Question

A Grätzel cell is to be designed using strontium titanate as the semiconductor and the iodide/triode mediator discussed in the Connection to Energy: Solar Energy Conversion. Where should the energy levels of the sensitizer lie for maximum solar-to-electrical conversion efficiency?

Short answer

In sensitizing dye, the efficiency of the lowest unoccupied molecular orbitals (LUMO) is shown slightly above the energy of the semiconductor conduction band, and the efficiency of the highest occupied molecular orbitals (HOMO) is shown at lower possible energy.

Step-by-step solution

Gratzel cell.

Sensitizing dyes are adsorbed or chemically bound to semiconductors inGratzel cells.

Semiconductors are of wide band gap and nanoparticles. Nanoparticles increasessurface areaandincrease light absorption.

The cell efficiency depends on theelectronic energy levelsof dye molecules.

Working mechanism of Gratzel cell.

When light is absorbed by sensitizing dye, electrons are excited from energy levelsHOMO to LUMO.

Here, HOMO means the highest occupied molecular orbital, and LUMO means the lowest unoccupied molecular orbital.

Electrons are injected from the LUMO level into the semiconductor's conduction band. Then they pass through the external circuit to do work.

Energy levels.

The more efficient LUMOs of the sensitizing dye is located slightly above theenergy of the semiconductor conduction band. This will allow electrons to be easily injected.

At the same time, the HOMO of sensitizing dye is located at thelowest possible energy. This will allow maximum light fraction absorbed.