Novel Star-Shaped Helical Perylene Diimide Electron Acceptors for

Subscriber access provided by University of Sussex Library

Organic Electronic Devices

Novel Star-Shaped Helical Perylene Diimide Electron Acceptors for Efficient Additive-Free Non-Fullerene Organic Solar Cells Mingliang Wu, Jian-Peng Yi, Li Chen, Guiying He, Fei Chen, Matthew Y. Sfeir, and Jianlong Xia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06126 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 28, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21 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

ACS Applied Materials & Interfaces

Novel Star-Shaped Helical Perylene Diimide Electron Acceptors for Efficient Additive-Free Non-Fullerene Organic Solar Cells Mingliang Wu,1# Jian-Peng Yi,1# Li Chen,1# Guiying He,1 Fei Chen,1 Matthew Y. Sfeir, 2 Jianlong Xia1* 1. School of Chemistry, Chemical Engineering and Life Science; State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Email: [email protected] 2. Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA. #

These authors contributed equally to this work

Keywords: Star-Shaped, Perylene Diimide, Non-Fullerene, Organic Solar Cells, 3D structure

Abstract Two star-shaped helical perylene diimide (PDI) electron acceptors TPDI2 and FTPDI2 were designed and synthesized for non-fullerene organic solar cells (OSCs). The integration of helical PDIs into a 3D structure provides a new strategy to tune the intermolecular interactions, and the as-cast blend films with PTB7-Th show favorable morphology and efficient charge transfer and separation, as evidenced by the morphology and femtosecond transient absorption spectroscopy studies. A trade-off between suppressing the self-aggregation and maintaining the charge transfer properties was achieved by FTPDI2. Using PTB7-Th as the electron donor, the FTPDI2 based non-fullerene OSCs show a high power conversion efficiency (PCE) of 8.28%, without the assistance of any additives.

Introduction In the past decade, a great deal of attention has been devoted to organic non-fullerene acceptors (NFAs) due to the shortcomings of fullerene acceptors including limited energy level tunability, weak absorption in the visible region, and morphology instability.1-13 Owing to their outstanding characteristics such as strong absorption in the visible region, ease of chemical functionalization as well as relative low-cost, the use of perylene diimide (PDI) and its derivatives as electron acceptors for organic solar cells (OSCs) has made great achievements during the past five years, and PDI based

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

acceptors have become one of the most promising candidates for non-fullerene OSCs.14-25 PDI molecules usually display strong π-π intermolecular interactions owing to the planar structure of the PDI core unit, which increases the exciton diffusion lengths and thus achieves efficient charge transportation and separation.5, 26-27 While the planar PDI derivatives also exhibit the tendency of self-aggregation and crystallization from the donor-acceptor blend films, which are highly detrimental to the device performance.28-32 Therefore, restricting the crystallinity without adversely weakening the exciton diffusion lengths is a design principle for PDI-based acceptors. Many molecular design approaches have been reported to successfully deal with this trade-off,33 for examples, decreasing the intermolecular packing through the formation of helical oligomers,4, 17, 34-42 or integrating the planar perylene monomer into a star-shaped structure to form 3D network assembly.43-55 Motivated by the recent gram-scale synthesis of PDI dimer developed by Nuckolls et al.,34, 40 we describe here the design, synthesis and device applications of two novel PDI based acceptors TPDI2 and FTPDI2 (Scheme 1), in which the helical PDI dimers (PDI2) are incorporated into a star-shaped 3D structure. In TPDI2, three PDI2 units are connected to the center benzene ring with single bonds, a DFT optimized minimum energy structure suggests that an unfavorable intermolecular packing is achieved due to the highly distortion. For FTPDI2, the three PDI2 units are fused to the core benzene ring, DFT modeling structure shows the molecular torsion is significantly reduced in contrast to TPDI2. Femtosecond transient absorption spectroscopy studies revealed that the PTB7-Th:FTPDI2 blend film supports the highest internal quantum efficiency. By using the commercially available PTB7-Th as the electron donor, a PCE of 7.25% was obtained for TPDI2-based OSCs without the addition of additive, while a higher PCE of 8.28% was achieved for FTPDI2-based additive-free OSCs with simple thermal annealing treatment (80 oC for 10 min). This result indicates that the fused star-shaped 3D structure of FTPDI2 has achieved a trade-off between suppressing the molecular self-aggregation and maintaining the charge-transport properties. Combining with the gram-scale synthesis of PDI derivatives developed by Nuckolls and co-workers, the integration of non-planar PDI units into a 3D structure provides a promising molecular design strategy to develop novel PDI based electron acceptors for high efficient additive-free OSCs.34, 40


Page 2 of 21

Page 3 of 21 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


Scheme 1. Synthetic route of the TPDI2 and FTPDI2.

