All-Small-Molecule Nonfullerene Organic Solar Cells with High Fill

Aug 11, 2017 - All-Small-Molecule Nonfullerene Organic Solar Cells with High Fill Factor and High Efficiency over 10%. Beibei Qiu†‡, Lingwei ... S...
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All-Small-Molecule Nonfullerene Organic Solar Cells with High Fill Factor and High Efficiency over 10% Beibei Qiu,†,‡ Lingwei Xue,†,§ Yankang Yang,†,‡ Haijun Bin,†,‡ Yindong Zhang,∥,⊥ Chunfeng Zhang,*,∥,⊥ Min Xiao,∥,⊥ Katherine Park,# William Morrison,# Zhi-Guo Zhang,*,† and Yongfang Li*,†,‡,∇ †

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 ‡ School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China § Department of Chemistry & Chemical Engineering, School of Chemical & Biological Engineering, University of Science & Technology Beijing, Beijing 100083, China ∥ National Laboratory of Solid State Microstructures, School of Physics, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China ⊥ Synergetic Innovation Center in Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China # Molecular Vista, San Jose, California 95119, United States ∇ Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123, China S Supporting Information *

ABSTRACT: We synthesized two wide bandgap A−D−A structured p-type organic semiconductor (p-OS) small molecules with weak electron-withdrawing ester end groups: SM1 with cyano group (CN) on the ester group and SM2 without the CN group. SM1 showed stronger absorption, lower-lying HOMO energy level, and higher hole mobility in comparison with that of SM2 without the CN groups. The all-smallmolecule organic solar cell (SM-OSC) with SM1 as donor and a narrow bandgap n-OS IDIC as acceptor demonstrated a high power conversion efficiency (PCE) of 10.11% and a high fill factor (FF) of 73.55%, while the PCE of the device based on SM2:IDIC is only 5.32% under the same device fabrication condition. The PCE of 10.11% and FF of 73.55% for the SM1-based device are the highest values for the nonfullerene SMOSCs reported in the literature so far. The results indicate that the cyano substitution in SM1 plays an important role in improving the photovoltaic performance of the p-OS donors in the nonfullerene SM-OSC. In addition, the photoinduced force microscopy (PiFM) was first used in OSCs to characterize the morphology of its donor/acceptor blend active layer.

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INTRODUCTION

promote the development of OSCs, a number of novel nonfullerene n-type organic semiconductors (n-OS) have been designed and used as alternative electron acceptors recently.15−17 Among various n-OS acceptors, low bandgap A− D−A structured n-OS (such as IEIC,18 ITIC,19 IDIC,20 etc.) have shown distinguished advantages of easy tuning of energy levels, strong absorbance, good morphology stability, and being more suitable for flexible devices. To date, high power conversion efficiency (PCE) of 11−13% with single junction structure has been achieved by using medium bandgap conjugated polymer donor materials,21−27 demonstrating the great potential of the nonfullerene OSCs.

Bulk-heterojunction (BHJ) organic solar cells (OSCs) have been thought of as promising next-generation green energy technology,1−4 because of their attractive advantages in fabricating lightweight, low-cost, large-area, and flexible devices through a simple solution process. During the past several decades, both the innovation of materials and the optimization of devices have led to significant improvements in power conversion efficiency (PCE) of fullerene-based OSCs.5−12 Fullerene derivatives, such as phenyl-C61-butyric acid methyl ester (PC61BM) and phenyl-C71-butyric acid methyl ester (PC71BM), have played an important role in promoting the development of the OSCs; however, the fullerene acceptors have some inherent shortcomings, such as weak light absorption, difficult to tune energy levels, and morphological instability.13,14 To resolve these drawbacks and to further © 2017 American Chemical Society

Received: June 22, 2017 Revised: August 11, 2017 Published: August 11, 2017 7543

DOI: 10.1021/acs.chemmater.7b02536 Chem. Mater. 2017, 29, 7543−7553

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Chemistry of Materials Scheme 1. Synthesis Routes of the p-OS Donors SM1 and SM2

Figure 1. (a) Chemical structures of small-molecule donors SM1 and SM2 and low bandgap n-OS acceptor IDIC. (b) Device structure of the SMOSCs. (c) Schematic energy diagram of the materials involved in the SM-OSCs.

