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Incorporation of Designed Donor-Acceptor-Donor Segments in a Host polymer for Strong Near-Infrared Emission from a Large-Area Light-Emitting Electrochemical Cell Petri Murto, Shi Tang, Christian Larsen, Xiaofeng Xu, Andreas Sandström, Juuso Pietarinen, Benedikt Bagemihl, Birhan Alkadir, Wendimagegn Mammo, Mats Andersson, Ergang Wang, and Ludvig Edman ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00283 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018
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ACS Applied Energy Materials
Incorporation of Designed Donor-Acceptor-Donor Segments in a Host polymer for Strong Near-Infrared Emission from a Large-Area Light-Emitting Electrochemical Cell Petri Murto,†,‡,┴ Shi Tang,§,∆,┴ Christian Larsen,§,∆ Xiaofeng Xu,† Andreas Sandström,∆ Juuso Pietarinen,†,ϕ Benedikt Bagemihl,† Birhan A. Abdulahi,†,¶ Wendimagegn Mammo,†,¶ Mats R. Andersson,‡ Ergang Wang,*,† and Ludvig Edman*,§,∆ †
Department of Chemistry and Chemical Engineering/Applied Chemistry, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden ‡ Flinders Centre for Nanoscale Science and Technology, Flinders University, Sturt Road, Bedford Park, Adelaide, SA, 5042, Australia § The Organic Photonics and Electronics Group, Umeå University, SE-90187 Umeå, Sweden ∆ LunaLEC AB, Umeå University, SE-90187 Umeå, Sweden ϕResearch Unit of Sustainable Chemistry, University of Oulu, P.O. Box 3000, FI-90014 Oulu, Finland ¶ Department of Chemistry, Addis Ababa University, P.O. Box 33658, Addis Ababa, Ethiopia KEYWORDS: near-infrared, NIR, large-area device, light-emitting electrochemical cell, LEC, copolymer, solution processing ABSTRACT: Cost-efficient thin-film devices that emit in the near-infrared (NIR) range promise a wide range of important applications. Here, the synthesis and NIR application of a series of copolymers comprising poly[indacenodithieno[3,2-b]thiophene-2,8diyl] (PIDTT) as the host and different donor–acceptor–donor (DAD) segments as the guest are reported. We find that a key design criterion for efficient solid-state host-to-guest energy transfer is that the DAD conformation is compatible with the conformation of the host. Such host-guest copolymers are evaluated as the emitter in light-emitting electrochemical cells (LECs) and organic lightemitting diodes, and the best performance is invariably attained from the LEC devices because of the observed balanced electrochemical doping that alleviates issues with a non-centered emission zone. An LEC device comprising a host-guest copolymer with 4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene as the donor and benzo[c][1,2,5]thiadiazole as the acceptor delivers an impressive near-infrared (NIR) performance in the form of a high radiance of 1458 µW/cm2 at a peak wavelength of 725 nm when driven by a current density of 500 mA/cm2, a second-fast turn-on, and a good stress stability as manifested in a constant radiance output during 3 days of uninterrupted operation. The high-molecular-weight copolymer features excellent processability, and the potential for low-cost and scalable NIR applications is verified through a spray-coating fabrication of a >40 cm2 large-area device, which emits intense and uniform NIR light at a low drive voltage of 4.5 V.
INTRODUCTION Light-emitting devices that deliver strong and efficient emission in the near-infrared (NIR) region are of interest for a wide range of applications, spanning from optical communication to biosensing.1-5 Such emissive devices based on organic compounds are particularly attractive in that they can be thin, flexible, and carry a low cost, if fabricated with scalable solution-based methods.6-10 For implantable biomedical applications that exploit the semitransparency of biological tissue to NIR light, it is further preferable if the emissive device is free from toxic metal-based compounds.11-14 The principal constituent in a light-emitting device is the emitter, and a number of approaches for improving the commonly poor emission efficiency of organic compounds in the NIR range have been reported: Aggregation-induced emission (AIE) enhances the solid-state emission yield by restricting the intramolecular motion (rotation and/or vibration) of the emitter,15-18 and Ledwon and co-workers19 attained efficient emission with a peak wavelength of 688 nm from an organic lightemitting diode (OLED) comprising an AIE-active compound.
Thermally activated delayed fluorescence (TADF) harvests the commonly dark triplet excitons in organic compounds,20-24 and OLEDs based on TADF materials that emit with a peak wavelength exceeding 700 nm are now common.25-32 However, efficient OLED devices typically comprise nm-precision multi-layer stacks, which necessitate the use of vacuum processing with a concomitant penalty in fabrication cost and throughput. The light-emitting electrochemical cell (LEC) offers a much simplified device structure comprising a single active layer sandwiched between two air-stable electrodes,33-37 and LEC devices can accordingly be fabricated with cost-efficient and scalable solution-based methods.38-40 Pal et al.41 reported an LEC based on an Ir-complex as the emitter, which delivered a high radiance of 471 µW/cm2 centered at 705 nm when driven by a 1 kHz pulsed current density of 80 mA/cm2. Pertegás and co-workers42 also employed 1 kHz pulsed operation to obtain a radiance of 170 µW/cm2 at a peak wavelength 700 nm from a NIR-LEC using a metal-free organic cyanine dye as the emitter.
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Scheme 1. (top and middle rows) The chemical structures of the six different DAD segments. (bottom row) The chemical structures and assigned acronyms of the PIDTT-based host-guest copolymers with incorporated DAD guest segments.
S
S
C12H25 O O C12H25 S
S
O
O
O
S N
N
S
N
S
S
N
S
S
S
Si
Si N
S
S
C12H25 O O C12H25 S
N
S
N
O
O
EBRE
C12H25 O O C12H25 S
Si
N
O
S
N
EBE
S S
O
S
N
TBRT
TBT
O
S
Si N
S
N
SBRS
SBS C6H13
C6H13
S
S
S S
n C6H13
S
D A D
S
m
C6H13
PIDTT PIDTT-TBT PIDTT-TBRT PIDTT-EBE PIDTT-EBRE PIDTT-SBS PIDTT-SBRS
n:m = 100:0 n:m = 99.5:0.5 n:m = 99.5:0.5 n:m = 99.5:0.5 n:m = 99.5:0.5 n:m = 99.5:0.5 n:m = 99.5:0.5
poly[indacenodithieno[3,2-b]thiophene-2,8-diyl] (PIDTT)
However, for several applications a high-frequency power supply is not practical. In this context, it is important that Bideh and Shahroosvand43 were able to record a luminance of 742 cd/m2 at 690 nm at a constant current density of 222.4 mA/cm2 from an LEC using a Ru-complex as the emitter; unfortunately, the operational lifetime was short at
