Modulating Molecular Orientation Enables Efficient Nonfullerene

Mar 13, 2018 - Efficient photovoltaic small molecules (SMs) typically possess an acceptor–donor–acceptor structure that causes intrinsic anisotrop...
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Article Cite This: Chem. Mater. 2018, 30, 2129−2134

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Modulating Molecular Orientation Enables Efficient Nonfullerene Small-Molecule Organic Solar Cells Liyan Yang,†,∥ Shaoqing Zhang,*,†,‡ Chang He,*,†,∥ Jianqi Zhang,*,§,∥ Yang Yang,§,∥ Jie Zhu,† Yong Cui,†,∥ Wenchao Zhao,†,∥ Hao Zhang,†,∥ Yun Zhang,†,∥ Zhixiang Wei,§,∥ and Jianhui Hou*,†,∥ †

Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ School of Chemistry and Biology Engineering, University of Science and Technology Beijing, Beijing 100083, China § Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscience and Technology, Beijing 100190, China ∥ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: In bulk-heterojunction organic solar cells (BHJ-OSCs), exciton dissociation and charge transport are highly sensitive to the molecular packing pattern and phase separation morphology in blend films. Efficient photovoltaic small molecules (SMs) typically possess an acceptor−donor−acceptor structure that causes intrinsic anisotropy, limiting the control over molecular packing because of the lack of an effective method for modulating molecular orientation. In this report, we design a group of model compounds, named DRTB-T-CX (X = 2, 4, 6, and 8), to demonstrate that adjusting the length of the end alkyl chain can be used to modify the molecular orientation. A top-performance power conversion efficiency (PCE) of up to 11.24% is achieved with a DRTB-T-C4/ IT-4F-based device, which is the best performance reported for a state-of-the-art nonfullerene SM organic solar cell (NFSM-OSC).

S

orientation so far. Since exciton dissociation and charge transport are highly sensitive to the molecular packing pattern and phase separation morphology in the active layer,22,23 and there is a lack of research on molecular orientation and its influence on the photovoltaic performance of NFSM-OSCs, designing model compounds to study their molecular orientation and aggregation behavior and then devising a new method for modifying molecular orientation through molecular design are of great importance. Here, we designed and synthesized a group of SM compounds having identical π-conjugated backbones and end groups, but end-group alkyl chains with increasing lengths (named DRTB-T-CX; see Figure 1a). This design provides model compounds with identical photoelectric properties and allows us to focus on the significant influence of the end alkyl chains on the molecular orientation and intermolecular aggregation behavior in solid-state films. The 2D grazingincidence wide-angle X-ray scattering (GIWAXS) characterization of the DRTB-T-CX films revealed the obvious transition of orientation from edge-on to face-on relative to the substrate when the length of the end alkyl chain is extended. Then, we blended the model compounds DRTB-T-CX with IT-4F (a

olution-processed bulk-heterojunction organic solar cells (BHJ-OSCs) have been extensively studied because of their advantages of low cost, light weight, and mechanical flexibility.1−4 Compared to polymeric materials, small molecules (SMs) have intrinsic superiorities for applications in OSCs,5 such as a well-defined molecular structure and therefore excellent batch-to-batch reproducibility.6−8 In recent years, both SM donor materials and nonfullerene SM acceptors9−14 have been developed rapidly because of their easily modified molecular structure and highly tunable physical properties, such as their absorption profile, molecular orbital energy level, and mobility.15 All these factors contributed to the increased research attention in nonfullerene small-molecule organic solar cells (NFSM-OSCs).16−19 SM:fullerene-based OSCs with a power conversion efficiency (PCE) of over 11% have been achieved;20,21 however, the performance of NFSM-OSCs is still lower than that of fullerene-based devices, and the reported PCEs of single-junction NFSM-OSCs are rarely over 9%.16−18 The π-conjugated acceptor−donor−acceptor (A−D−A) backbone structure that has usually been employed in the design of NFSMs impart anisotropic properties to the material and thus limit control over molecular orientation and intermolecular aggregation. Although most nonfullerene acceptors have a faceon orientation, almost all the SM donors show an edge-on orientation. It is crucial to obtain face-on donors because of the lack of an effective method for modulating molecular © 2018 American Chemical Society

