Improving the Compatibility of Donor Polymers in Efficient Ternary

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Improving the compatibility of donor polymers in efficient ternary organic solar cells via post-additive soaking treatment Xiao-yu Yang, Fei Zheng, Wei-Long Xu, Peng-Qing Bi, Lin Feng, Jianqiang Liu, and Xiao-Tao Hao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11063 • Publication Date (Web): 13 Dec 2016 Downloaded from http://pubs.acs.org on December 15, 2016

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ACS Applied Materials & Interfaces

Improving the Compatibility of Donor Polymers in Efficient Ternary Organic Solar Cells via PostAdditive Soaking Treatment Xiaoyu Yang, †Fei Zheng,† Weilong Xu,† Pengqing Bi,† Lin Feng,† Jianqiang Liu#,† and Xiaotao Hao*,†,‡ †

School of Physics, State Key Laboratory of Crystal Materials, Shandong University,

Jinan, Shandong 250100, China ‡

School of Chemistry, The University of Melbourne, Parkville, Victoria 3010, Australia

Abstract: In dual-donor ternary organic solar cells, the compatibility between the donor polymers plays important roles to control the conformational change and govern the photo-physical behavior in the blend films. Here, we apply a post-additive soaking (PAS) approach to reconstruct the morphology in a ternary organic photovoltaic BHJ of PTB7-Th: PCDTBT: PC71BM. The PAS treated device has a maximum power conversion efficiency (PCE) of about 8.7% in this ternary system. From the analyses of GIWAXS and GISAXS, the superior device performance is attributed to the favorable nanomorphology with optimum crystallinity of PTB7-Th and good intermixing of PCDTBT with PTB7-Th:PC71BM, leading to improved charge transport in the vertical direction. AFM and TRPL measurements clearly demonstrate PAS-treated film envisages a homogeneous distribution of smaller PC71BM aggregates to facilitate the exciton dissociation and carrier extraction at the interface. The increased PCE ascribed to not only the enhancement of absorption and non-radiative Förster resonance energy 1

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transfer (FRET) between two donors (PCDTBT and PTB7-Th), but also the formation of a bicontinuous interpenetrating network of PC71BM.

Keywords:

Ternary organic solar cells; PAS; FRET; Photophysical process;

Morphology

1. INTRODUCTION In the past decades, bulk-heterojunction (BHJ) organic solar cells, composed of a mixture of various semiconducting polymers (as a donor) and fullerene derivatives (as an acceptor), have been undergoing a dramatic development, holding promise of flexible large-area organic solar cells fabrication using room-temperature solution processing.1 Presently, a single-junction polymer solar cell was confirmed to exhibit high PCE in excess of 10%.1 However, the solar spectrum could not be totally captured by the single polymer, restricting the efficiency of single-junction polymer solar cell developments.2 This could be due to relatively narrow absorption bands of conjugated polymers.2 Great efforts are being undertaken by the scientific community to expand the light absorption spectra of BHJ organic solar cells, via both chemical and physical approaches. One effective method is to synthesize narrow bandgap polymers to extend the absorption spectrum.3 The internal quantum efficiency of these materials systems can reach up to 100%.4 However, the inherent low open-circuit voltage (VOC) is a key limitation of the PSCs efficiency.5 An alternative approach is to perform a tandem organic solar cell with bottom cell and top cell absorbing complementary spectra.6,7

2


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However, the realization of a tandem cell requires a complicated technical process for stacking multiple photoactive layers, leading to high costs and limited availability. Furthermore, active layer thickness needs to readjust for different tandem cell systems, which is critical for high performance tandem solar cells. By comparison, ternary organic solar cells have drawn much attention recently by blending suitable conjugated polymers into a binary solar cell as the third component.8-11 The ternary BHJ solar cell can not only overcome aforementioned disadvantages and challenges, but also have many advantages of the enhanced absorbency, the low cost and the simple solution process.12 For the ternary organic solar cells, PCEs have been increased by 10–15% compared to binary solar cells. Recently, Goh and coworkers reported that non-radiative Förster resonance energy transfer (FRET) process is an effective strategy to greatly enhance the efficiency of polymer BHJ solar cells.13 In particular, in this work, we apply the FRET mechanism into ternary solar cells. PTB7-Th is selected as a dominating electron-donor polymer because it has a deeper HOMO energy, which can simultaneously give higher open circuit voltage (Voc) and short circuit current density (Jsc).1 Here, we select PCDTBT as a third component since not only complementary absorption spectrum and good air stability, but also roughly 100% internal quantum efficiency in PC71BM associated blends.14 However, for both PTB7-Th:PC71BM and PCDTBT:PC71BM binary system, the optimal active layer thickness is usually limited at ∼70–100 nm,1,15 which is unfavorable for the large-area solar cells fabrication with roll-to-roll techniques. On the other side, the increasing thickness leads to decrease Jsc and FF, resulting bad device 3



