A Double-Cable Poly(fluorene-alt-thiophene) with Bay-Substituted

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

A Double-Cable Poly(fluorene-alt-thiophene) with Bay-Substituted Perylenediimide Pendants: An Efficient Interfacial Material in BulkHeterojunction Solar Cells Naiwu Chen,† Jurong Lu,† Danbei Wang,‡ Chaoyue Zheng,† Huarui Wu,† Hongmei Zhang,*,‡ and Deqing Gao*,† †

Jiangsu National Synergistic Innovation Centre for Advanced Materials (SICAM), Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Nanjing Tech University (Nanjing Tech), 30 South Puzhu Road, Nanjing 211816, P. R. China ‡ Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, P. R. China S Supporting Information *

ABSTRACT: A solution-processable double-cable polymer (PFT-PDI), composed of the backbone poly(fluorene-altthiophene) (PFT), the n-butoxyl linker, and the pendants perylenediimides (PDI), was developed. PFT-PDI was almost nonconductive with the hole and electron mobilities in the order of 10−10. There was no charge transfer but energy transfer from the donor PFT chain to the acceptor PDI units. With a hole-transporting channel from the stacked PFT units and an electron-transporting channel along PDI chain, PFTPDI at the P3HT/PCBM interface facilitated the effective charge generation from P3HT excitons and charge transporting and enhanced the cell photocurrent. The encapsulated cell ITO/MoO3/P3HT:PCBM:PFT-PDI/LiF/Al with doping PFT-PDI of 3 wt % demonstrated the maximum power conversion efficiency (PCE) of 4.50%, increasing by 27.5%, relative to PCE of 3.53% from the cell without doping. The PFT-PDI doping much improved the cell’s stability with the loss of the initial PCE of 5.8%, in contrast to 29.7% from the reference device after being stored for 7 days.

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designed and synthesized.10−13 Cravino et al. first defined such polymer as a donor−acceptor double-cable polymer.14 Since then, a variety of double-cable polymers have been reported.15−19 However, only a few double-cable copolymers have presented OPV performance. Janssen et al. reported poly(p-phenylenevinylene) with C60-appended side chains, and the fabricated cell (ITO/PEDOT:PSS/copolymer/Al) showed PCE of 0.1%.20 Li’s group synthesized a double-cable polythiophene with high content of C60 pendants and achieved PCE of 0.52% for the PSC.21 Hashimoto et al. observed the formation of nanoscale phase separation of a block double-cable copolymer composed of rrP3HT and a C 60 -appended polythiophene, and the fabricated single-component solar cell showed PCE of 1.7%.22 Yang et al. synthesized a diblock double-cable copolymer PCBM-Ph-P3HT with linkages of C60 moieties as the side chain. By doping the double-cable polymer into the P3HT:PCBM system, the bicontinuous interpenetrat-

INTRODUCTION Organic photovoltaics (OPVs) have been playing an important role in the renewable energy sources due to their advantages of low-cost, large area feasibility, high flexibility, and light weight.1,2 Heeger3 and Friend4 invented the bulk-heterojunction (BHJ) solar cells which contained an interpenetrating network of a polymer donor and an acceptor as the photoactive layer. One of the most popular research models of BHJ solar cells consists of poly(3-hexylthiophene) (P3HT) and a soluble fullerene derivative [6,6]-phenyl C61-butyric acid methyl ester (PCBM). In the P3HT:PCBM system, severe phase segregation often exists, leading to low efficient charge generation.5−7 In theory, a nanoscale separation (i.e., high interfacial area) facilitates exciton dissociation, and the interpenetrating networks create bicontinuous pathways for charge carriers transporting to the respective electrodes.8,9 Instead of the “blend heterojunction”, the “molecular heterojunction” may have homogeneous distribution of the bicontinuous electron donor−acceptor domains. To demonstrate the molecular heterojunction, the polymers with acceptor units being covalently linked to a donor polymer chain were © XXXX American Chemical Society

Received: August 18, 2017 Revised: December 9, 2017

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DOI: 10.1021/acs.macromol.7b01792 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of the Polymer PFT-PDI, the Reference Materials PFT, and Alkoxy-PDI

single-component active layer.24 In general, the photovoltaic properties of double-cable copolymers reported to date are not satisfied. Instead of directly covalently bound to the donor polymer chain (i.e., double-cable architecture), an acceptor-appended nonconjugated chain may copolymerize with a conjugated donor polymer chain to form a block copolymer. Such block

ing network was promoted, and the BHJ cell presented PCE of 3.4%, increased by 12% compared to that of the P3HT:PCBM solar cells.23 Lanzi et al. synthesized a double-cable diblock copolymer polythiophene bearing C60 (20% in moles) attached via hexamethylenic side chain and the solar cell with the polymer as a single photoactive layer showed PCE of 4.19%, which was the highest record so far for the devices with a B


