Enhancing the Performance of Polymer Solar Cells by Using Donor

Research Article www.acsami.org

Enhancing the Performance of Polymer Solar Cells by Using Donor Polymers Carrying Discretely Distributed Side Chains Xue Gong,† Guangwu Li,† Yang Wu,‡ Jicheng Zhang,† Shiyu Feng,† Yahui Liu,† Cuihong Li,*,† Wei Ma,‡ and Zhishan Bo*,† †

Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, China ‡ State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, No. 28 Xianning West Road, Xi’an 710049, China S Supporting Information *

ABSTRACT: Conjugated polymers with three components, P1-1 and P1-2, were prepared by one-pot Stille polymerization. The two-component polymer P1-0 is only composed of a 5fluoro-6-alkyloxybenzothiadiazole (AFBT) acceptor unit and a thiophene donor unit, while the three-component polymers P1-1 and P1-2 contain 10% and 20% 5,6-difluorobenzothiadiazole (DFBT), respectively, as the third component. The incorporation of the third component, 5,6-difluorobenzothiadiazole, makes the side chains discretely distributed in the polymer backbones, which can enhance the π−π stacking of polymers in film, markedly increase the hole mobility of active layers, and improve the power-conversion efficiency (PCE) of devices. Influence of the third component on the morphology of active layer was also studied by X-ray diffraction (XRD), resonant soft X-ray scattering (R-SoXS), and transmission electron microscopy (TEM) experiments. P1-1/PC71BM-based PSCs gave a high PCE up to 7.25%, whereas similarly fabricated devices for P1-0/PC71BM only showed a PCE of 3.46%. The PCE of P1-1/PC71BM-based device was further enhanced to 8.79% after the use of 1,8-diiodooctane (DIO) as the solvent additive. Most importantly, after the incorporation of 10% 5,6-difluorobenzothiadiazole unit, P1-1 exhibited a marked tolerance to the blend film thickness. Devices with a thickness of 265 nm still showed a PCE above 8%, indicating that P1-1 is promising for future applications. KEYWORDS: solar cells, side chains, three-component conjugated polymers, power-conversion efficiency, film thickness, morphology

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INTRODUCTION

develop a new strategy to design conjugated polymers that can form close packing without losing their solubility and changing their energy levels. For most donor−acceptor-type conjugated polymers, the flexible side chains are homogeneously located at the polymer main chain.30−36 The design and synthesis of terpolymers for some systems was proven to be efficient in challenging the balance of solubility and photovoltaic properties.37 In addition, synthetic workload of this type of polymers will be reduced rapidly, and favorable properties arise.38 By the introduction of a third component, the side chains will be discretely distributed in the polymer main chain,39 which will be expected to enhance the “cross-talking” between polymer chains, leading to improved hole-transport ability and device performance.

Polymer solar cells (PSCs) with distinctive features of being cost-effective, lightweight, and flexible and having roll-to-roll processability have attracted considerable research interest in recent years.1−3 By materials design,4−7 interfacial modification,8−12 and device optimization,11,13,14 power-conversion efficiency (PCE) of PSCs has been recently boosted to above 11%.15−20 As for polymer donors, beside the polymer backbone, the side chains are also critical to their photovoltaic performance.21−26 To acquire high-performance PSCs, it is crucial to design conjugated polymers with suitable energy level, hole-transport ability, and solution processability. The energy levels of polymer donors are mainly determined by the main-chain structures,6,23,27 whereas the polymer solubility and their phase-separation behavior when blended with acceptors are greatly affected by the side chains.28,29 Higher hole mobility requires that conjugated polymers could form a close π−π stacking in film. However, strong π−π interaction usually leads to poor solubility for the polymers. To solve the dilemma, we © 2017 American Chemical Society

