Sexithiophene-Based Photovoltaic Cells with High Light Absorption Coefficient via Crystalline Polymorph Control

Herein, we report an efficient approach to control the crystalline polymorph of evaporated alpha-sexithiophene (6T) thin films by keeping them overnight (12 h) under vacuum. Further, we investigated the effects on the performance of organic photovoltaic devices of controlling the 6T polymorph via this vacuum technique so that the films take on the low-temperature (LT) polymorph (in which the backbones of the 6T molecules lie flatter on the substrate the so-called lying-down orientation). Our results revealed that when the organic layer was deposited directly onto a copper(I) iodide interlayer, the angle between the organic backbone and the substrate was reduced in the LT polymorph compared with the high-temperature (HT) polymorph. The power conversion efficiency of solar cells could thus be enhanced from 0.58 to 1.77% via a change in the crystal polymorph of the 6T layer from HT to LT by simply keeping the films in vacuum for 12 h.