Improving the Photovoltaic Performance and Mechanical Stability of

Jun 26, 2019 - On the other hand, low-crystalline p-type polymers with highly interconnected networks of ordered regions have shown high hole mobiliti...
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Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

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Improving the Photovoltaic Performance and Mechanical Stability of Flexible All-Polymer Solar Cells via Tailoring Intermolecular Interactions Minjun Kim,†,§ Hong Il Kim,†,§ Seung Un Ryu,† Sung Yun Son,† Sang Ah Park,† Nasir Khan,‡ Won Suk Shin,‡ Chang Eun Song,*,‡ and Taiho Park*,†

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Department of Chemical Engineering, Pohang University of Science and Technology, 77 Cheongam-Ro, Nam-Gu, Pohang 37673, Gyeongbuk, Korea ‡ Energy Materials Research Center, Korea Research Institute of Chemical Technology (KRICT), 141 Gajeong-Ro, Yuseong-Gu, Daejeon 34114, Korea S Supporting Information *

ABSTRACT: Naphthalene diimide (NDI)-based copolymers are promising polymer acceptors in all-polymer solar cells (all-PSCs), but their large crystal domains cause large-scale phase separation in all-polymer blend films. This limits the photovoltaic performance and mechanical stability of all-PSCs. Herein, we control all-polymer blend films by introducing a fluorinated copolymer of NDI and (E)-1,2-bis(3fluorothiophen-2-yl)ethene (FTVT) (PNDI−FTVT) as a polymer acceptor for flexible all-PSCs. The copolymer PNDI−FTVT has a less crystalline structure and higher electron mobility than its nonfluorinated copolymer counterpart (PNDI− TVT). A blended film incorporating PNDI−FTVT exhibits a well-mixed morphology and improves the chain interconnectivity with a polymer donor, providing better charge transport pathways and enhanced mechanical resilience. The PNDI−FTVTbased flexible all-PSC exhibits enhanced photovoltaic performance in comparison with a PNDI−TVT-based flexible all-PSC (5.11−7.14%) as well as excellent mechanical stability in a flexible all-PSC (7.14−5.78%), maintaining 81% of its initial performance at a bending radius of 8.0 mm after 1000 bending cycles.

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INTRODUCTION All-polymer solar cells (all-PSCs) are now considered as promising renewable energy technologies for applications in flexible and wearable photovoltaics because of their various advantages, including complementary light absorption with a polymer donor, a chemically tunable polymer acceptor, and excellent mechanical properties.1−3 Because the alternating copolymer of naphthalene diimide (NDI) and bithiophene (T2) (PNDI2OD-T2) has been reported as an electron acceptor, the power conversion efficiency (PCE) values of allPSCs have been developed rapidly to 11%.4,5 NDI-based polymers have been used extensively as polymer acceptors in all-PSCs because of their strong electron affinity and high electron mobility (μe).6,7 The high μe of NDI-based polymers mainly stems from the self-assembled crystalline structure formed by intermolecular interactions between the highly planar NDI units of adjacent polymer chains.8−10 High crystallinity can increase μe of the polymer acceptor, though this is accompanied by an increase in the crystal domains over the typical general exciton diffusion length of ∼20 nm.11,12 Furthermore, the large crystal domains of the polymer acceptors result in an undesirable morphology due to low intermixing with the polymer donors, leading to large-scale phase separation.13 This leads to inefficient exciton dissociation and charge transport between the polymer donor and © XXXX American Chemical Society

polymer acceptor, thereby reducing the short-circuit current (JSC) and fill factor (FF) of all-PSCs.14 Moreover, given that highly crystalline polymers are rigid and brittle, cracks appear in the polymer film after they undergo morphological stress.15,16 Several approaches have been reported to suppress largescale phase separation in NDI-based all-polymer films through processing techniques or molecular designs. Examples include a thermal treatment involving the quenching of the polymer in a melt state,17 rapid solvent evaporation using low-boilingpoint solvents,18 and random copolymerization of electron-rich and electron-deficient units.19 These approaches can effectively reduce the crystallization tendency of polymer acceptors and produce well-mixed blend morphologies with polymer donors, but they cannot increase the inherent μe of the polymer. On the other hand, low-crystalline p-type polymers with highly interconnected networks of ordered regions have shown high hole mobilities (μh) comparable to those of semicrystalline polymers. These polymers have continuous electrical connections with a low activation energy for efficient charge transport.20,21 Among NDI-based polymers, the fluorinated Received: February 13, 2019 Revised: June 26, 2019 Published: June 26, 2019 A

DOI: 10.1021/acs.chemmater.9b00639 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 1. (a) Chemical structures of PBDB-T, PNDI−TVT, and PNDI−FTVT. UV−vis absorption spectra of (b) polymer films and (c) allpolymer blend films.

