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Crystallization and Optical Compensation by Fluorinated Rod Liquid Crystals for Ternary Organic Solar Cells Xunfan Liao, Feiyan Wu, Lie Chen, and Yiwang Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05887 • Publication Date (Web): 05 Aug 2016 Downloaded from http://pubs.acs.org on August 6, 2016

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The Journal of Physical Chemistry

Crystallization and Optical Compensation by Fluorinated Rod Liquid Crystals for Ternary Organic Solar Cells Xunfan Liaoa, Feiyan Wua, Lie Chen*a,b and Yiwang Chena,b

a

College of Chemistry/Institute of Polymers, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China

b

Jiangxi Provincial Key Laboratory of New Energy Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China

*Corresponding author. Tel.: +86 791 83968703; fax: +86 791 83968830. E-mail: [email protected] (L. Chen).

ABSTRACT Cyano biphenyl type liquid crystal molecules used as additives for polymer solar cells (PSCs) have been proved as a promising strategy to increase donor crystallinity, short circuit current density (Jsc) and result in a higher power conversion efficiency (PCE) of PSCs. However, they would sacrifice light absorption in long wavelength region due to their large bandgap and weak absorbance. To overcome this shortage, a fluorinate

donor-acceptor-donor

rodlike

liquid

crystal

(RLC),

5,6-difluoro-4,7-bis(5'-hexyl-[2,2'-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole (DFBT-TT6), is designed and synthesized as additive for bulk-heterojunction organic solar cells. DFBT-TT6 shows a broad absorption from 300 nm to 650 nm. Quite different from the commonly used liquid crystal additives, DFBT-TT6 can effectively compensate the energy loss of P3HT in the short wavelength without sacrificing the absorbance in long wavelength region. Remarkably, grazing incident X-ray diffraction (GIXRD), electron microscopy and X-ray photoelectron spectroscopy (XPS) prove that driven by the cooperative effect of the RLC orientation and its surface segregation,

a

favorable

morphology

with

improved

crystallinity

of

poly(3-hexyl)thiophene (P3HT) in the active layer has been developed. Therefore,

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incorporation of DFBT-TT6 into P3HT: [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend achieved an improved PCE from 2.34% to 3.91% with an increased Jsc, fill factor (FF) and charge mobility.

1. INTRODUCTION Over the past two decades, polymer solar cells (PSCs) have intensively been developed for its advantages of lightweight, low cost, mechanical flexibility and would be large scale manufactured1-5. Recently, the single cells power conversion efficiencies (PCEs) over 10% have been reported due to synergistic effects through the optimization of energy levels, donor band gaps, and active layer morphology6-8. Though continuous improvement in PCE, the performance of those photovoltaic devices are still restricted by the low light absorption of active layer, weak crystallinity of donor materials, and difficulty in developing the desired nanosized phase separation of donor/acceptor materials9. Therefore, to achieve industrialization of organic photovoltaics (OPVs), further improvement in organic solar cells performance is still demanded. Currently, strategies to overcome the organic materials' absorption shortcomings are still in the development phase. For instance, the light absorption and harvesting can be improved by developing novel and promising low-bandgap conjugated copolymers10. Otherwise, tandem PSCs (polymer solar cells) consisting of two or more single cells with complementary absorption wavelength ranges are stacked together and show impressively improved efficiency11,12. However, fabrication of the tandem architecture is more complex, which leads to increased costs. Recently, an elegant alternative strategy of combining two complementary donor materials directly into a single active layer which is known as ternary cell can also increase the absorption. Those third donor materials mostly are the infrared (IR) sensitizers. The IR sensitizers can be used to extend the wide bandgap polymers light absorption into the near IR region13-16. The ternary method would not break the detailed balance limit, instead of improving the photon harvesting in thickness limited active layers, resulting in higher short circuit current densities (Jsc) and PCE in a simple single cells10.


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In addition to having better light absorption, the donor crystallinity and active layer morphology are also directly correlation with the performance of BHJ solar cells17-19. Presently, there are many methods such as thermal and solvent annealing as well as incorporation of LC additives have been used to improve the crystallinity and morphology of the blended film20-22. Compared to other methods, the LC additives technique is regarded as a promising approach to formation of nanosized phase separation morphology due to its nature of self-assemble ability17,21,22. According to the molecular structure, LC additives could be divided into two types, liquid crystalline copolymers and liquid crystalline small molecules23,24. Compared to copolymers, small molecules have promising advantages in easily structure design, well-defined structure, easily refine and higher charge carrier mobilities25. The most used LC small molecules applied for P3HT:PCBM-based PSCs are cyano biphenyl type

molecules,

such

as

nematic

liquid

crystals

(NLCs)

5CB

(4-cyano-40-pentylbiphenyl) and 8CB (4-cyano-40-octylbiphenyl)26, and discotic liquid crystals (DLCs)24,

