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This journal is © The Royal Society of Chemistry and the Chinese Chemical Society 2021 Mater. Chem. Front., 2021, 5, 6139–6144 | 6139
Cite this: Mater. Chem. Front.,
2021, 5, 6139
A large-bandgap copolymer donor for efficientternary organic solar cells†
Yue Luo,‡ab Xiujuan Chen,‡bc Zuo Xiao, *b Shengjian Liu, *a Meizhen Yin *c
and Liming Ding *b
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 thepower conversion efficiency (PCE) to 18% for organic solar cells(OSCs).1–9 The active layer of OSCs is usually a binary blend filmcomposed of an electron donor and an electron acceptor. Thedonor and acceptor undergo phase separation to form ‘‘bulkheterojunction (BHJ)’’ morphology, providing sufficient inter-faces for exciton dissociation and pathways for chargetransport.10,11 In recent years, ternary cells, containing threematerials in the active layer, have emerged as an efficient typeof OSCs.12 Ternary cells have shown great potential in terms ofenhancing light-absorption, reducing energy loss, improvingcharge carrier mobilities and optimizing film morphology.13–16
According to the composition of the active layer, ternary cellscan 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-typeternary cells.20,21 Especially, 1D2A cells based on a NFA and atraditional fullerene acceptor have integrated the strong light-harvesting advantage of NFA and the high electron-mobilityadvantage of fullerene, thus affording high PCEs.22–25 Veryrecently, Ding et al. reported 1D2A cells based on a copolymerdonor 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 laggedbehind.27–29 To improve the performance of 2D1A cells, wethink that developing efficient large-bandgap donors would bea promising direction. Large-bandgap donors have deep thehighest occupied molecular orbital (HOMO) energy levels andstrong absorption at the short-wavelength region. These prop-erties could enhance the open-circuit voltage (Voc) and theexternal quantum efficiency (EQE) at short wavelengthes,30–32
and making them a good component for 2D1A cells. Inthis 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 aromaticbenzene moieties and the strong electron-withdrawing proper-ties of PT, the donor C1 has a large bandgap of 2.18 eV and adeep HOMO level of �5.57 eV. By introducing a small amountof C1 into a host binary blend L1:N3, we improved the PCEfrom 15.22% to 16.32%.
Fig. 1 Chemical structures of C1, L1 and N3.
a School of Chemistry, South China Normal University, Guangzhou 510006, China.
E-mail: [email protected] Center for Excellence in Nanoscience (CAS), Key Laboratory of Nanosystem and
Hierarchical Fabrication (CAS), National Center for Nanoscience and Technology,
Beijing 100190, China. E-mail: [email protected], [email protected] School of Materials Science and Engineering, Beijing University of Chemical
Technology, Beijing 100029, China. E-mail: [email protected]
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1qm00835h‡ These authors contributed equally to this work.
Received 7th June 2021,Accepted 6th July 2021
DOI: 10.1039/d1qm00835h
rsc.li/frontiers-materials
MATERIALS CHEMISTRYFRONTIERS
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The synthetic route for C1 is shown in Scheme S1 and the detailsare provided in the ESI.† 5,10-Dibromophenanthro[9,10-c][1,2,5]thia-diazole (PT-Br)33 coupled with tributyl(4-(2-butyloctyl)thiophen-2-yl)stannane to give compound 1 in 33% yield. Bromination ofcompound 1 with N-bromosuccinimide gave the monomer com-pound 2 in 68% yield. Finally, C1 was obtained in 79% yield via aStille copolymerization of compound 2 and (4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)benzo[1,2-b:4,5-b 0]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. Insolution, 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 thehigh-energy peak becomes stronger. The blue shift and the intensi-fication of the high-energy peak suggest H-aggregation of C1 in solidstate.34 The absorption onset of C1 film is 570 nm, corresponding toan optical bandgap (Eg
opt) of 2.18 eV. The absorption spectra for thecopolymer donor L135 and the NFA N336 are also shown in Fig. 2a.The absorption spectra for C1, L1 and N3 are complementary. Thissuggests that the three materials may constitute an effective light-harvesting blend. The HOMO and the lowest unoccupied molecularorbital (LUMO) energy levels of C1 were estimated from cyclicvoltammetry (CV) measurements (Fig. S6, ESI†). An energy leveldiagram 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 shallowerthan 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 (mh) of6.44 � 10�4 cm2 V�1 s�1 for pure C1 film (Fig. S8, ESI†). The goodmh of C1 is due to the planar PT unit, which favors polymer packingand hole transport.