Small-molecule p-type organic semiconductor (p-OS) donors, compared to polymer donors, have the advantages of well-defined chemical structures, easy purification, and good

photovoltaic performance reproducibility without batch-tobatch variation.28−34 However, PCE of the nonfullerene all small molecule organic solar cells (SM-OSCs) lags behind the 7544

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Figure 2. (a) Absorption spectra of SM1, SM2, and IDIC in thin film; (b) cyclic voltammograms for SM1 and SM2; (c) molecular configuration, electron density distribution, and electronic energy levels of SM1 and SM2 calculated by DFT at the B3LYP/6-31G(d, p) level.

nonfullerene PSCs.35−41 The lower PCE of the nonfullerene SM-OSCs should be mainly due to the poorer interpenetrating network, unbalanced charge transportation ability, and mismatched absorption spectra of donor and acceptor materials. Therefore, it is essential to select appropriate p-OS donor and n-OS acceptor materials that possess complementary absorption spectra for enhancing short-circuit current density (Jsc), higher and balanced hole/electron mobilities (μh/μe) for enhancing fill factor (FF), and appropriate compatibility for forming nanoscale phase-separated D/A interpenetrating networks for facilitating exciton dissociation and charge carrier transportation. For both p-OS donors and n-OS acceptors, the conjugated acceptor−donor−acceptor (A−D−A) type backbone, consisting of an electron-rich unit (D) as central building block and two electron-deficient units (A) as end groups, is an effective strategy to tune the optical absorption and molecular energy level.13,32,33,42−44 In the A−D−A type nonfullerene n-OS acceptors, to facilitate light-induced ultrafast charge transfer in OSCs, end groups with stronger electron-withdrawing effects are commonly used, which leads to lower optical bandgap.13 Besides, it should be noted that, in comparison with fullerene derivatives, the electron-withdrawing property of nonfullerene acceptors are not as strong as fullerene derivatives. Therefore, the combination of a wide bandgap p-OS donor with weak electron-withdrawing end unit and a low bandgap n-OS A−D− A acceptor is a reasonable design strategy. Considering the analysis mentioned above, herein a weak electron-withdrawing ester end-capping group was selected to construct two wide bandgap A−D−A type p-OS small molecules, SM1 with cyano group (CN) on the ester group and SM2 without the CN group. SM1 with cyano substituents showed stronger absorption, lower-lying HOMO energy level, and higher hole mobility in comparison with that of SM2 without the CN groups. The photovoltaic performance of the p-OS molecules was investigated by fabricating nonfullerene

OSCs with the p-OS molecules SM1 and SM2 as donor and a narrow bandgap n-OS IDIC20 as acceptor. After optimizing the phase-separation morphology of the donor and acceptor in the active layers by thermal annealing, a PCE of 10.11% and a high fill factor (FF) of 73.55% were achieved for the nonfullerene SM-OSCs based on SM1/IDIC. To the best of our knowledge, the PCE of 10.11% is the highest efficiency reported in the literature to date for the nonfullerene SM-OSCs.

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RESULTS AND DISCUSSION Materials Synthesis. The synthetic routes and chemical structures of the p-OS molecules are depicted in Scheme 1. The target small molecules SM1 and SM2 were obtained through a Stille coupling reaction between compound 4 and compound 3a or 3b, respectively. Figure 1a shows the molecular structures of the p-OS donors of SM1 and SM2 and the n-OS acceptor IDIC, where SM2 without cyano group substituent was used as a control p-OS donor to SM1 for confirming the positive effect of the cyano group on the photovoltaic performance. The two small molecules were characterized by 1H and 13C NMR spectroscopy. Thermogravimetric analysis (TGA) demonstrated that both SM1 and SM2 possess excellent thermal stability with 5% weight loss at 378 and 411 °C, respectively (Figure S1a in the Supporting Information). Figure S1b in the Supporting Information shows the differential scanning calorimetry (DSC) thermograms of SM1 and SM2. The compound SM1 with CN substituents shows a high melting point (Tm) of 207.6 °C from ordered structure to amorphous structure with melting enthalpy (ΔHm) of 40.5 J/g, while SM2 displays a much lower Tm of 149.2 °C and a smaller ΔHm of 28.1 J/g, indicating that the ordered structure of SM1 possesses better thermostability. The XRD patterns of SM1 and SM2 films are shown in Figure S2 in the Supporting Information. Both pure SM1 and SM2 films exhibited strong (100) diffraction peaks, confirming the ordered structure of SM1 and SM2. The 2θ values of the (100) diffraction peaks of 7545

