Enhancement of Out-of-plane Mobility in P3HT Film by Rubbing: Aggregation and Planarity Enhanced with Low Regioregularity

Abstract

Poly(3-hexylthiophene) (P3HT) is used as a conjugated polymer material for the fabrication of organic solar cells. To enhance the performance of the solar cell, it is crucial to improve the charge transport properties of the out-of-plane direction, which corresponds to the direction normal to the cell. The out-of-plane hole mobility in P3HT films before and after rubbing was investigated. The mobility was measured by the time-of-flight method and was enhanced by a factor of 8 with rubbing. The enhancement of mobility is significantly dependent on the regioregularity (RR) of P3HT and is the largest with low RR (91%). The mobility of low RR becomes almost equal to that of high RR (98%) after rubbing. Polarized absorption and Raman spectral measurements, and optical, atomic force, and scanning electron microscopy observations indicate the mobility enhancement can be attributed to aggregation and molecular planarity enhanced by rubbing. Three enhancement factors (EFs) for the mobility, aggregation, and planarity were quantified by changing the RR. It was thus determined that subtle aggregation produces a large mobility enhancement. According to the results of grazing-incidence X-ray diffraction measurement, the increase of face-on component of P3HT by rubbing was observed and corresponds to the increase of π–π staking of thiophene ring along the out-of-plane direction. Evolution from the disordered to the ordered P3HT structure by rubbing significantly improves hole transport in the out-of-plane direction.