Isoindigo-dicyanobithiophene-Based Copolymer for High

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Isoindigo-dicyanobithiophene-Based Copolymer for High Performance Polymer−Fullerene Solar Cells Reaching 1.06 V Open Circuit Voltage and 8.36% Power Conversion Efficiency Song-Fu Liao,† Chin-Ti Chen,*,‡ and Chi-Yang Chao*,† †

Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan, Republic of China Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan, Republic of China

‡

S Supporting Information *

ABSTRACT: To investigate the effect of substitution of cyano groups (CN) on D-π-A type conjugated copolymer in photophysical and photovoltaic properties, a non-CN-substituted P4TIH and a CN-substituted P4TIN isoindigo-based copolymers were synthesized and characterized. Having dicyano-substituted bithiophene as electron-donating segment and isoindigo as electron-accepting segment, P4TIN exhibits a deeper highest occupied molecular orbital energy level (HOMO) than that of the non-CN-substituted P4TIH. Benefiting from the improved solubility via copolymer side-chain substituent (2decylteradecyl), inverted solar cells fabricated with a thick (∼200 nm) active layer (P4TIN:PC61BM, 1:2.0) have achieved a very high open circuit voltage of 1.06 V. High power conversion efficiency of 8.36% can be reached without thermal annealing treatment or processing solvent additives. olymer solar cells (PSCs) composed of π-conjugated polymers and fullerene derivatives with a bulk heterojunction (BHJ) structure have attracted many attentions because of the low-cost, lightweight, flexibility and solution processability for the large-area, roll-to-roll fabrication process.1 The donor-π-acceptor (D-π-A) along the π-conjugated copolymer backbone is usually employed as the chemical constitution for high performance PSCs. In recent years, a number of single cell PSCs based on D-π-A copolymers have achieved power conversion efficiency (PCE) over 10%.2−6 Since the open circuit voltage (Voc) is in general proportional to the energy offset between the highest occupied molecular orbital (HOMO) of copolymer and the lowest unoccupied molecular orbital (LUMO) of fullerene derivative,7 many copolymers are designed to lower the HOMO energy level for increasing the Voc of PSCs. An effective method of reducing the HOMO energy level is through the introduction of electronwithdrawing fluorine atom to the donor segment (becoming a weaker donor) of D-π-A copolymers.5,6,8−12 However, the HOMO-level-lowering approach usually leads to an increase in the energy gap of the copolymer and shortens the light absorption wavelength, which impairs the short circuit current

P

© XXXX American Chemical Society

(Jsc) of PSCs. Therefore, to overcome the drawback of such approach, a much stronger acceptor segment is necessary to accompany with a weaker donor segment of D-π-A copolymers.13 The isoindigo moiety has been considered as a promising strong acceptor segment for D-π-A copolymers toward high performance PSCs.14 Recently, isoindigo-based PSCs achieving high PCEs of over 6−8% have been reported.12,15−19 On the other hand, there are a few reports of D-π-A copolymers constituted with dicyano (CN)-substituted bithiophene as the acceptor (instead of donor) segment already exhibiting low HOMO energy levels and high Voc of 0.9 V for their PSCs.20,21 Considering a further weaker donor segment together with a strong acceptor segments, we have synthesized and characterized a P4TIN copolymer (Scheme 1), which has a D-π-A structure with an isoindigo moiety as the strong acceptor segment and a dicyano (CN)-substituted bithiophene as a Received: July 25, 2017 Accepted: August 21, 2017

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DOI: 10.1021/acsmacrolett.7b00547 ACS Macro Lett. 2017, 6, 969−974

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ACS Macro Letters

copolymer chains in solid state due to the polarized cyano group CN−π interactions between copolymer chains of P4TIN.26 Figure 1 displays the UV−visible absorption spectra of these two copolymers in dilute CF solution and thin film state. The

Scheme 1. Synthesis of Monomers A1 and D2 and Copolymers P4TIH and P4TIN

Figure 1. UV−visible absorption of the copolymers: (a) in dilute CF solution and (b) in solid thin film state coated on quartz substrate from CF/DCB (1/1, v/v) solution.