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Chemistry of Materials Table 1. Absorption and Electronic Energy Levels of SM1 and SM2 p-OS molecules

λmax solution (nm)

λmax film (nm)

λonset film (nm)

Egopt (eV)

Eox (V)

Ered (V)

EHOMO (eV)

ELUMO (eV)

Egec (eV)

SM1 SM2

500 466

566 521

660 615

1.88 2.02

0.88 0.68

−1.54 −1.66

−5.24 −5.04

−2.82 −2.70

2.42 2.34

Figure 2b, the φox/φred of SM1 and SM2 were 0.88 V/−1.54 V and 0.68 V/−1.66 V vs Ag/AgCl, respectively. The ELUMO/ EHOMO of SM1 and SM2 were calculated to be −2.82 eV/−5.24 eV and −2.70 eV/−5.04 eV, respectively, according to the equations mentioned above. In comparison with SM2, the EHOMO of SM1 with cyano group substitution is down-shifted by 0.20 eV, which is beneficial for higher Voc of the SM-OSCs with SM1 as donor. We also performed theoretical calculation on the two molecules by DFT at the B3LYP/6-31G(d, p) level, for understanding the effect of the cyano group on the electronic energy levels of the molecules. The results are shown in Figure 2c and Figure S7 in the Supporting Information. The calculated LUMO/HOMO energy levels of SM1 and SM2 are −2.80/−4.98 eV and −2.45/−4.84 eV, respectively. The calculated HOMO energy level of SM1 is down-shifted by 0.14 eV in comparison with that of SM2, which agrees with the results measured by the electrochemical method. It should be mentioned that, in compariosn with SM2, the dowshift of the DFT-calculated LUMO level of SM1 is 0.35 eV, which is ca. 0.2 eV larger than the downshift of the HOMO level and agrees with the reduction of Eg of SM1 with the CN substituents. However, the downshift of the LUMO level of SM1 measured by cyclic voltammetry is only 0.12 eV which is smaller than that of the HOMO level downshift. The relatively higher LUMO level of SM1 measured by cyclic voltammetry could be due to the higher charge transfer energy barrier for the reduction of SM1 with the bigger end group on the electrode, which makes the onset reduction potential of SM1 moved to a more negative value. For high performance OSCs, matching of the EHOMO and ELUMO of the donor and acceptor is very important. Generally, the EHOMO and ELUMO of the donor should be more higherlying than that of the acceptor. Besides, the energy difference (ΔELUMO and ΔEHOMO) between the donor and the acceptor should be larger than the binding energy of excitons for efficient exciton dissociation. The molecular energy level alignments in the two types of blends are presented in Figure 1c. For the SM1:IDIC system, ΔELUMO is 1.09 eV which is large enough for the exciton dissociation of the donor SM1 and the electron transfer from SM1 to IDIC. The ΔEHOMO between the SM1 and the IDIC acceptor is 0.45 eV, so the hole transfer from IDIC to SM1 should also be highly efficient, which is confirmed from the photoluminescence (PL) quenching measurements, as shown in Figure S8 in the Supporting Information. For the blend of SM2/IDIC, the ΔEHOMO and ΔEHOMO are 1.21 and 0.65 eV, respectively, indicating sufficient driving force for exciton dissociation. Photovoltaic Properties. SM-OSCs were fabricated with a conventional device structure of ITO/PEDOT:PSS/p-OS:IDIC/PDINO/Al and characterized to investigate the photovoltaic properties of the p-OS SM1 or SM2. It should be noted that, for both SM1/IDIC and SM2/IDIC, the optimal donor to acceptor weight ratio (D/A, w/w) with thermal annealing (TA) treatment was different from that without thermal annealing treatment, as shown in Tables S1−S3 in the Supporting Information. Finally, the photovoltaic performance of the device with the optimal D/A ratio (2:1) was further improved

