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A Star-Shaped Perylene Diimide Electron Acceptor for High-Performance Organic Solar Cells Yuze Lin, Yifan Wang, Jiayu Wang, Jianhui Hou, Yongfang Li, Daoben Zhu, and Xiaowei Zhan* Solution-processed bulk heterojunction (BHJ) organic solar cells (OSCs) are a promising, cost-effective alternative for utility of solar energy, and low-cost, light-weight, and flexibility are just some of their advantages.[1–3] Over the last decade, the development of OSCs has seen a dramatic rise in power-conversion efficiency (PCE) from less than 1%[4] in the earliest reports to 10% in recent publications.[5] Generally, record efficiencies directly result from the development of new electron-donor materials that exhibit improved properties such as better spectral sensitivity, enhanced hole transport and favorably tuned HOMO/LUMO energy levels that are matched well with those of existing acceptors. In fact, the electron acceptors are of the same importance as the electron donors for high performance OSCs, but the development of electron-acceptor materials has lagged far behind that of donor materials. Fullerenes and their derivatives have been the dominant electron–acceptor materials in BHJ OSCs, owing to their high electron mobility, large electron affinity, and charge-transport isotropy.[6] However, the need remains to develop non-fullerene electron acceptors that will not only retain the favorable properties of fullerenes, but can also overcome their insufficiencies, such as their weak absorption in the visible spectral region, the limited spectral breadth, and limited energy-level variability. Nowadays, solution-processed BHJ OSCs based on non-fullerene acceptors have shown PCEs up to 3–4%,[7–14] which have been much higher than those (generally < 1.5%) disclosed in earlier days,[15–17] but non-fullerene acceptors are still at the infant stage relative to fullerenes, and more novel non-fullerene acceptors should be explored. Perylene diimide (PDI) units are widely used for constructing n-type organic semiconductors, and PDI derivatives are the earliest and most common non-fullerene acceptors
Y. Lin, Y. Wang, Prof. J. Hou, Prof. Y. Li, Prof. D. Zhu Beijing National Laboratory for Molecular Sciences Institute of Chemistry Chinese Academy of Sciences Beijing 100190, P.R. China J. Wang, Prof. X. Zhan Department of Materials Science and Engineering College of Engineering Peking University Beijing 100871, P.R. China E-mail: [email protected] Y. Lin, Y. Wang University of Chinese Academy of Sciences Beijing 100049, P.R. China
DOI: 10.1002/adma.201400525
Adv. Mater. 2014, DOI: 10.1002/adma.201400525
used in OSCs.[18–21] PDIs generally show high thermal, chemical, and light stabilities, good electron-accepting abilities, and excellent electron mobilities.[18–21] Compared to fullerenes, PDIs show a strong and broad absorption in the visible spectral region and their optoelectronic properties are easily tuned by tailoring the substituents on the imide-N, bay, and/or nonbay positions. However, traditional PDI derivatives show a high planarity and suffer from strong aggregation, leading to the formation of large crystalline domains (typically micrometer scale). Large crystalline domains of PDI in BHJ films lead to large phase separations, reduced exciton diffusion/separation efficiencies, and finally low PCEs of the OSCs. Therefore, restricting the crystallinity without adversely weakening the charge-transport properties of PDIs is a design principle for PDI-based acceptors. Recently, PDI dimers with twisted structures have shown promising performance as electron acceptors in solution-processed OSCs.[10,11,14,22] Novel, planar, star-shaped triimides were reported for electron acceptors in OSCs.[23] Like other rylene diimides-based polymer acceptors,[24,25] PDI-based polymer acceptors also exhibited encouraging performance in all-polymer solar cells with PCEs of up to 3.45%.