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Cite this: Phys. Chem. Chem. Phys., 2013, 15, 19990 Received 23rd September 2013, Accepted 12th October 2013

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Donor–acceptor conjugated polymers based on a pentacyclic aromatic lactam acceptor unit for polymer solar cells† Qiaogan Liao,zab Jiamin Cao,zb Zuo Xiao,b Jianhe Liao*a and Liming Ding*b

DOI: 10.1039/c3cp54022g www.rsc.org/pccp

A series of donor–acceptor (D–A) conjugated polymers P1–P4 was synthesized by copolymerization of a novel pentacyclic aromatic lactam acceptor unit, thieno[2 0 ,3 0 :5,6]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)-dione (TPTI), with a donor unit, benzo[1,2-b:4,5-b 0 ]dithiophene (BDT) or dithieno[3,2-b:2 0 ,3 0 -d]silole (DTS). The effect of the donor units and the side chains on TPTI on polymer properties and solar cell performance was investigated. Bulk heterojunction solar cells based on P1 and PC71BM afforded the highest power conversion efficiency (PCE) of 5.30%.

Bulk heterojunction polymer solar cells (PSCs) based on a conjugated polymer donor and a fullerene acceptor are promising alternatives to traditional silicon solar cells due to their advantages like low cost, lightweight, flexibility, and roll-to-roll fabrication.1 However, the power conversion efficiency (PCE) of PSCs is still low compared with that of inorganic solar cells and needs to be further improved to make PSCs more competitive.2 The key approach for enhancing the efficiency of PSCs is to design and synthesize high-performance D–A polymer donors and fullerene acceptors.3 D–A conjugated polymers consisting of an electron-donating unit and an electron-accepting unit have attracted great attention due to their excellent light-harvesting capability and high mobilities.4 Up to 9.2% PCE has been achieved for a single solar cell based on a D–A conjugated polymer.5 For developing high-performance D–A polymers, polycyclic aromatic donor and acceptor units are superior building blocks since their extended conjugation can effectively reduce bandgap, enhance light absorbance, and improve charge carrier mobility.6 Recently, we developed a pentacyclic aromatic lactam acceptor unit, thieno[20 ,30 :5,6]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)dione (TPTI), which shows great potential for PSCs. The D–A

polymer PThTPTI, constructed from TPTI and a thiophene donor unit, afforded a PCE of 7.8% when blended with PC71BM. PThTPTI is among the best donor materials with a moderate bandgap.7 Being inspired by this success, we continue to develop new D–A polymers for PSCs by copolymerization of TPTI with other donor units with outstanding performance. Benzo[1,2-b:4,5-b0 ]dithiophene (BDT) and dithieno[3,2-b:2 0 ,3 0 -d]silole (DTS) are excellent donor units for constructing efficient polymer and small molecule donor materials.8 In this work, we report the synthesis of four D–A conjugated polymers, P1–P4, based on TPTI acceptor units and BDT or DTS donor units (Fig. 1). Solar cells based on P1–P4 and PC71BM afforded a PCE of up to 5.30%. The effect of donor units and the side chains on TPTI on physical properties and photovoltaic performance of P1–P4 will be discussed. The synthetic routes for monomers and polymers are illustrated in Scheme 1. TPTI-containing monomers 4a with a 2-hexyldecyl side chain and 4b with a 2-ethylhexyl side chain were synthesized

a

College of Materials and Chemical Engineering, Hainan University, Haikou 570228, China. E-mail: [email protected] b National Center for Nanoscience and Technology, Beijing 100190, China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental details including synthesis, measurements, and instruments. See DOI: 10.1039/c3cp54022g ‡ These authors contributed equally to this work.

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Fig. 1

The structures of P1–P4.

