Regioisomeric Non-Fullerene Acceptors Containing Fluorobenzo[c][1

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Regioisomeric Non-Fullerene Acceptors Containing Fluorobenzo[c][1,2,5]thiadiazole Unit for Polymer Solar Cells Wenkai Zhong, Baobing Fan, Jing Cui, Lei Ying, Feng Liu, Junbiao Peng, Fei Huang, Yong Cao, and Guillermo C. Bazan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12902 • Publication Date (Web): 06 Oct 2017 Downloaded from http://pubs.acs.org on October 8, 2017

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ACS Applied Materials & Interfaces

Regioisomeric Non-Fullerene Acceptors Containing Fluorobenzo[c][1,2,5]thiadiazole Unit for Polymer Solar Cells Wenkai Zhong,†,# Baobing Fan,†,# Jing Cui,‡ Lei Ying, Huang,† Yong Cao,† and Guillermo C. Bazan*,†,

†

*,†

Feng Liu,

⊥

Junbiao Peng,†,

‖

Fei

‖,§

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of

Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China ‡

Sinopec Shanghai Research Institute of Petrochemical Technology, Shanghai, 201208, China

⊥

Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai, 200240, P.

R. China ‖

MOE International Collaborative Laboratory for Advanced Functional Materials, South China

University of Technology, Guangzhou 510640, P. R. China §

Department of Chemistry and Biochemistry, Center for Polymers and Organic Solids,

University of California, Santa Barbra, California 93106, USA

1 ACS Paragon Plus Environment


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ABSTRACT: We designed and synthesized two isomeric non-fullerene acceptors, IFBR-p and IFBR-d. These molecular semiconductors contain indacenodithiophene (IDT) as the central unit, adjacent asymmetric 5-fluorobenzo[c][1,2,5]thiadiazole units, and are flanked with rhodanine as the peripheral units. The orientation of the two fluorine atoms (proximal, p, or distal, d), relative to IDT impacts most severely the film morphologies when blended with the electron-donating polymer PTzBI. Polymer solar cells based on PTzBI:IFBR-p give rise to a power conversion efficiency (7.3 ± 0.2%) that is higher than what is achieved with PTzBI:IFBR-d (5.2 ± 0.1%). This difference is attributed to the lower tendency for (over)crystallization by IFBR-p and the resulting more favorable morphology of the photoactive layer. These results highlight the subtle impact of substitution regiochemistry on the properties of non-fullerene acceptors through modulation of their self-assembly tendencies.

KEYWORDS: regiochemistry, fluorobenzo[c][1,2,5]thiadiazole, non-fullerene, acceptor, polymer solar cells


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INTRODUCTION The emergence of non-fullerene acceptors (NFAs) has revealed new opportunities for the fabrication of organic solar cells. 1 - 4 NFAs provide a range of merits, including versatile molecular structures that are convenient for scale-up and purification, tunable absorption coefficients with oscillator strengths higher than their fullerene counterparts, and absorption profiles that complement the donor polymer characteristics and thereby improve the light harvesting capability of the photoactive layer. Much of the molecular design of NFAs has focused on acceptor-donor-acceptor (A-D-A) type molecules, which may contain additional electron-deficient structural units on each side of the electron-donating core.5-7 The optical and electronic properties of such acceptors can be adjusted by modifying the molecular structures including the core units and the flanking groups 8 - 14 , and a series of NFAs containing 3ethylrhodanine flanked benzo[c][1,2,5]thiadiazole (BT) units have been developed and integrated into binary or ternary polymer solar cells (PSCs) with excellent power conversion efficiencies15-18. These opportunities have led to efforts to understand how molecular structure impacts optoelectronic properties.19-23 It has been established that the incorporation of fluorine atoms modifies both intramolecular features and intermolecular interactions that concurrently lead to improved photovoltaic performances.24-28 Incorporation of a fluorine atom into the 5- or 6-position of the BT unit provides a relevant platform to investigate the effects of the regiochemistry of fluorination on relevant NFA properties, both at the single molecule level and within the ensemble of molecules in the photoactive semiconducting layer.29-35 In this contribution, we investigate how the regio-chemistry of fluorine substitution impacts optoelectronic properties and film morphology by taking advantage of two A-D-A type NFAs containing the asymmetric 5-fluorobenzo[c][1,2,5]thiadiazole (FBT) unit, namely IFBR-p and



