Enhancing the Performance of Non-Fullerene Organic Solar Cells

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Enhancing the Performance of Non-Fullerene Organic Solar Cells Using Regioregular Wide-Bandgap Polymers Yahui Liu,† Hao Lu,‡ Miao Li,† Zhe Zhang,§ Shiyu Feng,† Xinjun Xu,*,† Youzhi Wu,‡ and Zhishan Bo*,†

Macromolecules Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 10/22/18. For personal use only.

†

Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, Beijing Normal University, Beijing 100875, P. R. China ‡ School of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, P. R. China § College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, P. R. China S Supporting Information *

ABSTRACT: To better utilize sunlight, organic solar cells (OSCs) should contain a narrow-bandgap non-fullerene acceptor blended with a wide-bandgap polymer donor as the active layer. Here we developed a regioregular widebandgap polymer (denoted as reg-PThE) as a donor material to enhance the performance of non-fullerene OSCs. Compared with the corresponding random polymer (ranPThE), reg-PThE in films showed closer packing of the polymer backbone and a larger absorption coefficient. Devices based on reg-PThE:FTIC achieved a high power conversion efficiency (PCE) of 12.07%. Conversely, devices based on ranPThE:FTIC only realized a PCE of 9.89%. When ITTC was used as the acceptor, PCEs of 11.21% and 8.38% were obtained for reg-PThE- and ran-PThE-based devices, respectively. Semitransparent OSCs with reg-PThE:ITTC as the active layer demonstrated a PCE of 8.69% and an average visible transmittance of ∼25%. Our results revealed that a regioregular polymer is better than the corresponding random polymer to maximize the PCE of non-fullerene OSCs.

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Choi et al.18 recently reported the wide-bandgap random polymer 3MT-Th, which was used in OSCs that exhibited a PCE of 9.73%. To investigate the influence of the structural order of polymer donors on the performance of non-fullerene OSCs, here we develop a similar polymer with regioregular structure (reg-PThE) and its random equivalent ran-PThE for comparison. The performance of non-fullerene OSCs with regPThE and ran-PThE as polymer donors is investigated. The absorption coefficient of a donor polymer can be improved with its regioregularity19 because of strengthened interplane interactions.20 This will lead to more efficient light absorption by an active layer with a regioregular polymer than that with a random one. The low carrier mobility and short diffusion length of excitons in organic materials limit the thickness of the active layer, leading to insufficient light absorption. This dilemma of improving the light absorption capability without increasing the photoactive layer thickness can be partially solved by using a polymer with a high absorption coefficient in the active layer. If the active layer can absorb enough sunlight in a single optical path, the thickness of the metal top electrode, which acts as a mirror to increase the

olymer solar cells (PSCs) with non-fullerene acceptors (NFAs) have been developed extensively recently. Compared with fullerene, NFAs have advantages like facile synthesis, tunable energy levels, and adjustable absorption.1,2 Non-fullerene organic solar cells (OSCs) have received widespread attention, and the power conversion efficiency (PCE) of single-junction devices has reached over 13%.3−7 Fullerene derivatives have long served as the acceptors in OSCs; accordingly, many kinds of high-efficiency conjugated polymer donor materials have been developed. However, devices containing blends of NFAs and reported polymer donors usually exhibit PCEs inferior to those with fullerene acceptors. Although numerous conjugated polymer donors have been developed, only several polymers blended with narrow-bandgap NFAs have provided high-performance devices. The polymer structural order, which can be tuned by the chain conformation, influences the generation of free charges in OSCs.8 Generally, the disordered structure is dominant in a random polymer, but in a regioregular polymer, the chains can planarize and assemble to form weakly coupled aggregates,9,10 which arrange to form closely packed lamellar structures via πstacking.11 Several regioregular polymers have been used as donor materials in fullerene-based OSCs.12−17 However, the use of a regioregular polymer in non-fullerene OSCs has not been reported. © XXXX American Chemical Society

Received: August 3, 2018 Revised: October 7, 2018

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DOI: 10.1021/acs.macromol.8b01677 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Syntheses of ran-PThE and reg-PThEa

Reagents and conditions: (i) Pd(PPh3)4, toluene/DMF, 110 °C; (ii) Pd2(dba)3, P(o-tol)3, toluene, reflux; (iii) LDA, −78 °C, CBr4.

a

Figure 1. UV−vis absorption spectra of ran-PThE and reg-PThE in (a) dilute CB solutions at room temperature and 100 °C and (b) thin films.