Received: March 28, 2017 Accepted: June 20, 2017 Published: June 20, 2017 24020

DOI: 10.1021/acsami.7b04369 ACS Appl. Mater. Interfaces 2017, 9, 24020−24026

Research Article

ACS Applied Materials & Interfaces Scheme 1. Synthetic Route of Co-polymers

Figure 1. UV−visible absorption of co-polymers P1-0 (a), P1-1 (b), and P1-2 (c) in solutions at 25 and 100 °C and as films, respectively.

thermal properties of P1-0, P1-1, and P1-2 were shown in the Supporting Information. Optical Properties. UV−vis absorption spectroscopy was used to reveal the optical properties of polymers. The corresponding absorption curves of P1-0, P1-1, and P1-2 in dilute DCB solutions (at 25 and 100 °C) and as films are presented in Figure 1. The detailed optical data are summed up in Table 1. As shown in Figure 1a, P1-0 in dilute DCB solution

To verify the feasibility of this idea, we synthesized a kind of three-component conjugated polymers consisting of thiophene, 5-fluoro-6-alkyloxybenzothiadiazole, and 5,6-difluorobenzothiadiazole. Polymers containing 0%, 10%, and 20% 5,6difluorobenzothiadiazole units are synthesized and denoted as P1-0, P1-1, and P1-2, respectively. Owing to the limited solubility of P1-2 in 1,2-dichlorobenzene (DCB), it is hard to fabricate devices with a reasonable thickness of active layer. As expected, by incorporating 10% and 20% 5,6-difluorobenzothiodiazole units to the polymer main chain, the π−π stacking distances of polymers P1-1 and P1-2 became closer in comparison to P1-0. The hole mobility of polymer/PC71BMbased devices tested by space charge limited current (SCLC) method was raised from 9 × 10−4 cm2 V−1 s−1 for P1-0 to 7 × 10−3 cm2 V−1 s−1 for P1-1. PSCs with the blends of P1-1/ PC71BM as the active layer exhibited a PCE of 7.25%, whereas P1-0/PC71BM-based PSCs using the same fabrication condition only showed a PCE of 3.46%. The PCE of P1-1/PC71BM-based device was further enhanced to 8.79% after using DIO as the solvent additive. Most importantly, after introducing a 10% 5,6difluorobenzothiadiazole unit, P1-1 exhibited marked insensitivity to the film thickness. The PCE can still be above 8% for devices with an active layer thickness of 265 nm, making P1-1 a promising donor material for future practical application.

Table 1. Optical and Electrochemical Properties of P1-0, P11, and P1-2 polymer

λmax (nm)a

λmax (nm)b

λedge (nm)b

Eg,opt (eV)c

HOMO (eV)

LUMO (eV)

P1-0 P1-1 P1-2

582 582 582

622, 671 622, 671 621, 671

718 719 720

1.73 1.73 1.73

−5.56 −5.66 −5.74

−3.83 −3.93 −4.01

DCB solution at 100 °C. bAs films. cCalculated on the basis of Eg,opt = 1240/λedge.

a

at 25 °C exhibited two absorption peaks located at 611 and 653 nm; after the temperature of the solution was increased to about 100 °C, P1-0 only exhibited a broad absorption peaked at 583 nm, indicating that increasing the temperature increases the molecular rotation to dissipate intermolecular interactions. The above result also indicated that strong intermolecular interactions existed even in dilute solution at 25 °C. P1-0 as film showed a broader absorption spectrum with an onset of 718 nm. According to the equation: Eg,opt = 1240/λonset, an optical band gap of 1.73 eV was obtained for P1-0. Additionally, P1-1 and P1-2 exhibited similar absorption behavior to that of P1-0. Electrochemical Properties. Cyclic voltammetry (CV) was used to survey the electrochemical behaviors of these polymers. As shown in Figure 2, the onset oxidation potentials of polymer films for P1-0, P1-1, and P1-2 are 0.85, 0.95, and 1.03 V, respectively. Using the absolute energy level of 4.8 eV for Fc/Fc+ in vacuum, HOMO levels of P1-0, P1-1, and P1-2 were calculated using the equation EHOMO = −e[Eox + 4.80