Received: January 21, 2018 Revised: March 13, 2018 Published: March 13, 2018 2129

DOI: 10.1021/acs.chemmater.8b00287 Chem. Mater. 2018, 30, 2129−2134

Article

Chemistry of Materials

Figure 1. (a) Molecular structure of DRTB-T-CX and IT-4F. (b) Energy level diagram of the donor and acceptor materials. (c) Normalized thin-film UV−vis absorption spectra of DRTB-T-CX and IT-4F. (d) 2D GIWAXS patterns of pristine C2, C4, C6, and C8.

nonfullerene acceptor; see Figure 1a)24 and constructed nonfullerene all-SM organic photovoltaic (OPV) devices, which afforded PCEs between 9.14% and 11.24%. In addition, we found that, in the solid state, the face-on orientation could enhance the charge mobility and increase the coherence length of π−π stacking, leading to a substantial increase in the efficiency of NFSM-OSCs. The four benzo[1,2-b:4,5-b′]dithiophene-based (BDTbased) SM compounds were synthesized by end-capping with rhodanine featuring different alkyl groups, namely, ethyl, nbutyl, n-hexyl, and n-octyl. (Hereafter, the names of the SMs are abbreviated and referred to as C2, C4, C6, and C8.) Compounds C4, C6, and C8 were synthesized according to a previous reported method,16 and the detailed synthetic procedures and compound characterization data can be found in the Supporting Information (SI). According to their absorption profiles in both solution and films (Figure S1a and Figure 1c), the four compounds show identical spectra with absorption maxima at 500−600 nm. The absorption onsets in the films were found in the range 620−629 nm, which corresponds to optical band gaps of 1.97−2.0 eV. The energy levels for C4, C,6 and C8 were estimated by cyclic voltammetry (CV) measurement, and their highest occupied molecular orbitals (HOMOs) were from ∼−5.50 to −5.52 eV (Figure S1b and Table S1), which are similar to that of the short ethylsubstituted analogue C2 (−5.51 eV). The molecular orbital level and optical absorption results of the DRTB-T-CX molecules indicate that the different lengths of the end alkyl chains have little influence on the optoelectronic properties of this family of SMs. To explore the solid-state molecular orientation of the model compounds, GIWAXS measurements and analyses were conducted. Figure 1d presents the 2D GIWAXS patterns of the SMs with different alkyl chains, and the corresponding

intensity profiles in the out-of-plane and in-plane directions are given in Figure S5. On the basis of the 2D GIWAXS patterns of a pristine C2 thin film, the scattering vector q of 0.3, 0.6, and 0.9 Å−1 on the qz axis originates from lamellar packing diffraction (100, 200, and 300), whereas the vector q of 1.73 Å−1 on the qxy axis can be attributed to the π−π stacking diffraction (010), suggesting a typical edge-on molecular orientation. However, compared to C2, C4 shows a completely different scattering pattern: the scattering intensity of the (010) peak is mainly in the out-of-plane direction, indicating a preferred face-on orientation. Interestingly, when the alkyl chain further increased in length to C6, the scattering intensity of the (010) peak mainly localizes at directions approximately 60° with respect to the surface of the substrate (Figure S5c). This observation implies that some of the crystallites have a preferred orientation (along 60°) that is in between an edge-on and a face-on orientation. As the alkyl chain increased in length to C8, the scattering intensity of (010) mainly distributed in the out-of-plane direction, indicating that face-on orientation dominates. These GIWAXS results demonstrate that the molecular orientation can be effectively tailored by changing the length of the alkyl chain on the end group. Because the molecular orbital level and optical absorption spectra of the four compounds are similar, we can infer that the variations in photovoltaic device performance are caused by the differences in the molecular orientation and phase separation morphology in the solid state. NFSM-OSCs based on blends of DRTB-T-CX with IT-4F (Figure 1a) in an inverted structure (ITO/ZnO/DRTB-TCX:IT-4F/MoO3/Al) were fabricated to investigate the photovoltaic properties of the four SMs. DRTB-T-CX and the SM acceptor IT-4F possess complementary absorption profiles in the region 300−800 nm (Figure 1c), and both the HOMO (−5.66 eV) and LUMO (−4.14 eV) levels of IT-4F are 2130

