Synthesis, Properties and Applications of Polymeric Matrix-Based Phosphor Hybrids

Phosphors are special types of compounds which emit lights when exposed to visible light, ultraviolet radiation or electron beam. In general, these compounds are prepared from inorganic transition metal or rare earth compounds. The radiation causes movement of its valence electron to the conduction or exciton band leaving behind a hole in the valence band. The electron–hole pairs moves to the impurities in the crystal of the phosphor which rapidly de-excite by emitting light. The inhomogeneity in the crystal structure of phosphor is created by addition of the impurities or dopants which is also called activator. Accordingly, a phosphor consists of a host which is oxides, nitrides, sulfides, halides, silicates or selenides of Zn, Cd, Mn, Al, Si or different rare earth metals and an activator metal such as Cu or Ag activated ZnS or Bi activated CaS phosphor. Apart from inorganic phosphors, more energy efficient phosphors for lighting and other optoelectronic applications are prepared from metal–organic frameworks (MOF) or coordination polymers. MOF consists of single metal ions or clusters of metal ions linked by organic ligands having multiple binding sites to form extended network structures. However, the inorganic or MOF based phosphors have several drawbacks like limited resources, high toxicity and also high cost. In contrast to inorganic or MOF based phosphors, metal-free small organic molecules or polymer based room temperature phosphors (RTP) are environment friendly and easy to process. These two types of RTPs are characterized by its long-lived triplet excitons and larger Stokes shift. However, easy processing, good flexibility and stretching ability, low cost, excellent electron mobility and thermal conductivity have made polymer based RTPs more attractive than small organic molecules based RTPs. Thus, Polymer based RTPs are widely used in organic light emitting diodes, solar cells, field effect transistors, memory devices and many other similar applications. The phosphors are prepared by (1) intersystem crossing (ICS) from the lowest excited singlet state (S₁) to a triplet state (T_n) and (2) radiative transition from the lowest excited triplet state (T₁) to the ground state (S₀). The emission from T₁ state is quenched at room temperature under ambient conditions. Accordingly, the challenge to get efficient RTP is to suppress nonradiative decay. Polymers are of high molecular weight with long chains that can cause entanglement and a high degree of rigidity making them ideal candidates to observe phosphorescence from organic lumiphores. In this book chapter synthesis, properties and applications of (1) non-doped and (2) doped polymer based RTPs will be discussed with reference to recent literature with the following possible.