[7,26] In this communication, we report a novel, nonplanar, starshaped PDI acceptor (S(TPA-PDI), Scheme 1) with a triphenylamine (TPA) core. As TPA has a special propeller starburst molecular structure because of the sp3 hybrid orbital of the nitrogen atom, S(TPA-PDI) possesses a quasi-3D nonplanar structure and isotropic optical and charge-transporting properties.[27] Like some other TPA-based or star-shaped molecules,[28–35] S(TPA-PDI) generally shows weak intermolecular interactions and molecular aggregation. Furthermore, S(TPAPDI) exhibits strong absorption in the visible region, a complementary absorption range, and appropriate energy levels matched with low-bandgap polymer donors, such as a copolymer of 5-alkylthiophene-2-yl-substituted benzo[1,2-b:4,5-b′] dithiophene and alkylcarbonyl-substituted thieno-[3,4-b]thiophene, PBDTTT-C-T (Scheme 1).[36] Solution-processed BHJ OSCs based on PBDTTT-C-T: S(TPA-PDI) showed a PCE as high as 3.32%, which is among the highest values reported for solution-processed OSCs based on non-fullerene acceptors. The compound S(TPA-PDI) was synthesized through a Suzuki coupling reaction between tris(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)amine and N,N′-di(2ethylhexyl)-1-bromo-7-(n-butoxyl)-3,4:9,10-perylene diimide using Pd(PPh3)4 as the catalyst (Scheme 1). Compound S(TPAPDI) was fully characterized by matrix-assisted laser desorption/ionization–time-of flight mass spectrometry (MALDI-TOF MS), 1H NMR, 13C NMR, and elemental analysis. This compound is readily soluble in common organic solvents such as dichloromethane and dichlorobenzene at room temperature,
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S(TPA-PDI) exhibits quasi-reversible reduction and oxidation waves; its reduction wave is obviously stronger than its oxidaO tion wave, because there are 3 PDI units but O N O only 1 TPA unit per S(TPA-PDI) molecule. O S The onset oxidation and reduction potenO S tials versus FeCp2+/0 are 0.60 and −1.10 V, S N O N O respectively, thus the HOMO and LUMO S n S O energies were estimated to be −5.40 and N S −3.70 eV from the onset oxidation and reducO O tion potentials, respectively, assuming the N absolute energy level of FeCp2+/0 to be 4.8 eV O below vacuum.[37] The energy gap calculated O O N O from CV (1.70 eV) was similar to its optical N O bandgap (1.76 eV). PBDTTT-C-T O The electron mobility of S(TPA-PDI) was measured by the space-charge limited current (SCLC) method with a S(TPA-PDI) device structure of Al/S(TPA-PDI)/Al O O (Figure S4, Supporting Information). O N O B The S(TPA-PDI) film exhibits an electron K aq. CO Pd(PPh3)4 2 3 mobility of 3.0 × 10−5 cm2 V−1 s−1, which is S(TPA-PDI) toluene + Br lower than those (ca. 10−4–10−2 cm2 V−1 s−1) O N 110 o C of fullerene derivatives[6] but is relatively O O high compared to other small molecular B B 46% O O non-fullerene acceptors.[1] O N O To demonstrate the potential application of S(TPA-PDI) in OSCs, we used S(TPAPDI) as an electron acceptor and a classical low-bandgap polymer PBDTTT-C-T Scheme 1. Chemical structures of compounds PBDTTT-C-T and S(TPA-PDI), and synthetic as the electron donor, and fabricated BHJ route of compound S(TPA-PDI). OSCs with a structure of ITO/PEDOT:PSS/ PBDTTT-C-T:S(TPA-PDI)/Ca/Al. The LUMO gap (0.45 eV) and HOMO gap (0.29 eV) between PBDTTT-C-T because of its solubilizing alkyl substituents. The thermal (LUMO/HOMO: −3.25/ −5.11 eV)[36] and S(TPA-PDI) should be properties of S(TPA-PDI) were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry large enough to promote efficient exciton dissociation.[38] The (DSC). This compound exhibits excellent thermal stability difference between the LUMO of S(TPA-PDI) and the HOMO with a decomposition temperature (5% weight loss) at 406 °C of PBDTTT-C-T is 1.41 eV, which could result in a high openin nitrogen atmosphere (Figure S1, Supporting Information). circuit voltage (VOC) of the solar cells.