DOI: 10.1021/acs.chemmater.8b00287 Chem. Mater. 2018, 30, 2129−2134

Article

Chemistry of Materials

Figure 2. (a) J−V curves. (b) EQE spectra. (c) Plots of PCE versus active layer thickness for the C2- and C4-based devices. (d) Hole and electron mobility of the blend films.

Table 1. Photovoltaic Properties of DRTB-T-CX:IT-4F BHJ Devices C2 C4 C6 C8

Jsc [mA cm−2]

Voc [V]

donor 0.893 0.909 0.929 0.928

(0.891 (0.917 (0.924 (0.921

± ± ± ±

0.005) 0.007) 0.004) 0.004)

b

16.66 18.27 17.92 16.15

(16.63 (18.04 (17.64 (16.12

± ± ± ±

Jsca [mA cm−2]

0.43) 0.23) 0.37) 0.11)

16.53 18.39 17.89 16.01

FF 0.64 0.68 0.63 0.61

(0.63 (0.67 (0.63 (0.61

PCE [%] ± ± ± ±

0.02) 0.01) 0.01) 0.01)

9.52 (9.50 ± 0.08) 11.24 (11.06 ± 0.10) 10.52 (10.33 ± 0.13) 9.14 (9.09 ± 0.12)

a

Calculated by integrating the EQE spectra. bThe average PCE values were obtained from over 10 devices, and the active layer thickness is approximately 100 ± 5 nm.

substantial increase in the efficiency of the NFSM-OSCs. The tests also indicated that C4 has good tolerance for the active layer thickness, which is important for roll-to-roll printing technology. In addition, the other two DRTB-T-CX:IT-4F-based devices (X = 6 and 8) also show relatively inferior performance compared to that of the C4-based devices; i.e., the replacement of the end alkyl chain with n-hexyl or n-octyl could lead to slightly decreased FF and Jsc, resulting in lower PCEs of 10.52% and 9.14%. Thus, we infer that varying the end-group alkyl chain length could significantly affect the molecular alignment and therefore the performance of the NFSM-OSCs. For insight into the impact of the end alkyl chains on charge transport properties, the charge carrier mobilities were measured using the space charge limited current (SCLC) model.27−29 The hole-only devices based on the blend films of C2, C4, C6, and C8 showed mobilities of 3.27 × 10−5, 1.74 × 10−4, 5.55 × 10−5, and 3.14 × 10−5 cm2 V−1 s−1, respectively, and the electron mobilities of those blend films in electron-only devices were 7.05 × 10−5, 1.68 × 10−4, 1.32 × 10−4, and 1.03 × 10−4 cm2 V−1 s−1, respectively (Figure S3a,b). The mobility values are summarized in Figure 2d, and the results indicate that C4 has the highest carrier mobility and the most balanced transport, which can explain the enhanced Jsc and FF in the NFSM-OSCs.

clearly lower than those of DRTB-T-CX (Figure 1b), thus suggesting that there is a sufficient driving force for efficient exciton dissociation in the blend film. Therefore, the combination of DRTB-T-CX and IT-4F is suitable for the active layer component for NFSM-OSCs.25,26 The best current density−voltage (J−V) curves and external quantum efficiency (EQE) spectra are shown in Figure 2a,b, and the device performance parameters are summarized in Table 1. The optimal devices were obtained by varying the blend ratios, solvent vapor annealing conditions, and active layer thickness. The details of the device optimization processes are shown and summarized in Table S3. In the NFSM-OSCs, an outstanding PCE of 11.24% with a particularly high Jsc of 18.27 mA cm−2 and a strong and broad EQE response approaching 75% efficiency at 640 nm in the region from 350 to 800 nm were obtained from the C4-based devices. The integrated current from the EQE spectrum is 18.39 mA cm−2, which is in good agreement with the current obtained from the J−V measurement. Remarkably, the devices based on C4/IT-4F can still retain a high PCE of 10% with active layer thicknesses up to 300 nm in single-junction solar cells. C2-based devices, which exhibit an obvious edge-on orientation in the active layer, exhibited a PCE of 9.52% (Figure 2c). Combined with the GIWAXS analysis, these results suggest that the preferred faceon orientation of the compound is the vital factor in the 2131

