A large-bandgap copolymer donor for efficient ternary organic solar cells

Yue Luo ab, Xiujuan Chen bc, Zuo Xiao *b, Shengjian Liu *a, Meizhen Yin *c and Liming Ding *b
aSchool of Chemistry, South China Normal University, Guangzhou 510006, China. E-mail: shengjian.liu@m.scnu.edu.cn
bCenter for Excellence in Nanoscience (CAS), Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS), National Center for Nanoscience and Technology, Beijing 100190, China. E-mail: xiaoz@nanoctr.cn; ding@nanoctr.cn
cSchool of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China. E-mail: yinmz@mail.buct.edu.cn

Received 7th June 2021 , Accepted 6th July 2021

First published on 6th July 2021


A 2.18 eV bandgap copolymer donor C1 based on a fused-ring unit phenanthro[9,10-c][1,2,5]thiadiazole (PT) was developed for 2D1A ternary organic solar cells. Incorporating a small amount of C1 into the L1:N3 blend deepened the HOMO of donor side, enhanced light absorption from N3, balanced hole/electron transport and optimized film mophogolgy, leading to an improved power conversion efficiency of 16.32%.

The rapid development of active layer materials has boosted the power conversion efficiency (PCE) to 18% for organic solar cells (OSCs).1–9 The active layer of OSCs is usually a binary blend film composed of an electron donor and an electron acceptor. The donor and acceptor undergo phase separation to form “bulk heterojunction (BHJ)” morphology, providing sufficient interfaces for exciton dissociation and pathways for charge transport.10,11 In recent years, ternary cells, containing three materials in the active layer, have emerged as an efficient type of OSCs.12 Ternary cells have shown great potential in terms of enhancing light-absorption, reducing energy loss, improving charge carrier mobilities and optimizing film morphology.13–16 According to the composition of the active layer, ternary cells can be divided into two types, the one donor and two acceptors (1D2A) type and the two donors and one acceptor (2D1A) type.17–19 The emerging low-bandgap nonfullerene acceptors (NFAs) have promoted the rapid development of 1D2A-type ternary cells.20,21 Especially, 1D2A cells based on a NFA and a traditional fullerene acceptor have integrated the strong light-harvesting advantage of NFA and the high electron-mobility advantage of fullerene, thus affording high PCEs.22–25 Very recently, Ding et al. reported 1D2A cells based on a copolymer donor D18-Cl, a NFA N3 and a fullerene acceptor PC61BM, delivering a record PCE of 18.69% (certified 18.1%).26 On the other hand, the performance of 2D1A-type cells lagged behind.27–29 To improve the performance of 2D1A cells, we think that developing efficient large-bandgap donors would be a promising direction. Large-bandgap donors have deep the highest occupied molecular orbital (HOMO) energy levels and strong absorption at the short-wavelength region. These properties could enhance the open-circuit voltage (Voc) and the external quantum efficiency (EQE) at short wavelengthes,30–32 and making them a good component for 2D1A cells. In this work, we designed a large-bandgap copolymer donor C1 (Fig. 1) by using a fused-ring acceptor unit phenanthro[9,10-c][1,2,5]thiadiazole (PT). Thanks to the strong aromatic benzene moieties and the strong electron-withdrawing properties of PT, the donor C1 has a large bandgap of 2.18 eV and a deep HOMO level of −5.57 eV. By introducing a small amount of C1 into a host binary blend L1:N3, we improved the PCE from 15.22% to 16.32%.
image file: d1qm00835h-f1.tif
Fig. 1 Chemical structures of C1, L1 and N3.

The synthetic route for C1 is shown in Scheme S1 and the details are provided in the ESI. 5,10-Dibromophenanthro[9,10-c][1,2,5]thiadiazole (PT-Br)33 coupled with tributyl(4-(2-butyloctyl)thiophen-2-yl)stannane to give compound 1 in 33% yield. Bromination of compound 1 with N-bromosuccinimide gave the monomer compound 2 in 68% yield. Finally, C1 was obtained in 79% yield via a Stille copolymerization of compound 2 and (4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(trimethylstannane). The number-average molecular weight (Mn) for C1 is 66.9 kDa, and the polydispersity index (PDI) is 1.72. In solution, C1 shows an absorption band at 400–570 nm, with a low-energy peak at 538 nm and a high-energy peak at 503 nm (Fig. 2a). The low-energy peak is stronger than the high-energy peak. In film, the two peaks shift to 532 nm and 499 nm, respectively, and the high-energy peak becomes stronger. The blue shift and the intensification of the high-energy peak suggest H-aggregation of C1 in solid state.34 The absorption onset of C1 film is 570 nm, corresponding to an optical bandgap (Egopt) of 2.18 eV. The absorption spectra for the copolymer donor L135 and the NFA N336 are also shown in Fig. 2a. The absorption spectra for C1, L1 and N3 are complementary. This suggests that the three materials may constitute an effective light-harvesting blend. The HOMO and the lowest unoccupied molecular orbital (LUMO) energy levels of C1 were estimated from cyclic voltammetry (CV) measurements (Fig. S6, ESI). An energy level diagram is shown in Fig. 2b. C1 has a HOMO of −5.57 eV, which is deeper than that of L1, and a LUMO of −2.70 eV, which is shallower than that of L1. The deep HOMO of C1 benefits the open-circuit voltage (Voc) in solar cells. The space charge limited current (SCLC) measurement37–42 indicates a hole mobility (μh) of 6.44 × 10−4 cm2 V−1 s−1 for pure C1 film (Fig. S8, ESI). The good μh of C1 is due to the planar PT unit, which favors polymer packing and hole transport.

