Angular-Shaped Dithienonaphthalene-Based Nonfullerene Acceptor

Oct 26, 2017 - Abstract: We synthesized two wide bandgap A–D–A structured p-type organic semiconductor (p-OS) small molecules with weak electron-w...
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Article Cite This: Chem. Mater. 2017, 29, 9775-9785

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Angular-Shaped Dithienonaphthalene-Based Nonfullerene Acceptor for High-Performance Polymer Solar Cells with Large Open-Circuit Voltages and Minimal Energy Losses Yunlong Ma,†,‡ Meiqi Zhang,†,‡ Yabing Tang,§ Wei Ma,§ and Qingdong Zheng*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao West Road, Fuzhou, Fujian 350002, China ‡ University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, China § State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China S Supporting Information *

ABSTRACT: The utilization of low bandgap copolymers has been considered as one of the most efficient ways to increase power conversion efficiencies (PCEs) of fullerene-based polymer solar cells (PSCs). However, an increase in the short-circuit current (JSC) value is usually counteracted by a decrease in the open-circuit voltage (VOC), which limits a further PCE enhancement of fullerene-based PSCs. As a result, nonfullerene acceptors with wide-range tunable energy levels are used as alternatives to the traditional fullerene acceptors to overcome the negative tradeoff between the JSC and VOC. Here, a novel nonfullerene acceptor is developed by using an angular-shaped dithienonaphthalene flanked by electron-withdrawing 3-ethylrhodanine units via benzothiadiazole bridges. The obtained nonfullerene acceptor exhibits a high-lying lowest unoccupied molecular orbital level of −3.75 eV with enhanced absorption. In combination with a benchmark low bandgap copolymer (PTB7-Th), a high PCE of 9.51% with a large VOC of 1.08 V was achieved for the nonfullerene PSCs, demonstrating an extremely low energy loss of 0.50 eV, which is the lowest among all high-performance (PCE > 8%) polymer-based systems with similar optical bandgaps. The results demonstrate the bright future of our nonfullerene acceptor as an alternative to the fullerene derivatives for PSCs with large JSC and VOC values and improved device stability.

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INTRODUCTION Organic solar cells (OSCs), which can be large-area solutionprocessed, have attracted increasing attention in the past decade due to their unique features of lightweight, good mechanical flexibility, and potentially low cost.1−6 The power conversion efficiencies (PCEs) of OSCs have increased rapidly due to the synergetic development in photoactive materials (both n-type and p-type), interfacial materials, and various morphology engineering and device engineering.7−14 Among them, the photoactive materials play the most significant role because they not only govern the highest attainable shortcircuit current (JSC), but also determine the largest achievable open-circuit voltage (VOC). Although the use of low bandgap copolymers has been proven successful to increase PCEs of fullerene-based polymer solar cells (PSCs), an increase in the JSC is usually concomitant with a decrease in the VOC, which prevents a further PCE enhancement of the resulting fullerenebased PSCs.15,16 It is known that the VOC of OSCs is related to the energy level difference between the highest occupied molecular orbital (HOMO, i.e., ionization potential) energy level of a donor and the lowest unoccupied molecular orbital (LUMO, i.e., electron affinity) energy level of an acceptor.17 As an important © 2017 American Chemical Society

parameter of solar cells, the energy loss (Eloss) is defined as Eloss = Eg − eVOC, where Eg is the optical bandgap of the organic semiconductor having the smaller bandgap.18,19 For the material with a given bandgap, a smaller Eloss indicates a higher VOC of the resulting OSC. By minimizing the Eloss, the negative trade-off between JSC and VOC can be overcome thereby boosting PCEs of OSCs further. For the best crystalline inorganic solar cells, low Eloss values of 0.34−0.48 eV can be realized with ease.20 However, it remains a challenge to achieve OSCs with a comparable low Eloss to those of inorganic solar cells. For example, for high-performance (PCE > 8%) bulkheterojunction (BHJ) OSCs, Eloss < 0.6 eV is seldom reported considering the suggested minimum Eloss of 0.6 eV.21−24 Unlike the traditional fullerene derivatives with relatively fixed energy levels, recently emergent nonfullerene acceptors exhibit wide-range tunable energy levels, which may lead to decreased Eloss of the resulting OSCs.25−31 For efficient nonfullerene acceptors, an excellent molecular design strategy is the A−D−A (acceptor−donor−acceptor) configuration by Received: September 5, 2017 Revised: October 26, 2017 Published: October 26, 2017 9775

DOI: 10.1021/acs.chemmater.7b03770 Chem. Mater. 2017, 29, 9775−9785

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Chemistry of Materials Scheme 1. Synthesis of the Nonfullerene Acceptor DTNR

parts, which show a PCE of 8.94% with a small VOC of 0.79 V and a large Eloss of 0.79 eV. We also investigated on the effects of solvent additive on the morphology, photovoltaic properties, charge generation, transport, and recombination dynamics. It should be noted that the PTB7-Th:DTNR system still shows efficient charge generation and separation despite the low LUMO energy offset of 0.13 eV, which is much smaller than the empirical threshold value of 0.3 eV. Our results open a new avenue for solving the negative trade-off between the JSC and VOC of OSCs by using a novel nonfullerene acceptor with proper energy levels. The high PCE with a low energy loss achieved in this work will have a fundamental impact on the development of OSCs in the long run.