ance can be ignored. Ran-PThE showed good solubility in chlorobenzene (CB) and 1,2-dichlorobenzene at room temperature, whereas reg-PThE only exhibited good solubility in these solvents at elevated temperature. UV−vis absorption spectra of ran-PThE and reg-PThE in dilute CB solutions and as thin films were measured. Figure 1a shows that in dilute CB solution at room temperature regPThE exhibited broad structured absorption from 300 to 600 nm with peaks located at 520 and 565 nm. After heating to 100 °C, the absorption spectrum became narrower and blue-shifted with only one featureless peak located at 483 nm, similar to that of ran-PThE. In comparison, ran-PThE displayed similar absorption spectra at both room temperature and 100 °C with a slightly narrower absorption band from 300 to 550 nm and a peak at 483 nm. These results indicated that reg-PThE formed aggregates at room temperature that disassembled after heating to 100 °C. As for ran-PThE, no aggregation was observed in the CB solution at room temperature. The room-temperature solutions of reg-PThE and ran-PThE were red and orange, respectively (inset of Figure 1a). The absorption coefficient of reg-PThE (6.98 × 103 cm−1) was much larger than that of ranPThE (5.23 × 103 cm−1). In films, these two polymers

optical path length for light absorption, in OSCs can be decreased to give high-performance semitransparent devices. Semitransparent OSCs are of great interest for buildingintegrated photovoltaic applications and power-generating windows.21,22 Here we fabricate semitransparent PSCs using reg-PThE in the active layer. The syntheses of ran-PThE and reg-PThE are outlined in Scheme 1. ran-PThE and reg-PThE were prepared in yields of 87% and 89%, respectively, by Stille polymerization of bis(stannyl) compound 1 with dibromo monomer 226 and dibromo monomer 5, respectively. Compound 4 was synthesized with a yield of 90% by Stille cross-coupling of compound 1 and monobromo compound 3.27 The abstraction of the two hydrogen atoms at the α-position of the thiophene unit by diisopropylamine (LDA) afforded the corresponding dianions, which were subsequently quenched with CBr4 to give dibromo monomer 5 in a total yield of 62%. The measured molecular weight (Mn) and polydispersity index (PDI) were 62 kg/mol and 1.73 for ran-PThE, respectively, and 82 kg/mol and 2.72 for reg-PThE, respectively. The polymers both have similar high Mn, and ran-PThE has a lower PDI than that of reg-PThE. Therefore, the influence of Mn on device performB

DOI: 10.1021/acs.macromol.8b01677 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 2. Calculated structures of simplified linkage units.

Table 1. Photovoltaic Parameters of ran-PThE:ITTC and reg-PThE:ITTC DIO (%)

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

0.5

0.891 ± 0.003 0.899 ± 0.003 0.917 ± 0.005

15.96 ± 0.18 (15.44)a 17.35 ± 0.20 (16.94)a 18.04 ± 0.03 (17.76)a

0.589 ± 0.006 0.649 ± 0.008 0.677 ± 0.006

8.38 (8.29)b 10.14 (10.01)b 11.21 (11.07)b

ran-PThE reg-PThE a

Calculated by EQE measurements. bAverage PCE of ten devices.

Figure 3. (a) J−V and (b) EQE curves of ran-PThE:ITTC- and reg-PThE:ITTC-based devices.

polymer chains and film crystallinity can be improved. This behavior was further verified by X-ray diffraction measurements. Both polymers exhibited two diffraction peaks (Figure S3). The peak located at smaller angle reflected the distance between polymer backbones separated by alkyl side chains, and the one at higher angle represented the π−π stacking distance between polymer backbones. The calculated π−π stacking distances were 3.94 Å for ran-PThE and 3.74 Å for reg-PThE. The photovoltaic properties of these two wide-bandgap polymers were investigated in devices with a structure of ITO/ ZnO (30 nm)/active layer/MoO3 (80 Å)/Ag (100 nm). The ZnO layer was prepared according to previous literature.28 The active layer was composed of ran-PThE or reg-PThE and an NFA (ITTC). Polymer concentration, active-layer composition, spin-coating rate, and additives were systematically screened. For both polymers, the optimized weight ratio of polymer donor to acceptor was 1:1. For ran-PThE, the optimized active-layer thickness was ∼150 nm, which was deposited from a CB solution with a polymer concentration of 10 mg/mL at a spin-coating rate of 1800 rpm. For reg-PThE, the optimized active-layer thickness was ∼100 nm, which was obtained by spin-coating a hot (95 °C) CB solution with a polymer concentration of 4 mg/mL at a rate of 1600 rpm. The poor solubility of reg-PThE in CB limited the solution concentration for device fabrication. Details are described in the Supporting Information. The parameters of the optimized devices with ran-PThE and reg-PThE are presented in Table 1.

