Photoluminescence Signatures of Energy Transfer in CuInS₂ Colloidal Quantum Dot Films

In this work, we demonstrate spectroscopic signatures of energy transfer (ET) in a solid film prepared from CuInS₂ colloidal quantum dots (QDs).<br/>Energy transfer in colloidal QDs has been a highly studied area in pursuit of applications such as solar cells. The size-dependent optical properties of QDs make them ideal candidates for such exploration. The environmentally safer QD materials such as CuInS₂ (CIS) have not been explored as much as Pb and Cd chalcogenide QDs in this regard. The significant Stokes shift of CIS and the scope for changing its stoichiometry provide additional parameters, which may influence ET efficiency. Here, we study steady-state photoluminescence (PL) and time-resolved photoluminescence (trPL) on QD films and solutions. Comparison of the results between the film and solution samples unveils three signatures of ET in QD films: (i) red-shift of the film PL spectrum, (ii) increase/decrease of the relative PL decay rate at the blue/red end of the film PL spectrum, and (iii) presence of a PL rise dynamics at the red end of the film PL spectrum. In agreement with studies on other QD systems, all these effects point to an ET from small to large QDs within the ensemble.<br/>The CIS QDs studied here are synthesized at 230°C for 1 minute with Cu-to-In precursor ratio of 1:4. The PL spectrum of QDs in solution form peaks at 652 nm and exhibits a broad full width at half maximum (FWHM) of about 100 nm. This is the sign of the presence of highly inhomogeneous QD size distribution in the ensemble. The PL peak of the same ensemble when drop cast to form a solid film is red-shifted by over 50 nm (140 meV) to 705 nm. Since the FWHM of the film PL hardly changes from the solution PL, we interpret the red-shift as a signature of non-radiative ET from donor QDs of smaller sizes to the larger acceptor QDs when deposited as a film. The spectrally integrated PL lifetime in solution phase is of the order of 300 ns while for the films is shorter, i.e., of the order of 200 ns. The faster PL decays point to the existence of additional recombination channels in the case of films. Furthermore, the spectrally resolved trPL studies unveil the shortening of PL lifetimes from the red end of the spectrum to the blue end. In solution phase, the lifetimes at the red and blue ends relative to that of the peak are 1.3 and 0.6, respectively. This shows that the QD ensemble exhibits an intrinsic spectral dependence of the lifetimes. On the other hand, in the film, the lifetimes at the red and blue ends relative to that of the peak are 1.6 and 0.32, respectively. Thus, in the film form, an additional contribution to the spectral dependence of lifetimes is manifested. An analogous effect was previously ascribed to non-radiative ET within inhomogeneous ensembles of QDs. Accordingly, we interpret this additional contribution as a result of ET between donors and acceptors in the film. This conclusion is further supported by the observation of PL rise dynamics at the red end of the film spectrum, which is absent in the case of the solution trPL. The existence of rise time at the red end together with very short decay at the blue end can be explained by the non-radiative transfer of energy from the small QDs into the larger QDs. Similar effects are observed with varying degrees in 4 other CIS QD ensembles made of different QD sizes and Cu-to-In precursor ratios. Results for all samples support the conclusion that ET occurs in CIS QD films. For more detailed insight into this effect, the future research will include temperature dependent studies and fine selection of QD-sizes to assert an improved control over ET.