Chien Lu

A chameleon-inspired stretchable electronic skin with interactive colour changing controlled by tactile sensing

Some animals, such as the chameleon and cephalopod, have the remarkable capability to change their skin colour. This unique characteristic has long inspired scientists to develop materials and devices to mimic such a function. However, it requires the complex integration of stretchability, colour-changing and tactile sensing. Here we show an all-solution processed chameleon-inspired stretchable electronic skin (e-skin), in which the e-skin colour can easily be controlled through varying the applied pressure along with the applied pressure duration. As such, the e-skins colour change can also be in turn utilized to distinguish the pressure applied. The integration of the stretchable, highly tunable resistive pressure sensor and the fully stretchable organic electrochromic device enables the demonstration of a stretchable electrochromically active e-skin with tactile-sensing control. This system will have wide range applications such as interactive wearable devices, artificial prosthetics and smart robots.

Highly stretchable polymer semiconductor films through the nanoconfinement effect

Trapping polymers to improve flexibility Polymer molecules at a free surface or trapped in thin layers or tubes will show different properties from those of the bulk. Confinement can prevent crystallization and oddly can sometimes give the chains more scope for motion. Xu et al. found that a conducting polymer confined inside an elastomer—a highly stretchable, rubber-like polymer—retained its conductive properties even when subjected to large deformations (see the Perspective by Napolitano). Science, this issue p. 59; see also p. 24 A high-performance conjugated polymer is combined with an elastomer to produce a fully stretchable transistor. Soft and conformable wearable electronics require stretchable semiconductors, but existing ones typically sacrifice charge transport mobility to achieve stretchability. We explore a concept based on the nanoconfinement of polymers to substantially improve the stretchability of polymer semiconductors, without affecting charge transport mobility. The increased polymer chain dynamics under nanoconfinement significantly reduces the modulus of the conjugated polymer and largely delays the onset of crack formation under strain. As a result, our fabricated semiconducting film can be stretched up to 100% strain without affecting mobility, retaining values comparable to that of amorphous silicon. The fully stretchable transistors exhibit high biaxial stretchability with minimal change in on current even when poked with a sharp object. We demonstrate a skinlike finger-wearable driver for a light-emitting diode.

Single‐Crystal C60 Needle/CuPc Nanoparticle Double Floating‐Gate for Low‐Voltage Organic Transistors Based Non‐Volatile Memory Devices

Low-voltage organic field-effect transistor memory devices exhibiting a wide memory window, low power consumption, acceptable retention, endurance properties, and tunable memory performance are fabricated. The performance is achieved by employing single-crystal C60 needles and copper phthalocyanine nanoparticles to produce an ambipolar (hole/electron) trapping effect in a double floating-gate architecture.

A silole copolymer containing a ladder-type heptacylic arene and naphthobisoxadiazole moieties for highly efficient polymer solar cells

We report a combination of a silole containing ladder-type heptacylic arene and naphthobisoxadiazole moieties for highly efficient polymer solar cells. This new class of PSiNO polymer possesses a planar, rigid backbone and a low-ordering framework. This unique feature facilitates chain extension, leading to high hole mobility and hence a high PCE of 8.37% without further thermal annealing.

Biaxially extended quaterthiophene-thiophene and -selenophene conjugated polymers for optoelectronic device applications

New biaxially extended quaterthiophene (4T) conjugated polymers, including poly(5,5′′′-di-(2-ethylhexyl)[2,3′;5′,2′′4′′,2′′′]quaterthiophene) (P4T) and their copolymers with thiophene(P4TT), bithiophene (P4T2T), selenophene(P4TSe) and biselenophene (P4T2Se) were synthesized by Stille coupling reactions under microwave heating. The effects of the ring number of thiophene and selenophene moieties on the physical properties and polymer structures were systematically investigated experimentally and theoretically. With the increased ring number of the unsubstituted thiophene and selenophene moieties, the band gaps and the main-chain torsional angles were reduced. However, the side-chain torsional angles were increased with increasing the ring number, and thus significantly affected the carrier transporting characteristics. Among these studied conjugated polymers, the field-effect transistor (FET) based on P4TSe showed the highest hole mobility of up to 4.28 × 10−2 cm2 V−1 s−1 and an on/off ratio of 1.12 × 104. The photovoltaic device prepared from P4TSe/PC71BM exhibited the highest power conversion efficiency (PCE) of 2.6%, which resulted from more balanced hole/electron mobility and a smaller band gap. The above results revealed that the conformation, charge-transporting and optoelectronic device characteristics of biaxially extended 4T-based conjugated copolymers could be manipulated by incorporating the heteroaromatic ring spacer.

Significance of the double-layer capacitor effect in polar rubbery dielectrics and exceptionally stable low-voltage high transconductance organic transistors.

