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New-structure perylene diimide oligomers by the linkage of the bay- and imide-position for nonfullerene solar cells
Dyes and Pigments 163 (2019) 356–362
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New-structure perylene diimide oligomers by the linkage of the bay- and imide-position for nonfullerene solar cells
T
Zhenghui Luoa,b, Kailong Wub, Yuan Zhaob, Beibei Qiuc,d, Yongfang Lic,d,∗, Chuluo Yanga,b,∗∗ a
Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen, 518060, China Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan, 430072, China c CAS Research/Education Center for Excellence in Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China d School of Chemical Science, University of Chinese Academy of Sciences, Beijing, 100049, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Perylene diimide Polymer solar cells Non-fullerene Photovoltaic performance
A new family of perylene diimide (PDI) oligomers, T-PDI and H-PDI, were designed and synthesized via dehydration condensation reactions with two PDI monomers linked together at bay- and imide-positions. The as-cast non-fullerene polymer solar cells (PSCs) based on H-PDI as acceptor achieved a power conversion efficiency (PCE) of 1.91%. In comparison with the device based on H-PDI, the as-cast T-PDI based PSC gave a PCE of 3.50% with higher open-circuit voltage (VOC), short-circuit density (JSC) and fill factor (FF). The higher VOC of the PSC based on T-PDI was consistent well with its shallower LUMO energy level compared with that of H-PDI. Meanwhile, the higher JSC and FF for the T-PDI-based device should be mainly attributed to the stronger light absorption, more balanced carrier transport and favorable morphology relative to those of the device based on H-PDI. These results reveal the relationship between the PDI oligomers and the photovoltaic performance.
1. Introduction Over the past two decades, bulk-heterojunction (BHJ) polymer solar cells (PSCs) have been studied extensively [1–4]. The promise of flexibility, light weight and low cost has attracted a number of researchers [5–10]. In generally, the active layers of BHJ PSCs include a fullerene derivative as acceptor and a polymer as donor [11–13]. However, in recent three years, non-fullerene acceptors have shown great potential in the PSCs [14–19], especially 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno-[2,3d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene (ITIC) and perylene diimide (PDI) derivatives [20–40], which can be comparable or even superior to the fullerene derivatives [41,42]. PDI dimers and trimers, a very pivotal type of PDI derivatives, are developed by PDI monomers coupling via chemical bonds (singly-, doubly-, and triply-linked), without other bulky bridge-blocks such as spirofluorene [35,43], tetraphenyl ethylene [36] and benzene [32,33,44]. PDI dimers and trimers play an important role in the nonfullerene acceptors due to the desirable combined properties of good
film-forming, high molar extinction coefficient and excellent electron mobilities [40,45–48]. As known in Fig. 1, the PDI monomer includes three different reaction positions: ortho- (α-), bay- (β-) and imide position. Most reported PDI dimers and trimers are the homologous ones, which are usually synthesized by linking two PDI monomers via β- or the imide positions (Fig. 1a and b) [40,45,46]. Until recently, the first heterologous PDI arrays (di-PDI and tri-PDI) were synthesized by Suzuki cross-coupling reactions between the β-position bromide of PDI monomer and the α-position borate of PDI monomer, which exhibited unique photophysical and electrochemical properties relative to the homologous ones [49]. And the PSC based on tri-PDI as acceptor achieved the high power conversion efficiency (PCE) of 4.55%. These results indicate that developing heterologous PDI oligomers provides an alternative way to investigate fullerene-free electron acceptors for high efficient PSCs. Inspired by the above considerations, we present two heterologous PDI molecules (T-PDI and H-PDI) in Fig. 1c. The shapes of T-PDI and HPDI resemble the capital letters ‘‘T’’ and ‘‘H’’, respectively. The two heterologous molecules were efficiently synthesized through
∗ Corresponding author. CAS Research/Education Center for Excellence in Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China. ∗∗ Corresponding author. Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen, 518060, China. E-mail addresses: [email protected] (Y. Li), [email protected] (C. Yang).
https://doi.org/10.1016/j.dyepig.2018.12.015 Received 22 October 2018; Received in revised form 10 December 2018; Accepted 10 December 2018 Available online 11 December 2018 0143-7208/ © 2018 Elsevier Ltd. All rights reserved.
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Fig. 1. a) Structures of PDI dimers and trimers via β-β positions; b) Structures of PDI dimers and trimers via the imide-imide positions; c) Structures of T-PDI and HPDI via the imide-β positions.
dehydration condensation reactions between the β-position amino group of one PDI monomer and the imide position of the other PDI monomer. The two acceptors possess quite twisted structures, which can suppress aggregations of PDIs. In comparison with H-PDI, T-PDI shows higher LUMO level, stronger ability of light absorption and more favorable orbital distribution, which make T-PDI a more promising electron acceptor. PSCs based on PBDB-T:T-PDI without thermal annealing and additive treatment achieved a PCE of 3.50%. A lower PCE of 1.91% for the as-cast device of PBDB-T:H-PDI was obtained mainly due to its poor JSC and VOC.
chloroform (CF).