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performances (Figure S1 and Table S1). This may be resulted from the augmented charge recombination due to limited carrier mobility and poor charge transport in the blends. Alternatively, FRET process taken place from PCDTBT to PTB7-Th in the ternary blends can provide an additional non-radiative decay path for the PCDTBT to minimize recombination loss with thick active layers. Plenty of effort has been made to enhance the performance and exploring the working mechanisms of the ternary blend solar cells.8 Meanwhile, the conjugated polymers with ternary structure is found to exceed 10% of PCE that is suitable for fabrication of organic solar cells.16 In general, the selected third component plays significant role in systems through unique optoelectronic properties and morphology effects.10 It can serve as a light-absorbing species and/or a charge relay component of the electronic effect. Structurally, it can act as an inducer to enhance molecular crystallinity and orientation in the blend film, which facilitates the charge separation and transport.17 Unfortunately, although plenty of materials with excellent performance are used for binary systems, relatively few successful material combinations in ternary systems have been reported.11 It is not surprising that, in many cases, organic solar cells with the third component may have even worse performances owing to the dissimilar chemical and physical features of the two polymers. Yang et al.18 reported that compatible polymer donors could exist harmoniously, but the incompatibility between the polymers resulting harsh molecular disorder and cause low device performance. Thompson et al.19-22 systemically studied the influence of polymer compatibility on the open-circuit voltage in ternary BHJ solar cells. The incompatibility of third component 4


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makes the morphology of active layer exhibit different features compared to binary organic solar cells. To work out the issue of incompatibility, processing modifications for improving the morphology of binary BHJ devices can be effectively exploited to the ternary solar cells, such as thermal annealing treatment, adding solvent additive, solvent vapor treatment, mixed solvents and casting solvent.10,23 Among them, the post-additive soaking (PAS) approach for reconstructing BHJ nanomorphology has received much attention because it can be widely applied to various binary BHJ solar cells.24,25 Using this method, the morphology of the BHJ films can be effectively optimized, resulting in finely remixed nanophases. Therefore, it is necessary to apply this method into the ternary systems. In this work, the PAS method was developed to increase the compatibility in ternary BHJ organic solar cells. The ternary system is demonstrated based on two donors,

Poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-

benzothiadiazole)] (PCDTBT), Poly([2,6′-4,8-di(5-ethylhexylthienyl)benzo [1,2-b;3,3b]dithiophene]{3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]

thiophenediyl})

(PTB7-Th) and one acceptor, [6,6]-Phenyl C71 butyric acid methyl ester (PC71BM). The PAS treated device has a maximum power conversion efficiency (PCE) in the order of 8.7% in our system. The PAS treatment has a strong impact on intermolecular π-π stacking of the polymer and the morphology of the active layer revealed by grazing incidence X-ray scattering and atomic force microscopy. The underlying photophysical mechanism in the ternary organic films was investigated by time-resolved 5



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photoluminescence (TRPL) measurement.

2. EXPERIMENTAL SECTION Materials. Polymer PTB7-Th, PCDTBT and PC71BM were purchased from 1Materials. All the solvents were bought from Sigma-Aldrich without further purification.

Characterization. The absorption spectra were measured by a TU-1900 dual beam spectrophotometer. Steady-state photoluminescence was recorded with PG2000 Pro spectrophotometer. Fluorescence up-conversion method was used to record the time-resolved

decay

profiles.

