Multifunction-oriented high-mobility polymer semiconductors

Recent progress in multifunction-oriented high-mobility polymer semiconductors is profiled, with current challenges and future directions proposed in this perspective.


Multifunction-oriented high-mobility polymer semiconductors
Mingliang Zhu 1 , 2 , Yunlong Guo 1 , 2 , * and Yunqi Liu 1 , 2 , * The discovery of conductive polyacetylene has initiated interdisciplinary research into condensed matter physics and synthetic chemistry in organic electronics.Unlike their inorganic and small-molecule counterparts, polymer semiconductors possess the inherent advantages of facile solution processing, large-area manufacturing, excellent mechanical strength and intrinsic flexibility.To date, they have delivered over-thebenchmark mobility of 10 cm 2 V -1 s -1 , while device technology and controllable doping/blending further enable performance enhancement and ambient stability [ 1 ].Beyond high mobility, integration of additional mechanical (stretchabi lity), optical ( luminescence and photopatterning) and thermal (thermoelectric conversion) properties contributes to multimode fusion and intelligent manufacturing, wherein adjustable π -conjugation components can functionalize as-prepared materials towards diverse applications (Fig. 1 ).In this perspective, we wi l l summarize recent progress in functional high-mobility polymer semiconductors, with current challenges and future directions proposed.

HIGH MOBILITY INTEGRATED WITH HIGH STRETCHABILITY
Next-generation wearable and implantable electronic devices require skin-like stretchable semiconducting materials.In contrast to the complex manufacturing of structure engineering and long-term metastable-state instability in the blending system, intrinsically stretchable polymer semiconductors possess an innate strain-accommodating capability and excellent mobility retention under strain.
Bao et al .initially introduced amidety pe flex ible-segment 2,6-pyridine dicarboxamide (PDCA) into an allconjugated polymer backbone with the aim of efficient stress dissipation via dynamic non-covalent crosslinking [ 2 ].They achieved a desirable balance between electrical and mechanical properties at the modification ratio of 10%, and this material exhibited mobility up to 1.32 cm 2 V -1 s -1 initially and preserved 1.12 cm 2 V -1 s -1 at 100% strain along the direction perpendicular to the strain, delivering a maximum mobility of 0.6 cm 2 V -1 s -1 in fully stretchable transistors.In particular, a self-healing ability was also observed, with filed-effect mobilities fully recovered after heating or solvent annealing.Taking advantage of coordination interactions between Fe(III) ion and PDCA, the researchers blended this polymer semiconductor with an insulating poly(dimethylsiloxanealt-2,6pyridinedicarbozamine) elastomer to confer strain sensitivity, stretchability and self-healing capabilities onto the target skin-like active semiconductor [ 3 ].After partial metal-ligand bonding exchange and rational controlling of bicomponent percolation, blending films exhibited high sensitiv ity w ith an unprecedented gauge factor of 5.75 × 10 5 at 100% strain in stretchable transistors, yielding excellent stretchability over 1300% and room-temperature self-healing.
Without breaking intrachain conjugation or weakening interchain order, Zhang et al .incorporated different noncentrosymmetric spiro-fluorine units into the reference conjugated polymer [ 4 ].Randomly distributed spiro-fluorene units undermined film crystallinity, reduced bulky side chains, and shortened lamellar and π −π stacking distances.In particular, the cyclopropane-inserted spiro-fluorene unit endowed the designed polymer with a low elastic modulus of 83.7 MPa, and record-high mobility of 3.1 cm 2 V -1 s -1 at 150% strain and 1.4 cm 2 V -1 s -1 after 10 0 0 stretching-releasing cycles at 50% strain.Generally, whether a conjugation interruption is involved or not, these materials have been established as a trade-off between film crystallinity and mechanical stretchability, but the essential issue of (micro)structure-property relationship is ambiguous and high-mobility stretchable n-type semiconductors are nearly blank.Inspired by microscopic morphology, Facchetti et al .reported an amphiphilic semiconducting polymer combined with uniform honeycomb-like microstructure, realizing intrinsically stretchable and electrochemically redoxfree organic electrochemical transistors (OECTs) [ 5 ].Through biaxial prestretching to stabilize electronic-ionic transport, OECTs with initial normalized transconductance of 16.16 S cm -1 delivered excellent biaxial stretchability (up to 140%), mechanical durability (10 4 stretching-releasing cycles at 30% strain) and stable signal amplification for electrocardiogram and synapse response under strain.