which the HOMO and LUMO energy levels of the target molecules can be varied separately by using different donor cores and different acceptor terminals, respectively.32−37 To obtain a low Eloss, acceptors with high-lying LUMO energy levels are preferred, and they can be synthesized by attaching weak electron withdrawing groups at the terminals. In recent years, indacenodithiophene (IDT) has been frequently used as the donor core for efficient nonfullerene acceptors.38,39 However, as an excellent aromatic fused-ring system, laddertype angular-shaped dithienonaphthalene (DTN) has seldom been used for constructing nonfullerene acceptors, although it has a more extended π-conjugation system in comparison with IDT. Moreover, studies have shown that the fused aromatic moieties with angular shapes can further tune the frontier molecular orbital energy levels, leading to higher-lying LUMO energy levels than their linear-shaped counterparts.40 Very recently, using dithienonaphthalene as the donor unit, we reported an A−D−A-type nonfullerene acceptor (DTNIC8), which exhibited a decent PCE of 9.03% with a VOC of 0.96 V and an Eloss of 0.77 eV.41 The much larger Eloss than those of inorganic solar cells is mainly attributed to the deep-lying LUMO energy level of the nonfullerene acceptor (DTNIC8) caused by the strong electron-withdrawing ability of the 2-(3oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (INCN) units. To further improve the PCE of OSCs, the Eloss should be reduced to a minimal value. In this context, a new nonfullerene acceptor DTNR is designed and synthesized by using the ladder-type angularshaped DTN core flanked by two 3-ethylrhodanine units via two benzothiadiazole bridges, for the first time. Here, the combination of benzothiadiazole and rhodanine, namely 5(benzo[c][1,2,5]thiadiazol-4-ylmethylene)-3-ethyl-2-thioxothiazolidin-4-one (BR), was used as the electron-deficient group, which possesses a weaker electron-withdrawing ability compared to INCN.32,38 In comparison with DTNIC8, the nonfullerene acceptor based on BR should exhibit a higherlying LUMO energy level, which is in favor of a higher VOC and a decreased Eloss for the corresponding PSC. By incorporating a benchmark low bandgap copolymer (PTB7-Th), BHJ PSCs with an inverted device configuration are fabricated. With the high-lying LUMO energy level of the target acceptor, a high PCE of 9.51% with a large VOC of 1.08 V was achieved, demonstrating an extremely low Eloss of 0.50 eV, which is the lowest value achieved for high-performance (PCE > 8%) OSCs with similar optical bandgaps. In addition, the PTB7Th:DTNR-based PSCs exhibit a higher PCE and better shelf life stability compared to the PTB7-Th:PC71BM-based counter-

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RESULTS AND DISCUSSION

Synthesis and Characterization. The chemical structure and the corresponding synthetic route for DTNR are shown in Scheme 1. The detailed synthetic procedures are described in the Experimental Section. The key compound 1 was synthesized according to a previously established method by us.42 Bis-stannylated derivative 2 was obtained in 89% yield by lithiation of compound 1 using n-butyllithium followed by a treatment with trimethyltin chloride. A Stille coupling reaction between compound 2 and 7-bromobenzo[c][1,2,5]thiadiazole4-carbaldehyde using Pd(PPh3)4 as the catalyst rendered compound 3 in 77% yield. Finally, the target small molecule DTNR was synthesized in 90% yield by a triethylaminecatalyzed Knoevenagel condensation of compound 3 with 3ethylrhodanine. The chemical structures of all new compounds were characterized by 1H NMR, high resolution mass spectrometry (HRMS), and elemental analysis. DTNR is readily soluble in common organic solvents such as chloroform, dichloromethane, and chlorobenzene at room temperature, which is important for material purification and device fabrication. Optical and Electrochemical Properties. UV−vis absorption spectra of DTNR and PC71BM in chlorobenzene solutions (4 × 10−6 M) are shown in Figure 1a. DTNR shows a broad absorption band in the range of 300−700 nm with two intense peaks located at 400 and 618 nm, which can be ascribed to the localized π−π* transition and intramolecular charge transfer transition, respectively. The maximum molar extinction coefficient of DTNR in chlorobenzene is determined to be 8.8 × 104 M−1 cm−1 at 618 nm. In contrast, PC71BM exhibits relatively weak absorption in the visible region with a smaller maximum extinction coefficient of 2.0 × 104 M−1 cm−1 at 462 nm. The absorption data of DTNR and PC71BM are 9776

DOI: 10.1021/acs.chemmater.7b03770 Chem. Mater. 2017, 29, 9775−9785

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Chemistry of Materials

energy level is 4.80 eV below vacuum. Cyclic voltammograms of DTNR and PC71BM are shown in Figure 2a, and the

Figure 1. (a) UV−vis absorption spectra of DTNR and PC71BM in chlorobenzene. (b) Normalized UV−vis absorption spectra of DTNR, PTB7-Th, and PTB7-Th:DTNR blend films.

