Organic PV Technology 3

Continuing from Organic PV Technology 2:

DSSC is a different kind of organic solar cell.  It is a photoelectrochemical system consisting of TiO₂ nanoparticles, dye particles/sensitiser, an electrolyte, and a platinum contact.  The photoactive dye sensitiser is the electron donor, while the TiO₂ nanoparticles are the electron acceptors.  They are mixed together to form an organic bulk heterojunction solar cell.  The dye photosensitiser is ruthenium polypyridine.

When a photon is absorbed by the dye, an electron is excited from its ground state S (the HOMO) to an excited state S* (the LUMO).  The S* state is at a higher energy level compared with the energy level of the conduction band of TiO₂.  This causes the light excited electrons to be injected into the TiO₂ nanoparticles.  The dye molecules (see diagram above) will remain positively charged, while the electrons in the TiO₂ will diffuse to the TCO based back contact, and across the external electric circuit to the other contact/electrode (the counter electrode).

There is an electrolyte between the counter electrode and dye.  The usual electrolyte contains iodine, where the positively charged oxidised dye molecule is neutralised by a negatively charged iodide.  3 negatively charged iodides neutralise 2 dye molecules to create 1 negatively charged triiodide.  This triiodide will move to the counter electrode and be reduced by 2 electrons into 3 negatively charged iodides.

A DSSC requires an expensive platinum (Pt) back contact to catalyse the reactions.  Hence, it depends on the HOMO and LUMO level of the dye, the Fermi level of the TiO₂, and the redox potential of the iodide and triiodide reactions.  The best efficiency of DSSC is 14.1%.  DSSCs are relatively low cost, but the electrolyte may freeze at low temperatures, stopping power production and possibly damaging the DSSC.  Higher temperatures will expand the electrolyte significantly, so encapsulation of DSSC is more difficult.

To improve the DSSC, materials cheaper than Pt must be discovered.  More stable and resistive electrolyte materials must be developed.  Finally, dyes with improved spectral and band gap utilisation must be developed.



Reference:
5.5 Organic PV Technology, Delft University of Technology, https://www.youtube.com/watch?v=jCtgMm55nBA