SM1 and SM2 are 4.36° and 4.92°, corresponding to their lamellar stacking distances of 20.24 and 17.94 Å, respectively. The larger lamellar stacking distance of SM1 could be ascribed to its larger end group. Hole mobilities of pure SM1 and SM2 were measured by SCLC and FET methods. The SCLC hole mobilities of pure SM1 and SM2 films are calculated to be 5.39 × 10−5 cm2 V−1 s−1 and 4.77 × 10−5 cm2 V−1 s−1, respectively (see Figure S3 in the Supporting Information). The FET hole mobilities of pure SM1 and SM2 films are 3.50 × 10−2 cm2 V−1 s−1 and 9.92 × 10−3 cm2 V−1 s−1, respectively (see Figure S4 in the Supporting Information). These results indicate that SM1 with the CN substituents possesses slightly higher hole mobility. Absorption Spectra and Electronic Energy Levels. The UV−vis absorption spectra of the p-OS donors (SM1 and SM2) were measured in dilute CHCl3 solution and thin film spun casted on quartz plate, as shown in Figure S5 in the Supporting Information, Figure 2a, and Table 1. In solution, SM1 presents an absorption band with a molecular absorption coefficient ε1 of 8.78 × 104 M−1 cm−1 at peak wavelength λmax of 500 nm, as shown in Figure S5 in the Supporting Information. In contrast, SM2 shows a shorter λmax at 466 nm with a slightly lower absorption coefficient ε2 of 7.64 × 104 M−1 cm−1. SM1 film exhibits the maximum absorption peak at 566 nm with a red shift of 66 nm relative to its solution. Similarly, the maximum absorption peak of SM2 film redshifted by 55 nm relative to its solution. The absorption coefficients of pure SM1 and SM2 films are 9.45 × 104 cm−1 and 7.95 × 104 cm−1, respectively (see Figure S5 in the Supporting Information), confirming the stronger absorption coefficient of SM1 with the CN substituents. The optical bandgaps (Egopt) of SM1 and SM2 films were calculated to be 1.88 and 2.02 eV, respectively, estimated from the absorption edge of their films. The results of absorption spectra indicate that attaching the cyano substituent on the end-capping group could lead to an increase of absorption coefficient and a red shift of the absorption band. We also measured the absorption spectra of the blend films of the p-OS and the n-OS IDIC, as shown in Figure S6 in the Supporting Information. The blend film of SM1 and IDIC provides a better complementary absorption range from 400 to 750 nm with and without thermal annealing, in comparison with that of the blend film of SM2 and IDIC. This strong and broad absorption of the blend film of SM1 and IDIC will benefit the Jsc enhancement of the OSCs based on SM1:IDIC. To estimate the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the two small molecules, we performed cyclic voltammetry (CV) measurements of the molecules. The HOMO/LUMO energy levels (EHOMO/ELUMO) can be calculated from the onset oxidation/reduction potentials (φox/φred) in the cyclic voltammograms according to the equations of EHOMO/ELUMO = −e(φox/φred + 4.8 − φFc/Fc+) (eV).23 Here, φFc/Fc+ was measured to be 0.44 V vs Ag/AgCl in this measurement system, and then the calculation equations are expressed as EHOMO/ELUMO = −e(φox/φred + 4.36) (eV). From the cyclic voltammograms of SM1 and SM2 shown in 7546

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Figure 3. (a) J−V characteristics of the optimized SM-OSCs based on SM1 (or SM2)/IDIC with D/A weight ratio of 2:1 and with thermal annealing at 115 °C for 10 min for the SM1-based device and at 110 °C for 10 min for the SM2-based device; (b) IPCE spectra of the corresponding devices; (c) light intensity dependence of Jsc of the devices; (d) light intensity dependence of Voc of the devices.

Table 2. Photovoltaic Performance of the Optimized SM-OSCs Based on SM1:IDIC and SM2:IDIC

a

active layer

Voc (V)

Jsc (mA cm−2)

FF (%)

PCE (%)

Rs (Ω cm2)

Rsh (kΩ cm2)

SM1:IDIC SM2:IDIC

0.905 (0.906 ± 0.004)a 0.768 (0.768 ± 0.002)

15.18 (15.16 ± 0.17) 10.77 (10.79 ± 0.17)

73.55 (71.72 ± 0.85) 64.40 (62.84 ± 0.81)

10.11 (9.85 ± 0.13) 5.32 (5.21 ± 0.12)

7.41 (8.15 ± 0.69) 11.79 (12.97 ± 2.40)

1.33 (1.41 ± 0.24) 0.58 (0.57 ± 0.12)

The values in parentheses are average values from 20 devices.