2.2. Optical properties and electronic energy levels The optical properties of T-PDI and H-PDI were characterized by measuring their ultraviolet–visible (UV–vis) absorption spectra in thin films and in chloroform solution (10−5 M). As can be seen in Fig. 2a, in dilute chloroform solution, T-PDI exhibits strong absorption peaks in the wavelength range of 410–560 nm with a maximum extinction coefficient (εmax) of 1.43 × 105 M−1 cm−1 at 529 nm, which is higher than that of H-PDI (1.11 × 105 M−1 cm−1 at 523 nm). Neat T-PDI and H-PDI films show similar absorption spectrum to the corresponding solution one, which is similar to the reported spiro-fused PDI dimer (SDP) [48], indicative of less aggregation tendency among PDI cores. However, for the single chromophore PDI, red-shifted and broader absorption spectrum was observed relative to that of T-PDI and H-PDI (Fig. S1), which is recognized as H-aggregate formation. In addition, the energy levels of T-PDI and H-PDI were also investigated by employing electrochemical cyclic voltammetry (CV) method, and the corresponding data were outline in Table 1. T-PDI and H-PDI show multiple reduction waves. The two different reduction potentials were observed for T-PDI, with the higher attributed to the PDI with ‘PDI’ at the imide position due to the weaker electron donating ability of a
2. Results and discussions 2.1. Synthesis and characterization The synthetic routes of the two heterologous PDI molecules (T-PDI and H-PDI) were outlined in Scheme 1. The reduction of PDI-NO2 via stannous chloride reduction method yielded the compound PDI-NH2. Then dehydration condensation reactions between PDI-NH2 and PDAC6/PDA generated T-PDI/H-PDI, respectively. The new compounds were fully characterized by 1H NMR, 13C NMR, MALDI-TOF and elemental analysis. They show quite good solubility in common organic solvents, such as o-dichlorobenzene (o-DCB), chlorobenzene (CB) and
Scheme 1. Synthesis of T-PDI, H-PDI. 357
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Fig. 2. a) UV–vis absorption spectra of T-PDI and H-PDI in chloroform solution (10−5 M); b) Normalized absorption spectra of T-PDI, H-PDI and PBDB-T in films; c) Energy level diagram of PBDB-T, T-PDI and H-PDI.
achieved a higher VOC of 0.837 V, a JSC of 7.64 mA cm−2 and a FF of 0.547, thereby an enhanced PCE of 3.50% was obtained. The poor VOC for the device of H-PDI is consistent with its lower LUMO energy level relative to that of T-PDI. Meanwhile, the higher JSC and FF for the TPDI-based device could be ascribed to the stronger ability of light absorption and more favorable morphology compared with the device of H-PDI. Narayan and coworkers reported that monomer PDI and a PDI dimer achieved PCEs of 0.13% and 2.78% [47], respectively. The worse PCE of monomer PDI resulted from the oversize phase separation. Xiao's group also reported a 2.84% efficiency by using heterologous di-PDI [49]. The significantly higher PCE of T-PDI is mainly ascribed to the larger VOC compared to the two mentioned PDI dimer above. The external quantum efficiency (EQE) curves of the PSCs based on T-PDI and H-PDI are shown in Fig. 4d. Both of the PBDB-T:T-PDI and PBDB-T:H-PDI-based devices displayed a broad response from 300 to 700 nm. The calculated current density values integrated from the EQE spectra for the PBDB-T:T-PDI and PBDB-T:H-PDI-based devices are 7.65 and 5.16 mA/cm2, respectively, which are in good agreement with the JSC values measured from J−V curves.
single alkyl chain compared with the PDI with imide alkyl chains, and the lower assigned to the other PDI. Similarly, for H-PDI, the highest reduction potential is ascribed to the PDI with ‘PDI’ at the imide position”, and the other two are ascribed to the PDI with imide alkyl chains. Also, the HOMO and LUMO energy levels of T-PDI were estimated to be −5.85 eV and −3.95 eV from the onset oxidation and reduction potentials. Lower HOMO energy level (−5.92 eV) and LUMO energy level (−4.02 eV) were obtain for H-PDI. The higher LUMO energy level for TPDI facilitates the realization of higher VOC. 2.3. Theoretical analysis Density functional theory (DFT) calculations were employed to obtain the analogous models of T-PDI and H-PDI (Fig. 3). Both T-PDI and H-PDI display quite twisted molecular geometries, which can suppress intermolecular aggregation. For T-PDI, the dihedral angle between the two PDIs is calculated to be 74°. A smaller dihedral angle between adjacent two PDIs for H-PDI is 66°. As shown in Fig. 3, the imide positions of the terminal PDIs have two alkyl chains with weak electron donating ability, while the imide positions of the core-PDI are connected to the β positions of the strong electron-withdraw PDI, which make the LUMO locate on the core-PDI. The HOMO/LUMO values of TPDI and H-PDI are −5.98/-3.66 eV and −6.02/-3.83 eV through DFT calculations, respectively. The calculated values are in good agreement with the values by the CV measurements.
2.5. Charge carrier mobility Charge transport property has an important effect on the device performance. Thus we employed space-charge-limited-current (SCLC) method to survey the electron and hole mobilities (see Fig. S2), and the corresponding parameters were summarized in Table 1. The hole and electron mobility of PBDB-T:H-PDI blend films were 1.02 × 10−4 and 5.17 × 10−5 cm2 V−1 s−1 (μh/μe = 1.97), respectively. The higher hole and electron mobility (1.10 × 10−4 cm2 V−1 s−1/ 8.42 × 10−5 cm2 V−1 s−1) and more balanced μh/μe (μh/μe = 1.31) were obtained for the PBDB-T:T-PDI-based blend films, which can be a reason for the higher JSC and FF in the corresponding device. In addition, electron mobilities of neat T-PDI and H-PDI films were also measured by employing SCLC method (Fig. S3). In comparison with HPDI, T-PDI gives higher electron mobility (9.87 × 10−5 cm2 V−1 s−1), which contributes to the carrier transport.
