Iodine/Iodide-Free Dye-Sensitized Solar Cells

Abstract

Dye-sensitized solar cells (DSSCs) are built from nanocrystalline anatase TiO₂ with a 101 crystal face (nc-TiO₂) onto which a dye is absorbed, ruthenium complex sensitizers, fluid I⁻/I₃⁻ redox couples with electrolytes, and a Pt-coated counter electrode. DSSCs have now reached efficiencies as high as 11%, and G24 Innovation (Cardiff, U.K.) is currently manufacturing them for commercial use. These devices offer several distinct advantages. On the basis of the electron lifetime and diffusion coefficient in the nc-TiO₂ layer, DSSCs maintain a diffusion length on the order of several micrometers when the dyed-nc-TiO₂ porous layer is covered by redox electrolytes of lithium and/or imidazolium iodide and their polyiodide salts. The fluid iodide/iodine (I⁻/I₃⁻) redox electrolytes can infiltrate deep inside the intertwined nc-TiO₂ layers, promoting the mobility of the nc-TiO₂ layers and serving as a hole-transport material of DSSCs. As a result, these materials eventually give a respectable photovoltaic performance. On the other hand, fluid I⁻/I₃⁻ redox shuttles have certain disadvantages: reduced performance control and long-term stability and incompatibility with some metallic component materials. The I⁻/I₃⁻ redox shuttle shows a significant loss in short circuit current density and a slight loss in open circuit voltage, particularly in highly viscous electrolyte-based DSSC systems. Iodine can also act as an oxidizing agent, corroding metals, such as the grid metal Ag and the Pt mediator on the cathode, especially in the presence of water and oxygen. In addition, the electrolytes (I⁻/I₃⁻) can absorb visible light (λ = ∼430 nm), leading to photocurrent loss in the DSSC. Therefore, the introduction of iodide/iodine-free electrolytes or hole-transport materials (HTMs) could lead to cost-effective alternatives to TiO₂ DSSCs. In this Account, we discuss the iodide/iodine-free redox couple as a substitute for the fluid I⁻/I₃⁻redox shuttle. We also review the adaptation of solid-state HTMs to the iodide/iodine-free solid-state DSSCs with an emphasis on their pore filling and charge mobility in devices and the relationship of those values to the performance of the resulting iodide/iodine-free DSSCs. We demonstrate how the structures of the sensitizing dye molecules and additives of lithium or imidazolium salts influence device performance. In addition, the self-organizing molecular interaction for electronic contact of HTMs to dye molecules plays an important role in unidirectional charge diffusion at interfaces. The poly(3,4-ethylenedioxythiophene) (PEDOT)-based DSSCs, which we obtain through photoelectrochemical polymerization (PEP) using 3-alkylthiophen-bearing ruthenium dye, HRS-1, and bis-EDOT, demonstrates the importance of nonbonding interface contact (e.g., π−π-stacking) for the successful inclusion of HTMs.