average values from 20 devices were listed in Table 2 for a clear comparison. Compared to SM2, the p-OS SM1 with cyano substitution shows higher average Voc of 0.906 V for the SMOSCs with the molecule as donor, which is consistent with the lower-lying HOMO of SM1. The photovoltaic performance of the large area (1 cm2) OSCs based on SM1:IDIC (2:1) was also investigated checking the possible application of the OSCs. Figure S9 in the Supporting Information shows the J−V curve of the 1 cm2 OSCs with thermal annealing treatment (115 °C for 10 min). The OSC demonstrated a PCE of 7.67% with a Voc of 0.903 V, a Jsc of 14.95 mA cm−2, and a FF of 56.81%. The lower FF value could be due to the higher resistance of the ITO electrode, and it could be improved by using a low resistance transparent electrode. The results indicate that SM1 is promising for future application in fabricating large area devices. The input photon to converted current efficiency (IPCE) spectra of the optimized devices are shown in Figure 3b. Both of them demonstrate broad photoresponse from 300 to 800 nm, which indicates that both the small molecule donors and the IDIC acceptor make contributions to the photocurrent. For the SM1-based device, the IPCE spectra exhibit a broad plateau higher than 60% at approximately 475−725 nm, and the IPCE spectra in the wavelength range of 660−800 nm are corresponding to the exciton dissociation of the IDIC acceptor with the hole transfer from the HOMO of IDIC to that of SM1. For the SM2-based devices, the IPCE curve presents a broad

by adjusting thermal annealing temperature for the OSCs. Table S3 in the Supporting Information displays the photovoltaic parameters of the OSCs based on SM1:IDIC (2:1, w/w) and SM2:IDIC (2:1, w/w) as cast or with thermal annealing treatment at different temperatures under the illumination of AM 1.5G, 100 mW cm−2. The as-cast device of SM1:IDIC exhibited a PCE of 5.45%, while it was distinctly enhanced to 8.48% after thermal annealing at 100 °C for 10 min. After being treated at 115 °C for 10 min, the PCE was further improved to 10.11%, with an open-circuit voltage (Voc) of 0.905 V, a shortcircuit current density (Jsc) of 15.18 mA cm−2, and a high fill factor (FF) of 73.55%. Up to now, the PCE of 10.11% and FF of 73.55% are the highest values for the nonfullerene SM-OSCs. Compared to the as-cast devices, the enhanced PCE of the devices with thermal annealing treatment is largely a result of the increased Jsc and FF. When the devices were treated at 120 °C for 10 min, the values of Jsc and FF were slightly reduced. For the as-cast OSCs based on SM2:IDIC, a PCE below 1% was obtained with rather low Jsc and FF. After treated at 110 °C for 10 min, the PCE was improved to 5.32% with a Voc of 0.768 V, a Jsc of 10.77 mA cm−2, and a FF of 0.64. Figure 3a shows the J−V characteristics of the optimized SM-OSCs based on SM1 (or SM2):IDIC with D/A weight ratio of 2:1 and with thermal annealing at 115 °C for 10 min for the SM1-based device and at 110 °C for 10 min for the SM2-based device under the illumination of AM 1.5G, 100 mW cm −2 , and the corresponding photovoltaic performance data including the 7547

