Synergistic Effect of Chlorination and Selenophene: Achieving

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

pubs.acs.org/Macromolecules

Synergistic Effect of Chlorination and Selenophene: Achieving Elevated Solar Conversion in Highly Aggregated Systems Xiaowei Zhong,† Hui Chen,† Meijing Wang,† Shenglong Gan,‡ Qiming He,§ Wei Chen,*,‡,§ and Feng He*,† †

Downloaded via EAST CAROLINA UNIV on March 12, 2019 at 11:30:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Department of Chemistry and Shenzhen Grubbs Institute, Southern University of Science and Technology, Shenzhen 518055, P. R. China ‡ Materials Science Division, Argonne National Laboratory, 9700 Cass Avenue, Lemont, Illinois 60439, United States § Institute for Molecular Engineering, The University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637, United States S Supporting Information *

ABSTRACT: Chlorination and selenophene were used to develop benzothiadiazole-based polymers for high-performance polymer solar cells (PSCs). The introduction of selenophene can increase crystallinity due to the metalloid nature of selenium and thus facilitate charge transport. Chlorination can tune the energy levels and induce strong aggregation due to its unique features. A non-chlorinated polymer with selenophene, PBT3TSe, shows a highly crystalline structure and a dominant face-on orientation, consequently attaining a high short-circuit current (JSC). Chlorinated PBT3TClSe displays synergy between the advantages of chlorination and selenophene to achieve elevated photovoltaic performance, with a power conversion efficiency (PCE) approaching 9.89% in PC71BM-based devices. Interestingly, chlorination has an important influence on morphology of the polymer and polymer blend films, resulting in a severe aggregation and mixed face-on and edge-on orientation in the blend film. But the sufficient intermingling of donor and acceptor and the closer distances between molecules from the introduction of the chlorine and selenophene offset their morphological inferiority to achieve higher solar conversion.

■

INTRODUCTION Currently, polymer solar cells (PSCs) have been attracting increasing attention from researchers due to their advantages of low cost, light weight, fine-tuned molecular structure, and potential for large-area solution processing and in flexible electronics. 1−3 Phenyl-C 71 -butyric acid methyl ester (PC71BM), a mature, commercially available product, is widely used as the acceptor material in PSCs with various donor polymers, and the resulting devices achieve high photovoltaic performance with power conversion efficiency (PCE) approaching 12%.4,5 In developing well-performing donor materials, many efforts have been made to tune their aggregation and morphological characteristics through molecular design and device processing.6−9 The morphology of polymers and their blend films is of great importance because it affects the exciton dissociation and charge transport in the active layer.10 A desirable morphology requires suitable miscibility.11 Each material should have an intrinsic domain size and suitable orientations that carriers can transfer to the corresponding electrode smoothly.12−14 Herein, the morphology of the blend films is related to the polymers’ backbone torsion, crystallinity, interaction with acceptors, and so forth.15 To adjust the aggregation and morphology in a neat blend film, molecular modification of the donor polymer is often useful.16,17 The length of alkyl chains and substitution of © XXXX American Chemical Society

heterocyclic compounds are frequently explored as potential excellent donor materials.18−24 In previous studies, a selenophene unit was found to increase the crystallinity of molecules due to the metallic properties of selenium.25,26 PSCs based on selenophene-enhanced donors often exhibit wellordered aggregation in the active layer and high short-circuit current (JSC) under operational conditions.27−29 However, the open-circuit voltage (VOC) of these devices is usually moderate, and these types of donor polymers have not been widely investigated. Additionally, chlorination has been shown to have great potential for regulating blend film aggregation and morphology to improve the photovoltaic performance of polymers.30−33 Chlorine atoms cause torsion in the backbone of the polymer due to their large size and further influence polymer chain packing.34 In addition, the strong electronegativity of chlorine atoms can readjust the charge distribution of the molecules, thus affecting the electronic properties of the polymer.35,36 Moreover, the empty 3d-orbitals on the chlorine atom can cause strong intramolecular and intermolecular connections, which can further enhance aggregation in both solutions and Received: November 14, 2018 Revised: March 3, 2019

A

DOI: 10.1021/acs.macromol.8b02445 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Table 1. Intrinsic Properties of PBT3TClSe and PBT3TSe polymer PBT3TClSe PBT3TSe

Mn (kDa) 34.7 38.4

PDI

εa (L mol−1 cm−1)

λsol max (nm)

λfilm max (nm)

Egopt (eV)

ELUMO (eV)

EHOMO (eV)

EgC (eV)

1.6 1.1

9.4 × 10 5.8 × 104

700 710

700 710

1.65 1.59

−3.62 −3.63

−5.32 −5.38

1.70 1.65

4

Calculated from per repeat unit. bEgopt = 1240/λonset; λonset was calculated in the solid state.

a

Scheme 1. Synthetic Route to PBT3TClSe and PBT3TSe

films.37,38 Meanwhile, PSC devices based on chlorinated polymers often show increased VOC because the highest occupied molecular orbital (HOMO) is lowered and the lowest unoccupied molecular orbital (LUMO) is closely maintained.39−41 In this study, selenophene and chlorination were applied together with the aim of obtaining the relationship between the morphological characteristics and photovoltaic performance. Two polymers, named PBT3TClSe and PBT3TSe, were designed and synthesized for comparison in this study. The polymer with only a selenophene unit, PBT3TSe, shows a PCE of 9.07% with a JSC of 18.70 mA cm−2 when blended with PC71BM as active materials. Unsurprisingly, PBT3TSe and its blend films exhibit a highly crystallinity and favorable face-on orientation with the introducing of selenophene, which is beneficial for charge transport in devices. However, there is a large difference in the morphology after the chlorine substitution on the adjacent thiophene unit. In detail, the polymer PBT3TClSe with chlorine and selenophene mod-

ifications shows a multiscale-length morphology with clustered domain size up to 100 nm mingled with fibers inside. The PBT3TClSe-based device presents an improved performance, with a higher VOC of 0.77 V and a slightly improved JSC of 18.93 mA cm−2, promoting PCE to as much as 9.89%.