Results & discussion Enabled by the gram-scale synthesis of PDI dimer (PDI2, Figure S1), the synthesis of TPDI2 and FTPDI2 are quite straightforward and scalable. The TPDI2 was synthesized by a Suzuki coupling reaction between bromo-PDI2 and commercially available 1,3,5-Benzenetriboronic acid tris(pinacol) ester in a yield of 74% (Scheme S1). TPDI2 underwent efficient photocyclization reaction under the catalyst of I2 by using a home-built visible light photochemical flow reactor,40 the FTPDI2 (Scheme S2) was obtained by column chromatography and subsequently recrystallized from methanol and cyclohexane in a yield of 90%. It is worth to point out that the flow photochemistry allows for synthetic scalability of this type of nanoribbons. Both TPDI2 and FTPDI2 were characterized by 1H NMR,

13

C NMR (supporting information) and MALDI-TOF mass spectrum

(supporting information). Thermogravimetric analysis (TGA, Figure S2a) and differential scanning calorimetry (DSC, Figure S2b) were applied to investigating the thermal properties of TPDI2 and FTPDI2. The TGA data exhibit the decomposition temperature (5% weight loss) under nitrogen ACS Paragon Plus Environment


atmosphere was over 370 oC, which implied that both of TPDI2 and FTPDI2 have good thermal stability. In the DSC curve of TPDI2, there is inconspicuous melting peaks of 183 oC, during heating process, while the FTPDI2 has a sharp melting peak at 207 oC.

Figure 1. DFT optimized molecular structures of TPDI2 ((a) top view, (b) side view) and FTPDI2 ((c) top view, (d) side view)

Figure 1 shows the DFT calculated molecular structures of possible energy minimized conformations of TPDI2 and FTPDI2. The helical structure of PDI dimer is well persevered in TPDI2 and FTPDI2, as evidenced by the twist angles between two PDI units of 25 °, 6.8°, 25.2° and 26°, 6°, 24.7°, respectively. The dihedral angles between the inner PDI units and the central benzene ring for TPDI2 and FTPDI2 are 51°, 56°, 46° and 6.5°, 36°, 5.25°, respectively (Figure S5, S6). This


Page 4 of 21

Page 5 of 21

suggests that after incorporating the helical PDI dimers into a 3D structure, the TPDI2 displays a more twisted conformation in comparison to fused molecular FTPDI2 and thus can significantly suppress the intermolecular interactions. The electronic properties of TPDI2 and FTPDI2 in comparison to PDI2 were accessed by using UV−Vis spectroscopy (Figure S3), DFT calculations, and cyclic voltammetry (CV) (Table 1, Figure S4). Figure 2a shows the UV-Vis spectra of PDI2, TPDI2, FTPDI2 and the polymeric donor PTB7-Th in the thin film state (UV-Vis in solution showed in Figure S3). FTPDI2 display broad absorptions in the range of 350~450 nm and its absorption onset is red-shifted in contrast to PDI2 and TPDI2. According to the TDDFT calculation (Table S1, Figure S7, S8), the characteristic absorption band centered at 400 nm for FTPDI2 may arise from the fused conjugation between the central benzene ring and the three outside PDI2 units. All of the PDI2, TPDI2, FTPDI2 show complementary absorptions with PTB7-Th in the 350~800 nm. DFT calculated HOMO and LUMO orbitals of TPDI2 and FTPDI2 are showed in the Supporting Information (Figure S9). The HOMO and LUMO of TPDI2 are located on two different PDI2 units, whereas the HOMO and LUMO for FTPDI2 consist of appreciable degenerate orbital configurations that are centered on the core benzene ring. The CV patterns of PDI2, TPDI2 and FTPDI2 are presented in the supporting information (Figure S4). The LUMO levels of PDI2, TPDI2 and FTPDI2 calculated from their reduction onsets are listed in Table 1. The HOMO levels and bandgaps of PDI2, TPDI2 and FTPDI2 evaluated by combining the UV-Vis absorption with CV data are presented in Table 1. The energy levels of PDI2, TPDI2 and FTPDI2 are well aligned with PTB7-Th (Figure 2b), in conjunction with their complementary UV−Vis absorption, indicating that PTB7-Th (Figure S1) is a suitable electron donor for engineering the PDI-based OSC devices.20 2.5

(a) Absorption (norm.)

