New Terthiophene-Conjugated Porphyrin Donors for Highly Efficient

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New Terthiophene Conjugated Porphyrin Donors for Highly Efficient Organic Solar Cells Liangang Xiao, Song Chen, Ke Gao, Xiaobin Peng, Feng Liu, Yong Cao, Wai-Yeung Wong, Wai-Kwok Wong, and Xunjin Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09790 • Publication Date (Web): 12 Oct 2016 Downloaded from http://pubs.acs.org on October 14, 2016

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ACS Applied Materials & Interfaces

New Terthiophene Conjugated Porphyrin Donors for Highly Efficient Organic Solar Cells Liangang Xiao, ‡ a Song Chen, ‡ b Ke Gao, ‡a Xiaobin Peng,* Feng Liu,*c Yong Cao,a Wai-Yeung Wong,*b Wai-Kwok Wongb and Xunjin Zhu*b a

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of

Luminescent Materials and Devices, South China University of Technology, 381 Wushan Road, Guangzhou 510640, China. b

Institute of Molecular Functional Materials, Research Centre of Excellence for Organic

Electronics, Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, China. c

Materials Sciences Division, Lawrence Berkeley National Lab, Berkeley, CA, 94720, United

States. KEYWORDS: Porphyrin, organic solar cells, terthiophene, peripheral substitutions, molecular packing

ABSTRACT: To mimic the natural photosynthetic systems utilizing chlorophylls to absorb light and store light energy, two new porphyrin-based small molecules of PTTR and PTTCNR have

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been developed for photovoltaic applications. The highest power conversion efficiency of 8.21% is achieved, corresponding to a short-circuit current of 14.30 mA cm−2, open-circuit voltage of 0.82 V, and fill factor of 70.01%. The excellent device performances can be ascribed to the engineering of molecule structure and film morphology. The horizontal conjugation of 3,3''dihexyl-terthiophene to porphyrin-core with the vertical aliphatic 2-octylundecyl peripheral substitutions, can not only effectively increase the solar flux coverage between the conventional Soret and Q bands of porphyrin unit, but also optimize molecular packing through polymorphism associated with side-chains and the linear π-conjugated backbones. And the additive of 1,8diiodooctane and subsequent chloroform solvent vapor annealing facilitate the formation of the blend films with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) characteristics of bicontinuous, interpenetrating networks required for efficient charge separation and transportation.

1. Introduction Solution-processed small molecule (SM) bulk heterojunction organic solar cells (BHJ OSCs) have received intense study very recently because of the defined molecular structure and weight, high purity, and good batch-to-batch reproducibility in cell performance. These advantages make it easier to understand the relationships between the molecular structures and device performance and realize the ultimate performance limit of these materials and devices. Not surprisingly, dramatically increased PCEs up to 10% have been reported for single layer SM BHJ solar cells,110

comparable to the best of polymer solar cells.11-15 Inspired by the natural photosynthetic

systems which utilize chlorophylls to absorb light and carry out photochemical charge separation to store light energy, porphyrins and their derivatives have been explored for a long time as the


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active materials in dye-sensitized solar cells with the highest efficiency of 13%,16 and in BHJ OSCs but with very low PCEs.17-28 Recently, we adopted an A-π-D-π-A structural design strategy to synthesize porphyrin-based donor materials, and successfully achieved high PCEs more than 8% employing diketopyrrolopyrrole (DPP) or 3-ethylrhodanine as the acceptor units,3, 29-31

demonstrating the potential of porphyrins for OSCs by their virtues of high molar absorption

coefficients, easy chemical structure modification, and unique photophysical properties. To date, however, a very limited number of porphyrin-based materials have been developed with efficiencies higher than 8% in BHJ OSCs, since it is very challenging to achieve a balance between solubility and intermolecular interactions for porphyrin molecules, simultaneously targeting optimally positioned energy levels (to ensure a high VOC) and an increased solar flux coverage and charge carrier mobility.