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Macromolecules

Figure 1. UV−vis absorption spectra of PFT-PDI (black line), PFT:alkoxy-PDI blend (red line), PFT (green line), alkoxy-PDI (blue line), and PDI (purple line) in CHCl3 solution (a) and in films (b).

derivatives which facilitated the exciton diffusion into the interface of P3HT/PCBM.36 As the side chains, PDI units were used in double-cable polymers as well. Gómez et al. reported poly(fluorene-altphenylene) endowed with alkyl chain-linked pendant PDI units16 and poly(fluorene-alt-thiophene) endowed with benzylbridged pendant PDI units,37 but no photovoltaic property from two polymers has been mentioned. Wei et al. reported a double-cable copolymer composed of poly(cyclopentadithiophene-2,7-linked carbazole) with pendant PDI units, and the cell from the copolymer:PCBM blend showed PCE of 0.454%.38 Iraqi et al. synthesized a 2,7-linked carbazole-based double-cable polymer with pendant perylenediimide, and the cell with the polymer as the single photoactive layer showed PCE of 0.011%.39 It is known that the bay substitution not only affects molecular-level electronic and optical properties but also the aggregation and solubility. 40 Taking these factors into consideration, we have designed and synthesized a novel double-cable polymer PFT-PDI with poly(fluorene-alt-thiophene) backbone covalently linked PDI groups. The polymer was composed of three components: the acceptor perylenediimide (PDI), the linkage n-butoxyl chain on the bay position, and the donor poly(fluorene-alt-thiophene) (PFT) backbone. Compared with polyfluorene,16,41,42 PFT has stronger electrondonating ability than PDI. It was worth highlighting the function of the linkage n-butoxyl chain on the bay position. The substitution on the bay position attenuated the strong π−π interaction to avoid excimer formation, but the remaining π−π interaction was strong enough to create the long-range πstacking characteristics for efficient carrier transfer. The oxygen atom on the bay position extended the absorption spectrum to the high wavelength range and broadened the absorption spectrum as well. Together with imide alkanes at two ends of PDI, the n-butoxyl chain made PFT-PDI being well arranged and soluble in the common solvents. The structure and photophysical properties of PFT-PDI have been characterized by nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), UV−vis, fluorescence spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and cyclic voltammetry (CV). The effects of PFT-PDI doped in the P3HT:PCBM blend film were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. The photovoltaic devices with a configuration of ITO/MoO3/P3HT:PCBM:PFT-PDI/ LiF/Al were fabricated by adding PFT-PDI with different

copolymers which are composed of a donor block and an acceptor block are supposed to have optoelectronic property and nanoscale phase separation. Hadziioannou et al. first reported a diblock copolymer in which one block was poly(pphenylenevinylene) (PPV) and the other was a C60-functionalized polystyrene and prepared the solar cell with the copolymer as a single component of the photoactive layer.25 Zhang et al. synthesized a diblock copolymer in which the donor block was rrP3HT and the acceptor block was a polyacrylate with pendant perylenediimide (PDI) groups. The cell ITO/PEDOT:PSS/copolymer/LiF/Al exhibited PCE of 0.49%, higher than that of the device made from the corresponding P3HT and PDI-appended polyacrylated blend.26 In addition to the above-reported function as a single component for the photoactive layer, these copolymers have been applied as the compatibilizers to control morphology of the blend film in the bulk heterojunctions. Fréchet et al. added a PDI-appended copolymer to the mixture of P3HT and PDI derivative and achieved PCE of 0.55%.27 Jo et al. reported a diblock polymer rrP3HT-b-C60 and showed that the charge transfer of the P3HT:PCBM blend with addition of P3HT-bC60 was more effective and observed the formation of nanoscale phase separation and the enhancement of high-temperature stability with compared to the P3HT:PCBM blend.28,29 Wudl et al. reported a rod−coil diblock copolymer P3HT-b-P(SxAy)C60 which was synthesized by a straightforward synthetic strategy of living polymerization and subsequent cycloaddition. By doping the copolymer into the P3HT:PCBM blend film, the BHJ morphology was improved and the cell exhibited PCE of 3.5%, 35% higher than that from a pristine P3HT:PCBM device.30 As is well-known, PDI has a strong cofacial π−π interaction, easily aggregates in the solid, and can form micrometer-sized crystallites which are too large for efficient exciton transporting to the donor−acceptor interface for carrier creation; moreover, PDI tends to form excimers which act as traps for excitation energy.27,31,32 By introducing sterically hindered condensed aromatic cores or highly sterically hindered groups on perylene’s bay position, the intermolecular π−π interaction between the perylene planes can be efficiently suppressed. The application of PDI derivatives in OPVs has been reported.33,34 By adding 1 wt % a n-type perylenediimide derivative into a P3HT:PCBM layer, the enhancement of PCE was achieved compared with the reference cell without the additive.35 ̇ alp et al. reported the bay-substituted perylenediimide Dinç C