summarized in Table 1. These absorption properties demonstrate that DTNR has an appreciable advantage over the traditional PC71BM regarding the solar light harvesting in the visible region. Figure 1b shows the normalized UV−vis absorption spectra of pure DTNR, PTB7-Th, and their blend in cast films. Compared with its solution absorption spectrum, the absorption spectrum of DTNR in thin film exhibited a 32 nm bathochromic shift and a clear shoulder peak, indicating the possible formation of more ordered molecular structure in the solid state. The absorption onset of DTNR film is located at ∼722 nm, which corresponds to a medium optical bandgap of 1.72 eV. This should afford a relatively complementary absorption with the benchmark low bandgap polymer PTB7Th. As shown in Figure 1b, the blend film of PTB7-Th with DTNR provides a rather broad absorption in the range from 300−800 nm, which is beneficial for an enhanced light absorption. Matched energy levels between the donor and acceptor materials are important to achieve high-performance solar cells. Here, cyclic voltammetry (CV) was used to determine the energy levels of DTNR, PC71BM, and PTB7-Th. The HOMO and LUMO energy levels are estimated from their oxidation and reduction potentials in the cyclic voltammograms, and the measurement was calibrated by ferrocene, whose absolute

Figure 2. (a) Cyclic voltammograms of DTNR, PC71BM, and PTB7Th films (vs Ag/AgNO3). (b) Energy level diagram of DTNR, PC71BM, and PTB7-Th.

corresponding results are summarized in Table 1. The oxidation and the reduction onset potentials of DTNR are 0.85 eV and −1.07 eV, respectively, corresponding to a HOMO level of −5.67 eV and a LUMO level of −3.75 eV. As expected, the LUMO level of DTNR is higher than that of PC71BM (−3.94 eV). This is beneficial for achieving high VOCs in the corresponding OSCs. The HOMO and LUMO levels of PTB7Th were measured to be −5.26 and −3.62 eV, respectively, coinciding with the reported values.27,43 Figure 2b displays the energy level diagram of DTNR, PC71BM, and PTB7-Th. The HOMO energy offset between PTB7-Th and DTNR is 0.41 eV, which should be sufficient for efficient hole transfer from DTNR to PTB7-Th. While the LUMO energy offset (ΔEL) between PTB7-Th and DTNR is only 0.13 eV, it is smaller than the generally accepted empirical threshold value of 0.3 eV.17 In addition to the CV measurements, ultraviolet photoelectron spectroscopy (UPS) was also used to estimate the energy levels of these molecules, and the results are shown in Figure S1 (Supporting Information). Although there are differences

Table 1. Optical and Electrochemical Properties of DTNR and PC71BM molecules

ε [104 M−1 cm−1]

λmaxsolution [nm]

λmaxfilm [nm]

Egopt [eV]a

HOMO [eV]b

LUMO [eV]b

DTNR PC71BMc

8.8 2.7 (2.0)

618 373 (462)

650 479

1.72 1.68

−5.67 −5.94

−3.75 −3.94

Estimated from the onset of the absorption spectrum of thin film. bEnergy levels evaluated by CV. cParameters for the shoulder peak are shown in parentheses.

a

9777

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Chemistry of Materials between the absolute energy levels determined from the CV and UPS measurements, the energy change trends are similar. The ΔEL between PTB7-Th and DTNR obtained from UPS was calculated to be 0.18 eV, which is close to that obtained from the CV measurement (0.13 eV). Theoretically, to achieve OSCs with both high VOC and large JSC, it is desirable to minimize the LUMO energy offset between the donor and the acceptor while still allowing for efficient electron transfer from the donor to the acceptor. In this work, the electron transfer from PTB7-Th to DTNR is efficient despite the small LUMO energy offset of 0.13 eV, as evidenced by the photoluminescence quenching experiments and the external quantum efficiency measurements discussed below. Consequently, we would expect that the PTB7-Th:DTNR-based PSCs can achieve both high JSC and large VOC values. Photoluminescence. Photoluminescence (PL) quenching experiment was carried out to investigate the exciton dissociation and charge transfer behavior in the blend film. Figure S2 (Supporting Information) depicts the PL spectra of PTB7-Th, DTNR and their blend films. According to the absorption features of DTNR and PTB7-Th in thin film, pump wavelengths of 410 and 720 nm were selected to excite DTNR and PTB7-Th, respectively, in either the pure or blend films. It can be seen that with an excitation at 410 nm, the PL intensity of DTNR decreases significantly compared to that of the PTB7Th:DTNR blend, suggesting the highly efficient hole transfer from DTNR to PTB7-Th. More importantly, with an excitation at 720 nm, the PL emission of PTB7-Th was also largely quenched by adding DTNR, which is mainly attributed to the efficient electron transfer from PTB7-Th to DTNR, although the LUMO offset between the donor and acceptor is only 0.13 eV. These results demonstrate that an energy offset of over 0.3 eV might not be an essential requirement for efficient exciton separation and charge transfer in nonfullerene PSCs. The efficient exciton dissociation and charge transfer in PTB7Th:DTNR blend is in favor of effective photocurrent generation for the resulting PSCs. Photovoltaic Properties. To evaluate the photovoltaic performance of DTNR, solar cells were fabricated using PTB7Th as a donor with a device architecture of (indium−tin-oxide) ITO/ZnO/PTB7-Th:DTNR/MoO3/Ag. The inverted configuration was chosen due to improved stability of the device in relative to that based on the conventional architecture. The active layer was prepared by spin-coating the PTB7-Th:DTNR blend solution in chlorobenzene, and the optimized active layer thickness was around 90 nm. For comparison, the control devices based on PTB7-Th:PC71BM with the identical inverted configuration were also fabricated according to a previously optimized procedure.43 Figure 3 depicts the J−V curves for the optimized devices. The photovoltaic parameters were obtained under simulated AM 1.5G, 100 mW cm−2 illumination, and summarized in Table 2. More detailed device parameters under different device fabrication conditions are given in Tables S1 and S2 (Supporting Information). The influence of donor/acceptor (D/A) blend ratios on the photovoltaic performance of PSCs was studied. The PTB7Th:DTNR-based device with a blend ratio of 1:1.5 (by weight) gave the best PCE of 8.17% with a VOC of 1.08 V, a JSC of 13.88 mA cm−2, and a fill factor (FF) of 0.54. Then, under this optimal D/A weight ratio, 1-chloronaphthalene (CN) was used as solvent additive to further optimize the photovoltaic performance. With an addition of 1% CN (by volume), the PCE was improved to 9.31% with a VOC of 1.07 V, a JSC of

