Regulation of the Polar Groups in n-Type Conjugated Polyelectrolytes

Oct 8, 2018 - Conjugated polyelectrolytes based on n-type backbone (n-CPEs) were recently developed as highly efficient cathode interlayers for polyme...
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Regulation of the Polar Groups in n‑Type Conjugated Polyelectrolytes as Electron Transfer Layer for Inverted Polymer Solar Cells Yun Tan,† Lie Chen,*,†,‡ Feiyan Wu,†,‡ Bin Huang,† Zhihui Liao,† Zoukangning Yu,† Lin Hu,§ Yinhua Zhou,§ and Yiwang Chen*,†,‡

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†

College of Chemistry and ‡Institute of Polymers and Energy Chemistry (IPEC), Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China § Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China S Supporting Information *

ABSTRACT: Conjugated polyelectrolytes based on n-type backbone (n-CPEs) were recently developed as highly efficient cathode interlayers for polymer solar cells (PSCs), but the relevance between structure and property of n-CPEs has not been fully understood yet. Herein, three new self-doping n-CPEs based on the diketopyrrolopyrrole (DPP) alternated fluorene framework were reported. The effect of the number and location of polar groups on the properties of the new n-CPEs has been systematically studied. It can be found that the photoelectric properties of n-CPEs were sensitive to the number and location of polar groups. A tunable work function (WF), interfacial interaction, and conductivity of these n-CPEs can be readily realized by regulating the number and location of polar groups on the electron skeleton. The polar groups directly appending on the electron-withdrawing DPP unit can promote a stronger n-type self-doping and lower WF than the ones attached on the electron-pushing fluorene unit. Increasing the number of the polar groups can further optimize the photoelectric properties of conjugated electrolytes. As a result, upon incorporation of new n-CPEs as cathode buffer layer, the PSC performance was significantly improved to 8.3% for the fullerene system and 10.57% for the non-fullerene system. These results demonstrated that without complex molecular design, simply regulating the number and location of polar groups of the n-CPEs can provide a facile way to develop highly efficient cathode interlayers for high performance polymer solar cells.

1. INTRODUCTION Polymer solar cells (PSCs) have drawn significant attention due to their potential for low cost, light weight, printable, and high-throughput solution processing.1−4 It is well-known that a typical PSC device is usually composed of two materials that work as electron donor and acceptor to form a bulk heterojunction. During the past decades, plenty of excellent active layer materials have been designed and synthesized. The power conversion efficiency (PCE) of a single-junction device based on polymer and fullerene has surpassed 10%, and a striking 14% can also be obtained by energy level modulation of fullerene-free acceptors.5,6 In addition to the synthesis of novel photoactive materials, it is also crucially important to design appropriate interlayer materials to efficiently extract the charge carriers from the active layer to the electrodes.7−9 It is noted that an excellent interlayer can not only tune the energy level between the contact interface but also improve the interface affinity. Great efforts have been made to develop plenty of interlayer materials to improve the performance of PSCs. Among these interlayer materials, the organic interfacial materials are attracting the most attention for the reason that the chemical © XXXX American Chemical Society

structure can be tailored and the interfacial compatibility can be greatly improved to achieve brilliant optoelectronic properties. Conjugated polyelectrolytes (CPEs) have been employed as one of the most distinguished organic interlayers to improve electron injection or extraction.10−14 Most of CPEs possess good solubility in green solvents, e.g., water and alcohol, due to the various polar groups attached on the main chain. At the same time, the polar groups can create favorable dipoles at the electrode/active layer interface, which was critical for the CPEs to minimize the interfacial energy barrier.15 These electrolytes have been successfully used as cathode interlayer to decrease the electron transport barrier by reducing the work function (WF) of cathode.16 However, most of the reported polyelectrolytes were based on p-type conjugated or nonconjugated backbones, such as polythiophenes, polyfluorenes, polycarbazoles, and polyethylenimine.17−20 These electrolytes could not Received: July 17, 2018 Revised: September 26, 2018