[39] Moreover, PBDTTTThe DSC trace for this compound shows no melting peak, and C-T and S(TPA-PDI) show complementary absorption spectra; the X-ray diffraction (XRD) pattern on the film shows no reflecthe PBDTTT-C-T: S(TPA-PDI) (1:1 w/w) blend film exhibits a tion peak (Figure S2, Supporting Information), suggesting that very broad absorption ranging from 400 to 800 nm (Figure 2a). S(TPA-PDI) is amorphous. Blending of PBDTTT-C-T with S(TPA-PDI) results in fluoresFigure 1a shows the normalized spectra of the optical cence quenching of over 95% and 65% of that of PBDTTT-Cabsorption of S(TPA-PDI) in dichloromethane solution T and S(TPA-PDI), respectively, indicating that an effective (10−6 M) and in thin solid film. S(TPA-PDI) in solution exhibits photoinduced charge transfer occurred between PBDTTT-C-T and S(TPA-PDI) in the blend film. Especially for PBDTTT-Ca strong absorption with a maximum extinction coefficient of T almost all photoinduced excitons were separated (Figure 2b), 9.09 × 104 M−1 cm−1 at 536 nm. Compared to the absorption in which is beneficial to a high short-circuit current density (JSC) solution, the thin film of S(TPA-PDI) shows a broader absorption with a similar profile, suggesting there is a weak interin the OSC. molecular interaction and molecular aggregation in the film, Table 1 summarizes VOC, JSC, fill factor (FF), and PCE of the because of the quasi-3D molecular structure of S(TPA-PDI) devices at different donor/acceptor (D/A) weight ratios. These (Figure S3, Supporting Information). The optical bandgap devices yielded a relatively high VOC, as expected, and the VOC of the S(TPA-PDI) film estimated from the absorption edge values of 0.86–0.87 V were almost independent of the donor/ (703 nm) was 1.76 eV. acceptor weight ratio. The blend at a D/A weight ratio of 1:1 The electrochemical properties of S(TPA-PDI) were invesgave the best performance: VOC of 0.86 V, JSC of 6.48 mA cm−2, tigated by cyclic voltammetry (CV) in film on a glassy carbon FF of 0.33, and PCE of 1.84% (Figure 3a). On the basis of the working electrode in 0.1 M [nBu4N]+[PF6]− CH3CN solution at optimal D/A ratio of 1:1, the effect of solvent additive 1,8-diiodooctane (DIO) content on the device performance was also a potential scan rate of 100 mV s−1. As shown in Figure 1b,
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investigated. When 5% DIO was used, an enhanced JSC of 11.92 mA cm−2 and enhanced PCE of 3.32% were obtained. The incident photon to converted current efficiency (IPCE) values of the blend films with a donor/acceptor weight ratio of 1:1 without and with 5% DIO are shown in Figure 3b. These blend films showed broad IPCE plateau spectra from 450 to 730 nm, suggesting that both donor and acceptor in the BHJ blend made a considerable contribution to the IPCE and JSC. A solvent additive improved the IPCE maximum from 30% to 50%, which is consistent with an enhancement in the JSC. To evaluate the accuracy of the photovoltaic results, the JSC values were calculated from the integration of the IPCE spectra. The calculated average JSC was 6.10 and 10.45 mA cm−2 for the devices without and with 5% DIO, which entail a 2% and 7% mismatch with that measured by J–V measurements, respectively. The morphology of the BHJ active layer was examined by atomic force microscopy (AFM) in the tapping mode. The actual surface morphology of the blend films of PBDTTT-C-T: S(TPA-PDI) (1:1, w/w) without and with 5% DIO is shown in
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Figure 2. a) UV-vis absorption spectra of S(TPA-PDI), PBDTTT-C-T, PBDTTTC-T: S(TPA-PDI) (1:1, w/w) film; b) photoluminescence spectra of S(TPA-PDI) (excitation at 536 nm), PBDTTT-C-T (excitation at 652 nm), and PBDTTT-C-T: S(TPA-PDI) (1:1, w/w) (excitation at 536 and 652 nm) in thin films.