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RESULTS AND DISCUSSION Material Synthesis and Characterization. The syntheses of polymers are outlined in Scheme 1; M129 and M340 were synthesized according to the reported procedures. Polymers were prepared by Stille reaction with catalyst of Pd(PPh3)4 and solvent of toluene/DMF at 110 °C. P1-0, P1-1, and P1-2 are fully soluble in hot 1,2-dichlorobenzene. Because the volume of side chains greatly influence the morphology of active layer and the performance of PSCs, P1-0 with smaller 2-butyloctyl side chains was also synthesized for comparison. 2-Butyloctyl substituted P1-0 exhibited a poor solubility in DCB at 100 °C (less than 5 mg/mL), and it was difficult to fabricate thin blend films for device applications. Molecular weights and 24021


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ACS Applied Materials & Interfaces

Photovoltaic Properties. To shed light on the photovoltaic performance of the co-polymers, solar cells were fabricated. First, the weight ratio of donor to acceptor (D/A) was optimized, and a weight ratio of 1:1 showed the best device performance for P1-0 and P1-1. Next, DIO and 1-CN as additives were tried to further enhance PCE of devices. For P10, devices prepared from DCB solution provided a PCE of 3.46% with a Voc of 0.82 V, a Jsc of 8.50 mA cm−2, and an FF (fill factor) of 0.50. When DIO (4%, by volume) was employed as the processing additive, PCE was largely enhanced to 7.25% with a Voc of 0.80 V, a Jsc of 13.38 mA cm−2, and an FF of 0.68. The increasing of PCE mainly originated from the improved Jsc and FF. For P1-1, devices fabricated from DCB solution gave a PCE of 7.12% with a Voc of 0.83 V, a Jsc of 13.42 mA cm−2, and an FF of 0.65. When DIO (1%, by volume) was employed as the processing additive, the highest PCE of 8.38% was obtained with a Voc of 0.82 V, a Jsc of 14.39 mA cm−2, and an FF of 0.71. It was worth noting that the real content of 5,6difluorobenzothiodiazole might be higher than 10% due to the purification procedure of polymer P1-1, the chloroform fraction of P1-1 might have a higher content of 5,6difluorobenzothiodiazole (see the Supporting Information). Due to the very limited solubility of P1-2 (less than 5 mg/mL at 100 °C in DCB), it is hard to form visually qualified films for P1-2 and PC71BM. The devices fabricated from P1-2 and PC71BM can afford a PCE of only 5.69%. The average PCEs were calculated from 10 devices. All of the photovoltaic characteristics are summarized in Table 2. After the preliminary optimization of the D-to-A ratio and volume of additives, P1-0- and P1-1-based solar cells were subjected to further optimization by changing the blend film thickness. It is worth mentioning that P1-1 exhibited certain toleration to the thickness of active layer in contrast to P1-0, which is much more sensitive to the blend film thickness. The J−V characteristics of related devices are shown in Figure 4a, and the detailed data were summarized in Table 3. For P1-0, when the active-layer thickness was raised to about 172 nm, PCE was decreased to be 6.73%, whereas, after the active layer thickness for P1-1/PC71BM-based devices were increased to be 180 nm, the best PCE of 8.79% was achieved, as well as a Jsc of 15.98 mA cm−2, a Voc of 0.81 V, and an FF of 0.68. Further raising the thickness of active layer to 265 nm, the Jsc increased to 16.20 mA cm−2, but FF dropped to 0.62, resulting in a slightly decreased PCE of 8.02%. After increasing the active layer thickness to 371 nm, a PCE of 6.81% was still obtained. To further study the photovoltaic behavior of P1-1, holetransport properties of blend films based on P1-0 and P1-1 with PC71BM were measured, and the data are summarized in Table 3. The hole mobility of P1-1/PC71BM-blend films were determined to be 9 × 10−4 cm2 V−1 s−1, which is nearly 10 times higher than that of P1-0-based blend films (7 × 10−3 cm2 V−1 s−1). Higher hole mobility is favorable to the charge

Figure 2. Cyclic voltammograms of P1-0, P1-1, and P1-2 films.

