Quasi‐1D Polymer Semiconductor–Diarylethene Blends: High Performance Optically Switchable Transistors

Abstract

Optically switchable field‐effect transistors (OSFETs) are non‐volatile photonic memory devices holding a great potential for applications in optical information storage and telecommunications. Solution processing of blends of photochromic molecules and π‐conjugated polymers is a low‐cost protocol to integrate simultaneously optical switching and charge transport functions in large‐area devices. However, the limited reversibility of the isomerization of photochromic molecules due to steric hindrance when embedded in ordered polymeric matrices represents a severe limitation and it obliges to incorporate as much as 20% in weight of the photochromic component, thereby drastically diluting the electronic function, limiting the device performance. Herein, a comparative study of the photoresponsivity of a suitably designed diarylethene molecule is reported when embedded in the matrix of six different polymer semiconductors displaying diverse charge transport properties. In particular, this study focuses on three semi‐crystalline polymers and three quasi‐1D polymers. It is found that 1% w/w of 1,2‐bis(5‐(3,5‐di‐tert‐butylphenyl)‐2‐methylthiophen‐3‐yl)cyclopent‐1‐ene in a blend with poly(indacenodithiophene‐co‐benzothiadiazole) is sufficient to fabricate OSFETs combining photo‐modulation efficiencies of 45.5%, mobilities >1 cm2 V−1s−1, and photo‐recovered efficiencies of 98.1%. These findings demonstrate that quasi‐1D polymer semiconductors, because of their charge transport dominated by intra‐molecular processes, epitomize the molecular design principles required for the fabrication of high‐performance OSFETs.

Diarylethene-Type Photochromic Electron Traps Extending Narrow Linear Dynamic Range of Organic Photodetectors. Design Principles and Fate After Electron Transfer. Self-Destructive or Self-Protective?

“…However, the source of the electron herein is the photoelectron transferred from the hosting donor polymer (P3HT herein) and would be available only if P3HT more easily gives up its photoelectron from its LUMO, i.e., only if the EA of P3HT is lower than the EA of DAE-c or only if the LUMO energy (conduction band minimum) of P3HT is higher (less negative) than the LUMO energy of DAE-c. In fact, the compounds considered above (4F/6F-T-H-Ph; EA 2.0 eV, LUMO −3.2/–3.3 eV) would fail in their current forms because their EA (LUMO energies) are not sufficiently larger (lower or more negative) than those of P3HT, which have been estimated as 3.2 eV (EA) , and −3.5 to −2.8 eV (LUMO). − ,,,,,,− …”

“…Therefore, considering a broad range of HOMO (valence band maximum) and LUMO (conduction band minimum or EA) energies, −5.2 to −4.4 and −3.5 to −2.8 eV, respectively, − ,,,,,,− reported for the hosting donor polymer P3HT, we state additional criteria as follows.…”

“…DAE has also been blended with highly conductive organic semiconductors (e.g., poly-3-hexylthiophene, P3HT, Figure b) in a bulk heterojunction layer to make various optically modulated multifunctional organic devices such as switches, transistors, memories, diodes, and photodetectors. − In such bicomponent photoactive layers, the closed-ring isomer DAE-c can act as a photoswitchable acceptor (or trap) of holes or electrons transferred from the semiconductor via the highest occupied MO (HOMO) or the lowest unoccupied MO (LUMO), respectively, depending on the electrical nature of the semiconductor. In fact, DAE have been mostly used as p-type (hole) traps utilizing their photoswitched shifts in HOMO energy levels. − It is probably because a majority of high-mobility organic semiconductors (such as P3HT, PEDOT:PSS, and push–pull-type photoelectron donor polymers) are p-type and their hole transport is faster than electron transport. Less attention has been paid to DAE as n-type dopants (i.e., electron acceptors or traps) utilizing the photoinduced shifts of their LUMO energy levels, e.g., blending DAE with n-type organic semiconductors such as phenyl-C 61/71 -butyric acid methyl ester (PCBM; Figure b) or adsorbing DAE on nonorganic-but-flexible WSe 2 flakes …”

“…If cycloreversion occurs so fast after trapping electrons (to a level of immediate self-destruction of the DAE-c electron acceptor), it raises a question whether such electron traps would survive long enough to induce the desired photomultiplication behavior, although it may be beneficial for open-isomer recovery for the next cycle. However, information on the LUMO energy ,, and anionic/reductive , behavior of DAE is much less available than for their HOMO energy − and cationic − /oxidative − behavior.…”

New insights into the photocyclization reaction of a popular diarylethene switch: a nonadiabatic molecular dynamics study

“…Diarylethenes (DAEs) are an outstanding family of molecular switches, with numerous applications in fundamental science and new technologies. 1–5 They are known for their high thermal stability, 6 fatigue resistance 7–9 and ultra-fast response time. 10–12 Furthermore, they can be synthesised with relative ease, 13 and are reactive in a variety of environments, including a wide range of solvents, gels, crystals, and nanoparticles.…”