The Study Of Organic Semiconductor Optoelectronic Devices Fabricated By Terpene Solvents

In the past two decades, organic optoelectronic devices, such as organic light emitting diodes(OLEDs), organic field-effect transistors(OFETs), and organic photovoltaic cells(OPVs), have attracted great attentions. Organic semiconductors for the active layers of the organic devices can be fabricated by vacuum depositions of small molecules or solution depositions for polymers and larger molecules. In the solution processing, a solvent is needed to dissolve an organic material, and then its thin solid film can be dopsited typically by spincoating, screen printing, and blade-coating. In comparison to vacuum technology, the solution processing is relatively convenient to fabricate a thin solid film. Moreover, the solution procesing has big potential to supply a large area and low cost film, from which polymer optoelectronic devices based on conjugated polymers have received wide interest from scientific communities.Normally, a conjugated polymer, based on aromatic units, possesses a rigid backbone. In order to dissolve conjugated polymers, benzene, toluene, p-xylene, chloroform, dichloromethane, chlorobenzene(CB), o-dichlorobenzene(DCB), and 1,2,4-trichlorobenzene, are the common solvents. However, these solvents are toxic and hazardous to human health and environment. In many countries, more and more strict policies have been established to protect the environment and human health. Therefore, green solvents are highly desirable for the solution processing of optoelectronic devices based on conjugated polymers, especially for commercial purpose.We have noticed that d-limonene is a possible green solvent with some unique merits. Dlimonene is a natural product that exists in some fruits, including orange, lemon, mandarin, lime, and grapefruit. Currently, d-limonene is widely used as a flavor and fragrance additive in perfumes, soaps, foods, chewing gum, and beverages. D-limonene is also listed in the Code of Federal Regulation as generally recognized as safe(GRAS) for a flavoring agent. The boiling point for d-limonene is 176 °C, very close to 180 °C for DCB. Delightedly, we found dlimonene could show good solubility to dissolve several kinds of conjugated polymers in our lab。This thesis is concentrated on the performance of orangic solar cells, orangic light-emitting diodes, and organic field effect transistors. In the Chapter 3, we measured the solubilities of many polymers in all kinds of solvents. Then, d-limonene, l-limonene and terpenolenene were determined to be the low toxic solvents for processing organic solar cells. In order to explan the different salvation abilities of solvents, Hansen solutoin ability parameters were used. According to choosing the kinds of additives, purification method, processing temperatures, and low boiling point additives, the good device performance have been got. For the dlimonene-deposited OPVs, identical PCE of 6.99% can be obtained. The l-limonene-deposited OPVs have a similar PCE of 6.72%。They are both high than the PCEs of 1.24, which deposited by terpinolene.In Chapeter 4, The reference device casted with the p-xylene solution displayed a LEmax of 3.11 cd/A and an EQEmax of 3.05%, whose maximum brightness(Lmax) was 9340 cd/m2. Delightedly, using d-limonene as the processing solvent for the emissive layer could achieve better device performances. The EL device fabricated with d-limonene solution in N2 atmosphere could elevate LEmax and EQEmax to 3.66 cd/A and 3.57%, respectively. The Lmax of the device increased to 11540 cd/m2. Notable LEmax of 3.42 cd/A and EQEmax of 3.35% were achieve when the emissive layer was deposited with d-limonene solution in air.For the reference OFET fabricated with the DCB solution, its calculated mh was 1.00 cm2/(V s), with on/off current ratio(Ion/Ioff) of 106 and threshold voltage(Vth) of -3.0 V. The OFET device fabricated with d-limonene in N2 atomsphere could show a comparable performance: mh of 1.06 cm2/(V s), Ion/Ioff of 106, and Vth of -1.5 V. The results indicate that D-limonene is a promising solvent for high-performance OFETs. When the OFET device was fabricated with d-limonene in air, a notable μh of 0.7 cm2/(V s) could be achieved.