Figure 4. The as-cast film exhibits a smooth and uniform morphology with a root-mean-square (RMS) roughness of 0.66 nm. The AFM height histograms and section curves (Figure 4) show that a 5% DIO solvent additive increased the film roughness (4.1 nm) and crystalline size. The crystalline domains are attributed to the self-organization of PBDTTT-C-T, which Table 1. The average (at least 8 devices) and best (in brackets) device data of OSCs based on PBDTTT-C-T:S(TPA-PDI) under the illumination of AM 1.5G, 100 mW cm−2. PBDTTT-C-T:S(TPA- DIO PDI) (w/w) [%]
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0.85 (0.86) 5.04 (5.07)
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0.339 (0.341)
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0.324 (0.340)
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0.318 (0.331)
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3.22 (3.32)
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0.88 (0.89) 6.10 (6.14)
2.10 (2.14)
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0.381 (0.391)
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In summary, a novel, nonplanar, star-shaped PDI acceptor (S(TPA-PDI)) with a TPA core was explored. S(TPA-PDI) exhibited a quasi-3D structure, weak intermolecular interactions and molecular aggregation, a strong absorption in the visible region, a complementary absorption range to and appropriate energy levels matched with the low-bandgap polymer donor PBDTTT-C-T. Solution-processed OSCs based on a PBDTTTC-T:S(TPA-PDI) (1:1, w/w) blend film with 5% DIO solvent additive exhibited PCEs as high as 3.32%, which is among the highest values reported for solution-processed OSCs based on non-fullerene acceptors. These preliminary results demonstrate that this PDI-based, quasi-3D structure can suppress molecular aggregation, facilitate miscibility with donors, and in turn improve the PCE.
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is beneficial to the ordered structure formation and charge transport in the thin film.[7] Suitable crystalline domains and phase separation of the blend film are beneficial to achieving a high JSC. Further increasing the DIO content to 7%, both the film roughness (4.5 nm) and crystalline size of the blend film increased (Figure S5, Supporting Information). Over-aggregation and larger size phase separation of the blend film lowered the JSC and PCE of the device.[14] In order to investigate the influence of the DIO additive and morphology on the charge-carrier transport, the hole and electron mobilities of the PBDTTT-C-T:S(TPA-PDI) (1:1, w/w) blend film were measured by the SCLC method (Figure S6 and S7, Supporting Information). The average hole and electron mobilities for the blend without DIO were found to be 5.31 × 10−4 cm2 V−1 s−1 and 1.04 × 10−5 cm2 V−1 s−1, respectively. The blend with the 5% DIO additive exhibited enhanced charge transport with a hole mobility of 7.17 × 10−4 cm2 V−1 s−1 and electron mobility of 2.32 × 10−5 cm2 V−1 s−1. The better charge transport leads to an enhancement in the JSC of the OSC. However, the electron/hole mobility ratios (0.02–0.03) in this system are relatively low, which may be responsible for the relatively low FF (
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A Star-Shaped Perylene Diimide Electron Acceptor for High-Performance Organic Solar Cells Yuze Lin, Yifan Wang, Jiayu Wang, Jianhui Hou, Yongfang Li, Daoben Zhu, and Xiaowei Zhan* Solution-processed bulk heterojunction (BHJ) organic solar cells (OSCs) are a promising, cost-effective alternative for utility of solar energy, and low-cost, light-weight, and flexibility are just some of their advantages.[1–3] Over the last decade, the development of OSCs has seen a dramatic rise in power-conversion efficiency (PCE) from less than 1%[4] in the earliest reports to 10% in recent publications.