HIGH MOBILITY INTEGRATED WITH STRONG LUMINESCENCE
Revolutionary miniatured optoelectronic devices, including electrically pumped polymer lasers, organic light-emitting transistors (OLETs) and intrinsic drive-integrated displays, require integration of high mobility and strong solid-state luminescence.Generally, dense π -stacking and delocalized electron-cloud distribution facilitate carrier transport, whereas intense emission often requires attenuated intermolecular packing and localized excitons.
To simultaneously improve mobility and fluorescence, Sirringhaus et al .designed a class of near-amorphous laddertype conjugated polymers with gradually extended rigid backbones and promoted close-crossing points for low energetic disorder and three-dimensional carrier-percolation pathways [ 6 ].Interchain high-degree short contacts and an appropriate energy level of 2.0 eV contributed to a high mobility exceeding 2.4 cm 2 V -1 s -1 and a luminescence quantum yield over 15% enabled by pinned internal charge transfer.Taking intramolecular electronic coupling into account, Yu et al .designed a ladder-type copolymer with enhanced crystallinity and backbone coplanarity based on the weak donor-acceptor-coupling strategy, which adopted ideal J-aggregation with a large π −π stacking distance of 4.0 Å [ 7 ].The polymer exhibited a high fluorescence quantum yield of 77%, and ambipolar characteristics, with hole and electron mobilities of 1.26 × 10 −3 and 3.53 × 10 −4 cm 2 V -1 s -1 , respectively, delivering a desirable external quantum efficiency of 5.3% with electroluminescence intensity of 414 nW in multi-layered OLET devices.
Different from quasi-twodimensional hori zontal transport, vertical devices are suitable for the vigorous requirements of even carrier injection, transport and recombination luminescence.Liu et al .developed a selfassembled three-dimensional penetrating nano-network comprising equal-mass super-yellow poly( p -phenylene vinylene) and polyacrylonitrile, realizing intrinsic stretchability over 100% and 5-6 -fold mobility enhancement with photoluminescence well maintained [ 8 ].Assisted by a highly efficient stretchable electroninjection layer, intrinsically stretchable polymer light-emitting diodes yielded an excellent current efficiency of 2.35 cd A -1 and a maximum luminescence of 3780 cd m -2 with a turn-on voltage of 6.5 V, maintaining 54% of the initial luminescence at 30% strain.Regardless of the device structure, when qualifying strong luminescence, the highest possible carrier mobility could enable efficient exciton recombination towards highly efficient light-emitting devices.

HIGH MOBILITY INTEGRATED WITH PHOTOLITHOGRAPHY PATTERNING
Facile photopatterning of high-mobility polymer semiconductors is of vital importance for fabricating multilayer functional devices and high-density integrated circuits.High photopatterning resolution, mobility retention and processing stability are critical factors for all-photolithography patterning.
Wei et al .developed a semiconducting photoresist involving the polymer semiconductor poly(tetrathienoacenediketopyrrolopyrrole), a cross-linkable small-molecule monomer, a radical photo-initiator and a thiol additive for ensuring efficient photo-crosslinking [ 9 ].The nano-interpenetrating microstructure enabled submicron resolution and dense π −π stacking, thus delivering high-density transistor arrays reaching 1.1 × 10 5 units cm −2 with the highest mobility being 1.64 cm 2 V -1 s -1 .Bao et al .proposed the covalently embedded in-situ rubber matrix (iRUM) strategy to endow high-mobility conjugated polymers with additional mechanical elasticity, solvent resistance and high-precision photopatterning [ 10 ].Leveraging the reactiv ity discrepanc y of azide groups with C −H and C = C bonds, more reactive azide/C = C cycloaddition resulted in the polymer semiconductor network being evenly embedded into the elastic rubber matrix without disrupting semiconductor aggregation.In fully stretchable transistors, the iRUM-poly(indacenodithiopheneco -benzothiadiazole) film retained initial mobility at 100% strain and maintained over 1 cm 2 V -1 s -1 after 500 stretching-releasing cycles at 50% strain, accompanied by a stable cycling life extended 50 0 0-fold.Simultaneously, interfacial crosslinking of semiconducting and dielectric layers avoided interfacial delamination under strain, further facilitating the fabrication of fully patterned elastic transistors.
Photolithographic species generally contain dual-/multi-components with photo-reactive azide and diazirine groups attached, accompanied by puzzles like mutual miscibility, phase separation and film reproducibility.Zhang et al .developed a mono-component semiconducting photoresist via appending azide groups in alkyl chains of conjugated polymers [ 11 ].Upon UV-light irradiation, transformed nitrene units crosslinked alkyl side chains towards great discrepancy in solubility and thus high-resolution photopatterning (5 μm).Patterned thin films showed an average mobility of 0.61 cm 2 V -1 s -1 with satisfactory uniformity, and maintained π −π stacking.Unfortunately, single-component semiconducting photoresists require complex synthesis, and photo-reaction products might reside as trapping or scattering sites for carrier transport.