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Figure 1. Chemical structures for PDPP-FNBr, PF-DPPNBr, and PDPPNBr-FNBr. In addition, schematic illustration of n-type self-doping effect in the n-CPEs and their plausible dipole formation at ZnO interfaces. The purple arrows represent the ion-induced dipoles (μI) and the pink for dipole direction of polaron-induced dipoles (μP).

transport the electrons effectively, so they only worked as cathode modifiers with an ultrathin film (normally 2−5 nm).21,22 In fact, the backbones of conjugated electrolytes also have an important influence on the characteristics of interlayer materials. Recently, our group reported a series of highly efficient CPE cathode interlayers based on the n-type backbone, showing better performance than the p-type analogues. This was because n-type CPEs (n-CPEs) acted not only as a cathode modifier but also as an electron transport layer (ETL), which makes CPEs work well in a much thicker film with good charge selectivity. More intriguingly, n-type self-doping has been regarded as an effective method to further improve the charge transport of n-CPEs and promote the PSCs performance. n-Type self-doping was a charge transfer process that occurs from the polar pendent dopants to n-type backbones, resulting in a high electron affinity or a low ionization potential of the host CPEs. Pendent polar groups, e.g., quaternary ammonium, sulfonic, anionic carboxyl, etc., can produce a strong n-type self-doping effect on the backbone to further improve the conductivity of n-CPEs. Moreover, different from unstable n-type doped organic semiconductor materials obtained by introducing extrinsic dopants, n-type self-doped CPEs were much more stable in ambient conditions. However, most research of n-CPEs focused on the effect of various types of backbones on the photoelectric properties of conjugated electrolytes. Because the polar groups impact so greatly on the formation of the dipoles and the n-type self-doping process, it was meaningful to explore the relevance between polar groups and properties of n-CPEs as well as the eventual device performance. We expect that simply tuning the number and location of polar groups for electrolytes can effectively modulate the photoelectric properties of n-CPEs and improve devices performance. In this work, we designed and synthesized three new selfdoping n-CPEs based on a diketopyrrolopyrrole (DPP) alternated fluorene framework with different number and location of polar groups on the n-type electron skeleton, as shown in Figure 1. It can be found that a tunable WF, interfacial

interaction, and conductivity of these n-CPEs can be readily realized by regulating the number and location of polar groups on the electron skeleton. The polar groups directly appending on the electron-withdrawing DPP unit can promote a stronger n-type self-doping and lower WF than the ones attached on the electron-pushing fluorene unit. Increasing the number of the polar groups can further optimize the photoelectric properties of conjugated electrolytes. Subsequently, these n-type CPEs can work well as ETLs for both fullerene and non-fullerene PSCs, showing the dependence of the device performance on the chemical polarity of n-CPEs. Notably, the inverted device based on polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7):(6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) and poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione))] (PBDB-T):3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC)23,24 obtained an improved power conversion efficiency (PCE) of 8.3% and 10.57%, respectively. These results provided better understanding on the relationship between structure and properties of n-CPEs and demonstrated more insights into interlayer material design for high performance polymer solar cells.

2. RESULTS AND DISCUSSION The synthesis route and molecular structure of the n-CPEs are shown in Figure 1 and Scheme S1 (synthesis details are provided in the Supporting Information). Obviously, the conjugated backbones of three neutral precursor polymers were prepared by the simple Suzuki cross-coupling reaction with different fluorene and DPP monomer. Impurities and products of low molecular weight were eliminated by Soxhlet extraction using methanol, acetone, and hexane. The products were collected by chloroform and then quaternization with trimethylamine to introduce the quaternary ammonium cationic polar groups. The final n-CPEs with different number and location of polar groups B

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Figure 2. (a) Ultraviolet−visible (UV−vis) absorption spectra of the PDPP-FNBr, PF-DPPNBr, and PDPPNBr-FNBr films and solution. (b) Electron paramagnetic resonance (EPR) spectra of the PDPP-FNBr, PF-DPPNBr, and PDPPNBr-FNBr in the solid state.