DOI: 10.1021/acs.chemmater.8b00287 Chem. Mater. 2018, 30, 2129−2134

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Chemistry of Materials

Figure 3. (a) Pole profiles extracted from the lamellar diffraction of the blend films. The scattering intensity was normalized with the intensity at 90°. (b) Ratio of face-on orientation for the blend films and pristine IT-4F. (c) CCLs of the four donors and acceptor for the blend films. (d) GISAXS profiles (black symbols) and corresponding fitting (red curves).

of the four systems is consistent with that of the hole and electron mobility, as shown in Figure 2d. The CCL of the π−π stacking has a strong correlation with the charge mobility, especially for the system consisting of two conjugated SMs. To address the nanoscale morphology of the active layer, grazing-incidence small-angle X-ray scattering (GISAXS) measurements were performed. Figure 3d plots the in-plane intensity profiles, and the 2D GISAXS patterns are given in Figure S7. Information on the lateral structures can be extracted by fitting the intensity profile with a 1D paracrystal model.33 Two distinct structures (small and large ones) are identified by the proper fitting of the intensity profiles (solid red lines in Figure 3d). Comparing the scattering intensity of different samples with the same thickness (see Figure S8), the crystallinity of donors is much higher than that of the acceptor. Thus, we propose that the large domains originate from the aggregation of the donor because of its high crystallinity and that the small domains originate from the acceptor. For C2, C4, C6, and C8, the extracted smaller domain sizes are 16, 12, 16, and 20 nm, and the larger domain sizes are 35, 22, 30, and 50 nm, respectively. These results agree with the AFM measurements, which show increased roughness due to formation of larger domains (Figure S9). The Jsc is well-known to depend on the domain size. The C4 system has the smallest domain size, which should lead to a larger interfacial area and thus an improved exciton separation efficiency, whereas the C8 system has the largest domain size, which might be too large for efficient exciton dissociation. Thus, the difference in the domain size could explain the variation in the Jsc values in the present systems and the low performance of C8. In summary, we have used a group of model compounds, DRTB-T-CX (X = 2, 4, 6, and 8), to demonstrate that adjusting the length of the end alkyl chains can effectively modify the

We employed GIWAXS measurements to further understand the impact of the variable molecular orientation on the device performance. Figure S6 presents the 2D GIWAXS patterns for the DRTB-T-CX:IT-4F blend films and pristine IT-4F. For the acceptor IT-4F, the (010) peak is located in the out-of-plane direction, whereas the (100) peak is located in the in-plane direction, indicating a typical face-on orientation. Compared with the pristine donors shown in Figure 1d, the blend films present strong (010) peaks in the out-of-plane direction, suggesting that the face-on orientation is prevalent, which facilitates the charge transport. For a quantification of the incidence of the face-on orientation, the scattering intensity of the (100) peak with the azimuth angle was integrated, namely, the pole profile.21 Figure 3a shows pole profiles for the four samples. The face-on ratio can be obtained from these profiles by a previously reported method.30 The areas integrated with the azimuth ranges 0−20°, 160−180°, and 70−110° correspond to the ratios of face-on and edge-on crystallites. The calculated results are given in Figure 3b. Notably, the face-on ratio increases as the alkyl chain length on the end group increased. Conversely, the decrease in the face-on fraction of C6 compared with those of C4 and C8 is attributed to the formation of the preferred orientation at 30° with respect to the surface of the substrate (see the blue curve in Figure 3a). The mean crystallite size can be obtained by calculating the crystal coherence length (CCL) using the Scherrer equation.31,32 The CCL calculated by fitting the (010) peaks of the blend films is shown in Figure 3c. The (010) peaks can be fitted by accumulating the individual peaks of the pure donor and acceptor. Thus, the CCLs of the donor and acceptor in the blends can be extracted separately. Clearly, the CCLs of the donors and acceptor have a similar trend, which means that they have a synergic effect. Furthermore, the trend in the CCL 2132