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


PTB7-Th PDI2 TPDI2 FTPDI2

2.0 1.5 1.0 0.5 0.0 300

400

500

600

700

800

Wavelength (nm)



Page 6 of 21

Figure 2. (a) The absorption spectra of PTB7-Th (solid spheres), PDI2 (solid squares), TPDI2 (solid circles), and FTPDI2 (solid triangles) thin films. (b) The energy-level diagram for PTB7-Th, PDI2, TPDI2, FTPDI2. The energy levels of PTB7-Th were obtained from the literature.20

Table 1. Summary of photophysical and electrochemical properties of PDI2, TPDI2, FTPDI2.

εa/M-1 cm-1

Eredb/V

EHOMOc/eV

ELUMOc/eV

Egd/eV

PDI2

1.1×104 (548 nm)

-0.46

-6.06

-3.91

2.15

TPDI2

1.6×104 (550 nm)

-0.50

-5.89

-3.80

2.09

FTPDI2

2.3×104 (538 nm)

-0.49

-5.83

-3.81

2.02

Compound

a

ε was determinated in the CHCl3 solutions of a concentration of 5.0×10-6 M.

potentials were obtained through cyclic voltammetry (CV) method.

c

b

The reduction

The LUMO levels were

calculated by the following equations: ELUMO = - (Ered - EFc + 4.8) eV, where Ered results were obtained from the onset of reduction, respectively, while EFc was the half-wave potential of ferrocene. d

The optical bandgap was estimated from the onset positions of their absorption spectra and

calculated by the equation: Eg = 1240/λ onset. In order to compare the photovoltaic behaviors of TPDI2 and FTPDI2 with PDI2, we fabricated solar cells with an inverted device structure of indium tin oxide (ITO)/ZnO/PTB7-Th:TPDI2 or FTPDI2/MoO3/Ag, using PTB7-Th as electron donor and TPDI2 or FTPDI2 as electron acceptors, while PDI2-based OSCs was prepared as control devices (Figure S1). The device performance of as-cast and annealing treatment were summarized in Table S2-S4 and the result of optimal D/A ratio and with/without DIO additive were listed in Table S5-S8. The optimized D/A ratio was 1:1 (w/w) and the addition of DIO additive have no positive effect on the performance of the devices. The characteristic current density-voltage (J-V) curves of the optimized OSCs are shown in Figure 3a, and the corresponding device photovoltaic parameters are summarized in Table 2. The as-cast PTB7-Th:PDI2 devices exhibit a PCE of 4.67%, and the PCE was increased to 5.57% after thermal annealing at 80 oC for 10 min. This device performance is comparable to previously reported results. With regard to the as-cast TPDI2-based devices, a much higher PCE of 7.25% was obtained and simple thermal annealing treatment (annealed upon 80 oC for 10 min) can further increase the PCE to 7.84%. A PCE of 7.65% was achieved for the FTPDI2-based as-cast devices without addition


Page 7 of 21

of additives, after 10 min thermal annealing at 80 oC the PCE was increased to 8.28% with a Voc of 0.79 V, Jsc of 16.87 mA/cm2 and FF of 61.8% (Table 2, S2-S4). Comparing the device performances of these three PDI based OSCs, the highest PCE of 8.28% indicates that a trade-off between suppressing the self-aggregation and preserving the charge transportation properties of PDI based acceptors was achieved for FTPDI2. The results demonstrate that the integration of helical PDI oligomers into 3D architectures is a promising design strategy to develop nonfullerene acceptors for high efficient additive-free OSCs. The short current density (Jsc) increased from 12.50 to 15.98 and 16.87 mA/cm2, and the high

