(375d) Engineering Energy Levels At the TiO2:P3HT Interface Using Atomic Layer Deposition

It has been shown over the last two decades that conversion efficiencies up to 12% can be achieved using TiO₂ based solar cells, such as in liquid electrolyte dye sensitized solar cell. Recently, there has been a push to replace these hazardous electrolytes with organic materials to create environmentally friendly devices with extended lifetimes. However, one of the limitation of these hybrid solar cells is electron-hole interaction across the metal-oxide and organic hole transporter interface.

In this work, a core-shell nanostructured hybrid solar cells is synthesized with a single crystal core in order to increase electron mobility and light scattering without additional recombination effects. A combination of hydrothermal and atomic layer deposition (ALD) was used to fabricate TiO₂ nanowires (NWs) in order to take advantage of the directed electric field within the structures. During the growth process a Sn⁴⁺ dopant is introduced to create a doped Sn:TiO₂ nanostructured array with increased electron mobility. After the NW growth, an additional ALD step is used to deposit a TiO₂ layer, creating the core-shell structure. The dopant gradient within the core-shell structure causes the electrons to migrate toward the core of the nanowire due to the lower energy conduction band, potentially reducing electron-hole recombination at the TiO₂:P3HT interface. The conduction band engineering has a similar effect as that seen with dipole modification of the interface. Additionally, the surface can be further modified with dye molecules. This doped core-shell structure has resulted in a conversion efficiency of 2 % with a surface treatment of the squaraine dye SQ2. This  efficiency is due to the contribution of the dye and the P3HT in the photon current generation.. The engineered energy levels and interfacial modifiers have a significant effect on the external quantum efficiency and internal resistances, as determined using various characterization methods.