27

. Despite those LC molecules can manipulate the

morphology of the activelayer, they usually sacrifice some absorption in long wavelength region due to their large band energy which could limitation for short-circuit current (Jsc). Inspired

by

successful

donor-acceptor-donor

application

(A-D-A)

rodlike

of LC

LC

additives, (RLC)

a

fluorinated

small

molecule

5,6-difluoro-4,7-bis(5'-hexyl-[2,2'-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole (DFBT-TT6) is designed and synthesized to optimize active layer morphology and improve performance in P3HT:PCBM based ternary PSCs. Benzothiadiazole (BT) is chosen as acceptor unit for its strong electron accepting ability, planar backbone and commercial availability28,29. DFBT-TT6 is easily synthesized as shown in Scheme 1. Compared to those common LC small molecules24,27, the p-conjugated RLC small molecules DFBT-TT6 exhibits a broad absorption in the wavelength range from 300 nm to 650 nm, which would not sacrifice the light response, but well provide a complementary absorption in the active layer instead. It has reported that the low surface energy materials, such as fluorinated compounds, prefer to transfer to the



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air/liquid interface during coating30. Therefore, incorporation of fluorine atoms into the RLC small molecule can further manipulate the morphology of active layer. Grazing incident X-ray diffraction (GIXRD), electron microscopy and X-ray photoelectron spectroscopy (XPS) demonstrate that when small amount of fluorinated DFBT-TT6 as additive blend with P3HT:PCBM, the cooperative effect of the RLC orientation and the surface segregation of the F atoms not only can improve the crystallinity of the P3HT chains, but also can drive the DFBT-TT6 to spontaneous assemble on blended film surface.

2. RESULTS AND DISCUSSION Scheme 1 shows the synthetic route of LC small molecule DFBT-TT6. The detailed synthesis of DFBT-TT6 is presented in Supporting Information. The product structure has been verified by structural analysis, as shown in Figure S1-2 (Supporting Information). Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and polarized optical microscopy (POM) were used to investigate the thermal and phase behaviors of DFBT-TT6. The temperatures with 5% weight loss for DFBT-TT6 is up to 400 ºC, as shown in Figure 1a, indicating that the thermal stability of DFBT-TT6 is sufficient for long-term photovoltaic application. In the DSC measurement as shown in Figure 1b, two endothermic peaks at 112.8 and 137.6 °C upon heating are emerged, which corresponding to LC mesophase and isotropic phase transitions of DFBT-TT6, respectively. The liquid crystallinity of DFBT-TT6 can also been confirmed by the POM photomicrographs (see Figure 1c and d). The liquid crystalline molecule DFBT-TT6 showed birefringent in the liquid crystalline states during heating. The absorption spectra of the DFBT-TT6 and P3HT solution with the same concentrations (0.01 g/L) and films are shown in Figure 2, the correlated optical parameters are presented in Table S1. As shown in Figure 2a, the absorption intensity of DFBT-TT6 solution is only slightly weaker than that of P3HT. Additionally, benefited from its D–A molecular structures, DFBT-TT6 exhibit an absorption in the wavelength range from 300 nm to 600 nm in chloroform. In the solid state, the


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absorption of DFBT-TT6 become broader and extend to more than 700 nm compared to its absorption in solution, as shown in Figure 2b. Additionally, a new vibronic peak in 552 nm emerged, indicating that π-π* stacking interaction exist in the solid films. The position of the vibronic peak of DFBT-TT6 is similar with P3HT, where the maximum absorption peaks are located at ~520 nm. More interestingly, DFBT-TT6 shows a relatively strong absorption coefficients in the region from 300 nm to 430 nm compared to P3HT, which can effective compensate the energy loss of the P3HT in the short wavelength. The absorption of DFBT-TT6 film after annealing with different temperature also are presented in Figure S3. DFBT-TT6 film exhibit a distinct bathochromic shift after thermal annealing in the liquid crystalline state (130 °C) compared to the pristine film. The optical band gaps (Egopt) is estimated to 1.86 eV from DFBT-TT6 films absorption edges (ca. 665 nm), as shown in Table S1. Cyclic voltammetry (CV) measurement was performed to calculate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of DFBT-TT631. As shown in Figure 3a, the onset oxidation potentials (Eox) and onset reduction potentials (Ered) are 1.19 V and −0.75 V for DFBT-TT6. Thus, the HOMO and LUMO for DFBT-TT6 are calculated to be −5.59 and −3.65 eV, respectively (shown in Table S1). The electrochemical band gaps of DFBT-TT6 is calculated to be 1.94 eV. It is worth noting that the energy levels of DFBT-TT6 are positioned in between P3HT and PCBM (see Figure 3b). Such a cascade energy schematic of P3HT:DFBT-TT6:PCBM matrix provides a feasibility of exciton dissociation and charge carriers transfer at both interfaces of P3HT/PCBM and DFBT-TT6/PCBM32,33. The application of the rodlike liquid crystalline molecule in organic solar cells was explored. Photovoltaic properties of DFBT-TT6:PCBM based devices were investigated with a configuration of ITO/ZnO/active layer/MoO3/Ag. DFBT-TT6 based devices show very low PCE of 0.4–0.6% with low FF and Jsc as shown in Figure S4, and the corresponding parameters are summarized in Table S2. This extremely low photovoltaic performance mainly results from the poor blended film



