited state as previous reports.54 Combined the contrast variations here, it is clearly seen that efficient charge transfer occurs within either P3HT- or PSBTBT-rich domains. To quantify the charging effects, the absolute values of surface potential were recorded over the same area corresponding to Figure 5a. As shown in Figure 5c, a steep step appeared when light was applied. The surface potential decreases, revealing a net accumulation of negative charge at the surface.54 The cross sections of SP values were also listed in Figure 5c. The cross section of SP shows a deeper well when excited at 532 nm than excited at 650 nm laser. From these results, we could find the contribution to overall photogenerated charges from the two polymers is not equal; the P3HT-rich domains provide more effective charges in this ternary-blend system. Thus, our limited conversion efficiency here is originated from the PSBTBT-rich domains. Combined with the AFM images in Figure 3, parts b and d, we deduce that the uncontinuous nanoparticles of PSBTBT may disrupt the efficient charge transport in blend matrix and induce bimolecular recombination.

Figure 4. Proposed HINM model of the active layer structure in ternary-phase PSC. (a) Ternary phase diagram of P3HT/PSBTBT/ PC71BM. (b) 3D schematic illustration of intermixing morphology. (c) 3D schematic illustration of binary-phase micrometer scale phase separation morphology. (d) 3D schematic illustration of ternary-phase hierarchical phase separation morphology.

separation, and the complicated active layer could be simply separated into two parts: P3HT-rich region and PSBTBT-rich region. As the charge separation length is often within the tens of nanometers, the separated micrometer scale morphology could effectively reduce the recombination loss when introducing a second donor in the ternary-phase active layer. Second, mesoscopic phase separation of donor and acceptor was revealed in the corresponding micrometer scale domains. The bicontinuous phase composed of polymer and PC71BM plays the same role as in a binary-phase active layer, which promote exciton separation and charge transport in the active layer. This hierarchical architecture offers a maximum interface for exciton separation and continuous pathways for charge transport; on the other hand it could effectively reduce the charge recombination between the two polymers. Although a rather ideal model was formed, the efficiency was not as high as expected. As the macroscopic performance of a solar cell is a sum of local contributions, quantitative information and relevance between photovoltaic performance and local phase separation are of vital importance for explaining the working mechanism. KPFM could directly measure the local optoelectronic properties with high lateral resolution to gain profound insights for the working mechanism of optoelectronic devices. In our HINM system, KPFM was carried out to explain how the photovoltaic property was controlled by the hierarchical phase separation. The typical KPFM image in dark is shown in Figure 5a, (height image in Figure S4) which corresponds to the topography image acquired simultaneously (shown as Figure S5). KPFM is used to measure surface potential distribution; it can provide intuitive surface potential information and is highly sensitive to the incident light. As described by previous reports,53 the surface potential contrast was originated from the different energy level positions between different components.

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CONCLUSION In summary, by rationally controlling the hierarchical phase separation as a result of changing the process route, an HINM model is proposed, which is a hierarchical phase separation synergistically improving the overall PCE of the ternary-phase PSCs. Combining with photoluminescence quenching spectroscopy and KPFM, we revealed the optoelectronic process in this complicated system. The ordered phase separation structure could change the photophysical processes between polymers, FRET instead of CT, which is different from previous reports by other groups. Our results provide a method to change the cascade charge transfer PSCs to parallel-like charge transfer PSCs just by adjusting the active layer morphology. This process might break through the limited polymer loading, thus further improve the absorption intensity. We believe that our findings are generally applicable and outlined a rational design rule for the active layer of ternary-phase PSCs. This novel morphology mainly optimized charge transporting matrix with decreasing charge recombination losses. And this could serve as a guideline for optimizing other material systems to achieve higher PCE.

