This book describes Opto-electronic properties of three new carbazole based co-polymers that can be implemented as host in phosphorescence polymer light-emitting diode. We present absorption, steady-state emission and time resolved emission spectroscopy of carbazole based conjugated polymers. The decay kinetics of triplet emission of the co-polymers was observed after optical excitation. Several findings such as optical band-gap, singlet to singlet transition energies, triplet to triplet transition energies, exchange energies (?EST), Stoke''s shift have been presented. The onset of absorption is at 3.10-3.25 eV for P1, P2 and P3 polymers respectively. One emission band originating from singlet excited state is observed at RT. The gaps between singlet and triplet excited states are 0.44-0.47 eV. From decay kinetics the triplet excited state life-times were found to be several orders of ms at 10-13 K that reduces to less than 3 ms at room temperature. At room temperature delayed fluorescence has been observed for all three polymers. An energy band diagram including lower lying excited states has been constructed and compared.