Jsc value (16.87 mA/cm2) for PTB7-Th/FTPDI2 without additives is comparable with the complexly engineered PTB7-Th/PC71BM based solar cells (15-19 mA/cm2).56-57 The enhancement of Jsc values was also confirmed by the EQE measurements as shown in Figure 3b. All these three devices cover a broad EQE spectral from 300 nm to 800 nm, which are corresponding to the absorption spectra of the donor and acceptors. Compared to PDI2, the TPDI2 and FTPDI2-based devices show improved EQEs, for instance, the EQE at ~550 nm was increased from 51.7% to 63.3% and 72.5%. A high EQE of 76.5% (683 nm) without any aid of solvent additives was presented by the FTPDI2-based OSCs. It is notable that the FTPDI2-based devices display an obvious EQE maximum near 420 nm, which is in good agreement with the characteristic UV-Vis absorption of FTPDI2 in the 350-450 nm range. The integration of the EQE of TPDI2 and FTPDI2 (Figure S10) is 13.10 mA/cm2 and 15.87 mA/cm2, respectively, which is much lower to the best device Jsc 15.85 mA/cm2 and 16.77 mA/cm2. We ascribed this to the degradation of the devices.

5

80 (a)

70

(b)

2

Current Density (mA/cm )

60

0

PTB7-Th:PDI2 PTB7-Th:TPDI2 PTB7-Th:FTPDI2

-5

EQE (%)

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


-10

50 40 30 20

-15 -20 -0.2

PTB7-Th:PDI2 PTB7-Th:TPDI2 PTB7-Th:FTPDI2

10 0.0

0.2

0.4 0.6 Voltage (V)

0.8

1.0

0 300

400

500

600

700

800

Wavelength (nm)

Figure 3. (a) The J-V curves of the solar cells based on PDI2 (solid/ squares), TPDI2 (solid ACS Paragon Plus Environment


Page 8 of 21

circles) and FTPDI2 (solid triangles) under illuminated conditions. (b) The external quantum efficiency (EQE) spectra for the devices based on PDI2 (solid squares), TPDI2 (solid circles), and FTPDI2 (solid triangles). Table 2. Photovoltaic performance parameters of OSCs based on PTB7-Th:PDI2, PTB7-Th:TPDI2 and PTB7-Th:FTPDI2.

Voc (V)

Jsc (mA/cm-1)

FF (%)

PCEa (%)

PTB7-Th : PDI2-as cast

0.778±0.003

12.30±0.20

45.60±0.20

4.36 (4.67)

PTB7-Th : PDI2-annealed

0.760±0.003

12.13±0.18

57.00±0.24

5.25 (5.57)

PTB7-Th : TPDI2-as cast

0.770±0.005

15.08±0.15

57.75±0.25

6.71 (7.25)

PTB7-Th : TPDI2-annealed

0.777±0.004

15.85±0.13

62.55±0.25

7.70 (7.84)

PTB7-Th : FTPDI2-as cast

0.786±0.003

15.64±0.11

61.25±0.15

7.53 (7.65)

PTB7-Th : FTPDI2-annealed

0.790±0.003

16.77±0.10

61.67±0.13

8.17 (8.28)

Device

a

These values were obtained from 5 devices for each condition. The optimum PCEs are shown in

parentheses. The excimer formation has been previous observed in PDI based OSCs,52,

58

and the

introduction of twist conformation is beneficial to form a domain smaller than the excimer diffusion length and may also prevent the excimer formation.52,

59

As shown in Figure S11a-c, the

photoluminescence (PL) of PDI2 in the film displays a broader and red-shifted emission band compare to that in the dilute solution, indicating the formation of PDI2 excimers. While the red-shift of the emission peaks of TPDI2 and FTPDI2 in the films are significantly reduced, suggested that the combination of the dimeric structure and 3D conformation could suppress the formation of excimers. To probe the extent of exciton dissociation, fluorescence quenching measurements have been carried out. The PL intensity of the blend film and neat films were shown in Figure S11d-i. The PL quenching efficiencies (summarized in Table S9) were calculated to be 81.9%, 98.0% and 99.2% (excimers from PDI2 and hole transfer quenching of TPDI2 and FTPDI2) and 97.6%, 94.3%, and 96.0% (electron transfer quenching of PTB7-Th), suggesting highly efficient exciton dissociation for the TPDI2 and FTPDI2 based blends. In addition, relatively weak bimolecular recombination was observed for both the TPDI2 and FTPDI2-based devices, as evidenced by the dependence of Jsc on the illumination light density studies (Figure S12, see the discussion in Supporting information). The


