7. Molecular Oxygen in Photoresponsive Organic Materials

The presence of molecular oxygen in organic materials designed for use in photoresponsive devices (e.g., solar cells, photon up-converters) can adversely influence the performance of the device in several ways. The most important of these is arguably through reactions that oxygenate and/or oxidize the organic components and thereby change properties relevant for a functioning device. The ground electronic state of molecular oxygen is a spin triplet, O₂(X³Σ_g ⁻). As such, it behaves as a biradical in its chemical reactions, trapping adventitious organic free radicals to yield reactive peroxyl radicals. The lowest excited electronic state of molecular oxygen is a spin singlet, O₂(a¹Δ_g), and can be formed in appreciable yield by energy transfer from a photoexcited organic molecule to O₂(X³Σ_g ⁻). Functional groups common to molecules used in photoresponsive materials (e.g., double bonds, sulfides) can react with O₂(a¹Δ_g) to form peroxides, which likewise are reactive and propagate disruption. The quenching of an excited-state organic component by O₂(X³Σ_g ⁻) also points to other ways in which oxygen can influence device performance. For example, the oxygen-mediated deactivation of comparatively long-lived triplet states can adversely influence the energy fusion process essential for some up-conversion devices. Likewise, electron transfer from an excited-state organic molecule to O₂(X³Σ_g ⁻) and/or O₂(a¹Δ_g) can not only interfere with desired charge movement (e.g., in a photovoltaic device), but it will also produce the superoxide radical ion that, in turn, can contribute to oxygenation reactions. Thus, to exploit fully the functional capabilities of photoresponsive organic materials and to prolong device longevity, and in lieu of completely excluding oxygen, it is necessary to monitor, understand, and ultimately control the behavior of oxygen in such systems.