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respectively, with the improved μh/μe ratio of 0.97. The better photovoltaic performance of the SM-OSCs after thermal annealing mentioned above could be ascribed to the higher and more balanced charge carrier mobilities of the annealed film. For the SM2:IDIC blend film, the μh and μe values are 0.37 × 10−4 cm2 V−1 s−1 and 1.05 × 10−4 cm2 V−1 s−1, respectively, with the μh/μe ratio of 0.35 for the film without thermal annealing. The unbalanced charge mobility could be one reason for the relatively poor performance of the as-cast SM2-based device, while after thermal annealing treatment, the μh and μe values were improved to 1.47 × 10−4 cm2 V−1 s−1 and 1.56 × 10−4 cm2 V−1 s−1, respectively, with the enhanced μh/μe ratio of 0.94, which should also be responsible for the improvement of its photovoltaic performance of the SM2based device with thermal annealing. In addition to carrier mobilitiy, charge recombination behavior is another critical factor that could have great influence on the Jsc and fill factor as well as device performance of SM-OSCs.45,46 Here, we measured the Jsc versus light intensity (Plight) curves to investigate the charge recombination behavior, as shown in Figure 3c. In general, the relationship between Jsc and Plight could be described as Jsc ∝ Plightα.47 For the devices based on SM1:IDIC and SM2:IDIC, the exponents α are calculated to be 0.979 and 0.953, respectively. The higher α value for the SM1-based device indicates very weak bimolecular recombiantion at the short circuit condition for the cyano group substituted p-OS SM1-based device. In addition, we further investigated the charge recombination properties of the SM-OSCs through measuring the dependence of Voc on Plight curves. The relationship between Voc and Plight could be described as Voc ∝ ln Plight, and the slope of the fitting straight line of Voc vs ln(Plight) should be kT/q (where q is the elementary charge, k is the Boltzmann constant, and T is the Kelvin temperature) if bimolecular recombination is the exclusive recombination form.48 As shown in Figure 3d, the slope of the fitted line of Voc vs ln Plight are 1.014kT/q for the SM1-based device and 1.386kT/q for the SM2-based devices. The slope close to kT/q for the SM1-device indicates lower monomolecular recombination at the open circuit condition. The results of less bimolecular and monomolecular charge recombinations agree well with the higher FF and higher PCE values for the SM-OSCs based on SM1:IDIC. Morphological Characterization. In order to further understand the role of the thermal annealing treatment on enhancing the photovoltaic performance, the morphology of the SM1:IDIC blend film with or without thermal annealing treatment is investigated and compared through photoinduced force microscopy (PiFM),49−51 as shown in Figure 4. The PiFM, which can measure the sample polarizability by detecting the force gradient between the interaction of the optically driven molecular dipole and its mirror image dipole in a metalcoated atomic force microscopy (AFM) tip, is a relatively new technique that combines the high spatial resolution of the AFM in the sub-10 nm range with the analytical capability provided by optical spectroscopy.48 PiFM has been used to demonstrate chemical mapping on block copolymers with full pitch of 40 nm, clearly demonstrating ∼10 nm spatial resolution in identifying different molecules based on their absorption peaks.49 Compared to the normal AFM phase image, the PiFM could present better resolution and point out which phase regions belong to the donor or acceptor more clearly. As shown in Figure S12 in the Supporting Information, PiFM spectra for the two samples (as-cast and thermal-annealed)

plateau higher than 40% at approximately 450−735 nm, which is relatively lower than that of the SM1-based device, with a concave in the range of 565−635 nm causing by weak absorption of blend SM2:IDIC, leading to a much lower Jsc than that of the SM1-based device. The Jsc values of the OSCs based on SM1 and SM2 calculated from integration of the EQE spectra with the AM 1.5G reference spectrum are 14.814 mA cm−2 and 10.525 mA cm−2, respectively, which are in good agreement with the Jsc values measured from J−V curves (with ∼2.5% mismatch). The exciton dissociation and charge transfer in the pOS:IDIC blends were measured by PL quenching experiments. Figure S8 in the Supporting Information displays the PL spectra of the p-OS (SM1 excited at 550 nm and SM2 excited at 520 nm) and the n-OS IDIC (excited at 710 nm) films as well as the blend films of SM1:IDIC (2:1, w/w) (excited at 550 and 710 nm) and SM2:IDIC (2:1, w/w) (excited at 520 and 710 nm). For the blend of SM1:IDIC with or without thermal annealing treatment, the PL emission of SM1 is completely quenched by IDIC, indicating efficient electron transfer from SM1 to IDIC. The broad PL emission peak of IDIC in the range of 720−860 nm is also completely quenched by SM1. Similar results were obtained for the blend of SM2:IDIC as cast. However, for the blend of SM2:IDIC with thermal annealing, the PL emission of SM2 is stronger than that without thermal annealing, while the PL emission of IDIC is well quenched by SM2. This phenomenon should be ascribed to the enhanced aggregation of SM2 in the blend of SM2:IDIC after thermal annealing treatment, as the absorption coefficient of SM2 in the blend of SM2:IDIC was obviously enhanced after thermal annealing treatment (see Figure S6 in the Supporting Information). To better understand the effect of thermal annealing on the photovoltaic performance of the SM-OSCs, the hole and electron mobilities (μh and μe) of the p-OS:IDIC blend films with or without thermal annealing treatment were measured by the space charge limited current (SCLC) method as shown in Figures S10 and S11 in the Supporting Information, and the corresponding data are summarized in Table 3. The hole Table 3. Hole and Electron Mobilities of SM1:IDIC and SM2:IDIC Blend Films with or without Thermal Annealing Treatment active layer

treatment

SM1:IDIC

as cast

SM2:IDIC

115 °C for 10 min as cast 110 °C for 10 min

hole mobility (μh) (cm2 V−1 s−1) 10−4 −1 −1

0.61 × cm2 V s 1.60 × 10−4 cm2 V−1 s−1 0.37 × 10−4 cm2 V−1 s−1 1.47 × 10−4 cm2 V−1 s−1

electron mobility (μe) (cm2 V−1 s−1) 10−4 −1 −1

1.07 × cm2 V s 1.65 × 10−4 cm2 V−1 s−1 1.05 × 10−4 cm2 V−1 s−1 1.56 × 10−4 cm2 V−1 s−1