image file: d1qm00835h-f2.tif
Fig. 2 (a) Absorption spectra for C1 solution, C1 film, L1 film and N3 film. (b) Energy level diagram.

Next, we investigated the performance of C1 in OSCs. The device structure is ITO/PEDOT:PSS/L1:C1:N3 (D1:D2:A)/PDIN/Ag. The (D1 + D2):A ratio was fixed at 1[thin space (1/6-em)]:[thin space (1/6-em)]1.4, and the content of C1 in donors was gradually increased from 0 wt% to 100 wt%. The J–V curves and the photovoltaic parameters are shown in Fig. 3a and Table 1, respectively. The host L1:N3 (1[thin space (1/6-em)]:[thin space (1/6-em)]1.4) binary cells gave a PCE of 15.22%, with a Voc of 0.780 V, a short-circuit current density (Jsc) of 26.81 mA cm−2 and a fill factor (FF) of 72.75%. Incorporating a small amount of C1 in donors improved device performance. With 20 wt% C1 in donors, the L1:C1:N3 (0.8[thin space (1/6-em)]:[thin space (1/6-em)]0.2[thin space (1/6-em)]:[thin space (1/6-em)]1.4) ternary cells gave the best PCE of 16.32%, with simultaneously enhanced Voc, Jsc and FF as compared to the L1:N3 cells. Further increasing the content of C1 led to higher Voc but lower Jsc and FF. The C1:N3 (1[thin space (1/6-em)]:[thin space (1/6-em)]1.4) binary cells gave a PCE of 13.24%, with a Voc of 0.860 V (Table S1–S3, ESI). As shown in Fig. 3b, Voc increases linearly with C1 content in donors, suggesting that L1 and C1 could form a polymer alloy.43–45 Since C1 has a deeper HOMO than L1, increasing the content of C1 deepens the HOMO of the alloy and enhances Voc. The external quantum efficiency (EQE) spectra for L1:N3 (1[thin space (1/6-em)]:[thin space (1/6-em)]1.4), L1:C1:N3 (0.8[thin space (1/6-em)]:[thin space (1/6-em)]0.2[thin space (1/6-em)]:[thin space (1/6-em)]1.4) and C1:N3 (1[thin space (1/6-em)]:[thin space (1/6-em)]1.4) cells are shown in Fig. 3c. The integrated photocurrent densities from EQE spectra confirmed the highest Jsc from L1:C1:N3 (0.8[thin space (1/6-em)]:[thin space (1/6-em)]0.2[thin space (1/6-em)]:[thin space (1/6-em)]1.4) ternary cells (Table 1). Interestingly, despite of strong absorption at 400–570 nm, C1 does not help to boost EQE response at this region for ternary cells. Instead, it largely enhances the EQE at 750–900 nm which comes from the absorption of N3. To understand this, we measured the absorption spectra for the three blend films (Fig. 3d). With 20 wt% C1 in donors, the L1:C1:N3 ternary blend film shows an absorbance increment at NIR region as compared to L1:N3 film, indicating that C1 can improve the light absorption from N3. In contrast, the absorbance at 400–570 nm does not change much. This explains the EQE enhancement at NIR region for the ternary cells. To understand why the ternary cells gave optimal FF, we investigated charge transport and recombination in the active layers. SCLC measurements (Fig. S9–S10, Table S4, ESI) indicate a μh of 4.10 × 10−4 cm2 V−1 s−1 and an electron mobility (μe) of 4.82 × 10−4 cm2 V−1 s−1 for L1:N3 binary blend film. With 20 wt% C1 in donors, μh increased to 4.16 × 10−4 cm2 V−1 s−1 and μe decreased to 4.32 × 10−4 cm2 V−1 s−1, leading to more balanced hole/electron transport (μh/μe = 0.96) in the ternary blend. Balanced charge transport should account for high FF. The bimolecular recombination was studied by plotting Jsc against the light intensity.46–51 The α values for L1:N3, L1:C1:N3 (0.8[thin space (1/6-em)]:[thin space (1/6-em)]0.2[thin space (1/6-em)]:[thin space (1/6-em)]1.4) and C1:N3 cells are 0.981, 0.982 and 0.973, respectively (Fig. S11, ESI). This indicates that although more bimolecular recombination existing in C1:N3 cells, the incorporation of a small amount of C1 into L1:N3 blend does not increase bimolecular recombination. To figure out whether there is energy transfer from C1 to L1, we carried out photoluminescence (PL) measurements (Fig. S12, ESI). Compared with pure L1 film with a PL peak at 645 nm, the L1:C1 (0.8[thin space (1/6-em)]:[thin space (1/6-em)]0.2) blend film shows weaker PL at 645 nm, suggesting that there is no obvious energy transfer.52,53 We further fabricated single-component C1 cell and L1 cell, and L1:C1 (0.8[thin space (1/6-em)]:[thin space (1/6-em)]0.2) binary cell to find out whether there is photoinduced charge transfer between C1 and L1 (Fig. S13, ESI).54 The Jsc for L1:C1 (0.8[thin space (1/6-em)]:[thin space (1/6-em)]0.2) cell is between that for C1 cell and L1 cell, suggesting negligible charge transfer between C1 and L1.