2.4. Photovoltaic properties In order to estimate the photovoltaic performance of T-PDI and HPDI, PSCs were constructed with a conventional device configuration architecture of ITO/PEDOT:PSS poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate)/PBDB-T: acceptors/PDINO (perylene diimide functionalized with amino N-oxide)/Al (Fig. 4b). The current density-voltage (J-V) curves of PBDB-T:T-PDI and PBDB-T:H-PDI-based devices without thermal annealing and additives are shown in Fig. 4c, and the corresponding photovoltaic data are presented in Table 2. The as-cast solar cell based on PBDB-T:H-PDI exhibited a PCE of 1.91% with a VOC of 0.734 V, JSC of 5.14 mA cm−2 and FF of 0.506. In comparison with the device of H-PDI, the as-cast T-PDI-based device Table 1 Optoelectronic data of T-PDI and H-PDI. Acceptor
εmaxa (105 M−1 cm−1)
λmaxfilm (nm)
λonsetfilm (nm)
b
T-PDI H-PDI
1.43 1.11
529 523
571 577
2.18 2.19
a b
Egopt (eV)
HOMODFT (eV)
LUMODFT (eV)
HOMOCV (eV)
LUMOCV (eV)
μe (cm2 V−1 s−1)
−5.98 −6.02
−3.66 −3.83
−5.85 −5.92
−3.95 −4.02
9.87 × 10−5 7.80 × 10−5
In chloroform solution. Calculated from Egopt = 1240/λonset. 358
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Fig. 3. Optimal molecular geometries, HOMO and LUMO electron density distributions using density functional theory (DFT) by Gaussian 09 at B3LYP/6-31G(d) level.
active layer, which is favorable for the charge collection and transport. In the TEM image, more evident and interconnected phase-separated domain sizes are clearly observed in the PBDB-T: T-PDI blend films, which match well with the AFM phase image. These results should be responsible for higher FF and JSC in the PBDB-T:T-PDI-based device.
2.6. Morphology characterization To gain further insight into the effect of surface morphology on the photovoltaic performance, transmission electron microscopy (TEM) and atomic force microscopy (AFM) were utilized to investigate the morphology of the active layers (Fig. 5). The blend films of PBDB-T:T-PDI and PBDB-T:H-PDI exhibited smooth and uniform morphologies with root-mean-square (RMS) surface roughness of 1.38 and 1.30 nm, respectively. And the higher RMS value for the blend film of PBDB-T and T-PDI enlarges the contact area between the cathode interface and
3. Conclusions In summary, a new family of PDI oligomers, heterologous PDI dimers and trimers, were designed and efficiently synthesized via
Fig. 4. a) Chemical structure of donor PBDB-T; b) Device configuration of the studied PSCs; c) J-V characteristics curves and (d) EQE spectra of the best performing PSCs of PBDB-T:T-PDI (1:1 w/w) and PBDB-T:H-PDI (1:1 w/w). 359
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Table 2 Summary of photovoltaic parameters of PSCs based on PBDB-T/acceptors under standard AM 1.5G illumination, 100 mW/cm2. Devices PBDB-T:T-PDI PBDB-T:H-PDI a
Voca (V) 0.837 0.734
Jsca (mA cm−2) 7.64 5.14
FFa % 54.7 50.6
PCEa % 3.50 (3.36 ± 0.07) 1.91 (1.81 ± 0.06)
μh (cm2 v1 s−1) −4
1.10 × 10 1.02 × 10−4
μe (cm2 v−1 s−1) −5
8.42 × 10 5.17 × 10−5
μh/μe 1.31 1.97
The values in parentheses are average values obtained from 10 devices.
dehydration condensation reactions between the β-position of one PDI monomer and the imide position of the other PDI monomer. The absorption spectra indicate that the intermolecular aggregations in the solid state of the two PDI oligomers were effectively suppressed due to the highly twisted molecular configurations. Meanwhile, T-PDI displayed higher molar extinction coefficient and LUMO levels relative to that of H-PDI. Employing the two PDI oligomers as electron acceptors, the as-cast non-fullerene PSC based on H-PDI achieved a PCE of 1.91% with a VOC of 0.734 V, a JSC of 5.14 mA cm−2 and a FF of 0.506. In comparison with the H-PDI-based device, the as-cast T-PDI-based device gave a satisfactory PCE of 3.50% with a VOC of 0.837 V, a JSC of 7.64 mA cm−2 and a FF of 0.547. The higher VOC for the device of T-PDI is consistent well with the shallower LUMO energy level compared with that of H-PDI. Meanwhile, the higher JSC and FF for the T-PDI-based device should be mainly attributed to the stronger ability of light absorption, more balanced carrier transport and favorable morphology relative to that of the device based on H-PDI. Though the device performance is not ideal, the new-structure PDI oligomers provide an alternative way to study fullerene-free electron acceptors for PSCs.
