Growth And Applications Of High Quality Ultra-thin Rubrene Films

Rubrene, as a benchmark organic semiconductor, has attracted wide attention recent years for its high charge carrier mobility and highly delocalized transport mechanism. However, these superior performances were mainly conducted on single crystals rather than films that are more practical for large-area and low-cost device application. Thus we hope to obtain rubrene crystalline films to realize applications of this kind of molecule with high device performance. The obstacle is that forming homogenous crystalline films on amorphous substrates for molecules like rubrene is particularly difficult, owing to their complicated non-planar and twist conformation. This issue was addressed by growing on substrates that pre-modified by crystalline ultra-thin films or self-assembled-monolayer. However, rubrene maynot directly from crystal films until the initial generated amorphous clusters experience a phase transition process that happens when a certain critical thickness is achieved. And it remains a challenge to guarantee the uniformity and reproduction owing to this complicated growth behaviors. So our main research purposes are（1） figuring out the growth behaviors of rubrene molecules and obtaining methods for rubrene to form into crystalline phase directly which contributing to the high quality rubrene thin films with high homogeneity and reproducibility.（2） Researching the performance of electronic devices based on crystalline ultra-thin rubrene films including organic thin-film transistor and gas sensors.1. We have conducted a deep research on the weak epitaxy growth of rubrene using p-6P as inducing layer. Experiments indicate that double layer p-6P is more benefit of growing crystalline rubrene films compared to single layer p-6P. In the assist of p-6P, rubrene films exhibit three kinds of morphologies: dot-like, chain-like and terrace-like films. And the chain-like film is the transition state between amorphous dot-like and crystalline terrace-like films. That is to say the formation of crystalline rubrene films has experienced a phase transistion.2. We have obtained high quality crystalline ultra-thin rubrene films through introducing foreign seeds C₆₀. It turns out that rubrene crystalline films were fabricated in terms of a spontaneous crystallization process, which were triggered by the imposing of C₆₀. We have studied the OTFTs performance based on this kind of heterogeneous crystalline ultra-thin films subsequently. For heterogeneous film with thickness of only 3 nm, the field effect signal can be detected, indicated a better crystallinity from the very initial stage. A high holes’ mobility of 1.0-1.4 cm2/V·s was obtained when rubrene thickness is 10 nm, which are about one order of magnitude higher than ultra-thin films without C₆₀ imposed（0.006-0.25 cm2/V·s）. Results indicate that the ultra-thin rubrene crystalline films can possess a fairly good charge transport ability which is closely related to the highly ordered and homogeneous film morphology.3. We have researched the performance of gas sensors based on heterogeneous crystalline rubrene ultra-thin films. The relative response sensitivity（%） to NO₂ reaches to 3400 and 5200 for NH₃. This sensitivity is comparable to the most NH₃ gas sensors based on organic semiconductors. The results demonstrate that sense performance of rubrene films to both NO₂ and NH₃ represents high level for organic semiconductors and it is the first case that revealed the sensor functionality for rubrene molecules, benefited from this direct crystallization approach, and provide new methods for non-planar configuration molecules like rubrene to be applied in gas sensors.