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Scheme 1

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Synthetic routes for monomers and polymers.

according to a three-step route.7 The coupling of N-alkyl thiophen-3-amines (1a and 1b) with 2,5-dibromoterephthaloyl dichloride followed by Pd-catalyzed intramolecular cyclization and NBS bromination afforded 4a and 4b in 37% and 42% overall yields, respectively. BDT and DTS organotin reagents were prepared according to the procedure reported in the literature.9 Polymers were synthesized through Stille-coupling reactions. P1 and P2 were prepared by copolymerization of BDT with 4a and 4b in 93% and 92% yields, respectively. P3 and P4 were prepared by copolymerization of DTS with 4a and 4b in 47% and 64% yields, respectively. Impurities and low molecular weight fractions of the polymers were removed by methanol and hexane in a Soxhlet extractor. All polymers are soluble in CHCl3 and toluene. Compared with P2 and P4 with short alkyl chains on the TPTI unit, P1 and P3 show higher solubility. The molecular weights of P1, P3 and P4 were determined by gel permeation chromatography (GPC) against polystyrene standards in the THF eluent, while the molecular weight of P2 cannot be measured due to its poor solubility in THF. The Mn values of P1, P3 and P4 are 30.2, 12.2 and 11.8 kDa, respectively, and the PDI values of P1, P3 and P4 are 4.13, 1.76 and 1.74, respectively. All the polymers show good thermal stability. The decomposition temperatures (Td) (5% wt loss) for P1–P4 are 316, 328, 393 and 306 1C, respectively (Fig. S1, ESI†). The normalized absorption spectra of P1–P4 in chloroform and as films are shown in Fig. 2. The optical data are listed in Table S1 (ESI†). All polymers show two absorption peaks in the

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Fig. 2

Absorption spectra of P1–P4: (a) in chloroform; (b) as thin films.

visible region. In both solution and films, P3 and P4 with DTS units show broader absorption spectra than P1 and P2 with BDT units, indicating the superior light harvesting capability of P3 and P4. The absorption spectra of all polymer films present red-shifts compared with that of solution, suggesting the p–p stacking of the polymer chains in the solid state. The absorption onsets for P1–P4 films are 612, 613, 670, and 666 nm, respectively, corresponding to the optical bandgaps of 2.03, 2.02, 1.85, and 1.86 eV, respectively. The smaller optical bandgaps of P3 and P4 than those of P1 and P2 indicate stronger interaction between donor and acceptor units in backbones of P3 and P4, since DTS possesses stronger electron-donating capability than BDT.10 The electrochemical properties of P1–P4 were investigated by a cyclic voltammetry (CV) method. Cyclic voltammograms of P1–P4 are shown in Fig. S2 and the electrochemical data are listed in Table S1 (ESI†). All potentials were calibrated against the Fc/Fc+ redox couple. The oxidation onset potentials (Eon ox ) of P1–P4 are 0.65, 0.63, 0.53, and 0.54 V, respectively, while the reduction onset potentials (Eon red) of P1–P4 are 2.12, 2.09, 2.08, and 2.11 V, respectively. According to the