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due to the very low molecular weight of DFBT-TT6. To investigate how RLC additives affects the performance of PSCs with a P3HT:PCBM:DFBT-TT6 BHJ layer, photovoltaic devices with the inverted structure were fabricated. Scheme S1 shows the schematic structure of solar cells as well as the chemical structures of the materials that used for device fabrication. A thickness of ~120 nm for active layers were obtained after spin-coated from their 1,2-dichlorobenzene (DCB) solutions. The current density−voltage (J−V) characteristics of P3HT:PCBM:DFBT-TT6-based solar cells without annealing treatment were measured, as shown in Figure 4a. The corresponding photovoltaic parameters of ternary solar cells with DFBT-TT6 are presented in Table 1. Obviously, the performances of PSCs are increased by doping DFBT-TT6 as LC additives. The open-circuit voltage (Voc) is kept at 0.6 V while Jsc and FF values of ternary PSC are increased from 6.27 mA cm-2 to 7.89 mA cm-2 and from 61.5% to 65.6% by doping 4 wt % DFBT-TT6, respectively, resulting in a higher PCE of 3.14%. The J-V characteristics of PSCs that were measured without illumination are shown in Figure 4b. The leakage current of the device with DFBT-TT6 is extraordinary restrained, and the 4% weight fraction of DFBT-TT6 yields the lowest leakage current. It is indicates that the recombination of carriers could be restrained by DFBT-TT6. Moreover, a lower leakage current suggests that charge losses decrease, which result in a larger shunt resistance (Rsh) and higher FF17,34. The Rsh increases from 529.6 Ω cm2 for pristine P3HT:PCBM to 2308.7 Ω cm2 with the addition of 4% DFBT-TT6. The series resistance (Rs) of the pristine P3HT:PCBM solar cells is about 3.8 Ω cm2, but reduces to 1.9 Ω cm2 with the incorporation of 4% DFBT-TT6 (see Table 1). Therefore, the the improvement of Jsc and FF could be owing to the increased Rsh, reduced Rs and leakage currents. In order to further improve the performance of PSCs, the performance of P3HT:PCBM:4 wt%DFBT-TT6 based devices treatment with different temperature for 10 min were investigated. The device configuration is also ITO/ZnO/active layer/MoO3/Ag. The J−V characteristics of ternary solar cells with thermal treatment are illustrated in Figure 4c and the corresponding photovoltaic parameters are


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summarized in Table 2. The pristine P3HT:PCBM based device annealed in 150 °C as the standard solar cells is used for comparison. The standard PSCs show a best PCE of 3.18% with Voc = 0.628 V, Jsc = 8.12 mA cm−2 and FF = 62.4%. Compared to the standard PSCs, the Jsc and FF values of ternary solar cells with 4 wt% DFBT-TT6 are significantly enhanced with different temperature treatment, leading to a better PCE. Intriguingly, after annealed at liquid crystalline state temperature of 130 °C, the performance of the ternary solar cells further improved to 3.91% with Jsc = 9.27 mA cm-2, FF = 67.3% and Voc = 0.626. In addition, after annealed at liquid crystalline state temperature, the Rs of the device decreases from 3.8 Ω cm2 to 3.1 Ω cm2 while the Rsh increases from 643.3 Ω cm2 to 1513.2 Ω cm2, consequently, contributing to a highest Jsc. This was also confirmed by the external quantum efficiency (EQE) spectra of the standard solar cells and the ternary PSCs with a liquid crystalline state temperature treatment, as shown in Figure 4d. The calculated current from EQE is good agreement with the values measured from J–V characteristics. Compared to the standard cells, the EQE spectra of ternary solar cells is increased in the region from 390 nm to 630 nm. The improved efficiency can be attributed to the self-assembled liquid crystalline molecules, which can increase P3HT crystallinity and develop a beneficial interpenetrating network. The UV-vis absorption spectra of the active layers can be applied to detect the polymer chains' conformation and level of orientation35. Figure 5a shows absorption spectra of P3HT:PCBM films with and without different doping concentration of RLC additives. The absorption spectra of P3HT:PCBM films with 4% RLC additives after thermal annealing are presented Figure 5b. There are three vibronic peaks A0–2, A0–1 and A0–0 located at 523, 558 and 608 nm in the absorption spectra, respectively36,37. It has reported that the existence of the peak at 608 nm indicate that there are strong π-π interaction between P3HT chains and infer that P3HT has a high degree of crystallinity38,39. The amplitude ratio of A0–0/A0–1 and A0–0/A0–2 of the active layers with DFBT-TT6 are shown in Table 3. Compared to the standard P3HT:PCBM film, both the A0–0/A0–1 and A0–0/A0–2 ratios increase for 4%



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DFBT-TT6 doping film. After annealed at 130 °C, the A0–0/A0–1 and A0–0/A0–2 ratios increase more. The increased A0–0/A0–1 and A0–0/A0–2 ratios indicate that a small quantity of DFBT-TT6 could increase the crystallization of P3HT chains. H-aggregate model was used to further investigated the degree of crystallinity and the ordering of P3HT:PCBM:DFBT-TT6 blend film. The free exciton bandwidth (W) can be calculated from the equation:

A=

A0 −0  1 − 0.24W / EP  =  A0−1  1 + 0.073W / EP 

2

where EP is the vibrational energy of the main intramolecular transition corresponding to a symmetric vinyl stretch and the value is assumed as a constant of 180 eV40. Table