copolymer thin films were prepared from a CF and DCB (1/1, v/v) mixed solvent which is same as that used in PSC fabrication. P4TIH exhibits two major absorption bands in the region of 350−800 nm in both solution and thin film states. The absorption band in the high-energy region (350−500 nm) can be assigned to the π−π* transition of the donor segments (oligothiophene),16 and that in the low-energy region (550− 800 nm) is related to the intramolecular charge transfer (ICT) of the copolymers. For P4TIN, the absorption band around 350−500 nm seems to be red-shifted to a longer wavelength around 450−600 nm, which partially overlaps with the ICT absorption band around 550−800 nm. Such red-shifted π−π* absorption wavelength may be attributed to the ICT feature, which is caused by the electron-withdrawing CN substituent of oligothiophene. We believe that the CN substituent also causes the onset absorption wavelength a bit shorter and hence a larger optical energy gap (Egopt) of 1.67 eV (vs 1.59 eV of P4TIH). From the thin film absorption spectra (Figure 1b), both P4TIH and P4TIN have a similar absorptivity of 0.72− 0.75 × 105 cm−1, around 633−637 and 685−691 nm. Comparing both solution and thin film absorption spectra, the vibronic emission feature seems to be more revealing in the thin film spectrum of P4TIH, whereas it is about the same for P4TIN. To understand the interaction of copolymer chains further, UV−visible absorption spectra of these two copolymers in heated solution were measured (Figure S6). Comparing with room temperature solution spectra, P4TIH has a more blueshifted and vibronic-structure reduced absorption band than P4TIN, indicating the copolymer chain of P4TIN possesses a stronger tendency of aggregation in solution. These spectroscopic observations suggest that P4TIH develops a larger change on copolymer chain interaction when dissolved in solution to aggregation in thin film than that of P4TIN. In addition, a somewhat larger red-shifting of absorption wavelength of the P4TIH thin film spectrum is consistent with the inference above. This result is also in accordance with the higher Tm and a poorer solubility observed for P4TIN, comparing with those of P4TIH. To understand the influence of CN substituents on the donor segment of D-π-A copolymers, we employed cyclic voltammetry (CV) to measure the HOMO and LUMO energy levels of P4TIH and P4TIN (see S1 in SI, Figure S7). The HOMO and LUMO energy levels and energy gap (EgCV) were estimated to be about −5.37, −3.58, and 1.79 eV for P4TIH

weaker donor segment than difluoro-substituted bithiophene. As a comparison, a non-CN-substituted P4TIH copolymer was also synthesized and characterized. A detailed study of thermal property, absorption spectroscopy, electrochemistry, DFT calculation, hole mobility, crystallinility, and morphology, as well as the PSCs characteristics of P4TIN and P4TIH, is presented. The copolymers P4TIH and P4TIN were prepared via a Stille cross coupling reaction of the isoindigo monomer (A1) and bithiophene (D1) or dicyanobithiophene (D2) monomer in chlorobenzene at a mild temperature of 90 °C for 24 h. The compounds 1,22 4,20 monomer D1,23 2-decyl-1-tetradecyl bromide,24 and (4-dodecylthiophen-2-yl)trimethylstannane25 were synthesized according to literatures. The synthesis routs are illustrated in Scheme 1 (see SI for details of synthesis and characterization). The number-average molecular weight (Mn), weight-average molecular weight (Mw), peak molecular weight on gel permeation chromatography (GPC) trace (Mp), and polydispersity indexes (PDIs) of two copolymers were determined by GPC and calibrated against polystyrene standards. P4TIH and P4TIN have Mn of 76.8 and 59.9 kDa, Mw of 131.5 and 152.4 kDa, Mp of 153.6 and 294.4 kDa, and PDI of 1.7 and 2.5, respectively (see Figures S3 and S4 for GPC traces). In order to ensure the solubility of the copolymers in organic solvent, two long and branched alkyl chains (2-decyltetradecyl) and two long and straight alkyl chains (n-dodecyl) were introduced to the isoindigo moiety and its two flanked thiophene π-spacers, respectively. As a result of the long and branched alkyl chains, these copolymers can be readily dissolved in halogenated solvents, such as chloroform (CF), chlorobenzene (CB), and 1,2-dichlorobenzene (DCB) at room temperature. However, relevant to the solubility of two copolymers, the viscosity of P4TIN solution is apparently higher than that of P4TIH, indicating relatively poor solubility of P4TIN. As revealed by thermogravimetric analysis (TGA) (Figure S5a), both copolymers exhibit good thermal stability, with 5% weight loss at nearly same temperature (Td) around 405 °C. The melting temperatures (Tms) were determined from the first scan of heating thermography of the copolymers by differential scanning calorimetry (DSC) (Figure S5b). The Tms are 271 and 353 °C for P4TIH and P4TIN, respectively. Particularly, P4TIN has a higher Tm and a lower solubility, indicative of stronger intermolecular interactions between 970