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empirical formulae: HOMO = (Eon ox + 4.8) eV, LUMO = (Eon red + 4.8) eV, the HOMO and LUMO energy levels of the polymers are determined to be 5.45 and 2.68 eV for P1, 5.43 and 2.71 eV for P2, 5.33 and 2.72 eV for P3, and 5.34 and 2.69 eV for P4, respectively. P1 and P2 (or P3 and P4) show similar HOMO and LUMO levels, indicating that the alkyl chains on the TPTI unit slightly affect the energy levels of the polymers. P3 and P4 with DTS units possess higher HOMO levels, about 0.1 eV higher than those of P1 and P2 with BDT units. Donor units change LUMO levels slightly. P3 and P4 show lower electrochemical bandgaps, 2.61 and 2.65 eV, than those of P1 and P2, 2.77 and 2.72 eV, respectively. The electrochemical bandgaps are 0.70–0.79 eV higher than the optical bandgaps, which is caused by the interface barrier between polymer films and the electrode surface.11 The conventional device structure, ITO/PEDOT:PSS/polymer: PC71BM/Ca/Al, was adopted to evaluate the photovoltaic performance of P1–P4. The optimized D–A ratios for P1–P4 solar cells are 1 : 1.6, 1 : 1, 1 : 1.2 and 1 : 1, respectively. It was interesting to find that for the cells of P1 and P3 with long side chains, more fullerenes are needed to achieve better performance (Table S2, ESI†). The 1,8-diiodooctane (DIO) additive has been frequently used to adjust nanoscale morphology of the active layer.12 The optimized DIO contents for P1–P4 solar cells are 5 vol%, 0.5 vol%, 3 vol%, and 1 vol%, respectively (Table S3, ESI†). The optimized film thicknesses for P1–P4 solar cells are around 95 nm. J–V curves for the best devices are shown in Fig. 3a, and the performance parameters are listed in Table 1. DIO significantly increased the PCE from 3.34% to 5.30% for P1 cells, from 2.30% to 4.87% for P2 cells, from 1.00% to 3.64% for P3 cells, and from 1.50% to 3.48% for P4 cells, respectively. The PCE increments for P1–P4 cells with DIO are due to significant enhancement in short-circuit current (Jsc), which are more remarkable for DTS-containing P3 and P4 cells. Besides, the addition of DIO decreased open-circuit voltage (Voc) for all cells and reduced FF for P2, P3, and P4 cells. The change in device performance parameters results from the morphological change of the active layers. We studied the morphology of polymer:PC71BM blend films without and with DIO by atomic force microscopy (AFM). AFM height images indicate that the addition of DIO reduced the surface roughness for all blend films. The root mean square (RMS) roughness was reduced from 3.29 to 2.78 nm for P1:PC71BM films, from 3.15 to 2.34 nm for P2:PC71BM films, from 11.40 to 3.22 nm for P3:PC71BM films, and from 2.77 to 1.94 nm for P4:PC71BM films, respectively (Fig. S4, ESI†). The reduction in film roughness suggests that DIO facilitates the mixing of PC71BM with P1–P4 and prevents the formation of large domains.13 The increased D/A interfaces induced by DIO lead to enhanced Jsc. All solar cells showed high Voc due to deep HOMO levels (5.45 to 5.33 eV) of P1–P4.14 P4 cells gave the highest Jsc due to good light-absorbing capability of P4. External quantum efficiency (EQE) spectra confirmed that P4 cells gave the best spectral response (Fig. 3b). The highest EQE for P4 cells is 65% at 510 nm. The mobilities of the blend films were measured by the space-charge limited current (SCLC) method (see ESI†). The hole mobilities of P1–P4 are 4.83  104, 2.33  104,

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Fig. 3

Table 1

(a) J–V curves for P1–P4 solar cells; (b) EQE spectra.

Performance of solar cells based on P1–P4 and PC71BM

Active layer P1 : PC71BM (1 : 1.6) P2 : PC71BM (1 : 1) P3 : PC71BM (1 : 1.2) P4 : PC71BM (1 : 1)

DIO [vol%]

Voc [V]

Jsc [mA cm2]

FF [%]

PCE [%]

None 5 None 0.5 None 3 None 1

0.96 0.90 0.95 0.91 0.90 0.87 0.87 0.80

6.47 9.26 4.16 9.60 1.75 7.62 3.19 10.31

53.8 63.6 58.2 55.8 63.2 54.9 53.9 42.2

3.34 5.30 2.30 4.87 1.00 3.64 1.50 3.48

9.62  105 and 3.55  104 cm2 V1 s1, respectively. P1 possesses the highest hole mobility among the four polymers, which might explain the best performance of P1 cells. In summary, four D–A conjugated polymers, P1–P4, based on a pentacyclic aromatic lactam acceptor unit, TPTI, and a BDT or DTS donor unit, were synthesized and applied in polymer solar cells. P3 and P4 with DTS units show better light-harvesting capability, lower bandgaps, and higher HOMO levels than P1 and P2 with BDT units. P1 consisting of a 2-hexyldecyl-functionalized

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TPTI acceptor unit and a BDT donor unit demonstrates the best photovoltaic performance owing to its higher mobility. Currently, our efforts are focusing on designing new donor units to combine with TPTI acceptor units to develop high-performance D–A conjugated polymers for efficient organic solar cells.

Acknowledgements This work was supported by the ‘‘100 Talents Program’’ of Chinese Academy of Sciences. We thank Xiaoyan Du for doing AFM measurements.

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Phys. Chem. Chem. Phys., 2013, 15, 19990--19993

19993

Donor-acceptor conjugated polymers based on a pentacyclic aromatic lactam acceptor unit for polymer solar cells.

A series of donor–acceptor (D–A) conjugated polymers P1–P4 was synthesized by copolymerization of a novel pentacyclic aromatic lactam acceptor unit, t...
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