Two-dimensional

time-resolved

imaging

was

implemented by the time-correlated single photon counting (TCSPC) method combined with high-resolution optical microscopy. The SPCImage software was used to analyze the fluorescence decay data. Atomic force microscopy (AFM, NanoScope IIIA) was employed to investigate the surface morphology of the produced films. 2D grazingincidence wide-angle X-ray scattering (GIWAXS) and grazing-incidence small-angle X-ray scattering (GISAXS) measurements were performed at the Shanghai Synchrotron Radiation Facility on beamline BL16B1, using a MAR165 CCD detector, at 10 keV radiation energy, and with an incident angle of 0.3 °. The distance of the sample to the detector was 260 mm for the GIWAXS and 2020 mm for the GISAXS measurements. The samples for both measurements were prepared by spin-coating on PEDOT:PSS blocking layer that coated with the silicon wafers.

Device Fabrication and Testing. The ternary organic solar cells with the inverted architecture of ITO/ZnO/PTB7-Th:PCDTBT:PC71BM/MnOx/Ag were 6


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fabricated according to the procedure described in this section. The fabricated device had an active area of 9 mm2. The patterned ITO substrates were washed in a bath of deionized water, acetone and isopropanol with ultra-sonication for 15 min in duration for each consecutive step. The method reported by Heeger et al.26 was used to prepare the ZnO precursor. 1 g of zinc acetate dehydrate and 0.28 g of ethanolamine in 2methoxyethanol (10 ml) were dissolved under vigorous stirring for 24 h in air and the precursor was aged at room temperature for one day. Subsequently, the ZnO precursor solution was spun on the substrates at 3000 rpm for 40 s. Immediately after processing, the samples were baked in air at 200 °C for 60 min in order to convert the zinc acetate to ZnO. The PTB7-Th:PCDTBT:PC71BM (1-x: x: 2) and PTB7-Th:PC71BM (1:2) composites were dissolved in chlorobenzene (CB) with polymer concentrations of 10 mg/ml. These solutions were stirred at 55 °C for 24 h. The pristine films of binary and ternary cells were fabricated by spin-casting blend solutions on the ZnO-coated substrates at 450 rpm for 100 s. The DIO-treated ternary film was spin coated from the solution with DIO (3 vol. %) on the substrate at 450 rpm for 100 s. The PAS-treated films were prepared by dropping 60 µL mixed solutions of additive/buffer-solvent onto the surface of the dried pristine ternary active layer for 3 seconds duration, followed by spinning of the residual solution at 3000 rpm for 40 s. The appropriate additive/buffer solvent mixture consists of DIO:cyclohexane:n-hexane (6:29:71). The MoOx (10 nm) and Ag (100 nm) were deposited by thermal evaporation in vacuum at a pressure of about 1.0×10-4 torr. The devices were finally encapsulated with epoxy paste. A Keithley 236 source meter unit under AM1.5G illumination of 100 mW cm-1 was used to 7



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measure the J-V curves. External quantum efficiency (EQE) profiles were recorded with a QEX10 system (PV Measurement, Inc.).

3. RESULTS AND DISCUSSION Controlling the morphology and crystallinity plays a key role in the recent progress to improve the power conversion efficiency of OSCs.27,28 The use of solvent additive offers the possibility to tailor the morphology and crystallinity in the self-assembly process of the active layer of the solar cells due to low evaporation rate and interaction with the polymers.29 Thus, we apply the PAS approach for ‘first-film-casting and second-additive-soaking’ to reconstruct the morphology in ternary organic solar cell active layer. Figure 1a illustrates the molecular structure of PTB7-Th, PCDTBT and PC71BM. The inverted cell device structure corresponding to the energy levels of the component materials is depicted in Figure 1b. It should be noted that the PTB7-Th and PCDTBT have the same HOMO energy level, which indicates no charge transportation taking place between them. As it can be seen in Figure 1c and 1d, the two polymers possess good complementary absorption spectra. The PTB7-Th spectrum demonstrates two prominent bands at 630 nm and 710 nm in the spectral range of 500-800 nm. PCDTBT exhibits two bands from 350 nm to 600 nm with two peaks at 400 nm and 570 nm. The PCDTBT can compensate the weak absorption of PTB7-Th at short wavelength range. Therefore, the polymer PCDTBT as the third component is appropriate to improve the light harvesting. Figure 1c shows the UV-vis spectra of the blended PTB7Th:PCDTBT:PC71BM on glass substrates with 0% and 30% PCDTBT by weight. 8