Figure 3. (a) J−V curves for PTB7-Th:DTNR-based solar cells without and with solvent additive CN (1%, v/v) under AM 1.5G irradiation at 100 mW cm−2. (b) EQE and corresponding integrated JSC curves of the PSCs.

Table 2. Device Parameters of PSCs Based on PTB7Th:DTNR or PTB7-Th:PC71BM acceptor

D/A ratio [w/w]

VOC [V]

JSC [mA/cm2]

FF [%]

DTNR DTNR DTNR DTNR DTNRc PC71BM

1:1 1:1.5 1:2 1:1.5b 1:1.5b 1:1.5d

1.08 1.08 1.08 1.07 1.08 0.79

13.73 13.88 13.35 15.01 15.72 16.74

0.51 0.54 0.53 0.58 0.56 0.67

PCE [%]a 7.59 8.17 7.70 9.31 9.51 8.94

(7.44 (8.08 (7.52 (9.18 (9.23 (8.64

± ± ± ± ± ±

0.21) 0.12) 0.22) 0.15) 0.29) 0.31)

a

Average PCEs with standard deviations in parentheses are based on 16 devices. b1% CN is added. cSolar cells using an inverted device structure of ITO/ZnO:EDTA/PTB7-Th:DTNR/MoO3/Ag. d3% 1,8diiodooctane is added.

15.01 mA cm−2, and a FF of 0.58. For the control device based on PTB7-Th:PC71BM, a PCE of 8.94% with a VOC of 0.79 V, a JSC of 16.74 mA cm−2, and a FF of 0.67 was achieved. The VOC of the PTB7-Th:DTNR-based device is much higher than that of the control device, which is mainly attributed to the higherlying LUMO level of DTNR. In addition, the high VOC of PTB7-Th:DTNR-based device can sufficiently compensate for its relatively lower JSC and FF, leading to an improved device efficiency in comparison with the PC71BM-based counterpart. These results indicate that DTNR is a promising alternative to the conventional fullerene derivatives for high-performance PSCs. Apart from efficiency, the device stability was also studied to evaluate the performance of PSCs. For the stability test, devices were encapsulated with epoxy and left in ambient conditions. The normalized PCEs versus storage time are shown in Figure S3 (Supporting Information). After 50 day of storage, the PTB7-Th:DTNR-based device still showed a PCE up to 9.02%, 9778

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Figure 4. (a) Jph versus Veff characteristics for the devices under constant incident light intensity. (b) Double-logarithmic plots of JSC versus light intensity for both devices. α stands for the slope of the linear fitting.

Figure 5. J−V characteristics of the (a) hole- and (b) electron-only devices based on PTB7-Th:DTNR with and without CN (μ values were calculated by fitting J−V curves in the SCLC regime).

is also consistent with the results from the PL quenching experiment. Charge Carrier Generation, Transport, and Recombination. To know the effect of CN on the exciton dissociation and charge collection efficiency, photocurrent density (Jph) as a function of effective voltage (Veff) is plotted in double logarithmic coordinates (Figure 4a). Jph is defined as Jph = JL − JD, where JD and JL are the current densities in the dark and under illumination conditions, respectively. Veff is written as Veff = V0 − Va, where Va is the applied bias voltage and V0 is the voltage when Jph is 0.46 As shown in Figure 4a, at a high Veff (Veff > 2.5 V), Jph reaches saturation (where the saturation current density Jsat was achieved), which indicates that the photogenerated excitons are split into free charge carriers and then the carriers are collected by the electrodes efficiently. The Jph/Jsat ratio is a parameter to determine the overall efficiency of exciton dissociation as well as charge collection. Under a shortcircuit condition, the Jph/Jsat ratios are 81% and 88% for the blend films without and with the treatment of 1% CN, respectively, suggesting considerably more efficient exciton dissociation and charge collection in the CN treated solar cells. Furthermore, the CN additive in the PTB7-Th:DTNR blend could reduce the series resistance from 12.3 to 9.0 Ω cm2 and increase the shunt resistance from 0.4 to 0.7 KΩ cm2, indicating a better ohmic contact formed in the CN treated devices. To investigate the charge recombination behavior in the solar cells, JSC values in different light intensities (P) were obtained.

