Fabrication And Application Of Novel Organic Semiconductor Nano/micro-wires

Organic semiconductor nano/micro-wires have attracted enormous attention for its unique photoelectric properties. Compared with the amorphous organic thin films, the nano/micro-wire fabricated by self-assembly assisted by intermolecular interaction has less grain boundaries, smaller quantity of surface defects, and the π-orbital overlaps in high degree, it will greatly increase the mobility of the charge carriers in organic semiconductor. The charge transport in a nano/micro-wire is directional, which makes it a proper candidate as building blocks of organic nano/micro integrated circuits, facilitating the minimization and integration of the devices. Compared with its bulk crystalline counterpart, the large aspect ratio of a nano/micro-wire endows it excellent flexibility, showing promising future in high performance flexible devices. As so far, organic semiconductor nano/micro-wires have been applied in the fields of organic field transistors, organic solar cells, organic light detection and organic field emission, et al. However, most of the nano/micro-wires are homogeneous in materials without p-n heteroj unction interface for photoelectric conversion, which is still remained to be developed further. Therefore, it’s necessary to design and fabricate novel nano/micro-structure with heterojunction to improve the photoelectric performance of organic nano/micro-wires and discover new phenomenon and mechanism. In light of this, the main purpose of this study is to fabricate novel high performance organic semiconductor nano/micro-wires, and study their photoelectric property.1. Poly(3-hexylthiophene) (P3HT) and aluminum tris(8-Hydroxyquinolinate) (Alq3) have been taken as electron donor and acceptor respectively to fabricate a novel radial heterojunction organic microwire, and a high performance photodiode based on a single microwire has been constructed. The photodiode was a bottom contact device with parallel silver/aluminum heteroelectrodes and the self-assembled radial heterojunction microwire as working medium, demonstrating excellent diode property. In dark, the rectification ratio of the device was more than 103. The heterojunction interface in such a radial heterojunction microwire is quite large, which is beneficial to the dissociation of photogenerated excitons, as well as the generation of photogenerated charge carriers. Under white light illumination (79.4 mW/cm2), the device showed an excellent photosensitivity with an on/off ratio of about 1.7×103 at-10 V bias, and exhibited high EQE values in the visual spectral region, especially around the wavelength of 400nm, the EQE was larger than 60%. Meanwhile, the photocurrent of the device demonstrated good linearity, under the illumination of 400 nm, the detectivity of the device is about 4.6×1011 Jones, compared to some of its inorganic counterparts. The unique radial heterojunction microwire is a promising candidate to be applied in flexible, light portable electronics.2. A new p-type organic semiconductor has been designed and synthesized using dithieno[2,3-b:2’,3’-e]pyrazine (DTP) as a building block, then self-assembled onto the surface of an air-stable n-type TDCNQI microwire in solution, forming a core-shell p-n heteroj unction hybrid microwire. DTP has a similar rigid and planar structure as benzo[1,2-b:4,5-b’]dithiophene (BDT) unit, the star moiety in photovoltaic material, and improve resistance to oxidization via replacing the C atoms by N atoms at the 4- and 8-positions of BDT, and it is a good electron donor. Such a solution-processed method for preparation of 1D core-shell wires was convenient, energy-saving, and suitable for device fabrication. The synthesized 1D core-shell structure has been verified by confocal fluorescence microscopy and TEM, the microwire is of good crystalline, which is good for charge transport. Photoconductive device has been fabricated based on single microwire and exhibited excellent photoconductive property. Under illumination, photogenerated excitons will be dissociated and generate charge carriers at the heterojunction interface along the microwire, and the alkyl side chains of the two components at the interface form an ultrathin insulate layer which will prevent the recombination of the photogenerated charge carriers. The photogenerated charge carriers will form a conductive channel at the heterojunction interface, it will enhance the conductivity of microwire tremendously. At 15 V bias, the on/off ratio of the single microwire device exceeds 106 with a good air stability. This study provides both a good p-type semiconducting material and a convenient preparation method for organic microwire with good photoconductivity.3. A unique ternary organic hybrid microwire radial heterojunction has been fabricated by solvent-assisted self-assembly. PCBM nanoparticles were absorbed onto the surface of p-type TCTA microwire, forming a corn cob-like binary hybrid microwire. Thereafter,4CzIPN were absorbed on the surface of the binary hybrid microwire to form a ternary hybrid microwire radial heterojunction. Device of a single ternary hybrid microwire showed excellent photoconductivity. Compared with TCTA/PCBM binary microwire, the photoconductivity of the ternary microwire has enhanced by more than 3 times. Experiments manifested that the thermally activated delayed fluorescence (TADF) material 4CzIPN facilitates the generation of free charge carriers by upconverting triplet excitons to singlet excitons which dissociate more easily via reversed intersystem crossing. Thus it will take advantage of the triplet excitons that improve the density of charge carriers, thereby increase the conductivity of hybrid microwire. Such a radial heterojunction ternary microwire has brought in an effective method to improve the photoconductivity of organic one-dimension material, and it will be instructing to developing advanced integrated optoelectronic devices.