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Figure 2. Fluorescence spectra of PFT-PDI (black line), PFT:alkoxy-PDI (red line), PFT (green line), and alkoxy-PDI (blue line): in CHCl3 excited at 408 nm (a) (inset: magnified fluorescence spectrum of PFT-PDI) and at 555 nm (b); in films excited at 408 nm (c) and at 555 nm (d).

CHCl3 solution and in films are shown in Figures 1a and 1b, respectively. PFT-PDI exhibited three absorption maxima peaked at 408, 518, and 555 nm (Figure 1a). The peak at 408 nm corresponding to the π−π* transition in the fluorene− thiophene polymeric backbone was assigned to the absorption of the PFT moiety, and the peaks at 518 and 555 nm were attributed to the absorption of alkoxy-PDI component.43,44 The change of the ratio of the intensities of the (0, 0) and the (0, 1) transitions of a perylene derivative is often used as an indication of aggregation. When PDI chromophores are stacking in Haggregates, the (0, 1) transition becomes intense and the ratio trends smaller.16,45 In the diluted solution, free alkoxy-PDI molecules showed a maximum of the (0, 0) vibronic transition at 557 nm and a maximum of (0, 1) vibronic at 519 nm, and the ratio of the (0, 0) transition to the (0, 1) was close to 1.5, which was smaller than 1.6 in the case of PDI,46,47 indicating that the bay substitution in alkoxy-PDI weakened the intermolecular aggregation to a certain extent. For PFT-PDI, the (0, 1) vibronic band showed an increase relative to the (0, 0) transition and the ratio of (0, 0) transition to (0, 1) was 1.3, indicating that PDI units in PFT-PDI were stacked by face to face in H-aggregates.16,48−51 Considering that PDI chromophores bearing long alkyl chains did not tend to aggregate up to 10−3 M in the solvents,52 the π-stack of PDI units in PFT-PDI should have an intramolecular aggregate and not an intermolecular aggregate in the 10−6 M CHCl3 solution for UV−vis measurement, being in agreement with the reported conclusion.16 In contrast, the absorption spectrum of the PFT:alkoxy-PDI blend was the sum of that of PFT and that of alkoxy-PDI, and neither the intensity of the (0, 0) transition nor that of the (0, 1) transition of the alkoxy-PDI component experienced change compared with the absorption of alkoxy-

ratios, and their photovoltaic properties were determined under 100 mW/cm−2 illumination of simulated AM 1.5G sunlight.

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RESULTS AND DISCUSSION Synthesis and Characterization. Scheme 1 shows the synthetic routes for preparation of PFT-PDI, PFT, and alkoxyPDI. N,N′-Bis(1-pentylbutyl)perylene-3,4,9,10-tetracarboxylic diimide (PDI) was prepared from 3,4,9,10-perylene tetracarboxylic dianhydride with the reagents of 1-butylpentylamine. N,N′-Bis(1-pentylbutyl)perylene-3,4,9,10-tetracarboxylic diimide was brominated to give compound 1. Compound 1 underwent the nucleophilic substitution, and compound 2 was collected. Compound 3 which was synthesized from the reaction between 2 and 3-(4-bromobutyl)thiophene was brominated under the exclusion of the light, and the monomer M1 was obtained with the high yield. The polymer PFT-PDI was prepared by Suzuki coupling reaction of M1 and M2. Using the same polymerization method for PFT-PDI preparation, the polymer PFT was synthesized by Suzuki coupling reaction of M2 and M3. Alkoxy-PDI was synthesized from compound 2 and 1-bromohexane. The number-average molecular weight (Mn) of PFT-PDI was found to be 14 kDa with PDI of 1.24. The Mn of PFT was 107 kDa with PDI of 2.33. The detailed description of the synthesis is available in the Supporting Information. It was noteworthy that the alkyl side chains on the fluorene units and those at the PDI pendant moieties made PFT-PDI soluble in common organic solvents, enabling the complete characterization by NMR electrochemical spectroscopy. Electronic Absorption and Fluorescence Properties of PFT-PDI. The UV−vis absorption spectra of PFT, PDI, PFTPDI, alkoxy-PDI, and PFT:alkoxy-PDI blend (1:1 by mol) in D