3 shows the calculated free exciton bandwidth W. It is found that the W for P3HT:PCBM:4%DFBT-TT6 film decreased from 158.2 meV to 132.72 meV, in contrast to the pristine P3HT:PCBM film. After annealed at liquid crystalline state temperature, the W decreased to 124.6 meV. As reported, a reduced W suggest similar interchain ordering in polymer40. Therefore, the P3HT chain ordering increased after the addition of 2–4 wt% DFBT-TT6, and more ordered P3HT chain arrangement is obtained when 4 wt% DFBT-TT6 is incorporated as well as treatment with liquid crystalline state temperature. The result confirms that when DFBT-TT6 incorporated into the P3HT:PCBM blend films, the crystallization and ordering of P3HT chains are improved, which benefit for charge mobility, Jsc, FF and PCE. Additionally, the as-cast P3HT:PCBM films processed with the RLC molecule after thermal treatment, compared to the standard film of P3HT:PCBM with treatment at 150 °C, shows a red-shift absorption peaks with significantly increased intensity. It is also implied that incorporation of RLC molecule with thermal annealing can increase crystallization of P3HT and could improve the spectral overlap with solar emission, which is in agreement with their EQE spectra. To further understand the morphology of the active layers and confirm the effect


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of DFBT-TT6 on the ordering of the P3HT chains, Out-of-plane grazing incident X-ray diffraction (GIXRD) measurements of P3HT:PCBM films with or without DFBT-TT6 were performed. The X-ray diffraction (XRD) patterns of the pristine DFBT-TT6 film are shown in Figure 6a. DFBT-TT6 displays a strong diffraction peak at 2θ around 3.54°, corresponding to d-spacing between molecular layers is 2.77 nm, indicating a high crystallinity of DFBT-TT6. After annealing at 110 °C, the intensity of this peak becomes stronger. When the film annealed at 130 °C from the liquid crystalline state, a new diffraction peak at 5.88° emerged, indicating a better arrangement of the molecules. Further increasing the annealing temperature to 150 °C, the peak at 3.54° disappeared and the peak of 5.88° became much weaker. This is because the annealing temperature exceeded the melting temperature of DFBT-TT6 (137.6 °C see in Figure 1b) and lost the feature of the liquid crystal. The out-of-plane GIXRD of P3HT:PCBM film with or without various doping concentration of RLC additive are presented in Figure 6b. The diffraction peak positions of P3HT are at 2θ = 5.5°, 10.8° and 16.4°, corresponding to the (100), (200) and (300) reflections of the lamellae, respectively. When add the RLC molecules into the active blend, the intensity of the P3HT (100) reflection enhance (Figure 6b), indicative of an improvement in P3HT crystallinity. Figure 6c shows the out-of-plane GIXRD of P3HT:PCBM:4 wt%DFBT-TT6 film undergoing thermal treatment. The peak in 3.54 degree disappeared, which illustrated that the additive DFBT-TT6 dispersed well in P3HT:PCBM:4%DFBT-TT6 blend film after annealing. Meanwhile, the (100) diffraction increased when the ternary blended film after thermal annealing, especially annealed at the liquid crystalline state temperature. This increased intensity can also partly owing to the contribution of DFBT-TT6 due to the peak position of (100) for P3HT very close to the peak position of DFBT-TT6. From the GIXRD we can find that the thermal-stimulate orientation of the RLC DFBT-TT6 can promote packing arrangement of the P3HT chains and form a well-intermixed phase to facilitate charge transfer and transportation. Additionally, the in-plane cut diffraction patterns are also performed (see Figure S5). No diffraction peaks located at low angles was observed, implying that the P3HT crystal orientation has not changed.



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In order to gain further insight into the effect of DFBT-TT6 on P3HT crystal size in blend film, the crystalline correlation length (CCL) of active layer was calculated from Scherrer's formula41:

CCL =

2π FWHM

where FWHM is the full-width-at-half-maximum of the fitted Pseudo-Voigt function. CCL is a value of the crystallite size. Fig. S6 and Table S3 show the fitting peaks and parameters calculated from GIXRD. The CCL of P3HT increase from 8.49 nm to 9.26 nm as the DFBT-TT6 content up to 6%. A larger CCL value of 10.96 nm was achieved when the P3HT:PCBM:4%DFBT-TT6 blend film annealed at the liquid crystalline state temperature of 130 °C. The increased CCL values indicate that DFBT-TT6 can induce P3HT to form larger size crystals. Atomic force microscopy (AFM) was used to further investigate the effect of RLC additives on the performance of ternary solar cells. The surface morphology of the blend films are shown in Figure 7. Figure 7a-d are AFM images of P3HT:PCBM with or without different doping concentrations DFBT-TT6. The pristine P3HT:PCBM film exhibit an root-mean-square (RMS) roughness of 9.1 nm. With the addition of 2 and 4 wt% DFBT-TT6, the active layers roughness decrease to 7.5 and 6.8 nm, respectively. This smooth surface can provide good contact between electrodes and active layer, favoring for more efficient charge transport, reducing Rs (see Table 1) and improving performance42. Upon further increasing the doping concentrations of DFBT-TT6 to 6 wt%, the roughness of ternary blend films increases to 10.8 nm. This rough surface would be infaust for charge separation and transport, leading to decreased PCE. Figure 7e-h are AFM images of P3HT:PCBM standard film and with 4 wt%DFBT-TT6 annealed from different temperature. Compared to P3HT:PCBM:4 wt%DFBT-TT6 film without annealing, the RMS roughness of the blend films with 110, 130 and 150 °C annealing decreased to 6.2, 5.4 and 3.8 nm, respectively. The P3HT:PCBM film with 150 °C annealing (a RMS of 3.2 nm) is provided for comparison in Figure 7h. It is notable that quite different from the samples annealed from other temperatures, the one annealed from the liquid


