read the paper: https://www.nature.com/articles/s41467-022-34117-8    

     With a wide direct bandgap of 3.1 eV, p-type transparent copper iodide (CuI) has attracted increasing attentions in recent years because of its relatively high hole mobility and large exciton binding energy (~62 meV). A variety of intriguing optoelectronic applications of CuI, such as efficient hole transport materials, thin-film transistors, p-n heterojunctions, thermoelectrics etc, have been demonstrated. Despite numerous efforts devoted to the fundamental optoelectronic properties and device applications of CuI, some fundamental properties including the defect physics/evolution, excitonic properties, and particularly the ultrafast carrier dynamics (i.e., carrier generation, relaxation and trapping/recombination), are still quite controversial or has rarely been investigated.

     In our study, we synthesized the CuI thin films with a wide range of hole density from ~10¹⁷ to ~10²⁰ cm^-3 by controlling the concentration of native defects via post thermal annealing in different conditions. The optoelectronic properties and ultrafast carrier dynamics of CuI thin films were comprehensively studied by using a variety of analytical techniques including Hall-effect measurements, photoluminescence (PL) spectroscopy, spectroscopic ellipsometry, and femtosecond transient absorption (fs-TA) spectroscopy. Interestingly, distinct excitonic resonance of CuI was observed to persist at high hole density up to ~5×10¹⁹ cm^-3 (around one order of magnitude higher than its Mott density), indicating the possible formation of Mahan exciton. The prominent PL peak at ~420 nm arising from the radiative recombination of electrons in the conduction band with the neutral copper vacancies is verified by the power-dependent steady-state PL measurements. By varying the excitation density (N₀), the photogenerated carrier density dependent ultrafast physical processes including thermalization, hot carrier cooling, and recombination are elucidated. Both the effects of hot-phonon bottleneck and Auger heating are found to slow down the cooling rate in case of high excitation density. Effects of defects on the carrier recombination and the two-photon absorption induced carrier/exciton dynamics are also investigated.  Fig. 1 schematically illustrates the overall relaxation pathways of photogenerated carriers in the CuI thin films .

     Our work provides detailed and in-depth understandings of the optoelectronic properties, excitonic prperties and ultrafast carrier dynamics of CuI, which are crucial to their optoelectronic applications. The discovery further demonstrates that the high mobility p-type transparent CuI is a unique material not only for optoelectronic applications but also for unveiling fundamental physical processes.