Research Article www.acsami.org
Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
All-Small-Molecule Solar Cells Incorporating NDI-Based Acceptors: Synthesis and Full Characterization Jisu Hong,†,∇ Yeon Hee Ha,‡,∇ Hyojung Cha,∥ Ran Kim,‡ Yu Jin Kim,⊥ Chan Eon Park,† James R. Durrant,∥ Soon-Ki Kwon,*,§ Tae Kyu An,*,# and Yun-Hi Kim*,‡ †
POSTECH Organic Electronics Laboratory, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea ‡ Department of Chemistry and RINS and §Department of Materials Engineering and Convergence Technology and ERI, Gyeongsang National University, Jinju 660-701, Republic of Korea ∥ Department of Chemistry, Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, U.K. ⊥ Center for Nanoscale Materials, Argonne National Laboratory, 9700 S. Cass Avenue, Lemont, Illinois 60439, United States # Department of Polymer Science & Engineering and Department of IT Convergence, Korea National University of Transportation, Chungju 380-702, Republic of Korea S Supporting Information *
ABSTRACT: A series of naphthalene diimide (NDI)-based small molecules were synthesized as nonfullerene acceptors and incorporated in all-small-molecule solar cells. Three NDI-based small molecules, NDICN-T, NDICN-BT, and NDICN-TVT, were designed with different linkers between two NDI units to induce the different conjugation length and modulate the geometric structures of the NDI dimers. The small NDI-based dimer electron acceptors with slip-stacked structures that facilitate π−π stacking interactions and/or hinder excessive aggregation exhibited different morphological behaviors, such as miscibility or crystallinity in bulk heterojunction blends with 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (DTS-F) electron donors. The photovoltaic devices prepared with NDICN-TVT gave the highest power conversion efficiency (PCE) of 3.01%, with an open-circuit voltage (Voc) of 0.75 V, a short-circuit current density (Jsc) of 7.10 mA cm−2, and a fill factor of 56.2%, whereas the DTS-F:NDICN-T and DTS-F:NDICN-BT devices provided PCEs of 1.81 and 0.13%, respectively. Studies of the charge-generation properties, charge-transfer dynamics, and charge-transport properties for understanding the structure−property relations revealed that DTS-F:NDICN-TVT blend films with welldeveloped domains and well-ordered crystalline structures performed well, whereas an excessive miscibility between DTS-F and NDICN-BT disrupted the crystallinity of the material and yielded a poor device performance. KEYWORDS: all-small-molecule solar cell, nonfullerene acceptor, NDI-based small molecule, bulk heterojunction morphology, charge-transfer dynamics
■
dielectric constant of organic semiconducting materials.3,4 For electron donors, a wide range of conjugated polymers or small molecules have been synthesized in an effort to lower the band gap and improve intramolecular charge transfer (ICT) and intermolecular charge transport, and power conversion efficiencies (PCEs) exceeding 10% for conjugated polymer donors and 9% for small molecules have been achieved, whereas electron acceptors are typically fullerene derivatives, such as [6,6]-phenyl-C61-butyric acid methyl ester, [6,6]phenyl-C71-butyric acid methyl ester, and indene-C60-bisadduct,
INTRODUCTION Solar energy has been explored as a clean and sustainable future energy source over the past decade.1,2 Solution-processed organic photovoltaic devices show promise as lightweight, flexible, large-area devices with potential utility in wearable or built-in photovoltaic devices.3−6 High-performance organic solar cell designs have been achieved through tremendous synthetic strategies for producing photoactive materials and device engineering, including device configuration and interlayer selection.7−13 Bulk heterojunction (BHJ) active layers consisting of blends of electron donors and electron acceptors are considered to be the most efficient active layers for use in organic solar cells due to the limited exciton diffusion length (∼10 nm) caused by low © XXXX American Chemical Society
Received: October 22, 2017 Accepted: December 1, 2017
A
DOI: 10.1021/acsami.7b16004 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces which have high electron affinities and form films with good morphologies for efficient charge separation and electron transport.14−20 However, the high synthetic costs, poor structural tunability, poor light absorption properties, and morphological instability under thermal stress have been considered as limitations of fullerene derivatives as an electron acceptor on the applicable photovoltaic devices.21−23 Alternative electron acceptor materials are needed to replace fullerene-based acceptors, reduce synthetic costs, obtain complimentary absorption spectra among the electron donor and acceptor materials for better light harvesting, and facilitate energy-level tuning for a higher open-circuit voltage.21,23 Novel nonfullerene acceptors that include a variety of electron-deficient moieties have been blended with conjugated polymer electron donors to obtain photovoltaic performances that surpass those of fullerene acceptors.24−26 However, bulk heterojunction solar cells that employ nonfullerene smallmolecule acceptors and small-molecule donors have not been extensively studied.27−31 Semiconducting small molecules form well-defined intermolecular structures, have readily tunable energy levels, are easy to synthesize, and provide low batch-tobatch variations in the products, and high performances of solar cells incorporating semiconducting small molecules have been reported.9,10,17−22 However, small-molecule donor:small-molecule acceptor blends are difficult to optimize, and the chargetransfer and charge-transport properties tend to be limited.27,29 Therefore, only a few studies of all-small-molecule solar cells have been reported. More recently, Yang et al. have reported the highest PCE yet achieved, 9.08%, using a new small molecule as an electron donor and IC-C6IDT-IC as an electron acceptor.27 The high photovoltaic performances obtained from the new wide-band-gap small-molecule