displayed the same absorption ranges but slightly different absorption features to those in solution. ran-PThE displayed a stronger absorption peak at 532 nm and a weaker one at 573 nm. reg-PThE showed absorption peaks at almost the same wavelengths, but that at 532 nm was weaker than the one at 573 nm. These phenomena indicated that reg-PThE interacted more strongly than ran-PThE in thin film (solid state).20 The optical bandgaps of ran-PThE and reg-PThE calculated from the onsets of their absorption spectra were both 1.99 eV. The energy levels of the two polymers were determined by cyclic voltammetry (Figure S2 and Table S1). The highest occupied and lowest unoccupied molecular orbitals of reg-PThE were almost the same as those of ran-PThE. The dihedral angle of simplified linkage units of the polymers was investigated via density functional theory calculations at the B3LYP/6-31G(d) level. Figure 2 depicts the possible adjacent units named BDTh-1 and BDTh-2, which have different dihedral angles. BDTh-1 with the ester group located away from the benzodithiophene unit has a small dihedral angle of 13°, whereas BDTh-2 with the ester group close to the benzodithiophene unit exhibits a larger dihedral angle of 30°. If the dihedral angle of the ester group is distributed randomly in the polymer main chain, it is difficult for the polymer to form ordered aggregates in films. For a regioregular polymer, the ester group is regularly distributed in the polymer main chain, which could lead to more ordered packing in the solid state. Therefore, the ordered π-stacking of C

DOI: 10.1021/acs.macromol.8b01677 Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. GIWAXS 2D patterns of (a) ran-PThE, (b) reg-PThE, (c) ran-PThE:ITTC, and (d) reg-PThE:ITTC and (e−h) corresponding scattering profiles in the in-plane and out-of-plane directions.

Table 2. Photovoltaic Parameters of Optimized Devices with ran-PThE:ITIC, reg-PThE:ITIC, ran-PThE:FTIC, and regPThE:FTIC donor

acceptor

ran-PThE reg-PThEa ran-PThEa reg-PThEa

ITIC ITIC FTIC FTIC

Jsc (mA/cm2)

Voc (V) 0.992 0.980 1.043 1.005

± ± ± ±

0.001 0.002 0.006 0.006

15.84 17.41 15.71 17.03

± ± ± ±

0.05 0.33 0.26 0.35

FF 0.547 0.619 0.628 0.702

± ± ± ±

PCE (%) 0.005 0.012 0.007 0.011

8.61 (8.54)b 10.57 (10.51)b 9.89 (9.47)b 12.07 (11.62)b

a

Photovoltaic performance with 0.5% DIO as additives. bAverage PCE of ten devices.

Grazing-incidence wide-angle X-ray scattering (GIWAXS) was used to investigate the molecular packing and orientation of the polymers. Generally, the pure ITTC film exhibited no obvious diffraction peaks in the out-of-plane and in-plane orientation (shown in the Supporting Information), indicating unordered molecular packing. reg-PThE showed a strong (010) π−π stacking diffraction peak in the out-of-plane direction and a (100) lamellar stacking diffraction peak (Figure 4). However, there were different behaviors for the blend films, as the (010) and (100) diffraction peaks of ran-PThE polymers weakened after blending with ITTC, indicating that ITTC decreased polymer crystallinity. Whereas in the reg-PThEbased blend film, it maintained the face-on orientation for the polymer backbones, and relatively stronger diffraction peaks are observed in comparison with ran-PThE. The polymer crystallinity in reg-PThE:ITTC blend films was higher than that in ran-PThE:ITTC ones, which helped to improve charge transport and photovoltaic performance. The hole and electron mobilities of the blend films were measured by the space charge limited current (SCLC) method using the device structures of ITO/PEDOT:PSS (30 nm)/ active layer/Au (100 nm) and ITO/ZnO/active layer (100 nm)/Al (100 nm), respectively. As shown in Figure S6 and Table S7, the calculated hole and electron mobilities of ranPThE based devices were 1.12 × 10−5 and 2.65 × 10−5 cm2 V−1 s−1 (μh/μe = 0.42), respectively. In comparison, the hole and electron mobilities of reg-PThE based devices were 5.24 × 10−5 and 5.76 × 10−5 cm2 V−1 s−1 (μh/μe = 0.91). The higher carrier mobility can explain the improved Jsc of reg-PThEbased devices compared with that of ran-PThE-based ones. Moreover, the more balanced hole and electron mobilities are