were named as (6-(2-methyl-7-(5-(4-(5-methylthiophen-2-yl)-2, 5-bis(2-octyldodecyl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-yl)thiophen-2-yl)-9-(6-(trimethylammonio)hexyl)-9Hfluoren-9-yl)hexyl)(trimethylammonio)bromate(I) bromide (PDPP-FNBr), 6,6′-(3-(5-(7-methyl-9,9-dioctyl-9H-fluoren-2-yl)thiophen-2-yl)-6-(5-methylthiophen-2-yl)-1,4-dioxopyrrolo[3,4c]pyrrole-2,5(1H,4H)-diyl)bis(N,N,N-trimethylhexan-1-aminium) bromide (PF-DPPNBr), and 6,6′-(2-(5-(2,5-bis(6-(bromotrimethyl-l5-azanyl)hexyl)-4-(5-methylthiophen-2-yl)-3,6-dioxo2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-yl)thiophen-2-yl)-7methyl-9H-fluorene-9,9-diyl)bis(N,N,N-trimethylhexan-1-aminium) bromide (PDPPNBr-FNBr). The molecular weights of the three neutral precursor polymers were measured by gel permeation chromatography (GPC) by using tetrahydrofuran (THF) as eluent, and polystyrene was used as the internal standard. GPC provided a number-average molecular weight (M̅ n) and molecular weight and polydispersity (PDI) of 29.1 kDa and 1.48, 16 kDa and 1.27, and 12.1 kDa and 2.10 for PDPP-FBr, PF-DPPBr, and PDPPBr-FBr, respectively (see Figure S1). All GPC relevant data are recorded in Table S1, and corresponding nuclear magnetic resonance (NMR) spectra are shown in Figure S2. To investigate the torsional angles (φ) between fluorene and DPP, density functional theory (DFT) was performed. As shown in Figure S3, the optimal geometries of the backbone were presented at multiple sides. The backbone exhibits a planar geometry, and the small torsional angle between fluorene and the DPP unit was ∼17.58°, which evidenced that the new n-CPEs can tightly contact the underlying interface. The ultraviolet−visible (UV−vis) absorption spectra of PDPP-FNBr, PF-DPPNBr, and PDPPNBr-FNBr are provided in Figure 2a. Owing to the same conjugated backbone, three CPE solutions maintained similar absorption profiles. In solid films, three polyelectrolytes displayed a visible red-shift of maximum absorption peak, which indicated a better molecular packing in the solid than in solution.25−27 The optical energy gap (Eg) values were calculated to be 1.72, 1.67, and 1.63 eV for PDPPFNBr, PF-DPPNBr, and PDPPNBr-FNBr, respectively. The orbital energies of PDPP-FNBr, PF-DPPNBr, and PDPPNBrFNBr were calculated by the cyclic voltammetry (CV) measurement, as plotted in Figure S4. Based on the reduction potential, the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of PDPP-FNBr, PFDPPNBr, and PDPPNBr-FNBr were estimated to be −3.81 eV/ −5.31 eV, −3.95 eV/−5.17 eV, and −3.97 eV/−5.02 eV, respectively. Detailed data are provided in Table 1. Taking the relatively high HOMOs in these polyelectrolytes into account, we

Table 1. Summary of Electrochemical Properties of PDPPFNBr, PF-DPPNBr, and PDPPNBr-FNBr CPEs

Eox

Ered

LUMO (eV)

HOMO (eV)

Egcv (eV)

Egopt (eV)