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Compared to the absorption of PTB7-Th:PC71BM, the absorption intensity of PTB7Th:PCDTBT:PC71BM film is increased from 350 to 600 nm, while the absorption in this spectral domain of 650-750 nm is shortening. The change of absorption intensity in the region can be ascribed to increase 30% PCDTBT content in the blend and decreased 30% PTB7-Th accordingly. In comparison to the pristine ternary film without treatment, the PAS-treated film exhibits increased absorption in the range of 650-750nm. The increased absorption feature originates from the enhanced intermolecular π-π interaction of PTB7-Th polymer that is compatible with the GIWAXS data. Figure 1e represents the diagrammatic sketch about the preparation process of the PAS-treated ternary blend active layer. First, the ternary blends film on the ZnO/ITO substrate prepared by spin-coating technique. Next, the PAS-treated film was produced by spin-casting the optimal additive/buffer-solvent mixture onto the dried pristine ternary film. The solvent mixture is comprised of a small amount of DIO additive and most two buffer solvents, cyclohexane and n-hexane. DIO, as a well-known solvent additive, is utilized to enhance the miscibility of the donor and acceptor as well as further to increase charge transfer.30 The n-hexane is chosen owing to the miscibility with DIO neither the donor polymers nor PC71BM. The effect of cyclohexane (a marginal solvent for polymers and PC71BM31-33) is to weakly dissolve the film surface in order to make DIO impenetrate in the film. The device characteristics of solar cells fabricated with various treatments from blend films were investigated. The ternary organic solar cells were fabricated with an inverted structure of ITO/ZnO/PTB7-Th:PCDTBT:PC71BM/MnOx/Ag. The optimal 9



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blend ratio of active layer was 0.7:0.3:2 (Figure S2 and S3, Table S2 and S3). The thickness of PAS film is 289 nm obtained by a cross-sectional view of SEM image (Figure S4). The current-voltage (J-V) characteristics are shown in Figure 2a. The corresponding device parameters are summarized in Table 1. The PCE of PCDTBT:PC71BM device is 1.56% with low Jsc and FF, which can be attributed to the large thickness of the active layer, as shown in Figure S1a. The Jsc (11.2 mA/cm-2) value of the PTB7-Th:PC71BM binary cell was decreased to 7.1 mA/cm-2 when the PCDTBT of 30% was mixed into blends, which should be attributed to the incompatibility between the two polymers. The nature of immiscibility of two polymers disrupted the interpenetrated network leading to a poor charge transportation. In contrast, the ternary devices with DIO or PAS treatment have remarkably increased of Jsc and fill factor (FF) in devices. The BHJ film with PAS treatment not only exhibits a high short current density of 18.8 mA/cm-2, but also the highest FF in the order of 0.62 compared to other cells. Adversely, the open circuit voltage is higher (0.77 V) for ternary solar cell without treatment than the PAS-treated solar cell (0.71 V). For the high Voc of pristine solar cell, it may be ascribed to the lack of electronic interaction among the polymer chains, which can leave HOMO level of the donor at a more negative value.34 However, for the decreased Voc for PAS-treated solar cell, further study is necessary to find out the underlying mechanism. At the PAS-treated PTB7Th:PCDTBT:PC71BM system, the gain in Jsc overcompensated the Voc losses, leading to increase in efficiency (PCE=8.7%) significantly. In addition, the external quantum efficiency (EQE) measurement displays an overall improvement in the broad spectra in 10