Page 9 of 21 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


efficient exciton dissociation and weak bimolecular recombination are accounted for the high PCEs of the OSCs. Space-charge-limited-current (SCLC) method was used to determine the hole and electron mobility (µh, µe) of the optimal PTB7-Th:PDI2, PTB7-Th:TPDI2 and PTB7-Th:FTPDI2 blend films (Figure S13). Hole-only and electron-only devices were fabricated with the structures of ITO/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/blend film/MoO3/Ag and ITO/ZnO/blend film/LiF/Al, respectively. The results are listed in Table 3. For the annealed PTB7-Th:TPDI2 blend film, the hole and electron mobilities are 1.2×10-4 cm2 V-1 s-1 and 4.7×10-4 cm2 V-1 s-1, respectively. A much higher hole mobility of 3.0×10-4 cm2 V-1 s-1 was obtained for annealed PTB7-Th:FTPDI2 blend film. The µe/µh ratio of 1.6 for the PTB7-Th:FTPDI2 film is much lower than that of PTB7-Th:TPDI2 blend film (3.9), indicating a more balanced carrier transport within the PTB7-Th:FTPDI2 film.

Table 3. The SCLC results of PTB7-Th:TPDI2 and PTB7-Th:FTPDI2 blend films. Hole mobility (cm2 V-1 s-1)

Electron mobility (cm2 V-1 s-1)

µe/µh

PTB7-Th:TPDI2

1.2×10-4

4.7×10-4

3.9

PTB7-Th: FTPDI2

3.0×10-4

4.7×10-4

1.6

Device

*All of the device were annealed at 80 oC for 10 mins. The microscopic morphologies of the blend films were studied by atomic force microscope (AFM) and transmission electron microscope (TEM), all the as-cast blend films exhibit good film morphologies (Figure S14, S15). The root-mean-square (rms) roughness values for the as-cast PTB7-Th:PDI2, PTB7-Th:TPDI2 and PTB7-Th:FTPDI2 blend films are 0.628, 0.604, and 0.493 nm, respectively. The results of DFT calculation (Figure S5, S6) support that FTPDI2 has a less twisted structure than TPDI2, conbined with its small roughness can strongly indicate that FTPDI2 reached a better structure twist balance. And the best miscibility between donor and acceptor was obtained from the PTB7-Th:FTPDI2 blend film. The morphologies show slight rms roughness variation after 10 min thermal annealing at 80 oC. The good film morphologies are further confirmed by the TEM studies (Figure S15). The XRD spectra (Figure S16) of neat film (PDI2, TPDI2 and FTPDI2) and blend film (PTB7-Th:PDI2, PTB7-Th:TPDI2 and PTB7-Th:TPDI2) indicated that the films are



amorphous. All the as-cast and annealed blend films display uniform and homogeneous distribution without evident phase separation or aggregation. The good morphology of the blend films is highly desirble for efficient exciton diffusion. To gain further insight into the device performance difference, femtosecond transient absorption (TA) spectroscopy was used to probe the exciton dynamics of the blend films. Figure 4 shows the TA spectra and normalized kinetics of PTB7-Th:PDI2 and PTB7-Th:FTPDI2 blend films excited at 520 nm (PTB7-Th:TPDI2 blend film in Supporting Information, Figure S17). There is a large degree of spectral overlap between the different species, which can be seen by comparing the transient spectra of the neat films and the blends (Figure 4a, 4b). In addition to ground-state bleaching and excited state absorption assigned to PTB7-Th (minimum near 720 nm) and PDI2 derivatives (minimum near 550 nm for PDI2 and 575 nm for FTPDI2), we also observe a broad and relative strong excited state absorption at wavelengths greater than 750 nm that is associated with both the PDI exciton (green dashed line in Figure 4a, 4b) and the hole polaron. At the excitation wavelength of 520 nm, photocarrier generation occurs in both PTB7-Th and the PDI2 derivatives (mimicking solar conditions), allowing us to simultaneously monitor electron transfer from PTB7-Th to the acceptor and hole transfer from the acceptor to PTB7-Th. We find that PTB7-Th:PDI2 and PTB7-Th:FTPDI2 blends differ primarily by the fraction of excitons that undergo charge transfer rather than their charge transfer kinetics. This result supports the idea that the FTPDI2 acceptor suppresses self-aggregation. In both blends, the fast rise of GSB and ESA signals indicate that rapid charge (hole and electron) transfer occurs at the interface of the heterojunctions. The diffusive part of the charge transfer process can be resolved in PTB7-Th:PDI2 and PTB7-Th:TPDI2 films, where we observe TA features that rise with time constants of ~4.5 ps and 2.5 ps, respectively. We note that when we preferentially excited the polymer at 700 nm (shown in Figure S18), electron transfer occurred in < 1 ps, indicating that hole transfer is the limiting step under solar illumination. No discernable rise of PTB7-Th:FTPDI2 transient features resolved, indicate that charge transfer occurs on time scales below 100 fs, as has been seen in other high efficiency polymer systems.60-61 Still, the slightly slower charge transfer in PDI2 would not materially affect the overall photocurrent efficiency, since charge transfer is still much faster than other decay processes. The more essential features for solar cell operation can be seen in the decay of the kinetic traces for the different blends (Figure 4c, 4d). In PTB7-Th:FTPDI2, the decay kinetics ACS Paragon Plus Environment

