Xianyi Meng , Ke Jin , Zuo Xiao , and Liming Ding

1Center for Excellencein Nanoscience (CAS), Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS), National Center for Nanoscience and Technology,Beijing100190, China

2University of ChineseAcademy of Sciences,Beijing100049, China

Donor–acceptor (D–A) conjugated copolymers containing fused-ring acceptor units demonstrate outstanding performanceinorganic solarcells (OSCs)[1−13]. We haveinvented 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 approachtoimprove the performance of conjugated polymers in optoelectronic devices[14−16]. The alkyl side chains not only determine polymers’solubility, butalsoinfluence theircrystallinity andmobility.In this work, wedeveloptwo efficientdonors D18-B and D18-Cl-Bviaside chain engineering on D18 polymers (Fig. 1(a)).These donors offer PCEs upto18.74% (certified 18.2%) in ternary OSCs.

The structural difference between D18 and D18-B (or D18-ClvsD18-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 andD18-Cl-B, the side chainis 3-butylnonyl. The synthesis details forD18-B and D18-Cl-B are given in theSupporting Information. To figureout theinfluence 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 wereachievedbyadjustingtheratiobetweenthe donor and acceptor monomersduringthe polymerization. Thewellstudied D18-B and D18-Cl-B presentmoderate numberaverage molecular weights (Mn) 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 theoptical properties of the polymers (Fig. S7). The 0–1 transitionpeak 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 occupiedmolecular orbital(HOMO) and the lowestunoccupied molecularorbital(LUMO)levels are–5.51 and–2.71eVfor D18-B,and –5.56 and –2.68 eV for D18-Cl-B, respectively. Theholemobilities(μh) fromspace-charge limited current (SCLC)measurementsare 8.64 × 10–4and6.93×10–4cm2/(V·s)for D18-Band D18-Cl-B, respectively (Fig. S9).Ternarysolar cellswith D18-B orD18-Cl-B asthe donorand N3[19]/PC61BM asthe acceptors were made. Device fabricationconditionswere optimized (Tables S1–S8). Thebest D18-B:N3:PC61BM (1 :1.4 : 0.2) cellsgave a PCE of 18.53%, with an open-circuitvoltage(Voc)of0.823 V, ashort-circuit current density (Jsc) of28.50mA/cm2anda fill factor(FF) of79.0%.ThebestD18-Cl-B:N3:PC61BM (1 : 1.4 :0.2)cellsoffereda PCE of18.74%, with aVocof0.836 V, aJscof 28.50 mA/cm2and a FFof 78.7%(Fig.1(b),Table 1). D18-Cl-B:N3:PC61BMcellsafforded the highestexternal quantumefficiency (EQE) of 90%at 550 nm (Fig. 1(c)). TheadditionofPC61BMenhancedJscand FF for both D18-B and D18-Cl-Bternarycells (Tables S2 and S5),suggesting thatfullerene balancescharge transport in thedevices[20,21]. The best D18-Cl-Bdevices were alsomeasuredat theNational Institute ofMetrology(NIM),and a certifiedPCEof 18.2% (Voc, 0.835 V;Jsc,27.64 mA/cm2; FF,78.9%; effective area, 2.580 mm2)was recorded(Fig. S10). The active layer morphology was investigatedby atomic force microscope(AFM).Both D18-B:N3:PC61BMand D18-Cl-B:N3:PC61BM blend films presenttypical nano-structures (Fig.S11).Wealso tested theperformanceof low-Mnand high-MnD18-Bor D18-Cl-B in ternarysolar cells (Table1).Low-Mnandhigh-MnD18-B deliver 17.69% and 17.36% PCEs, respectively,while low-Mnand high-MnD18-Cl-B give17.87%and 17.39% PCEs,respectively. Optimizing the molecular weight of polymers is important for achieving theoptimal performance.

Table 1. 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.

Fig. 1. (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.

In short, we create two polymer donors D18-B and D18-Cl-Bviaside 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 NaturalScience Foundation of China (51773045, 21772030, 51922032 and21961160720)for financial support.

Appendix A. Supplementary data

Supplementary datatothisarticlecan befound online at https://doi.org/10.1088/1674-4926/42/10/100501.