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copolymer of NDI and (E)-1,2-bis(3-fluorothiophen-2-yl)ethene (FTVT) (PNDI−FTVT) is suitable as an efficient and flexible polymer acceptor because of its granular structure with a high μe value of 3.20 cm2 V−1 s−1.22 It should also be noted that the μe value of PNDI−FTVT is higher than that of the nonfluorinated copolymer of NDI and 1,2-di(2-thienyl)ethene (TVT) (PNDI−TVT) with a self-assembled crystalline structure (μe = 2.31 cm2 V−1 s−1).23 Although backbone fluorination improved the μe of PNDI−TVT, the underlying of crystal size changes due to the fluorinated electron-donating unit, and the effect on the polymer blend morphology remains unclear. Moreover, the need to gain a better understanding of these effects also depends on the fact that most descriptions of the structural features of fluorinated polymers are confined to those exhibiting close intermolecular packing in the solid state.24,25 Herein, we introduce PNDI−FTVT as a polymer acceptor, which is expected to achieve a well-mixed donor/acceptor (D/ A) blend morphology and to enhance the mechanical resilience of flexible all-PSCs. Moreover, PNDI−FTVT can improve the chain interconnectivity to construct better charge transport pathways in all-polymer D/A blend films via the increased interaction with the polymer donor through fluorination. For comparison, PNDI−TVT was synthesized as a control polymer. Poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo-[1′,2′-c:4′,5′c′]dithiophene-4,8-dione))] (PBDB-T) was used as a polymer donor because PBDB-T exhibits complementary absorption and a suitable energy level for NDI-based polymer acceptors.26−29 The PBDB-T:PNDI−FTVT-based flexible all-PSC device showed an improved PCE of 7.14% compared to the PBDB-T:PNDI−TVT-based control device (PCE = 5.11%). In addition, an excellent mechanical stability of the flexible allPSC with PNDI−FTVT, maintaining 81% of its initial performance (7.14% → 5.78%) after 1000 bending cycles at a bending radius of 8.0 mm, was achieved.

EXPERIMENTAL SECTION

Materials. Patterned polyethylene naphthalate (PEN)/indium tin oxide (ITO) (15 Ω/sq) was purchased from Nano Clean Tech. The PBDB-T and NDI-based polymers (1:0.8 w/w, 18 wt %, filtered by 0.45 μm PTFE) were dissolved in chloroform. These solutions were stirred at 60 °C for 1 day. Device Fabrication. All-PSCs were fabricated with glass or PEN/ ITO/ZnO NPs/PEIE/PBDB-T:PNDI−TVT or PNDI−FTVT (∼100 nm)/MoO3/Ag. The patterned ITO-coated substrate was ultrasonicated using a universal detergent in deionized water, ethanol, acetone, and isopropanol and dried at 80 °C for 30 min. After an ozone plasma treatment for 10 min, ZnO NPs and PEIE were each spin-coated at 3000 rpm for 30 s. For deposition of the active layer, the D/A blend solutions were spin-coated at 4500 rpm for 25 s. All of the devices were then dried in an Ar-filled glovebox. Finally, MoO3 (25 nm) and Ag (100 nm) were thermally deposited at 2.7 × 10−7 Torr. The photo-active area is 0.09 cm2. The photovoltaic parameters were estimated at 100 mW cm−2 (450 W Xe lamp). The illumination intensity was calibrated using a 91150-KG5 reference cell and meter. The incident photon-to-current efficiency (IPCE) spectra were measured in the wavelength range of 350−900 nm under a 100 W Xe lamp (IQE-200B model). Measurements. Gel permeation chromatography (GPC) measurements were carried out using a Shimadzu LC solution, and polystyrene was used as a control to determine the number-average molecular weights (Mn) and polydispersity (D̵ , Mw/Mn) of the polymers. UV−vis absorption of the polymers in solution and film states was recorded using a Mecasys Optizen Pop UV−vis spectrophotometer between 300 and 1000 nm. The photoluminescence (PL) spectra were analyzed by a spectrofluorometer (FP-6500, JASCO) and thin films were excited at 500 nm. Cyclic voltammogram (CV) measurements were carried out via a three-electrode system (glassy carbon disk, platinum wire, and reference electrode) using a PowerLab/AD instrument with a 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) solution in acetonitrile. Ferrocene was used as the internal standard material for CV voltammogram calibration. The atomic force microscopy (AFM) images were measured using a VEECO Dimension 3100+ Nanoscope V device. The transmission electron microscopy (TEM) images were measured using a JEOL JEM-2200FS instrument. GIWAXS Analysis. The grazing incidence wide-angle X-ray scattering (GIWAXS) experiments were conducted at the 3C B