μh/μe 0.57 0.97 0.35 0.94

mobilities were measured with a hole-only device structure, ITO/PEDOT:PSS/p-OS:IDIC/Au, and the electron mobilities were measured with an electron-only device structure, ITO/ ZnO/p-OS:IDIC/PDINO/Al. For the as-cast film of SM1:IDIC, the hole (μh) and electron (μe) mobilities of SM1 are calculated to be 0.61 × 10−4 cm2 V−1 s−1 and 1.07 × 10−4 cm2 V−1 s−1, respectively, with μh/μe ratio of 0.57, while after thermal annealing treatment μh and μe values were increased to 1.60 × 10−4 cm2 V−1 s−1 and 1.65 × 10−4 cm2 V−1 s−1, 7548

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Figure 4. AFM topography images of SM1:IDIC film: (a) as cast, (e) thermal annealing. PiFM images of as cast SM1:IDIC film based on IR absorption at different wave numbers: (b) based on peaks associated with SM1 donor at 1717 cm−1 and (c) based on peaks associated with IDIC acceptor at 1701 cm−1; (d) images (b) and (c) combined to provide chemical map of SM1 and IDIC molecules. In situ test PiFM images of thermal annealed SM1:IDIC film at different wave numbers of IR absorption: (f) based on peaks associated with SM1 donor 1717 cm−1 and (g) based on peaks associated with IDIC acceptor at 1701 cm−1; (h) images (f) and (g) combined to provide chemical map of SM1 and IDIC molecules.

Figure 5. Hole transfer dynamics of the SM-OSCs based on SM1:IDIC: (a) Transient absorption spectra of the annealed film of blend SM1:IDIC sample in comparison with those in the neat films of SM1 and IDIC at 5 ps with pump at 710 nm. (b) Temporal evolution of signal probed at 570 and 730 nm in the blend film of SM1:IDIC and the signal probed at 730 nm in the neat film of IDIC, respectively. (c) Schematic diagram of the hole transfer in the blend of SM1:IDIC.

without distinct phase separation, indicating that there is no interpenetrating networks formed for charge transportation in the active layer, which results in relatively low Jsc. However, after thermal annealing treatment, the image in Figure 4f (based on peaks associated with SM1 donor) clearly shows SM1 rich domains, which formed a continuous network throughout the film. Similarly, the image in Figure 4g (based on peaks associated with IDIC acceptor) shows the continuous network of the IDIC rich domain, clearly demonstrating the SM1 domains with respect to the IDIC domains. As shown in Figure 4h, the combination of the PiFM image at 1717 and 1701 cm−1 demonstrates clearly that ∼20 nm phase separation was formed for both SM1 and the IDIC phase. Compared to the as-cast film, the enhanced SM1 and IDIC domains indicate the improved charge transfer ability, which is consistent with the mobility and J−V measurement. These results demonstrate that thermal annealing treatment can tune the morphology effectively, and the enhanced photovoltaic properties of the

clearly show the sum of various peaks associated with the Fourier transform infrared spectroscopy (FTIR) of SM1 and IDIC which was measured in order to determine the IR absorption peak wavenumber of the donor and acceptor molecules for the PiFM measurement. There are strong absorption peaks at 1717 cm−1 for SM1 and at 1701 cm−1 for IDIC causing by the different functional groups in the two molecules (SM1 and IDIC). Therefore, in order to image the SM1 (or IDIC) component, we tuned the excitation laser to one of its absorption peaks at 1717 cm−1 (or 1701 cm−1). As shown in Figure 4b, when PiFM selectively imaged the as cast SM1:IDIC film at 1717 cm−1, the red signal represented the distribution of the SM1 phase, which distributes relatively evenly over the whole film. Similarly, as shown in Figure 4c, when selectively imaged at 1701 cm−1, the even-distributed IDIC phase was also obtained. Figure 4d displays the combination of the PiFM image at 1717 and 1701 cm−1. Both SM1 and IDIC distributed evenly over the whole image 7549