200 mL of 2 M Na2CO3, then extracted by CHCl3 three times, next washed with water for two times, and dried over Na2SO4. The crude product was purified by silicon chromatography (petroleum ether (PE):ethyl acetate (EA), v/v(5:1)) to get pure product (1.30 g, 85%). 1H NMR (CDCl3, 400 MHz): δ [ppm]: 8.88 (d, J = 8.0 Hz, 1H), 8.57–8.45 (m, 5H), 8.27 (d, J = 8.0 Hz, 1H), 5.30–5.25 (m, 4H), 2.27–2.18 (m, 4H), 1.88–1.82 (m, 4H), 1.36–1.21 (m, 32H), 0.84–0.81 (m, 12H); 13C NMR (100 MHz, CDCl3): δ [ppm]: 165.76, 144.35, 136.03, 132.75, 130.84, 129.23, 128.45, 127.46, 126.95, 125.31, 124.50, 123.93, 122.22, 118.51, 54.57, 48.17, 32.37, 30.98, 29.25, 29.22, 26.93, 26.90, 22.66, 22.59, 14.07. MS (MALDI-TOF): calcd. for (C50H63N3O4), 769.5; found, 770.6. Elemental anal. calcd for C50H63N3O4: C, 77.99; H, 8.25; N 5.46. Found: C, 77.52; H, 8.33; N 5.83. Synthesis of T-PDI: Compound PDI-NH2 (770 mg, 1.00 mmol), Compound PDA-C6 (570 mg, 1.00 mmol), zinc acetate (0.11 g, 0.6 mmol), and quinoline (10 mL) was stirred at 180 °C for 12 h under Ar atmosphere. After cooling to room temperature, the reaction mixture was added with HCl aqueous solution (2 M, 200 mL), then extracted by CHCl3 three times, and dried over Na2SO4. The crude product was purified by silicon chromatography (petroleum ether (PE):CHCl3, v/v (1:3)) to get pure product (1.11 g, 80%).1H NMR (CDCl3, 400 MHz): δ [ppm]: 8.78–8.74 (m, 12H), 8.67 (d, J = 8.1 Hz, 1H), 8.58 (d, J = 12.0 Hz, 1H), 8.41 (d, J = 8.0 Hz, 1H), 5.20–5.04 (m, 3H), 2.30–2.20 (m, 6H), 1.99–1.79 (m, 6H), 1.45–1.20 (m, 48H), 0.85–0.77 (m, 18H). 13C NMR (CDCl3, 100 MHz): δ [ppm]: 163.50, 136.32, 134.04, 133.93, 132.97, 132.65, 130.20, 129.47, 129.29, 129.25, 128.25, 126.99, 126.80, 126.38, 125.23, 123.91, 123.77, 123.33, 123.26, 122.27, 119.29, 54.90, 54.68, 32.36, 31.77, 31.76, 31.70,
4. Experimental section The synthesis of PDI-NO2 and PDA-C6 were synthesized according to literature [40,50]. Synthesis of PDI-NH2: Compound PDI-NO2 (1.6 g, 2.0 mmol), HCl (4 mL), SnCl2•2H2O (1.80 g, 8.0 mmol) were added into 50 mL tetrahydrofuran (THF), after stirring for 12 h at 90 °C. Then reaction mixture was cooled to room temperature, the reaction mixture was poured into
Fig. 5. Morphology images of the blend films: a) AFM height, b) AFM phase, and c) TEM images for PBDB-T:T-PDI (1:1, w/w) blend film; d) AFM height, e) AFM phase, and f) TEM images for PBDB-T:H-PDI (1:1, w/w) blend film. 360
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29.25, 29.24, 29.17, 26.89, 26.82, 22.61, 22.55, 18.45, 14.07, 14.03. MS (MALDI-TOF): calcd for (C87H96N4O8), 1324.7; found, 1324.5. Elemental anal. calcd for C87H96N4O8: C, 78.82; H, 7.30; N 4.23. Found: C, 79.21; H, 7.39; N 4.58. Synthesis of H-PDI: Compound PDI-NH2 (770 mg, 1.00 mmol), PDA (120 mg, 0.30 mmol), zinc acetate (0.11 g, 0.6 mmol) and quinoline (10 mL) was stirred at 180 °C for 24 h under Ar atmosphere. After cooling to room temperature, the reaction mixture was added to HCl aqueous solution (2 M, 200 mL), then extracted by CHCl3 three times, and dried over Na2SO4. The crude product was purified by silicon chromatography (petroleum ether (PE):CHCl3, v/v(1:4)) to get pure product (360 mg, 66%). 1H NMR (CDCl3, 400 MHz): δ [ppm]: 8.89–8.74 (m, 16H), 8.72–8.69 (m, 2H), 8.61–8.57 (m, 2H), 8.49–8.46 (m, 2H), 5.23–5.04 (m, 4H), 2.32–2.20 (m, 8H), 1.99–1.78 (m, 8H), 1.45–1.11 (m, 64H), 0.85–0.70 (m, 24H). 13C NMR (CDCl3, 100 MHz): δ [ppm]: 163.45, 163.40, 139.80, 135.83, 135.75, 134.06, 132.79, 130.14, 130.08, 129.84, 129.33, 128.90, 128.28, 127.85, 127.07, 126.99, 126.84, 126.69, 125.26, 124.14, 123.84, 123.42, 123.04, 122.78, 58.48, 54.92, 54.73, 32.28, 31.76, 31.71, 29.26, 29.18, 26.89, 26.83, 22.62, 22.56, 18.44, 14.08, 14.05. MS (MALDI-TOF): calcd for (C124H130N6O12), 1894.9; found, 1895.2. Elemental anal. calcd for C124H130N6O12: C, 78.53; H, 6.91; N 4.43. Found: C, 78.24; H, 7.22; N 4.71.