HIGH MOBILITY INTEGRATED WITH EFFICIENT THERMOELECTRIC CONVERSION
The Internet of Things requires lightweight power-supplying elements, and thermoelectric materials could directly convert renewable heat sources into electricity via the Seebeck effect.High-mobility conjugated polymers are expected to break the trade-off relationship of thermoelectric parameters, delivering significantly improved electrical conductivity upon chemical doping.
Leclerc et al .substituted linear alkyl-chains with isometric singleether-functionalized side chains to produce the slightly polarized poly(2,5bis(3-dodecyl-2-thienyl)-co -thieno[3,2b]thiophene) analogue with the aim of achieving enhanced cohesive forces and polymer-dopant intercalation within side-chain layers [ 12 ].Polymer chains could be uniaxially aligned towards high dichroic ratios up to 20, and dopants were randomly distributed in amorphous side-chain regions, bringing about a unidirectionally high electrical conductivity of 5 × 10 4 S cm -1 and a record power factor (PF) of 2.9 mW m -1 K -2 .Limited to electron mobility, doping efficiency and ambient stability, the thermoelectric performance of n-doped materials lags far behind.Huang et al .developed the side-chain-free and in-situ reductive n-doped conducting polymer poly(benzodifurandione), which has excellent conductivity approaching 2 × 10 3 S cm -1 [ 13 ].A negatively charged conjugated backbone with ∼0.9 charges per repeating unit enabled it to have intrinsic solubility in polar solvents, and condensed films exhibited a metallic state and coherent carrier transport with PF ∼90 μW m -1 K -2 .In addition to achieving balanced thermoelectric parameters towards maximum conversion efficiency, theory-guided chemical doping, intrinsic host-dopant interactions and all-level stability are problems that deserve more attention.

OUTLOOK
Established on mutual trade-offs between electrical, mechanical, optical and thermal characteristics, high-mobility polymer semiconductors are marching towards multifunctionalization.Apart from mobility and comprehensive performance improvement, there sti l l exist many puzzles and bottlenecks for practical application to overcome.First, a rigid coplanar backbone and strong donoracceptor coupling contribute to superior carrier mobility, commonly accompanied by mechanical brittleness, fluorescence quenching and low-efficiency doping.Amorphous conjugated polymers with efficient intramolecular charge transport could afford unexpectedly outstanding molecular-weight-dependent functionalization.Second, electronic devices require environmental, temporal and operational stability, resulting in high demand for encapsulation layers, functional layers and layer-by-layer interfaces.Besides material selection and device technology, developing advanced orthogonal reactions and crosslinking chemistry might be feasible methods.Finally, multifunctional integration with mutual compatibility/promotion is the future of high-mobility polymer semiconductors.Additionally, expanding and fusing more special characteristics, such as magnetic, electrochemical and acoustic properties, could be equally important.Novel molecular design, doping/blending strategies and alignment solution processing all provide brand new pathways towards all-round multifunctionalization.