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crystalline state temperature (130oC) shows a lot of nanofibers dispersed in surface of the active film, as shown in Figure 7f, due to the assemble of the RLC with F atoms (discuss later). These nanofibers induced by the rodlike liquid crystals are favorable for charge carrier transport, thus correlated to improved Jsc, FF, and PCE43. Transmission electron microscopy (TEM, Figure 8 and Figure S7) measurement was also conducted to clearly detect the morphological changes. For the P3HT:PCBM film with 4 wt%DFBT-TT6 loading annealed from liquid crystalline state (130°C), the nanofibers induced by the rodlike liquid crystals DFBT-TT6 are also observed in the set image in Figure 8c (the scale bar is 500 nm). Furthermore, compared with the films treated from110 and 150 °C, a favorable nano-scale phase separation is also developed undergoing 130 °C annealing, as shown in Figure 8. Therefore, an optimal domain size can be formed in ternary film, in which the domain size is sufficiently large for an effective continuous interpenetrating D–A charge transport pathway but small enough to match the short effective exciton diffusion length44,45。 To probe the origin behind the surface assembly of active layer treated with fluorinated RLC molecules during thermal annealing, X-ray photoelectron spectroscopy (XPS) was performed. The films were prepared by spin coating the P3HT:PCBM:4 wt%DFBT-TT6 solutions on ITO/ZnO-coated glass substrates. Figure

9a shows the survey XPS spectra of the P3HT:PCBM:4 wt%DFBT-TT6 film with different temperature treatment. The high-resolution XPS spectra clearly exhibit F 1s and N 1s peaks at a binding energy of ~687 and ~400 eV, respectively, as shown in

Figure 9b and c. With the temperature increased from room temperature (no annealing) to 150 °C, the intensity of both F 1s and N 1s peaks gradually increased in favor of a low surface energy of fluorinated DFBT-TT6. The increased F and N element content in the surface of active layer demonstrates that fluorinated DFBT-TT6 migrates to the interface of the ternary film during thermal annealing. The surface aggregation of the fluorinated DFBT-TT6 can also be detected by contact angle measurements in Figure S8. The contact angles and calculated surface energies of the P3HT:PCBM:4 wt%DFBT-TT6 film with different temperature treatment on ITO/ZnO substrates are shown in Table S4. The surface energy of the standard film of



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P3HT:PC61BM is 25.42 mN/m. The surface energy of ternary film decreases to 24.98 mN/m with the temperature increasing. From the results we can conclude that the optimized morphology of the active layer not only originates from the orientation of rodlike liquid crystals improving the crystallinity of P3HT, but also is correlated with the surface organization of DFBT-TT6 driven by the F atoms, which can supported by the assembled nanofibers on the surface of the active layer. Furthermore, the surface assembly could be develop a vertical phase separation in the active layer, which is favorable for hole selective abstraction and electron blocking at the cathode interface. To verify the beneficial exciton dissociation and charge transfer in the blends, steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements of the blend films were carried out, as shown in Figure 10a. The PL emission peak located at ~ 650 nm is attribute to P3HT:PCBM blend film. The emission intensity decreases significantly after blending with DFBT-TT6 and incorporation of 4% DFBT-TT6 shows the most quenching of the PL spectrum, indicating that the versatile BHJ interfaces can offer multi-channels for charge transfer more efficiently46. Compared with the optimal P3HT:PCBM:4% DFBT-TT6 blend film, the PL intensity was increased with the doping concentrations added to 8 wt%. The increased PL intensity was owing to the adverse morphology of the active layer, confirmed by the AFM result (Figure 7). TRPL was used to research the charge-transfer dynamics (see Figure 10b). The fitted lifetime of the P3HT:PCBM film is 585 ps. When incorporation of 4% DFBT-TT6 into P3HT:PCBM, the fitted lifetime become to 550 ps which is slightly faster than that (585 ps) for the pristine P3HT:PCBM film. It indicates that the DFBT-TT6 in the ternary blend film reveals better intermixing, contributing to the faster charge transfer kinetics. The value of the lifetime for P3HT:PCBM with 4% DFBT-TT6 after annealing in 130 ºC are 470 ps, which is the shortest time compared to formers. The P3HT:PCBM:4 wt%DFBT-TT6 blend film upon annealed at liquid crystalline state temperature of 130 °C shows the shortest exciton lifetime, suggesting that the fastest exciton dissociation in the active layer. This favorable charge separation efficiency could contribute to the higher Jsc and photovoltaic performance.