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between 300-800 nm after PAS treatment or DIO treatment as shown in Figure 2b. Especially, the increased EQE greatly in the 500-700 nm is mainly due to an efficient energy transfer process from PCDTBT to PTB7-Th. Compared to pristine binary solar cell, the pristine ternary solar cell has lower EQE, which can be ascribed to the unfavorable interactions between two donors functioning as ‘morphological traps’ and recombination centers in complex ternary BHJ systems.18 Compared to DIO-treated solar cell, the PAS-treated device has a dramatic increase in EQE, implying good continuous channels for the electrons and holes reconstructed by PAS treatment. The stability of the devices of PTB7-Th:PCDTBT:PC71BM solar cells with PAS treatment has also been enhanced. Normally, PTB7-Th:PC71BM system shows poor environmental stability.7 However, this limitation can be improved by a post-treatment with solvents.7 Lee et al.25 reported that the DIO-treated devices keep only 45% of their initial performances after 60 days, while the devices in PAS treatment still displayed 60%. In our work, the ternary cells treated with PAS still showed 73% of their initial performances after 60 days (Figure S5). The use of PCDTBT as the third component can be a stabilizer for the ternary solar cells. Zhang et al.35 reported that there were the PCDTBT:PC71BM based PSCs giving good stability in outdoor conditions during a year. Besides, the PAS treatment has made the microstructure more stable over time which is beneficial for a stable performance of the ternary solar cells. The Förster resonance energy transfer has been studied extensively in the last century,36 and it has been employed to enrich PCE of the ternary solar cells recently.9,37,38 As shown Figure 1d, the absorption of PTB7-Th overlaps well with an 11



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emission spectrum of PCDTBT, implying the possible existence of FRET between PTB7-Th and PCDTBT. Gupta et al.9 revealed the occurrence of FRET between PTB7 and PCDTBT through PL, PL decay, and ultrafast transient absorption measurements. The main parameter Förster radius R0 can be given as follows: 𝑅06

9000(ln 10)𝑄𝐷 𝑘 2 ∫ 𝐹𝐷 (𝜆)𝜀𝐴 (𝜆)𝜆4 𝑑𝜆 = × 128𝜋 5 𝑛4 𝑁𝐴 ∫ 𝐹𝐷 (𝜆)𝑑𝜆

Where QD is the PL quantum efficiency of donor, k is the orientation factor of the donor and acceptor dipoles, n is the refractive index, NA is the Avogadro’s number, FD is the donor emission spectrum, and 𝜀𝐴 is the FRET acceptor molar extinction coefficient. We assume a random orientation of the donor and the acceptor dipole (k2=2/3), n=2 and QD=0.309.9 The Förster radius R0 is 7.7 nm calculated from the spectral overlap integral. The value of R0 is in accordance with the length of 3-10 nm, where FRET can take place.39 Figure 3a shows the molecular orbital schematic diagram for FRET. The incident light excites the electron in the ground state of the PCDTBT molecule to the excited state. When FRET occurs, the electron in the excited state of PCDTBT returns to the ground state. Simultaneously the energy can be transferred to the nearby ground state PTB7-Th molecule by the resonance, leading to the electron in the PTB7-Th goes to the excited state orbital. Then, it may emit again or be transferred to the acceptor like PC71BM. The recombination loss energy of PCDTBT non-radiation relaxation can be partially “reused” by transferring energy for exciton generation in the PTB7-Th molecule. Note that there is no intermediate photon in FRET process.40 The photoluminescence (PL) decay is the convenient tool to probe the FRET. Figure 3b 12


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shows the emission spectra of PTB7-Th, PCDTBT and PTB7-Th:PCDTBT blend films. For neat PCDTBT film the PL emission peak is at 680 nm, while for PTB7-Th the peak is at 770 nm. It is seen that the PTB7-Th emission intensity is remarkably increased by mixing 30% PCDTBT into PTB7-Th film, although the PTB7-Th concentration is smaller than the neat film. At the same time, the PCDTBT emission intensity is strongly decreased in the blend film, suggesting an energy transfer process from PCDTBT to PTB7-Th. To further demonstrate the efficiency of FRET, fluorescence decay profiles of neat PCDTBT film and PTB7-Th mixed with 30% PCDTBT were probed at 680 nm under 400 nm excitation, as shown in Figure 3c. The decay profile of the PCDTBT film is bi-exponential with τ1=63 ps and τ2=991 ps. The decay data for the two donor polymer blend film can be fitted by triple-exponential function, revealing two major decay components of τ1 = 1.6 ps and τ2 = 28 ps and a third residual component of τ3 = 989 ps, respectively. The efficiency of FRET can be obtained by the equation of 1-τDA/τD where τDA and τD are the fluorescence lifetime of the donor in the presence and absence of the energy acceptor.37 From the decay profiles, we could obtain the transfer efficiency of 86.8% in the PTB7-Th/PCDTBT blend film with 30 wt % PCDTBT addition. To further clarify the effect of PAS treatment on the performance of ternary solar cells, the change in surface morphology of binary and ternary blend films was characterized by atomic force microscopy (AFM) as shown in Figure 4. The root mean square (RMS) roughness was 3.84 nm for the pristine binary film and 6.08 nm for the ternary film. DIO and PAS treatments resulted in a much smoother surface of the films, 13



