Introducing Fluorine and Sulfur Atoms into Quinoxaline-Based p-type

Jun 5, 2019 - Introducing Fluorine and Sulfur Atoms into Quinoxaline-Based ... Gradually Improve the Performance of Fullerene-Free Organic Solar Cells...
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Letter Cite This: ACS Macro Lett. 2019, 8, 743−748

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Introducing Fluorine and Sulfur Atoms into Quinoxaline-Based p‑type Polymers To Gradually Improve the Performance of Fullerene-Free Organic Solar Cells Jing Yang,†,‡ Peiqing Cong,§ Lie Chen,§ Xiaochen Wang,† Jianfeng Li,† Ailing Tang,† Bao Zhang,∥ Yanfang Geng,† and Erjun Zhou*,†,‡,∥

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†

CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China ‡ Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China § College of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China ∥ Henan Institutes of Advanced Technology, Zhengzhou University, Zhengzhou 450003, China S Supporting Information *

ABSTRACT: Three quinoxaline-based “D−π−A” conjugated polymers, named as PE61, PE62, and PE63, are utilized to investigate the effect of introducing fluorine and sulfur atoms into the thiophene side chains on the photovoltaic performance when paired with a nonfullerene Y6. The open-circuit voltage (VOC) and power conversion efficiency (PCE) can be improved from 0.66 V and 8.61% for PE61:Y6 to 0.78 V and 12.02% for PE62:Y6, and then to 0.83 V and 13.10% for PE63:Y6, respectively. The results provide a simple and effective strategy to fine-tune the optoelectronic properties and thus improve the photovoltaic performance.

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acceptor, a Qx-based polymer can realize the improvement of the power conversion efficiency (PCE) from an initial 0.5% in 200618 to 7.5% in 201219 and then to 9.2% in 2018.20 In addition, a Qx-based polymer can also pair with an n-type polymer to fabricate all-polymer solar cells, and the PCE increased from 4.1%21 in 2014 to 7.1%22 in 2017. Although a large improvement has been achieved, the PCEs are still relatively low, which can be attributed to the large band gap, low crystallinity, and inferior hole mobility of Qx-based polymers. With the emergence of 1,1-dicyanomethylene-3-indanone (IC)-based nonfullerene small molecular acceptors (NFSMAs) in 2015,23 the photovoltaic performance of PSC has been largely improved with the highest PCE beyond 15% in this year by using a new acceptor of Y6.24 In 2017, Hou et al. employed three Qx-based conjugated polymers named as PBQ-0F, PBQQF, and PBQ-4F to blend with ITIC and achieved PCEs of 6.7%, 8.9%, and 11.3%, respectively.25 In 2018, Li et al. synthesized a low cost Qx-based polymer PTQ10, which could achieve a high PCE of 12.7% when blending with IDIC.26 Obviously, Qx-based polymer can match well with NFSMAs, but the photovoltaic performance still lagged behind the other

rganic solar cells (OSCs) have achieved rapid development in recent years due to the exploration of novel photovoltaic materials and the optimization of device fabrication methods.1−4 The active layer of a typical OSC consists of a p-type material as the electron donor and an ntype semiconductor as the electron acceptor.5,6 Due to the great efforts of researchers, many effective molecule design rules have been developed to modify the physical and chemical properties of the photovoltaic materials. For example, the donor−acceptor (D-A) alternating structure, composed of an electron-donating unit (D) and an electron-accepting unit (A), is proved to be an efficient strategy to construct high-efficient photovoltaic materials. Among the large amount of electrondonating units, benzodithiophene (BDT) is the most widely used one and contributes to many high performance p-type polymers such as PBDB-T7 and PTB7-Th.8 On the other hand, benzothiadiazole (BT)9−12 and benzotriazole (BTA)13−15 are classic electron-deficient units to construct many promising photovoltaic polymers and small molecules. Besides classic BT and BTA, quinoxaline (Qx) is also a kind of important electron-deficient building block to construct photovoltaic materials, including both electron donors16 and electron acceptors.17 As a p-type photovoltaic material, Qxbased polymers were first synthesized in 2006,18 and then they were strongly investigated and promoted the rapid progress of polymer solar cells (PSCs). When blending with a fullerene © 2019 American Chemical Society

Received: May 14, 2019 Accepted: May 31, 2019 Published: June 5, 2019 743

DOI: 10.1021/acsmacrolett.9b00368 ACS Macro Lett. 2019, 8, 743−748

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Figure 1. Chemical structures of PE61, PE62, PE63, and Y6.

