Materials science is an interdisciplinary field that investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. Materials science aims at creating or transforming existing materials as well as enhancing materials to give better performance for particular applications.
Charge carrier dynamics in semiconductors
Charge carrier dynamics in semiconductors are determined by the architecture and function of the respective device and directly reflect the nature and quality of wafer materials. This makes precise and efficient measurement techniques of the free charge carrier lifetime essential for characterizing these systems. For particular classes of semiconductors, the characteristic charge carrier lifetime is highly dependent on the nature and dimensions of the materials and interfaces involved. Furthermore, surface effects, passivation and the energy transfer efficiency of sensitizers as well as the presence of dopants, impurities and defect sites can introduce significant variations in this parameter. Since the photoluminescence of semiconductors is a direct monitor of the charge carrier dynamics, the general methodology of time-resolved photo-luminescence (TRPL) via time-correlated single photon counting (TCSPC) and the periphery technology are highly suited for the analysis of the phenomena that determine fast charge carrier dynamics in a semiconductor. As a result, the mechanism that determines the charge carrier dynamics within a particular system can be characterized directly down to the sub-nanosecond time scale.
Direct observation of charge carrier dynamics
The general methodology of time-resolved photoluminescence can be expanded by lifetime imaging of the charge carrier dynamics. This can be exploited for, e.g., determining the effect of carrier diffusion and its influence on the total lifetime measured in conjunction with intensity dependent photoluminescence lifetimes measurements. It brings an exceptional component to semiconductor analysis with respect to material and architectural substructures, spatial inhomogenities and process dependent morphology. Using TRPL imaging, charge carrier diffusion processes and the effect of localized inhomogeneities and defect sites can be identified. With this multi-dimensional approach, a versatile and powerful methodology for the analysis of semiconductor materials can be achieved.
Two-photon process leads to emission at shorter wavelengths
Fluorescence upconversion particles absorb light in the near infrared (NIR) and emit light in the visible range. These particles are currently in the focus of research because of their prospective application for, e.g., in vivo optical imaging, as they allow excitation in the NIR spectral range where light absorption and scattering from biological tissues is minimized. Another possible application are dye sensitized solar cells. A standard solar cell does not absorb in the near infrared spectral range, as the energy of the NIR light is too low for injecting electrons into the conducting band of the solar cells semiconducting material. A possible option to overcome this limitation, and thus to create solar cells with a higher light conversion efficiency, are dye sensitized solar cells in which upconversion particles are used to convert the NIR light into visible light, which then has enough energy to leap the band gap of the semiconducting material.