■

RESULTS AND DISCUSSION The synthetic route of these two polymers is shown in Scheme 1. 4,7-Bis(5-bromo-4-(2-octyldodecyl)thiophen-2-yl)-5,6difluorobenzo[c][1,2,5]thiadiazole (FBT) was prepared according to the literature.5,8 8-Trimethyl(selenophen-2-yl)stannane was coupled with 2-bromo-3-chlorothiophene or 2bromothiophene, and both products were subjected to stannylation to produce (4-chloro-5-(5-(trimethylstannyl)selenophen-2-yl)thiophen-2-yl)trimethylstannane (STCl-Sn) and trimethyl(5-(5-(trimethylstannyl)selenophen-2-yl)thiophen-2- yl)stannane (ST-Sn), respectively. Later, copolymerizing FBT with ST-Sn or STCl-Sn via Stille coupling resulted in polymers PBT3TClSe and PBT3TSe, respectively. B

DOI: 10.1021/acs.macromol.8b02445 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. Normalized UV−vis absorption spectra of PBT3TClSe (a) and PBT3TSe (b) in chlorobenzene solution and in a thin film and normalized temperature-dependent UV−vis absorption spectra of PBT3TClSe (c) and PBT3TSe (d) in chlorobenzene solution.

tend to enter the d-orbitals of chlorine atoms due to the metalloid properties of selenium and the strong electronegativity of chlorine. This tendency further strengthens the mutual linking among polymer chains. The double action of enhancing the interaction consequently leads to powerful aggregations. Therefore, PBT3TClSe exhibits similar aggregations in both the solution and the solid state, thus similar UV− vis absorption. While selenophene can help crystallization, such aggregation might also have something to do with the solvent. When PBT3TSe forms thin films from chlorobenzene solution, a nonequilibrium state, the evaporation of solvent may reshape the aggregates, resulting in different degrees of aggregation. Hence, the synergy of chlorine and selenophene can enhance aggregation in neat films. To study the aggregation behaviors of PBT3TClSe and PBT3TSe, temperature-dependent UV−vis spectra of these two polymers were measured in chlorobenzene solution and are shown in Figures 1c and 1d.8,28 As the testing temperature increases, λmax of PBT3TClSe and PBT3TSe slowly blue shifts at first but shifts quickly at high temperature later, with 151 and 161 nm blue shifts at 100 °C compared to 30 °C, respectively, which illustrates that there is rather high aggregation in both polymers. The interchain π−π* transition peak of the two polymers declined gradually as the temperature increased from 30 to 100 °C. Cyclic voltammetry was used to estimate the electrochemical properties of the two polymers. The HOMO and LUMO of PBT3TSe were −5.28 and −3.63 eV, respectively, and the related band gap was 1.65 eV. After chlorination, the HOMO and LUMO of PBT3TClSe were −5.32 and −3.62 eV, and the related band gap was 1.70 eV. These results are consistent with the UV−vis spectra data. In the CV curve of PBT3TSe, the magnitude of current under reduction was relatively small,

The molecular weights of the polymers were determined by high-temperature gel permeation chromatography at 180 °C using 1,2,4-trichlorobenzene as eluent. The number-average molecular weights of PBT3TClSe and PBT3TSe were 34.7 and 38.4 kDa, respectively, with polydispersity indices of 1.6 and 1.1, respectively. In the UV−vis absorption spectra shown in Figure 1, with a broad visible-region-covered absorption band, PBT3TClSe shows the highest absorption at 700 nm in chlorobenzene solution, which is unexpectedly the same value as that of its solid state. This phenomenon is also found in PBT3TSe with a λmax of 710 nm in both solution and solid state, indicating that selenophene cause strongly aggregation in these two polymers even in solution. The 10 nm hypsochromic shift of PBT3TClSe to PBT3TSe should result from chlorine substitution, whose strong electronegativity can lower the HOMO energy level and then widen the band gap, as verified by cyclic voltammetry. In addition, the mole absorbance coefficient of PBT3TClSe is 9.4 × 104 L mol−1 cm−1, which is larger than that of PBT3TSe (5.8 × 104 L mol−1 cm−1), suggesting that the ability of PBT3TClSe to harvest light is higher than that of PBT3TSe. The optical band gap of PBT3TClSe and PBT3TSe is calculated to be 1.65 and 1.59 eV, respectively. To take a deeper look at PBT3TClSe, its spectrum in solution parallels that of its solid state very closely, implying that aggregates in solution may be analogous to those in a solid state. However, there is an obvious difference in the spectrum of PBT3TSe between solution and the solid state. This dissimilarity could be attributed to the chlorine atoms, whose empty 3d-orbitals could hold additional electrons. These empty 3d-orbitals can accommodate the π-electrons of the molecular backbones to reinforce the interactions among polymers.37 Additionally, d-electrons of selenium atoms might C

DOI: 10.1021/acs.macromol.8b02445 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 2. Cyclic voltammograms (a) of polymers in thin films drop-casted on glassy carbon electrodes at a scan rate of 50 mV s−1, the reduction part of PBT3TSe is enlarged with 50 times, and energy level diagrams (b) for the polymers and PC71BM. DSC traces for PBT3TClSe (c) and PBT3TSe (d) at a heating/cooling rate of 10 °C min−1 in N2.