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Figure 3. (a) Cyclic voltammograms of PFT-PDI, PFT, and alkoxy-PDI films at a typical scan rate of 200 mA/s, and the potentials were referenced versus the ferrocene/ferrocenium couple. (b) TGA curve of PFT-PDI, measured at a heating rate of 10 °C/min and up to 800 °C.

the PFT backbone to the PDI units.53 The intramolecular aggregation of PDI units also resulted in the fluorescence quenching of the PDI units. In contrast, the fluorescence intensity of PFT:alkoxy-PDI blend was slightly weaker than that of PFT (Figure 2a) and nearly the same as that of alkoxy-PDI (Figure 2b), indicating the energy transfer from the PFT polymer to alkoxy-PDI was weak. This was understandable when considering the closer contact of the PDI units to the PFT backbone by the covalent linkage via the n-butoxyl bridge in PFT-PDI than in the physical PFT:alkoxy-PDI blend. The energy transfer was even more intense in film. As shown in Figure 2c,d, PFT-PDI had nearly no fluorescence upon excitation at 408 or 555 nm. The PFT:alkoxy-PDI blend only presented the relatively weak emission at 625 nm from the alkoxy-PDI component and no emission from PFT upon excitation at 408 nm, due to the smaller intra- and intermolecular distance in film than in solution and the corresponding strong energy transfer. Electrochemical Property. Cyclic voltammetry (CV), with 0.1 mol/L tetrabutylammonium hexafluorophosphate (Bu4NPF6) in CH3CN as the supporting electrolyte and a conventional three-electrode configuration consisting of a platinum working electrode, a counter electrode, and a Ag/ AgCl reference electrode, was applied to examine the electrochemical property of PFT-PDI and two reference materials PFT and alkoxyl-PDI. Accordingly, the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), and the band gap of PFT-PDI were determined.54,55 The ferrocene/ferrocenium (Fc/Fc+) couple was used as the internal reference. Each film sample was prepared by drop-casting CHCl3 solution of the corresponding material on a platinum electrode at room temperature under a nitrogen atmosphere. The bay substitution had an effect not only on UV−vis and photoluminescence (PL) but also on redox potentials.56 As shown in Figure 3a, on the negative scan alkoxy-PDI had two distinct reversible reduction waves at −1.01 and −1.20 V (vs Fc/Fc+), and PFT-PDI presented two reversible reduction waves at −1.04 and −1.25 V which were ascribed to alkoxy-PDI moiety. On the positive scan PFT-PDI exhibited the onset oxidation potential at 0.9 V, which was 0.1 V higher than that of PFT, and was assigned to the oxidation of the polymer chain. According to the onset oxidation potential (Eox) at 0.9 eV and the onset reduction potential (Ered) at −1.02 eV of PFT-PDI, HOMO was calculated to be −5.70 eV

PDI. This further indicated that the PDI units in PFT-PDI were arranged in π-stack which created the electron-transporting channel. Compared with the absorption maxima of PDI at 490 and 526 nm, the absorption peaks at around 518 and 555 nm of PFT-PDI experienced the red-shift of 28 and 29 nm, respectively. This can be interpreted by the enhancement of the conjugation of the mesomeric donating effect of the alkoxy groups, which appeared to predominate toward the perylene core. Relative to poly(fluorene-alt-thiophene) with covalently linked-PDI side groups via imide N,37 PFT-PDI showed the red-shift absorption spectrum with the onset of 600 nm instead of 550 nm of PDI, which benefited the efficient absorption of solar light. In films (Figure 1b), PFT-PDI exhibited the absorption at 409, 517, and 555 nm. Compared with the absorption in solution, the maximum absorption of the PDI units presented a hypsochromic shift, which was caused by the H-aggregation of perylene moiety in the solid state,31 being the same as the case of PFT:alkoxy-PDI and alkoxy-PDI. The fluorescence properties are useful for exploring the intramolecular interaction between donor and acceptor units. Figure 2 shows the fluorescence spectra of PFT-PDI with comparison to those of PFT:alkoxy-PDI blend, PFT, and alkoxy-PDI with selective excitation of the PFT backbone (λex = 408 nm) and the PDI side groups (λex = 555 nm) in CHCl3 (Figure 2a,b) and in films (Figure 2c,d). PFT presented an emission maximum at 465 nm, and alkoxy-PDI showed the characteristic emission at 576 and 616 nm. Upon excitation at 408 nm (Figure 2a and inset) the emission spectrum of PFTPDI in CHCl3 showed the maxima at 450 nm from the PFT backbone and at 574 and 618 nm from PDI unit. Considering that the PDI part hardly absorbed at 408 nm, the appearance of the emission at 574 and 618 nm indicated that the energy transfer from the PFT chain donor to the PDI units acceptor took place when PFT component was excited, in agreement with the observation of two PDI-appended copolymers containing poly(fluorene-alt-phenylene)16 and poly(carbazolealt-dithienyl).39 Upon excitation at 555 nm (Figure 2b) the emission spectrum of PFT-PDI showed the maxima at 558, 574, and 616 nm from the PDI unit. It was obvious that the fluorescence intensity of PFT-PDI was much weaker than those of PFT and alkoxy-PDI, almost by 2 orders of magnitude. The fluorescence quenching of the PFT chain and the PDI units of PFT-PDI was attributed to the efficient energy transfer from E