Both high gain and transconductance at low operating voltages are essential for practical applications of organic field-effect transistors (OFETs). Here, we describe the significance of the double-layer capacitance effect in polar rubbery dielectrics, even when present in a very low ion concentration and conductivity. We observed that this effect can greatly enhance the OFET transconductance when driven at low voltages. Specifically, when the polar elastomer poly(vinylidene fluoride-co-hexafluoropropylene) (e-PVDF-HFP) was used as the dielectric layer, despite a thickness of several micrometers, we obtained a transconductance per channel width 30 times higher than that measured for the same organic semiconductors fabricated on a semicrystalline PVDF-HFP with a similar thickness. After a series of detailed experimental investigations, we attribute the above observation to the double-layer capacitance effect, even though the ionic conductivity is as low as 10–10 S/cm. Different from previously reported OFETs with double-layer capacitance effects, our devices showed unprecedented high bias-stress stability in air and even in water.

Stretchable Polymer Dielectrics for Low-Voltage-Driven Field-Effect Transistors

A stretchable and mechanical robust field-effect transistor is essential for soft wearable electronics. To realize stretchable transistors, elastic dielectrics with small current hysteresis, high elasticity, and high dielectric constants are the critical factor for low-voltage-driven devices. Here, we demonstrate the polar elastomer consisting of poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP):poly(4-vinylphenol) (PVP). Owing to the high dielectric constant of PVDF-HFP, the device can be operated under less than 5 V and shows a linear-regime hole mobility as high as 0.199 cm2 V-1 s-1 without significant current hysteresis. Specifically, the PVDF-HFP:PVP blends induce the vertical phase separation and significantly reduce current leakage and reduce the crystallization of PVDF segments, which can contribute current hysteresis in the OFET characteristics. All-stretchable OFETs based on these PVDF-HFP:PVP dielectrics were fabricated. The device can still keep the hole mobility of approximately 0.1 cm2/(V s) under a low operation voltage of 3 V even as stretched with 80% strain. Finally, we successfully fabricate a low-voltage-driven stretchable transistor. The low voltage operating under strains is the desirable characteristics for soft and comfortable wearable electronics.

n‐Type Doped Conjugated Polymer for Nonvolatile Memory

This study demonstrates a facile way to efficiently induce strong memory behavior from common p-type conjugated polymers by adding n-type dopant 2-(2-methoxyphenyl)-1,3-dimethyl-2,3-dihydro-1H-benzoimidazole. The n-type doped p-channel conjugated polymers not only enhance n-type charge transport characteristics of the polymers, but also facilitate to storage charges and cause reversible bistable (ON and OFF states) switching upon application of gate bias. The n-type doped memory shows a large memory window of up to 47 V with an on/off current ratio larger than 10 000. The charge retention time can maintain over 100 000 s. Similar memory behaviors are also observed in other common semiconducting polymers such as poly(3-hexyl thiophene) and poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene], and a high mobility donor-acceptor polymer, poly(isoindigo-bithiophene). In summary, these observations suggest that this approach is a general method to induce memory behavior in conjugated polymers. To the best of the knowledge, this is the first report for p-type polymer memory achieved using n-type charge-transfer doping.

Manipulation of electrical characteristics of non-volatile transistor-type memory devices through the acceptor strength of donor–acceptor conjugated copolymers

We report a nonvolatile organic transistor-type memory based on biaxially extended quaterthiophene (4T) and dithienothiophene-based donor (D)–acceptor (A) copolymers. In this work, the effects of donor–acceptor chemical structures on the memory characteristics were investigated. We have found that the charge storage capability can be manipulated by the acceptor strength of the donor–acceptor copolymers. 4T-based D–A copolymers with strong electron-accepting benzothiadiazole (P4T-BT) and diketopyrrolopyrrole (P4T-DPP) exhibited large memory windows (55 and 79 V, respectively). However, the donor-only copolymer, poly(4T-dithenothiophene) (P4T-DTT), and the D–A copolymer with weak electron-withdrawing qunioxaline (P4T-Q) showed smaller memory windows of 7 and 11 V, respectively. This indicates that the strong acceptors play a crucial rule in enhancing memory behaviors. The ON and OFF states of the D–A polymer devices can be maintained over 3 × 104 s with the Ion/Ioff current ratios of 102–105, and the write–read–erase–read (WRER) cycles can be operated over 200 cycles, indicating excellent nonvolatile flash-type memory behaviors. To the best of our knowledge, this study is the first work systematically investigating the influence of the chemical structures of the acceptor moieties on the nonvolatile flash polymer memory applications.

Ambipolar field-effect transistors using conjugated polymers with structures of bilayer, binary blends, and paralleled nanofibers

In this paper, we explore ambipolar organic field-effect transistor (FET) characteristics using bilayer, binary blends, and paralleled nanofibers of poly(3-hexylthiophene) (P3HT; p-type) and poly{[N,N′-bis(2-decyltetradecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl] -alt-(thiophene-2,5-diyl)} (P(NDI-T); n-type). Ambipolar transistors with paralleled single P3HT and P(NDI-T) electrospun nanofibers showed high and well-balanced mobilities of 8.25 × 10−2 cm2 V−1 s−1 for holes and 7.51 × 10−2 cm2 V−1 s−1 for electrons with Ion/Ioff ≈ 104. This ambipolar nanofiber FET was also applied to a complementary inverter with a gain of up to 20.5.