Figure 3. J−V curves (a) and EQE spectra (b) of the devices using PBT3TClSe and PBT3TSe as donor materials under AM 1.5G illumination at 100 mW cm−2. Current density−voltage curves (c) for SCLC mobility estimation and current density−light density curves (d) of polymers.

D

DOI: 10.1021/acs.macromol.8b02445 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Table 2. Performance Parameters of the PC71BM-Based Devices with Polymers polymer

VOC (V)

PBT3TClSe PBT3TSe

0.77 (0.77 ± 0.002) 0.69 (0.69 ± 0.001)

a

JSC (mA cm−2)

FF (%)

PCE (%)

18.93 (18.69 ± 0.23) 18.70 (18.53 ± 0.18)

67.83 (67.45 ± 0.42) 70.26 (69.67 ± 0.52)

9.89 (9.68 ± 0.21) 9.07 (8.91 ± 0.15)

The average value ± standard deviation was calculated from 20 independent devices.

a

and external quantum efficiency (EQE) spectra of the devices were measured and are depicted in Figure 3. The measured device parameters are summarized in Table 2. The PBT3TSebased devices showed a PCE of 9.07% with a VOC of 0.69 V, a JSC of 18.7 mA cm−2, and a relatively high FF of 70.26%. As expected, the PBT3TClSe-based devices exhibited a higher PCE of 9.73% with a higher VOC of 0.76 V, a high JSC of 18.89 mA cm−2, and a FF of 67.64%. To understand the origin of the fairly similar JSC of the two types of devices, the EQE spectra were determined. From the EQE profiles, although the PBT3TClSe had an obvious blue shift, the PBT3TClSe presented a higher response in the range 500−700 nm. In this range, the value of EQE was nearly 80%. The efficient charge separation compensated for the JSC loss resulting from the blue shift. The chlorination in this family of polymers promoted the VOC and maintained JSC. The JSC calculated from the EQE of PBT3TClSe and PBT3TClSe is 18.56 and 17.46 mA cm−2, respectively, agreeing well with the data from the J−V curves. Indeed, the selenophene unit could help polymer assembly because of its metalloid properties. Therefore, polymers in the active layer had a strong tendency to form crystalline regions, which contributed to charge transfer. Thus, both devices exhibited well-performing JSC and FF. For the PBT3TClSebased devices, VOC increased due to the lower HOMO induced by chlorination. The slightly increased JSC of PBT3TClSebased devices is probably due to the high extinction coefficient of the polymer seen in the UV−vis spectrum. To explore the carrier transport in devices, the hole mobility of the blend films was measured by space charge limit current (SCLC) characterization. The hole-only devices with a structure of ITO/PEDOT:PSS/polymer:PC71BM/MoO3/Ag were prepared as shown in Figure 3c. The hole mobility was calculated to be 7.4 × 10−4 cm2 V−1 s−1 for PBT3TClSe and 5.2 × 10−4 cm2 V−1 s−1 for PBT3TSe, supporting the improved JSC of PBT3TClSe-based devices. Subsequently, a function of JSC versus light intensity for the device was developed from measurements, as depicted in Figure 3d. The extracted S values are 0.94 for PBT3TClSe and 0.96 for PBT3TSe, indicating that the PBT3TClSe blend film was slightly less bimolecular than that of PBT3TSe. Because of the strong temperature-dependent aggregation of the polymer, the crystallization rate of the polymer and the quality of neat films could be controlled by the temperature during casting.8 During casting, PBT3TClSe disaggregates and mixes well with PC71BM at 110 °C initially. Then, when the solvent evaporates and temperature decreases, the rate of precipitating PBT3TClSe to solids with a reasonable domain size could occur as quickly as that of PC71BM because PBT3TClSe has a strong tendency to cluster, as proven by the UV−vis and DSC analyses, resulting in a satisfied mingling of PBT3TClSe with PC71BM. Thus, the better carrier splitting and less bimolecular recombination could result from the favorable degree of intermingling of donor and acceptor. Atomic force microscopy (AFM) was used to analyze active layer films to understand how their morphology influences the