Figure 1. Structures of porphyrin small molecules PTTR and PTTCNR. To extend the backbone conjugation and enhance intermolecular π-π interaction, we designed and prepared two porphyrin small molecules PTTR and PTTCNR (Figure 1) in which 3,3''dihexyl-terthiophene (TT) were symmetrically conjugated to 10,20-bis-(2-octylundecyl)-


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porphyrin core (P), then ended with 3-ethylrhodanine (R) and 2-(1,1-dicyanomethylene)-3ethylrhodanine (CNR) terminal units, respectively. Detailed studies show that the appropriate alkyl chains and terthiophene ethynylene backbone can not only enhance the directional intermolecular π-π stacking in films but also form the blend films with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) with characteristics of bi-continuous, interpenetrating networks required for efficient charge separation and transportation. The processing engineering for their blend films with PC71BM afforded impressive power conversion efficiencies of 7.66% and 8.21% in BHJ OSC based on PTTR and PTTCNR, respectively.

Br

S

S

(a)

S CHO

(b)

S

H

S

S CHO

2

1 Br C8H17 C10H21

N N Zn N N

C8H17 C10H21

(c)

Br 3 C8H17 C10H21 PTTR and

OHC S (d)

PTTCNR

S

S

N N Zn N N

S

S S CHO

C8H17 C10H21 PTTCHO

Scheme 1. Synthesis of PTTR and PTTCNR. Reaction conditions: a) trimethylsilylacetylene, PdCl2(PPh3)2, CuI, THF/Et3N, 40°C, overnight; b) TBAF, THF, 30 min; c) Pd(PPh3)4, CuI, THF/Et3N, 50°C, overnight; d) 3-ethylrhodanine or 2-(1,1-dicyanomethylene)-3-ethylrhodanine, dry CHCl3, piperidine, reflux, overnight. 2. Results and Discussion


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2.1 Synthesis and UV-visible Absorption The synthesis of PTTR and PTTCNR is shown in Scheme 1. 5''-Bromo-3,3''-dihexyl-[2,2':5',2''terthiophene]-5-carboxaldehyde (1) were synthesized according to the procedures reported previously,32 and 5''-ethynyl-3,3''-dihexyl-[2,2':5',2''-terthiophene]-5-carboxaldehyde (2) was prepared through Sonogashira coupling between 1 and trimethylsilylacetylene, and then the deprotection of trimethylsilyl group with tetrabutylammonium hydrogen fluoride trihydrate (TBAF). The precursor PTTCHO was synthesized by Pd-catalyzed Sonogashira coupling of 5,15-Dibromo-10,20-bis(2-octyl-undecyl)-porphyrin (3) with 2 according to our previous work.29, 31 Then, 3-ethylrhodanine and 2-(1, 1-dicyano-methylene)-3-ethylrhodanine were condensed with PTTCHO by Knoevenagel condensation reaction to produce PTTR and PTTCNR, respectively. The two compounds were purified on silica-gel column, followed by Soxhlet extraction in acetone (24 h each) to remove the trace catalyst. Their chemical structures and purity were verified by NMR and electrospray ionization (ESI)/matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy.

(a) 1.0

PTTR PTTCNR

0.8

(b)

PTTR PTTCNR

1.4 1.2

Absorption

Absorption

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0.6 0.4 0.2

1.0 0.8 0.6 0.4 0.2

0.0 300

400

500

600

700

Wavelength (nm)

800

0.0 300

400

500

600

700

800

900

1000

Wavelength(nm)

Figure 2. UV–visible absorption spectra of PTTR and PTTCNR (a) in chloroform solution and (b) in film.


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Table 1. Optical and electrochemical data of PTTR and PTTCNR.

PTTR

λmax λmax λonset (nm)a (nm)b (nm)b 465,682 535,755 815

Eox (V) 0.43

Ered (V) −1.15

EHOMO (eV) c −5.14

ELUMO (eV) d −3.56

Eg (eV)e 1.58

Eg (eV)f 1.52

PTTCNR

463,684 514,755 855

0.46

−1.08

−5.17

−3.63

1.54

1.45

Compound

a

Absorption in CH2Cl2; b Absorption in film; c EHOMO = −(Eox + 4.71) (eV); d ELUMO = −(Ered + 4.71) (eV); e The electrochemical bandgap; f The optical bandgap.