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IFBR-d (Figure 1). Here, “p” and “d” indicate that the fluorine atoms are situated “proximal” and “distal”, respectively, relative to a central indaceno[1,2-b:5,6-b']dithiophene (IDT) electrondonating core unit. As will be shown, the two regioisomers have comparable frontier molecular orbital energy levels; however IFBR-p exhibits a red-shifted absorption profile both in solution and in the solid state. Both acceptors can be used with the wide bandgap polymer PTzBI, which consists of benzodithiophene as the electron-donating unit and imide-functionalized benzotriazole unit as the electron-withdrawing unit.36 One finds that the PTzBI:IFBR-d blend attains a more pronounced crystallization and larger scale phase separation. These features cause PTzBI:IFBR-d to exhibit a lower photovoltaic performance than the PTzBI:IFBR-p counterpart.

RESULTS AND DISCUSSIONS Synthesis and Characterizations. Compounds IFBR-p and IFBR-d were obtained by Knoevenagel condensation of 3-ethylrhodanine with acceptor-donor-acceptor type intermediates functionalized with aldehyde end-groups. More critical are the requisite regiospecific intermediates, which require a multistep reaction sequence using 5-fluoro-3-methyl-2nitroaniline or 4-fluoro-2-nitroaniline as the starting materials. Complete details are provided in Scheme 1. Differences in their molecular structures are revealed in their 1H NMR spectra (Figure S1 in the supporting information (SI)), where one notes that the chemical shifts of protons nearby the fluorine atom change with the orientation of fluorine atom. The molecular structures of all the intermediates and target molecules were confirmed by NMR spectroscopy and matrixassisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF). No obvious thermal transition characteristics were observed from the differential scanning calorimetry measurements with the temperature ranging from 30 to 300 oC (Figure S2, SI).


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Scheme 1. Synthesis of IFBR-p and IFBR-d.

Theoretical Calculations. Density function theory (DFT) calculations at the B3LYP/6-31G* level were used to gain insight into structural and electrostatic differences between IFBR-p and IFBR-d. Alkyl groups in the molecules were truncated to methyl groups to simplify calculations. Potential energy surface (PES) scans were also employed to evaluate the conformational freedom of the FBT-rhodanine fragments as function of fluorine orientation (FBT-R-p and FBT-R-d, see Figure 1). From the PES plots of FBT-R-p, one notes that the relative energy is at a minimum at a dihedral angle of about 0o; the relative energy is ~1 kcal mol-1 higher at 180o (Figure 1c). For FBT-R-d there is a wider range of rotational isomers with similar energy to each other, from 0o to 24o and close to 180o (Figure 1d). Differences in the angular dependence of the PES of FBTR-p and FBT-R-d give rise to different distribution of conformers at 298 K (Figure S4, SI), where the peripheral rhodanine ring has a slight preference to orient away from thiadiazole 5 ACS Paragon Plus Environment


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moiety in IFBR-p and in the same direction for IFBR-d. These observations can be understood as the slightly different minimum energy of two regioisomers of IFBR-p and IFBR-d. For IFBR-p, the geometry at the minimum energy can be associated with the C-H···N interaction between the H-atom (in the bridged double bond) and N-atom (in the FBT unit). For IFBR-d, the geometry at the minimum energy can be associated with the simultaneous non-bonding interactions of CH···F that is between the H-atom (in the bridged double bond) and the F-atom (in the FBT unit), and the N···S that is between the S-atom (in the rhodanine unit) and N-atom (in the FBT unit). Such simultaneous non-bonding interactions may lead to the more preferential to flip over relative to FBT, and thus resulted in different molecular geometry. Additionally, the electrostatic potential (ESP) maps from DFT calculations show that the negative potential at the S atom (C=S in rhodanine unit) of IFBR-d (Figure 1f) is stronger than in IFBR-p (Figure 1e), which can be attributable to the different orientation of fluorine atoms within the molecular conjugated backbone. For IFBR-d, the fluorine atoms with strong electron-negativity are distal to the central core, which can lead to more negative ESP in the C=S in rhodanine ring than that of IFBR-p.