Next, we investigated the performance of C1 in OSCs. Thedevice structure is ITO/PEDOT:PSS/L1:C1:N3 (D1:D2:A)/PDIN/Ag. The (D1 + D2):A ratio was fixed at 1 : 1.4, and the content ofC1 in donors was gradually increased from 0 wt% to 100 wt%.The J–V curves and the photovoltaic parameters are shown inFig. 3a and Table 1, respectively. The host L1:N3 (1 : 1.4) binarycells gave a PCE of 15.22%, with a Voc of 0.780 V, a short-circuitcurrent density ( Jsc) of 26.81 mA cm�2 and a fill factor (FF) of72.75%. Incorporating a small amount of C1 in donorsimproved device performance. With 20 wt% C1 in donors, theL1:C1:N3 (0.8 : 0.2 : 1.4) ternary cells gave the best PCE of16.32%, with simultaneously enhanced Voc, Jsc and FF ascompared to the L1:N3 cells. Further increasing the contentof C1 led to higher Voc but lower Jsc and FF. The C1:N3 (1 : 1.4)binary cells gave a PCE of 13.24%, with a Voc of 0.860 V (TableS1–S3, ESI†). As shown in Fig. 3b, Voc increases linearly with C1content in donors, suggesting that L1 and C1 could form apolymer alloy.43–45 Since C1 has a deeper HOMO than L1,increasing the content of C1 deepens the HOMO of the alloyand enhances Voc. The external quantum efficiency (EQE)spectra for L1:N3 (1 : 1.4), L1:C1:N3 (0.8 : 0.2 : 1.4) and C1:N3(1 : 1.4) cells are shown in Fig. 3c. The integrated photocurrentdensities from EQE spectra confirmed the highest Jsc fromL1:C1:N3 (0.8 : 0.2 : 1.4) ternary cells (Table 1). Interestingly,despite of strong absorption at 400–570 nm, C1 does not helpto boost EQE response at this region for ternary cells. Instead, itlargely enhances the EQE at 750–900 nm which comes from theabsorption of N3. To understand this, we measured the absorp-tion spectra for the three blend films (Fig. 3d). With 20 wt% C1in donors, the L1:C1:N3 ternary blend film shows an absor-bance increment at NIR region as compared to L1:N3 film,indicating that C1 can improve the light absorption from N3. Incontrast, the absorbance at 400–570 nm does not change much.This explains the EQE enhancement at NIR region for theternary cells. To understand why the ternary cells gave optimalFF, we investigated charge transport and recombination in theactive layers. SCLC measurements (Fig. S9–S10, Table S4,ESI†) indicate a mh of 4.10 � 10�4 cm2 V�1 s�1 and anelectron mobility (me) of 4.82 � 10�4 cm2 V�1 s�1 for L1:N3 binary blend film. With 20 wt% C1 in donors, mh
increased to 4.16 � 10�4 cm2 V�1 s�1 and me decreased to4.32 � 10�4 cm2 V�1 s�1, leading to more balanced hole/electron transport (mh/me = 0.96) in the ternary blend. Balancedcharge transport should account for high FF. The bimolecularrecombination was studied by plotting Jsc against the lightintensity.46–51 The a values for L1:N3, L1:C1:N3 (0.8 : 0.2 : 1.4)and C1:N3 cells are 0.981, 0.982 and 0.973, respectively(Fig. S11, ESI†). This indicates that although more bimolecu-lar recombination existing in C1:N3 cells, the incorporation of
Fig. 2 (a) Absorption spectra for C1 solution, C1 film, L1 film and N3 film.(b) Energy level diagram.
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a small amount of C1 into L1:N3 blend does not increasebimolecular recombination. To figure out whether there isenergy transfer from C1 to L1, we carried out photolumines-cence (PL) measurements (Fig. S12, ESI†). Compared withpure L1 film with a PL peak at 645 nm, the L1:C1 (0.8 : 0.2)blend film shows weaker PL at 645 nm, suggesting that thereis no obvious energy transfer.52,53 We further fabricatedsingle-component C1 cell and L1 cell, and L1:C1 (0.8 : 0.2)binary cell to find out whether there is photoinducedcharge transfer between C1 and L1 (Fig. S13, ESI†).54 TheJsc for L1:C1 (0.8 : 0.2) cell is between that for C1 cell and L1cell, suggesting negligible charge transfer between C1and L1.
The morphology for L1:N3 (1 : 1.4), L1:C1:N3 (0.8 : 0.2 : 1.4) andC1:N3 (1 : 1.4) blend films was studied by using atomic forcemicroscope (AFM) (Fig. 4). The phase images present nanofiberstructures. The diameters for the nanofibers in above films areB27 nm, B18 nm, and B8 nm, respectively. The height imagesindicate the coarsest surface of L1:N3 film. The root-mean-squareroughnesses 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 suggestthat C1 might have higher miscibility with N3 than L1. To verifythis hypothesis, we measured the surface free energy (g) of L1, C1and N3 by carrying out the contact angle experiments with waterand ethylene glycol as the liquids. The g was determined by usingOwens–Wendt model.55–57 The g for L1, C1 and N3 are18.27 mJ m�2, 17.18 mJ m�2 and 16.42 mJ m�2, respectively(Table S5, ESI†). The miscibility can be quantified by theFlory–Huggins interaction parameter (wij).
58,59 wij is propor-
tional toffiffiffiffigip � ffiffiffiffigjp� �2
, where gi and gj are the surface free
energies of the two interacting components.60 A smaller wmeans higher miscibility. Owing to the closer g values of C1and N3, wC1–N3 should be lower than wL1–N3, suggesting thehigher miscibility between C1 and N3. This explains theweaker phase separation in C1:N3 film. The addition of C1into L1:N3 blend could increase the miscibility between donorand acceptor, yielding an optimal phase separation to achieveefficient charge generation and fast charge transport.
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%] Voc [V] Jsc [mA cm�2] FF [%] PCE [%]
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)
a Data in parentheses are integrated current densities from EQEspectra. b Data in parentheses are averages for 8 cells.
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Conclusions
A new copolymer donor C1 was developed for 2D1A-type ternaryOSCs. 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. Byadding a small amount of C1 into the L1:N3 blend, the Voc, Jsc andFF were simultaneously enhanced and the PCE was improved to16.32%. Developing large-bandgap donors will be a promisingdirection for highly efficient 2D1A OSCs.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We thank the National Key Research and Development Pro-gram of China (2017YFA0206600) and the National NaturalScience Foundation of China (51773045, 21772030, 51922032,21961160720 and 21805097).
Notes and references
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