donor and low-bandgap small-molecule acceptor relied on good morphological control and demonstrated the possibility of fabricating highperformance all-small-molecule solar cells. Badgujar et al. reported a notable PCE of 7.09% using BDT3TR as the electron donor and O-IDTBR as the electron acceptor.28 Outside of these specific outstanding results, however, the PCEs of all-small-molecule solar cells have remained stagnant at 3−5%.29−31 The barrier to further performance improvements has been attributed to a lack of information about the structure−property relationships among the small molecule donors and acceptors. The benefits of semiconducting small molecules could be exploited and the photovoltaic performances of all-small-molecule solar cells could be improved by designing new electron-accepting small molecules and by exploring the structure−property relationships. The naphthalene diimide (NDI) moiety is a versatile electron-deficient unit and its low-lying lowest unoccupied molecular orbital (LUMO) energy level and high electron mobility have been reported in the literature.32−34 These properties suggest that NDI-based electron acceptors could replace fullerene derivatives. NDI-based electron acceptors can also potentially replace perylene diimide-based electron acceptors, which promote large-scale phase separation due to high molecular planarity and strong intermolecular interactions.32 Herein, we report the preparation of all-small-molecule solar cells that incorporate NDI-based small molecules as electron acceptors and 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5b′]dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (DTS-F) as electron donors. It has been reported that the most efficient structure of the high-performing NDI-based electron acceptors is a
twisted NDI dimer structure prepared by incorporating a linker between the two NDI units to disrupt excessive aggregation in a BHJ blend but preserve the charge-transport properties via effective π−π stacking.32,35−37 The electrical and morphological properties of the NDI-based nonfullerene materials in a BHJ blend were optimized by designing three NDI dimers with different linkers, thiophene (T), bithiophene (BT), and thiophene−vinylene−thiophene (TVT), between the two NDI units. The linkers extended the conjugation length, lowered the band gap, and modulated the geometric structure of the twisted NDI dimers.10,38 The three NDI-based small molecules with different linkers exhibited different blend morphologies, either miscibility or crystallinity, in BHJ blends that incorporated these NDI-based small molecules as electron acceptors and DTS-F as an electron donor. NDICN-BT and NDICN-TVT, which were more planar than NDICN-T, formed crystalline structures in pure small molecule films, whereas NDICN-T formed amorphous films. The crystalline structure of NDICN-BT was interrupted, however, due to an excessive miscibility with DTS-F. The DTS-F:NDI-based smallmolecule blends were deployed in all-small-molecule solar cells to provide devices with distinctly different properties. Promising photovoltaic characteristics were achieved from the DTS-F:NDICN-TVT device, with a high power conversion efficiency (PCE) of 3.01%, an open-circuit voltage (Voc) of 0.75 V, a short-circuit current density (Jsc) of 7.10 mA cm−2, and a fill factor (FF) of 56.2%. The DTS-F:NDICN-T and DTSF:NDICN-BT devices, on the other hand, gave PCEs of 1.81 and 0.13%, respectively. The relationship between the morphology and the photovoltaic characteristics of the DTSF:NDI-based acceptor all-small-molecule solar cells were explored by characterizing the charge-generation properties and transfer dynamics in the blend films using femtosecond transient absorption spectroscopy (fs-TAS). The chargetransport properties of the blend films were characterized using the space-charge limited-current (SCLC) model. The excited-state dynamics and charge-transport properties revealed that the high electron mobility in the DTS-F:NDICN-TVT photovoltaic device induced by well-developed domains with a crystalline structure could provide excellent performances, despite a low charge-transfer efficiency due to large-scale phase separation.
■
EXPERIMENTAL SECTION
Solar Cell Fabrication and Characterization. The solar cell devices were fabricated to have a conventional structure of glass/ indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/active layer (DTS-F:NDI-based small molecules)/LiF/Al. The ITO-coated glass substrates were cleaned following a reported procedure.39 The glass/ITO substrates were then exposed to UV−ozone for 30 min. Subsequently, a PEDOT:PSS (Clevios P VP AI 4083, filtered using 0.45 μm poly(vinylidene difluoride)) layer with a thickness of ca. 40 nm was spin-coated and the glass/ITO/PEDOT:PSS substrates were baked at 120 °C for 20 min. Before the active layer formation, the active solutions were prepared by blending DTS-F and NDI-based small molecules in a blend ratio of 1:1 with a total concentration of 30 mg mL−1 in chlorobenzene and were stirred overnight in a nitrogen-filled glovebox. The active solution was filtered using a 0.20 μm poly(tetrafluoroethylene) filter, and the active layer was spin-coated onto the PEDOT:PSS layer to a thickness of ca. 100 nm. Finally, 0.8 nm LiF and 100 nm Al layers were deposited onto active-coated substrates using a thermal evaporator under high vacuum (5 × 10−6 Torr). The current density−voltage (J−V) characteristics and external B
DOI: 10.1021/acsami.7b16004 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Scheme 1. Synthesis of the NDI-Based Small-Molecule Acceptors
quantum efficiency (EQE) measurements were performed according to our previously reported procedure.39 Transient Absorption Spectroscopy Measurement. fs-TAS was carried out using a commercially available transient absorption spectrometer, HELIOS (Ultrafast Systems). Samples were excited with a pulse train generated by an optical parametric amplifier, TOPAS (light conversion). Both the spectrometer and the parametric amplifier were seeded with 800 nm,