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Since the high degree crystallinity of the P3HT, the homogenous morphology and vertical phase separation of the active layer can influence the carrier mobility of active layers47,48, the charge mobility of the active layers with and without DFBT-TT6 are calculated by the space-charge-limited current (SCLC) method according to the Mott–Gurney equation49. Figure S9 shows the electron-only and hole-only J1/2~V curve of the active layers with or without different doping concentrations of liquid crystalline molecules. The electron mobility increases when incorporate DFBT-TT6 into active layer. The corresponding calculated electron mobility for pristine P3HT:PCBM and P3HT:PCBM with 2, 4, 6, 8 wt% DFBT-TT6 are 2.83 × 10−4, 3.07 × 10−4, 5.6 × 10−4, 6.38 × 10−4 and 8.66 × 10−4 cm2 V−1 s−1, respectively. In addition,

Figure S9c shows the electron-only J1/2~V characteristics of the P3HT:PCBM film with 150 ºC annealing and the modified film with different temperature treatment. The electron mobility are 3.82 × 10-4 cm2 V−1 s−1 for the standard device of P3HT:PCBM film with 150 ºC annealing. Compared to the standard device, thermal annealing insured the devices based on P3HT:PCBM:4 wt%DFBT-TT6 with the higher electron mobility, and a highest electron mobility of 2.1 × 10-3 cm2 V−1 s−1 was achieved when the film was annealed at the liquid crystalline state temperature (130 °C). Similarly, the maximum hole mobility uh = 1.63 × 10-3 cm2 V−1 s−1 is achieved for the blend with 4% DFBT-TT6 after annealed at the liquid crystalline state temperature. These fitting electron and hole mobility values are shown in Table

S5 and S6. The increased electron and hole mobilities are good agreement with the increased crystallinity of the P3HT chains and the possible existence of vertical phase separation in the blend films, which are important for performance improvement in organic solar cells.

3. CONCLUSIONS In summary, a novel fluorinated D-A-D type rodlike liquid crystal small molecule (LCSM) DFBT-TT6 has been designed and synthesized for BHJ PSCs. DFBT-TT6 exhibits broader absorption compared to commonly used LC additives, which would not sacrifice the light response, but well provide a complementary absorption in the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

active layer instead. The efficiency of ternary PSCs with appropriate DFBT-TT6 doping concentrations show an obvious improvement, especially annealed at the liquid crystalline state temperature of DFBT-TT6. The impressive increased performance is owing to the orientation of the LC that can improve the crystallinity of the P3HT chains, while the surface segregation of the F atoms can drive the DFBT-TT6 to spontaneous assemble on the surface of the blended film to promote a well-intermixed microphase morphology. It is demonstrated that this LCSM not only improved crystallinity of P3HT without sacrificing light absorbance but also had the propensity to develop a vertical phase separation in the active layer, which benefit for charge separation and transportation. Therefore, incorporation of the fluorinated D-A-D type rodlike liquid crystal small molecule into the active blend can enable us with better understanding on the morphology control of the ternary blend films and provide a promising material for high performance device.

Electronic Supplementary Information (ESI) available The detailed experimental sections and the corresponding characterization are in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements This work was financially supported by the National Science Fund for Distinguished Young Scholars (51425304), National Natural Science Foundation of China (51263016, 51473075 and 21402080), National Basic Research Program of China (973 Program 2014CB260409), and the Natural Science Foundation of Jiangxi Province (20143ACB20001 and 20151BAB203016).

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 1. Synthetic Route of DFBT-TT6


Page 20 of 33

Page 21 of 33

(b)

(a)

115.8 °C

8

endoHeat flow (W/g) exo

100

5% mass loss at 400 °C Mass loss (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60 40 20 200

400

600

800

DFBT-TT6

6 4 2 0 -2 -4

112.8 °C 137.6 °C

-6 -8 50

Temperature (°C)

100

150

Temperature (°C)

c）

d）

50 um

50 um

Figure. 1 (a) Thermogravimetric analysis (TGA) plot of DFBT-TT6 with a heating rate of 10 °C min-1 under nitrogen atmosphere; (b) Differential scanning calorimetry (DSC) thermograms of DFBT-TT6 at 5 °C min-1; Polarized optical micrographs (POM) of DFBT-TT6 in room temperature (c) and (d) in the liquid crystalline state temperature of 130 °C.


(a)

P3HT sol DFBT-TT6 sol

0.8

Absorbance

0.6 0.4 0.2 0.0 P3HT

-0.2 300

400

DFBT-TT6

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

500

600

700

Absorbance coefficient (cm-1)


Page 22 of 33

(b)

P3HT film DFBT-TT6 film

5

1.6x10

5

1.2x10

4

8.0x10

4

4.0x10

0.0 300

400

Wavelength (nm)

500

600

700

800

Wavelength (nm)

Figure 2. (a) Absorption spectra of DFBT-TT6 and P3HT in CHCl3 solution and (b) in the solid state.


(b) -2

LUMO=-3.65

-0.0002

-0.0004 -2.0 -1.5 -1.0 -0.5 0.0

0.5

1.0

1.5

-3.65

-4 Ag -4.2 -5

-5.0 -5.3

-6

-5.59

-4.4

-5.93

-7

2.0

Potential (V vs Ag/AgCl)

-3.91

-8

ITO -4.7

ZnO ZnO

0.0000

-3.0

-3

PC61BM

2

HOMO=-5.59

-2.3

DFBT-TT6

0.0002

electron

P3HT

Electronic Energy (eV)

(a) Current (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MoO3

Page 23 of 33

hole -7.7

Figure 3. (a) Cyclic voltammogram (CV) of the DFBT-TT6 film in a 0.1 mol/L Bu4NPF6/CH3CN solution at a scan rate of 50 mV s-1; (b) The HOMO and LUMO energy levels of P3HT, DFBT-TT6, PCBM, MoO3, ZnO, and the work functions of the ITO cathode and Ag anode (curved arrows indicate the possible pathways of charge carrier transport in the ternary blend).