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ACS Macro Letters Table 1. Electrochemical Data, DFT Simulation, and Related Energy Levels of the Copolymers cyclic voltammetry

P4TIH P4TIN

DFT calculation

HOMO (eV)

ΔEHOMOa (eV)

LUMO (eV)

ΔELUMOb (eV)

EgCVc (eV)

Egoptd (eV)

HOMO (eV)

ΔEHOMOa (eV)

LUMO (eV)

ΔELUMOb (eV)

EgDFTe (eV)

−5.37 −5.77

0.40

−3.58 −3.70

0.12

1.79 2.07

1.59 1.67

−4.94 −5.33

0.39

−2.73 −2.93

0.20

2.21 2.40

ΔEHOMO is the HOMO energy level difference compared with P4TIH. bΔELUMO is the LUMO energy level difference compared with P4TIH. EgCV is the energy gap determined from cyclic voltammetry. dEgopt is the energy gap calculated from the intersection of the tangent on the lowest energetic edge of the copolymer thin film absorption spectrum with the baseline by Egopt = 1240/λonset. eEgDFT is the energy gap obtained from DFT calculation. a c

Table 2. Inverted PSCs Performance and Hole Mobility of P4TIH and P4TIN devicea P4TIH P4TIN

Vocb (V) 0.76 1.06

Jscb (mA cm−2) 18.75 15.17

FFb (%) 57 52

PCEb (%) 8.12 8.36

PCEavgc (%)

Jsc

7.79 7.92

IPCEd

(mA cm−2)

18.59 15.04

thickness (nm) 210 180

μhe (cm2 V−1 s−1) −4

3.52 × 10 4.39 × 10−5

μhf (cm2 V−1 s−1) 7.72 × 10−5 1.82 × 10−5

a For P4TIH, the copolymer/PC61BM blend ratio is 1:1.5 (w/w); for P4TIN, the copolymer/PC61BM blend ratio is 1:2.0 (w/w). bResults of the highest PCE device. cAverage data from over 15 devices. dSpectral integral from IPCE spectrum of the best devices. eHole mobility of pure copolymer thin films. fHole mobility of copolymer:PC61BM blended thin films.

