Solution-processed Orgainc Semiconductor Crystalline Thin Films And Their Applications In Field Effect Transistors

Organic field effect transistors(OFETs) are of potential importance for electronic applications such as flexible circuits, display backplanes and sensors due to their solution processability and chemical versatility over their inorganic counterparts. In addition to low expense and compatibility in the roll-to-roll process, solution processed high-crystalline OFETs have been attracting enormous attention because of their higher mobility and better environmental stability than the amorphous and polycrystalline OFETs. Although there are several solution methods to fabricate organic crystalline films, most of these techniques are in the exploratory stage and always need high temperature treatment process. Therefore, these methods are still not suitable for convenient, large-area commercial production. Besides, the influence of the experimental conditions on the packing arrangement of crystalline thin films is not clearly understood. After systematically investigating a traditional solution method—solution shearing, two novel room-temperature solution methods have been developed to produce highly aligned crystalline films. We focus on the relationship between the experimental conditions and the packing arrangement of crystalline thin films. Furthermore, the performance of the OFETs is explored as well as related applications in red light detection and non-volatile memory devices. The main contents and efforts are as follows:1. The solution shearing method is utilized to fabricate large-area crystalline thin films of 1,3-bis[(3,3-dimethylindolin-2-ylidene)methyl]squaraine(ISQ). The crystalline orientation under different experimental conditions is systematically investigated by GIXRD method in order to accumulate experience for empoldering new solution methods. Based on the highly ordered crystalline thin film, the OFET devices are fabricated based on it, exhibiting the on/off ratio of 104 and field-effect mobility of 0.04 cm2 V-1 s-1,which is to the theoretical calculation values. Under weak red light illumination as low as 0.2-0.4 μW/cm2, the phototransistors show high responsibility up to 4800 A W-1, which is higher than the previously reported red light photosensitive OFETs.2. We have developed a novel room-temperature solution method of volatilize-controlled oriented growth(VOG) to produce highly aligned crystalline thin films by vacuum control. First, by using a single solvent of carbon tetrachloride, we fabricate OFET devices based on the crystalline films of TIPS-PEN(6,13-Bis(triisopropylsilylethynyl)pentacene). Besides, the stability and the non-volatile memory properties of the devices have been explored. The OFET devices show good non-volatile memory properties with a high memory ratio of 102 and a long retention time of 103 s. Then, we use 1,2-dichloroethane as the solvent of the TIPS-PEN, and the cyclohexane to provide the atmosphere during the crystal growth,large area aligned single crystalline thin films can be achieved on several substrates at the same time. Without the conventional photolithography for patterning, the TIPS-PEN crystalline thin films cover over 90% on the substrates. The VOG growth direction is in the evacuating direction, along with the smallest lattice parameter found. On the basis of the single crystalline thin films, OFETs with field-effect mobility of 0.42 cm2V-1s-1 and on/off current ratio of 105 are achieved. The electrical characteristics strongly depend on the direction of crystal orientation,which further proves the high alignment of the crystalline thin films.3. We have developed another novel solution method of Marangoni effect-controlled oriented growth(MOG) to fabricate large area high-crystalline thin films of TIPS-PEN. We investigate the growth mechanism under the combined action of the marangoni flow and the convective flow. The thickness of the crystalline thin film can be controlled based on the MOG method, which further influence the electrical performance of the OFETs. The on/off current ratio is 105 and the field-effect mobility is 0.92 cm2V-1s-1, which is among the highest performance OFETs fabricated under room-temperature. This work shows that the MOG method is a potential technology for industrial applications of organic semiconductors.