Comparison among Perylene Diimide (PDI), Naphthalene Diimide

Apr 13, 2017 - Rylene dimides are widely used as the building blocks for n-type semiconducting polymers due to the tunable electronic properties. To e...
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Comparison among Perylene Diimide (PDI), Naphthalene Diimide (NDI), and Naphthodithiophene Diimide (NDTI) Based n‑Type Polymers for All-Polymer Solar Cells Application Jing Yang,†,‡ Bo Xiao,†,‡ Keisuke Tajima,§ Masahiro Nakano,§ Kazuo Takimiya,§ Ailing Tang,† and Erjun Zhou*,† †

CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China § RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan S Supporting Information *

ABSTRACT: Rylene dimides are widely used as the building blocks for n-type semiconducting polymers due to the tunable electronic properties. To elucidate their potentials as the electron acceptors in all-polymer solar cells, systematic comparisons of the properties among the derivatives are necessary. Herein, we used perylene diimide (PDI), naphthalene diimide (NDI), and naphthodithiophene diimide (NDTI) with the same alkyl chains combined with dithienothiophene (DTT) unit to obtain three polymer acceptors PPDI-DTT, PNDI-DTT, and PNDTI-DTT, respectively. Light absorption, carrier mobility, film morphology, and molecular orientation were characterized and compared. The photovoltaic devices based on PPDI-DTT, PNDI-DTT, and PNDTI-DTT achieved power conversion efficiency (PCE) of 3.49, 2.50, and 5.57%, respectively, in combination with BDDT as the donor polymer. The high performance of PNDTI-DTT was attributed to the strong absorption profile in the near-infrared (NIR) region, high and balanced electron and hole mobilities, and the preferable face-on orientation for the polymer chains in the blend films. The results indicate that NDTI is a promising building block to construct n-type photovoltaic polymers, and higher photovoltaic performance is anticipated with the further development of novel NDTI-based polymers.

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photovoltaic performance.7 After modulation of the chemical structures of PDI-based polymer and utilization of promising ptype polymers, the photovoltaic performance improved gradually. In 2011 and 2014, PCEs of 2.2%8 and 4.1%9 were realized by utilizing PDI-co-carbarzole and PDI-co-thiophene copolymers, respectively. In 2016, Yan et al. adopted a vinylenebridged PDI-based polymer and achieved a high PCE of 7.57%, which is also the highest value for PDI-based polymers.10 Studies on NDI-based acceptor polymers11 for organic photovoltaic began with the work of Loi and co-workers in 2011.12 They studied the photovoltaic properties of the classic NDI-co-bithiophene copolymer (N2200) in the blend with the P3HT and achieved a high fill factor of nearly 70%, suggesting high charge separation efficiency and balanced electron and hole mobility. In 2013, Zhou et al. synthesized a n-type polymer of NDI-co-carbazole and achieved a PCE of 3.6%,13 and almost at the same time, Jenekhe et al. also realized a PCE

INTRODUCTION All-polymer solar cells (all-PSCs) based on the blend films of a p-type conjugated polymer as the electron donor (D) and a ntype conjugated polymer as the electron acceptor (A) have been attracting a lot of attention due to the advantages of effectively tunable electron energy levels and absorption spectra and superior flexibility in controlling solution viscosity compared with the polymer/fullerene blends.1 Although the history of the polymer acceptors started as early as the fullerenes in 1995, their development lags far behind. Polymer acceptors primarily include cyanated polyphenylenevinylenes,2−4 benzothiadiazole,5,6 and rylene diimide-based polymers. The polymers based on rylene diimide such as perylene diimide (PDI), nanphthalene diimide (NDI), and naphtho[2,3-b:6,7-b′]dithiophene-4,5,9,10-diimide (NDTI) are among the most efficient acceptors for all-PSCs. Figure 1 summarizes the development of the rylene diimide-based polymers for organic photovoltaic (OPV) application in 2007− 2016. In 2007, Zhan et al. synthesized the first perylene dimidebased polymer, PDI-co-dithienothiophene, and investigated the © 2017 American Chemical Society

Received: February 24, 2017 Revised: April 8, 2017 Published: April 13, 2017 3179