−E(Fc/Fc+)] and were found to be −5.56, −5.66, and −5.74 eV, respectively; the data are also displayed in Table 1. The LUMO levels of P1-0, P1-1, and P1-2 were −3.83, −3.93, and −4.01 eV, respectively, determined by the equation of ELUMO = EHOMO + Eg,opt. By the incorporation of 10% and 20% 5,6difluorobenzothiodiazole units to the polymer main chain, the HOMO and LUMO levels of P1-1 (or P1-2) were lower than those of P1-0. The Voc values of P1-1 and P1-2 devices are expected to be higher than that of P1-0-based devices. X-ray Diffraction Patterns. Powder X-ray diffraction (XRD) analysis was applied to study the packing behavior of polymers in film. As shown in Figure 3, two obvious diffraction

Figure 3. Powder XRD patterns of co-polymers.

peaks were observed in the powder XRD curves for all the polymer samples. The first peak in the small angle region, which reflexes the packing distance of polymer backbones isolated by the flexible side chains, appeared at 2θ of 4.27 o for P1-0, 4.49 o for P1-1, and 4.69 o for P1-2 with the corresponding distance of 20.71, 19.67, and 18.86 Å, respectively. The peak in the wide-angle region attributed to the π−π stacking of polymer backbones is situated at 2θ of 24.57° for P1-0, 24.98° for P1-1, and 25.19° for P1-2, corresponding to a distance of 3.62, 3.57, and 3.54 Å, respectively. By the incorporation of 10% and 20% 5,6difluorobenzothiodiazole units to the polymer main chain, the π−π stacking distances of polymers P1-1 and P1-2 became closer in comparison to P1-0.

Table 2. Photovoltaic Performances of Polymer/PC71BM Blend Film D-to-A ratio

solvents

Jsc (mA cm−2)

Voc (V)

FF

P1-0

1:1

P1-1

1:1

P1-2a,b

1:1

DCB DCB + 4% DIO DCB DCB + 1% DIO DCB

8.50 13.38 13.42 14.39 11.11

0.82 0.80 0.83 0.82 0.84

0.50 0.68 0.65 0.71 0.61

polymer

a

PCE 3.46% 7.25% 7.12% 8.38% 5.69%

(3.02%) (7.04%) (6.78%) (8.16%) (5.11%)

thickness (nm) 74 80 75 85 90

fabricated from its not fully dissolved solution with poor morphology. bsubstrate needs to be preheated at 100 °C. 24022


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Figure 4. (a) J−V characteristics and (b) EQE curves of P1-0/PC71BM devices spin-coated from DCB solutions (containing 4% DIO, by volume) and P1-1/PC71BM devices spin-coated from DCB solutions (containing 1% DIO by volume) in different thicknesses of the active layer.

Table 3. Effect of the Film Thickness on the Photovoltaic Performances for P1-0/PC71BM- and P1-1/PC71BM-Blend Films, with 10 Devices Used for the Calculation of the Average PCE polymer

thickness (nm)

P1-0

80 172 85 137 185 265 371

P1-1

solvents DCB DCB DCB DCB DCB DCB DCB

+ + + + + + +

4% 4% 1% 1% 1% 1% 1%

DIO DIO DIO DIO DIO DIO DIO

Jsc (mA cm−2)

Voc (V)

FF

PCE (average)

13.38 14.86 14.39 15.13 15.98 16.20 15.41

0.80 0.80 0.82 0.81 0.81 0.81 0.81

0.68 0.52 0.71 0.70 0.68 0.62 0.55

7.25% 6.73% 8.38% 8.50% 8.79% 8.02% 6.81%

(7.12%) (6.44%) (8.16%) (8.33%) (8.65%) (7.61%) (6.21%)