image file: d1qm00835h-f3.tif
Fig. 3 (a) J–V curves. (b) The effect of C1 content on Voc. (c) EQE spectra. (d) Absorption spectra for blend films.
Table 1 Device performance changing with C1 content in donors
C1 in donors [wt%] V oc [V] J sc [mA cm−2] FF [%] PCE [%]
a Data in parentheses are integrated current densities from EQE spectra. b Data in parentheses are averages for 8 cells.
0 0.780 26.81 (25.94)a 72.75 15.22 (15.13 ± 0.08)b
20 0.797 27.55 (26.18) 74.38 16.32 (16.20 ± 0.09)
40 0.811 26.56 (25.66) 71.90 15.49 (15.35 ± 0.19)
60 0.825 25.24 (24.57) 71.87 14.96 (14.78 ± 0.19)
80 0.841 24.32 (23.12) 68.30 13.97 (13.67 ± 0.26)
100 0.860 22.44 (21.49) 68.58 13.24 (13.19 ± 0.07)

The morphology for L1:N3 (1[thin space (1/6-em)]:[thin space (1/6-em)]1.4), L1:C1:N3 (0.8[thin space (1/6-em)]:[thin space (1/6-em)]0.2[thin space (1/6-em)]:[thin space (1/6-em)]1.4) and C1:N3 (1[thin space (1/6-em)]:[thin space (1/6-em)]1.4) blend films was studied by using atomic force microscope (AFM) (Fig. 4). The phase images present nanofiber structures. The diameters for the nanofibers in above films are ∼27 nm, ∼18 nm, and ∼8 nm, respectively. The height images indicate the coarsest surface of L1:N3 film. The root-mean-square roughnesses are 2.06 nm, 1.93 nm and 1.79 nm for L1:N3, L1:C1:N3 and C1:N3 films, respectively. The above results suggest that C1 might have higher miscibility with N3 than L1. To verify this hypothesis, we measured the surface free energy (γ) of L1, C1 and N3 by carrying out the contact angle experiments with water and ethylene glycol as the liquids. The γ was determined by using Owens–Wendt model.55–57 The γ for L1, C1 and N3 are 18.27 mJ m−2, 17.18 mJ m−2 and 16.42 mJ m−2, respectively (Table S5, ESI). The miscibility can be quantified by the Flory–Huggins interaction parameter (χij).58,59 χij is proportional to image file: d1qm00835h-t1.tif, where γi and γj are the surface free energies of the two interacting components.60 A smaller χ means higher miscibility. Owing to the closer γ values of C1 and N3, χC1–N3 should be lower than χL1–N3, suggesting the higher miscibility between C1 and N3. This explains the weaker phase separation in C1:N3 film. The addition of C1 into L1:N3 blend could increase the miscibility between donor and acceptor, yielding an optimal phase separation to achieve efficient charge generation and fast charge transport.

image file: d1qm00835h-f4.tif
Fig. 4 AFM height (left) and phase (right) images for the blend films. (a and b) L1:N3 (1[thin space (1/6-em)]:[thin space (1/6-em)]1.4) film; (c and d) L1:C1:N3 (0.8[thin space (1/6-em)]:[thin space (1/6-em)]0.2[thin space (1/6-em)]:[thin space (1/6-em)]1.4) film; (e and f) C1:N3 (1[thin space (1/6-em)]:[thin space (1/6-em)]1.4) film.


A new copolymer donor C1 was developed for 2D1A-type ternary OSCs. C1 possesses a large bandgap of 2.18 eV, a deep HOMO of −5.57 eV and a good hole mobility of 6.44 × 10−4 cm2 V−1 s−1. By adding a small amount of C1 into the L1:N3 blend, the Voc, Jsc and FF were simultaneously enhanced and the PCE was improved to 16.32%. Developing large-bandgap donors will be a promising direction for highly efficient 2D1A OSCs.

Conflicts of interest

There are no conflicts to declare.


We thank the National Key Research and Development Program of China (2017YFA0206600) and the National Natural Science Foundation of China (51773045, 21772030, 51922032, 21961160720 and 21805097).

Notes and references

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Electronic supplementary information (ESI) available. See DOI: 10.1039/d1qm00835h
These authors contributed equally to this work.

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