Subscriber access provided by UNIV OF WINNIPEG
Article
Low Bandgap Small Molecule as Non-Fullerene Electron Acceptor Composed of Benzothiadiazole and Diketopyrrolopyrrole for All Organic Solar Cells Jae Woong Jung, and Won Ho Jo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b02480 • Publication Date (Web): 11 Aug 2015 Downloaded from http://pubs.acs.org on August 17, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 7
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
Chemistry of Materials
Low Bandgap Small Molecule as Non-Fullerene Electron Acceptor Composed of Benzothiadiazole and Diketopyrrolopyrrole for All Organic Solar Cells Jae Woong Jung†‡, Won Ho Jo†* †
Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151–744, Korea. ABSTRACT: Small molecules composed of benzothiadiazole and diketopyrrolopyrrole have affirmative optoelectronic properties as non-fullerene electron acceptors in photovoltaic device such as high crystallinity, decent electron mobility, low bandgap and proper molecular energy levels. The organic solar cell device combined with PTB7 as electron donor exhibits a promising PCE of 5.0%, demonstrating that the small molecules are promising electron acceptors for non-fullerene organic solar cells.
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
1. INTRODUCTION
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 61 62 63 64 65 66 67 68 69 70
of low-cost production of organic solar cells. In this regard, benzothiadiazole (BT) and diketopyrrolopyrrole (DPP) are attractive units for synthesis of small-molecular non-fullerene electron acceptors, because they are commonly used for synthesis of various semi-conducting organic materials.25−29 Since they have strong electron-withdrawing nature, the combination of the two units makes it possible to obtain n-type organic semiconductors, which can be utilized as non-fullerene electron acceptors in NFOSCs. They also possess excellent optoelectronic characteristics such as strong light absorption, highly conjugated and coplanar structure, and good photochemical stability.30-31 Particularly, the synthesis and purification of BT and DPP units are relatively simple and easy. Recently, a few small-molecular electron acceptors based on BT or DPP have been reported with promising PCEs up to 4 %.32−37 Despite their appropriate energy levels and extended light absorption range as compared to fullerene derivatives, most of NFOSCs exhibited unsatisfactory JSC lower than 10 mA/cm2, which are far below that of typical fullerenebased organic solar cells, probably because of its low crystallinity and poor charge-transporting property. Therefore, it is imperative to develop better electron acceptors to further improve JSC and thus the PCE of NFOSCs. In this work, we synthesized a small molecular electron acceptor using 4,7-dithien-2 -yl-2,1,3benzothiadiazole (DTBT) as a core unit and DPP as a flanking unit. We also synthesized another small mo lecule co mpr ising fluor o -sub stituted DT BT (DTDfBT) and DPP to investigate the effect of fluorine substitution on the photovoltaic performance of NFOSCs. Both electron acceptors exhibit low bandgap of 1.5 eV with decent absorptivity in the range of 500 to
For the last decade, organic solar cells have intensively been investigated as a renewable energy source because they can generate the electricity from sunlight at very low cost.1−4 Rapid progress in organic solar cells based on bulk heterojunction (BHJ) structure comprising a conjugated organic semiconductor and a fullerene derivative as electron donor and acceptor, respectively, have led a dramatic advance in the power conversion efficiency (PCE).5−9 Although [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) has been recognized as the most promising electron acceptor in organic solar cells, its weak absorption in UV−Vis region, unfavorable energy levels with low tunability, and high production cost limit further performance enhancement of fullerenebased conventional organic solar cells.10,11 To overcome these drawbacks of PCBM, non-fullerene electron acceptors have been developed to replace the conventional fullerene derivatives in the organic solar cells.12−15 Recently, significant progress has been made in development of non-fullerene electron acceptors based on solution-processable small-molecular organic semiconductors, because their optical properties are easily tuned with tailored molecular orbital energy levels, which may enhance the photocurrent (JSC) and photovoltage (VOC) of non-fullerene organic solar cells (NFOSCs).16−20 However, the state-of-the-art solutionprocessable non-fullerene electron acceptors exhibit PCEs lower than 4% even when combined with high performance electron donor polymers, although a few non-fullerene electron acceptors with >5% PCE have very recently been reported.21−24 Furthermore, most of small-molecular electron acceptors require multi-step synthesis and purifications with low yield, which may increase the production cost and thus diminish the merit ACS Paragon Plus Environment
Chemistry of Materials
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
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
Figure 1. (a) Chemical structures, (b) UV-Vis absorption spectra, (c) cyclic voltammograms, (d) the energy band diagram relative to the vacuum level of molecules studied in this work, and (e) schematic device configuration used in this work.
800 nm. Since the combination of DTBT and DPP exhibits high electron affinity and favorable energy levels as an electron acceptor, it is expected to quench efficiently the photoluminescence (PL) of p-type conjugated polymer. In addition, their coplanar backbone structures along with extended conjugation may afford high crystallinity to the small molecules. As a result, the NFOSCs based on the electron acceptors exhibit promising PCE up to 5.00% with a high JSC over 12 mA/cm2, which is among the highest photocurrent of organic solar cells based on small-molecular nonfullerene electron acceptor. 2. RESULTS and DISCUSSION The chemical structures of non-fullerene electron acceptors (DTBT(TDPP)2 and DTDfBT(TDPP)2) and ptype conjugated polymer (PTB7) used in this study are shown in Figure 1a. The non-fullerene electron acceptors composed of DTBT and DPP were synthesized via the Stille coupling reaction with high yield over 75% (see synthetic details and characterizations in Supporting Information). As shown in Fig. 1b, UV–Vis absorption spectra of the two electron acceptors reveal almost the identical absorption in dilute chloroform solution, while they exhibit broad and extended light absorption over 800 nm in film state, indicating that DTBT(TDPP)2 and
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
Page 2 of 7
Figure 2. (a) UV-Vis absorption and photoluminescence spectra of the BHJ blend films, (b) GIXD spectra of DTBT(TDPP)2 and DTDfBT(TDPP)2 thin films.