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with 1.67 nm RMS roughness for the DIO-treated film and 2.23 nm for the PAS-treated film. The pristine binary film (Figure 4a) has a rough surface morphology with many large domains, which is unfavorable for the exciton dissociation at donor/acceptor (D/A) interface. As shown in Figure 4b, when added PCDTBT into binary blend films, the morphology deteriorated even further, which can be ascribed to the incompatibility between the polymers. After DIO or PAS treatment, the ternary films have a smoother surface (Figure 4c and 4d) and show a homogeneous distribution of fullerene domains. In the PAS-treated film, there are some polymer fibrils compared to the DIO-treated film, which is a benign nanomorphology in the similar system.41 To further investigate the exciton dissociation at the interface, we measured the time resolved imaging by means of confocal optical microscopy with a TCSPC module.42,43 Figure 5 demonstrates the time resolved images of blend films with different treatment conditions. The average fluorescence lifetime image of PAS-treated film (Figure 5d) reveals a finely homogeneous lifetime distribution compared to other films (Figure 5a,b,c). As shown in Figure 5a1-d1, the average lifetime of PAS-treated film is 33.5ps which is the shortest compared to 50.8ps, 215ps and 223ps of DIO-treated, pristine binary film and ternary film, respectively. We assume that the green regions are wellmixed zones of three materials. The green region provides a large D/A interfacial area for exciton dissociation and charge separation.44 Figure 5a2-d2 show the distribution histograms of an average fluorescence lifetime in the blend films with different treatments. An average fluorescence lifetime of PTB7-Th:PC71BM (Figure 5a1) and PTB7-Th:PCDTBT:PC71BM (Figure 5a2) films concentrate around 200 to 230 ps. 14


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Especially, the average lifetime distribution of the ternary film without treatment becomes much more dispersed owing to the coarse morphology. The lifetime distribution histogram of DIO-treated film (Figure 5c2) ranges from 42 to 58 ps with a broad peak around 50 ps. The shorter average lifetime of the PAS-treated film (Figure 5d2) presents the higher compositional homogeneity compared with DIO-treated film, whose lifetime distribution is concentrated in the range of 31 to 40 ps with a narrow peak around 33 ps. Therefore, the best performance of the device with PAS treatment can be due to the smooth and fibrous morphology leading to improved exciton dissociation and enhanced electron transport as Chiu et al.45 reported previously. Grazing incidence X-ray scattering (GIXS), including GIWAXS and GISAXS, can provide unique information about the complex inner morphology of active layers in organic solar cells.46 Here, GIWAXS was used to probe changes of the crystal structure and the molecular orientation of the ternary PTB7-Th:PCDTBT:PC71BM blend film.47 Figure 6 shows the two-dimensional GIWAXS images of blend films with different treatments and the corresponding integrated profiles along Qz and Qxy direction. PCDTBT is largely amorphous and has good miscibility with PC71BM.48 In previous studies, the GIWAXS image of PCDTBT:PC71BM film shows that π–π stacking peak (010) of the PCDTBT are barely visible.49,50 The first order scattering peak (100) is very weak, but it is located at ~0.36 Å–1, the same position as in the pure PCDTBT phase.50 Therefore, the (010) peak of the ternary blends is only originated from the PTB7-Th. When PAS treatment or DIO treatment was employed, the reflection peak (100) can be clearly observed from the in-plane and out-plane of 2D GIWAXS as shown 15