Figure 3. The optimum geometries and electron-state-density distributions of PTTR and PTTCNR. As shown in Figure 2, both PTTR and PTTCNR in dilute chloroform and solid films show broad absorption spectra in the range of 300–700 nm since the conjugation of 3,3''-dihexylterthiophene can effectively increase the solar flux coverage between the Soret (464 nm) and Q band (682 nm) of porphyrin. Impressively, the polymorphism associated with long pendant sidechains on both porphyrin-core and terthiophene backbone triggered much stronger intermolecular self-assembly in the condensed solid state related to those porphyrin-based small molecules reported previously.3, 23, 24 As a result, both the Soret and Q bands in the solid films become much red-shifted in comparison with those in solution (Table 1). In addition, the


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intensity of Q bands (760 nm) became stronger than the Soret bands in the solid films while the opposite is true in solution. On the other hand, the high planar optimum geometries of PTTR and PTTCNR (Figure 3) definitely make the optimal molecular packing possible in the solid films. The optical band gaps estimated from the onsets of the absorption spectra in film are 1.45 and 1.52 eV for PTTCNR and PTTR, respectively. Based on cyclic voltammetry (CV) measurements (Figure S1 and Table 1), the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels) were estimated to be −5.14, −3.56 eV for PTTR and −5.17, −3.63 eV for PTTCNR, respectively. The electrochemical bandgaps were then calculated to be 1.58 and 1.54 eV for PTTR and PTTCNR, respectively, which are in agreement with the optical energy gaps. Apparently, the ending unit of CNR with stronger electron-withdrawing property slightly deepened both HOMO and LUMO energy levels and spontaneously lowered the energy gap of PTTCNR related to PTTR. 4

70

(a) 60

0 PTTR PTTCNR

-4 -8 -12

40 30 20 10

-16 -0.2

PTTR PTTCNR

(b)

50

EQE (%)

Current density (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0

0.2

0.4

0.6

Voltage (V)

0.8

1.0

0 300

400

500

600

700

800

900

Wavelength (nm)

Figure 4. The J−V (a) and EQE (b) curves of the devices based on PTTR and PTTCNR under optimal condition and simulated AM 1.5G irradiation (100 mW cm−2).


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Table 2. Device performances based on PTTR and PTTCNR.

Donor materials

Processing conditions

JSC

VOC

FF

PCE

mA cm−1 (V) (%) (%) CF 10.80 0.86 43.66 4.05a/3.89b PTTR CF/DIO 14.49 0.80 61.86 7.17a/7.04b CF/DIO/SVA 14.93 0.80 64.18 7.66a/7.47b CB 9.28 0.89 42.13 3.47a/3.31b PTTCNR CB/DIO 13.22 0.84 66.78 7.42a/7.30b CB/DIO/SVA 14.30 0.82 70.01 8.21a/8.13b a b and indicate the best and the average value of 10 devices, respectively.

2.2 Photovoltaic Properties Solution-processed BHJ OSCs were fabricated using either PTTR or PTTCNR as the electron donor material and PC71BM as the electron acceptor material with a conventional device structure of ITO/PEDOT:PSS/Donor:PC71BM/PFN/Al (ITO: indium tin oxide, PEDOT: poly(styrene sulfonate), PSS: poly(3,4-ethylenedioxythiophene), PFN: poly[(9,9-bis(3’-(N,Ndimethylamino)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl-fluorene)]).33 In general, chlorobenzene (CB) with high boiling point is selected in solution processing, since the slow evaporation of CB can facilitate the molecular self-assembly and the formation of interpenetrate networks with good crystallinity. However, it was found that a good PTTCNR blend film can be obtained by only using chlorobenzene (CB) solution while only chloroform (CF) solution produced a smooth PTTR blend film. Therefore, CB and CF were chosen for the fabrication of PTTR- and PTTCNR-based devices, respectively. The weight ratios of the donors to PC71BM were optimized to be 1:1 for PTTR:PC71BM and 4:3 for PTTCNR:PC71BM with an active layer thickness about 100 nm. The J-V curves of the devices are presented in Figure 4 and Figure S2–