Page 10 of 21

Page 11 of 21 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


for the ground state bleaches of the donor and acceptor are identical to the polaron signal (The normalized GSB decay of thermal annealed and as-cast PTB7-Th:FTPDI2 blend film was shown in Figure S19). This indicates that all of the excited state population is contained in the polaron, and internal quantum efficiencies approaching 100% can be theoretically achieved. In contrast, the decay kinetics of PTB7-Th:PDI2 and PTB7-Th:TPDI2 are not identical throughout the probe spectral region, indicating that a large fraction of the excited state population decays without undergoing charge transfer. TA measurements further proved our design strategy of using star-shaped structure to suppress self-aggregation between PDI dimers, and the rigid structure of fused FTPDI2 can achieve a better balance to suppress the self-aggregation and maintain efficient exciton diffusion than nonfused TPDI2. And Together, these results suggest the films of PTB7-Th:PDI2 and PTB7-Th:TPDI2, contain large domains of pure donor and acceptor phases. And this is also in good accordance with the device characterization results described above.

Figure 4. TA spectra and normalized dynamics of the (a, c) PTB7-Th:PDI2 and (b, d) PTB7-Th:FTPDI2. For comparison, the spectra of pure PTB7-Th, PDI2 and FTPDI2 are presented in arbitrary unit.



Conclusion In summary, two new star-shaped helical PDI-based electron acceptors TPDI2 and FTPDI2 have been designed and synthesized. Both TPDI2 and FTPDI2 display promising potential as electron acceptors for photovoltaic device applications. DFT calculations show that TPDI2 has a more twisted structure, while morphology studies suggest that FTPD2 displays better miscibility with PTB7-Th, which was also confirmed by the PL quenching and femtosecond transient absorption (TA) spectroscopy studies. We found that FTPDI2 achieves a better balance in suppressing the molecular self-aggregation and maintaining the charge transfer properties. Using the commercially available PTB7-Th as electron donor, FTPDI2-based devices show a high PCE of 8.28% with a Voc of 0.79 V, Jsc of 16.87 mA/cm2 and FF of 61.8%, without the assistance of any additive. The combination of helical conformation with 3D structure provided an efficient way to tune the intermolecular packing of PDI based acceptors, this strategy can be used to design novel PDI based acceptors for high-efficient additive-free OSCs.

Experimental Section Synthesis of TPDI2. A 100 ml flask was charged with 1.5 g molecule 1 (0.993 mmol), 118.6 mg molecule 2 (0.26 mmol), 200 mg Tetrakis(triphenylphosphine)palladium(0) , 30 ml THF and 15 ml Na2CO3 (8 g) aqueous solution, the mixture was degassed under N2 for 45 minutes, then the mixture was refluxed under 70 oC for 48 h. After cooling down to room temperature, the mixture was poured into excessive 1M HCl aqueous solution and stirred overnight. The precipitate was filtrated and washed with H2O, CH3OH. After dried under vacuum, the crude solid was purified by silica gel chromatography eluted with petroleum ether/CH2Cl2 from 2:1 to 1:2, further recrystallization with cyclohexane afforded the target molecule TPDI2 as dark red laminar solid (900 mg, 74%).1H NMR (500 MHz, CDCl3, 323 K) δ 10.39 (s, 13H), 9.38 (d, J = 120.0 Hz, 20H), 8.59 (s, 3H), 5.30 (d, J = 44.4 Hz, 12H), 2.38 (s, 24H), 2.02 (s, 24H), 1.34 (d, J = 55.8 Hz, 192H), 0.84 (s, 72H).13C NMR (101 MHz, CDCl3, 323 K) δ 164.90, 163.78, 134.04, 130.36, 127.34, 127.13, 126.54, 126.18, 125.87, 124.33, 123.95, 55.14, 32.60, 32.40, 31.80, 31.70, 29.73, 29.69, 29.40, 29.30, 29.22, 27.08, 26.90, 22.73, 22.64, 22.58, 22.51, 14.17, 14.09, 14.02, 13.92.HRMS (MALDI-TOF): [TPDI2+Na]+ calculated for 4684.731, found 4684.714. Synthesis of FTPDI2. TPDI2 (200 mg, 0.0429 mmol) was dissolved in 500 mL toluene in a