DOI: 10.1021/acs.chemmater.9b00639 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 2. GIWAXS images of (a) polymer films and (b) all-polymer blend films. (c) δb−h2 curves of PNDI−TVT and PNDI−FTVT films extracted from (h00) of in-plane (left) and out-of plane (right) GIWAXS patterns.

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beamline of the Pohang Accelerator Laboratory (PAL). The sampleto-detector distance and the X-ray radiation beam energy of the 3C beamline were 212 mm and 10.26 keV, respectively. The incident angle of the radiation beam to the polymer films was set to 0.12°. For the GIWAXS experiments, thin films were prepared by spin-coating onto Si wafers (10 mg mL−1 in chloroform). The diffraction results of the thin films measured under a vacuum were collected on a Rayonix 2D MAR165 image plate. To calculate the true paracrystal size (LT), a Hosemann plot was devised. h is the diffraction order and δb is the reciprocal value of the full width at half maximum, yielded from (h00) diffractions in the qxy and qz directions. Through the intercept of linear fitted line, LT can be obtained. SCLC Device Fabrication and Testing. The architectures of hole- and electron-only devices consisted of an ITO/PEDOT:PSS/ active layer/Au and an ITO/ZnO NPs/active layer/LiF/Al, respectively. The mobility levels were calculated by the spacecharge-limited current (SCLC) model, JSCLC = 9ε0εrμV2/8L3, where JSCLC is the dark current density, ε0 is the dielectric constant, εr is the relative permittivity, μ is the electron or hole mobility, V is the internal voltage, and L is the thickness of the active layer. Photocurrent Analysis. The maximum exciton generation rate (Gmax) was calculated from the photocurrent density (Jph = JL − JD) versus the effective voltage (Veff = V0 − Va), where JL and JD represent the current density under illumination and dark conditions, respectively, V0 is the voltage at Jph = 0, and Va represents the applied bias voltage. The Jph values become saturated from Jsat at a sufficiently high Veff. Thus, Gmax is calculated from Jsat = q × Gmax × L, where q is the electronic charge and L is the thickness of the active layer.

RESULTS AND DISCUSSION The chemical structures of the polymers used in this study are shown in Figure 1a. The polymer donor, PBDB-T, was purchased from 1-Materials with a number-average molecular weight (Mn) of 33 kg mol−1 (polydispersity index, PDI = 2.10). The two polymer acceptors, PNDI−TVT and PNDI−FTVT, were synthesized according to a previous report;30 their Mn values determined by GPC were 69 kg mol−1 (PDI = 2.09) and 79 kg mol−1 (PDI = 2.58), respectively. The energy levels of the polymers were estimated by CV (Figure S1 and Table S1). PNDI−FTVT exhibited a downshifted lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) energy levels compared to those of PNDI−TVT, by 0.07 and 0.14 eV, respectively, because of the electron withdrawing effect of fluorine. Fluorination of TVT considerably affected the HOMO energy level of PNDI−FTVT, leading to an increased band-gap for PNDI−FTVT (1.54 eV) compared to that of PNDI−TVT (1.47 eV). The LUMO and HOMO energy levels of PBDB-T were determined to be −3.49 and −5.34 eV, respectively. Considering the LUMO energy level of PBDB-T, the two polymer acceptors exhibited a sufficient LUMO offset of more than 0.3 eV for efficient exciton dissociation at the D/ A interfaces.31 We measured differential scanning calorimetry (DSC) thermograms for polymer acceptors and their blends with PBDB-T (55 wt %) under heating at 10 °C min−1 with N2 C