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
New-structure perylene diimide oligomers by the linkage of the bay- and imide-position for nonfullerene solar cells
T
Zhenghui Luoa,b, Kailong Wub, Yuan Zhaob, Beibei Qiuc,d, Yongfang Lic,d,∗, Chuluo Yanga,b,∗∗ a
Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen, 518060, China Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan, 430072, China c CAS Research/Education Center for Excellence in Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China d School of Chemical Science, University of Chinese Academy of Sciences, Beijing, 100049, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Perylene diimide Polymer solar cells Non-fullerene Photovoltaic performance
A new family of perylene diimide (PDI) oligomers, T-PDI and H-PDI, were designed and synthesized via dehydration condensation reactions with two PDI monomers linked together at bay- and imide-positions. The as-cast non-fullerene polymer solar cells (PSCs) based on H-PDI as acceptor achieved a power conversion efficiency (PCE) of 1.91%. In comparison with the device based on H-PDI, the as-cast T-PDI based PSC gave a PCE of 3.50% with higher open-circuit voltage (VOC), short-circuit density (JSC) and fill factor (FF). The higher VOC of the PSC based on T-PDI was consistent well with its shallower LUMO energy level compared with that of H-PDI. Meanwhile, the higher JSC and FF for the T-PDI-based device should be mainly attributed to the stronger light absorption, more balanced carrier transport and favorable morphology relative to those of the device based on H-PDI. These results reveal the relationship between the PDI oligomers and the photovoltaic performance.
1. Introduction Over the past two decades, bulk-heterojunction (BHJ) polymer solar cells (PSCs) have been studied extensively [1–4]. The promise of flexibility, light weight and low cost has attracted a number of researchers [5–10]. In generally, the active layers of BHJ PSCs include a fullerene derivative as acceptor and a polymer as donor [11–13]. However, in recent three years, non-fullerene acceptors have shown great potential in the PSCs [14–19], especially 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno-[2,3d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene (ITIC) and perylene diimide (PDI) derivatives [20–40], which can be comparable or even superior to the fullerene derivatives [41,42]. PDI dimers and trimers, a very pivotal type of PDI derivatives, are developed by PDI monomers coupling via chemical bonds (singly-, doubly-, and triply-linked), without other bulky bridge-blocks such as spirofluorene [35,43], tetraphenyl ethylene [36] and benzene [32,33,44]. PDI dimers and trimers play an important role in the nonfullerene acceptors due to the desirable combined properties of good
film-forming, high molar extinction coefficient and excellent electron mobilities [40,45–48]. As known in Fig. 1, the PDI monomer includes three different reaction positions: ortho- (α-), bay- (β-) and imide position. Most reported PDI dimers and trimers are the homologous ones, which are usually synthesized by linking two PDI monomers via β- or the imide positions (Fig. 1a and b) [40,45,46]. Until recently, the first heterologous PDI arrays (di-PDI and tri-PDI) were synthesized by Suzuki cross-coupling reactions between the β-position bromide of PDI monomer and the α-position borate of PDI monomer, which exhibited unique photophysical and electrochemical properties relative to the homologous ones [49]. And the PSC based on tri-PDI as acceptor achieved the high power conversion efficiency (PCE) of 4.55%. These results indicate that developing heterologous PDI oligomers provides an alternative way to investigate fullerene-free electron acceptors for high efficient PSCs. Inspired by the above considerations, we present two heterologous PDI molecules (T-PDI and H-PDI) in Fig. 1c. The shapes of T-PDI and HPDI resemble the capital letters ‘‘T’’ and ‘‘H’’, respectively. The two heterologous molecules were efficiently synthesized through
∗ Corresponding author. CAS Research/Education Center for Excellence in Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China. ∗∗ Corresponding author. Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen, 518060, China. E-mail addresses: [email protected] (Y. Li), [email protected] (C. Yang).
https://doi.org/10.1016/j.dyepig.2018.12.015 Received 22 October 2018; Received in revised form 10 December 2018; Accepted 10 December 2018 Available online 11 December 2018 0143-7208/ © 2018 Elsevier Ltd. All rights reserved.
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Fig. 1. a) Structures of PDI dimers and trimers via β-β positions; b) Structures of PDI dimers and trimers via the imide-imide positions; c) Structures of T-PDI and HPDI via the imide-β positions.
dehydration condensation reactions between the β-position amino group of one PDI monomer and the imide position of the other PDI monomer. The two acceptors possess quite twisted structures, which can suppress aggregations of PDIs. In comparison with H-PDI, T-PDI shows higher LUMO level, stronger ability of light absorption and more favorable orbital distribution, which make T-PDI a more promising electron acceptor. PSCs based on PBDB-T:T-PDI without thermal annealing and additive treatment achieved a PCE of 3.50%. A lower PCE of 1.91% for the as-cast device of PBDB-T:H-PDI was obtained mainly due to its poor JSC and VOC.
chloroform (CF).
