Novel erbium and erbium–ytterbium complexes based on 4-pentylbenzoate (PBa) groups and 1,10-phenanthroline (Phen) ligands have been synthesized and structurally characterized. The X-ray single crystal structure of the erbium–ytterbium complex indicates that it is a binuclear complex model with a short metal-to-metal distance (4.269 Å), which facilitates the intramolecular Yb–Er energy-transfer process in the erbium–ytterbium complex. The optical properties for solid state films of the erbium–ytterbium complex prepared by spin-coating technique were studied in detail. The NIR PL spectrum of the film shows considerable room temperature emission at 1.53 µm with a large bandwidth (full width at half maximum) of 72 nm. The planar waveguide of the complex is capable of guiding 1550 nm light with low loss. The mechanism for the formation of high-quality films is also proposed according to the structural investigation. Our results show that the erbium–ytterbium complex film has promising potential for the planar optical amplifier application.