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Figure 3. AFM height, phase images and TEM images of (a) PBDB-T:PNDI−TVT blend film and (b) PBDB-T:PNDI−FTVT blend film, respectively.

plot extracted from the (200) scattering peak provides information regarding the orientation distribution of the polymer crystallites (Figure S4).38 The face-on:edge-on ratio of PNDI−TVT was 73:27 and the ratio of PNDI−FTVT was 57:43. This indicates that the polymer crystallites of PNDI− TVT adopted strong face-on orientation, whereas those of PNDI−FTVT have a broad orientation at all polar angles. This mixed orientation of polymer crystallites can be beneficial to vertical and parallel charge carrier transport in the device.38 To calculate the true paracrystal size (LT), the Hosemann plot was performed (Figure 2c).39 LT to the (h00) plane of the PNDI− FTVT film (L(h00) = 239 Å, L(00l) = 144 Å in qxy, and L(h00) = T T T 132 Å in qz) was smaller than that of the PNDI−TVT film (L(h00) = 279 Å, L(00l) = 174 Å in qxy, and L(h00) = 141 Å in qz) T T T (Table S2). The reduced value of LT of the PNDI−FTVT film is due to the strong aggregation, which interferes with the crystal growth, leading to the optimum crystal size relative to the exciton diffusion length.11,12 The PBDB-T:PNDI−TVT and PBDB-T:PNDI−FTVT blend film showed similar scattering patterns overlapping the scattering peaks of the polymer donor and polymer acceptor (Figures 2b and S5). The PBDB-T:PNDI−FTVT blend film showed lower scattering peak intensities in qxy compared to the PBDB-T:PNDI−TVT blend film due to the reduced crystallinity of PNDI−FTVT; thus, the blending of these two polymers is thought to form a well-mixed film morphology. In addition, the PBDB-T:PNDI−FTVT blend film exhibited a stronger (010) scattering peak in qz compared to the PBDBT:PNDI−TVT blend film. The estimated π−π stacking spacing (d010) of the PBDB-T:PNDI−FTVT blend film was 3.69 Å, revealing slightly tighter π−π stacking compared to the PBDB-T:PNDI−TVT blend film (d010 = 3.71 Å). The crystalline features of PNDI−FTVT were well preserved after blending with PBDB-T, and the enhanced (010) scattering peak is favorable for efficient charge transfer between the πstacked domains of the polymer donor and the polymer acceptor.27,40 The morphologies of the D/A blend films were also investigated using AFM and TEM (Figure 3). Both of the AFM images revealed an interpenetrated morphology. The PBDB-T:PNDI−TVT blend film exhibited a well-defined fibril

(Figure S2). PNDI−FTVT exhibited a higher melting temperature (Tm) (Tm = 284 °C) than PNDI−TVT (Tm = 274 °C) because of the increased intermolecular interactions by backbone fluorination.32−34 The Tm depression values for the PBDB-T:PNDI−TVT blend and the PBDB-T:PNDI− FTVT blend were 9 and 21 °C, respectively. This result implies increased interaction between the polymer donor and the acceptor; thus, the PBDB-T:PNDI−FTVT blend is expected to have a better mixed blend morphology than the PBDBT:PNDI−TVT blend.35,36 The two polymer acceptors exhibited similar UV−vis absorption spectra with two absorption bands at 400 and 750 nm, respectively, facilitating complementary absorption with the polymer donor, PBDB-T (Figure 1b and Table S1). The PNDI−TVT film exhibited an absorption edge (λedge) at 840 nm, whereas the PNDI−FTVT film exhibited a relatively blue-shifted λedge at 805 nm, implying a widened band gap because of the reduced HOMO level, as mentioned above. Another absorption feature of the PNDI−FTVT film was its pronounced vibronic shoulder, indicative of the strong intermolecular coupling of aggregates caused by backbone fluorination.32,33 The PBDB-T:PNDI−FTVT blend film exhibited a higher absorption coefficient over the entire range compared to the PBDB-T:PNDI−TVT blend film (Figure 1c). This is advantageous for better light harvesting, leading to a higher JSC.37 Microstructural analyses of the polymer acceptor and allpolymer blend films were conducted via a GIWAXS analysis (Figures 2a and S3). The PNDI−TVT film exhibited strong scattering peaks of lamellar (h00) stacking and backbone repeats (00l) in qxy and π−π stacking (010) in qz, whereas the PNDI−FTVT films showed arc-type scattering patterns and weak scattering peaks in qxy and qz. In particular, the PNDI− FTVT film exhibited a relatively much weaker intensity of (00l) scattering peaks than those of the PNDI−TVT, indicating poor structural order along the backbone direction. The PNDI−FTVT film showed a 0.14 Å shorter π−π stacking distance (d010) than the PNDI−TVT film because of the strong intermolecular interactions by fluorination (Table S2). From the GIWAXS results, the PNDI−FTVT film has a less crystalline structure than the PNDI−TVT film. The azimuthal D

