Guocheng Zhang

Solution-Processed Organic Thin-Film Transistor Arrays with the Assistance of Laser Ablation

A key step toward commercialization of organic thin-film transistors (OTFTs) is to manufacture large-area OTFT arrays with desired uniform device performance. In this work, for the first time, solution-processed OTFT arrays were fabricated with the assistance of laser ablation. The source-drain electrodes and the whole devices were patterned by precise control of laser intensity and process path. Compared with traditional methods, this approach significantly simplifies the fabrication process of OTFT arrays with high quality and high yield. A careful selection of laser processing parameters is key to obtaining high quality and high performance OTFT arrays. The grazing incidence X-ray diffraction experiments and device performance tests ensured the selection of proper laser ablation intensity. Eventually, the OTFT arrays on silicon wafer and ITO glass exhibited uniform electrical characteristics with the mean mobility of 0.16 and 0.10 cm2 V-1 s-1, respectively. These results demonstrated that the laser ablation process provides a promising tool to simplify the fabrication of solution-processed OTFT arrays with low cost and high yield, which has great potential in upscaling of high performance OTFT arrays for display and circuits.

Solution-processed metal oxide arrays using femtosecond laser ablation and annealing for thin-film transistors

In this study, a femtosecond (fs) laser is proposed to pattern and anneal a metal oxide active layer for the facile fabrication of metal oxide thin-film transistor (TFT) arrays. Compared with conventional photolithography and lift-off techniques, fs-laser ablation significantly simplified the patterning process due to its advantages such as no contact, high yield and high resolution. Uniform patterned arrays with sharp edges and small feature size (down to 32 μm) were achieved using fs-laser ablation. Moreover, fs-laser annealing was also applied for rapid conversion of the precursor to metal oxide lattices. The results showed that an increase in laser intensity lead to an improvement in charge carrier mobility and an improvement in the on–off current ratio of indium zinc oxide (IZO) thin film transistors (TFTs) along with a negative shift of the threshold voltage. X-ray photoelectron spectroscopy (XPS) analysis indicates that an increase in laser intensity enhanced the breakdown of weak or metastable chemical bonds within IZO films, leading to the removal of hydroxide-related (OH) species and enhancement of metal oxide composition and oxygen vacancies (Ov); it facilitated carrier transport and led to an average mobility up to 9.0 cm2 V−1 s−1. Furthermore, compared with thermal annealing (TA), fs-laser annealing could reduce the thermal budget and more effectively realize the dehydroxylation behavior for OH-related species, resulting in higher charge mobility. These results clearly demonstrated that fs-laser ablation and annealing could provide a promising tool to significantly simplify the fabrication of metal oxide TFT arrays due to their advantages such as high resolution, high yield and low cost. Hence, these techniques show great potential to be applicable in the fabrication of large-area metal oxide TFT arrays for use in display devices and circuits.

Importance of Solvent Removal Rate on the Morphology and Device Performance of Organic Photovoltaics with Solvent Annealing

Solvent vapor annealing has been widely used in organic photovoltaics (OPV) to tune the morphology of bulk heterojunction active layer for the improvement of device performance. Unfortunately, the effect of solvent removal rate (SRR) after solvent annealing, which is one of the key factors that impact resultant morphology, on the morphology and device performance of OPV has never been reported. In this work, the nanoscale morphology of small molecule (SM):fullerene bulk heterojunction (BHJ) solar cell from different SRRs after solvent annealing was examined by small-angle neutron scattering and grazing incidence X-ray scattering. The results clearly demonstrate that the nanoscale morphology of SM:fullerene BHJ especially fullerene phase separation and concentration of fullerene in noncrystalline SM was significantly impacted by the SRR. The enhanced fullerene phase separation was found with a decrease of SRR, while the crystallinity and molecular packing of SM remained unchanged. Correlation to device performance shows that the balance between pure fullerene phase and mixing phase of SM and fullerene is crucial for the optimization of morphology and enhancement of device performance. Moreover, the specific interfacial area between pure fullerene phase and mixing phase is crucial for the electron transport and thus device performance. More importantly, this finding would provide a more careful and precise control of morphology of SM:fullerene BHJ and offers a guideline for further improvement of device performance with solvent annealing.