Photocalorimetric study on nonradiative relaxation processes and photoluminescence quantum yields in Alq3 single crystals

Heat generation caused by nonradiative relaxation processes in tris-8-hydroxyquinoline aluminum (Alq3) was measured at 2 K by using a photocalorimetric (PC) detecting system. At lower temperatures, the photoluminescence (PL) quantum yield is much higher than that at room temperature because of the quenching of thermal disturbance. At 2 K, the PL quantum yield of a single crystal specimen and of a vapour-deposited thin film of Alq3 was estimated to be 0.76±0.03 and 0.79±0.03, respectively, under excitation above the absorption edge. On the other hand, under excitation below the absorption edge, a weak PC signal response is clearly resolved, even in the highest quality single crystal, indicating the existence of nonradiative recombination centres in the transparent energy region.