DOI: 10.1021/acs.chemmater.8b00287 Chem. Mater. 2018, 30, 2129−2134

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Chemistry of Materials

which is well-defined by a metal mask with an aperture aligned with the device area. The EQE curves were measured through the solar cell spectral response measurement system QE-R3011 (Enli Technology Ltd.), which was calibrated with a crystal silicon photovoltaic cell before use. The film thickness data were obtained via a surface profilometer (Dektak XT, Bruker). Hole Mobility Measurements. The hole mobilities were measured by the SCLC method,31 employing a device architecture of ITO/PEDOT:PSS/blend film/Au. The mobilities were obtained by taking the dark current−voltage curves in the range 0−6 V and fitting the results to a space charge limited form, where the SCLC is described by

molecular orientation of the SM donor materials. The transition of molecular orientation from edge-on to face-on could be observed as the end alkyl chain of the compounds was extended. A top performance of up to 11.24% is achieved with the DRTB-T-C4/IT-4F-based NFSM-OSCs. Remarkably, devices based on DRTB-T-C4/IT-4F with active layer thicknesses up to 300 nm can still retain a high PCE of 10%, which is beneficial for practical roll-to-roll printing technology. The high PCE and tolerance of film thickness are closely related to the enhancement of charge mobility and the correlation length of π−π stacking. Importantly, we have found that modifying the end alkyl chains in a group of SM donors is an effective and facile method for controlling the molecular orientation.

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J=

9ε0εrμV 2 3

8L

⎡ V exp⎢0.89 ⎢⎣ E0L

⎤ ⎥ ⎥⎦

where J is the current density, L is the film thickness of the active layer, μ is the hole mobility, E0 is the characteristic field, εr is the relative permittivity of the material, ε0 is the permittivity of free space (8.85 × 10−12 C V−1 m−1), V (=Vappl − Vbi) is the internal voltage in the device (where Vappl is the applied voltage to the device, and Vbi is the built-in voltage due to the relative work function difference of the two electrodes). Electron Mobility Measurements. The electron mobilities were measured by the SCLC method, employing a device architecture of ITO/ZnO/blend film/Al. The mobilities were obtained by taking the dark current−voltage curves in the range 0−6 V and fitting the results to a space charge limited form, where the SCLC is described by