probably because electron richness of the selenium atom makes PBT3TSe unstable under reduction. In contrast, in PBT3TClSe, the chlorine atoms can help dissipate the electron density due to its electronegativity to stabilize PBT3TClSe under reduction. Because thermal stability is of great importance for efficient polymers, thermogravimetric analysis (TGA) (Figure S2) was conducted on these two polymers. Under a N2 atmosphere, the decomposition temperature (Td) with 5% weight loss of PBT3TClSe and PBT3TSe is 359 and 410 °C, respectively, which shows sufficient thermal stability for their study and application in polymer solar cells. Later, differential scanning calorimetry (DSC) was used to estimate the crystallinity of the polymers and their corresponding blends, as shown in Figures 2c and 2d. According to the colligative property, a compound’s melting point lowers when other compounds exist. If the mass ratio of the two compounds is fixed, the decrease of the melting point can reflect the degree of intermingling of these two compounds.42,43 Within a certain range, the larger the decrease in the melting point is the higher the degree of intermingling of compounds. Herein, by comparison of the melting points obtained from DSC curves between polymers and blends, a rough understanding of crystallinity and the degree of intermingling can be obtained. PC71BM shows a negligible thermal effect in the range of 50−300 °C under a N2 atmosphere. With regard to PBT3TClSe, there is a glass transition at ∼167 °C (Tg) and an indistinct melting point at ∼280 °C. The glass transition temperature could arise from the clustered aggregation of the polymer, and the melting point might arise due to the semicrystallinity. After blending with PC71BM, the melting point at ∼280 °C blurred, and the glass transition temperature at 166 °C could still be observed. This change indicates that PBT3TClSe mixed well with PC71BM, with large crystals becoming obscure. However, the blend remained in a strongly clustered state since the glass transition temperature does not obviously change. PC71BM could be viewed as a π system that can interact with chlorine atoms, so its existence does not destroy the aggregation of polymers, and the blends retain well-clustered aggregations. There is a sharp peak of melting point on the DSC curve of PBT3TSe near 272 °C, indicating a highly crystalline state. This peak reduces to ∼255 °C after mixing with PC71BM, still displaying a considerably high crystallinity. This decrease suggests that PC71BM has merged into PBT3TSe, but the polymer remains highly crystalline. Consequently, chlorination is found to have a great influence on the crystallinity of polymers and the intermingling between the polymer and PC71BM, which can have impacts on their photovoltaic performance. To study the photovoltaic properties of PBT3TClSe and PBT3TSe, the devices were fabricated with an inverted structure of ITO/ZnO/polymer:PC71BM/MoO3/Ag. The D−A weight ratio was 1:1.2, and the total solid concentration was 20 mg L−1 dissolved in chlorobenzene solution with 3% 1,8-diiodooctane (DIO) as an additive. The devices without encapsulation were tested under AM 1.5 G illumination at 100 mW cm−2. The characteristic current−voltage (J−V) curves E

DOI: 10.1021/acs.macromol.8b02445 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 4. AFM height images (5 μm × 5 μm) of the film blends of PBT3TClSe-PC71BM (a) and PBT3TSe-PC71BM (b). TEM images of the film with a blend of PBT3TClSe-PC71BM (c) and PBT3TSe-PC71BM (d).

Figure 5. 2D GIWAXS patterns of PBT3TClSe:PC71BM (a) and PBT3TSe:PC71BM (b). Linecut in the in-plane (c) and out-of-plane (d) directions. F

DOI: 10.1021/acs.macromol.8b02445 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Table 3. Relationship between the Molecular Weight and Photovoltaic Performances polymer PBT3TClSe-1 PBT3TClSe-2 PBT3TClSe-3 PBT3TClSe-4

Mn (kDa) 16.2 22.1 24.2 34.7

JSC (mA cm−2)

VOC (V) 0.79 0.79 0.78 0.77

(0.79 (0.79 (0.78 (0.77

± ± ± ±

a

0.003) 0.002) 0.002) 0.002)

15.58 14.19 15.92 18.93

(15.38 (14.08 (15.67 (18.69

± ± ± ±

0.17) 0.12) 0.24) 0.23)

FF (%) 72.21 62.92 65.32 67.83

(71.59 (61.45 (64.94 (67.45

± ± ± ±

PCE (%) 0.62) 0.48) 0.36) 0.42)

8.89 7.05 8.11 9.89

(8.65 (6.94 (7.99 (9.68

± ± ± ±

0.22) 0.09) 0.13) 0.21)

The average value ± standard deviation was calculated from 20 independent devices.

a

calculated.46,47 For PBT3TClSe, the ratio is 0.11 in pristine films and increases to 0.35 after intermingling with PC71BM. This ratio for PBT3TSe is 2.11 in pristine films and 7.31 in blend films. It seems that PC71BM helps these two polymers increase the proportion of face-on orientation. The higher proportion of edge-on orientation for PBT3TClSe is ascribed to the possibility that the empty d-orbitals would interfere with the lone pair electrons on ZnO, making the polymer chains perpendicular to the surface, namely, edge-on orientation. PBT3TSe and its blend films have a considerably large face-on orientation fraction, which is thought to be beneficial for charge transport between electrodes, and the corresponding devices do perform well. Although PBT3TClSe does not have an ideal orientation distribution, it may still be sufficient for satisfactory device performance. PBT3TClSe-based devices have outperformed PBT3TSe devices with respect to VOC and are competitive with respect to JSC, probably because of the small d-spacing between π−π stacking, which is favorable for charge hopping between molecules and better charge transport. In addition, according to the literature,48,49 which used the Gaussian disorder model (GDM) to analyze the amorphous film, amorphous aggregation with a narrower distance could provide more detour pathways for carriers than films with high crystallinity, facilitating carrier transport. Hence, it could be inferred that in PBT3TClSe blend films carriers successfully going through detour pathways to each electrode take less time than those in the highly crystalline PBT3TSe, improving carrier mobility and enhancing the photovoltaic performance. In addition, the improved miscibility of donor and acceptor materials enhances exciton splitting for well-performing photovoltaic properties. Such superior performance compensates for the disfavored orientation distribution of PBT3TClSe:PC71BM blend films and enables the devices to achieve high JSC and PCE. As observed in the TEM and AFM profiles, the PBT3TClSebased blend films show large aggregates, which are thought to be great obstacles for charge transport. Herein, PBT3TClSe polymers with lower molecular weights were synthesized to reduce severe aggregation to study the relationship between morphology and photovoltaic performance. The AFM and TEM profiles of these three polymers are shown in Figure S5, illustrating that severe aggregations could be effectively ameliorated by reducing molecular weight. There is a negligible change in VOC. However, in terms of the newly synthesized polymers, the JSC and PCE are significantly less than the initial polymer with higher molecular weight and greater aggregation. Compared with PBT3TClSe-1 and PBT3TClSe-4, reduced clustering to set the charge free to some extent seems to have the potential to tune the devices’ performance due to the maintenance of VOC and improvements to FF. The decreased JSC mainly results from the short length of polymer chains, which restricts the electron flow. Nevertheless, slightly increasing the molecular weight does not improve JSC, as