PDPP-FNBr PF-DPPNBr PDPPNBr-FNBr

0.91 0.77 0.62

−0.60 −0.45 −0.43

−3.81 −3.95 −3.97

−5.31 −5.17 −5.02

1.40 1.22 1.05

1.72 1.67 1.63

spin-coated the three CPEs on the zinc oxide (ZnO)28 to act as the cathode interfacial layers. The diverse electron transfer intensities of n-CPEs were affirmed by electron paramagnetic resonance (EPR) spectroscopy, which can afford powerful testimony for the formation of radical anions.29 Three n-CPEs were handled under the same conditions. As shown in Figure 2b, all three n-CPEs displayed strong signals of radical anions, demonstrating the self-doping properties. The efficient electron transfer occurred from polar groups to the n-type skeleton which were stabilized by the pendant cations for electrostatic interaction; therefore, stable and intrinsic n-doping could be achieved.30 PF-DPPNBr and PDPPNBr-FNBr showed much higher signal intensities than PDPP-FNBr with polar groups. This result implied that polar groups directly attached to the electron-deficient DPP unit can favor a much stronger electron transfer and self-doping than those appending on the fluorene unit. In addition, increasing the number of polar groups further enhanced the n-type selfdoping effect as revealed by the most strongest signal intensities of PDPPNBr-FNBr, thanks to the increased charge transfer from polar groups to the n-type skeleton. The conductive properties of ZnO/PDPP-FNBr, ZnO/ PF-DPPNBr, and ZnO/PDPPNBr-FNBr interlayers were researched by conductive atomic force microscopy (C-AFM) measurements. In Figure 3a−d, three ZnO/n-CPEs all displayed a much higher and more homogeneous current intensity than the bare ZnO film, which would improve the charge transport and effectively enhance short-circuit current density (Jsc) in the inverted devices. The current intensity of bare ZnO was calculated to be 0.358 nA, and the current intensity of ZnO/PDPPFNBr was calculated to be 0.848 nA. After the polar group was changed from fluorene to the DPP unit, the current intensity of ZnO/PF-DPPNBr was improved to 2.01 nA. Further appending the polar groups on both fluorene and the DPP unit afforded the ZnO/PDPPNBr-FNBr with the most homogeneous current intensity of 2.52 nA. In addition, the conductivity of the n-CPEs interlayer was also investigated and compared with the device of ITO/n-CPEs/Al. As shown in C

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Figure 3. Conductive atomic force microscopy (C-AFM) images of (a) ZnO/PDPP-FNBr, (b) ZnO/PF-DPPNBr, (c) ZnO/PDPPNBr-FNBr, and (d) ZnO at Vbias = 2 V and representative current curve of C-AFM for (e) ZnO/PDPP-FNBr, (f) ZnO/PF-DPPNBr, (g) ZnO/PDPPNBr-FNBr, and (h) ZnO.

Figure 4. (a) High-resolution X-ray photoelectron spectra of (XPS) of N 1s. (b) High-resolution XPS of Br 3d. (c) High-resolution XPS of Zn 2p. (d) High-resolution XPS of O 1s.

Figure S5 and Table S2, the PDPPNBr-FNBr with the highest self-doping displayed a more superior conductivity of 6.35 × 10−6 S cm−1 over PF-DPPNBr (5.20 × 10−6 S cm−1) and even was 2 times higher than that of PDPP-FNBr (3.51 × 10−6 S cm−1). These results were in good agreement with their n-type self-doping effects. To examine the influence between polyelectrolytes and ZnO, X-ray photoelectron spectroscopy (XPS) has been conducted (see Figure S6). In Figure 4, the characteristic peaks N(1s) and Br(3d) can be observed for PDPP-FNBr, PF-DPPNBr, and PDPPNBr-FNBr, indicating that three n-CPEs have been successfully deposited on the surface of the ZnO. Clearly, PDPPNBrFNBr possessed the strongest Br(3d) peak at 68.7 eV, which was attributed to the highest content of Br− in polymer. The characteristic peak of N(1s) observed at 399.6 eV was assigned to atom N in the DPP core, while the peak at 402.4 eV corresponded to the positively charged amine (N+). In addition, to confirm the disparity of interface interaction for PDPP-FNBr, PF-DPPNBr, and PDPPNBr-FNBr on ZnO, the XPS spectra of Zn(2p) and O(1s) for ZnO/n-CPEs also had been explored. Different from that of bare ZnO exhibiting the O(1s) peak at 530.57 eV, the ZnO/PDPP-FNBr, ZnO/PF-DPPNBr, and ZnO/PDPPNBr-FNBr films showed a O(1s) peak at 530.37, 530.23, and 530.14 eV with a deviation of 0.20, 0.34, and 0.43 eV, respectively (see Figure 4c). The core level shift toward the lower binding implied the higher negative electric charge density of O atoms by electrostatic interaction.31