(b) 1000

(a)

0

-4

Current (mA/cm2)

Current (mA/cm2)

-2

DFBT-TT6 in P3HT:PCBM

100

DFBT-TT6 in P3HT:PCBM 0 wt% 2 wt% 4 wt% 6 wt% 8 wt%

-6 -8

10 1 0.1

0 wt% 2 wt% 4 wt% 6 wt% 8 wt%

0.01 1E-3 1E-4

-10 0.0

1E-5

0.2

0.4

0.6

-2

-1

(d)

1

2

P3HT:PCBM@150 °C P3HT:PCBM:4 wt% DFBT-TT6@130 °C

0 P3HT:PCBM@150 °C P3HT:PCBM:4 wt% DFBT-TT6@110 °C P3HT:PCBM:4 wt% DFBT-TT6@130 °C P3HT:PCBM:4 wt% DFBT-TT6@150 °C

60

EQE (%)

-2

0

Voltage (V)

Voltage (V) (c)

Current (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

-4 -6

40

20

-8 0

-10 0.0

0.1

0.2

0.3

0.4

0.5

0.6

300

400

Voltage (V)

500

600

700

Wavelength (nm)

Figure. 4 (a) Current density−voltage (J−V) characteristics of BHJ-PSCs with various DFBT-TT6 doping concentrations; (b) Dark current–voltage (J–V) characteristics of BHJ-PSCs

with

various

DFBT-TT6

doping

concentrations;

(c)

Current

density−voltage (J−V) characteristics of BHJ-PSCs with different temperature treatment; (d) External quantum efficiency (EQE) spectra of P3HT:PCBM standard device and with 4 wt%DFBT-TT6 doping concentrations followed by 130 °C annealing.


Page 25 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Device Performance Parameters for Ternary BHJ-PSCs with Different DFBT-TT6 Doping Concentrations. DFBT-TT6 in

Jsc

Voc

FF

PCE (%)

Rs

Rsh

P3HT:PC61BM

(mA/cm2)

(V)

(%)

best averagea

(Ω cm2)

(Ω cm2)

0 wt%

6.27

0.606

61.5

2.34

2.19

3.8

529.6

2 wt%

7.05

0.608

62.1

2.66

2.56

2.8

903.8

4 wt%

7.89

0.606

65.6

3.14

3.04

1.9

2308.7

6 wt%

7.51

0.608

58.1

2.65

2.58

3.0

559.2

8 wt%

7.17

0.597

56.4

2.41

2.3

3.9

328.3

a

Average values of ten devices

Table 2. Device Performance Parameters for P3HT:PC61BM:4wt%DFBT-TT6 Annealing in Different Temperature. DFBT-TT6 in

Jsc

Voc

FF

P3HT:PC61BM

(mA/cm2)

(V)

(%)

0 + 150 ºC

8.12

0.628

62.4

3.18

4% + 110 ºC

8.93

0.607

65.2

4% + 130 ºC

9.27

0.626

4% + 150 ºC

8.29

0.627

a

PCE (%)

Rs

Rsh

(Ω cm2)

(Ω cm2)

3.12

3.8

643.3

3.53

3.46

19.8

686.6

67.3

3.91

3.82

3.1

1513.2

62.6

3.25

3.2

3.2

10058.3

best

averagea

Average values of ten devices



(a) A0-2

Absorbance (a.u.)

1.0

A0-1

0.8 0.6

A0-0

DFBT-TT6 in P3HT:PCBM 0 wt% 2 wt% 0.2 4 wt% 6 wt% 0.0 8 wt% 0.4

300

400

500

600

700

800

700

800

Wavelength (nm)

(b)

4 wt% DFBT-TT6 in P3HT:PCBM A0-2

1.0

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 33

A0-1

0.8 0.6 0.4 0.2 0.0 300

A0-0

no annealing 110 °C 130 °C 150 °C P3HT:PCBM@150 °C 400

500

600

Wavelength (nm) Figure. 5 (a) Absorption spectra of P3HT:PCBM:DFBT-TT6 films with various DFBT-TT6 doping concentrations from 0 to 8 wt%; (b) Absorption spectra of P3HT:PCBM:4 wt%DFBT-TT6 films with different temperature annealing from 110 °C to 150 °C.


