Excitons in organic-inorganic hybrid compounds (CnH2n+1NH3)2PbBr4 (n = 4, 5, 7 and 12)

Thin films of microcrystalline (CnH2n+1NH3)2PbBr4 (n = 4, 5, 7 and 12) have been prepared by a modified spin-coating method, and the effect of the number of carbon atoms of the alkyl chain length (n) on optical properties has been investigated. Absorption spectra reveal that (CnH2n+1NH3)2PbBr4 films show stable excitons with a binding energy of a few hundred meV. The excitonic structure of (CnH2n+1NH3)2PbBr4 varies with the number of carbon atoms. The lowest-energy exciton splits into a few fine-structure levels at low temperature. (CnH2n+1NH3)2PbBr4 films (n = 5, 7 and 12) show not only singlet excitons but also triplet excitons at low temperature, while (C4H9NH3)2PbBr4 films show only singlet excitons. The intersystem crossing from excited singlet state to triplet state plays an important role in the relaxation process of excitons.