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S5, and the key device parameters are summarized in Table 2 and Table S1. Upon the addition of 0.4% volume 1,8-diiodooctane (DIO) and further chloroform solvent vapor annealing (SVA) for 80 s, the PCEs were significantly improved to 7.66% for PTTR-based device with a short circuit current (JSC) of 14.93 mA cm-2, an open circuit voltage (VOC) of 0.80 V, and a fill factor (FF) of 64.18%, and 8.21% for PTTCNR-based device with a JSC of 14.30 mA cm-2, a VOC of 0.82, and a FF of 70.1%. The absorption spectrum of PTTCNR:PC71BM (Figure S6) also indicate that the DIO additive and SVA are effective at improving the self-assembly of PTTCNR and the crystallinity the active layer of PTTCNR:PC71BM, leading to the enhancement of FF. As listed in Table 2 and Table S1, all PTTCNR-based devices fabricated under different conditions show higher VOC values possibly due to the deeper HOMO energy level of PTTCNR induced by the strong electron-withdrawing CNR groups. On the other hand, the energy loss (Eloss), which is defined as Eloss = Eg − eVOC, for the optimized PTTCNR-based devices is as low as 0.63 eV, which is responsible for the high VOC of 0.82 V despite the low band gap of PTTCNR (Eg = 1.45 eV).30 No surprisingly, high external quantum efficiencies (EQEs) up to 65% were obtained from 300 to 850 nm under the optimal condition for PTTCNRbased devices (Figure S7), and the JSC value integrated from the EQE curve is 13.89 mA cm−2, which are within 5% mismatch with the experimental data. And the EQE of PTTCNR-based devices is slightly broader but lower than that of PTTR in the range of 600 to 750 nm, which is also consistent with the absorption spectra of their pure and blend films (Figure S6 and Figure S8).34 In addition, hole-only devices were fabricated and the dark J−V curve was measured to estimate the hole mobility for each of the materials (Figure S9). A moderate hole mobility of 1.67 × 10−4 cm2 V−1 s−1 was observed for the as-cast blend film of PTTCNR:PC71BM, while it


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was increased to 4.14 × 10−4 cm2 V−1 s−1 for the blend film upon DIO additive and SVA under the same optimal conditions. Similarly, the blend film of PTTR:PC71BM under the same optimal condition exhibit a relatively higher hole mobility of 3.62 × 10−4 cm2 V−1 s−1. The results are consistent with higher FF and JSC, thus better device performances upon the DIO additive and SVA.

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Jph (mA cm-2)

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1 PTTR CB/DIO/SVA PTTCNR CB PTTCNR CB/DIO PTTCNR CB/DIO/SVA

0.1 0.01

0.1

1

Veff (V)

Figure 5. Jph-Veff characteristics of PTTR- and PTTCNR-based solar cells under different processing conditions. We further measured the photocurrent density (Jph) versus the effective voltage (Veff) or light intensity (Pin) of the cells to study their charge generation, dissociation and collection properties of the optimized solar cells based on PTTCNR and PTTR.35 A Veff value determines the electric field in the BHJ region and thereby can reflect the carrier transport and the photocurrent extraction properties. Jph becomes saturated if the Veff is high enough, suggesting that all the photogenerated excitons are dissociated into free charge carriers and collected at electrodes. In this case, the saturation current density (Jsat) is only limited by total amount of absorbed incident photons. And the ratio Jph/Jsat is essentially the product of exciton dissociation efficiency and charge collection efficiency.36 As shown in Figure 5, at a large Veff of 2 V, the Jph values of both the optimal PTTCNR- and PTTR-based devices reach saturation with very similar Jsat values,