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Figure 4. (a) J−V and (b) IPCE characteristic curves for PBDB-T:PNDI−TVT- and PBDB-T:PNDI−FTVT-based flexible all-PSCs. (c) SCLC characteristic curves of all-polymer blend based electron-only devices. (d) PL quenching spectra for all-polymer blend films. (e) Photocurrent analysis and (f) dependence of the JSC factor on the light intensity for PBDB-T:PNDI−TVT- and PBDB-T:PNDI−FTVT-based devices, respectively.

the flexible all-PSCs based on the PBDB-T:PNDI−FTVT presented a remarkable PCE of 7.14%, while the PBDBT:PNDI−TVT device exhibited a relatively low PCE of 5.11%. The higher PCE for the PBDB-T:PNDI−FTVT-based device was closely associated with the increases in the JSC and FF values of 9.67 to 14.5 mA cm−2 and 62.0−68.0%, respectively, compared to those for the PBDB-T:PNDI−TVT-based device. The integrated JSC values for the devices based on PBDBT:PNDI−TVT and PBDB-T:PNDI−FTVT were 9.43 and 14.0 mA cm−2, respectively. These integrated JSC values were well matched with the JSC values within an error of approximately 3%, which is an acceptable error range. The dramatic increases in the JSC and FF values for the PBDBT:PNDI−FTVT-based device are consistent with analysis results of the morphological and charge-transport behavior, indicating that the PBDB-T:PNDI−FTVT-based device provided efficient exciton dissociation and charge-transport at the D/A interface. To examine the charge-transport behavior of the all-polymer D/A blend films, the electron (μe) and hole mobility (μh) values were calculated from the SCLC depending on certain structural differences (Figures 4c and S9). The μe value for PBDB-T:PNDI−FTVT was 8.36 × 10−5 cm2 V−1 s−1; this value was higher than that obtained for PBDB-T:PNDI−TVT (2.80 × 10−5 cm2 V−1 s−1). In addition, the μh/μe values were 1.96 and 4.79 for the PBDB-T:PNDI−FTVT and PBDB-

structure with a larger crystalline size, as observed for NDIbased polymers with a high degree of packing. Meanwhile, a collapsed fibril structure was observed for the PBDB-T:PNDI− FTVT blend film because of the reduced crystallinity of PNDI−FTVT. The surface root-mean-square values for the PBDB-T:PNDI−TVT and PBDB-T:PNDI−FTVT blend films were correspondingly 1.63 and 1.36 nm, implying that the surface morphology for the PBDB-T:PNDI−FTVT blend film was considerably smooth, which is beneficial for charge extraction from the active layer to the counter electrode.41 Morphological differences between the two blend films were more evident from the TEM images. The PBDB-T:PNDI− TVT blend film exhibited larger, isolated domains, whereas the PBDB-T:PNDI−FTVT blend film exhibited smaller interconnected domains. The observed interconnected domains in all-polymer D/A blend films facilitate efficient exciton dissociation, thus increasing the JSC and FF values for the devices.42,43 The photovoltaic characteristics of the all-PSCs based on the PBDB-T:PNDI−TVT and PBDB-T:PNDI−FTVT blend active layers were investigated, and their current density− voltage (J−V) curves and the IPCE spectra of the devices were obtained (Figures 4 and S6), respectively. The photovoltaic parameters are summarized in Tables S3 and S4. Details of the optimal device conditions are described in the Supporting Information (Figures S7 and S8, Tables S5 and S6). As a result, E

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Figure 5. (a) Schematic illustration for defining bending radius. (b) Normalized PCE for PBDB-T:PNDI−TVT-and PBDB-T:PNDI−FTVT-based flexible all-PSCs with different bending radii for 100 cycles (left) and different bending cycles at 8.0 mm bending radius (right).

