Energy transfer from inorganic excitons to naphthalene in organic-inorganic hybrid quantum-well materials

Phosphorescence from a triplet excited state in organic materials has been extensively studied because of its high potentiality to increase the efficiency of organic light emitting diodes. In this paper, we report strongly enhanced phosphorescence from the triplet state of naphthalene molecules which are doped into an organic-inorganic hybrid quantum-well material. The enhanced phosphorescence is caused by energy transfer from excitons in the inorganic well layer to the triplet state of naphthalene molecules. We investigate the dynamics of the energy transfer and find out that the mechanism is Dexter-type transfer. An organic-inorganic hybrid quantum-well material, (C~H~NH~)~P~B~J, is a layered perovskite-type quantum-well material. It has inorganic well layers composed of two-dimensional network of comer-sharing [PbBr6]" octahedra between organic barrier layers of alkyl ammonium chains. Excitons are tightly confined in the inorganic well layers and have extremely large binding energy (- 400 meV)'. Strong excitonic photoluminescence (PL) are clearly observed at 3.014 eV. On the other hand, (Cldl7C.H2.+1NH,)2PbBr4 and (ClaH70C.H2,+INH3)zPbBr4 in which naphthalene molecules are doped into organic bamier layers show little PL from the excitons but show strongly e-nhanced phosphorescence from the triplet state of naphthalene molecules. The PL excitation. measurements' have shown that the enhanced phosphorescence is caused by energy transfer from the excitons as shown in Fig.1. We investigate the dynamics of the energy transfer by time-resolved PL measurements using the samples which are controlled the distance between the naphthalene molecules and the excitons. The excitons in the inorganic well layer have spin fine structure levels'. As shown in Fig.1, the dipole-allowed T< excitons which are created by optical excitation relax to r; and rl- excitons due to the fast spin relaxation (- 4 ps). The r; and r; excitons have relatively long lifetime ( - Ions) because they are mainly composed of triplet excitons. We can estimate the energy transfer rate by time-resolved measurements of PL from the The width and energy of the excitation pulse are 200 fs and 3.5 eV which is above the bandgap energy of the inorganic well layer. All measurements were performed at temperature of 10 K. Figure 2 shows the result, where R represents the distance between the naphthalene and the exciton. The PL of naphthalene doped materials has faster decay compared to that of (C4H9NH3)'PbBr4, and the samples with smaller R show faster decay times. Figure 3 shows and rl- excitons.