Figure 2. Calculated molecular geometries and frontier molecular orbitals for the dimer model compounds of PE61, PE62, and PE63.

To find out the influence of introducing F atoms and S atoms on the geometries and electronic properties of the polymers, theoretical calculations were carried out by using density functional theory (DFT) with Gaussian 09 at the B3LYP/6-31G (d) level. The dimers of the repeat units with isobutyl as the side chains were used as the model compounds for simplicity in the calculation. Figure 2 displays the optimized molecular geometries and the frontier molecular orbitals. Three polymers have similar geometries with HOMO and the lowest unoccupied molecular orbital (LUMO) electrons delocalized throughout the backbone. With the introduction of the F atoms and S atoms, both the HOMO and LUMO energy levels of the three polymers were deepened. The calculated HOMO levels were −4.73, −4.84, and −4.94 eV, and the LUMO levels were −2.46, −2.55, and −2.64 eV for PE61, PE62, and PE63, respectively. The results indicate that gradually introducing F and S atoms could effectively decrease the HOMO energy levels of BDT-based polymers, which could contribute to the high open-circuit voltage (VOC). This result agrees well with our previous work, which introduced F and S atoms into BTA-based polymers.30 The optical properties of PE61, PE62, PE63, and Y6 in thin films were investigated by UV−vis-NIR spectroscopy. As shown in Figure 3a, all three donor polymers exhibited broad

polymers based on benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD)27 or pyrrolo[3,4-f ]benzotriazole-5,7-dione (TzBI).28 In this paper, we also chose thienyl-substituted benzodithiophene (BDT), thiophene, and fluorine-substituted Qx as the D part, π bridge, and A part, respectively, to obtain three “D−π−A” conjugated polymers. Besides introducing fluorine (F) atom to the 4-position of thiophene side chain as reported,25 we further added sulfur (S) atom to the 5-position of thiophene, which was effective to downshift the highest occupied molecular orbital (HOMO) energy levels for BDD29 or BTA-based polymers.30 The final polymers, renamed as PE61, PE62, and PE63, were in conjunction with the emerging NFSMA of Y624,31,32 to investigate the effect of F and S atoms in the side chain on the photovoltaic performance of these polymers. Figure 1 displays the chemical structures of PE61, PE62, PE63, and NFSMA of Y6. The detailed synthesis procedure of the conjugated polymers is described in Scheme S1. All three polymers have a comparable weight-average molecular weight (Mw) of 54.1−66.2 kDa and polydispersity index (PDI) of 2.9−3.2. All the polymers showed good solubility in commonly used organic solvents such as chlorobenzene (CB), odichlorobenzene (ODCB), and chloroform (CF). 744

DOI: 10.1021/acsmacrolett.9b00368 ACS Macro Lett. 2019, 8, 743−748

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Figure 3. (a) UV−vis-NIR absorption spectra of the three polymers and Y6 in a thin film. (b) Energy level diagrams of the polymers measured by CV. (c) J−V characteristics of the optimized devices based on polymer: Y6. (d) EQE spectra of the corresponding devices.

Table 1. Photovoltaic Performance Parameters of the Optimized OSCs Based on Polymer:Y6 D:A PE61:Y6 PE62:Y6 PE63:Y6

VOC (V) 0.66 0.78 0.83

JSC (mA cm−2) 23.41 24.64 24.68

μe (cm2 V−1 s−1)

FF (%)

PCE (%)

55.30 62.22 63.74

8.61 (8.44 ± 0.24) 12.02 (11.84 ± 0.25)a 13.10 (13.01 ± 0.13)a a

−4

1.40 × 10 2.56 × 10−4 3.15 × 10−4

μh (cm2 V−1 s−1) 1.18 × 10−4 1.76 × 10−4 1.96 × 10−4

a

Average value and standard deviation calculated from eight cells.