photovoltaic performance. The AFM height images in Figures 4a and 4b show that the root-mean-square (rms) roughness of the PBT3TClSe-PC71BM blend film is 5.38 nm and that of the PBT3TSe-PC71BM blend film is 5.52 nm. Although the rms values of the two blend films were quite close, their images exhibited great differences in appearance. It is postulated that there might be less amorphous miscibility but more liquidus miscibility in the PBT3TClSe-PC71BM blend film compared to the PBT3TSe-PC71BM blend film, which means that PC71BM mixed with PBT3TClSe would tend to be more aggregated.11 The FF loss of PBT3TClSe-based devices compared to PBT3TSe-based devices might be attributed to the tendency of liquidus miscibility over amorphous miscibility. To prove this postulation, transmission electron microscopy (TEM) was performed on the blend active layers as shown in Figure 4c,d. For PBT3TSe, the TEM image showed welldistributed fibrous blends. However, at first glance, PBT3TClSe blend films showed special and large domain sizes of nearly 100 nm. However, after careful examination, fibers could also be seen in the clustered region. It is suggested that PBT3TClSe:PC71BM adopts a multiscale-length morphology.15,43 This polymorphous morphology demonstrated that PBT3TClSe and PC71BM have mixed and clustered well in solution and then form a small group in solid state. In each cluster, donor and acceptor mingle well to improve carrier splitting. For each cluster, a relatively independent part might reduce the chances for charge recombinaton, benefiting charge transport. Hence, this morphology guarantees prior mobility, as observed in the devices. To gain a deeper understanding of the morphology and aggregation in vertical phase separation, two pristine polymer blend films were subjected to the grazing-incidence wide-angle X-ray scattering (GIWAXS) method. The 2D GIWAXS patterns and the corresponding linecut profiles are shown in Figure 5. The (100) scattering peak indicated the lamellar structures of the polymers. The scattering peak of lamellar structures of PBT3TSe is located at 0.57 Å−1 with d-spacing of 11.0 Å, and that of PBT3TClSe is located at 0.67 Å−1 with dspacing of 9.37 Å.44,45 The (300) scattering ring represents the PC71BM, which is nearly amorphous and evenly distributed in both blend films. The (010) scattering peak reflects π−π stacking. PBT3TClSe blend films exhibited a ring-like (010) scattering pattern, which means that their direction of π−π scattering is slightly irregular. However, PBT3TSe blend films show a strong scattering peak in the out-of-plane direction, indicating high crystallinity with a good face-on orientation. The PBT3TClSe:PC71BM films’ out-of-plane π−π stacking scattering peak is situated at 1.75 Å−1 (3.59 Å), and that of PBT3TSe is situated at 1.71 Å−1 (3.68 Å). This demonstrates that the chlorinated polymer has a shorter π−π stacking distance, improving charge transport. To evaluate the orientation distribution of the polymers’ pristine films and blend films, profiles of I sin(χ) against χ are plotted in Figure S4, and the out-of-plane to in-plane ratio was G

DOI: 10.1021/acs.macromol.8b02445 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Shenzhen Nobel Prize Scientists Laboratory Project (C17783101). X.Z. is particularly grateful to the Undergraduate Research Program of Department of Chemistry at SUSTech. We also thank Dr. Joseph Strzalka and Dr. Zhang Jiang for the assistance with GIWAXS measurements. Use of the Advanced Photon Source (APS) at the Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357.

PBT3TClSe-2 shows. The resulting stronger aggregation likely offsets the benefits of the longer polymer chains. PBT3TClSe-3 was obtained by further increasing the molecular weight. Interestingly, the optoelectronic properties rebound. Hence, it would be reasonable to illustrate that severe aggregation and large domain size are not always an impediment to polymer solar cells. For such polymers, clustering could contribute to the narrower distance between polymer chains, facilitating charge hopping for better charge transport, which overcomes the drawbacks of extreme aggregation. Meanwhile, similar to the TEM discussion, the supposed multiscale-length morphology could still assist the process of charge transport, alleviating the negative effects of a large domain size. Such observations of the PBT3TClSe with lower molecular weight agree with the discussion that the synergy of selenophene and chlorination can benefit from severe aggregation.