DTDfBT(TDPP)2 are effectively aggregated in the film state due to strong intermolecular π−π interaction derived from highly conjugated backbone structure of the small molecules. Since it is well known that high crystallinity of conjugated molecule is beneficial to efficient charge transport to the electrode, high crystallinities of DTBT(TDPP)2 and DTDfBT(TDPP)2 in conjunction with their low bandgaps would lead to high photocurrent generation of NFOSCs. When the molecular energy levels of DTBT(TDPP)2 and DTDfBT(TDPP)2 were measured by cyclic voltammetry, the ionization potentials (IPs) of DTBT(TDPP)2 and DTDfBT(TDPP)2 are −5.70 and −5.85 eV, respectively, as estimated from an empirical equation of EIP=−(Eonset(ox.)+4.8) eV (Fig. 1c). The electron affinities (EAs) of DTBT(TDPP)2 (−4.18 eV) and DTDfBT(TDPP)2 (−4.33 eV) were estimated by extracting the optical bandgap from the corresponding ionization potentials. It should be noted that the estimation of EA from optical and electrochemical measurements only provide approximations of molecular fundamental levels.38 Given that the IP and EA of PTB7 are −5.15 and −3.50 eV, respectively, the two electron
2 ACS Paragon Plus Environment
Page 3 of 7
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
Chemistry of Materials
27 Table 1. Photovoltaic parameters of NFOSCs based 28 on DTBT(TDPP)2 and DTDfBT(TDPP)2. Acceptor
Figure 3. (a) Current density–voltage curves and (b) the IPCE spectra of NFOSCs fabricated using PTB7:DTBT(TDPP)2 and PTB7:DTDfBT(TDPP)2 blends as an active layer.
acceptors exhibit large EA offsets to PTB7 (0.68 and 0.83 eV for DTBT(TDPP)2 and DTDfBT(TDPP)2, respectively). Since the exciton binding energy in organic semicondcutor is 0.3−0.5 eV,39 the EA offsets by DTBT(TDPP)2 and DTDfBT(TDPP)2 afford enough driving force for efficient exciton dissociation and effective electron transfer in the active layer. The UV−Vis absorption and PL spectra of the two blend films (PTB7:DTBT(TDPP)2 and PTB7:DTDfBT(TDPP)2) are shown in Figure 2a. Both blend films show broad light absorption of 500−850 nm, while PTB7:DTDfBT(TDPP)2 exhibits slightly redshifted absorption than PTB7:DTBT(TDPP)2. As expected, both blend films show excellent PL quenching over 90% of PTB7, indicating that both DTBT(TDPP)2 and DTDfBT(TDPP)2 are highly compatible with PTB7 for high performance NFOSCs. One of the most important properties of conjugated small molecule for optoelectronic application is crystallinity which may be derived from well-ordered molecular structure. The grazing-incident X-ray
VOCa) [V]
JSCa) 2
[mA/cm ]
[%]
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 61 62 63 64 65 66 67 68 69 70 71 72 73
diffraction (GIXD) patterns of DTBT(TDPP)2 and DTDfBT(TDPP)2 reveal that both molecules exhibit distinct lamellar diffractions at qz = 0.31, 0.61, 0.92, 1.23 Å−1 corresponding to (100), (200), (300), (400) reflections, respectively, indicating that both molecules are highly crystalline in solid film state (Fig. 2b). Particularly, DTDfBT(TDPP)2 shows a strong peak of (010) diffraction at 1.62 Å−1 with a π−π stacking distance of 0.39 nm while DTBT(TDPP)2 exhibits much weaker (010) diffraction. This intensity difference arises from the difference in the crystal orientation: DTBT(TDPP)2 crystals take predominantly edge-on orientation while DTDfBT(TDPP)2 crystals adopts mixed orientation of face-on and edge-on. Note here that the face-on orientation affords more efficient charge transport in the solar cell devices. The high crystallinities of DTBT(TDPP)2 and DTDfBT(TDPP)2 were further evidenced by strong melting and crystallization peaks in differential scanning calorimetry (DSC) thermograms (Fig. S1). For measurement of photovoltaic performance of the electron acceptor-based solar cells, we fabricated solution-processed NFOSCs with an inverted architecture (ITO/ZnO/active layer/MoO3/Ag), where the active layer is PTB7:DTBT(TDPP)2 or PTB7:DTDfBT(TDPP)2. The detailed fabrication procedure of NFOSCs is described in Supporting Information. The current density–voltage (J– V) curves and corresponding device parameters measured under AM1.5G illumination (100 mW/cm2) are summarized in Fig. 3 and Table S1. When odichlorobenzene (DCB) was used as a processing solvent, the devices fabricated from PTB7:DTBT(TDPP)2 (1:2 in weight) and PTB7:DTDfBT(TDPP)2 (1:2 in weight) show very low PCEs (
