Evaluation of semiconducting molecular thin films solution-processed via the photoprecursor approach: the case of hexyl-substituted thienoanthracenes†
Organic electronic devices are expected to be easily scalable and highly cost-effective, presuming the good solution processability of high-performance organic semiconductors. However, there are cases where an organic compound with promising semiconducting properties lacks adequate processability and does not form well-performing thin films through conventional solution-based deposition techniques. The photoprecursor approach, in which a soluble photoprecursor is solution-deposited on a substrate and then converted to a target material by in situ photoreaction, can be an effective means to evade such a problem. Herein, we describe a comparative evaluation of thin films deposited by three different methods; i.e., vacuum deposition, photoprecursor approach, and direct spin coating. Two highly crystalline molecular semiconductors, hexyl-substituted anthra[1,2-b:4,3-b′:5,6-b′′:8,7-b′′′]tetrathiophene (C6-ATT) and anthra[1,2-b:5,6-b′]dithiophene (or bent anthradithiophene, C6-BADT), are employed in this study along with the corresponding newly synthesized α-diketone-type photoprecursors. In the case of C6-ATT, thin films prepared through the photoprecursor approach are as good as those obtained by vacuum deposition in terms of surface smoothness and space-charge-limited-current (SCLC) mobility, while direct spin coating affords highly inhomogeneous films. For C6-BADT, on the other hand, employment of the photoprecursor approach is not as effective, albeit it is still advantageous as compared to direct spin coating. These results highlight the power and limitations of the photoprecursor approach, and will serve as a basis for the preparation of practically useful organic devices through this unique approach.