Titanium suboxide as an optical spacer in polymer solar cells

The use of a layer of titanium suboxide $\left({\text{TiO}}_x\right)$ between the bulk heterojunction (BHJ) thin film and the metal contact in “plastic” solar cell has been shown to increase both the short-circuit current and the fill factor. Using a combination of optical modeling and device experiments, we clarify the role of the ${\text{TiO}}_x$ layer as an optical spacer. For an appropriate choice of the BHJ thickness and composition, the ${\text{TiO}}_x$ layer can enhance the absorption within the active layer, thereby increasing the photocurrent and the power conversion efficiency.