Morphology control for highly efficient organic–inorganic bulk heterojunction solar cell based on Ti-alkoxide

• An organic–inorganic bulk-heterojunction photoactive layer was used.
• Electron donor was a semiconducting polymer and electron acceptor was Ti-alkoxide.
• Demonstration of morphology control by Ti-alkoxide molecules.
• Determination of Jsc value by the phase-separation structure in an ultra-thin film.
• Charge-separation efficiency of Ti-alkoxide system equivalent to fullerene system.

The number of publications concerned with typical bulk-heterojunction solar cells that use fullerene derivatives and inorganic materials as electron acceptors has grown very rapidly. In this work, we focus on Ti-alkoxides as electron acceptors in the photoactive layers of fullerene-free bulk-heterojunction solar cells. We show that it is possible to control the morphology by adjusting the molecular structure and size of the Ti-alkoxides. The short-circuit current density (Jsc) increased to 191 μA/cm2 from 25 μA/cm2 with a maximum, when the phase-separation structure was continuously formed to within about 20 nm below the exciton diffusion length by using either titanium(IV) ethoxide or isopropoxide as an electron acceptor. Within a thickness of 30 nm, the photoactive layer is not influenced by the electron transfer ability; thus, we demonstrate that the charge-separation efficiency is equivalent to that of a fullerene system.