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Figure 1. Molecular structure of IFBR-p and IFBR-d (a, b), potential energy surface scan of the end-capping groups of FBT-R-p and FBT-R-d (c, d), and ESP maps (in Hartree units) of IFBR-p and IFBR-d (e, f).

Electrochemical and Optical Properties. The ionization potentials (IPs) and electron affinities (EAs) of IFBR-p and IFBR-d were estimated to be –5.64/–3.73 eV and –5.67/–3.74 eV, respectively, as evaluated by cyclic voltammetry (CV) measurement in thin films (Figure S9c, SI), indicating that the orientation of fluorine atom does not significantly change the frontier molecular orbitals. Figure 2a shows the normalized optical absorption spectra of IFBR-p and IFBR-d. One notes that IFBR-p exhibits an absorption maximum at 671 nm, which is red-shifted relative to IFBR-d (647 nm); a similar difference can be also observed in chlorobenzene (CB) solutions (Figure S5, SI). The absorption onset of IFBR-p and IFBR-d are 744 and 726 nm, corresponding to optical gaps of 1.67 and 1.71 eV, respectively. The absorption profiles of both molecules are complimentary to the absorption profile of the wide bandgap polymer PTzBI (Figure 2a), indicating that these molecules are good pairings with PTzBI to fabricate bulkheterojunction (BHJ) devices with broad light harvesting ability. Both the absorption spectra of the PTzBI:IFBR-p and PTzBI:IFBR-d (1:1.5, wt:wt) blend films processed without solvent additives of 1,8-diiodooctane (DIO) showed the combined absorption characteristics of each single material (Figure 2b). However, it is interesting to note that a new sharp peak located at 720 nm emerges for the thermally annealed (TA, at 120oC) PTzBI:IFBR-d blend film processed in the presence of the solvent additive DIO 0.5 vol%, while no obvious variation for the absorption profile of PTzBI:IFBR-p film processed with DIO (Figure 2b).



Figure 2c and d provide the absorption spectra of blends processed with CB in the presence of DIO (0.5 vol%) and thermally annealed at various temperatures. The UV-vis absorption of PTzBI:IFBR-p blend films did not present apparent variation upon thermal annealing at different temperatures (Figure 2c). From Figure 2d, one notes that the UV-vis absorption spectrum of the PTzBI:IFBR-d blend film processed with DIO did not show any shoulder peak beyond 700 nm before thermal treatment, however, one can clearly observe the emergence of the sharp peak at 720 nm for the PTzBI:IFBR-d blend film upon thermal annealing at 60, 80 and 120 oC. These findings implied that the emerged sharp absorption peak located at 720 nm might be attributable to the self-organization of IFBR-d molecules upon thermal treatment in the presence of DIO. (a) 1.5

(b) 1.5 IFBR-p (film)

Normalized Absorbance


PTzBI (film) IFBR-d (film) 1

0.5

0 300

400

500

600

700

1

o

TA @120 C

0.5

0 300

800

PTzBI:IFBR-p (w/o DIO) PTzBI:IFBR-p (w DIO) PTzBI:IFBR-d (w/o DIO) PTzBI:IFBR-d (w DIO)

400

500

Wavelength (nm)

(c) 1.5

(d) 1.5

As-cast

PTzBI:IFBR-p (w DIO)

TA @ 60 C


o

o

TA @ 120 C

0.5

0 300

400

500

700

800

As-cast

PTzBI:IFBR-d (w DIO) o

TA @ 80 C 1

600

Wavelength (nm)

o


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600

700

800

Wavelength (nm)

TA @ 60 C o

TA @ 80 C o

1

TA @ 120 C

0.5

0 300

400

500

600

700

800

Wavelength (nm)

Figure 2. Normalized UV-vis absorption spectra: pure films (a), PTzBI:IFBR-p and PTzBI:IFBR-d blend films without (w/o) and with (w) DIO upon thermal annealing (TA) at 120


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o

C (b), PTzBI:IFBR-p blend film processed from CB with DIO (c); and PTzBI:IFBR-d blend

film processed from CB with DIO (d).