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EXPERIMENTAL SECTION 1. Fabrication of PSCs. Indium−tin oxide (ITO) coated glass (15 Ω sq−1) was cleaned successively with deionized water, ethanol, acetone, and isopropyl alcohol thrice for 15 min 10556

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Figure 5. (a) KPFM images (2 μm × 2 μm) of the ternary-phase sample prepared from p-x in dark. (b) Histogram distribution of the surface potential contrast in the dark and under illumination at 532 and 650 nm. (c) 3D schematic images of surface potential and the corresponding crosssection curves.

and then dried at 150 °C for 30 min. After complete drying, the ITO glass was exposed to UV−ozone for 15 min. The PEDOT:PSS was filtered through a 0.45 μm syringe filter, spin coated for 40 nm thickness, and then annealed at 140 °C for 10 min in an ambient atmosphere. A mixed 1 mL solution of P3HT (10 mg/mL):PSBTBT (10 mg/mL) solutes in p-x was heated to 80 °C, staying for half an hour. Then it slowly was cooled down to room temperature at a rate of 20 °C per hour. It was placed in glovebox for 36 h and finally mixed with 20 mg/mL PC71BM. Stirring for 20 min took place before use. The mixed solutions of P3HT (10 mg/mL):PC71BM (10 mg/ mL) and PSBTBT (10 mg/mL):PC71BM (10 mg/mL) were also prepared following this method. For comparison, solutions of P3HT (10 mg/mL):PSBTBT (10 mg/mL): PC71BM (20 mg/mL), P3HT (10 mg/mL): PC71BM (10 mg/mL) and PSBTBT (10 mg/mL): PC71BM (10 mg/mL) in CB was also prepared by directly solute them in solvent. The solutions were stirred for the same time at room temperature. The PSCs were processed in a N2-filled glovebox. The spin-coated PSCs were fabricated using the different solutions. To improve the device performance, thermal annealing was performed prior to electrode deposition at different conditions for each polymer. After the whole film-forming process, Al (80 nm)/Ca (20 nm) were thermally evaporated on top of the active layer under vacuum lower than 2 × 10−6 mbar. The active area of the standard PSC device was 0.04 cm2.

2. Measurements and Characterizations. For in situ measurement and characterization, the samples were processed using the same method as the PSC production. X-ray diffraction (XRD) (Philips X’Pert using Cu Kα line 0.15419 nm) was conducted directly on these film samples. Optical absorption spectra of the samples were recorded on a UV−visIR spectrometer (PE Lambda 650/850/950 UV−vis spectrophotometer). And the corresponding photoluminescence (PL) spectra of samples were measured on a PL spectrophotometer (LS-45/55 Fluorescence Spectrometer) with a Xe lamp. The morphology of the active layer was observed using tapping-mode AFM (Dimension icon, Bruker Nano). KPFM investigations were performed in the dark and under continuous wave laser illumination at 532 and 650 nm using a commercial diode-pumped solid state laser at 50 mW. This wavelength has been selected because it fits well within the peak of the absorption band of the polymers. The photovoltaic performance was measured using a Keithley 2400 sourcemeter under AM1.5G (100 mW/cm2) simulated by a solar simulator (Newport Oriel). The light intensity was calibrated using a photodiode and light source meter prior to measurement. Poly(3-hexylthiophene) (P3HT, Sigma-Aldrich, Mn ∼64 000, 98.5% regioregular), poly[(4,4′-bis(2-ethylhexyl)dithienol[3,2b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PSBTBT, Luminescence Technology Corp., Mn 10000− 30000), and PC71BM (American Dye Source, Inc., 99.5%) 10557

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were used as received. Poly(3,4-ethylenedioxy thiophene):poly(styrenesulfonate) (PEDOT:PSS) (Baytron PVP Al 4083) was purchased from H. C. Stark. 1,2-Dichlorobenzene (CB) and pxylene (p-x) were distilled prior to use.

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ASSOCIATED CONTENT

S Supporting Information *

Absorption spectroscopy, AFM characterization, XRD characterization, AFM image corresponding to KPFM measurement, KPFM images under illumination, and device optimization. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Authors

*(Z.W.) Fax: 86-10-62656765. Telephone: 86-10-82545565. Email: [email protected]. *(Y.Y.) Fax: 86-10-62656765. Telephone: 86-10-82545559. Email: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation of China (Grants 21125420, 91027031), The Ministry of Science and Technology of China (2009CB930400, 2010DFB63530, 2011CB932300), and the Chinese Academy of Sciences.

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