2.2. Optical properties and electronic energy levels The optical properties of T-PDI and H-PDI were characterized by measuring their ultraviolet–visible (UV–vis) absorption spectra in thin films and in chloroform solution (10−5 M). As can be seen in Fig. 2a, in dilute chloroform solution, T-PDI exhibits strong absorption peaks in the wavelength range of 410–560 nm with a maximum extinction coefficient (εmax) of 1.43 × 105 M−1 cm−1 at 529 nm, which is higher than that of H-PDI (1.11 × 105 M−1 cm−1 at 523 nm). Neat T-PDI and H-PDI films show similar absorption spectrum to the corresponding solution one, which is similar to the reported spiro-fused PDI dimer (SDP) [48], indicative of less aggregation tendency among PDI cores. However, for the single chromophore PDI, red-shifted and broader absorption spectrum was observed relative to that of T-PDI and H-PDI (Fig. S1), which is recognized as H-aggregate formation. In addition, the energy levels of T-PDI and H-PDI were also investigated by employing electrochemical cyclic voltammetry (CV) method, and the corresponding data were outline in Table 1. T-PDI and H-PDI show multiple reduction waves. The two different reduction potentials were observed for T-PDI, with the higher attributed to the PDI with ‘PDI’ at the imide position due to the weaker electron donating ability of a
2. Results and discussions 2.1. Synthesis and characterization The synthetic routes of the two heterologous PDI molecules (T-PDI and H-PDI) were outlined in Scheme 1. The reduction of PDI-NO2 via stannous chloride reduction method yielded the compound PDI-NH2. Then dehydration condensation reactions between PDI-NH2 and PDAC6/PDA generated T-PDI/H-PDI, respectively. The new compounds were fully characterized by 1H NMR, 13C NMR, MALDI-TOF and elemental analysis. They show quite good solubility in common organic solvents, such as o-dichlorobenzene (o-DCB), chlorobenzene (CB) and
Scheme 1. Synthesis of T-PDI, H-PDI. 357
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Fig. 2. a) UV–vis absorption spectra of T-PDI and H-PDI in chloroform solution (10−5 M); b) Normalized absorption spectra of T-PDI, H-PDI and PBDB-T in films; c) Energy level diagram of PBDB-T, T-PDI and H-PDI.
achieved a higher VOC of 0.837 V, a JSC of 7.64 mA cm−2 and a FF of 0.547, thereby an enhanced PCE of 3.50% was obtained. The poor VOC for the device of H-PDI is consistent with its lower LUMO energy level relative to that of T-PDI. Meanwhile, the higher JSC and FF for the TPDI-based device could be ascribed to the stronger ability of light absorption and more favorable morphology compared with the device of H-PDI. Narayan and coworkers reported that monomer PDI and a PDI dimer achieved PCEs of 0.13% and 2.78% [47], respectively. The worse PCE of monomer PDI resulted from the oversize phase separation. Xiao's group also reported a 2.84% efficiency by using heterologous di-PDI [49]. The significantly higher PCE of T-PDI is mainly ascribed to the larger VOC compared to the two mentioned PDI dimer above. The external quantum efficiency (EQE) curves of the PSCs based on T-PDI and H-PDI are shown in Fig. 4d. Both of the PBDB-T:T-PDI and PBDB-T:H-PDI-based devices displayed a broad response from 300 to 700 nm. The calculated current density values integrated from the EQE spectra for the PBDB-T:T-PDI and PBDB-T:H-PDI-based devices are 7.65 and 5.16 mA/cm2, respectively, which are in good agreement with the JSC values measured from J−V curves.
single alkyl chain compared with the PDI with imide alkyl chains, and the lower assigned to the other PDI. Similarly, for H-PDI, the highest reduction potential is ascribed to the PDI with ‘PDI’ at the imide position”, and the other two are ascribed to the PDI with imide alkyl chains. Also, the HOMO and LUMO energy levels of T-PDI were estimated to be −5.85 eV and −3.95 eV from the onset oxidation and reduction potentials. Lower HOMO energy level (−5.92 eV) and LUMO energy level (−4.02 eV) were obtain for H-PDI. The higher LUMO energy level for TPDI facilitates the realization of higher VOC. 2.3. Theoretical analysis Density functional theory (DFT) calculations were employed to obtain the analogous models of T-PDI and H-PDI (Fig. 3). Both T-PDI and H-PDI display quite twisted molecular geometries, which can suppress intermolecular aggregation. For T-PDI, the dihedral angle between the two PDIs is calculated to be 74°. A smaller dihedral angle between adjacent two PDIs for H-PDI is 66°. As shown in Fig. 3, the imide positions of the terminal PDIs have two alkyl chains with weak electron donating ability, while the imide positions of the core-PDI are connected to the β positions of the strong electron-withdraw PDI, which make the LUMO locate on the core-PDI. The HOMO/LUMO values of TPDI and H-PDI are −5.98/-3.66 eV and −6.02/-3.83 eV through DFT calculations, respectively. The calculated values are in good agreement with the values by the CV measurements.
2.5. Charge carrier mobility Charge transport property has an important effect on the device performance. Thus we employed space-charge-limited-current (SCLC) method to survey the electron and hole mobilities (see Fig. S2), and the corresponding parameters were summarized in Table 1. The hole and electron mobility of PBDB-T:H-PDI blend films were 1.02 × 10−4 and 5.17 × 10−5 cm2 V−1 s−1 (μh/μe = 1.97), respectively. The higher hole and electron mobility (1.10 × 10−4 cm2 V−1 s−1/ 8.42 × 10−5 cm2 V−1 s−1) and more balanced μh/μe (μh/μe = 1.31) were obtained for the PBDB-T:T-PDI-based blend films, which can be a reason for the higher JSC and FF in the corresponding device. In addition, electron mobilities of neat T-PDI and H-PDI films were also measured by employing SCLC method (Fig. S3). In comparison with HPDI, T-PDI gives higher electron mobility (9.87 × 10−5 cm2 V−1 s−1), which contributes to the carrier transport.