Page 27 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3 Peak positions of A0–0 and A0–1 transitions from UV-vis absorption spectra, and A0–0/A0–1 intensity of the active layers with different fractions of DFBT-TT6, and with 4% DFBT-TT6 after annealing with different temperature. active layer

λA0-1

λA0-0

A0-0/A0-1

W

A0-0/A0-2

(nm)

(nm)

P3HT:PCBM

548

600

0.55

158.2

0.43

+2% SMLC +4% SMLC

550 550

602 602

0.65 0.61

116.73 132.72

0.5 0.48

+6% SMLC +8% SMLC

549 548

600 598

0.52 0.53

171.33 167.1

0.41 0.42

+4%@110 ºC +4%@130 ºC

552 552

602 602

0.63 0.63

124.6 124.6

0.51 0.51

+4%@150 ºC P3HT:PCBM@150 ºC

551 550

601 602

0.61 0.58

132.73 145

0.5 0.46

(meV)



(a) DFBT-TT6 DFBT-TT6@110 °C DFBT-TT6@130 °C DFBT-TT6@150 °C

Intensity (a.u.)

4000 3000 2000 1000 0 5

10

15

20

25

30

2θ (degrees)

(b)

(100)


Intensity (a.u.)

30000

20000

(200)

(300)

10

15

10000

0 0

5

20

25

30

2-Theta (degrees) (c) 40000

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33

(100)

30000

P3HT:PCBM@150 °C P3HT:PCBM:4 wt% DTBT-TT6@110 °C P3HT:PCBM:4 wt% DTBT-TT6@130 °C P3HT:PCBM:4 wt% DTBT-TT6@150 °C

(200)

(300)

20000 10000 0 0

5

10

15

20

25

30

2-Theta (degrees)

Figure. 6 (a) X-ray diffraction (XRD) patterns of DFBT-TT6 film with different temperature treatment; (b) Out-of-plane grazing incident X-ray diffraction (GIXRD) of ternary films with different DFBT-TT6 doping concentrations; (c) Out-of-plane GIXRD of P3HT:PCBM standard film and P3HT:PCBM:4 wt%DFBT-TT6 with thermal annealing.


Page 29 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a)

b)

c)

d)

-30 nm

30 nm

e)

f)

g)

h)

-20 nm

-20 nm

Figure. 7 5 µm × 5 µm tapping-mode Atomic force microscopy (AFM) topography height images of (a) P3HT:PCBM as-cast, (b) P3HT:PCBM with 2 wt%DFBT-TT6, (c) P3HT:PCBM with 4 wt%DFBT-TT6, (d) P3HT:PCBM with 6 wt%DFBT-TT6 (e) P3HT:PCBM with 4 wt%DFBT-TT6 annealed at 110 °C, (f) annealed at 130 °C and (g) annealed at 150 °C, (h) The standard PSCs of P3HT:PCBM annealed at 150 °C. The thermal treatments were all performed under N2 atmosphere.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure. 8 Transmission electron microscopy (TEM) images of the active layers with P3HT:PCBM:4 wt%DFBT-TT6 (a) as cast, (b) annealed at 110 °C, (c) annealed at 130 °C (the insert figure scale bar is 500 nm), (d) annealed at 150 °C. The scale bar is 200 nm.


Page 30 of 33

Page 31 of 33

(a)

4 wt%DFBT-TT6 in P3HT:PCBM

Intensity (a.u.)

120000

no annealing 110 °C 130 °C O1s 150 °C

100000 80000

C1s S2p

60000 40000

F1s N1s

20000 0 800

600

400

200

0

Binding Energy (eV) (b) no annealing 110 °C 130 °C 150 °C

Intensity (a.u.)

12000 10000

F1s

8000 6000 4000 2000 695

690

685

680

Binding Energy (eV) (c)

N1s

no annealing 110 oC 130 oC 150 oC

6000

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5000 4000 3000 2000 406

404

402

400

398

396

394

Binding energy (eV) Figure. 9 (a) Survey X-ray photoelectron spectra (XPS) and high-resolution XPS of (b) F 1s, (c) N 1s on the surface of P3HT:PCBM:4 wt%DFBT-TT6 blended film with different temperature treatment.



(a)

PL Intensity (a.u.)


550

600

650

700

750

800

Wavelength (nm) (b) P3HT:PCBM P3HT:PCBM:4% DFBT-TT6 P3HT:PCBM:4% DFBT-TT6 @130 °C

1000

PL (counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 33

100

10

0

10

20

30

40

50

Time (ns) Figure.

10

(a)

Steady-state

photoluminescence

(PL)

spectra

of

P3HT:PCBM:DFBT-TT6 films with various DFBT-TT6 doping concentrations from 0 to 8 wt%; (b) Time-resolved photoluminescence (TRPL) decay spectra of P3HT:PCBM and P3HT:PCBM:4%DFBT-TT6 film with and without thermal annealing. The fitted lifetimes were τ = 585, 550 and 470 ps for P3HT:PCBM, P3HT:PCBM:4%DFBT-TT6 and P3HT:PCBM:4%DFBT-TT6 film after annealed at 130 °C.


Page 33 of 33

Table of content: P3HT:PCBM@150 ºC

Ag

Ag

Ag

Ag

MoO3 P3HT:PCBM:DFBT-TT6

Ag

P3HT:PCBM@150 °C +4% DFBT-TT6@130 °C

5

+ -

4

P3HT:PCBM:4%DFBT -TT6 @150 ºC

J0.5 (A0.5/cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.1 × 10-3 m2 v-1 s-1 3

3.82 × 10-4 m2 v-1 s-1

2 1

ZnO 0

Glass/ITO 0

1

2

3

4

Voltage (V)


5

6

Crystallization and Optical Compensation by ... - ACS Publications

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