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Figure 4. (a) J−V characteristics of P3HT:PCBM:PFT-PDI devices with doping PFT-PDI of 0 wt % (black line), 1 wt % (red line), 3 wt % (blue line), and 5 wt % (dark cyan line). (b) Scheme of ITO/MoO3/P3HT:PCBM:PFT-PDI/LiF/Al device architecture. The ratio of P3HT to PCBM keeps 1:1 in weight.

by the formula HOMO = −(4.8 + Eox), and LUMO was calculated to be −3.78 eV by the formula LUMO = −(4.8 + Ered) for PFT-PDI. Thermal Analysis of PFT-PDI. TGA and DSC were measured at a heating rate of 10 °C/min under flowing nitrogen to evaluate the thermal stability of PFT-PDI. As shown in Figure 3b, the onset decomposition temperature (Td) of PFT-PDI was determined to be 390 °C, corresponding to initial 5% of weight loss. When PFT-PDI was heated to 490 °C, the loss weight is about to 40% due to the complete decomposition of fluorene-alt-thiophene units, which quite fitted with the calculated content of 42% of fluorene-altthiophene in PFT-PDI. Figure S23 shows DSC of PFT-PDI, where melting point (Tm) was determined to be 180 °C. Photovoltaic Properties of P3HT:PCBM:PFT-PDI BulkHeterojunction Solar Cells. The bulk-heterojunction solar cells with the structure of ITO/MoO3/P3HT:PCBM:PFTPDI/LiF/Al (PFT-PDI doping ratio: 0, 1, 3, and 5 wt %, respectively) (as seen in Figure 4b) were fabricated and encapsulated in a glovebox (described in the Supporting Information). Under an AM 1.5 G solar simulator irradiating, the photovoltaic parameters (open-circuit voltage (VOC), shortcircuit current density (JSC), fill factor (FF), power conversion efficiency (PCE)) were obtained and compiled in Table 1. The

average PCE of 4.50% and 4.16%, respectively, being 27.5% and 19.5% higher than those of the reference P3HT:PCBM device (see Table 1). With doping PFT-PDI of 5 wt %, PCE was clearly decreased. In order to find the reason for the increase of the PCE with PFT-PDI doping, each photovoltaic parameter was taken into account. As shown in Table 1, the VOC values of the photovoltaic devices were in the range 0.59−0.60 V, indicating that a small amount of PFT-PDI did not influence the energy offset between the HOMO level of P3HT-based donor system and the LUMO level of the PCBM-based acceptor system. Similarly, FF was changed slightly with and without PFT-PDI doping (Table 1). It was noteworthy that JSC was increased from 10.1 mA/cm2 without PFT-PDI doping to 10.7 mA/cm2 (ca. 5.9% enhanced) with doping PFT-PDI of 1 wt % and 12.4 mA/cm2 (ca. 22.8% enhanced) with doping PFT-PDI of 3 wt %, respectively. The PCE improvement with doping PFT-PDI of 1 and 3 wt % was mainly attributed to the JSC increase. One reason was that the HOMO and LUMO energy levels of PFT-PDI were positioned between those of P3HT and PCBM (as shown in the energy diagram of PSC (Figure 5)), and PFT-PDI built an energetic cascade and injected electrons into PCBM and holes into P3HT, playing the similar role as the additive silicon phthalocyanine derivative (SiPc) at the P3HT/PCBM interface.57 Another reason was that PFT-PDI which was composed of an electron-donating

Table 1. Photovoltaic Characteristics of Devices Doping ratio of PFT-PDI (wt %)

JSC (mA/cm−2)

VOC (V)

