Development of a Simple, Scalable, and Vacuum-Free Method to Fabricate Organic Semiconductor Single Crystals with Area Selectivity for High Performance Organic Field-Effect Transistor Applications

2020 Fiscal Year Research-status Report

• Project/Area Number: 20K22421
• Research Institution: Institute of Physical and Chemical Research
• Principal Investigator: Bulgarevich Kirill 国立研究開発法人理化学研究所, 創発物性科学研究センター, 特別研究員 (60880268)
• Project Period (FY): 2020-09-11 – 2022-03-31
• Keywords: 有機トランジスタ / 単結晶有機半導体 / 空気中蒸着 / パターン化結晶成長 / 表面エネルギーパターン

Outline of Annual Research Achievements

According to the proposed research plan, the organic semiconductor single-crystal (SC) formation of rubrene, pentacene, DNTT and 1,3,6,8-tetrakis(methylthio)pyrene (MT-pyrene) by microspacing in-air sublimation (MAS) method was successfully achieved. It was confirmed that the SCs show good charge transport properties comparable to those grown by physical vapor transport (PVT) method by fabricating organic field-effect transistors (OFETs).
Interestingly, MT-pyrene OFETs showed extremely high carrier mobility of > 30 cm2/Vs at almost ideal low voltage operation, which exceeded rubrene and DNTT OFETs fabricated using the same method (9 and 2 cm2/Vs, respectively). This could be achieved because of successful manipulation of crystal structure from dimeric herringbone of parent pyrene to the new type of 2D pi-stack by methylthiolation. The article regarding the results on MT-pyrene is currently under revision.
Also, during the optimization of conditions for MAS method, a new method for SC formation was discovered. It was found that by using the similar setup to the MAS method, but in a semi-enclosed environment, the SC formation can occur more uniformly (almost independent on initial source material distribution) at much larger area. In this setup, the direct paths from source material to the crystallization area are intentionally blocked. However, the crystals can still form probably from saturated vapor. This "indirect sublimation" method is expected to be much more scalable and material efficient than PVT method and the article regarding it is in preparation.

Current Status of Research Progress

2: Research has progressed on the whole more than it was originally planned.

Reason

The usefulness of the MAS method in producing high quality SCs suitable for high-performance OFETs was successfully confirmed. For the proposed research, it is also important to achieve SC formation via the liquid like melt for the lyophobic/lyophilic patterning. Conditions for such SC formation were found for rubrene and MT-pyrene, although the formation of free-standing crystals is not yet completely suppressed.
The formation of highly lyophobic CYTOP and octadecyltrichlorosilane layers, and their photopatterning process was also established with the help of vacuum ultraviolet light exposure through a photomask.
The application of lyophobic/lyophilic patterns for SC patterning is currently under investigation.

Strategy for Future Research Activity

Planning to proceed according to the proposed research plan. First, the MAS method conditions for patterned SC growth has to be found and optimized. Free-standing SC formation that can also occur on lyophobic areas has to be suppressed.
Next, SCs grown on lyophilic areas will be studied in terms of crystallographic orientation, OFET properties, trap density and others. Surface modifications of lyophilic areas such as phenylalkyltrichlorosilane treatment are probably required for improving the performance of OFETs and will be tested.
Finally, the high-performance OFETs arrays will be fabricated. Actual OFET-based devices such as pseudo-CMOS inverters will also be tested.

Causes of Carryover

We planned to purchase quartz masks for vacuum ultraviolet light, drew blueprints, and made adjustments through repeated communication with the vendor, but unfortunately, we were unable to produce a finished product in time for procurement during the fiscal year. After persistent exchanges of opinions, we were finally able to place an order with the vendor in May 2021. In the next fiscal year, we will make up for this delay by testing, designing and ordering quartz masks more efficiently.