Effect of Substituents of Thienylene-Vinylene- Thienylene-Based

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C: Energy Conversion and Storage; Energy and Charge Transport

Effect of Substituents of Thienylene-VinyleneThienylene-Based Conjugated Polymer Donors on the Performance of Fullerene and Non-Fullerene Solar Cells Hee Su Kim, Tack Ho Lee, Jiwoo Yeop, Hye Won Cho, Jae Won Kim, Song Yi Park, Jong Baek Park, Jin Young Kim, and Do-Hoon Hwang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05184 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018

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The Journal of Physical Chemistry

Effect of Substituents of Thienylene-VinyleneThienylene-Based Conjugated Polymer Donors on the Performance of Fullerene and NonFullerene Solar Cells Hee Su Kim†,‡, Tack Ho Lee¶,‡, Jiwoo Yeop¶, Hye Won Cho¶, Jae Won Kim¶, Song Yi Park¶, Jong Baek Park†, Jin Young Kim*,¶, and Do-Hoon Hwang*,†

†

Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 46241, Republic of Korea ¶

Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea

Corresponding Author * E-mail address: [email protected], [email protected]

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ABSTRACT Semiconducting polymers consisting of (E)-1,2-di(thiophen-2-yl)ethene (TVT) derivatives and benzo[1,2-b:4,5-b']dithiophene with conjugated thiophene side chains (BDTT) were designed and synthesized to investigate the effect of fluorine and cyano groups in the 3position of the thiophene ring in TVT on the photovoltaic properties. The corresponding PBDTT-TVT, PBDTT-FTVT, and PBDTT-CNTVT copolymers containing TVT, di-fluoro TVT (FTVT), and di-cyano TVT (CNTVT), respectively, demonstrated considerable variations in optical, electrochemical, morphological, and charge transporting properties. PBDTT-FTVT showed suitable frontier orbital energy levels, favorable face-on orientation, and a well-mixed and smooth morphology in the blends with 3,9-bis(2-methylene-(3-(1,1dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-sindaceno[1,2-b:5,6-b']dithiophene (ITIC) and [6,6]-phenyl-C71-butyric acid methyl ester (PCBM). In contrast, PBDTT-CNTVT showed unfavorable frontier orbital energy levels and bimodal orientation in the thin film state, which interrupted efficient charge transport in organic photovoltaic devices. The device fabricated using PBDTT-FTVT exhibited the highest power conversion efficiency (PCE) of up to 6.50% with ITIC and a slightly lower PCE of 6.35% with PCBM.


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INTRODUCTION Polymer solar cells (PSCs) are a promising solar energy technology because of their light weight, low cost, and potential for fabricating flexible and large-area devices through solution processing.1-5 A typical bulk-heterojunction (BHJ) PSC comprises a p-type conjugated polymer as the donor and an n-type organic semiconductor as the acceptor. In the past two decades, fullerene derivatives have been the dominant n-type acceptor materials and fullerene-based PSCs have achieved power conversion efficiencies (PCEs) of over 10% because of their low-lying lowest unoccupied molecular orbital (LUMO), high electron affinity, and high electron mobility.6-9 Recently, non-fullerene acceptors (NFAs) are rapidly emerging as competitors of fullerene derivatives because of their advantages such as relatively easy synthesis, tunable energy levels, strong absorption in the visible region, and good morphological stability.10-13 In particular, PSCs using NFAs have made rapid progress in the past three years, mainly as a result of intensive research on the design and synthesis of new NFAs and the wide-bandgap (Eg ≥ 1.8 eV) donor polymers that have complementary absorption to that of the NFAs.14-18 (E)-1,2-Di(thiophen-2-yl)ethene (TVT) has recently been recognized as a promising electron donating building block for low-bandgap (Eg < 1.8 eV) polymers used in organic photovoltaics (OPVs) and organic thin film transistors (OTFTs). This is because the incorporation of TVT in the polymer backbone enhances its planarity and leads to strong π–π interactions, which in turn increases the crystallinity of the conjugated polymers.19-23 Furthermore, the physical and chemical properties of the conjugated polymers can be easily controlled by modification of the TVT building block with substituents such as alkyl, fluoro, and cyano groups.24-28 Among these TVT derivatives, those with fluoro and cyano groups in the 3-position of the thiophene ring in TVT were developed as OTFT materials to tune the



energy levels, control the morphology, and enhance the n-type characteristics. Recently, we reported the synthesis and characterization of diketopyrrolopyrrole-based polymers with cyano substituted TVT that showed an electron mobility of 1.2 cm2 V–1 s–1.29 Chen et al. have studied fluorinated TVT-naphthalenediimide copolymers for high mobility n-channel fieldeffect transistors with an electron mobility of 3.20 cm2 V–1 s–1.30 Despite these various developments in TVT derivatives, no attention has yet been paid to the effects of these substitutions developed for PSC materials with wide-bandgap. Therefore, it is worthwhile investigating the effect of substituents in TVT on the physical, optical, electrochemical, and morphological properties of TVT-based semi-conducting polymers for PSCs. In this study, a series of new wide-bandgap polymers, namely, poly{4,8-bis(5-(2octyldodecyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-alt-(E)-1,2-di(thiophen-2-yl) ethene}

