Colloidal Quantum Dots (QDs) are proposed as building blocks for novel devices, e.g., solar cells and LEDs. For these applications, understanding and controlling the energy transfer (ET) is essential. Considerable research has been devoted to Pb-/Cd-chalcogenide QDs in this regard. Group I-III-VI QDs offer a lesser explored and environmentally friendlier alternative. Here, we demonstrate spectroscopic signatures of ET in CuInS₂ QD films. We measure steady-state photoluminescence (PL) and time-resolved photoluminescence (trPL) in solution phase and on films in vacuum. We study two samples with Cu-to-In molar ratios of 1 and 0.25 (S₁ and S₂) covered with dodecanethiol (DDT) ligands. As a reference, we use a sample with Cu-to-In molar ratio of 0.25 encapsulated in micelles.

As observed routinely for CuInS₂ QDs, the PL spectra exhibit linewidths of about 100 nm. For S₁ and S₂, we observe a significant redshift (15 nm and 30 nm, respectively) of the film PL spectra compared to the solution spectra. This redshift is absent in the reference sample. This points to non-radiative transfer of energy in the films, from donor species emitting on the blue side of the PL, to the acceptor species emitting on the red side. The spectrally integrated trPL showed that the PL lifetime for solution samples is of the order of 200 ns. Importantly, we observe a shortening of the lifetime of the films compared to the solutions (by a factor of 3 and 1.8 in S₁ and S₂, respectively). The faster decays point to the existence of additional non-radiative recombination channels in films, possibly including the effects of ET and diffusion-related quenching. Crucially, in the reference sample, the PL lifetimes are comparable for solution and film, in line with the absence of ET effects. The spectrally resolved trPL studies revealed that PL lifetimes increase from the blue end of the spectrum to the red end. For solutions and the reference sample, we observed a variation by about a factor of 3. For S₁ and S₂ films, the change was stronger - by a factor of 10. This effect supports the ET hypothesis. We therefore conclude that the relatively short ligand length of DDT-capped samples provides small enough inter-QD distance of about 6 nm^[2],[3], enabling the observation of ET. For micelle-encapsulated QDs, the ET is inefficient owing to larger (9 nm)^[4] inter-QD distance.