SCLC mobility (cm2 V−1 s−1) 9 × 10−4 7 × 10−3

film of P1-0 and PC71BM, which were unfavorable for the transport and separation of excitons. After DIO (4%, by volume) was added as the processing additive, the blend films became much smoother without obvious large-scale phase separation. The blend film of P1-1 and PC71BM exhibited much-better film morphology than that of P1-0-based blend film, but still, some large-size domains could be observed, which is probably formed due to the aggregation of polymers; after the addition of DIO (1%, by volume), the blend films became smoother and homogeneous, and fibril networks could be observed. Clearly, the enhancement of photovoltaic performance after the addition of DIO can be well-interpreted by the evolution of film morphology. The influence of 5,6-difluorobenzothiodiazole unit on the packing of polymer chains was also studied by grazingincidence wide-angle X-ray scattering (GIWAXS). As shown in Figure 6, the out-of-plane (OOP) profiles of P1-0/PC71BM exhibit a sharp (010) diffraction peak at 1.74 Å −1 , demonstrating the π−π stacking distance of P1-0 is 3.61 Å. The (100) peak in the OOP direction of P1-0/PC71BM is not very clear, whereas in-plane (IP) profiles of P1-0/PC71BM displays a sharp (100) diffraction peak at 0.30 Å−1 and (200) peak and at 0.60 Å−1, revealing the lamellar stacking between the lateral alkoxy chains. For the OOP profiles of P1-1/ PC71BM-based blend films, the (010), (100), and (200) peaks were located in 0.32, 0.64, and 1.76 Å−1, respectively, corresponding to a closer π−π stacking and the lamellar stacking distance of 3.57 and 19.63 Å, respectively. The diffraction peak at 1.4 Å−1 could be attributed to the aggregation of PC71BM. After the incorporation of 20% 5,6difluorobenzothiodiazole to the polymer backbones, the π−π stacking and lamellar distance of P1-2/PC71BM was further decreased to 3.55 and 18.92 Å, respectively, consist with the results of powder XRD experiments. In addition, the strength of a π−π stacking peak of P1-1/PC71BM was larger than that of P1-2/PC71BM, revealing that introducing too much fluorine substitutions would reduce the crystallinity of polymers. For

separation and transport. As shown in Figure 4b, the external quantum efficiencies (EQEs) of P1-0- and P1-1-based devices were measured and used to check of the accuracy of Jsc values obtained from J−V curves. The deviation between the Jsc value determined from the J−V curve and the one calculated by integrating EQE curve with the solar spectrum (AM 1.5G) is within 5%, indicating that the Jsc values obtained from J−V curves are believable. Transmission electron microscopy (TEM) was used to explore the morphology of P1-0/PC71BM and P1-1/PC71BM blend films. Considering that the PC71BM domains have higher electron-scattering density than polymer domains, the dark and white domains were therefore attributed to the PC71BM and conjugated polymer domains, respectively. As shown in Figure 5a, the spherical black domains can be observed for the blend

Figure 5. TEM images of (a) P1-0-based blend layers without additive, (b) P1-0-based blend layers with additive, (c) P1-1-based blend layers without additive, and (d) P1-1-based blend layers with additive; the scale bar was 200 nm. 24023


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Figure 6. GIWAXS profiles for the blend films of P1-0/PC71BM, P1-1/PC71BM, and P1-2/PC71BM.

Figure 7. R-SoXS profile in log scale for the blend films of P1-0/PC71BM, P1-1/PC71BM, and P1-2/PC71BM.