DOI: 10.1021/acs.macromol.7b00414 Macromolecules 2017, 50, 3179−3185

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PPDI-DTT, PNDI-DTT, and PNDTI-DTT. The same 2decyltetradecyl alkyl chain was introduced to the imides to ensure the high solubility of the polymers. The p-type polymer BDDT25 (also known as PBDTBDD-T or PBDB-T) was used as the electron donor in all-PSC because of its outstanding performance in fullerene-free organic solar cells.26,27

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RESULTS AND DISCUSSION Figure 2 shows the chemical structures and energy level diagrams of the three rylene diimide-based polymers PPDI-

Figure 1. Summary of the development of three kinds of rylene diimide polymers for all-PSC application.

of 3.3% by using a polymer of NDI-co-selenophene.14 In 2014 and 2015, all-PSCs based on NDI-based polymers realized high PCE of 5.7%15 and 7.7%16 by both the material designs and the device optimizations. In 2016, Li et al. utilized medium bandgap benzodithiophene-alt-benzotriazole copolymers (J51) as the donor and N2200 as the acceptor to achieve the highest PCE of 8.27% with a high FF of 70.24%.17 Up to now, NDIbased polymers were the most efficient polymer acceptors in all-PSCs.18−21 Compared with above-mentioned two rylene dimides, NDTI is a relatively new class of building block22 with an extended πplane of thiophenes. It was first used in all-PSCs by Zhou and co-workers in 2014.23 A polymer acceptor based on NDTI and bithiophene exhibited an extended absorption in NIR region up to 900 nm and achieved a PCE of 2.59% in all-PSCs with PTB7 as the donor. In 2016, a comprehensive study was carried out on NDTI-based copolymers with thiophene (PNDTI-T), thienothiophene (PNDTI-TT), and dithienothiophene units (PNDTI-DTT).24 Interestingly, PNDTI-T and PNDTI-DTT showed a face-on orientation, but PNDTI-TT gave an edge-on orientation to the substrate due to the difference of the linearity for the polymer main chain. A highest PCE of 3.6% and Jsc of 10.7 mA/cm2 were achieved in PSC with PNDTI-DTT as the electron acceptor and PTB7 as the electron donor. Because of the strong absorption spectra in the near-infrared region for NDTI-based polymers, higher Jsc could be obtained after choosing suitable large band gap p-type polymers. As shown in Figure 1, PDI- and NDI-based photovoltaic materials have achieved a rapid growth in recent years with PCEs exceeding 7.5% and 8.2%, respectively. Although NDTIbased polymers still show lower PCE than PDI- and NDI-based polymers, considering the limited number of the PNDTI-based materials and only preliminary investigations on the photovoltaic applications, there is much room for improvement. Thus, comprehensive comparisons among PDI-, NDI-, and NDTI-based polymers under the same conditions would be very helpful to understand the structure−property−performance relationship of these rylene diimide-based polymers. In this paper, we choose dithienothiophene (DTT) as the electron-rich counterpart of PDI, NDI, and NDTI and synthesized three donor−acceptor alternative copolymers:

Figure 2. Chemical structures and energy level diagrams of the three rylene diimide-based polymers and the p-type polymer, BDDT.

DTT, PNDI-DTT, PNDTI-DTT, and the p-type polymer BDDT. Because the bulky side chains of 2-decyltetradecyl were used, the three polymers show good solubility in common organic solvents such as chloroform (CF), chlorobenzene (CB), and o-dichlorobenzene (DCB). All the n-type polymers have comparable molecular weight and polydispersity index (PDI). The weight-average molecular weight (Mw) and number-average molecular weight (Mn) are in the range of 56.9−68.5 and 27.1−30.5 kDa, respectively, with PDIs of 1.9− 2.4. The normalized UV−vis absorption spectra of BDDT, PPDIDTT, PNDI-DTT, and PNDTI-DTT in CB solutions (∼10−5 mol L−1) and in films are shown in Figure 3. The molar extinction coefficients in the solutions and the absorption spectra of the blend films with BDDT are shown in Figures S1 and S2, respectively. PPDI-DTT in CB solution showed a broad and strong absorption in the region of 300−700 nm with a maximum molar extinction coefficient of 3.21 × 104 L mol−1 cm−1 at 349 nm. The absorption spectrum of PNDI-DTT in solution showed two distinct absorption peaks at 300−400 nm which belong to the π−π* or n−π* transition of the polymer main chain and the peak at 723 nm which corresponds to the intramolecular charge transfer (ICT) band of the D/A copolymer,28 and the maximum molar extinction coefficient is 3.16 × 104 L mol−1 cm−1. Notably, due to the strong electron accepting ability of the NDTI unit and the electron donating ability of the DTT units, the absorption spectrum of PNDTIDTT expanded in the near-IR region,24 and the maximum molar extinction coefficient is 6.8 × 104 L mol−1 cm−1 at 833 3180

DOI: 10.1021/acs.macromol.7b00414 Macromolecules 2017, 50, 3179−3185

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Figure 3. UV−vis absorption spectra of BDDT, PPDI-DTT, PNDI-DTT, and PNDTI-DTT (a) in CB solutions and (b) in films on quartz plates.