[5] Generally, record efficiencies directly result from the development of new electron-donor materials that exhibit improved properties such as better spectral sensitivity, enhanced hole transport and favorably tuned HOMO/LUMO energy levels that are matched well with those of existing acceptors. In fact, the electron acceptors are of the same importance as the electron donors for high performance OSCs, but the development of electron-acceptor materials has lagged far behind that of donor materials. Fullerenes and their derivatives have been the dominant electron–acceptor materials in BHJ OSCs, owing to their high electron mobility, large electron affinity, and charge-transport isotropy.[6] However, the need remains to develop non-fullerene electron acceptors that will not only retain the favorable properties of fullerenes, but can also overcome their insufficiencies, such as their weak absorption in the visible spectral region, the limited spectral breadth, and limited energy-level variability. Nowadays, solution-processed BHJ OSCs based on non-fullerene acceptors have shown PCEs up to 3–4%,[7–14] which have been much higher than those (generally < 1.5%) disclosed in earlier days,[15–17] but non-fullerene acceptors are still at the infant stage relative to fullerenes, and more novel non-fullerene acceptors should be explored. Perylene diimide (PDI) units are widely used for constructing n-type organic semiconductors, and PDI derivatives are the earliest and most common non-fullerene acceptors
Y. Lin, Y. Wang, Prof. J. Hou, Prof. Y. Li, Prof. D. Zhu Beijing National Laboratory for Molecular Sciences Institute of Chemistry Chinese Academy of Sciences Beijing 100190, P.R. China J. Wang, Prof. X. Zhan Department of Materials Science and Engineering College of Engineering Peking University Beijing 100871, P.R. China E-mail: [email protected] Y. Lin, Y. Wang University of Chinese Academy of Sciences Beijing 100049, P.R. China
DOI: 10.1002/adma.201400525
Adv. Mater. 2014, DOI: 10.1002/adma.201400525
used in OSCs.[18–21] PDIs generally show high thermal, chemical, and light stabilities, good electron-accepting abilities, and excellent electron mobilities.[18–21] Compared to fullerenes, PDIs show a strong and broad absorption in the visible spectral region and their optoelectronic properties are easily tuned by tailoring the substituents on the imide-N, bay, and/or nonbay positions. However, traditional PDI derivatives show a high planarity and suffer from strong aggregation, leading to the formation of large crystalline domains (typically micrometer scale). Large crystalline domains of PDI in BHJ films lead to large phase separations, reduced exciton diffusion/separation efficiencies, and finally low PCEs of the OSCs. Therefore, restricting the crystallinity without adversely weakening the charge-transport properties of PDIs is a design principle for PDI-based acceptors. Recently, PDI dimers with twisted structures have shown promising performance as electron acceptors in solution-processed OSCs.[10,11,14,22] Novel, planar, star-shaped triimides were reported for electron acceptors in OSCs.[23] Like other rylene diimides-based polymer acceptors,[24,25] PDI-based polymer acceptors also exhibited encouraging performance in all-polymer solar cells with PCEs of up to 3.45%.[7,26] In this communication, we report a novel, nonplanar, starshaped PDI acceptor (S(TPA-PDI), Scheme 1) with a triphenylamine (TPA) core. As TPA has a special propeller starburst molecular structure because of the sp3 hybrid orbital of the nitrogen atom, S(TPA-PDI) possesses a quasi-3D nonplanar structure and isotropic optical and charge-transporting properties.