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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 27894−27901

Novel Star-Shaped Helical Perylene Diimide Electron Acceptors for Efficient Additive-Free Nonfullerene Organic Solar Cells Mingliang Wu,†,§ Jian-Peng Yi,†,§ Li Chen,†,§ Guiying He,† Fei Chen,† Matthew Y. Sfeir,‡ and Jianlong Xia*,†

ACS Appl. Mater. Interfaces 2018.10:27894-27901. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 01/29/19. For personal use only.

†

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 ‡ Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States S Supporting Information *

ABSTRACT: Two star-shaped helical perylene diimide (PDI) electron acceptors TPDI2 and FTPDI2 were designed and synthesized for nonfullerene organic solar cells (OSCs). The integration of helical PDIs into a three-dimensional structure provides a new strategy to tune the intermolecular interactions, and the as-cast blend films with PTB7-Th show favorable morphology as well as efficient charge transfer and separation, as evidenced by the morphology and femtosecond transient absorption (fs-TA) 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 nonfullerene OSCs show a high power conversion efficiency of 8.28%, without the assistance of any additives. KEYWORDS: star-shaped, perylene diimide, nonfullerene, organic solar cells, 3D structure

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INTRODUCTION

monomer into a star-shaped structure to form a threedimensional (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 central benzene ring with single bonds; a density functional theory (DFT) optimized minimum energy structure suggests that an unfavorable intermolecular packing is achieved because of high distortion. For FTPDI2, the three PDI2 units are fused to the core benzene ring; the DFT modeling structure shows the molecular torsion is significantly reduced in contrast to that of TPDI2. Femtosecond transient absorption (fs-TA) 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 power conversion efficiency (PCE) of 7.25% was obtained for TPDI2-based OSCs without the addition of an additive, whereas a higher PCE of 8.28% was achieved for FTPDI2based additive-free OSCs with a simple thermal annealing treatment (80 °C for 10 min). This result indicates that the

In the past decade, a great deal of attention has been devoted to organic nonfullerene acceptors (NFAs) because of the shortcomings of fullerene acceptors including limited energylevel 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 acceptors have become one of the most promising candidates for nonfullerene 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 However, 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 example, decreasing the intermolecular packing through the formation of helical oligomers4,17,34−42 or integrating the planar perylene © 2018 American Chemical Society

Received: April 16, 2018 Accepted: July 27, 2018 Published: July 27, 2018 27894

DOI: 10.1021/acsami.8b06126 ACS Appl. Mater. Interfaces 2018, 10, 27894−27901

Research Article

ACS Applied Materials & Interfaces Scheme 1. Synthetic Routes of TPDI2 and FTPDI2

good thermal stability. In the DSC curve of TPDI2, there is an inconspicuous melting peak at 183 °C, during the heating process, whereas FTPDI2 shows a sharp melting peak at 207 °C. Figure 1 shows the DFT-calculated molecular structures of the possible energy-minimized conformations of TPDI2 and FTPDI2. The helical structure of the PDI dimer is wellpersevered in TPDI2 and FTPDI2, as evidenced by the twist angles between the 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 (Figures S5 and S6). This suggests that after incorporating the helical PDI dimers into a 3D structure, TPDI2 displays a more twisted conformation in comparison to the 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 displays broad absorptions in the range of 350−450 nm, and its absorption onset is red-shifted in contrast to those of PDI2 and TPDI2. According to the time-dependent DFT calculations (Table S1, Figures S7 and 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 outer PDI2 units. All of the PDI2, TPDI2, and FTPDI2 show

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 nonplanar 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

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RESULTS AND DISCUSSION Enabled by the gram-scale synthesis of PDI dimer (PDI2, Figure S1), the synthesis of TPDI2 and FTPDI2 is quite straightforward and scalable. TPDI2 was synthesized by the 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 an efficient photocyclization reaction, with I2 as the catalyst, by using a home-built visible light photochemical flow reactor,40 and 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 the synthetic scalability of the nanoribbons of this type. Both TPDI2 and FTPDI2 were characterized by 1H NMR, 13C NMR (Supporting Information), and MALDI-TOF mass spectrometry (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 exhibits the decomposition temperature (5% weight loss) under nitrogen atmosphere was over 370 °C, which implied that both TPDI2 and FTPDI2 have 27895