oxidation and reduction potential. The CV curves are shown in Figure S1 (Supporting Information) and the energy levels diagrams of the polymers and Y6 were depicted in Figure 3b. When introducing the F atoms and S atoms into the polymer, the HOMO and LUMO energy levels of the polymers gradually downshifted, from −5.19 and −3.44 eV for PE61 to −5.37 and −3.48 eV for PE62, then to −5.41 and −3.50 eV for PE63. The trend in HOMO and LUMO agrees well with the DFT calculation results. The HOMO and LUMO energy levels of Y6 were −5.65 and −4.10 eV as reported.24 It is noted that the deeper HOMO energy level of the polymer donor relative to the acceptor was beneficial to improve the VOC of the devices. To investigate the effect of the structural modification of the donor polymers on the photovoltaic performance, OSCs based on PE61, PE62, and PE63 were fabricated. Y6 was selected as the acceptor due to its complementary absorption spectrum,

and strong absorption in the range of 300−700 nm, which was well complementary with the absorption of the Y6 film. PE61 showed a maximum absorption peak (λmax) at 600 nm with a slight shoulder peak in the longer wavelength region. When introducing the F atoms and S atoms into the thiophene side chain, the λmax shifted to 584 nm for PE62 and 580 nm for PE63 with a more distinct shoulder peak, which indicates that ordered aggregation existed in the polymer films to some extent.33 From PE61, PE62, to PE63, a slightly blue-shifted absorption can also be seen from the absorption onset (λonset) of PE61 (714 nm), PE62 (704 nm), and PE63 (692 nm), corresponding to the optical band gap (Egapopt) of 1.74, 1.76, and 1.79 eV, respectively. Furthermore, the electrochemical properties were also measured by cyclic voltammetry (CV) to investigate the energy level alignments of the polymers and Y6. The HOMO and LUMO energy levels were estimated from the onset of the 745

DOI: 10.1021/acsmacrolett.9b00368 ACS Macro Lett. 2019, 8, 743−748

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Figure 4. J−V curves of the (a) hole-only and (b) electron-only devices.

Figure 5. AFM height images (5 μm × 5 μm) and TEM images of active layers based on (a,d) PE61:Y6, (b,e) PE62:Y6, and (c,f) PE63:Y6.

matched energy levels and excellent performance in OSCs.24 The device structure is ITO/PEDOT:PSS/active layer/ PDINO/Al. The optimized donor/acceptor weight ratio (D/ A) was 1:1.2, and chloroform was used as processing solvent with 0.5% diphenyl ether (DPE) as the additive. The detailed photovoltaic data are summarized in Table S1. Figure 3c shows the current density−voltage (J−V) curves of the best devices based on PE61 (PE62 and PE63):Y6, and the corresponding photovoltaic parameters are listed in Table 1. The PE61:Y6 based device obtained a PCE of 8.61%, with a VOC of 0.66 V, JSC of 23.41 mA cm−2, and fill factor (FF) of 55.30%. When

introducing the F atoms into the thiophene side chain, PE62 based device exhibited a higher VOC of 0.78 V, JSC of 24.64 mA cm−2, and FF of 62.22%, affording a higher PCE of 12.02%. When further adding S atoms into the thiophene side chain, PE63 based device showed an increased VOC of 0.83 V, a JSC of 24.68 mA cm−2 and FF of 63.74%, achieving the highest PCE of 13.10%. This result is different from our previous work. In our previous work, the introduction of F and S atoms in J52 obtained a small ΔEHOMO value of 0.02 eV between the donor and acceptor, which limited the charge separation and obtain a low PCE.30 However, in this work, Y6 has low-lying HOMO 746

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In summary, three Qx-based p-type polymers PE61, PE62, and PE63 were utilized to investigate the influence of introducing F atoms and S atoms into the thiophene side chain on the photovoltaic performance. After optimized device fabrication, PE61:Y6 based OSC showed PCE of 8.61% with a lower VOC of 0.66 V. When introducing F atoms to the thiophene, PE62:Y6 based device obtained a PCE of 12.02% with an increased VOC of 0.78 V. Further adding S atoms to the thiophene side chain, an improved VOC of 0.83 V, JSC of 24.68 mA cm−2, and FF of 63.74% was achieved, affording the highest PCE of 13.10%, which is also the highest value for solar cells based on Qx-based polymers. The gradually improved photovoltaic performance of fullerene-free OSCs based on PE61, PE62, and PE63 was attributed to the fine-tuned energy levels, increased charge carrier mobilities, and optimized morphology of the active layers. Our results indicate that introducing F atoms and S atoms into the side chains was an effective method to tune the energy levels and improve the photovoltaic performance of the p-type polymers.

