THIN FILM MICROSTRUCTURE OF HIGH-MOBILITY DIKETO PYRROLO-PYRROLE POLYMER SEMICONDUCTORS WITH BRANCHED ALKYL SIDE CHAINS
Xinran Zhang,1 Lee J. Richter,2 R. Joseph Kline,1 Dean M. DeLongchamp1
1 Polymers Division, Materials Measurement Laboratory
2 Surface and Microanalysis Science Division, Materials Measurement Laboratory
The adoption of edge-on orientation of the conjugated plane was thought to be essential for solution-processable polymer semiconductors to achieve field-effect mobilities above 0.1 cm2 V-1 s-1. However, this criterion was recently challenged by emerging high-mobility materials with plane-on orientation of the conjugated plane. In this work, a series of highly soluble diketo pyrrolo-pyrrole (DPP)-bithiophene copolymers were found to exhibit hole mobilities ≈ 0.4 cm2 V-1 s-1 or even higher, in spite of the presence of disordered, bulky 2-octyldodecyl side groups. Diversity in microstructure, i.e., a transition from a preferentially edge-on orientation of the conjugated plane to a preferentially plane-on orientation/bimodal orientation distribution upon increasing attachment density of the branched alkyl side chains, was revealed by spectroscopic and microscopic measurements. Although the highest mobilities were obtained from thin films with edge-on orientation of the conjugated plane and in-plane liquid-crystalline textures, those with plane-on orientation/bimodal orientation distribution and no discernable in-plane texture still exhibited decent mobilities. Based on the fact that the backbones of all these DPP polymers are strongly oriented within the film plane, we argue that it is the primary cause for achieving excellent field-effect mobilities, while other factors, such as in-plane liquid-crystalline texture, might further enhance charge transport.