EXPERIMENTAL SECTION

Instruments and Measurements. 1H NMR and 13C NMR spectra were recorded on a Bruker AVANCE 300 and 75 MHz NMR spectrometer. Elemental analyses of the newly synthesized smallmolecular donors were conducted on the Flash EA1112 analyzer. MS spectra (MALDI−TOF−MS) were determined on a Bruker Autoflex III TOF mass spectrometer. Cyclic voltammetry was performed on a CHI650D electrochemical workstation using glassy carbon, Pt wire, and Ag/AgCl electrode as the working electrode, counter electrode, and reference electrode, respectively. A 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile solution was used as an electrolyte, and ferrocene/ferrocenium (Fc/Fc+) was used as the external standard. Absorption spectra in chloroform solution and as thin films of the small-molecular donors were measured on a Hitachi UH4150 UV−vis spectrophotometer. All film samples were spin-cast on quartz substrates. Atomic force microscopy measurements were acquired by using a Nanoscope V AFM instrument in tapping mode. Transmission electron microscopy measurements were performed by using Tecnai G2 F20 U-TWIN TEM instruments. Device Fabrication and Characterization. Prepatterned ITOcoated glass with a sheet resistance of ∼15 Ω □−1 was purchased from CSG HOLDING Co., Ltd. The ITO-coated glass substrates were cleaned by ultrasonic treatment in detergent, deionized water, acetone, and isopropanol, successively. The washed substrates were treated under UV-ozone for 15 min. A ZnO electron transport layer was spincoated onto the ITO substrate at a spinning rate of 3000 rpm for 30 s. After being baked in air at 200 °C for 1 h, the substrates were transferred into a nitrogen-filled glovebox. Active layer solutions (D/A ratio 1:1, total concentration 20 mg mL−1) were prepared in chloroform (CF) and stirred at room temperature for ca. 3 h until they were intensively dissolved. Subsequently, active layers were spincoated at 2000 rpm for 30 s to obtain a film thickness of approximately 100 nm. Thinner films were produced by varying the spin rate of 2500 and 3000 rpm to obtain a film thickness of approximately 90 and 80 nm. DRTB-T-CX:IT-4F in a 40 mg mL−1 chloroform solution was spin-coated at 4000, 3000, and 1500 rpm for 30 s to obtain a film thickness of approximately 125, 150, and 200 nm. DRTB-T-CX:IT-4F in a 60 mg mL−1 chloroform solution was spin-coated at 3000 and 2000 rpm for 30 s to obtain a film thickness of approximately 250 and 300 nm. Then, the substrates were placed in a glass Petri dish containing 200 μL of tetrahydrofuran (THF) for solvent vapor annealing (SVA). Finally, 10 nm thick MoO3 film and a 100 nm thick Al layer were evaporated sequentially under high vacuum. Conventional devices were fabricated with a structure of ITO/MoO3/active layer/Al. The ITO-coated glass substrates were cleaned by the same procedure with inverted devices. A thin layer of MoO3 (10 nm) was evaporated on top of the ITO. The mixture of small molecules and acceptors was intensively dissolved in CF with a total concentration of 20 mg mL−1. Subsequently, the active layer was spin-coated from blend chloroform solutions. Finally, 100 nm Al layers were evaporated under high vacuum. Device J−V characteristics were measured with a Keithley 2400 source measure unit under 100 mW cm−2 standard AM 1.5G light. Typical cells have device areas of approximately 4 mm2,

J=

9 V2 ε0εrμ 3 8 L

where J is the current density, L is the film thickness of the active layer, μ is the electron mobility, εr is the relative permittivity of the material, ε0 is the permittivity of free space (8.85 × 10−12 C V−1 m−1), V (=Vappl − Vbi) is the internal voltage in the device (where Vappl is the applied voltage to the device, and Vbi is the built-in voltage due to the relative work function difference of the two electrodes). GIWAXS Characterization. GIWAXS measurements were performed by using a XEUSS SAXS/WAXS system. Samples were prepared on Si substrates using blend solutions identical to those devices used. The wavelength of the X-ray beam is 1.54 Å, and the incident angle was 0.2°. Scattered X-rays were detected by using a Dectris Pilatus 300 K photon counting detector. The crystal coherence length (CCL) was obtained using the Scherrer equation.

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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.8b00287. Synthesis route of DRTB-T-CX, UV−vis absorption spectra, cyclic voltammetry curves, TGA plots, hole and electron mobility curves, Jph−Veff curves, Jsc−P curves, 2D GIWAXS and GISAXS patterns, detailed photovoltaic parameters, and NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail:

[email protected]. [email protected]. [email protected]. [email protected].

ORCID

Chang He: 0000-0002-9804-5455 Zhixiang Wei: 0000-0001-6188-3634 Jianhui Hou: 0000-0002-2105-6922 2133

DOI: 10.1021/acs.chemmater.8b00287 Chem. Mater. 2018, 30, 2129−2134

Article

Chemistry of Materials Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the NSFC (521734008, 21325419, 91333204, 1373181, 91633301, 51773047, and 21604017), and the Chinese Academy of Sciences (XDB12030200 and KJZD-EW-J01).

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DOI: 10.1021/acs.chemmater.8b00287 Chem. Mater. 2018, 30, 2129−2134