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in Figure 6e and 6f, further indicating the improvement of lamellar stacking of the blend films. The (010) π–π stacking packing reflection peak is the charge transport direction. The intense (100) diffraction peak at Qz=3.1 nm-1 in Figure 6e (the broken black line) corresponds to an ordered lamellar structure of face-on PTB7-Th, whereas the peak at Qz=17.3 nm-1 of (010) peak associating with the π−π stacking. For PTB7-Th:PC71BM blend film (red line), the intensity of (010) peak becomes weaker compared to the neat PTB7-Th, which is due to the destruction of the crystallinity of PTB7-Th by the PC71BM. In addition, a broad ring-like feature of PC71BM is seen for all systems (Figure 6a-d), indicated that the PC71BM crystals have no preferred orientation. For the PTB7-Th:PCDTBT:PC71BM ternary blend film without any treatment (blue line), two lamellar diffraction peaks were observed, implying the incompatibility of two donors. The peak of PCDTBT disappeared when films were treated by DIO or PAS, indicating that the treatments indeed enhanced the compatibility between the two polymers. The face-on π−π stacking peak (010) of PAS-treated film (magenta line) becomes more prominent in out-of-plane direction compared to DIO-treated film (orange line), which was mainly due to the enhanced intermolecular π−π stacking of PTB7-Th polymer chains. The result is consistent with previous reports.24 Furthermore, compared to neat PTB7-Th, the position of (010) peak is shifted to higher q value. According to the Debye-Scherrer equation 𝑑(010 ) = 2𝜋⁄𝑞(010) , the π-stacking distances are reduced, which can increase the electron transport due to better orbital overlap between neighboring polymer chains.51,52 Therefore, the enhanced face-on ordering of PTB7-Th with the shorter π–π interchain stacking distances caused by the PAS treatment may 16


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promote carrier transport and collection in the perpendicular orientation. GISAXS was used to probe the nano-scale structure inside the films.46,53 Figure 7 presents the 2D GISAXS intensity maps of ternary blend films with and without PAS treatment as well as the corresponding cuts in the Qy direction. It should be noted that scattering features in the low-Q range can provide the detailed information of PC71BM aggregates as reported by Chiu.54 As displayed in Figure 7b, the scattering intensity of the pristine blend film is drastically reduced in the low-Q range (0.05–0.2 nm−1), while that of the PAS film is smoothly decreased. In order to evaluate the radius R of a PC71BM aggregate, the Guinier approximation can be used as: 𝑙𝑛(𝐼(𝑄)) = 𝑙𝑛(𝐼(0)) −

𝑅2 2 𝑄 5

where I(Q) is the scattering intensity and I(0) the scattering intensity at Q=0, with Q given by Q = 4𝜋 sin(𝜃/2) /𝜆, and with θ and λ as the scattering angle and the incident wavelength, respectively. The Guinier analysis law was employed to fit the scattering curve of ln(I(Q)) versus 𝑄 2 shown in Figure 7c. The average R can be calculated from the two slopes of the two fitted curves. After PAS treatment, the average PC71BM domain sizes decreased from 27 nm to 15 nm, which is considered to be the optimal value for PC71BM of the active layer blend film.54,55 Besides, GISAXS was used to obtain structural information with high statistics and about the inner film morphology.29 Therefore, PAS treatment can make DIO impenetrate in the film to feature a 3D compositional homogeneity besides the surface modification. The improved device performance can be accounted for by the formation of the large D/A interface area and 17



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a bi-continuous interpenetrating network for carrier transportation to the two electrodes.

4. CONCLUSIONS In this work, PAS method was exploited to resolve the issue of incompatibility between two donor polymers in the ternary organic solar cells. A high PCE of 8.7% was reached with the PAS treatment. Meanwhile, enhanced light absorption and improved charge transportation were realized in the ternary organic solar cells. Meanwhile, GIWAXS and GIWAXS results reveal that PAS treatment can not only enhance π–π stacking of PTB7-Th and decrease the PC71BM aggregation, but also can significantly improve the structure compatibility between the two polymers in the ternary system, which together promote charge transport and collection in the vertical direction. At the same time, AFM and TRPL measurements show that the PAS-treated film exhibits a 3D compositional homogeneity, which facilitates efficient exciton dissociation and the carrier extraction at the interface.

 ASSOCIATED CONTENT Supporting Information Average device characteristics of the PCDTBT:PC71BM and PTB7-Th:PCB71M solar cells with different spin-coating speeds, the optimal blend ratio of active layer with and without PAS treatment, the cross-sectional view of SEM image of PAS-treated solar cell, and device environmental stability of PTB7-Th:PCDTBT:PC71BM solar cells with PAS treatment

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 AUTHOR INFORMATION Corresponding Authors *X.T. Hao. E-mail: [email protected] # J. Q. Liu. E-mail: [email protected]

Notes The authors declare no competing financial interest.