2.4. Photovoltaic properties In order to estimate the photovoltaic performance of T-PDI and HPDI, PSCs were constructed with a conventional device configuration architecture of ITO/PEDOT:PSS poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate)/PBDB-T: acceptors/PDINO (perylene diimide functionalized with amino N-oxide)/Al (Fig. 4b). The current density-voltage (J-V) curves of PBDB-T:T-PDI and PBDB-T:H-PDI-based devices without thermal annealing and additives are shown in Fig. 4c, and the corresponding photovoltaic data are presented in Table 2. The as-cast solar cell based on PBDB-T:H-PDI exhibited a PCE of 1.91% with a VOC of 0.734 V, JSC of 5.14 mA cm−2 and FF of 0.506. In comparison with the device of H-PDI, the as-cast T-PDI-based device Table 1 Optoelectronic data of T-PDI and H-PDI. Acceptor
εmaxa (105 M−1 cm−1)
λmaxfilm (nm)
λonsetfilm (nm)
b
T-PDI H-PDI
1.43 1.11
529 523
571 577
2.18 2.19
a b
Egopt (eV)
HOMODFT (eV)
LUMODFT (eV)
HOMOCV (eV)
LUMOCV (eV)
μe (cm2 V−1 s−1)
−5.98 −6.02
−3.66 −3.83
−5.85 −5.92
−3.95 −4.02
9.87 × 10−5 7.80 × 10−5
In chloroform solution. Calculated from Egopt = 1240/λonset. 358
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Fig. 3. Optimal molecular geometries, HOMO and LUMO electron density distributions using density functional theory (DFT) by Gaussian 09 at B3LYP/6-31G(d) level.
active layer, which is favorable for the charge collection and transport. In the TEM image, more evident and interconnected phase-separated domain sizes are clearly observed in the PBDB-T: T-PDI blend films, which match well with the AFM phase image. These results should be responsible for higher FF and JSC in the PBDB-T:T-PDI-based device.
2.6. Morphology characterization To gain further insight into the effect of surface morphology on the photovoltaic performance, transmission electron microscopy (TEM) and atomic force microscopy (AFM) were utilized to investigate the morphology of the active layers (Fig. 5). The blend films of PBDB-T:T-PDI and PBDB-T:H-PDI exhibited smooth and uniform morphologies with root-mean-square (RMS) surface roughness of 1.38 and 1.30 nm, respectively. And the higher RMS value for the blend film of PBDB-T and T-PDI enlarges the contact area between the cathode interface and
3. Conclusions In summary, a new family of PDI oligomers, heterologous PDI dimers and trimers, were designed and efficiently synthesized via
Fig. 4. a) Chemical structure of donor PBDB-T; b) Device configuration of the studied PSCs; c) J-V characteristics curves and (d) EQE spectra of the best performing PSCs of PBDB-T:T-PDI (1:1 w/w) and PBDB-T:H-PDI (1:1 w/w). 359
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Table 2 Summary of photovoltaic parameters of PSCs based on PBDB-T/acceptors under standard AM 1.5G illumination, 100 mW/cm2. Devices PBDB-T:T-PDI PBDB-T:H-PDI a
Voca (V) 0.837 0.734
Jsca (mA cm−2) 7.64 5.14
FFa % 54.7 50.6
PCEa % 3.50 (3.36 ± 0.07) 1.91 (1.81 ± 0.06)
μh (cm2 v1 s−1) −4
1.10 × 10 1.02 × 10−4
μe (cm2 v−1 s−1) −5
8.42 × 10 5.17 × 10−5
μh/μe 1.31 1.97
The values in parentheses are average values obtained from 10 devices.
dehydration condensation reactions between the β-position of one PDI monomer and the imide position of the other PDI monomer. The absorption spectra indicate that the intermolecular aggregations in the solid state of the two PDI oligomers were effectively suppressed due to the highly twisted molecular configurations. Meanwhile, T-PDI displayed higher molar extinction coefficient and LUMO levels relative to that of H-PDI. Employing the two PDI oligomers as electron acceptors, the as-cast non-fullerene PSC based on H-PDI achieved a PCE of 1.91% with a VOC of 0.734 V, a JSC of 5.14 mA cm−2 and a FF of 0.506. In comparison with the H-PDI-based device, the as-cast T-PDI-based device gave a satisfactory PCE of 3.50% with a VOC of 0.837 V, a JSC of 7.64 mA cm−2 and a FF of 0.547. The higher VOC for the device of T-PDI is consistent well with the shallower LUMO energy level compared with that of H-PDI. Meanwhile, the higher JSC and FF for the T-PDI-based device should be mainly attributed to the stronger ability of light absorption, more balanced carrier transport and favorable morphology relative to that of the device based on H-PDI. Though the device performance is not ideal, the new-structure PDI oligomers provide an alternative way to study fullerene-free electron acceptors for PSCs.