and −5.77, −3.70, and 2.07 eV for P4TIN, respectively (Table 1). The difference in HOMO energy levels between P4TIH and P4TIN is ∼0.40 eV, but the difference in the LUMO energy levels between them is only ∼0.12 eV. This result indicates that the introduction of electron-withdrawing CN groups on the bithiophene donor segment effectively lowers HOMO energy level, but lowers the LUMO energy level slightly. The CV results also reveal that the LUMO energy level of the copolymers is mainly determined by the strong acceptor segment (isoindigo), even though the CN substituted bithiophene donor features electron-deficient cyano-group substituents. Otherwise, although EgCVs are larger than Egopts determined from the on-set of absorption wavelength, P4TIH has a smaller energy gap compared with that of P4TIN, regardless of EgCV or Egopt. To further understand the frontier molecular orbitals and the torsion (dihedral) angle of the copolymers, density functional theory (DFT) calculation was applied using Gaussian 09 with a hybrid B3LYP correlation functional and a split valence 6-31G (d, p) basis set. The DFT calculation results are summarized in Table 2. Although the theoretically estimated energy levels of the copolymers are not quite the same as those obtained from experimental CV, the trend of energy levels variation due to the CN substituents remains the same in both experimental and theoretical results. From DFT calculation, when two CN substituents are introduced on bithiophene donor segment, the HOMO energy level is lowered by ∼0.39 eV, which is larger than ∼0.20 eV decline of the LUMO energy level. From the electron density contour plots of HOMO and LUMO (Figure S8), P4TIH exhibits a more pronounced ICT characteristics than P4TIN. Due to the electron-withdrawing CN substituents of bithiophene donor segment, both HOMO and LUMO of P4TIN are more sporadic (and hence less ICT) over both isoindigo and dicyanobithiophene moieties compared with those of P4TIH. This result is consistent with the UV−visible absorption spectrum of the copolymers in solution state: P4TIH has a higher ICT characteristics and a broader absorption band with a longer wavelength of absorption onset compared with those of P4TIN. The PSCs of the copolymers were fabricated with an inverted configuration: ITO/ZnO/PEIE/active layer/MoO3/Ag having a photoactive area of 0.04 cm−2. Ethoxylated polyethylenimine

(PEIE) was applied to modify the surface and work function of ZnO layer.27 The detail of fabrication procedures is described in Supporting Information (see S2 in SI). The preparation of the active layers was first optimized by adjusting the weight ratios of the copolymer and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) in a solution mixture. The optimized copolymer: PC61BM weight ratio is 1:1.5 and 1:2.0 for P4TIH and P4TIN, respectively. In order to increase the thickness and to control the morphology of active layers, a mixed solvent of CF and DCB (1:1, v/v) was found to be an ideal choice.28 The thickness of P4TIH blended active layer was optimized to be 210 nm, which is a relatively thick active layer compared to other PSCs. In order to have a fair comparison, P4TIN blended active layer was optimized to a similar thickness of 180 nm by changing the concentration of the blended solution and the spin rate of spin coater. The measured J−V curves were shown in Figure 2a, and the corresponding PSC characteristics are

Figure 2. (a) J−V curves and (b) IPCE spectrum of P4TIH and P4TIN PSCs.

listed in Table 2. As a result, the P4TIN-based PSC exhibits a prominent PCE of 8.36% with a remarkable Voc of 1.06 V. It is apparent that the high Voc of PSCs is mainly due to the lowlying HOMO energy level of P4TIN compared with that of P4TIH. To our best knowledge, PCE of 8.36% is the highest value compared with other isoindigo based PSCs reported in the literature so far.17 Recently, Wang et al. have reported an indenothiophene-difluorobenzothiadiazole copolymer PIT2FBT, showing a high PCE of 9.01% with an outstanding Voc of 1.0 V, Jsc of 14.42 mA cm−2, and FF of 62.48%.29 Although PCE of P4TIN PSC is lower than that of PIT2FBT, the Jsc of 15.17 mA cm−2 of P4TIN PSC is still the highest in the single-junction PSCs with Voc of over 1.0 V.30 Nevertheless, 971