The current density−voltage (J−V) and external quantum efficiency (EQE) curves of the devices are displayed in Figure 3. ran-PThE:ITTC-based devices exhibited a PCE of 8.38% with an open-circuit voltage (Voc) of 0.89 V, short-circuit current density (Jsc) of 15.96 mA/cm2, and fill factor (FF) of 0.59. However, as shown in Table S4, the photovoltaic performance display negligible improvement after using 0.5% DIO as additives. The reg-PThE-based ones exhibited a higher PCE of 10.14% with a Voc of 0.90 V, Jsc of 17.35 mA/cm2, and FF of 0.65. The PCE was raised to 11.21% by including 0.5% 1,8-diiodoctane (DIO) as an additive. Compared with ranPThE, reg-PThE increased device PCE by 34%. EQE was measured to verify Jsc obtained from the J−V curves. Figure 3b and Table 1 show that Jsc calculated from EQE curves were within 5% of those obtained from J−V measurements. Semitransparent OSCs were obtained using a similar device structure of ITO/ZnO (30 nm)/reg-PThE:ITTC (100 nm)/ MoO3 (80 Å)/Ag with a thinner Ag electrode. The average visible transmittance (AVT; 370−740 nm) and average transmittance (AT) from 400 to 600 nm were well controlled by the thickness of the Ag electrode. J−V curves, transmission spectra, and photographs of the semitransparent devices are shown in Figure S4, and the results are also summarized in Table S3. Devices with a 10 nm thick Ag layer displayed a PCE of 7.54% with an AVT of 27.9% and AT of 28.7%. Devices with a thicker Ag layer of 20 nm exhibited a PCE of 8.69%, AVT of 24.2%, and AT of 25.5%. Considering an AVT of 25% is the benchmark for window applications,21 reg-PThE shows potential to fabricate semitransparent solar cells for use in windows that utilize solar energy. D

DOI: 10.1021/acs.macromol.8b01677 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

51673028), Program for Changjiang Scholars and Innovative Research Team in University, and the Fundamental Research Funds for the Central Universities is gratefully acknowledged.

beneficial for higher FF, which is consistent with device parameters. To test whether the performance of devices with reg-PThE could surpass that of ones with ran-PThE when other NFAs were used, ITIC and FTIC were also used as acceptors in OSCs. Table 2 reveals that ran-PThE:ITIC-based devices fabricated using CB as the processing solvent displayed a PCE of 8.61%, which is consistent with the reported value.18 In comparison, reg-PThE:ITIC based devices displayed a higher PCE of 10.57% with improved Jsc and FF. A PCE of 12.07% was achieved for reg-PThE:FTIC-based devices, which was much higher than that of ran-PThE:FTIC-based ones (9.89%). These results indicate reg-PThE is a promising wide-bandgap polymer donor for non-fullerene OSCs. See the Supporting Information for detailed photovoltaic parameters. We developed a regioregular wide-bandgap donor polymer reg-PThE, which was blended with narrow-bandgap NFAs (ITTC, ITIC and FTIC) to fabricate high-efficiency OSCs. Compared with the parent random polymer ran-PThE, regPThE exhibited a shorter π−π stacking distance because of the regularly distributed ester groups in the polymer main chain. reg-PThE films showed a larger absorption coefficient than that of ran-PThE films. The hole mobility of reg-PThE-based blend films was about 5 times higher than that of ran-PThE-based ones. OSCs with reg-PThE:ITTC as the active layer showed a PCE of 11.21%, which is much higher than the 8.38% obtained for corresponding ran-PThE-based ones. Semitransparent OSCs with reg-PThE:ITTC as the active layer exhibited a PCE exceeding 8% with an AVT near 25%. Other acceptors such as ITIC and FTIC gave similar results to those achieved using ITTC as the acceptor. reg-PThE:FTIC-based devices displayed excellent photovoltaic performance with a PCE of 12.07%. reg-PThE reveals the promise of regioregular widebandgap polymers to raise the photovoltaic performance of non-fullerene OSCs.

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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.8b01677. Experimental details, CV curves, SCLC measurements, XRD data, AFM and TEM images, and OPV fabrication and measurements (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

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

Yahui Liu: 0000-0002-3572-2308 Xinjun Xu: 0000-0002-0750-352X Zhishan Bo: 0000-0003-0126-7957 Author Contributions

Y.L. and H.L. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (91233205, 20774099, 21574013, and E

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DOI: 10.1021/acs.macromol.8b01677 Macromolecules XXXX, XXX, XXX−XXX