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corresponding to Jph/Jsat of 92% and 91% at a short circuit current density, respectively, suggesting that both devices show high dissociation efficiency or charge collection efficiency. However, at the low effective voltage range, the Jph-Veff characteristics for the two devices show large differences. For example, at the maximal power output conditions, Jph/Jsat are 80% and 74% for the PTTCNR- and PTTR-based devices, respectively. This indicates either a higher exciton dissociation efficiency or charge collection efficiency for the PTTCNR-based devices, with the latter suggesting lower bimolecular recombination.33 For PTTCNR-based solar cells under different processing conditions, the Jph of the devices from pure CB solution shows a stronger field dependence across a large bias range and becomes slightly saturated at Veff = 3 V, suggesting a serious geminate and/or bimolecular recombination and/or less efficient interfacial contact, thus leading to a low FF of only 42.13%.37-39 But the Jph values of the devices fabricated from CB/DIO with/without SVA become saturated at much lower Veff, suggesting that the photogenerated excitons can be efficiently dissociated into free charge carriers and subsequently collected at the electrodes with little geminate or bimolecular recombination,5 leading to higher FF values of 70.01%/66.78%. The maximum exciton generation rate (Gmax) values, which can be calculated by the formula of Jsat = qLGmax (where q is elementary charge and L is the thickness of the active layer),40 are 7.80 × 1027 (Jsat = 124.81 A m–2), 8.92 × 1027 (Jsat = 146.25 A m–2) and 9.83 × 1027 m–3 s–1 (Jsat = 157.31 A m–2) for the three devices fabricated from pure CB, CB/DIO additive, and CB/DIO/SVA, respectively. The gradually enhanced Gmax values can be attributed to the more ordered self-assembly of PTTCNR and the better film morphology upon DIO additive and SVA (vide infra).


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Figure 6. GIXD data of the blend films based on PTTR:PC71BM (a-c) and PTTCNR:PC71BM (d-f) under different processing conditions; g) out-of-plane and h) in-plane line-cut profiles of GIXD of the blend films. 2.3 Structural Characterization The structure order of porphyrins in BHJ blends was further characterized by grazing incidence X-ray diffraction (GXID) method. As shown in Figure 6, both PTTR and PTTCNR in blends take an edge-on orientation. The (100) peak intensity in out-of-plane direction is much stronger than in in-plane direction. Line cut profiles in out-of-plane direction and in-plane direction are summarized in Figure 6 (g) and (h). As for spun PTTR blends, intensive (100) peaks with high order diffractions show up at 0.306 A−1, corresponding to an inter-spacing of 20.5 A−1. And the crystal size is estimated to 10.5 nm. The PTTR blend processed from CF/DIO shows a reduced (100) spacing of 20.1 A−1 and an increased crystal size of 12.8 nm. CF/DIO/SVA processing further increased the crystal size to 13.7 nm and reduced the (100) distance to 20.0 A−1. For PTTCNR blends, (100) crystal sizes are 9.8, 16.4 and 14.3 nm respectively for pure CB,


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CB/DIO and CB/DIO/SVA processed blend films, respectively. Similar (100) stacking distances of 19.5 A−1 were observed for all these samples. Apparently, the peak intensities are much stronger for CB/DIO and CB/DIO/SVA processed blend films, indicating more ordered selfaggregation. In addition, CB/DIO processed blend film shows a larger crystal size than that of CB/DIO/SVA processed film, but the relative intensity for CB/DIO/SVA processed blend film is much stronger. The comparable data indicates that the SVA processing leads to new nucleation and growth of crystallites, and the large population of small crystallites in blends drags down the average crystal value of crystals for PTTCNR blends. Also, DIO/SVA processing increases the crystal quality of PTTR blends as shown in Figure 6 with decreased crystal-packing distances. The π-π stacking peaks in these molecules are weak for all these samples and the azimuthal distributions are quite broad. It is also interference with the PC71BM broad diffraction peak in high q region and thus hard to accurately analyze. For PTTR and PTTCNR blends, the π-π stacking peaks are located at ~1.77 A−1, corresponding to a distance of 3.55 Å. This value is quite small among those conjugated molecules,10, 30 which benefits from the strong intermolecular interactions and the reduced steric ring with less bulk alkyl chain substituents. The small π−π stacking distance is favorable for charge transport,41 thus leading to a higher FF. The atomic force microscopy (AFM) height images of the BHJ blends shown in Figure 7 and Figure S10 indicate that the films processed from pure CF and CB exhibit a rather smooth morphology with root-mean-square (RMS) roughnesses of 0.6 and 0.7 nm for PTTR and PTTCNR, respectively, and are characteristic of bi-continuous, interpenetrating networks required for efficient charge separation and transportation. And DIO additive led to an increased surface roughness and topological features of ~100 nm for PTTR blend films, while it does not