which is 97% of its initial PCE. However, the PTB7Th:PC71BM-based device retained only 89% of its original PCE after a storage of the same time. Therefore, the nonfullerene acceptor DTNR has advantage over the fullerene derivative (PC71BM) in terms of device shelf life. This improved device stability is probably related with the nonspherical structure of DTNR, which may lead to the increased morphology stability of active layer over time.44,45 External quantum efficiencies (EQEs) of the PSCs were investigated to know the spectral response of the devices. As shown in Figure 3b, the device based on PTB7-Th:DTNR showed a broad EQE spectrum in the wavelength range of 300−800 nm, which is consistent with the absorption spectrum of PTB7-Th:DTNR blend film (Figure 1b). With the CN additive in the active layer, the maximum EQE value at 640 nm was enhanced from 68% to 74%. The JSC values calculated from integration of the EQE data with the AM 1.5G solar spectrum were 13.57 and 14.50 mA cm−2 for PTB7-Th:DTNR devices without and with CN additive, respectively, which are in good agreement with the JSC values obtained from the J−V curves within 3.4% error. According to the absorption spectra of PTB7-Th and DTNR films (Figure 1b), the EQE response in the 670−800 nm range is mainly attributed to the absorption of PTB7-Th. The relatively high EQE values for the PSCs in this spectra range (670−800 nm) indicate that the electron transfer from the LUMO of PTB7-Th to that of DTNR is efficient. This 9779

DOI: 10.1021/acs.chemmater.7b03770 Chem. Mater. 2017, 29, 9775−9785

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Chemistry of Materials The light intensity was varied from 1 to 100 mW cm−2. In general, the correlation between JSC and light intensity can be written as JSC ∝ Pα, where α is a constant reflecting the bimolecular recombination degree. When all the dissociated charges can be collected at the electrodes with negligible bimolecular recombination, the α value should be close to 1.47,48 As shown in Figure 4b, the α values of the devices based on PTB7-Th:DTNR blend films with and without CN were determined to be 0.96 and 0.92, respectively, indicating the bimolecular recombination can be suppressed after the addition of 1% CN. These results are further supported by the J−V curves in the dark (Figure S4, Supporting Information) where the CN-treated device shows a lower leakage current in the reverse direction. The combination of decreased bimolecular recombination with enhanced exciton dissociation and charge collection results in the enhanced JSC and FF values for the CN treated OSCs. To investigate the influence of the solvent additive (CN) on the charge carrier mobility of PTB7-Th:DTNR (1:1.5, w/w) blend films, hole (μh) and electron (μe) mobilities were measured using hole- and electron-only devices with the device structures of ITO/PEDOT:PSS/PTB7-Th:DTNR/MoO3/Au and ITO/ZnO/PTB7-Th:DTNR/Ca/Al, respectively. Both μh and μe mobilities were estimated by the space-charge limited current (SCLC) method. Meanwhile, the μe of neat DTNR film was also measured (Figure S5, Supporting Information). The J−V curves of the devices are plotted in logarithmic scale and shown in Figure 5 and the corresponding results are summarized in Table S3 (Supporting Information). The μe for the neat DTNR film was 8.85 × 10−5 cm2 V−1 s−1, while that for the as-cast blend film decreased to 2.20 × 10−5 cm2 V−1 s−1. This is probably because the favorable backbone orientation of DTNR for vertical charge transport is disrupted in the blend film. Given that the FF is highly dependent on efficient charge transport, the decreased mobility can partially explain the modest FF of the as-cast solar cells based on PTB7-Th:DTNR. Moreover, hole mobility for the as-cast PTB7-Th:DTNR film was determined to be 1.24 × 10−5 cm2 V−1 s−1. After addition of 1% CN into the PTB7-Th:DTNR blend, the hole and electron mobilities increased to 1.84 × 10−5 and 2.45 × 10−5 cm2 V−1 s−1, respectively, demonstrating more balanced hole and electron mobilities with a μe/μh ratio of 1.33. The relatively higher and more balanced hole and electron mobilities are beneficial for less carrier recombination, thus contributing to the higher JSC and FF values of the CN-treated devices. Morphology. It is well-known that the morphology of active layer plays a significant role in determining the performance of OSCs. To understand the effects of CN on the photovoltaic performance, the morphologies of the PTB7Th:DTNR films with and without CN were investigated by tapping-mode atomic force microscopy (AFM) and transmission electron microscopy (TEM). As shown in Figure 6a and d, the root−mean−square surface roughness (Rq) is 1.10 nm for the as-cast PTB7-Th:DTNR film without any treatment. After the addition of 1% CN (by volume) to the blend, the roughness decreased to 0.79 nm, which was probably due to the removal of some relatively large-size aggregations. This smooth surface morphology is beneficial to minimize the electrical leakage. From AFM phase images (Figure 6b,e), nanoscale phase separation can be observed in both cases. However, PTB7-Th:DTNR blend film with CN shows relatively smaller domain size compared to the pristine blend film, which suggests that the CN additive induces finer phase separation in the BHJ

Figure 6. Tapping-mode (a, d) AFM topography and (b, e) phase images and (c, f) TEM images of the PTB7-Th:DTNR-based solar cells without (a−c) and with (d−f) solvent additive CN (1%, v/v).