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Chemistry of Materials

Figure 6. Temporal dynamic of photoexcited bleaching signals at featured probe wavelengths for the blend films of SM1:IDIC prior to (a) and post(b) thermal annealing, and for the blend films of SM2:IDIC prior to (c) and post- (d) thermal annealing. The pump wavelength is 480 nm.

transfer is estimated to be ∼ 6 ps with the TA spectra measured with the pump at 480 nm (Figure S14 in the Supporting Information). Besides, Figure 5b shows the different dynamics between the neat IDIC and the SM1:IDIC blend. For the early stage, the relaxation rate of the blend film becomes dramatically faster than that of neat IDIC, indicating the presence of an additional relaxation channel of hole transfer in the SM1:IDIC blend. Moreover, it is worth noting that the GSB signals at 570, 630, and 730 nm in the SM1:IDIC blend persist to the nanosecond scale, which is much longer than those in a neat sample of SM1 and IDIC (Figure S13c in the Supporting Information). These results suggest the existence of the longlived dissociated excitons in the blend, which is beneficial for the carrier generation. Thermal annealing treatment dramatically boosts photovoltaic efficiency in devices made of blend films of SM1:IDIC and SM2:IDIC. To gain more insight about the underlying mechanism, we conduct a comparison study on the blend films prior to and post-thermal annealing with TA spectroscopy. The kinetics of GSB signals in resonance with the absorption peaks of donors and acceptors are compared in Figure 6. The early stage kinetic differences suggest a different role played by the charge transfer dynamics in these samples, which is possibly relevant to different morphologic heterogeneities. In annealed samples with larger crystalline grains, the intrinsic electron− hole recombination in single domains plays a more important role with early stage dynamics comparable to that in neat films of SM1 and IDIC (Figure S15 in the Supporting Information). In contrast, the interfacial recombination plays a more important role in untreated samples as evidenced by similar decay dynamics in both donor and acceptor. The similarity persists to a much longer time scale with nearly identical decay behaviors (lifetimes of ∼1100 ps) in the SM1:IDIC blend film, which can be understood if the geminate recombination at the D/A interface is efficient. Nevertheless, the recombination becomes much slower in the annealed samples at long time scale (∼2600 ps), suggesting that the geminate recombination

devices with thermal annealing treatment should be ascribed to the larger phase domains and the more continuous donor/ acceptor nanoscaled phase-separated interpenetrating networks. The results indicate that PiFM is an effective method to analyze phase separation involved in the field of OSCs. Photophysics. To further understand the charge transfer dynamics in the blend film of p-OS:IDIC, we performed transient absorption spectroscopy measurement.23,38 The primary absorption peaks for SM1, SM2, and IDIC are well separated in the spectral domain (Figure 2a), so we can extract the spectral and temporal characteristics of hole transfer dynamics with selected excitation. The pump wavelengths were selected to be 480 and 710 nm for selective excitation of SM1 and IDIC, respectively. The excitation density is kept in a weak regime ( 12%. Nat. Photonics 2016, 11, 85−90. (12) An, Q.; Zhang, F.; Zhang, J.; Tang, W.; Deng, Z.; Hu, B. Versatile Ternary Organic Solar Cells: A Critical Review. Energy Environ. Sci. 2016, 9, 281−322. (13) Nielsen, C. B.; Holliday, S.; Chen, H. Y.; Cryer, S. J.; McCulloch, I. Non-Fullerene Electron Acceptors for Use in Organic Solar Cells. Acc. Chem. Res. 2015, 48, 2803−2812. (14) Cheng, P.; Zhan, X. Stability of Organic Solar Cells: Challenges and Strategies. Chem. Soc. Rev. 2016, 45, 2544−2582. (15) Lin, Y.; Zhan, X. Designing Efficient Non-Fullerene Acceptors by Tailoring Extended Fused-Rings with Electron-Deficient Groups. Adv. Energy Mater. 2015, 5, 1501063. (16) Meng, D.; Fu, H.; Xiao, C.; Meng, X.; Winands, T.; Ma, W.; Wei, W.; Fan, B.; Huo, L.; Doltsinis, N. L.; Li, Y.; Sun, Y.; Wang, Z. Three-Bladed Rylene Propellers with Three-Dimensional Network

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y. Li). 7551

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