To further investigate whether the emerged sharp peak located at 720 nm was originated from PTzBI, we measured the absorption spectra of neat films of PTzBI and IFBR-d casting from CB solution with or without DIO (Figure S6, SI). It is noted that the UV-vis absorption spectra of PTzBI films cast from CB with or without DIO are nearly identical (Figure S6a, SI). In comparison to the neat IFBR-d film cast from CB without DIO, one notes that the neat IFBRd film cast from CB containing DIO significantly red-shifted, associated with the emergence of a sharp peak located at 720 nm (Figure S6b, SI). These observations clearly demonstrated that the sharp peak originated from IFBR-d. We attribute the 720 nm peak to absorption from wellordered domains or different polymorphs of IFBR-d. That this peak is not observed for IFBR-p suggests that the two regioisomers exhibit different tendencies to organize within the film, with IFBR-d exhibiting a greater tendency to achieve more ordered phases. Furthermore, in order to identify the generality of the emergence of such sharp peak at about 720 nm, we blending IFBRd with three different polymers IFBT-TT32, PBTA-BO37, and PTB7-Th (molecular structures shown in Figure S8, SI) with the weight ratio of 1:1.5, and the measured UV-vis absorption spectra of these blends cast without or with DIO (0.5 vol%) were provided in Figure S8 (SI). Similar to what observed in the UV-vis absorption of the PTzBI:IFBR-d blend film, the UV-vis absorption spectra of the other three blend films of IFBT-TT:IFBR-d, PBTA-BO:IFBR-d and PTB7-Th:IFBR-d cast from CB with DIO also displayed red-shifted absorption associated with the emergence of the new sharp peak at about 720 nm relative to those cast without DIO. These



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findings demonstrated that the emergence of new absorption peak at about 720 nm is not specifically for the PTzBI:IFBR-d film.

Photovoltaic Properties. How fluorine orientation impacts photovoltaic performance was investigated

by

fabricating

polymer

solar

cells

with

device

architecture

of

ITO/PEODT:PSS/PTzBI: IFBR-p or IFBR-d/PFNDI-Br/Al. Photoactive layers (PTzBI:acceptor = 1:1.5, wt:wt) with thickness ~ 95 nm were deposited atop PEDOT:PSS layers from a CB solution in the presence of solvent additive DIO (0.5 vol%). A survey of processing conditions led to optimal performance upon thermally annealing the resulting films at 120 oC for 10 minutes. A thin layer of PFNDI-Br (~ 5 nm) was integrated as the cathode interfacial layer to facilitate charge extraction.38,39 Finally, an 80 nm-thick aluminum cathode was deposited through thermal evaporation under vacuum. Measurements of current density-voltage (J-V) characteristics were carried out under an illumination of AM 1.5 G, 100 mW cm-2, and the results of these studies are provided in Figure 3a and corresponding photovoltaic parameters are collected in Table 1. Devices based on as-cast film exhibit similar moderate performances, with PCEs of 5.4 ± 0.2% (VOC = 1.05 V, JSC = 9.5 ± 0.3 mA cm-2, FF = 54.3 ± 0.8%) and 5.0 ± 0.4% (VOC = 1.03 V, JSC = 9.1 ± 0.3 mA cm-2, FF = 54.0 ± 3.0%) for IFBR-p and IFBR-d, respectively. Thermal treatment increases the PCE of IFBR-p based devices to 7.3 ± 0.2%, with an open-circuit voltage (VOC) of 1.00 V, a short circuit current density (JSC) of 11.6 ± 0.3 mA cm-2, and a fill factor (FF) of 62.3 ± 0.5%. In contrast, IFBR-d devices show slightly enhanced JSC and slightly decreased VOC values thermal treatment; the combination of these variations results in a nearly unchanged PCE of 5.2 ± 0.1% (VOC = 0.99 V, JSC = 9.4 ± 0.1 mA cm-2, FF = 55.9 ± 1.3%). Insight into the