A similar trend was also observed in O(1s) peaks, as shown in Figure 4d. Compared to bare ZnO, the Zn(2p) peaks of ZnO/ n-CPEs were shifted to a lower binding energy by 0.12, 0.19, and 0.25 eV for PDPP-FNBr, PF-DPPNBr, and PDPPNBrFNBr, respectively. The core level shift of O(1s) and Zn(2p) peaks indicated the strong interface interactions between ZnO and n-type CPEs, and the ZnO/PDPPNBr-FNBr possessed the strongest interfacial interaction because of the largest core level shift. Such strong interfacial interaction would contribute to suppress the interfacial energy barrier and improve the compatibility between electrode and active layer. CPEs employed as favorable electron transfer layers (ETLs) should have the ability to modify the WF of electrodes and diminish the interfacial energy barrier between active layer and electrode. As we mentioned above, the downshifted WF was realized by the formation of dipole moments when polar groups anchored at the interface of the metal/organic material.15 Therefore, the WFs of these new CPEs modified electrodes were measured by ultraviolet photoelectron spectroscopy (UPS), and the WF of bare ZnO was also measured for comparison. As shown in Figure 5a, the WF value of ZnO was 4.49 eV, which was reduced by 0.31 eV for ZnO/PDPP-FNBr, 0.36 eV for ZnO/PF-DPPNBr, and 0.45 eV for ZnO/PDPPNBr-FNBr. The consistent results were also further explored by Kelvin probe microscopy (KPM). In Figure 5b, the WF of ZnO, ZnO/ PDPP-FNBr, ZnO/PF-DPPNBr, and ZnO/PDPPNBr-FNBr was 4.49, 4.03, 4.00, and 3.95 eV, respectively. The reduced D

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Figure 5. (a) Ultraviolet photoelectron spectroscopy (UPS) spectra of ZnO, ZnO/PDPP-FNBr, ZnO/PF-DPPNBr, and ZnO/PDPPNBr-FNBr. (b) Work function images from the matrix of the ZnO, ZnO/PDPP-FNBr, ZnO/PF-DPPNBr, and ZnO/PDPPNBr-FNBr films, measured by Kelvin probe microscopy. (c) Schematic energy-level diagram of each component of the device.

The process for devices fabrication is provided in the Supporting Information. In the classic fullerene system, PTB7 and PC71BM blend were selected as active layer materials. The current densities versus voltage (J−V) characteristics of device are shown in Figure 6b. Compared to bare ZnO, the three CPEs all can improve open-circuit voltage (Voc), fill factor (FF), and Jsc of the devices simultaneously. Note that the efficiency was improved by 22.2% by incorporation of ZnO/PDPPNBr-FNBr as interlayer (PCE = 8.30%, Jsc = 15.75 mA/cm2, Voc = 0.76 V, FF = 69%). The improved all performance parameters mainly originated from the improved energy alignment and higher conductivity of the interlayer. As expected, with the same number of polar pendants, the PF-DPPNBr displays a higher PCE (PCE = 7.65%, Jsc = 15.23 mA/cm2, Voc = 0.75 V, FF = 67%) than PDPP-FNBr (PCE = 7.58%, Jsc = 15.52 mA/cm2, Voc = 0.74 V, FF = 66%). To explore the general and universal applicability, the non-fullerene system has also been evaluated with same device structure. Similarly, the device with PDPPNBrFNBr exhibited the best photovoltaic performance of 10.57% with a Jsc of 16.64 mA/cm2, a Voc of 0.880 V, and a FF of 72.2%. PF-DPPNBr (PCE = 10.13%) and PDPP-FNBr (PCE = 9.59%) can also boost the device performance with respect to the bare ZnO (PCE = 9.03%). All data are summarized in Table 2. To further verify the Jsc, the corresponding the external quantum efficiency (EQE) is measured in Figure 6. The results demonstrated that the EQE had been overall enhanced by introducing three n-CPEs compared with bare ZnO. Obviously, PDPPNBr-FNBr showed the highest EQE values than the other counterparts. The calculated Jsc agreed well with those values