[27] Like some other TPA-based or star-shaped molecules,[28–35] S(TPA-PDI) generally shows weak intermolecular interactions and molecular aggregation. Furthermore, S(TPAPDI) exhibits strong absorption in the visible region, a complementary absorption range, and appropriate energy levels matched with low-bandgap polymer donors, such as a copolymer of 5-alkylthiophene-2-yl-substituted benzo[1,2-b:4,5-b′] dithiophene and alkylcarbonyl-substituted thieno-[3,4-b]thiophene, PBDTTT-C-T (Scheme 1).[36] Solution-processed BHJ OSCs based on PBDTTT-C-T: S(TPA-PDI) showed a PCE as high as 3.32%, which is among the highest values reported for solution-processed OSCs based on non-fullerene acceptors. The compound S(TPA-PDI) was synthesized through a Suzuki coupling reaction between tris(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)amine and N,N′-di(2ethylhexyl)-1-bromo-7-(n-butoxyl)-3,4:9,10-perylene diimide using Pd(PPh3)4 as the catalyst (Scheme 1). Compound S(TPAPDI) was fully characterized by matrix-assisted laser desorption/ionization–time-of flight mass spectrometry (MALDI-TOF MS), 1H NMR, 13C NMR, and elemental analysis. This compound is readily soluble in common organic solvents such as dichloromethane and dichlorobenzene at room temperature,
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S(TPA-PDI) exhibits quasi-reversible reduction and oxidation waves; its reduction wave is obviously stronger than its oxidaO tion wave, because there are 3 PDI units but O N O only 1 TPA unit per S(TPA-PDI) molecule. O S The onset oxidation and reduction potenO S tials versus FeCp2+/0 are 0.60 and −1.10 V, S N O N O respectively, thus the HOMO and LUMO S n S O energies were estimated to be −5.40 and N S −3.70 eV from the onset oxidation and reducO O tion potentials, respectively, assuming the N absolute energy level of FeCp2+/0 to be 4.8 eV O below vacuum.[37] The energy gap calculated O O N O from CV (1.70 eV) was similar to its optical N O bandgap (1.76 eV). PBDTTT-C-T O The electron mobility of S(TPA-PDI) was measured by the space-charge limited current (SCLC) method with a S(TPA-PDI) device structure of Al/S(TPA-PDI)/Al O O (Figure S4, Supporting Information). O N O B The S(TPA-PDI) film exhibits an electron K aq. CO Pd(PPh3)4 2 3 mobility of 3.0 × 10−5 cm2 V−1 s−1, which is S(TPA-PDI) toluene + Br lower than those (ca. 10−4–10−2 cm2 V−1 s−1) O N 110 o C of fullerene derivatives[6] but is relatively O O high compared to other small molecular B B 46% O O non-fullerene acceptors.[1] O N O To demonstrate the potential application of S(TPA-PDI) in OSCs, we used S(TPAPDI) as an electron acceptor and a classical low-bandgap polymer PBDTTT-C-T Scheme 1. Chemical structures of compounds PBDTTT-C-T and S(TPA-PDI), and synthetic as the electron donor, and fabricated BHJ route of compound S(TPA-PDI). OSCs with a structure of ITO/PEDOT:PSS/ PBDTTT-C-T:S(TPA-PDI)/Ca/Al. The LUMO gap (0.45 eV) and HOMO gap (0.29 eV) between PBDTTT-C-T because of its solubilizing alkyl substituents. The thermal (LUMO/HOMO: −3.25/ −5.11 eV)[36] and S(TPA-PDI) should be properties of S(TPA-PDI) were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry large enough to promote efficient exciton dissociation.[38] The (DSC). This compound exhibits excellent thermal stability difference between the LUMO of S(TPA-PDI) and the HOMO with a decomposition temperature (5% weight loss) at 406 °C of PBDTTT-C-T is 1.41 eV, which could result in a high openin nitrogen atmosphere (Figure S1, Supporting Information). circuit voltage (VOC) of the solar cells.