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electronic characteristics of the blends was sought by measuring charge carrier mobilities as determined by the space-charge-limited current (SCLC) model. The hole/electron mobilities obtained in this way were 5.8×10-5/1.5×10-4 cm2 V-1 s-1 and 5.9×10-5/4.9×10-5 cm2 V-1 s-1 for PTzBI:IFBR-p and PTzBI:IFBR-d blend films, respectively, see Figure S11 for relevant J1/2–V characteristics. The higher electron mobility for IFBR-p is consistent with the larger FF of IFBRp based devices. It is also worth noting that the VOC of the resulting devices slightly dropped after thermal treatment, which might be attributable to the variation of film morphology and thus the non-radiative or radiative recombination loss.

However, clarifying this issue requires

systematical photophysical characterizations and the relevant film morphology investigations, which are beyond the scope of our current study. As shown in Figure 3b, the external quantum efficiency (EQE) of the optimal PTzBI:IFBRp device covers a wavelength range from 300 to 750 nm, with maximal values of ~ 60% at 560 nm. The EQE response of IFBR-d based devices extends up to 770 nm, in agreement with the emergence of new aggregation peak observed in Figure 3b, however they are lower than 40%. The JSC values integrated from the EQE curves are 11.3 and 8.8 mA cm-2 for devices based on PTzBI:IFBR-p and PTzBI:IFBR-d, respectively, which agree with those measured from the J-V characteristics (Figure 3b). In addition, it is noted that the PTzBI:IFBR-d blend film exhibited relatively low EQE response at low energy band (over 700 nm), which can be attributed to the emerged absorption band peaked at about 720 nm. This EQE response is consistent in low energy band (over 700 nm) that extended absorption of the blend film of PTzBI:IFBR-d regarding to that of PTzBI:IFBR-p (Figure 2b), since the absorption profile of pure IFBR-d film is blue-shifted regarding to that of the pure IFBR-p film (Figure 2a). Considering that the EQE response is not only correlated to the light harvesting ability of the blend film, but also correlated



with various factors such as the exciton diffusion and separation, charge transport and collection, etc, thus it seems reasonable that the (over)crystallization is unfavorable for the charge transportation, as can be confirmed by SCLC measurements; thus, the EQE response for PTzBI:IFBR-d blend was relatively low regarding to the PTzBI:IFBR-p blend film. In contrast, for the blend film of PTzBI:IFBR-p that lacks of (over)crystallization, the blend film showed favorable morphology with nano-fibrillar characteristics and appropriate phase separation, leading to higher charge mobilities and more efficient charge extraction, and thus high EQE response. (a)

(b)

PTzBI:IFBR-p

-2

PTzBI:IFBR-d PTzBI:IFBR-p

60

12

40

8

20

4

PTzBI:IFBR-d

-8

SC

EQE (%)

-4

-2

(mA cm )

Current Density (mA cm )

0

Integrated J

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-12 0

0.2

0.4

0.6

0.8

0 300

1

400

Voltage (V)

500

600

700

0 800

Wavelength (nm)

Figure 3. J-V curves of devices based on PTzBI:IFBR-p and PTzBI:IFBR-d (1:1.5, wt:wt) with 0.5 vol% DIO under the illumination of AM 1.5 G, 100 mW cm-2 (a) and the corresponding EQE curves (b).

Table 1. Detailed Photovoltaic Parameters of Solar Cells Processed with 0.5 vol% DIO. D:A

VOC

JSCa

FFa

PCEa

PCEbest

(1:1.5)

(V)

(mA cm-2)

(%)

(%)

(%)

PTzBI:IFBR-p

1.00

11.6 ± 0.3

62.3 ± 0.5

7.3 ± 0.2

7.44

PTzBI:IFBR-d

0.99

9.4 ± 0.1

55.9 ± 1.3

5.2 ± 0.1

5.28


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a

Statistical parameters were calculated from 8 individual devices.