active layer. Figure 6c and f show TEM images of the blend films. It is also demonstrated that CN-treated PTB7-Th:DTNR film exhibits smaller and more finely dispersed phase separation than that without CN treatment. Furthermore, a bicontinuous interpenetrating network with nanofibrillar structures is visible for the CN-treated PTB7-Th:DTNR film, which should favor efficient exciton separation and charge transport. These results are in good agreement with the increased JSC and FF for the PTB7-Th:DTNR-based solar cells with CN additive. The microstructure of the neat and blend films were studied by grazing incidence wide-angle X-ray scattering (GIWAXS).49 The 2D GIWAXS patterns and 1D line-cuts of neat PTB7-Th, DTNR, and PTB7-Th:DTNR blend films with and without CN are shown in Figure S6 (Supporting Information). Both the neat PTB7-Th and DTNR films show similar face-on orientations with π−π stacking peaks (010) in the out-ofplane (OOP) direction and lamellar stacking peaks (100) in the in-plane (IP) direction. In their as-cast blend film, both the low q region and high q region are dominated by DTNR scattering peaks. However, the lamellar peak (100) and π−π stacking peak (010) in the GIWAXS patterns of the blend film are weaker than those in the patterns of the pure DTNR film. These observations suggest the addition of PTB7-Th disrupts the crystallization of DTNR. Compared to the as-cast blend film, the CN-treated blend film exhibits enhanced diffraction features in both lamellar packing and π−π stacking regions. We quantified the coherence lengths by the full width at halfmaximum of OOP (010) diffraction peaks via the Scherrer equation.50 The quantitative fitting gave the coherence lengths of 0.78 and 1.49 nm for the blend without CN. After the addition of CN, the two coherence lengths increase to 1.51 and 1.87 nm, respectively. The results demonstrate an enhanced molecule packing in the CN-treated blend film, which is favorable for more efficient charge transport, and increased JSC and FF. Interface Engineering and Energy Loss. Further optimization of the device performance was carried out by using a hybrid interlayer of EDTA-ZnO (EDTA: ethylenediaminetetraacetic acid), which was reported recently by Fang and co-workers.51 As shown in Table 2 and Figure 7, when EDTA-ZnO was introduced as the cathode interlayer, an improved the device performance was achieved with a PCE of 9.51%, a high VOC of 1.08 V, and a JSC of 15.72 mA cm−2. The 9780

DOI: 10.1021/acs.chemmater.7b03770 Chem. Mater. 2017, 29, 9775−9785

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Figure 7. (a) J−V characteristics and (b) EQE spectrum of OSCs based on PTB7-Th:DTNR with a device structure of ITO/ ZnO:EDTA/PTB7-Th:DTNR/MoO3/Ag.

Figure 8. (a) Plots of eVOC against Eg for various high-performance (PCE ≥ 8%) solar cell systems. (b) Plots of PCE against Eloss for various high-performance (PCE ≥ 8%) solar cell systems extracted from a. SM represents small molecule, and NF-A represents nonfullerene acceptor (see Table S4 in the Supporting Information for the detail data). The gray dashed line is the Eg − eVOC = 0.5 eV.

enhanced JSC value was also confirmed by the EQE measurements. As shown in Figure 7b, the devices exhibit a high EQE over 70% around 640 nm. The calculated JSC of 15.30 mA cm−2 from the EQE integration is close to the JSC value from the J−V measurement within 3% error. It is worthwhile mentioning that both large VOC and high JSC values were obtained for the PTB7Th:DTNR-based devices. This is rarely observed in fullerenebased OSCs. As we all know, to achieve both high JSC and large VOC in fullerene-based OSCs, the donor material requires both a narrow optical bandgap and a low-lying HOMO level, which was barely possible in the past decades. However, in this work, we demonstrate that nonfullerene OSCs have greater potential to minimize the trade-off between the JSC and VOC, and to achieve better photovoltaic performance in relative to the fullerene-based counterparts. As mentioned in the Introduction, one of the bottlenecks in the PCE improvement of OSCs is the large Eloss. For the nonfullerene OSCs based on the PTB7-Th:DTNR blend, the VOC is 1.08 V, and the Eg refers to the onset of film absorption of PTB7-Th (1.58 eV). Thus, the Eloss is estimated to be 0.50 eV. In contrast, for PTB7-Th:PC71BM system, a VOC of 0.79 V was obtained indicating a much larger Eloss of 0.79 eV. Figure 8a shows the plots of eVOC against Eg for the high-performance OSCs (PCE > 8%) reported in this work and in the literatures. The detailed data of the corresponding devices are summarized in Table S4 (Supporting Information). It can be seen that the Eloss value of PTB7-Th:DTNR system is the lowest among those for high-performance (PCE > 8%) polymer-based systems with similar optical bandgaps (Figure 8a, Table S4). Indeed, the Eloss of 0.50 eV approaches the values for perovskite

and some of inorganic solar cells, which are around 0.4−0.5 eV. More importantly, despite the low Eloss value, the PTB7Th:DTNR system can still achieve a PCE as high as 9.51%, which is also among the highest values reported so far for OSCs with Eloss ≤ 0.5 eV (Figure 8b). Such an exorbitantly small Eloss obtained for the PTB7-Th:DTNR-based solar cells is in part attributed to the small ΔEL of 0.13 eV between PTB7-Th and DTNR. At the same time, the small driving force (ΔEL) may enhance the radiative quantum efficiency of the PTB7Th:DTNR-based solar cell, thereby further reducing the Eloss by the decreased nonradiative recombination loss.24 It should be noted that the nonradiative recombination loss from the charge-transfer states is not clearly understood yet, and is under investigation now.