[39] Moreover, PBDTTTThe DSC trace for this compound shows no melting peak, and C-T and S(TPA-PDI) show complementary absorption spectra; the X-ray diffraction (XRD) pattern on the film shows no reflecthe PBDTTT-C-T: S(TPA-PDI) (1:1 w/w) blend film exhibits a tion peak (Figure S2, Supporting Information), suggesting that very broad absorption ranging from 400 to 800 nm (Figure 2a). S(TPA-PDI) is amorphous. Blending of PBDTTT-C-T with S(TPA-PDI) results in fluoresFigure 1a shows the normalized spectra of the optical cence quenching of over 95% and 65% of that of PBDTTT-Cabsorption of S(TPA-PDI) in dichloromethane solution T and S(TPA-PDI), respectively, indicating that an effective (10−6 M) and in thin solid film. S(TPA-PDI) in solution exhibits photoinduced charge transfer occurred between PBDTTT-C-T and S(TPA-PDI) in the blend film. Especially for PBDTTT-Ca strong absorption with a maximum extinction coefficient of T almost all photoinduced excitons were separated (Figure 2b), 9.09 × 104 M−1 cm−1 at 536 nm. Compared to the absorption in which is beneficial to a high short-circuit current density (JSC) solution, the thin film of S(TPA-PDI) shows a broader absorption with a similar profile, suggesting there is a weak interin the OSC. molecular interaction and molecular aggregation in the film, Table 1 summarizes VOC, JSC, fill factor (FF), and PCE of the because of the quasi-3D molecular structure of S(TPA-PDI) devices at different donor/acceptor (D/A) weight ratios. These (Figure S3, Supporting Information). The optical bandgap devices yielded a relatively high VOC, as expected, and the VOC of the S(TPA-PDI) film estimated from the absorption edge values of 0.86–0.87 V were almost independent of the donor/ (703 nm) was 1.76 eV. acceptor weight ratio. The blend at a D/A weight ratio of 1:1 The electrochemical properties of S(TPA-PDI) were invesgave the best performance: VOC of 0.86 V, JSC of 6.48 mA cm−2, tigated by cyclic voltammetry (CV) in film on a glassy carbon FF of 0.33, and PCE of 1.84% (Figure 3a). On the basis of the working electrode in 0.1 M [nBu4N]+[PF6]− CH3CN solution at optimal D/A ratio of 1:1, the effect of solvent additive 1,8-diiodooctane (DIO) content on the device performance was also a potential scan rate of 100 mV s−1. As shown in Figure 1b,
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0.5
1.0
0.8
800
900
0.6 0.4 0.2 0.0
1.5
600
650
Figure 1. a) UV–vis absorption spectra of S(TPA-PDI) in dichloromethane solution and in thin film; b) cyclic voltammogram for S(TPA-PDI) in CH3CN / 0.1 M [nBu4N]+[PF6]− at 100 mV s−1, the horizontal scale refers to an Ag/Ag+ electrode.
investigated. When 5% DIO was used, an enhanced JSC of 11.92 mA cm−2 and enhanced PCE of 3.32% were obtained. The incident photon to converted current efficiency (IPCE) values of the blend films with a donor/acceptor weight ratio of 1:1 without and with 5% DIO are shown in Figure 3b. These blend films showed broad IPCE plateau spectra from 450 to 730 nm, suggesting that both donor and acceptor in the BHJ blend made a considerable contribution to the IPCE and JSC. A solvent additive improved the IPCE maximum from 30% to 50%, which is consistent with an enhancement in the JSC. To evaluate the accuracy of the photovoltaic results, the JSC values were calculated from the integration of the IPCE spectra. The calculated average JSC was 6.10 and 10.45 mA cm−2 for the devices without and with 5% DIO, which entail a 2% and 7% mismatch with that measured by J–V measurements, respectively. The morphology of the BHJ active layer was examined by atomic force microscopy (AFM) in the tapping mode. The actual surface morphology of the blend films of PBDTTT-C-T: S(TPA-PDI) (1:1, w/w) without and with 5% DIO is shown in
700
750
800
850
900
Wavelength / nm
Potential / V
Adv. Mater. 2014, DOI: 10.1002/adma.201400525
700
S(TPA-PDI) blend @ 536 nm PBDTTT-C-T blend @ 652 nm
1.0
-0.10
600
Wavelength / nm
Wavelength / nm b) 0.10
-0.25
500
Figure 2. a) UV-vis absorption spectra of S(TPA-PDI), PBDTTT-C-T, PBDTTTC-T: S(TPA-PDI) (1:1, w/w) film; b) photoluminescence spectra of S(TPA-PDI) (excitation at 536 nm), PBDTTT-C-T (excitation at 652 nm), and PBDTTT-C-T: S(TPA-PDI) (1:1, w/w) (excitation at 536 and 652 nm) in thin films.