FF (%)

PCE (%) max/ava

0 1 3 5

10.1 10.7 12.4 9.7

0.59 0.60 0.60 0.60

59 58 60 57

3.53/3.48 3.76/3.67 4.50/4.16 3.54/3.21

a

Average PCE values of six devices were given.

average value of power conversion efficiency of each sample was evaluated from the values of six devices. The current density−voltage (J−V) curves of each sample which presented the highest PCE are compared in Figure 4a. The reference device of ITO/MoO3/P3HT:PCBM/LiF/Al exhibited VOC of 0.59 V, JSC of 10.7 mA/cm2, FF of 59%, the maximum PCE of 3.53%, and the average PCE of 3.48%. When PFT-PDI was doped in the P3HT:PCBM blend by 1 wt %, the maximum PCE was increased to 3.76% (ca. 6.5% enhanced) and the average PCE was 3.67% (ca. 5.5% enhanced). The device with doping PFT-PDI of 3 wt % presented the maximum and the

Figure 5. Energy diagram of the bottom electrode ITO, hole transporting layer MoO3, donor polymer P3HT, the compatibilizer PFT-PDI, acceptor PCBM, and the top electrode LiF/Al in the BHJPSC. The HOMO and LUMO energy levels of PFT-PDI were determined with cyclic voltammetry in the work, and these of other functional layers were cited from the literature.58−60 F


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Figure 6. (a) UV−vis absorption spectra of the P3HT:PCBM (1:1 in weight) blend films with doping PFT-PDI of 0 wt % (black line), 1 wt % (red line), 3 wt % (blue line), and 5 wt % (magenta line) and their absorption coefficient calculated from Beer’s law. (b) Fluorescence spectra of the P3HT:PCBM:PFT-PDI blend films with doping PFT-PDI of 0 wt % (black line), 1 wt % (red line), 3 wt % (blue line), and 5 wt % (magenta line), excited at 520 nm. The films were spin-coated on quartz glass from a dichlorobenzene solution and thermally annealed at 120 °C for 20 min.

UV−Vis Absorption of P3HT:PCBM:PFT-PDI Blend Films. Figure 6a shows UV−vis absorption spectra of P3HT:PCBM:PFT-PDI blend films with PFT-PDI doping of 0, 1, 3, and 5 wt %. In agreement with the literature,63 the P3HT:PCBM film exhibited three characteristic vibronic peaks at 508, 550, and 598 nm, 508 nm corresponding to π−π* absorption of polythiophene and 598 nm corresponding to the strong interchain interaction between P3HT chains.64 The blend films with PFT-PDI doping exhibited the same three characteristic vibronic peaks at 508, 550, and 598 nm. The absorption coefficients calculated from Beer’s law were 2.9 × 104, 3.7 × 104, 3.8 × 104, and 3.7 × 104 cm−1 for doping of 0, 1, 3, and 5 wt %, respectively. The blend with 3 wt % doping presented the highest value by increase of 31%, relative to P3HT:PCBM film. It came to the conclusion that 3 wt % doping of PFT-PDI improved packing of P3HT chains,29 enhanced light absorption, and promoted charge generation from P3HT excitons.24,58 Photoluminescence of P3HT:PCBM:PFT-PDI Blend Films. The fluorescence spectra of the P3HT:PCBM:PFTPDI blend films with PFT-PDI doping of 0, 1, 3, and 5 wt % were obtained when excited at 520 nm. As shown in Figure 6b, the films exhibited the emission peak at 635 nm. In contrast to the nondoping P3HT:PCBM film, the P3HT:PCBM films with PFT-PDI doping showed the decreasing photoluminescence, meaning that the PFT-PDI doping reduced the radiative decay of the excited electrons and induced more effective excitons to be dissociated in the P3HT:PCBM:PFT-PDI film. It is understandable that the blend with 5 wt % presented the highest photoluminescence decrease due to PFT-PDI having the electron-transporting channel and hole-transporting channel. But taking into account the UV−vis absorption, morphology, charge-carrier mobility, and other factors, the cell with PFT-PDI doping of 3 wt % presented the best performance. Effect of PFT-PDI on Crystallinity of P3HT:PCBM Blend Film. In general, the performance of the polymer and the effects of crystals packing dominate the charge carrier transportation. 2 9 Therefore, the XRD pattern of P3HT:PCBM:PFT-PDI (doping 3 wt %) blend film was compared with that of P3HT:PCBM blend film after thermal annealing at 120 °C for 20 min. Both films showed a sharp (100) diffraction peak at 2θ = 5.4° in the XRD spectra, which