WF modified by CPEs was attributed to the formation of the favorable dipole moments and strong interactions at the contact interface, which was beneficial to minimize the interfacial energy barrier and facilitate the charge abstraction and collection.7,9,11 Intriguingly, the position and number of the polar groups caused a distinct difference in the shift of WF modified by CPEs, which can be illuminated by Figure 1. The favorable dipole moments formed at the interface that point out from electrode to active layer can be classified as two types: one was the ionic polar group-induced electrostatic dipole moment (μI),18 and the other was the n-type self-doping effect which created an extra dipole moment (μP). For PDPP-FNBr and PF-DPPNBr with the same number of polar groups, they both have the equal μI; thereby the WF difference should be caused by the difference in μP. As observed by EPR results, PF-DPPNBr with polar groups appending on the electron-deficient DPP unit has stronger n-type self-doping than the PDPP-FNBr counterpart. Such stronger self-doping can induce stronger μP,32−34 resulting in the more distinct shift of the vacuum level (Evac) and WF of the ZnO/PDPP-FNBr. Further increasing the number of polar groups improved the coverage of favorable interfacial dipoles and μP, subsequently obtaining the lowest WF of in ZnO/PDPPNBr-FNBr in these CPEs to favor the best energy alignment in the device. Therefore, a tunable WF of ZnO/ CPEs can be easily realized by simple regulation of position and number of polar groups. To explore the effects of these n-CPEs on the PSCs device, these new CPEs have been employed in the inverted structure of ITO/ZnO/n-CPEs/active layer/MoO3/Ag (Figure 6a). E

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Figure 6. (a) Schematic device structure of the inverted PSC and the molecule structures of active layer materials studied in this work. (b) Performance of devices based on PTB7:PC71BM under simulated AM 1.5G illumination (100 mW/cm2). (c) Incident photon-to-current efficiency (IPCE) spectra of the inverted solar cells based on ITO/ZnO/n-CPEs/PTB7:PC71BM/MoO3/Ag. (d) Performance of devices based on PBDB-T:ITIC under simulated AM 1.5G illumination (100 mW/cm2). (e) IPCE spectra of the inverted solar cells based on ITO/ZnO/n-CPEs/ PBDB-T:ITIC/MoO3/Ag.

Table 2. Summary of Photovoltaic Parameters of Solar Cells with Different n-CPEs under AM 1.5G Illumination at 100 mW cm−2 a active layer

ETL

PTB7:PC71BM

ZnO ZnO/PDPP-FNBr ZnO/PF-DPPNBr ZnO/PDPPNBr-FNBr ZnO ZnO/PDPP-FNBr ZnO/PF-DPPNBr ZnO/PDPPNBr-FNBr

PBDB-T:ITIC

Jsc (mA/cm2)

Voc (V) 0.71 0.72 0.74 0.75 0.858 0.864 0.867 0.870

± ± ± ± ± ± ± ±

0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01

14.80 15.42 15.15 15.64 15.46 15.60 16.25 16.52

± ± ± ± ± ± ± ±

0.16 0.10 0.08 0.11 0.03 0.11 0.14 0.12

FF (%) 61.5 64.3 66.1 67.9 66.7 68.8 70.0 71.3

± ± ± ± ± ± ± ±

1.5 1.7 1.0 1.1 0.5 1.1 0.5 0.9

PCE (%) 6.79b 7.58b 7.65b 8.30b 9.03b 9.59b 10.13b 10.57b

(6.55 ± 0.24) (7.37 ± 0.21) (7.47 ± 0.18) (8.17 ± 0.13) (8.92 ± 0.11) (9.38 ± 0.21) (9.88 ± 0.25) (10.31 ± 0.26)

a

The average values and standard deviations of 10 solar cells are shown in parentheses. bBest device PCE.