and LUMO energy levels. Although the energy levels of the polymer were downshifted after introducing F and S atoms, the difference of energy levels between donor and acceptor is still large enough to provide a driving force for charge separation. As shown in Figure 3d, all three devices exhibited broad and strong external quantum efficiency (EQE) spectra in the region of 400−900 nm, which indicated that effective charge separation occurred between the donor and acceptor. The integrated JSC calculated from EQE spectra were 22.74, 23.38, and 23.32 mA cm−2 for PE61, PE62, and PE63 based devices, respectively, which agree well with the JSC values measured from J-V curves. To better understand the different photovoltaic performances, we further characterized the charge transport properties of the optimized devices using the space-charge limited current (SCLC) method. The device structures were ITO/PEDOT:PSS/active layer/Au for holes and ITO/TiOx/active layer/Al for electrons. Figure 4 shows the J−V characteristics of the hole-only and electron-only devices, and the values of electron (μe) and hole mobilities (μh) are summarized in Table 1. Both μe and μh increased from 1.40 × 10−4 and 1.18 × 10−4 cm2 V−1 s−1 for PE61, 2.56 × 10−4 and 1.76 × 10−4 cm2 V−1 s−1 for PE62 to 3.15 × 10−4, and 1.96 × 10−4 cm2 V−1 s−1 for PE63. The higher charge carrier mobilities for PE62 and PE63 based devices resulted in the improved FF and JSC, thus achieving the significantly increased photovoltaic performance. However, the values of FF are relatively low for all the three devices, so we measured the light intensity (I) dependence of JSC and VOC to evaluate the carrier recombination in the devices. As shown in Figure S2, the calculated slope of log(JSC) versus log(I) is 0.96 for all the three devices and the slope of VOC versus log(I) is 1.3−1.4kT/q, which indicates that both bimolecular recombination and trap-assisted monomolecular recombination are operative,34 and thus, the relatively low FF was obtained for all the three devices. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were also used to study the effect of the morphology of the active layer on the photovoltaic performance. Figure 5 displayed the AFM height images and TEM images for three blend films. The root-mean-square (RMS) roughness is 1.88, 1.85, and 1.74 nm for PE61:Y6, PE62:Y6, and PE63:Y6, respectively, suggesting that a smooth surface was formed in all three devices. Compared with the PE61:Y6 blend film, the TEM images of PE62:Y6 and PE63:Y6 showed more uniform phase separation, which might partly contribute to the improved JSC and FF in PE62 and PE63 based devices. The crystallinity of the polymer neat films and the blend films was examined by grazing incident wide-angle X-ray diffraction (GIWAXS). The 2D-GIWAXS patterns and the line cuts of inplane and out-of-plane are depicted in Figures S3 and S4. All the polymer films exhibited preferred face-on orientation, and the π−π stacking distance along the out-of-plane direction was 3.79 Å for PE61. All the polymer films exhibited preferred faceon orientation, and the π−π stacking distance along the out-ofplane direction was 3.79 Å for PE61. After introducing F atoms on the side chains, the π−π stacking distance decreased to 3.72 Å. Futher introducing S atoms, the π−π stacking distance changed to 3.74 Å. When blending with Y6, all the blend films showed similar π−π stacking distance of 3.65 Å along the out-of-plane direction. The coherence length (CL) calculated from the Scherrer equation35 is 3.69 nm for PE61 and 4.19 nm for PE62 and PE63, which explained the improved carrier mobility for PE62 and PE63.

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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/acsmacrolett.9b00368. Methods, material synthesis, fabrication and characterization of photovoltaic cells, 2D-GIWAXS results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lie Chen: 0000-0002-5288-5563 Xiaochen Wang: 0000-0001-8888-3503 Erjun Zhou: 0000-0003-1182-311X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for the support from the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (Grant No. QYZDB-SSW-SLH033), the National Natural Science Foundation of China (NSFC, Nos. 51673048, 21875052, 51673092, 21602040, 51873044), and the National Key Research and Development Program of China (2017YFA0206600).

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