200 mL of 2 M Na2CO3, then extracted by CHCl3 three times, next washed with water for two times, and dried over Na2SO4. The crude product was purified by silicon chromatography (petroleum ether (PE):ethyl acetate (EA), v/v(5:1)) to get pure product (1.30 g, 85%). 1H NMR (CDCl3, 400 MHz): δ [ppm]: 8.88 (d, J = 8.0 Hz, 1H), 8.57–8.45 (m, 5H), 8.27 (d, J = 8.0 Hz, 1H), 5.30–5.25 (m, 4H), 2.27–2.18 (m, 4H), 1.88–1.82 (m, 4H), 1.36–1.21 (m, 32H), 0.84–0.81 (m, 12H); 13C NMR (100 MHz, CDCl3): δ [ppm]: 165.76, 144.35, 136.03, 132.75, 130.84, 129.23, 128.45, 127.46, 126.95, 125.31, 124.50, 123.93, 122.22, 118.51, 54.57, 48.17, 32.37, 30.98, 29.25, 29.22, 26.93, 26.90, 22.66, 22.59, 14.07. MS (MALDI-TOF): calcd. for (C50H63N3O4), 769.5; found, 770.6. Elemental anal. calcd for C50H63N3O4: C, 77.99; H, 8.25; N 5.46. Found: C, 77.52; H, 8.33; N 5.83. Synthesis of T-PDI: Compound PDI-NH2 (770 mg, 1.00 mmol), Compound PDA-C6 (570 mg, 1.00 mmol), zinc acetate (0.11 g, 0.6 mmol), and quinoline (10 mL) was stirred at 180 °C for 12 h under Ar atmosphere. After cooling to room temperature, the reaction mixture was added with HCl aqueous solution (2 M, 200 mL), then extracted by CHCl3 three times, and dried over Na2SO4. The crude product was purified by silicon chromatography (petroleum ether (PE):CHCl3, v/v (1:3)) to get pure product (1.11 g, 80%).1H NMR (CDCl3, 400 MHz): δ [ppm]: 8.78–8.74 (m, 12H), 8.67 (d, J = 8.1 Hz, 1H), 8.58 (d, J = 12.0 Hz, 1H), 8.41 (d, J = 8.0 Hz, 1H), 5.20–5.04 (m, 3H), 2.30–2.20 (m, 6H), 1.99–1.79 (m, 6H), 1.45–1.20 (m, 48H), 0.85–0.77 (m, 18H). 13C NMR (CDCl3, 100 MHz): δ [ppm]: 163.50, 136.32, 134.04, 133.93, 132.97, 132.65, 130.20, 129.47, 129.29, 129.25, 128.25, 126.99, 126.80, 126.38, 125.23, 123.91, 123.77, 123.33, 123.26, 122.27, 119.29, 54.90, 54.68, 32.36, 31.77, 31.76, 31.70,
4. Experimental section The synthesis of PDI-NO2 and PDA-C6 were synthesized according to literature [40,50]. Synthesis of PDI-NH2: Compound PDI-NO2 (1.6 g, 2.0 mmol), HCl (4 mL), SnCl2•2H2O (1.80 g, 8.0 mmol) were added into 50 mL tetrahydrofuran (THF), after stirring for 12 h at 90 °C. Then reaction mixture was cooled to room temperature, the reaction mixture was poured into
Fig. 5. Morphology images of the blend films: a) AFM height, b) AFM phase, and c) TEM images for PBDB-T:T-PDI (1:1, w/w) blend film; d) AFM height, e) AFM phase, and f) TEM images for PBDB-T:H-PDI (1:1, w/w) blend film. 360
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29.25, 29.24, 29.17, 26.89, 26.82, 22.61, 22.55, 18.45, 14.07, 14.03. MS (MALDI-TOF): calcd for (C87H96N4O8), 1324.7; found, 1324.5. Elemental anal. calcd for C87H96N4O8: C, 78.82; H, 7.30; N 4.23. Found: C, 79.21; H, 7.39; N 4.58. Synthesis of H-PDI: Compound PDI-NH2 (770 mg, 1.00 mmol), PDA (120 mg, 0.30 mmol), zinc acetate (0.11 g, 0.6 mmol) and quinoline (10 mL) was stirred at 180 °C for 24 h under Ar atmosphere. After cooling to room temperature, the reaction mixture was added to HCl aqueous solution (2 M, 200 mL), then extracted by CHCl3 three times, and dried over Na2SO4. The crude product was purified by silicon chromatography (petroleum ether (PE):CHCl3, v/v(1:4)) to get pure product (360 mg, 66%). 1H NMR (CDCl3, 400 MHz): δ [ppm]: 8.89–8.74 (m, 16H), 8.72–8.69 (m, 2H), 8.61–8.57 (m, 2H), 8.49–8.46 (m, 2H), 5.23–5.04 (m, 4H), 2.32–2.20 (m, 8H), 1.99–1.78 (m, 8H), 1.45–1.11 (m, 64H), 0.85–0.70 (m, 24H). 13C NMR (CDCl3, 100 MHz): δ [ppm]: 163.45, 163.40, 139.80, 135.83, 135.75, 134.06, 132.79, 130.14, 130.08, 129.84, 129.33, 128.90, 128.28, 127.85, 127.07, 126.99, 126.84, 126.69, 125.26, 124.14, 123.84, 123.42, 123.04, 122.78, 58.48, 54.92, 54.73, 32.28, 31.76, 31.71, 29.26, 29.18, 26.89, 26.83, 22.62, 22.56, 18.44, 14.08, 14.05. MS (MALDI-TOF): calcd for (C124H130N6O12), 1894.9; found, 1895.2. Elemental anal. calcd for C124H130N6O12: C, 78.53; H, 6.91; N 4.43. Found: C, 78.24; H, 7.22; N 4.71.
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