DOI: 10.1021/acsami.8b06126 ACS Appl. Mater. Interfaces 2018, 10, 27894−27901

Research Article

ACS Applied Materials & Interfaces

are located on two different PDI2 units, whereas the HOMO and LUMO for FTPDI2 consist of appreciably degenerated 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 of CV are listed in Table 1. The HOMO levels and the band gaps of PDI2, TPDI2, and FTPDI2 evaluated by combining the UV−vis absorption and 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 spectra, indicating that PTB7-Th (Figure S1) is a suitable electron donor for engineering the PDI-based OSC devices.20 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/PTB7Th:TPDI2 or FTPDI2/MoO3/Ag, using PTB7-Th as the electron donor and TPDI2 or FTPDI2 as electron acceptors, whereas PDI2-based OSCs were prepared as control devices (Figure S1). The performances of the as-cast and thermalannealed devices are summarized in Tables S2−S4, and the results of the optimal D/A ratio and those with/without a DIO additive are listed in Tables S5−S8. The optimized D/A ratio was 1:1 (w/w), and the addition of the DIO additive had no positive effect on the performance of the present 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 °C 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 a simple thermal annealing treatment (annealed upon 80 °C 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 the addition of additives; after 10 min of thermal annealing at 80 °C, the PCE was increased to 8.28% with a Voc of 0.79 V, Jsc of 16.87 mA/cm2, and fill factor (FF) of 61.8% (Tables 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 selfaggregation and preserving the charge-transport 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 NFAs for high efficient additive-free OSCs.

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

Table 1. Summary of the Photophysical and Electrochemical Properties of PDI2, TPDI2, and FTPDI2 compound

εa/M−1 cm−1

Eredb/V

EHOMOc /eV

ELUMOc /eV

Egd /eV

PDI2 TPDI2 FTPDI2

1.1 × 104 (548 nm) 1.6 × 104 (550 nm) 2.3 × 104 (538 nm)

−0.46 −0.50 −0.49

−6.06 −5.89 −5.83

−3.91 −3.80 −3.81

2.15 2.09 2.02

ε was determined in the CHCl3 solution of a concentration of 5.0 × 10−6 M. bThe reduction potentials were obtained through the CV method. cThe LUMO levels were calculated by the following equations: ELUMO = −(Ered − EFc + 4.8) eV, where the Ered results were obtained from the onset of reduction peak, whereas EFc was the half-wave potential of ferrocene. dThe optical band gap was estimated from the onset positions of their absorption spectra and calculated by the equation: Eg = 1240/λonset. a

complementary absorptions with PTB7-Th in the 350−800 nm range. The DFT-calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of TPDI2 and FTPDI2 are shown in the Supporting Information (Figure S9). The HOMO and LUMO of TPDI2

Figure 2. (a) Absorption spectra of PTB7-Th (solid spheres), PDI2 (solid squares), TPDI2 (solid circles), and FTPDI2 (solid triangles) thin films. (b) Energy-level diagrams for PTB7-Th, PDI2, TPDI2, and FTPDI2. The energy levels of PTB7-Th were obtained from the literature.20 27896

DOI: 10.1021/acsami.8b06126 ACS Appl. Mater. Interfaces 2018, 10, 27894−27901

Research Article

ACS Applied Materials & Interfaces

Figure 3. (a) J−V curves of the solar cells based on PDI2 (solid/squares), TPDI2 (solid circles), and FTPDI2 (solid triangles) under illuminated conditions. (b) 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 device PTB7-Th:PDI2-as cast PTB7-Th:PDI2-annealed PTB7-Th:TPDI2-as cast PTB7-Th:TPDI2-annealed PTB7-Th:FTPDI2-as cast PTB7-Th:FTPDI2-annealed

Voc (V) 0.778 0.760 0.770 0.777 0.786 0.790

± ± ± ± ± ±

0.003 0.003 0.005 0.004 0.003 0.003

Jsc (mA/cm−1) 12.30 12.13 15.08 15.85 15.64 16.77

± ± ± ± ± ±

0.20 0.18 0.15 0.13 0.11 0.10

FF (%) 45.60 57.00 57.75 62.55 61.25 61.67

± ± ± ± ± ±

0.20 0.24 0.25 0.25 0.15 0.13

PCEa (%) 4.36 5.25 6.71 7.70 7.53 8.17

(4.67) (5.57) (7.25) (7.84) (7.65) (8.28)