■

(1) He, F.; Yu, L. How Far Can Polymer Solar Cells Go? In Need of a Synergistic Approach. J. Phys. Chem. Lett. 2011, 2 (24), 3102−3113. (2) Li, Y. Molecular Design of Photovoltaic Materials for Polymer Solar Cells: Toward Suitable Electronic Energy Levels and Broad Absorption. Acc. Chem. Res. 2012, 45 (5), 723−733. (3) Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L. Recent Advances in Bulk Heterojunction Polymer Solar Cells. Chem. Rev. 2015, 115 (23), 12666−12731. (4) Günes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated PolymerBased Organic Solar Cells. Chem. Rev. 2007, 107 (4), 1324−1338. (5) Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H. Efficient Organic Solar Cells Processed from Hydrocarbon Solvents. Nat. Energy 2016, 1 (2), 15027−15033. (6) Yang, X.; Loos, J.; Veenstra, S. C.; Verhees, W. J. H.; Wienk, M. M.; Kroon, J. M.; Michels, M. A. J.; Janssen, R. A. J. Nanoscale Morphology of High-Performance Polymer Solar Cells. Nano Lett. 2005, 5 (4), 579−583. (7) Campoy-Quiles, M.; Ferenczi, T.; Agostinelli, T.; Etchegoin, P. G.; Kim, Y.; Anthopoulos, T. D.; Stavrinou, P. N.; Bradley, D. D. C.; Nelson, J. Morphology Evolution via Self-Organization and Lateral and Vertical Diffusion in Polymer:Fullerene Solar Cell Blends. Nat. Mater. 2008, 7, 158−164. (8) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and Morphology Control Enables Multiple Cases of High-Efficiency Polymer Solar Cells. Nat. Commun. 2014, 5, 5293. (9) Jiang, K.; Zhang, G.; Yang, G.; Zhang, J.; Li, Z.; Ma, T.; Hu, H.; Ma, W.; Ade, H.; Yan, H. Multiple Cases of Efficient Nonfullerene Ternary Organic Solar Cells Enabled by an Effective Morphology Control Method. Adv. Energy Mater. 2018, 8 (9), 1701370. (10) Heeger, A. J. 25th Anniversary Article: Bulk Heterojunction Solar Cells: Understanding the Mechanism of Operation. Adv. Mater. 2014, 26 (1), 10. (11) Ye, L.; Collins, B. A.; Jiao, X.; Zhao, J.; Yan, H.; Ade, H. Miscibility-Function Relations in Organic Solar Cells: Significance of Optimal Miscibility in Relation to Percolation. Adv. Energy Mater. 2018, 8, 1703058. (12) Ayzner, A. L.; Mei, J.; Appleton, A.; DeLongchamp, D.; Nardes, A.; Benight, S.; Kopidakis, N.; Toney, M. F.; Bao, Z. Impact of the Crystallite Orientation Distribution on Exciton Transport in DonorAcceptor Conjugated Polymers. ACS Appl. Mater. Interfaces 2015, 7 (51), 28035−28041. (13) Ayzner, A. L.; Nordlund, D.; Kim, D. H.; Bao, Z.; Toney, M. F. Ultrafast Electron Transfer at Organic Semiconductor Interfaces: Importance of Molecular Orientation. J. Phys. Chem. Lett. 2015, 6 (1), 6−12. (14) Vohra, V.; Kawashima, K.; Kakara, T.; Koganezawa, T.; Osaka, I.; Takimiya, K.; Murata, H. Efficient Inverted Polymer Solar Cells Employing Favourable Molecular Orientation. Nat. Photonics 2015, 9 (6), 403−408. (15) Gao, K.; Miao, J.; Xiao, L.; Deng, W.; Kan, Y.; Liang, T.; Wang, C.; Huang, F.; Peng, J.; Cao, Y.; Liu, F.; Russell, T. P.; Wu, H.; Peng, X. Multi-Length-Scale Morphologies Driven by Mixed Additives in Porphyrin-Based Organic Photovoltaics. Adv. Mater. 2016, 28 (23), 4727.

■

CONCLUSIONS In conclusion, PBT3TClSe and PBT3TSe were designed and synthesized as donor materials, both of which have strong and temperature-dependent aggregation properties due to the metalloid properties of selenophene; the corresponding devices both exhibit good performance. The PBT3TClSe blend film has a greater clustering property induced by chlorination and exhibits a multiscale-length morphology, which facilitates the process of charge transport, leading to a high PCE approaching 9.89%. PBT3TSe and its blend film have a considerable preference for face-on orientation, which is key to tuning the morphology toward designing donor materials with good crystallinity and aggregation. Despite the fact that PBT3TClSe does not substantially adopt the face-on orientation, which benefits charge transport between electrodes, its closer packing facilitates exciton hopping among polymer chains and between donor and acceptor and can compensate for the detrimental effects of the less favorable orientation, contributing to an excellent PCE. Synergy with chlorination could be powerful tool to produce outstanding photovoltaic performances.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02445. Synthetic procedures; TGA; theoretical calculations data; GIWAXS profiles; AFM and TEM images (PDF)

■

REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Wei Chen: 0000-0001-8906-4278 Feng He: 0000-0002-8596-1366 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge financial support from SUSTech, the National Natural Science Foundation of China (51603100, 51773087, and 21733005), the Shenzhen Fundamental Research program (JCYJ20170817111214740), and the H