Table 1. Photovoltaic Performances of All-PSCs Based on BDDT and the Three Rylene Diimide Polymers acceptors PPDI-DTT PNDI-DTT PNDTI-DTT

Voc (V) 0.77 0.76 0.69

Jsc (mA cm−2) 8.24 6.09 13.68

FF 0.55 0.54 0.59

PCE (%) 3.49 (3.40 ± 0.06) 2.50 (2.31 ± 0.20) 5.57 (5.50 ± 0.11)

nm. The absorption edges showed the lowering of the optical band gaps in the order of PDI, NDI to NDTI (Figure 3a). BDDT and all the three polymer acceptors showed complementary absorption spectra to some degree. Particularly the blend polymer film of BDDT and PNDTI-DTT can cover the broad wavelength region from 300 to 900 nm (Figure S2). Compared with the absorption spectra in the solutions, the absorptions of BDDT, PPDI-DTT, and PNDI-DTT in the films were slightly red-shifted, and the relative intensity in the long wavelength region was slightly increased. In contrast, the absorption peak of PNDTI-DTT in the film showed a blue-shift by 19 nm compared with the absorption in solution. It suggests the packing manner of the main chains in PNDTI-DTT could be different from the others, maybe containing H-aggregation in the film. Photovoltaic Properties. To investigate the photovoltaic performance of the polymer acceptors, all-PSCs was fabricated in the combination with BDDT as the donor because of its suitable energy levels and high performance in all-PSCs.25,29 The device structure was ITO/PEDOT: PSS/polymer blend/ Ca/Al, and the D/A weight ratios are optimized and shown in Table S1. The highest efficiencies of all-PSCs were obtained by using the solvent of CB and the annealing temperature of 120 °C, with the D/A weight ratios of 1:1 for PPDI-DTT and PNDI-DTT and 1.5:1 for PNDTI-DTT. Table 1 summarizes the photovoltaic performance parameters including open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and power conversion efficiency (PCE) of all-PSCs based on BDDT and the three rylene diimide polymer acceptors under the optimized conditions. Figure 4 shows the current density− voltage (J−V) curves of the PSCs under the illumination of AM 1.5G, 100 mW/cm2. Voc and FF for PPDI-DTT and PNDIDTT devices were similar to values of about 0.76−0.77 V and 0.54−0.55, respectively. For PNDTI-DTT, Voc was slightly lower and FF was slightly higher (Voc = 0.69 V, FF = 0.59); the former could be attributed to the lower-lying LUMO energy levels. Thus, for three polymers, the difference in the efficiency comes mainly from the difference in the current density. Among the three diimide polymer acceptors, the all-PCSs based on PNDTI-DTT show the best performance with the highest

μh (cm2 V−1 s−1) −5

4.88 × 10 2.18 × 10−4 2.16 × 10−4

μe (cm2 V−1 s−1)

μh/μe

1.24 × 10−5 1.99 × 10−6 1.43 × 10−4

3.94 109.55 1.51

Figure 4. J−V curves of photovoltaic devices based on (a) BDDT:PPDI-DTT, (b) BDDT:PNDI-DTT, and (c) BDDT:PNDTI-DTT under the illumination of AM 1.5, 100 mW cm−2.

