Organic PV Technology 2

Continuing from Organic PV Technology 1:

Organic materials are different from inorganic semiconductors when it comes to light excitation.  In inorganic semiconductors, excitation will move an electron from the valence to conduction band, leaving behind a hole, and this electron-hole pair is weakly bound and can easily separate and diffuse away from each other.  In organic materials, excitation creates excitons, which are electron-hole pairs that are excited but still bound together due to mutual coulombic force between the electron and hole.  The excitons can move around through diffusion, but their diffusion lengths are short (about 10nm) because they recombine easily back to the ground state in a few ns.

To build an organic solar cell, a heterojunction based on 2 types of intrinsic materials can be used, where one type is an electron donor, and the other is an electron acceptor.  The materials used can be either semiconductor or conjugated compounds.  As an example, when we join an electron donor conjugated polymer to an electron acceptor conjugated polymer, the HOMO and LUMO levels can be seen in the diagram above.  The difference between the HOMO and LUMO leads to the creation of an electrostatic force between the 2 layers.  The excitons will hence diffuse to the interface between the 2 materials.  When the materials are chosen to enable the difference between the HOMO and LUMO to be sufficiently large, the local electric fields may be strong enough to break up the excitons.  Hence, electrons are injected into the electron acceptor, while the holes remain in the electron donor.

Due to the short diffusion lengths of excitons and the required absorption length of at least 100nm, the electron donor and acceptor materials are mixed together to form bulk heterojunction photovoltaic devices (see diagram above).  The short diffusion lengths can be reached in the bulk heterojunction blend.  Hence, a large proportion of the excitons can reach an interface to separate into electrons and holes.  The electrons will move through the acceptor material to the metal back electrode, while the holes will move through the donor material and collect at a TCO electrode like ITO.

The best efficiency reached by tandem organic solar cells is 12%.  The stability of these cells is unknown, but the production cost is low.  These cells can also be chemically engineered to have a wide variance in band gaps, and can be integrated into flexible substrates.  However, they have low efficiencies, stability and strength as compared with inorganic PV cells.  There is also a need for more expensive encapsulation materials to stabilise the organic PV.



Reference:

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