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ACS Macro Letters the electron-withdrawing CN substituents on the donor segment of the copolymers are detrimental to the Jsc of PSCs. As demonstrated in this study, the relatively low Jsc (compared with 18.75 mA cm−2 of P4TIH) of P4TIN can first be attributed to the comparatively narrow absorption band and the relatively short absorption wavelength (compared with those of P4TIH). Otherwise, the thickness of P4TIN active layer is thinner than that of P4TIH by ∼30 nm, which may be another reason for the smaller Jsc. Such analysis and inference can be further verified by their incident photon-to-current efficiency (IPCE) spectra (Figure 2b). From Figure 2b, P4TIH exhibits a broader photocurrent wavelength ranging from 350 to 800 nm and a higher efficiency with IPCE peak values of 94% (at ∼625 nm) and 65% (at ∼460 nm), both of which are higher than those of P4TIN. This is in good agreement with the UV−visible absorption spectra of P4TIH and P4TIN in thin film state (Figure 1b). Furthermore, the resulting JscIPCE integrated from IPCE spectrum (Table 2) is also greater for P4TIH than that for P4TIN, consistent with the Jsc determined from J−V measurements. Regarding fill factor (FF) of PSCs, both P4TIH and P4TIN show an inferior FF of 57% and 52%, respectively. Considering no thermal annealing and no processing solvent additive involved in the thin film fabrication process in this paper, it is foreseeable that the FF performance of P4TIH and P4TIN PSCs can be improved after the optimization of the fabrication process, including post-thermal treatments and solvent additives. The results from the aforementioned optimization process will be reported in due course. In order to understand why the Jsc of P4TIH PSCs is higher than that of P4TIN PSCs, we have studied the charge transporting property (hole mobility) of these copolymers and their PC61BM-blended thin films by space charge limited current (SCLC) method (see S3 of SI and Figure S9). The hole only device were fabricated with a configuration of ITO/ PEDOT:PSS (35 nm)/copolymer/Au (100 nm) and ITO/ PEDOT:PSS (35 nm)/copolymer:PCBM/Au (100 nm). Evidently, as hole mobility data shown in Table 2, whether the pure copolymer or the copolymer:PC61BM blended thin films, P4TIH has a better hole mobility compared with P4TIN. With a better hole transporting ability, this is one of the reasons that Jsc of P4TIH PSCs is higher than that of P4TIN PSCs. The crystallinility and crystalline-aggregate orientation of these copolymers or copolymer:PC61BM blended films are other factors that are influential on the Jsc of PSCs. Accordingly, we have conducted the grazing incidence wide-angle X-ray scattering (GIWAXS) measurement (see S4 of SI) to probe the corresponding property of the thin films. From the twodimensional GIWAXS patterns (Figure 3a−d), both P4TIH:PC61BM and P4TIN:PC61BM blended thin films show clear (100), (200), and (300) diffraction signals, but P4TIH:PC61BM blend film exhibits higher intensity of each (h00) diffraction (Figure 3e,f). The (h00) diffraction is usually ascribed to the lamellar side chain and backbone packing of the copolymer. Since these copolymers have the alkyl side chains with same length and number, these copolymer:PC61BM blended thin films exhibit the same (100) diffraction at qz ≈ 0.26 Å−1, which is corresponding to the distance (d) between the copolymer chains of 24.3 Å. In contrast, only P4TIN:PC61BM blended thin film was observed for the (010) diffraction peak at qz ≈ 1.74 Å−1 (d = 3.62 Å), which is ascribed to the π−π (face-to-face) stacking between the ordered copolymer conjugated backbones. Such GIWAXS

Figure 3. 2D-GIWAXS pattern of (a) P4TIH; (b) P4TIH:PC61BM (1:1.5); (c) P4TIN; (d) P4TIN:PC61BM (1:2.0). TEM image of blended thin film of (e) P4TIH:PC61BM (1:1.5, w/w); (f) P4TIN:PC61BM (1:2.0, w/w).