PCE of 5.57%, which is also the highest efficiency of the photovoltaic cells using PNDTI-based materials reported to date.23,24 The high PCE of PNDTI-DTT based PSCs mainly comes from the high Jsc of 13.68 mA/cm2. Figure 5 shows the external quantum efficiency (EQE) plots of the best all-PSCs with BDDT as the polymer donor and the diimide polymers as the acceptor. As shown in the EQE spectra, both the polymer donor BDDT and the diimide polymer acceptor (PPDI-DTT, PNDI-DTT, or PNDTI-DTT) contribute to the photocurrent. It is worth noting that the PNDTI-DTT-based PSC shows a strong and broad photocurrent response from 350 to 900 nm, and the maximum EQE is 0.55, which explains the high Jsc and PCE. PPDI-DTT-based PSC shows a relatively high photoresponse in the region of 350−700 nm with a maximum EQE of 0.49. PNDI-DTT-based PSC shows a relatively wide photoresponse from 350 to 800 nm, but the maximum EQE is only 0.33, resulting in the low Jsc and PCE. The calculated Jsc values under the simulated sunlight from the EQE curves are 8.26, 6.24, and 13.95 mA cm−2 for the 3181

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the vertical direction. The high charge mobilities in the direction perpendicular to substrate could contribute to the high photovoltaic performance.31 To study the charge-transport properties of all-PSCs, the hole (μh) and the electron mobilities (μe) of blend films were measured by the space charge limited current (SCLC) method with the Mott−Gurney equation. Figure 8 shows the typical J− V curves of the hole-only and electron-only devices, where Ohmic (slope = 1) and SCLC regions (slope = 2) are clearly observed.32,33 The values of hole and electron mobilities are summarized in Table 1. Difference in the charge carrier mobility for three combinations, together with the different absorption spectra, accounted for the difference in Jsc. μh and μe of the PNDTI-DTT:BDDT blend film were calculated to be 2.16 × 10−4 cm2 V−1 s−1 and 1.43 × 10−4 cm2 V−1 s−1 with μh/ μe of 1.51, indicating high and balanced charge carrier mobility. PPDI-DTT:BDDT blend film showed the μh of 4.88 × 10−5 cm2 V−1 s−1 and μe of 1.24 × 10−5 cm2 V−1 s−1, suggesting that the charge carrier mobilities were balanced, though the values were relatively low. Thus, a moderate Jsc of 8.24 mA cm−2 and PCE of 3.49% were achieved. For the PNDI-DTT:BDDT blend film, the unbalanced charge carrier mobility and the lowest electron mobility of 1.99 × 10−6 cm2 V−1 s−1could be a factor for the low Jsc and PCE.

Figure 5. External quantum efficiency (EQE) plots of all-PSCs based on BDDT:PPDI-DTT, BDDT:PNDI-DTT, and BDDT:PNDTI-DTT.

all-PSCs based on PPDI-DTT, PNDI-DTT, and PNDTI-DTT, respectively, which agree well with the observed Jsc. To explain the difference in the photovoltaic performance among the three diimide polymer-based all-PSCs, atomic force microscopy (AFM) height images of the blend films with BDDT were measured (Figure 6). There was no significant phase separation in all the blend films, which displayed relatively uniform and smooth surface, indicating good miscibility of the D/A components in their blend films.30 The values of root-mean-square (RMS) roughness were 1.14 nm for BDDT:PPDI-DTT, 2.63 nm for BDDT:PNDI-DTT, and 0.98 nm for BDDT:PNDI-DTT. However, AFM images only give the surface information on blend films; the crystallinity and molecular orientation of the polymers in the blend films need to be further investigated. Grazing Incident Wide-Angle X-ray Scattering (2D GIWAXS). To understand the crystallinity and molecular orientation of the polymers, in both the pristine and blend films, two-dimensional grazing incidence wide-angle X-ray scattering (2D GIWAXS) was measured, as shown in Figure 7. Similar to the previous result, BDDT adopted a face-on orientation of the pristine polymer film.25 For the three rylene diimide polymers, the degree of the crystallinity was highest for PNDI-DTT followed by PNDI-DTT and PPDI-DTT, judging from the diffraction intensity. In the three blend films with BDDT, the face-on orientation of BDDT could be maintained at least in part. Especially BDDT:PNDTI-DTT film showed signifcant perferential face-on orientation of the polymer chains, which is favorable for hole and electron transport in

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CONCLUSION Three n-type polymers PPDI-DTT, PNDI-DTT, and PNDTIDTT were compared from the viewpoint of the photovoltaic performance, absorption spectrum, morphology, and molecular orientation. The LUMO energy levels of the three reylene diimide polymer were similar, resulting in the similar Voc (0.77−0.69 V) when using the same donor BDDT. The absorption spectra red-shifted as the acceptor unit changed from PDI, NDI to NDTI. In particular, the broad and strong absorption spectra of PNDTI-DTT made the blend films show a strong photoresponse in 400−850 nm; thus, the Jsc and PCE were the highest among the three rylene diimide polymers. Moreover, the high and balanced carrier mobility and face-on orientation to the substrate also contributed to the high PCE of PNDTI-DTT-based devices. The results indicate that NDTI is a promising building block for polymer acceptors with broad absorption spectra and high performance.