chain PFT and the electron-accepting PDI pendants possessed fast transporting channels for electron and hole carriers. The external quantum efficiency curves to evaluate the photoresponse of the P3HT:PCBM blends with PFT-PDI doping of 0 and 3 wt % were collected (as presented in Figure S24). The EQE value of the P3HT:PCBM blend was 45%, and EQE of the P3HT:PCBM blend with doping PFT−PDI of 3% was 50% by increase of 11%. It was observed that JSC was decreased to 9.7 mA/cm2 with doping PFT−PDI of 5 wt %. It was partly because much nonconductive PFT-PDI doping declined the carrier transporting. In order to further understand the mechanism of the performance improvement of the bulkheterojunction solar cell with doping PFT-PDI, we measured the UV−vis absorption, fluorescence, crystallinity, and morphology of the P3HT:PCBM:PFT-PDI blends. Charge Transport Properties. The electron and hole mobilities were evaluated in the Mott−Gurney space-charge limited current (SCLC) method.61 The mobility of PFT-PDI was studied with the device structure of ITO/ZnO/PFT-PDI/ LiF/Al for the electron and ITO/PEDOT:PSS/PFT-PDI/ MoO3/Au for the hole. From five different devices, the average hole mobility was calculated to be 6.75 × 10−9 cm2 V−1 s−1, and the average electron mobility was calculated to be 5.60 × 10−10 cm2 V−1 s−1. It was obvious that PFT-PDI was almost nonconductive and the n-butoxyl linker prohibited the charge transfer in between the conductive backbone poly(fluorene-altthiophene) (PFT) and the perylenediimide pendants. The m o b i l i t i e s o f t h e p h o t o v o lt a i c a c t i v e l a y e r s o f P3HT:PCBM:PFT-PDI (PFT-PDI doping ratio: 0 and 3 wt %) were measured with the device structure of ITO/ZnO/ P3HT:PCBM:PFT-PDI/LiF/Al for the electron and ITO/ PEDOT:PSS/P3HT:PCBM:PFT-PDI/MoO3/Au for the hole (as shown in Figures S25 and S26). From five different devices, the average hole and electron mobilities were calculated to be 1.51 × 10−4 and 3.53 × 10−4 cm2 V−1 s−1 for the nondoping P3HT:PCBM blend, in agreement with the report.62 The average hole and electron mobilities were calculated to be 2.63 × 10−4 and 6.32 × 10−4 cm2 V−1 s−1 for the 3% doping P3HT:PCBM blend. Even though TFP-PDI was almost nonconductive, its doping made the blend P3HT:PCBM more conductive, increasing by 1.7 times of either hole or electron carriers. G


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Macromolecules corresponded to the (100) a-axis orientation of the P3HT crystallite (as shown in the inset of Figure 7).29 It was clear that

investigated by transmission electron microscopy (TEM) measurement. As shown in Figure 8, the bright-field TEM images in 100 nm scale showed the phase-separated structure of bright P3HT-rich phase and dark PCBM-rich phase. The reference P3HT:PCBM presented a uniform film (Figure 8a). Upon doping PFT-PDI of 1 wt % (b), 3 wt % (c), and 5 wt % (d), the bicontinuous network morphology was obviously observed, especially in the case of 3 wt % doping (c). It indicated that doping PFT-PDI improved the interfacial contact between P3HT and PCBM, as PFT-PDI as a “surfactant” had good amphiphilicity toward P3HT and PCBM and created the bicontinuous interpenetrating networks in the P3HT:PCBM blend. It was in agreement with the reports that introducing the compatibilizers controlled the crystalline rate and improved the contact between P3HT and PCBM.29,30 On the basis of the above detailed analysis, the enhancement of current density which was main contribution to the PCE improvement was attributed to the following factors. First, the P3HT:PCBM:PFT-PDI blend film with doping PFT-PDI of 3 wt % effectively absorbed the incident light compared to P3HT:PCBM blend film for high photocurrent generation. Second, PFT-PDI at the interface between the P3HT and PCBM phases induced more effective exciton dissociation. Third, the P3HT:PCBM blend film with doping PFT-PDI increased the crystallinity of P3HT. Fourthly, PFT-PDI as a “surfactant” which has the amphiphilicity toward P3HT and PCBM improved the bicontinuous interpenetrating networks of the P3HT and PCBM components and modified the morphology of the donor−acceptor phases. Fifthly, as a double-cable material which was composed of an electrondonating chain PFT and the electron-accepting PDI pendants, PFT-PDI created fast transporting channels for electron and hole carriers. In order to deeply understand the unique photovoltaic effect of PFT-PDI as the interface material, PFT and alkoxy-PDI were