results can be rationalized by the polarized CN groups of P4TIN, of which dipole−dipole interaction enhances the π−π stacking of P4TIN copolymer chains. Therefore, P4TIH has a higher crystallinity of lamellar structure, and P4TIN has a higher crystallinity of π−π (face-to-face) stacking structure. However, from our GIWAXS data analysis (see below), crystallinity of lamellar structure ((100) diffraction) is much greater than that of π−π (face-to-face) stacking structure ((010) diffraction) in both P4TIH and P4TIN copolymers. As shown in Figure S11a,b, both of these copolymer:PC61BM blended thin films exhibit a combination of out-of-plane (azimuthal angle χ = 45−90°) and in-plane (azimuthal angle χ = 0−45°) diffraction, indicating a coexisted edge-on and faceon orientations of copolymer chains in the blended thin films. The percentage of face-on orientation11 and the crystallite correlation length is 35.8%, 134.1 Å and 43.8%, 100.9 Å for P4TIH:PC61BM and P4TIN:PC61BM blended thin films, respectively (see Figures S10 and S11). Although P4TIN:PC61BM blended thin film reveals a π−π stacking feature and higher percentage of face-on which is beneficial for charge transporting along vertical direction in PSCs, the crystallinity and the crystallite correlation length of P4TIN:PC61BM blended thin film is inferior to those of P4TIH:PC61BM blended thin film. The results of our GIWAXS study are consistent with the hole mobility results by SCLC study and the Jsc of PSCs. Similar results of GIWAXS study can be found for pure copolymer thin films (see S4 in SI). The morphology of the copolymer:PC61BM blended thin films can be probed by transmission electron microscopy (TEM). As shown in Figure 3e and 3f (also see Figure S12), compared with those of P4TIN:PC61BM, P4TIH:PC61BM blended thin film features a more pronounced difference of bright (copolymer) and dark (fullerene) regions, and a clear and more extended fibril nanostructures found in the TEM images. For P4TIH, the copolymer chains precipitate out or 972

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ACS Macro Letters aggregate slowly in the film-forming process due to its good solubility so that the copolymer chains are more sustainable to the growth of aggregate during the solvent evaporation of thin film fabrication.17 Consequently, P4TIH develops a more phase separation morphology than P4TIN. It is noted that the crystallinity is proportional to the formation of fibril nanostructure and this result is in accordance with the GIWAXS data and SCLC hole mobility data. In summary, based on lowering HOMO energy levels of Dπ-A P4TIH copolymer, we have designed and synthesized a novel copolymer P4TIN, which has a dicyanobithiophene as the donor segment accompanying with a strong electronwithdrawing isoindigo moiety as the acceptor segment. With improved solubility, both P4TIN and P4TIH copolymers blended with PC61BM can form thick (with a thickness about 200 nm) thin films with good quality. Particularly, P4TIN shows a superb Voc of 1.06 V and a high PCE of 8.36%, which is the highest PCE among isoindigo-based PSCs. For an easy fabrication of active layer, high performance inverted PSCs without thermal annealing and solvent additives have been achieved in this report. A further study of the effects of thermal treatments and solvent additives will be reported in due course.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00547. Details of synthesis procedures, 1H NMR, GPC, TGA, DSC, absorption spectra, DFT simulations, SCLC data, GIWAXS analysis, and TEM images for both copolymers (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Chin-Ti Chen: 0000-0002-1493-2533 Author Contributions

All authors have an equal contribution. Funding

This research is supported by the Project of Technology Development for Deep Decarbonization, Academia Sinica. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully thank Mr. Shih-Hsin Huang (Institute of Chemistry, Academia Sinica) for helping with the experiments in TEM.

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REFERENCES

(1) Gao, Y.; Liu, M.; Zhang, Y.; Liu, Z.; Yang, Y.; Zhao, L. Recent development on narrow bandgap conjugated polymers for polymer solar cells. Polymers 2017, 9, 39. (2) He, Z.; Xiao, B.; Liu, F.; Wu, H.; Yang, Y.; Xiao, S.; Wang, C.; Russell, T. P.; Cao, Y. Single-junction polymer solar cells with high efficiency and photovoltage. Nat. Photonics 2015, 9, 174. (3) Liu, T.; Pan, X.; Meng, X.; Liu, Y.; Wei, D.; Ma, W.; Huo, L.; Sun, X.; Lee, T. H.; Huang, M.; Choi, H.; Kim, J. Y.; Choy, W. C. H.; Sun, Y. Alkyl side-chain engineering in wide-bandgap copolymers leading to 973

DOI: 10.1021/acsmacrolett.7b00547 ACS Macro Lett. 2017, 6, 969−974

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DOI: 10.1021/acsmacrolett.7b00547 ACS Macro Lett. 2017, 6, 969−974