(PBDTT-TVT),

poly{4,8-bis(5-(2-octyldodecyl)thiophen-2-yl)benzo[1,2-b:4,5-

b']dithiophene-alt-(E)-1,2-bis(3-fluorothiophen-2-yl)ethene} (PBDTT-FTVT), and poly{4,8bis(5-(2-octyldodecyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-alt-(E)-1,2-bis(3cyanothiophene-2-yl)ethene} (PBDTT-CNTVT), were designed and synthesized as shown in Scheme 1. To investigate the effect of the substitutions of the TVT units in BDTT-based polymers, fluorine and cyano groups were introduced to the 3-position of the thiophene ring in TVT. These TVT derivatives were then polymerized with the BDTT co-monomer having a long octyldodecyl branched side chain in order to enhance the solubility and control the morphology of the resulting polymers. The three copolymers, PBDTT-TVT, PBDTT-FTVT, and PBDTT-CNTVT, were characterized by UV−Visible absorption spectroscopy, cyclic voltammetry (CV), hole/electron mobility measurements, two-dimensional grazing-incidence X-ray diffraction (2D-GIXD), and atomic force microscopy (AFM). The photovoltaic characteristics of the resultant polymers were systematically studied in detail in terms of


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energy levels, molecular ordering, orientation, and morphology of the blended films. The device

using

PBDTT-FTVT

blended

with

either

3,9-bis(2-methylene-(3-(1,1-

dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-sindaceno[1,2-b:5,6-b']dithiophene (ITIC) or [6,6]-phenyl-C71-butyric acid methyl ester (PCBM) as the acceptor showed improved PCEs of 6.50 and 6.35%, respectively, as compared to the reference device using PBDTT-TVT as the donor polymer blended with ITIC or PCBM (PCEs of 5.62% and 4.13%, respectively). The fluorine substituent on the TVT moiety led to higher absorption co-efficiency, predominant face-on orientation, and a well-balanced hole/electron mobility ratio. The device using PBDTT-CNTVT as the donor copolymer blended with either ITIC or PCBM showed relatively poor PCEs of 0.05% and 1.35%, respectively, because of the low hole/electron mobility ratio and strong aggregation behavior of the blend films.



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EXPERIMENTAL SECTION Materials All reagents were purchased from Aldrich, Alfa Aesar, or TCI Korea, and used without further purification. Tetrakis(triphenylphosphine)palladium(0) was purchased from Strem. [6,6]-Phenyl-C71-butyric acid methyl ester (PCBM) and 3,9-bis(2-methylene-(3-(1,1dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-sindaceno[1,2-b:5,6-b']dithiophene (ITIC) were purchased from Organic Semiconductor Materials (OSM, Republic of Korea) and 1-Material, respectively. Solvents were dried and purified by fractional distillation over sodium/benzophenone and handled in a moisture-free atmosphere. Column chromatography was performed using silica gel (Electronic Materials Index, SL-60-60A). (E)-1,2-Bis(3-bromothiophen-2-yl)ethene, 2,6-dibromo-4,8-bis(5-(2octyldodecyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene (trimethylstannyl)thiophen-2-yl)ethene

(TVT),

(BDTT),

(E)-1,2-bis(5-

(E)-2,2'-(ethene-1,2-diyl)bis(5-

(trimethylstannyl)thiophene-3-carbonitrile) (CNTVT) were synthesized following previously reported procedures.29,31

Measurements 1

H and 13C NMR spectra were measured on a Varian Mercury Plus 300 MHz spectrometer.

Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry was performed on a Voyager DE-STR (Applied Biosystems) instrument. Thermal analyses were carried out on a TA Instrument Q600 (PH407 PUSAN, Korea Basic Science Institute (KBSI)) under inert N2 atmosphere with heating and cooling rates of 10 °C min−1. The molecular weights of the synthesized polymers were determined from the calibration curve


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based on polystyrene standards recorded by gel permeation chromatography (GPC) using the Shimadzu LC solution at a flow rate of 1.0 mL min−1 chlorobenzene solution at 40 °C. The UV-Visible absorption spectra were measured on a UV-1800 UV-VIS spectrophotometer. Cyclic voltammetry experiments were performed with a CH Instruments electrochemical analyzer in acetonitrile solutions containing 0.1 M tetrabutylammonium tetrafluoroborate (Bu4NBF4) as the supporting electrolyte, with Ag/AgNO3 as the reference electrode, a platinum wire as the counter electrode, and a platinum working electrode. Density functional theory (DFT) calculations were performed using the Gaussian 09W package with the Becke three-parameter Lee-Yang-Parr (B3LYP) function and the 6-31G(d) basis set to elucidate the HOMO and LUMO energy levels and backbone planarity. 2D-GIXD experiments were performed at the 3C beamlines of the Pohang Accelerator Laboratory (PAL), Korea. X-rays with a wavelength of 1.2217 Å (10.1485 keV) were used. The incidence angle (0.14–0.17°) was chosen to allow for complete penetration of the X-rays into the samples. The atomic force microscopy (AFM) images (1.5 µm × 1.5 µm) were obtained using a Veeco AFM microscope in the tapping mode.

Device fabrication and characterization Patterned ITO glasses were cleaned with distilled water, acetone, and isopropanol, in sequence. ZnO precursor solution was prepared using the sol-gel method, spin-coated onto the cleaned ITO substrate, and annealed at 110 °C for 10 min.32,33 To prepare the ITICblended BHJ films, the polymer was blended with ITIC in a 1:1 weight ratio (1:1.5 for PBDTT-TVT; 6 mg mL–1) in chlorobenzene (CB). To prepare PCBM-blended BHJ films, the polymer was blended with PCBM in a 1:1.5 weight ratio (12 mg mL–1) in a CB:1,8diiodooctane (97:3 v/v) mixed solvent. On top of the ZnO layer, BHJ films were spin-casted ACS Paragon Plus Environment


by hot solution processing at 80 °C in a N2-filled glove box. Next, the device was pumped down under vacuum (

Effect of Substituents of Thienylene-Vinylene- Thienylene-Based

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