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change the surface topology for PTTCNR blends. And the subsequent SVA treatment only induces slightly rougher surfaces for both PTTR and PTTCNR blend films.

Figure 7. Tapping-mode AFM height images for PTTR:PC71BM (a-c, a: W/O DIO, b: with DIO, and c: DIO +SVA) blend films and PTTCNR:PC71BM blend film (d-f, d: W/O DIO, e: with DIO, and f: DIO +SVA).


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Figure 8. RSoXS profiles of (a) PTTR:PC71BM and (b) PTTCNR:PC71BM blend films under different processing conditions. Length scale of phase separation for these BHJ blends was characterized using resonant soft X-ray scattering (RSoXS) in transmission mode.42 As shown in Figure 8 with scattering profiles for PTTR and PTTCNR blends, quite weak scattering peaks were observed at around 0.02 A−1 for both as spin-coated films, corresponding to a phase separation of 32 nm. For PTTR blends, DIO additive drastically increases the length scale of phase separation to 0.0056 A−1, corresponding to a distance of 112 nm. And the scattering intensity increases by more than one order of magnitude. Obviously, the DIO additive induces the phase separation due to the enhanced interaction of donor and acceptor materials. The subsequent SVA led to a slight decrease in both scattering intensity and length scale of phase separation, showing a peak at 0.0065 A−1 (97 nm). Regarding the correlation of interphase properties with device performances, a weak phase separation is detrimental in generating excitons and collecting charges. The CF/DIO/SVA processing reaches a balance between length scale of phase separation and extent of phase separation, corresponding to a better device performance than CF/DIO. For PTTCNR blends, as casted thin film shows an even poorer phase separation (barely can be seen at ~0.02 A−1.), which corresponds to poorer device performance than PTTR due to serious geminate and/or bimolecular recombination demonstrated by Jph−Veff characteristics. But DIO additive led to an obvious scattering peak located at 0.019 A−1, corresponding to distance of 33 nm. This unambiguously demonstrate that the DIO additive is very helpful to improve the crystals size of donor materials, and frame the phase-separated morphology (The crystal size in (100) direction is about half of the size of phase separation). The subsequent SVA further increased the scattering intensity and the peak also slightly shifted to


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0.017A−1 (37 nm). The suitable crystals size and length scale of phase separation significantly reduced recombination and led to the improvement of device performances from 3.47% to 8.21%. 3. Conclusion In conclusion, two new porphyrin small molecules of PTTR and PTTCNR, have been developed for BHJ OSCs. The structural engineering with the horizontal conjugation of 3,3''dihexyl-terthiophene to porphyrin-core with the vertical aliphatic 2-octylundecyl peripheral substitutions can not only effectively increase the solar flux coverage in the visible and near infrared region, but also optimize molecular packing through polymorphism associated with the long alkyl chain appended. At the same time, their blend film with PC71BM demonstrated efficient photo-generated exciton dissociation and charge collection when processed by DIO additive and the subsequent chloroform SVA. As a result, the excellent device performances with PCEs of 7.66% and 8.21% were achieved for PTTR- and PTTCNR-based OSCs, respectively. 4. Experimental Section 4.1 Materials. All air and water-sensitive reactions were performed under nitrogen atmosphere. All of the chemicals were purchased from Dieckmann Chemical Ltd, China. Organic solvents used in this work were purified using standard process. The other materials were of the common commercial level and used as received. 4.2 Synthesis. PTTR: To a solution of PTTCHO (160 mg, 0.086 mmol) in dry CHCl3, three drops of piperdine and 3-ethylrhodanine (128 mg, 0.80 mmol) were added sequentially, and the resulting mixture