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EXPERIMENTAL SECTION

Hole and Electron Mobility Measurement by Space Charge Limited Current (SCLC) Method. Hole- and electron-only devices were fabricated by using the device structures of ITO/PEDOT:PSS/ polymer blend/Au and ITO/TiOx/polymer blend/Al, respectively. TiOx layer was fabricated by the sol−gel method from titanium

Figure 6. AFM height images of (a) BDDT:PPDI-DTT, (b) BDDT:PNDI-DTT, and (c) BDDT:PNDTI-DTT films spin-coated from CB. 3182

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Figure 7. 2D-GIWAXS patterns of the pristine thin films of BDDT, PPDI-DTT, PNDI-DTT, PNDTI-DTT, and their blend films with BDDT.

Figure 8. J−V curves in the space charge limited region for (a) ITO/PEDOT:PSS/polymer blend/Au for holes and (b) ITO/TiOx/polymer blend/ Al for electrons. and dried at 150 °C for 15 min in air. Then the devices were transferred into a nitrogen glovebox, and the solution of BDDT:acceptors (PPDI-DTT, PNDI-DTT, or PNDTI-DTT) blend in CB with a total concentration of 20 mg mL−1 was spin-coated onto the PEDOT:PSS layer. The optimized D/A weight ratio is 1:1 for PPDI-DTT and PNDI-DTT and 1.5:1 for PNDTI-DTT, and the optimized spin-casting speed was 2000 rpm for PPDI-DTT, 2500 rpm for PNDTI-DTT devices, and 1500 rpm for PNDI-DTT devices at room temperature, followed by thermal annealing at 120 °C for 10 min in the glovebox. Then, a Ca/Al (20 nm/80 nm) cathode was evaporated onto the active layer under a pressure of 3 × 10−6 mbar through a shadow mask. The active area of the cells is 4 mm2. The current density−voltage (J−V) curves were measured by a Keithley 2420 Source-Measure Unit, and the photocurrent was measured under illumination using an Oriel Newport 150 W solar simulator (AM 1.5G). The EQE of the devices was measured using an Oriel Newport System (Model 66902). All the measurements were performed at room temperature in air.

(diisopropoxide) bis(2,4-pentanedionate), 75% in isopropanol liquid (TIPD). The active layers were spin-coated from CB with the total concentration of 20 mg mL−1. Both the hole and electron mobilities were calculated with the Mott−Gurney equation in the SCLC region (slope = 2 in log J vs log V plots): J=

9 V2 ε0εr μ 3 8 L

where ε0 is the permittivity of the vacuum, εr is the dielectric constant of the polymer (assumed to 3), and L is the thickness of the polymer layer. Device Fabrication and Measurements. A patterned indium tin oxide (ITO)-coated glass substrate was precleaned in detergent solution, deionized water, acetone, and 2-propanol for 15 min sequentially in an ultrasonic bath, followed by UV-O3 treatment (Ultraviolet Ozone cleaner, Jelight Company, USA) for 15 min. Then a PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)) was spin-coated onto the substrate at 3500 rpm for 30 s 3183

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00414. Scheme S1; Figures S1 and S2; Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (E.Z.). ORCID

Keisuke Tajima: 0000-0003-1590-2640 Masahiro Nakano: 0000-0002-9231-4124 Kazuo Takimiya: 0000-0002-6001-1129 Erjun Zhou: 0000-0003-1182-311X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Nos. 51473040, 51673048, and 21602040), the National Natural Science Foundation of Beijing (No. 2162045), and the Chinese Academy of Sciences (QYZDB-SSW-SLH033). 2D GIWAXS experiments were performed with the approval of the Japan Synchrotron Radiation Research Institute (JASRI; Proposal 2016B1875). We thank Dr. Tomoyuki Koganezawa (JASRI) for his support with the measurements.

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REFERENCES

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DOI: 10.1021/acs.macromol.7b00414 Macromolecules 2017, 50, 3179−3185

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DOI: 10.1021/acs.macromol.7b00414 Macromolecules 2017, 50, 3179−3185