Figure 4. The as-cast film exhibits a smooth and uniform morphology with a root-mean-square (RMS) roughness of 0.66 nm. The AFM height histograms and section curves (Figure 4) show that a 5% DIO solvent additive increased the film roughness (4.1 nm) and crystalline size. The crystalline domains are attributed to the self-organization of PBDTTT-C-T, which Table 1. The average (at least 8 devices) and best (in brackets) device data of OSCs based on PBDTTT-C-T:S(TPA-PDI) under the illumination of AM 1.5G, 100 mW cm−2. PBDTTT-C-T:S(TPA- DIO PDI) (w/w) [%]
VOC [V]
JSC [mA cm−2]
FF
PCE [%]
3:2
0
0.85 (0.86) 5.04 (5.07)
0.299 (0.306)
1.28 (1.33)
2:3
0
0.86 (0.87) 5.35 (5.48)
0.339 (0.341)
1.56 (1.62)
1:1
0
0.86 (0.86) 6.20 (6.48)
0.324 (0.340)
1.73 (1.84)
1:1
1
0.87 (0.87) 7.29 (7.49)
0.318 (0.331)
2.02 (2.08)
1:1
3
0.87 (0.88) 7.62 (7.84)
0.386 (0.391)
2.56 (2.63)
1:1
5
0.87 (0.88) 11.27 (11.92) 0.328 (0.336)
3.22 (3.32)
1:1
7
0.88 (0.89) 6.10 (6.14)
2.10 (2.14)
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
0.381 (0.391)
wileyonlinelibrary.com
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In summary, a novel, nonplanar, star-shaped PDI acceptor (S(TPA-PDI)) with a TPA core was explored. S(TPA-PDI) exhibited a quasi-3D structure, weak intermolecular interactions and molecular aggregation, a strong absorption in the visible region, a complementary absorption range to and appropriate energy levels matched with the low-bandgap polymer donor PBDTTT-C-T. Solution-processed OSCs based on a PBDTTTC-T:S(TPA-PDI) (1:1, w/w) blend film with 5% DIO solvent additive exhibited PCEs as high as 3.32%, which is among the highest values reported for solution-processed OSCs based on non-fullerene acceptors. These preliminary results demonstrate that this PDI-based, quasi-3D structure can suppress molecular aggregation, facilitate miscibility with donors, and in turn improve the PCE.
Current density / mA cm-2
a) 4 2 0 -2 -4 -6 -8 -10
Without DIO With 5% DIO
-12 -14 -0.2
0.0
0.2
0.4
0.6
0.8
1.0
Bias / V Experimental Section
b) 60
IPCE (%)
50
without DIO with 5% DIO
40 30 20 10 0 300
400
500
600
700
800
Wavelength / nm Figure 3. a) J–V curves and b) IPCE spectra of devices with the structure ITO/PEDOT:PSS/ PBDTTT-C-T:S(TPA-PDI) (1:1, w/w)/Ca/Al without and with 5% DIO additive.
is beneficial to the ordered structure formation and charge transport in the thin film.[7] Suitable crystalline domains and phase separation of the blend film are beneficial to achieving a high JSC. Further increasing the DIO content to 7%, both the film roughness (4.5 nm) and crystalline size of the blend film increased (Figure S5, Supporting Information). Over-aggregation and larger size phase separation of the blend film lowered the JSC and PCE of the device.[14] In order to investigate the influence of the DIO additive and morphology on the charge-carrier transport, the hole and electron mobilities of the PBDTTT-C-T:S(TPA-PDI) (1:1, w/w) blend film were measured by the SCLC method (Figure S6 and S7, Supporting Information). The average hole and electron mobilities for the blend without DIO were found to be 5.31 × 10−4 cm2 V−1 s−1 and 1.04 × 10−5 cm2 V−1 s−1, respectively. The blend with the 5% DIO additive exhibited enhanced charge transport with a hole mobility of 7.17 × 10−4 cm2 V−1 s−1 and electron mobility of 2.32 × 10−5 cm2 V−1 s−1. The better charge transport leads to an enhancement in the JSC of the OSC. However, the electron/hole mobility ratios (0.02–0.03) in this system are relatively low, which may be responsible for the relatively low FF (