T:PNDI−TVT blend films, respectively, as PNDI−FTVT induced more enhanced π−π stacking with PBDB-T compared to PNDI−TVT. Therefore, the PBDB-T:PNDI−FTVT device could achieve a higher JSC value as it appears from the IPCE curves and better charge-transport behavior compared to the PBDB-T:PNDI−TVT device despite the negligible HOMO offset.44 These results indicate that the PBDB-T:PNDI−FTVT films constructed efficient charge-transport pathways because of the well-mixed blend morphology with interpenetrating networks.42,43 To investigate the exciton dissociation and charge separation ability of the all-polymer D/A blend films, the steady-state PL values were assessed (Figures 4d and S10). The PL intensity for the PBDB-T:PNDI−FTVT blend film was considerably quenched compared to that for the PBDB-T:PNDI−TVT blend film at ∼670 nm. This result indicates that the PBDBT:PNDI−FTVT blend film exhibited more efficient exciton dissociation and charge separation compared to the PBDBT:PNDI−TVT blend film, revealing a well-mixed morphology for the PBDB-T:PNDI−FTVT blend film without a largephase separation.45−47 To determine the correlation between the light absorption and generation of photocurrent in all-PSCs, the maximum exciton generation rate (Gmax) was measured by a photocurrent analysis (Figure 4e).48 The PBDB-T:PNDI−FTVT-based device exhibited a higher Gmax value of 9.53 × 1027 m−3 s−1 compared to the PBDB-T:PNDI−TVT-based device (Gmax = 6.46 × 1027 m−3 s−1). These results are associated with the difference in the light absorption of the D/A blend films for exciton generation with respect to the absorption spectra of the blend film (Figure 1c). These findings also indicate that the changes in the JSC values for all of the devices were strongly related to the difference in the number of the absorbed photons in each D/A blend film. In addition, Jph/Jsat is essentially a product of the exciton dissociation and charge collection under a short-circuit condition (Veff = 2 V).49,50 The PBDB-T:PNDI−FTVT-based device exhibited a higher Jph/Jsat value of 98.7% compared to the PBDB-T:PNDI−TVT-based device (Jph/Jsat = 97.9%), suggesting that the bimolecular recombination of the PBDB-T:PNDI−TVT-based device begins to dominate and usually leads to a lower FF. For a more in-depth understanding of the bimolecular recombination kinetics of the all-polymer D/A blend films, the JSC, VOC, and FF values for the PBDB-T:PNDI−TVT- and PBDB-T:PNDI−FTVT-based devices as a function of the light