4.311 × 10−4 cm2 V−1 s−1. There was no doubt that better electron mobility led to the higher Jsc. All data are presented at Table S3. As we known, the surface roughness has a significant effect on the performance of PSCs; the increase of interface traps can facilitate the possibility of trap-assisted recombination of electron, leading to the low FF of the solar cells.35 Contact angles and atomic force microscopy (AFM) were applied to explore

obtained from the J−V curves within a 5% mismatch. The electron mobility of the polyelectrolytes was determined using a space-charge limited current (SCLC) method using electrononly devices with an ITO/ZnO/n-CPEs/PTB7:PC71BM/Al structure in Figure S7. The device with ZnO showed an electron mobility of 2.998 × 10−4 cm2 V−1 s−1, while the ones with PDPP-FNBr, PF-DPPNBr, and PDPPNBr-FNBr promoted the electron mobility to 3.505 × 10−4, 3.895 × 10−4, and F

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the topography of bare ZnO, ZnO/PDPP-FNBr, ZnO/ PF-DPPNBr, and ZnO/PDPPNBr-FNBr shown in Figures S8 and S9. Deposition of the three n-CPEs made the ZnO surfaces more hydrophobic, which would afford a more favorable contact with the upper active layer. As shown in Figure S9a−d, the rootmean-square roughness (RMS) of bare ZnO was 3.87 nm. The RMS was 3.35 nm for ZnO/PF-DPPNBr, 3.15 nm for ZnO/ PDPP-FNBr, and 3.07 nm for ZnO/PDPPNBr-FNBr. Obviously, the introduced three copolymers will efficiently reduce the RMS and form an intimate interface contact with the active layer. Thereby, the morphology change of the active layer induced by the n-CPEs was further investigated by AFM. The PTB7:PC71BM and PBDB-T:ITIC were spin-coated on these substrates. As shown in Figure S9e−l, after n-CPEs modification, the RMS of the active layer was remarkably reduced compared with ZnO. Therefore, the homogeneous ZnO/ n-CPEs can improve the interfacial contact and induce a smooth morphology of the upper active layer, leading to the FF enhancement.

Y.T.: College of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.C. thanks the National Natural Science Foundation of China (NSFC) (21762029, 51673092, and 51473075) for financial support. Y.C. thanks the National Natural Science Foundation of China (NSFC) (51833004) and National Science Fund for Distinguished Young Scholars (51425304) for support. The authors thank the Graduate Innovation Fund Projects of Jiangxi Province (YC2017-S051) for support.

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3. CONCLUSIONS In summary, three new self-doping n-CPEs based on the DPP alternated fluorene framework were synthesized, and the effect of the number and location of ion polar groups on the properties of n-CPEs and the resulting device performance has been fully investigated. Anchoring the polar groups on the n-type DPP unit can favor more efficient electron transfer from polar groups to backbone to realize a stronger n-type selfdoping effect than those on the p-type fluoroene unit, consequently improving the conductivity of the n-CPEs and enhancing the interfacial dipole moments. Increasing the number of polar groups can further optimize the interfacial modification. A decreased WF, improved interlayer conductivity, and strengthened interfacial interaction were obtained to facilitate charge abstraction, transportation, and collection. As a result, the new n-CPEs as cathode buffer layers were valid both in fullerene and in non-fullerene systems of PSCs, achieving a notable PCE of 10.57%. These results demonstrated that simply regulating the number and location of polar groups of the n-CPEs provides a facile way to develop highly efficient cathode interlayers for high performance polymer solar cells.

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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.8b01490. Synthesis of the three polymers, 1H NMR, GPC, and CV of three polymers, molecular backbone conformations simulate by DFT, I−V curves, conductivity, electron mobility and XPS of the PDPP-FNBr, PF-DPPNBr, and PDPPNBr-FNBr, RMS roughness of corresponding film (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.C.). *E-mail: [email protected] (Y.C.). ORCID

Yinhua Zhou: 0000-0001-6424-9962 Yiwang Chen: 0000-0003-4709-7623 G

DOI: 10.1021/acs.macromol.8b01490 Macromolecules XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.macromol.8b01490 Macromolecules XXXX, XXX, XXX−XXX