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was stirred and refluxed under argon for 12 h. After cooling and then quenching in water (30 mL), the reaction mixture was extracted with CHCl3 (3 × 20 mL). The organic layer was dried over Na2SO4. After the removal of the solvent, the residue was purified by chromatography on a silica gel column using CHCl3 as an eluant. The highly pure PTTR as a gray green solid was obtained after the recrystallization from CHCl3 and methanol mixture (102 mg, 55%). 1H NMR (400 MHz, CDCl3) δ (ppm): 0.73 (m, 12H), 0.94−1.69 (m, 96H), 1.63−1.70 (m, 4H), 1.78−1.86 (m, 4H), 2.66−2.71 (m, 4H), 2.79−2.83 (m, 4H), 2.90−2.94 (m, 8H), 4.19 (m, 4H), 5.17 (m, 2H), 7.18 (s, 2H), 7.24 (d, J = 1.6 Hz, 4H), 7.50 (s, 4H), 7.73 (s, 2H), 9.57 (d, J = 4.8 Hz, 2H), 9.64 (m, 6H). (MALDI−TOF, m/z) calculated for C122H158N6O2S10Zn: 2126.8884; found: 2126.8942. PTTCNR: Using 2-(1,1-dicyanomethylene)-3-ethylrhodanine instead and following a similar procedure for PTTR, the highly pure PTTCNR was also obtained as a gray green solid in a yield of 52%. 1H NMR (400 MHz, CDCl3) δ (ppm): 0.77 (m, 12H), 0.86−1.41 (m, 90H), 1.53 (m, 12H), 1.83 (m, 4H), 2.65 (m, 8H), 2.91 (m, 8H), 4.23 (q, J = 7.2 Hz, 4H), 5.17 (m, 2H), 6.78 (m, 2H), 7.11 (q, J = 4.4 Hz, 2H), 7.24 (m, 4H), 7.46 (m, 2H), 9.57−9.64 (m, 8H). (MALDI−TOF, m/z) calculated for C128H158N10O2S8Zn: 2189.9716; found: 2189.9780. 4.3 Device fabrication and Characterization. The methods and procedures in details were provided in Supporting Information. ASSOCIATED CONTENT Supporting information The supporting information is available including the synthesis and characterization of all intermediates, details of device fabrication and characterization.


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AUTHOR INFORMATION Corresponding Author * [email protected] (X. Zhu); [email protected] (X. Peng); [email protected] (W.-Y. Wong); [email protected] (F. Liu). Author Contributions ‡These authors contributed equally. Notes §

These authors contributed equally to this work.

ACKNOWLEDGMENT This work was financially supported by International Science & Technology Cooperation Program of China (2013DFG52740, 2010DFA52150), and the National Natural Science Foundation of China (51473053, 51073060, 91222201，91333206). X.Z., W.-K.W. and W.Y.W. thank Hong Kong Research Grants Council (HKBU 22304115-ECS, HKBU 203011), Hong Kong Baptist University (FRG1/14-15/058, FRG2/13-14/083) and Areas of Excellence Scheme ([AoE/P-03/08]) for the financial support. F.L. was financially supported by the U.S. Department of Energy, Office of Basic Energy Sciences (DE-SC0001087). Specifically, X.Z. and X.B.P. thank Chang Liu, Junbiao Peng (SCUT), and Ben Ong (HKBU) for their help and discussions in this work. REFERENCES


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Table of Contents

A new class of porphyrin donors conjugated with terthiophene have been developed with good solar flux coverage and ordered self-assembly properties, resulting in the highest efficiency of 8.21% in solution-processed bulk heterojunction organic solar cells with PC71BM as acceptor.


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New Terthiophene-Conjugated Porphyrin Donors for Highly Efficient

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