Morphology Study. We examined possible differences in blend morphologies by using transmission electron microscopy (TEM) and grazing incidence X-ray diffraction (GIXD). Blends in these studies were prepared under exactly the same conditions as those used for device fabrication. For as-cast films, there is no obvious difference in their TEM images, where both films appear uniform and contain fibrillar nano-structures (Figure S13 in the SI). After thermal annealing, the PTzBI:IFBR-p blend film retains similar fibrillar nano-structural characteristics (Figure 4a), however, for PTzBI:IFBR-d one can observe formation large clusters with size of 200-300 nm across the entire film (Figure 4b). Our thinking is that the emergence of these aggregated domains is responsible for the emergence of the peak at 720 nm in the absorption spectra of PTzBI:IFBR-d blend films. Such large phase separation is typically undesirable, for example it may hinder efficient exciton breakup and thus reduce EQE values.40,41 GIXD patterns of the thermally annealed blend films cast with 0.5 vol% DIO are shown in Figure 4c and d, and the line-cut profiles are provided in Figure S14 (see the SI). The PTzBI:IFBR-p blend film exhibited a π-π stacking (010) reflection at 1.77 Å-1 in out-of-plane (OOP) direction and a (100) diffraction peaked at 0.28 Å-1 in in-plane (IP) direction, which indicated the preferential face-on orientation. As for the PTzBI:IFBR-d blend film, the (010) reflection in OOP direction is weaker, and the (010) peak located at 1.80 Å-1 is also observed in IP direction, implying the mix of face-on and edge-on orientation for the π-π stacking (Figure 4d). Additionally, from the PTzBI:IFBR-d blend film (Figure 4d), one notes a series of diffraction peaks (0.38, 0.79, and 1.15 Å-1) emerged along the OOP direction, suggesting the formation of highly ordered crystalline properties. These observations are consistent with the



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emerged sharp peaks in the absorption spectra (Figure 2b) and larger phase separation in TEM image (Figure 4b).

Figure 4. TEM images (a, b) and GIXD patterns (c, d) of blend films based on PTzBI:IFBR-p and PTzBI:IFBR-d (1:1.5, wt:wt) with 0.5 vol% DIO and annealed at 120 oC for 10 min.

CONCLUSION In summary, we prepared and examined two 5-fluorobenzo[c][1,2,5]thiadiazole based electronaccepting small molecules, IFBR-p and IFBR-d, which differ in the relative positions of the fluorine atoms relative to the centers of their molecular structures. The differences in orientation lead to negligible differences in electrochemical properties and minor differences in their optical absorption in solution. However, one finds a much greater tendency for aggregation of IFBR-d when blended with the PTzBI donor polymer, as determined by TEM, grazing incidence X-ray diffraction and the emergence of sharper shifted absorption features. We point out that the 14 ACS Paragon Plus Environment

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orientations of the fluorine atoms lead to differences in the calculated conformational diversity and the electrostatic potential of the molecules. These two molecular characteristics are likely to influence the preferred lattice orientations and thereby the kinetics of crystallization, although a detailed picture of this process within the confines of a thin film remains poorly understood. Despite these uncertainties, we attribute the lower device PCE of PTzBI:IFBR-d blends, relative to PTzBI:IFBR-p, to a less optimal bulk-heterojunction morphology. Overall, the results provided here demonstrate how a trivial shift in the location of fluorine substituents leads to quite different solid-state properties by virtue of supramolecular organizational tendencies.

ASSOCIATED CONTENT Supporting Information. Experimental Section and Figure S1-S23. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author [email protected] (LY), [email protected] (GCB). Author Contributions #

These authors contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT



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This work was financially supported by the Natural Science Foundation of China (No. 21520102006, 91633301 and 51673069). L.Y. thanks Guangdong Natural Science Fund for Distinguished Young Scholar (No. 2017A030306011). Portions of this research were carried out at beamline 7.3.3 at the Advanced Light Source, Lawrence Berkeley National Laboratory, which was supported by the DOE, Office of Science, and Office of Basic Energy Sciences.

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23 Environment ACS Paragon Plus

Regioisomeric Non-Fullerene Acceptors Containing Fluorobenzo[c][1

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