Figure 7. XRD patterns of P3HT:PCBM (1:1 in weight) blend films with doping PFT-PDI of 0 wt % (black line) and 3 wt % (red line). The inset showed the chain packing of regioregular P3HT. The films were prepared by spin-coating a dichlorobenzene solution on ITO plates and then thermally annealed at 120 °C for 20 min.

the (100) intensity of the P3HT:PCBM:PFT-PDI blend film was stronger than that of the P3HT:PCBM blend film. Besides, the P3HT:PCBM:PFT-PDI blend film exhibited the (200) and (300) diffraction peaks at 2θ = 10.8° and 16.0°, respectively. These features indicated that the PFT-PDI doping increased the crystallinity of P3HT in the P3HT:PCBM:PFT-PDI blend film, in agreement with the conclusion from the UV−vis absorption (as Figure 6a). Effect of PFT-PDI on the Surface Morphology of P3HT:PCBM Blend Film. The film morphology was

Figure 8. TEM images of P3HT:PCBM (1:1 in weight) blend films with doping PFT-PDI of 0 (a), 1 (b), 3 (c), and 5 wt % (d). H


Article

Macromolecules

The enhancement of the cell performance was ascribed to the increase of the short-current density. By being deeply probed with optical and electronic characterization, it came to conclusion that PFT-PDI strongly increased the light absorption of the photoactive film and the crystallinity of P3HT component, created the bicontinuous interpenetrating networks, and increased the interfacial contact which resulted in efficient photoinduced charge transfer between P3HT and PCBM and effective exciton dissociation.

utilized to fabricate the solar cells with the structure of ITO/ MoO 3 /P3HT:PCBM:PFT/LiF/Al and ITO/MoO 3 / P3HT:PCBM:alkoxy-PDI/LiF/Al, respectively. By doping 3 wt %, the cells experienced the decrease of the short-circuit current density and the fill factor. The PCE of the PFT doping cell was decreased by 36% and PCE of the alkoxy-PDI doping cell was decreased by 25%. It indicated that the components PFT and alkoxy-PDI cannot alone functionalize like PFT-PDI which had a hole-transporting channel and an electrontransporting channel and efficiently improved the bulkheterojunction photovoltaic performance at the P3HT/ PCBM interface. Longevity Stability of Polymer Solar Cells. Longevity stability plays an important role in polymer solar cells in terms of commercial application. Figure 9 shows the degradation of

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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.macromol.7b01792. Experimental details, synthesis and NMR, device fabrication, DSC thermogram, and AFM images (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.Q.G.). *E-mail [email protected] (H.M.Z.). ORCID

Deqing Gao: 0000-0002-9920-3427 Author Contributions

N.C., J.L., and D.W. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China under Grant No. 21371096.

Figure 9. Long-term stability test of the encapsulated devices with the P3HT:PCBM:PFT-PDI blend films with doping PFT-PDI of 0 wt % (black line) and 3 wt % (red line) which was carried out with the storage time.

REFERENCES

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PCE of the devices with the P3HT:PCBM (1:1 in weight) blend films with doping PFT-PDI of 0 and 3 wt % as a function of storage time. The stability test was carried out with the time after the devices were encapsulated. As shown in Figure 9, the PCE of the control cell without doping PFT-PDI decreased from 3.53% to 2.48% and degraded by 29.7% after 7 days. Surprisingly, the device with P3HT:PCBM:PFT-PDI blend film with doping PFT-PDI of 3 wt % had the initial PCE of 4.50% and remained 4.24%, decreasing only by 5.8% after the same period of time. It was indicating that the bicontinuous interpenetrating networks created between P3HT and PCBM by doping PFT-PDI could strengthen the ability against photochemical degradation and retard diffusion under irradiation, as in the report.65

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CONCLUSIONS In the work we present a new double-cable non-fullerene polymer PFT-PDI with an electron-donating PFT chain bound with the electron-accepting perylenediimide pendants via the bay-substituted n-butoxyl linker. A strong intramolecular photoinduced energy transfer from the PFT backbone to PDI moieties upon photoexcitation was discovered. By doping PFTPDI of 3 wt %, the P3HT:PCBM:PFT-PDI device showed the maximum PCE of 4.50%, increased by 27.5% relative to the reference P3HT:PCBM device. The device exhibited much higher stability. After 7 days, the PCE of the device with doping PFT-PDI of 3 wt % lost 5.8%, while the control cell lost 29.7%. I


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A Double-Cable Poly(fluorene-alt-thiophene) with Bay-Substituted

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