I would like to add to the colleagues above that:

The htl must match the donor of the active layer in the sense its homolevel must lie slightly above the homolevel of the donor layer. At the same time its lumolevel must lie above that of the active layer to reflect electrons back to the active region. The htl must be rich with holes to act as a good collector of holes from the active layer in the sense it needs small drift field to affect large hole current.

These electronic properties is not sufficient for using the material since it must be compatible with the fabrication process. Also, the layer must be transparent to let light pass to the active material.

Best wishes

From my experience, there are several reasons:

1. The energy level of spiro may not match many organic BHJ systems.

2. In the inverted structure of organic solar cells, the spiro can't be well spin-coated on the top of the active layer.

Hi Nithya K S ,

Very good question. Leiping Duan already answered your question somewhat, but I would like to explain in a little more detail.

I agree with the second point of Leiping Duan . Spiro is usually processed using the same solvents you would use for the active layer in an OPV (CB, CF, ODCB, etc.), so if we you and spin-cast Spiro on top, you will dissolve the active layer underneath. This is exactly why PEDOT:PSS is so widely used as the bottom hole-transporting layer, as you process it from water and it won't dissolve in solvents like CB, CF and ODCB.

For the first point of Leiping Duan , I'm not so sure. I don't think that the energy levels would mismatch. I'm sure if you could find a way to process Spiro either underneath or above the active layer, then it would do a fine job at extracting holes. The HOMO levels between PEDOT:PSS and Spiro-OMeTAD match very well, so I don't see why Spiro would not match well with the HOMO level of typical donor polymers used in OPV. For example, I have used PEDOT:PSS to inject holes into Spiro-OMeTAD for some of my charge-transport studies:

Article Charge Transport in Spiro-OMeTAD Investigated through Space-...

Let me know if you have any more questions.

Jason

I would like to add to the colleagues above that:

The htl must match the donor of the active layer in the sense its homolevel must lie slightly above the homolevel of the donor layer. At the same time its lumolevel must lie above that of the active layer to reflect electrons back to the active region. The htl must be rich with holes to act as a good collector of holes from the active layer in the sense it needs small drift field to affect large hole current.

These electronic properties is not sufficient for using the material since it must be compatible with the fabrication process. Also, the layer must be transparent to let light pass to the active material.

Best wishes