 ACKNOWLEDGMENTS We acknowledge the National Natural Science Foundation of China (No.11574181, No.51372141), Research Fund for the Doctoral Program of Higher Education (Grant No. 20130131110004), Open Research Fund of State Key Laboratory Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, the Fundamental Research Funds of Shandong University and the ‘National Young 1000 Talents’ Program of China. We would like to thank beamline BL16B1 (Shanghai Synchrotron Radiation Facility) for providing the beam time.

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Figures: Figure 1. (a) Chemical structures of PTB7-Th, PCDTBT and PC71BM. (b) Energy level diagram of the component materials used in device fabrication. (c) The absorption spectra

of

PTB7-Th:PC71BM,

PTB7-Th:PCDTBT:PC71BM

and

PTB7-

Th:PCDTBT:PC71BM (PAS). (d) The absorption spectra of the polymer PCDTBT and PTB7-Th and photoluminescence spectra of PCDTBT. (e) The schematic drawing of the processing steps of PAS treatment.

23



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Figure 2. (a) J-V curves of organic solar cells with different treatments under AM 1.5G illumination at 100 mW cm-2. (b) The corresponding EQE measurement.

24


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Figure 3. (a) Molecular orbital schematic for resonance energy transfer. (b) Emission spectra of neat PCDTBT, PTB7-Th and PCDTBT: PTB7-Th (0.3:0.7). (c) The fluorescence decay profiles of neat PCDTBT and PTB7-Th films with 30% PCDTBT at 680 nm.

Figure 4. AFM morphology images (10x10 um) of (a) pristine PTB7-Th:PC71BM film, (b) pristine PTB7-Th:PCDTBT:PC71BM, (C) PTB7-Th:PCDTBT:PC71BM with DIO treatment, (d) PTB7-Th:PCDTBT:PC71BM with PAS treatment.

25



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Figure 5. Time resolved imaging (50x50 um) of (a) PTB7-Th:PC71BM, (b) PTB7Th:PCDTBT:PC71BM, (c) PTB7-Th:PCDTBT:PC71BM with DIO treatment, (d) PTB7Th:PCDTBT:PC71BM with PAS treatment. Corresponding decay curves in the selected spot (marked with red box) is shown in (a1)-(d1). Green curve represents IRF. And (a2)(d2) are the detailed distribution histogram of average fluorescence lifetime of corresponding 2D time-resolved imaging.

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Figure

6.

2D

GIWAXS

image

of

(a)

PTB7-Th:PC71BM,

(b)

PTB7-

Th:PCDTBT:PC71BM, (c) PTB7-Th:PCDTBT:PC71BM with DIO treatment, (d) PTB7Th:PCDTBT:PC71BM with PAS treatment. (e) Out-plane line, (f) In-plane line cuts from 2D GIWAXS patterns.

27



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Figure 7. (a) 2D-GISAXS images of PTB7-Th:PCDTBT:PC71BM blend film without treatment and with PAS treatment. (b) The corresponding cuts in the Qy direction. (c) plots of ln I(Q) versus Q2 for the blend films, with a typical Guinier approximation fit (red lines).

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Table 1. Photovoltaic parameters of the binary and ternary blend BHJ devices treated by several optimization method.

Treatment

Voc (V)

Jsc (mA/cm2)

FF

PCE a) (%)

PCE best (%)

PCDTBT:PC71BM

NO

0.89 ± 0.01

4.80± 0.16

0.31 ± 0.041

1.33 ± 0.23

1.56

PTB7-Th:PC71BM

NO

0.79 ± 0.01

11.17 ± 0.16

0.40 ± 0.011

3.52 ± 0.11

3.63


NO

0.77 ± 0.01

7.83 ± 0.21

0.46 ± 0.034

2.77 ± 0.29

3.06


DIO

0.70 ± 0.01

16.80 ± 0.60

0.52 ± 0.035

6.12 ± 0.11

6.23


PAS

0.73 ± 0.01

18.81 ± 0.64

0.61 ± 0.015

8.30 ± 0.4

8.70

Polymer

a) Average was obtained from 12 devices tested, with the best PCE values listed.

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For Table of Contents Only

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Improving the Compatibility of Donor Polymers in Efficient Ternary

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