Metal Complexes

Rationally Designed Metal Complexes for Organic Photovoltaics:

Organic cells are in a class fundamentally distinct from conventional solar cells. In the latter case, excitons are generated upon light absorption. These Frenkel excitons have a large binding energy (>0.1-1 eV) and short diffusion length (~3-20 nm). To get useful work, the photoexcited donor-acceptor (DA) pair (exciton) must migrate to the interface where fields are large, and can be split into free carriers. The latter must migrate to the electrodes before recombining. In the last five years, organic thin film PV cells have progressed rapidly.

There are significant challenges at every stage in the photogeneration process :
(1) Absorption: polymers tend to absorb in a limited energy window that does not normally include the near infrared region; also their HOMO-LUMO gaps are typically too large, and they have a small absorption coefficient (a-1~100-200 nm) to efficiently absorb light within an exciton diffusion length (~3-20 nm).
(2) The mobility is low, field-dependent and strongly dependent on morphology and microstructure. Conduction along polymer chains is very different than between chains; conduction within a (1D) chain depends much more sensitively on the chain topology than in 3D, owing to traps.
(3) Charge transfer across the D/A interface can be slow. This depends on the matrix element between the constituents, which in turn depends on their relative energy levels, symmetry and wave function overlap.
(4) Polymers that tend to be the most efficient for PV tend to have poor electrochemical and thermal stability under operating conditions.

We propose to solve some of these problems by developing stable molecular and polymeric materials with wider absorption band and longer exciton diffusion length based on metal complexes. Cyclometalated Ir and Pt complexes have been widely studied for organic light emitting device (OLEDs) applications. These compounds are mostly neutral, sublimable, and phosphoresce very strongly at room temperature, demonstrating very high electrochemical and thermal stability.