Inorganic-organic nanocomposites and their assemblies for solar energy conversion

Unlike a conventional inorganic semiconductor in which the charge carriers are immediately generated upon the exposure to light, an organic semiconductor forms a spatially localized electron-hole pair (that is, a Frenkel-type exciton).1,2 These excitons typically have large binding energy of around 0.5 eV, which means that the generated excitons need to migrate to the donor-acceptor interface for their dissociation. The excitons diffuse randomly and are not influenced by an electric field as they are electrically neutral. Hence, the length scale of organic phases must be comparable to the exciton diffusion length (L = (D·t)1/2 = ~10 nm, where t is the lifetime of the exciton, and D is the diffusion coefficient).3-5 To understand the exciton migration process, the energy transfer mechanism should be invoked and can be written as *D + A → D + *A where D and A are the donor and the acceptor, respectively. The asterisk represents the excited state of molecules. The energy transfer may occur either by the dipole-dipole interaction or by the electron exchange interactions. The energy transfer through electron exchange interactions requires an orbital overlap between molecules and is sometimes referred to as “Dexter energy transfer” or “orbital overlap mechanism.”6,7 In this case, the electron is transferred from the LUMO of the donor to that of the acceptor, and the hole is also simultaneously transferred from the HOMO of the donor to that of the acceptor. As the Dexter transfer is governed by the orbital overlap between the electron density of both the excited donor (*D) and the nearby ground state acceptor (A), the rate of the Dexter transfer can be expressed as kDexter ∝ <ψ(*D)ψ(A)| Hex |ψ(D)ψ(*A)>2 where Hex is the electron exchange operator. The form of the Hex operator is exp(-RDA), where RDA is the distance between the donor (D) and the acceptor (A).