P1-1/PC71BM-based blend films, the (010) peak in OOP direction was markedly sharper than that in IP direction, revealing that P1-1 has a preferential face-on orientation, whereas P1-2 was mostly oriented in an edge-on direction, resulting in an inferior photovoltaic performance in devices. Resonant soft X-ray scattering (R-SoXS) experiments were also employed to further investigate the phase separation of polymer and PC71BM in P1-0-, P1-1-, and P1-2-based optimized active layers. As shown in Figure 7, a resonant energy of 284.2 eV was selected to enhance contrast between polymer and PC71BM in the blend films. The peak position represents characteristic domain spacing of two components and the length value was evaluated by the equation ofξ = 2π/ qpeak.41 The peak at q = 0.15 nm−1 for P1-0/PC71BM suggested that the domain spacing of P1-0 and PC71BM was 41.88 nm, which is bigger than effective exciton diffusion length, leading to a relatively low performance in devices. After the conjugation

of 10% 5,6-difluorobenzothiodiazole to the backbone of polymer, the domain spacing of P1-1-based devices was reduced to a mode distribution of 34.91 nm, resulting in a higher PCE, whereas further increasing the content of 5,6difluorobenzothiodiazole would decrease the solubility of P1-2 and lead to a big ξ value of 70.02 nm when blended with PC71BM; an inferior PCE was therefore achieved. The relative average domain purity of polymer/PC71BM could also be revealed by the integration of scattering intensity (TSI) over the length scales. In comparison to the P1-0/PC71BM-based active layer, the P1-1/PC71BM-based one also possessed a higher TSI value, implying the domain purity could also be enhanced by introducing more fluorine substitution. Because purer domains are beneficial to the suppression of bimolecular recombination, higher Jsc and the resulting PCE were obtained for P1-1/PC71BM-based devices. 24024


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ACS Applied Materials & Interfaces ORCID

From the polymer structure of polymer P1-0 and P1-1, we can also find some clues for the unique properties of P1-1, as shown in Figure 8, P1-0 only has a thiophene spacer that could

Cuihong Li: 0000-0002-4071-6359 Zhishan Bo: 0000-0003-0126-7957 Author Contributions

G.L. and X.G. contributed equally to this work. Notes

The authors declare no competing financial interest.

pack with other molecules due to the bulky alkoxyl side chain, but P1-1 has a much-larger spacer with 5,6-difluorobenzothiodiazole and two linked thiophene units; this larger spacer allows more molecular stacking in the solid state, which is supported by XRD results. The hole mobilities of P1-1 were therefore higher than P1-0 due to the closer molecular stacking.

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REFERENCES

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CONCLUSIONS In this article, two-component polymer P1-0 and threecomponent polymers P1-1 and P1-2 were synthesized as the donor materials for polymer solar cells. The branched alkoxyl chain on benzothiadiazole unit endows polymer with good solubility for solution processing, while the incorporation of the third component, a 5,6-difluorobenzothiadiazole unit, into the polymer main chain can significantly enhance the π−π interaction at some sites on the polymer main chain and induces the polymer/PC71BM blend films with much-higher hole mobility and the devices with much-higher PCE. By the incorporation of a 10% 5,6-difluorobenzothiadiazole unit into the polymer main chain, the hole mobility of polymer/PC71BM blend films increased from 9 × 10−4 cm2 V−1 s−1for P1-0 to 7 × 10−3 cm2 V−1 s−1 for P1-1. PSCs with the P1-1/PC71BM blend as the active layer gave the highest PCE of 8.79%. Most importantly, after the incorporation of 10% 5,6-difluorobenzothiadiazole unit into the polymer main chain, the devices exhibited a considerable tolerance to the thickness of active layer, whereas for the two-component polymer P1-0, the devices are easily affected by the blend film thickness. After the investigation of the packing of polymers in the solid state, we concluded that the higher hole mobility and efficiency for P1-1based devices could be attributed to the closer packing of P1-1 than P1-0 in the blend films. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04369. Additional details on polymer solar cell fabrication and characterization, hole mobilities of blend film measured by SCLC, materials and instruments, and detailed synthesis of polymers. Figures showing TGA curves and NMR spectra. A table showing molecular weights and thermal properties of the co-polymers. (PDF)

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ACKNOWLEDGMENTS

We thank the financial support from the NSF of China (grant no. 21574013).

Figure 8. Illustration of repeat units in polymers P1-0 and P1-1.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 24025


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Enhancing the Performance of Polymer Solar Cells by Using Donor

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