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CONCLUSION In summary, we have developed a novel nonfullerene acceptor DTNR, which employs a ladder-type angular-shaped dithienonaphthalene flanked by two electron-withdrawing 3-ethylrhodanine units via two benzothiadiazole bridges. DTNR features a strong absorption in the visible region, a high-lying LUMO energy level, and a relatively high electron mobility. Through morphology and device optimization, BHJ OSCs based on PTB7-Th:DTNR exhibited a high PCE of 9.51% with a large VOC of 1.08 V. More importantly, the energy loss for the 9781

DOI: 10.1021/acs.chemmater.7b03770 Chem. Mater. 2017, 29, 9775−9785

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Chemistry of Materials

measured by a Bruker Dektak XT surface profilometer. Cyclic voltammetry was performed in a solution of tetra-n-butylammonium hexafluorophosphate (0.1 M) in acetonitrile on a CHI 604E electrochemical workstation with a three-electrode system at a scan rate of 100 mV s−1. The small molecule or polymer films were deposited on a Pt plate electrode through dipping the electrode into the corresponding solutions and then dried under ambience. Pt wire and Ag/AgNO3 were used as the counter electrode and reference electrode, respectively. The HOMO and LUMO energy levels of thin films were determined by using the following equations:

solar cells is as low as 0.50 eV, which is the smallest value for high-performance (PCE > 8%) OSCs with similar bandgaps and is close to those of crystalline inorganic or perovskite solar cells. Additionally, the PL quenching results indicate that the photoinduced charge separation is efficient in PTB7-Th:DTNR system, despite the small LUMO energy offset of 0.13 eV. As a result of the effective charge separation and relatively complementary absorption of the donor and acceptor, a high EQE of over 70% was achieved in PTB7-Th:DTNR-based solar cells. The PTB7-Th:DTNR-based PSC showed improved device stability compared to the PTB7-Th:PC71BM-based counterpart, demonstrating another salient advantage of the nonfullerene acceptor. This work not only affords a novel angular-shaped dithienonaphthalene-based nonfullerene acceptor for high-performance OSCs, but also demonstrates that the use of nonfullerene acceptor is an effective approach to minimize the trade-off between the VOC and JSC which can have a fundamental impact on organic photovoltaics in the long run. Further studies will focus on the optimization of the morphology of the blend film and the selection of better matched donor materials which may lead to OSCs with an even higher PCE.

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E HOMO = − (φox + 4.82) (eV)

E LUMO = − (φred + 4.82) (eV) UPS Characterization. UPS measurements were performed by using Axis Ultra delay-line detector (DLD). PTB7-Th, DTNR, and PC71BM neat films were spin-casted on Si substrates. The incident photon energy (He I) was 21.22 eV, and a sample bias of −10 V was used to measure the onset of photoemission. Preparation of ZnO Film. ZnO film was prepared using a sol−gel method.52 Briefly, ZnO precursor solution was obtained by dissolving zinc acetate dehydrate (0.6 g) and ethanolamine (0.17 g) in 12 mL of 2-methoxyethanol under stirring for 8 h. The ZnO thin film (ca. 30 nm) was prepared by spin-coating the ZnO precursor solution on the top of ITO-glass at 3000 rpm for 50 s. Then the film was preheated on a hot plate at 150 °C for 10 min and further transferred into an oven for annealing at 200 °C for 45 min in air. Preparation of ZnO-EDTA Hybrid Film. ZnO-EDTA precursor solution was produced according to a previously established procedure.51 EDTA was dissolved into the same mixed solvent as that of the ZnO precursor. When preparing ZnO-EDTA precursor solution, the two solutions were mixed together with EDTA concentration of 1 mg/mL. The ZnO-EDTA hybrid film was cast from the ZnO-EDTA precursor solution at 3000 rpm for 50 s, followed by thermal annealing at 150 °C for 30 min in air. Fabrication and Characterization of OSCs. The precleaned ITO glass was dried in an oven at 130 °C overnight and further treated by ultraviolet/ozone for 15 min at room temperature. The ZnO or ZnO-EDTA hybrid electron transport layer (ETL) was fabricated on the precleaned ITO substrate as mentioned above. A chlorobenzene solution (totally 20 mg mL−1) of PTB7-Th:DTNR blend was subsequently spin-coated (2300 rpm, 60 s) on the ETL layer to form an active layer (ca. 90 nm). At last, the MoO3 (10 nm) and Ag (100 nm) were thermally deposited onto the active layer through shadow masks. The active area of the devices was fixed at 6 mm2. The devices were tested in ambient atmosphere at room temperature after an encapsulation by epoxy kits (general purpose, Sigma-Aldrich) in glovebox. J−V measurements were carried out using a Keithley 2400 source meter. The PSC devices were measured under AM 1.5 G irradiation (100 mW·cm−2) with an Oriel sol3A simulator (Newport). The light intensity for J−V measurements was calibrated with a NREL-certified silicon reference cell. More than 16 devices were analyzed to provide the average PCEs of OSCs. EQE data were obtained using the QE/ IPCE Measurement kit from Newport (QE−PV-SI). Hole- or Electron-Only Device Fabrication and Characterization. Hole-only and electron-only diodes were fabricated to measure the hole and electron mobilities using the SCLC method. The SCLC devices were prepared by following the same procedure as that for the solar cell fabrication, except for the metal electrode. The SCLC mobilities were determined by fitting the dark current to the model of a single carrier SCLC, which was described as53

