Photovoltaic & Optoelectronic Device Group

Abstract:

Colour-tuned phosphors are promising for advanced security applications such as multi-modal anti-counterfeiting and data encryption. The practical adoption of colour-tuned phosphors requires these materials to be responsive to multiple stimuli (e.g., excitation wavelength, excitation waveform, and temperature) and exhibit excellent materials stability simultaneously. Here we report germanium silicon oxide (GSO) - a heavy-metal-free inorganic phosphor - that exhibits colour-tuned ultra-long phosphorescence and delayed fluorescence across a broad temperature range (300 - 500 K) in air. We developed a sol-gel processing strategy to prepare amorphous oxides containing homogeneously dispersed Si and Ge atoms. The co-existence of Ge and Si luminescent centres (LC) leads to an excitation-dependent luminescence change across the UV-to-visible region. GSO exhibits Si LC-related ultra-long phosphorescence at room-temperature and thermally activated delayed fluorescence at temperatures as high as 573 K. This long-lived PL is sensitized via the energy transfer from Ge defects to Si LCs, which provides PL lifetime tunability for GSO phosphors. The oxide scaffold of GSO offers 500-day materials stability in air; and 1-week stability in strong acidic and basic solutions. Using GSO/polymer hybrids, we demonstrated colour-tuned security tags whose emission wavelength and lifetime can be controlled via the excitation wavelength, and temperature, indicating promise in security applications.This work was supported by the National Natural Science Foundation of China (21905316, 22175201) and the Science and Technology Planning Project of Guangdong Province (2019A050510018). Z.Y. would like to acknowledge financial support from Sun Yat-sen University. M.W., J.A., S.T., and E.H.S. acknowledge the financial support from the Natural Sciences and Engineering Research Council (NSERC) of Canada. We thank Prof. B. Huang (Sun Yat-sen University) and Prof. B. Xu (South China Normal University) for their assistance with temperature-dependent PL studies. L. Wei is thanked for the figure design. The synchrotron studies were financially supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada. This research used resources of the SGM (11ID-1) beamline at the Canadian Light Source. The authors thank Dr. Tom Regier and Dr. Jay Dynes from SGM beamline for the assistance in conducting XAS measurements

Abstract:

Charge carrier mobility is a fundamental property of semiconductor materials that governs many electronic device characteristics. For metal halide perovskites, a wide range of charge carrier mobilities have been reported using different techniques. Mobilities are often estimated via transient methods assuming an initial charge carrier population after pulsed photoexcitation and measurement of photoconductivity via non-contact or contact techniques. For nanosecond to millisecond transient methods, early-time recombination and exciton-to-free-carrier ratio hinder accurate determination of free-carrier population after photoexcitation. By considering both effects, we estimate long-range charge carrier mobilities over a wide range of photoexcitation densities via transient photoconductivity measurements. We determine long-range mobilities for FA0.83Cs0.17Pb(I0.9Br0.1)3, (FA0.83MA0.17)0.95Cs0.05Pb(I0.9Br0.1)3 and CH3NH3PbI3-xClx polycrystalline films in the range of 0.3 to 6.7 cm2 V−1 s−1. We demonstrate how our data-processing technique can also reveal more precise mobility estimates from non-contact time-resolved microwave conductivity measurements. Importantly, our results indicate that the processing of polycrystalline films significantly affects their long-range mobility.