intensity were measured (Figures 4f and S11).51 The PBDBT:PNDI−FTVT-based device exhibited a higher slope (α = 0.96) compared to the PBDB-T:PNDI−TVT-based device (α = 0.91), indicating that fewer bimolecular recombinations occurred in the PBDB-T:PNDI−FTVT-based device. In addition, bimolecular recombinations are dominant at a slope of approximately 1kT/q. The PBDB-T:PNDI−FTVT-based device exhibited a slope of 1.18kT/q, whereas the PBDBT:PNDI−TVT-based device showed a slope of 1.36kT/q. Furthermore, the FF value for the PBDB-T:PNDI−TVT-based device decreased remarkably compared to that of the PBDBT:PNDI−FTVT-based device under high light intensity. This result indicates that with an increase in the number of free carriers via high light intensity, the PBDB-T:PNDI−TVTbased device can initiate the recombination of free carriers and induce a loss of the FF. Thus, these results indicate fewer bimolecular recombination for the PBDB-T:PNDI−FTVT device because of the more efficient exciton dissociation and charge-transport at the D/A interface. To compare the long-term and mechanical stability between the flexible all-PSCs based on PBDB-T:PNDI−TVT and PBDB-T:PNDI−FTVT, their performances depending on different bending radiuses (r), bending cycles, and amounts of time (days) were compared (Figures 5 and S12). The photovoltaic performance of the PBDB-T:PNDI−FTVT device decreased relatively slowly compared to that of the PBDB-T:PNDI−TVT device for 30 days. In addition, the photovoltaic performance of PBDB-T:PNDI−TVT dropped significantly as the r value decreased (5.11% at r = infinite → 2.64% at r = 8.0 mm), whereas PBDB-T:PNDI−FTVT showed a small decrease (7.14% at r = infinite → 6.59% at r = 8.0 mm) (Figure S13a and Table S7). Furthermore, the PBDB-T:PNDI−FTVT-based flexible device exhibited fewer sharp microcracks compared to the PBDB-T:PNDI−TVTbased device over the entire film surface according to the reduction of the bending radius because of the rigid and brittle properties of the highly crystalline PNDI−TVT (Figure S13c). Furthermore, the mechanical stability of the two flexible devices with the bending cycles was examined at r = 8.0 mm. The photovoltaic performance of the devices based on PBDBT:PNDI−TVT and PBDB-T:PNDI−FTVT maintained 13% (5.11% → 0.67%) and 81% (7.14% → 5.78%) of the initial corresponding performances, respectively, after 1000 bending cycles (Figure S13b and Table S8). Accordingly, these results indicate that the PNDI−FTVT-based flexible all-PSCs have F

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Foundation (NRF) of Korea (2015M1A2A2056216), Center for Advanced Soft Electronics under the Global Frontier Research Program (2012M3A6A5055225), the National Research Foundation of Korea (NRF) grant (Code No. 2018R1A2A1A05079144) funded by the Korea government (MSIP), and Korea Institute of Energy Technology Evaluation and Planning of Korea (no. 20183010013820). GIWAXS was measured at the 3C beam line in the Pohang Accelerator Laboratory (PAL).

excellent mechanical stability because of the highly interconnected network with the reduced crystal domains of PNDI−FTVT.

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CONCLUSIONS In summary, we introduced fluorinated NDI-based copolymer PNDI−FTVT as a polymer acceptor to improve the photovoltaic performance and mechanical stability of flexible all-PSCs. PNDI−FTVT has a smaller crystal domain size and a higher μe value in SCLC devices than its nonfluorinated copolymer counterpart PNDI−TVT. The PBDB-T:PNDI− FTVT blend film exhibited a well-mixed morphology without large-scale phase separation because of the reduced crystal domain size of PNDI−FTVT and increased interaction with PBDB-T, improving the interconnectivity to construct better charge-transport pathways in the D/A blend film. This morphological change could facilitate efficient exciton dissociation and charge-transport at the D/A interface, improving the JSC and FF values for flexible all-PSCs. Thus, the PBDB-T:PNDI−FTVT-based device exhibited greater efficiency compared to the PBDB-T:PNDI−TVT-based device (5.11% → 7.14%). In addition, excellent mechanical stability of PBDB-T:PNDI−FTVT device was achieved, as it maintained 81% of its initial performance (7.14% → 5.78%) after 1000 bending cycles at r = 8.0 mm. Furthermore, the PBDBT:PNDI−FTVT device showed improved long-term stability compared to the PBDB-T:PNDI−TVT device over a time period of 30 days. This performance is an outstanding result reported for spin-coated flexible all-PSCs. We believe that this study will provide a valuable molecular design strategy regarding the creation of polymer acceptors for use in soft electronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.9b00639. More characterization details using CV, DSC, GIWAXS, SCLC, PL, VOC and FF factors on the light intensity, J− V characteristic curves, IPCE, summary of the optical and electrochemical properties, crystallographic parameters, and photovoltaic parameters (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.E.S.). *E-mail: [email protected] (T.P.). ORCID

Nasir Khan: 0000-0003-3119-8702 Won Suk Shin: 0000-0001-7151-519X Chang Eun Song: 0000-0001-6910-8755 Taiho Park: 0000-0002-5867-4679 Author Contributions §

M.K. and H.I.K. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Technology Development Program to Solve Climate Changes of the National Research G

DOI: 10.1021/acs.chemmater.9b00639 Chem. Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.chemmater.9b00639 Chem. Mater. XXXX, XXX, XXX−XXX