Donor–acceptor (D–A) conjugated copolymers containing fused-ring acceptor units demonstrate outstanding performance in organic solar cells (OSCs)^[1-13]. We have invented highly efficient D–A copolymer donors D18 and D18-Cl by using a fused-ring acceptor unit, dithieno[3',2':3,4;2'',3'':5,6]benzo[1,2-c][1,2,5]thiadiazole (DTBT)^{[1, 2]}. OSCs with D18 or D18-Cl gave power conversion efficiencies (PCEs) of 18.56% and 18.69%, respectively^{[3, 4]}. Side chain engineering is an effective approach to improve the performance of conjugated polymers in optoelectronic devices^[14-16]. The alkyl side chains not only determine polymers’ solubility, but also influence their crystallinity and mobility. In this work, we develop two efficient donors D18-B and D18-Cl-B via side chain engineering on D18 polymers (Fig. 1(a)). These donors offer PCEs up to 18.74% (certified 18.2%) in ternary OSCs.






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Figure1.
(Color online) (a) Chemical structures. (b) J–V curves for D18-B:N3:PC₆₁BM and D18-Cl-B:N3:PC₆₁BM solar cells. (c) EQE spectra for D18-B:N3:PC₆₁BM and D18-Cl-B:N3:PC₆₁BM solar cells.




The structural difference between D18 and D18-B (or D18-Cl vs D18-Cl-B) polymers is the alkyl chains on thiophene bridge units. For D18 and D18-Cl, the side chain is 2-butyloctyl, and for D18-B and D18-Cl-B, the side chain is 3-butylnonyl. The synthesis details for D18-B and D18-Cl-B are given in the Supporting Information. To figure out the influence of molecular weight to photovoltaic performance of polymers, we prepared D18-B and D18-Cl-B samples with high, moderate and low molecular weights. Different molecular weights were achieved by adjusting the ratio between the donor and acceptor monomers during the polymerization. The well-studied D18-B and D18-Cl-B present moderate number-average molecular weights (M_n) of 47.2 and 60.6 kDa, respectively, with polydispersity indexes (PDI) of 1.89 and 1.95, respectively. D18-B and D18-Cl-B show similar absorption spectra in either solution or film, indicating that F or Cl substitution has negligible influence to the optical properties of the polymers (Fig. S7). The 0–1 transition peak intensifies in both polymer films, suggesting the H-aggregation in solid state^{[17, 18]}. The optical bandgaps for D18-B and D18-Cl-B are 1.97 and 1.98 eV, respectively. Energy levels for D18-B and D18-Cl-B were estimated by cyclic voltammetry (CV) (Fig. S8). The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels are –5.51 and –2.71 eV for D18-B, and –5.56 and –2.68 eV for D18-Cl-B, respectively. The hole mobilities (μ_h) from space-charge limited current (SCLC) measurements are 8.64 × 10^–4 and 6.93 × 10^–4 cm²/(V·s) for D18-B and D18-Cl-B, respectively (Fig. S9). Ternary solar cells with D18-B or D18-Cl-B as the donor and N3^[19]/PC₆₁BM as the acceptors were made. Device fabrication conditions were optimized (Tables S1–S8). The best D18-B:N3:PC₆₁BM (1 : 1.4 : 0.2) cells gave a PCE of 18.53%, with an open-circuit voltage (V_oc) of 0.823 V, a short-circuit current density (J_sc) of 28.50 mA/cm² and a fill factor (FF) of 79.0%. The best D18-Cl-B:N3:PC₆₁BM (1 : 1.4 : 0.2) cells offered a PCE of 18.74%, with a V_oc of 0.836 V, a J_sc of 28.50 mA/cm² and a FF of 78.7% (Fig. 1(b), Table 1). D18-Cl-B:N3:PC₆₁BM cells afforded the highest external quantum efficiency (EQE) of 90% at 550 nm (Fig. 1(c)). The addition of PC₆₁BM enhanced J_sc and FF for both D18-B and D18-Cl-B ternary cells (Tables S2 and S5), suggesting that fullerene balances charge transport in the devices^{[20, 21]}. The best D18-Cl-B devices were also measured at the National Institute of Metrology (NIM), and a certified PCE of 18.2% (V_oc, 0.835 V; J_sc, 27.64 mA/cm²; FF, 78.9%; effective area, 2.580 mm²) was recorded (Fig. S10). The active layer morphology was investigated by atomic force microscope (AFM). Both D18-B:N3:PC₆₁BM and D18-Cl-B:N3:PC₆₁BM blend films present typical nano-structures (Fig. S11). We also tested the performance of low-M_n and high-M_n D18-B or D18-Cl-B in ternary solar cells (Table 1). Low-M_n and high-M_n D18-B deliver 17.69% and 17.36% PCEs, respectively, while low-M_n and high-M_n D18-Cl-B give 17.87% and 17.39% PCEs, respectively. Optimizing the molecular weight of polymers is important for achieving the optimal performance.

Donor	Mn (kDa)	PDI	Voc (V)	Jsc (mA/cm2)	FF (%)	PCE (%)
D18-BL	33.0	1.84	0.825	27.31	78.6	17.69 (17.52)a
D18-BM	47.2	1.89	0.823	28.50	79.0	18.53 (18.40)
D18-BH	57.0	1.95	0.810	27.94	76.7	17.36 (17.18)
D18-Cl-BL	38.1	1.98	0.832	27.68	77.6	17.87 (17.52)
D18-Cl-BM	60.6	1.95	0.836	28.50	78.7	18.74 (18.52)
D18-Cl-BH	68.6	2.06	0.821	27.45	77.2	17.39 (17.22)
a Data in parentheses stand for the average PCEs for 10 cells.

Table1.
Performance data for D18-B:N3:PC₆₁BM (1 : 1.4 : 0.2) and D18-Cl-B:N3:PC₆₁BM (1 : 1.4 : 0.2) solar cells.



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Donor	Mn (kDa)	PDI	Voc (V)	Jsc (mA/cm2)	FF (%)	PCE (%)
D18-BL	33.0	1.84	0.825	27.31	78.6	17.69 (17.52)a
D18-BM	47.2	1.89	0.823	28.50	79.0	18.53 (18.40)
D18-BH	57.0	1.95	0.810	27.94	76.7	17.36 (17.18)
D18-Cl-BL	38.1	1.98	0.832	27.68	77.6	17.87 (17.52)
D18-Cl-BM	60.6	1.95	0.836	28.50	78.7	18.74 (18.52)
D18-Cl-BH	68.6	2.06	0.821	27.45	77.2	17.39 (17.22)
a Data in parentheses stand for the average PCEs for 10 cells.

In short, we create two polymer donors D18-B and D18-Cl-B via side chain engineering on D18 polymers. PCEs of 18.53% and 18.74% were achieved, respectively, demonstrating their potential in organic solar cells.

Acknowledgements

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

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1088/1674-4926/42/10/100501.