Page 12 of 21

Page 13 of 21 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


1-L round bottom flask, and 500 mg of I2 was added. The reaction mixture solution was drawn into a home-built flow reactor40 (equipped with four 450 W-mercury lamps). The reaction mixture was repeatedly pumped through the flow reactor with a retention time of ~32 hours. The solvent was removed using vacuum rotary evaporator and the crude residue was washed with methanol to remove excessive iodine. The crude red solid was then purified with silica gel column using petroleum ether/CH2Cl2 as eluent. And the final product of FTPDI2 was obtained as a red solid (180 mg, 90%) following the subsequent recrystallization from methanol and cyclohexane. 1H NMR (500 MHz, CDCl3, 323 K) δ 11.09 – 10.23 (m, 15H), 9.71 – 9.07 (m, 15H), 5.38 (d, J = 37.8 Hz, 12H), 2.45 (s, 24H), 2.11 – 1.98 (m, 24H), 1.46 – 1.23 (m, 192H), 0.91 – 0.78 (m, 72H).13C NMR (101 MHz, CDCl3, 323 K) δ 165.24, 165.03, 164.91, 134.19, 129.40, 127.60, 127.47, 127.22, 127.13, 126.12, 125.59, 124.68, 124.56, 123.99, 122.34, 55.27, 53.43, 32.68, 31.86, 31.63, 30.93, 29.73, 29.69, 29.39, 29.36, 28.98, 27.17, 26.97, 22.68, 14.10.HRMS (MALDI-TOF): [FTPDI2+Na]+ calculated for 4678.684; found 4678.735. Device fabrication. The inverted devices were fabricated with the structure of ITO/ZnO/active layer/MoO3/Ag. The pre-patterned (sheet resistance, 15 Ω/sq) ITO-glass substrates were sequentially cleaned in ultrasonic bath with detergent (Alconox Inc.), de-ionized water, acetone and isopropanol. The oven-dried substrates were then treated by an oxygen plasma (180 W) for 5 min. The ZnO precursor solution (110 mg/mL) was prepared by dissolving 0.22 g ZnAc2·2H2O in 2 mL 2-methoxyethanol and 0.056 mL ethanol amine and then stirred for at least 24h before use. The solution was filtered with polyether sulfone (PES) filters. The ZnO precursor solution was spin-cast onto ITO substrate with spinning rate of 5000 rpm for 60s and the thickness was ~32 nm. The as-cast film was then annealed in ambient circumstance upon 150 oC for 60 min to form a compact ZnO layer. The blend solutions of PTB7-Th:PDI2, PTB7-Th:TPDI2 and PTB7-Th:FTPDI2 (D/A ratio, 1:1, weight ratio) mixtures were all processed with o-dichlorobenzene (o-DCB) with an identical concentration of 35 mg mL-1. All these solutions were heated at 60 oC and stirred overnight (or 100 o

C for 3h) to obtain well-mixed blend solutions. The active layer thicknesses of these three devices

were carefully optimized through spinning rate variation (1500, 1800, 2000, 2500, 3000 and 3500 rpm). And, the spinning duration was fixed at 50s. The active layers were thermal annealed at 80 oC in glove box for 10 min to get rid of residual solvent. A MoO3 (8 nm) layer and an Ag layer (100 nm) electrode were sequentially deposited by thermal evaporation using a shadow mask under a vacuum ACS Paragon Plus Environment


of

Novel Star-Shaped Helical Perylene Diimide Electron Acceptors for

Recommend Documents