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

a

S9) were calculated to be 81.9, 98.0, and 99.2% (excimers from PDI2 and the hole transfer quenching of TPDI2 and FTPDI2) and 97.6, 94.3, and 96.0% (electron transfer quenching of PTB7-Th), suggesting a highly efficient exciton dissociation for the TPDI2- and FTPDI2-based blends. In addition, a relatively weak bimolecular recombination was observed for both the TPDI2- and FTPDI2-based devices, as evidenced by the dependence of Jsc on the light density studies (Figure S12; see the discussion in the Supporting Information). The efficient exciton dissociation and weak bimolecular recombination are accounted for the high PCEs of the OSCs. The 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 PTB7Th: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)/blend film/MoO3/Ag and ITO/ZnO/blend film/LiF/ Al, respectively. The results are listed in Table 3. For the

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 external quantum efficiency (EQE) measurements, as shown in Figure 3b. All these three devices cover a broad EQE spectral range from 300 to 800 nm, which corresponds 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 integrated current density calculated from the EQE patterns for TPDI2- and FTPDI2-based devices (Figure S10) are 13.10 and 15.87 mA/cm2, respectively, which is much lower to the best device Jsc of 15.85 and 16.77 mA/cm2. We ascribed this to the degradation of the devices. The excimer formation has been previously observed in PDI-based OSCs,52,58 and the introduction of the 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 compared to that in the dilute solution, indicating the formation of PDI2 excimers. A significant reduction in the red shift of the emission peaks of TPDI2 and FTPDI2 in the films suggests 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 are shown in Figure S11d−i. The PL quenching efficiencies (summarized in Table

Table 3. SCLC Results of the PTB7-Th:TPDI2 and PTB7Th:FTPDI2 Blend Filmsa

a

device

hole mobility (cm2 V−1 s−1)

electron mobility (cm2 V−1 s−1)

μe/μh

PTB7-Th:TPDI2 PTB7-Th:FTPDI2

1.2 × 10−4 3.0 × 10−4

4.7 × 10−4 4.7 × 10−4

3.9 1.6

All the devices were annealed at 80 °C for 10 min.

annealed PTB7-Th:TPDI2 blend film, the hole and electron mobilities are 1.2 × 10−4 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 the 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 the PTB7-Th:TPDI2 blend film (3.9), indicating a more balanced carrier transport within the PTB7-Th:FTPDI2 film. 27897

DOI: 10.1021/acsami.8b06126 ACS Appl. Mater. Interfaces 2018, 10, 27894−27901

Research Article

ACS Applied Materials & Interfaces

Figure 4. TA spectra and normalized dynamics of (a,c) PTB7-Th:PDI2 and (b,d) PTB7-Th:FTPDI2. For comparison, the spectra of pure PTB7Th, PDI2, and FTPDI2 are presented in arbitrary units.

The microscopic morphologies of the blend films were studied by atomic force microscopy and transmission electron microscopy (TEM); all the as-cast blend films exhibit good film morphologies (Figures S14 and S15). The root-meansquare (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 smaller roughness of the PTBT-Th:FTPDI2 blend, together with the less twisted structure of the energy minimum conformations (Figures S5 and S6) , further suggested that a better structure twist balance was acheived for FTPDI2. Further, the best miscibility between the donor and acceptor was obtained from the PTB7-Th:FTPDI2 blend film. The morphologies show a slight RMS roughness variation after 10 min of thermal annealing at 80 °C. The good film morphologies are further confirmed by the TEM studies (Figure S15). The X-ray diffraction spectra (Figure S16) of neat films (PDI2, TPDI2, and FTPDI2) and blend films (PTB7-Th:PDI2, PTB7Th: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 an evident phase separation or aggregation. The good morphology of the blend films is highly desirable for efficient exciton diffusion. To gain further insight into the device performance difference, femtosecond TA spectroscopy was used to probe the exciton dynamics of the blend films. Figure 4 shows the TA spectra and normalized kinetics of the PTB7-Th:PDI2 and PTB7-Th:FTPDI2 blend films excited at 520 nm (PTB7Th:TPDI2 blend film in the 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,b). In addition to the ground-state bleaching (GSB) and excited-state absorption (ESA) 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 relatively strong ESA at wavelengths greater than 750 nm, which is associated with both the PDI exciton (green dashed line in Figure 4a,b) 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 the electron transfer from PTB7-Th to the acceptor and the hole transfer from the acceptor to PTB7-Th. We find that the 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 the GSB and ESA signals indicate that a 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 the PTB7-Th:PDI2 and PTB7-Th:TPDI2 films, where we observe the TA features that rise with the time constants of ∼4.5 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