EXPERIMENTAL SECTION

Materials. PTB7-Th and PC71BM (99%) were purchased from Solarmer Materials Inc. and American Dye Source Inc., respectively. The other chemical reagents were purchased from Aldrich Inc., Aladdin-Reagent Inc., and Adamas-beta Ltd. and used without further purification. Compounds 1 and 2 were synthesized according to the procedure reported previously by us.42 Synthesis of Compound 3. To a degassed solution of compound 2 (0.55 g, 0.5 mmol), 7-bromobenzo[c][1,2,5]thiadiazole-4-carbaldehyde (0.36 g, 1.5 mmol) in toluene (30 mL), Pd(PPh3)4 (30 mg, 0.03 mmol) was added. Then the mixture was heated to reflux for 24 h under N2. After being cooled to ambient temperature, the solvent was evaporated to get the crude product, which was purified by column chromatography on silica gel using petroleum ether/CH2Cl2 (1:2) as eluent. A deep brown solid was obtained (0.42 g, 77%). 1H NMR (CDCl3, 400 MHz, ppm): 10.76 (s, 2H), 8.48 (d, J = 8.0 Hz, 2H), 8.30 (d, J = 8.0 Hz, 2H), 8.25 (d, J = 8.0 Hz, 2H), 8.14 (d, J = 8.0 Hz, 2H), 7.75 (d, J = 8.0 Hz, 2H), 2.29−2.11 (m, 8H), 1.03−0.51 (m, 60H). HRMS (MALDI) m/z: calc. for C66H80O2N4S4: 1088.5213; found: 1088.5237. Elemental analysis (%) calc. for C66H80O2N4S4: C, 72.75; H, 7.40; N, 5.14; found: C, 72.81; H, 7.41; N, 4.93. Synthesis of DTNR. Under N2, 0.5 mL of triethylamine was added to a mixture of compound 3 (0.44 g, 0.4 mmol) and 3-ethylrhodanine (0.52 g, 3.2 mmol) in 30 mL of chloroform. Then the mixture solution was heated to reflux for 24 h. After cooling to room temperature, the mixture was poured into methanol (100 mL) and the precipitate was filtered. Through the purification by column chromatography on silica gel using petroleum ether/CH2Cl2 (1:2) as the eluent, DTNR was obtained as a deep purple solid (0.49 g, 90%). 1H NMR (CDCl3, 400 MHz, ppm): 8.57 (s, 2H), 8.41 (m, 2H), 8.24 (d, J = 8.0 Hz, 2H), 8.10 (d, J = 8.0 Hz, 2H), 7.79 (d, J = 8.0 Hz, 2H), 7.73 (m, 2H), 4.29 (m, 4H), 2.27−2.09 (m, 8H), 1.38−0.50 (m, 66H). HRMS (MALDI) m/ z: calc. for C76H90O2N6S8: 1374.4891; found: 1374.4880. Elemental analysis (%) calc. for C76H90O2N6S8: C, 66.33; H, 6.59; N, 6.11; found: C, 66.38; H, 6.61; N, 5.97. General Characterization. High-resolution mass spectroscopy (HRMS) measurements were carried out on an IonSpec 4.7T spectrometer. 1H NMR (400 MHz) spectra were obtained using a Bruker AVANCE-400 spectrometer. Elemental analyses were performed on a Vario EL-Cube elemental analyzer. UV−vis absorption spectra were obtained on a Lambda 365 UV/vis spectrophotometer. Photoluminescence spectra were measured using a Cary spectrophotometer. The tapping-mode AFM images of the active layers were acquired by a Dimension Icon AFM. The thickness of the film was

J=

9 V2 εrε0μ 3 8 L

where J is the current density (mA/cm−2), εr is the relative dielectric constant of the active layer material (assumed to be 3), ε0 is the permittivity of empty space (8.85 × 10−12 F m−1), μ is the hole or electron mobility, V is the drop in voltage across the device (V = Vappl 9782

DOI: 10.1021/acs.chemmater.7b03770 Chem. Mater. 2017, 29, 9775−9785

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Chemistry of Materials − Vbi, where Vappl is the applied voltage to the SCLC device, and Vbi is the built-in voltage owing to the difference in work function of the two electrodes), and L is the film thickness of active layer. The hole or electron mobility can be calculated from the slope of the J1/2−V curves. GIWAXS Measurement. GIWAXS characterization of the thin films was performed at Advanced Light Source on beamline 7.3.3.54 Thin film samples were spin-coated on Si substrates. The 10 keV X-ray beam was incident at a grazing angle of 0.11−0.15° for an optimized signal-to-background ratio. The scattered X-rays were detected using a Dectris Pilatus 2 M photon counting detector.

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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.chemmater.7b03770. UPS results, PL spectra, device stability, dark currents, GIWAXS results, J−V characteristic of DTNR-based electron-only device, detailed photovoltaic properties and OSCs parameters (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wei Ma: 0000-0002-7239-2010 Qingdong Zheng: 0000-0002-6324-0648 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. U1605241, 61325026, 51561165011), the Key Research Program of Frontier Sciences, CAS (No. QYZDB-SSW-SLH032), the CAS/SAFEA International Partnership Program for Creative Research Teams, and the Strategic Priority Research Program of the Chinese Academy of Sciences, Grant No. XDB20000000. W.M. thanks the Ministry of science and technology (No. 2016YFA0200700), NSFC (21504066) for the support. X-ray data were acquired at beamlines 7.3.3 and 11.0.1.2 at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC0205CH11231. The authors thank Chenhui Zhu at beamline 7.3.3 for assistance with data acquisition.

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DOI: 10.1021/acs.chemmater.7b03770 Chem. Mater. 2017, 29, 9775−9785

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DOI: 10.1021/acs.chemmater.7b03770 Chem. Mater. 2017, 29, 9775−9785