DOI: 10.1021/acs.macromol.8b02445 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (16) Yao, H.; Ye, L.; Zhang, H.; Li, S.; Zhang, S.; Hou, J. Molecular Design of Benzodithiophene-Based Organic Photovoltaic Materials. Chem. Rev. 2016, 116 (12), 7397−7457. (17) Marszalek, T.; Li, M.; Pisula, W. Design Directed Self-Assembly of Donor-Acceptor Polymers. Chem. Commun. 2016, 52 (73), 10938−10947. (18) Sun, S. X.; Huo, Y.; Li, M. M.; Hu, X.; Zhang, H. J.; Zhang, Y. W.; Zhang, Y. D.; Chen, X. L.; Shi, Z. F.; Gong, X.; Chen, Y.; Zhang, H. L. Understanding the Halogenation Effects in Diketopyrrolopyrrole-Based Small Molecule Photovoltaics. ACS Appl. Mater. Interfaces 2015, 7 (36), 19914−19922. (19) Hu, H.; Jiang, K.; Yang, G.; Liu, J.; Li, Z.; Lin, H.; Liu, Y.; Zhao, J.; Zhang, J.; Huang, F.; Qu, Y.; Ma, W.; Yan, H. Terthiophene-based D-A Polymer with an Asymmetric Arrangement of Alkyl Chains that Enables Efficient Polymer Solar Cells. J. Am. Chem. Soc. 2015, 137 (44), 14149−14157. (20) Zheng, Y.-Q.; Wang, Z.; Dou, J.-H.; Zhang, S.-D.; Luo, X.-Y.; Yao, Z.-F.; Wang, J.-Y.; Pei, J. Effect of Halogenation in IsoindigoBased Polymers on the Phase Separation and Molecular Orientation of Bulk Heterojunction Solar Cells. Macromolecules 2015, 48 (16), 5570−5577. (21) Li, C. H.; Huang, C. H.; Kuo, M. Y. Halogenated 6,13Bis(triisopropylsilylethynyl)-5,7,12,14-tetraazapentacene: Applications for Ambipolar Air-Stable Organic Field-Effect Transistors. Phys. Chem. Chem. Phys. 2011, 13 (23), 11148−11155. (22) Tang, M. L.; Bao, Z. Halogenated Materials as Organic Semiconductors. Chem. Mater. 2011, 23 (3), 446−455. (23) Ashraf, R. S.; Meager, I.; Nikolka, M.; Kirkus, M.; Planells, M.; Schroeder, B. C.; Holliday, S.; Hurhangee, M.; Nielsen, C. B.; Sirringhaus, H.; McCulloch, I. Chalcogenophene Comonomer Comparison in Small Band gap Diketopyrrolopyrrole-based Conjugated Polymers for High-Performing Field-Effect Transistors and Organic Solar Cells. J. Am. Chem. Soc. 2015, 137 (3), 1314−1321. (24) Zhao, Z.; Yin, Z.; Chen, H.; Zheng, L.; Zhu, C.; Zhang, L.; Tan, S.; Wang, H.; Guo, Y.; Tang, Q.; Liu, Y. High-Performance, Air-Stable Field-Effect Transistors Based on Heteroatom-Substituted Naphthalenediimide-Benzothiadiazole Copolymers Exhibiting Ultrahigh Electron Mobility up to 8.5 cm V(−1) s(−1). Adv. Mater. 2017, 29 (4), 1602410. (25) Patra, A.; Bendikov, M. Polyselenophenes. J. Mater. Chem. 2010, 20 (3), 422−433. (26) Saadeh, H. A.; Lu, L.; He, F.; Bullock, J. E.; Wang, W.; Carsten, B.; Yu, L. Polyselenopheno[3,4-b]selenophene for Highly Efficient Bulk Heterojunction Solar Cells. ACS Macro Lett. 2012, 1 (3), 361− 365. (27) Dong, T.; Lv, L.; Feng, L.; Xia, Y.; Deng, W.; Ye, P.; Yang, B.; Ding, S.; Facchetti, A.; Dong, H.; Huang, H. Noncovalent Se···O Conformational Locks for Constructing High-Performing Optoelectronic Conjugated Polymers. Adv. Mater. 2017, 29 (34), 1606025. (28) Cao, F.-Y.; Tseng, C.-C.; Lin, F.-Y.; Chen, Y.; Yan, H.; Cheng, Y.-J. Selenophene-Incorporated Quaterchalcogenophene-Based Donor−Acceptor Copolymers to Achieve Efficient Solar Cells with Jsc Exceeding 20 mA/cm2. Chem. Mater. 2017, 29 (23), 10045− 10052. (29) Dou, L.; Chang, W. H.; Gao, J.; Chen, C. C.; You, J.; Yang, Y. A Selenium-Substituted Low-Bandgap Polymer with Versatile Photovoltaic Applications. Adv. Mater. 2013, 25 (6), 825. (30) Hu, Z.; Chen, H.; Qu, J.; Zhong, X.; Chao, P.; Xie, M.; Lu, W.; Liu, A.; Tian, L.; Su, Y.-A.; Chen, W.; He, F. Design and Synthesis of Chlorinated Benzothiadiazole-Based Polymers for Efficient Solar Energy Conversion. ACS Energy Lett. 2017, 2 (4), 753−758. (31) Hu, Z.; Chen, H.; Zhong, X.; Qu, J.; Chen, W.; Liu, A.; He, F. Design and Synthesis of Halogen Atom Substituted Benzothiadazolebased Donor Polymers for Efficient Solar Energy Conversion. Acta Polym. Sin. 2018, 2, 273−283. (32) Wu, Y.; An, C.; Shi, L.; Yang, L.; Qin, Y.; Liang, N.; He, C.; Wang, Z.; Hou, J. The Crucial Role of Chlorinated Thiophene Orientation in Conjugated Polymers for Photovoltaic Devices. Angew. Chem., Int. Ed. 2018, 57 (39), 12911−12915.

