Copper-doped semiconductors are classic phosphor materials that have been used in a variety of applications for many decades. Colloidal copper-doped semiconductor nanocrystals have recently attracted a great deal of interest because they combine the solution processability and spectral tunability of colloidal nanocrystals with the unique photoluminescence properties of copper-doped semiconductor phosphors. Although ternary and quaternary semiconductors containing copper, such as CuInS2 and Cu2ZnSnS4, have been studied primarily in the context of their photovoltaic applications, when synthesized as colloidal nanocrystals, these materials have photoluminescence properties that are remarkably similar to those of copper-doped semiconductor nanocrystals. This review focuses on the luminescent properties of colloidal copper-doped, copperbased, and related copper-containing semiconductor nanocrystals. Fundamental investigations into the luminescence of copper-containing colloidal nanocrystals are reviewed in the context of the well-established luminescence mechanisms of bulk copper-doped semiconductors and copper(I) molecular coordination complexes. The use of colloidal copper-containing nanocrystals in applications that take advantage of their luminescent properties, such as bio-imaging, solid-state lighting, and luminescent solar concentrators, is also discussed. a Measured at 20 K b Measured in a PV device 2. Synthesis and Structural Characterization of Colloidal Copper-Containing Nanocrystals Progress in the synthesis of luminescent copper-doped colloidal semiconductor NCs mirrors that in the synthesis of colloidal semiconductor NCs in general. The earliest reports from the mid-1980s through the 1990s describe the synthesis of copper-doped CdS and ZnS colloidal NCs by simply adding small amounts of copper salts to the aqueous-phase arrested precipitation reactions

The electronic and magnetic properties of the luminescent excited states of colloidal Cu(+):CdSe, Cu(+):InP, and CuInS2 nanocrystals were investigated using variable-temperature photoluminescence (PL) and magnetic circularly polarized luminescence (MCPL) spectroscopies. The nanocrystal electronic structures were also investigated by absorption and magnetic circular dichroism (MCD) spectroscopies. By every spectroscopic measure, the luminescent excited states of all three materials are essentially indistinguishable. All three materials show very similar broad PL line widths and large Stokes shifts. All three materials also show similar temperature dependence of their PL lifetimes and MCPL polarization ratios. Analysis shows that this temperature dependence reflects Boltzmann population distributions between luminescent singlet and triplet excited states with average singlet-triplet splittings of ∼1 meV in each material. These similarities lead to the conclusion that the PL mechanism in CuInS2 NCs is fundamentally different from that of bulk CuInS2 and instead is the same as that in Cu(+)-doped NCs, which are known to luminesce via charge-transfer recombination of conduction-band electrons with copper-localized holes. The luminescence of CuInS2 nanocrystals is explained well by invoking exciton self-trapping, in which delocalized photogenerated holes contract in response to strong vibronic coupling at lattice copper sites to form a luminescent excited state that is essentially identical to that of the Cu(+)-doped semiconductor nanocrystals.

Single-nanocrystal and ensemble photoluminescence measurements on CuInS2 semiconductor nanocrystals reveal intrinsically broad luminescence bandshapes attributable to strong electron-phonon coupling in the emissive excited state, similar to the single-nanocrystal luminescence spectra of Cu +-doped CdSe nanocrystals. This finding is consistent with the hypothesis of exciton self-trapping in CuInS2 NCs, which forms an emissive state similar to those of Cu +-doped nanocrystals. Blinking is observed that resembles that of other semiconductor nanocrystals. Ensemble luminescence measurements reveal the existence of a remarkably long-lived excited state in these nanocrystals that continues to emit photons over several orders of magnitude in time following the excitation pulse. This delayed luminescence is attributed to reversible electron trapping. The delayed luminescence overlaps in time and shows similar distributed kinetics to the blinking "off" times, supporting the proposal that these two phenomena arise from the same microscopic carrier-trapping and-detrapping processes. Excitation power dependence measurements illustrate that the delayed luminescence saturates at very low intensities at the power densities used in single-nanocrystal measurements, consistent with this metastable charge-trapped state being the "off" state of the luminescence blinking cycle.

The performance of colloidal CuInS2/CdS nanocrystals as phosphors for full-spectrum luminescent solar concentrators has been examined. Their combination of large solar absorption, high photoluminescence quantum yields, and only moderate reabsorption produces the highest projected flux gains of any nanocrystal luminophore to date.