State bulk heterojunction structures

Recently, Foreman demonstrated that fe-linear terms in semiconductor heterojunctions [47] are enhanced near the interface, and that the associated mixing of Fg hh and Ih states is increased so that it influences the quantum-well Pockels effect. [48] The reduced symmetry in quantum well structures (from Td in the bulk to C2v) increases the optical anisotropy. The work in Ref. [47] shows that the A -linear splitting contributes significantly to this anisotropy because the C coefficients are often an order of magnitude larger in the heterojunctions than in the bulk materials. 

The electronic structure of interfacial excitations was recently described by Beljonne, Herz, Friend, and coworkers [38,88]. Static dipoles in the ground state of one polymer chain are considered to modulate the exciton energy on the neighboring polymer chains by Coulomb interactions across the heterojunction. Depending on the relative position of the chains, attractive or repulsive interaction were found, which respectively stabilize or obstruct charge-transfer state formation [88]. This implies that the density of states of interfacial excitons is broader than that for bulk excitons. Indeed, this was experimentally observed by femtosecond time-resolved photoluminescence spectroscopy in the same study [88]. The broader density of states at the heterojunction is consistent with the fast directional motion toward the interface described above (see Section 13.3.1) [87]). Excitons could transfer quickly into lower energy interfacial sites when they approach the interfacial region. 

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