(33) Zhang, S.; Qin, Y.; Zhu, J.; Hou, J. Over 14% Efficiency in Polymer Solar Cells Enabled by a Chlorinated Polymer Donor. Adv. Mater. 2018, 30 (20), No. 1800868. (34) Yang, Z.; Chen, H.; Wang, H.; Mo, D.; Liu, L.; Chao, P.; Zhu, Y.; Liu, C.; Chen, W.; He, F. The Integrated Adjustment of Chlorine Substitution and Two-Dimensional Side Chain of Low Band gap Polymers in Organic Solar Cells. Polym. Chem. 2018, 9 (8), 940−947. (35) Chao, P.; Wang, H.; Qu, S.; Mo, D.; Meng, H.; Chen, W.; He, F. From Semi- to Full-Two-Dimensional Conjugated Side-Chain Design: A Way toward Comprehensive Solar Energy Absorption. Macromolecules 2017, 50 (24), 9617−9625. (36) Chao, P.; Wang, H.; Mo, D.; Meng, H.; Chen, W.; He, F. Synergistic Effects of Chlorination and a Fully Two-Dimensional Side-Chain Design on Molecular Energy Level Modulation toward Non-fullerene Photovoltaics. J. Mater. Chem. A 2018, 6 (7), 2942− 2951. (37) Tang, M. l.; Oh, J. H.; Reichardt, A. D.; Bao, Z. Chlorination: A General Route toward Electron Transport in Organic Semiconductors. J. Am. Chem. Soc. 2009, 131 (10), 3733−3740. (38) Chen, H.; Hu, Z.; Wang, H.; Liu, L.; Chao, P.; Qu, J.; Chen, W.; Liu, A.; He, F. A Chlorinated π-Conjugated Polymer Donor for Efficient Organic Solar Cells. Joule 2018, 2 (8), 1623−1634. (39) Qu, S.; Wang, H.; Mo, D.; Chao, P.; Yang, Z.; Li, L.; Tian, L.; Chen, W.; He, F. Fine Tuning of Open-Circuit Voltage by Chlorination in Thieno[3,4-b]thiophene−Benzodithiophene Terpolymers toward Enhanced Solar Energy Conversion. Macromolecules 2017, 50 (13), 4962−4971. (40) Wang, H.; Chao, P.; Chen, H.; Mu, Z.; Chen, W.; He, F. Simultaneous Increase in Open-Circuit Voltage and Efficiency of Fullerene-Free Solar Cells through Chlorinated Thieno[3,4-b]thiophene Polymer Donor. ACS Energy Lett. 2017, 2 (9), 1971−1977. (41) Mo, D.; Wang, H.; Chen, H.; Qu, S.; Chao, P.; Yang, Z.; Tian, L.; Su, Y.-A.; Gao, Y.; Yang, B.; Chen, W.; He, F. Chlorination of Low-Band-Gap Polymers: Toward High-Performance Polymer Solar Cells. Chem. Mater. 2017, 29 (7), 2819−2830. (42) Ye, L.; Hu, H.; Ghasemi, M.; Wang, T.; Collins, B. A.; Kim, J. H.; Jiang, K.; Carpenter, J. H.; Li, H.; Li, Z.; McAfee, T.; Zhao, J.; Chen, X.; Lai, J. L. Y.; Ma, T.; Bredas, J. L.; Yan, H.; Ade, H. Quantitative Relations between Interaction Parameter, Miscibility and Function in Organic Solar Cells. Nat. Mater. 2018, 17 (3), 253−260. (43) Liu, F.; Zhao, W.; Tumbleston, J. R.; Wang, C.; Gu, Y.; Wang, D.; Briseno, A. L.; Ade, H.; Russell, T. P. Understanding the Morphology of PTB7:PCBM Blends in Organic Photovoltaics. Adv. Energy Mater. 2014, 4 (5), 1301377. (44) Smilgies, D.-M. Scherrer Grain-Size Analysis Adapted to Grazing-Incidence Scattering with Area Detectors. J. Appl. Crystallogr. 2009, 42 (6), 1030−1034. (45) Jiang, Z. GIXSGUI: a MATLAB toolbox for grazing-incidence X-ray scattering data visualization and reduction, and indexing of buried three-dimensional periodic nanostructured films. J. Appl. Crystallogr. 2015, 48 (3), 917−926. (46) Page, K. A.; Kusoglu, A.; Stafford, C. M.; Kim, S.; Kline, R. J.; Weber, A. Z. Confinement-Driven Increase in Ionomer Thin-Film Modulus. Nano Lett. 2014, 14 (5), 2299−2304. (47) Johnston, D. E.; Yager, K. G.; Hlaing, H.; Lu, X.; Ocko, B. M.; Black, C. T. Nanostructured Surfaces Frustrate Polymer Semiconductor Molecular Orientation. ACS Nano 2014, 8 (1), 243−249. (48) Chen, L.; Dong, G.; Duan, L.; Wang, L.; Qiao, J.; Zhang, D.; Qiu, Y. One Order of Magnitude Enhancement of Electron Mobility by Rapid Cooling the Melt of an n-Type Organic Semiconductor. J. Phys. Chem. C 2009, 113 (37), 16549−16552. (49) Borsenberger, P. M.; Pautmeier, L.; Bassler, H. Charge Transport in Disordered Molecular Solids. J. Chem. Phys. 1991, 94 (8), 5447−5454.

I

DOI: 10.1021/acs.macromol.8b02445 Macromolecules XXXX, XXX, XXX−XXX