Time-resolved THz spectroscopy

Especially with LTG GaAs, materials became available that were nearly ideal for time-resolved THz spectroscopy. Due to the low growth temperature and the slight As excess incorporated, clusters are fonned which act as recombination sites for the excited carriers, leading to lifetimes of <250 fs [45], With such recombination lifetunes, THz radiators such as dipole anteimae or log-periodic spirals placed onto optoelectronic substrates and pumped with ultrafast lasers can be used to generate sub-picosecond pulses with optical bandwidths of 2-4 THz. Moreover, coherent sub-picosecond detection is possible, which enables both... 

Material response in THz frequency region, which corresponds to far- and mid-infrared electromagnetic spectrum, carries important information for the understanding of both electronic and phononic properties of condensed matter. Time-resolved THz spectroscopy has been applied extensively to investigate the sub-picosecond electron-hole dynamics and the coherent lattice dynamics simultaneously. In a typical experimental setup shown in Fig. 3.5, an... 

Due to the generation of charge carriers, conjugated polymers become electrically conductive when irradiated with UV/visible light. The quantum yield for charge carrier generation is increased by additives, such as fullerene compounds. With the aid of time-resolved THz spectroscopy, the time- and frequency-dependent complex conductivity with its real and imaginary parts can be measured. THz radiation is absorbed by the polymer due to the dielectric... 

Figure 2.32 Photogeneration of charge carriers in MEH-PPV (see Chart 2.11) as probed by time-resolved THz spectroscopy, (a) Change in the electric field strength of the THz probe pulses after the UV pump pulse...

Carrier Dynamics Studied by Time-Resolved THz Spectroscopy 303... 

In this section, the basic characteristics of time-resolved THz spectroscopy (TRTS) are discussed. Generation and detection of THz radiation is briefly introduced and the layout and operation of a common THz setup is described. Also, theoretical models suited to describe the response of carriers in the THz frequency window are presented for three different semiconductor geometries that are relevant for conventional and novel PV applications (Section 11.2.2.1) bulk semiconductors, (Section 11.2.2.2) polycrystals and mesoporous oxide films and (Section 11.2.2.3) semiconductor quantum dots (QDs). 

Figure 11.2 Schematic layout of a conventional time resolved THz spectroscopy setup.

In this chapter we have presented time-resolved THz spectroscopy as a powerful tool for the study of carrier dynamics in PV structures and materials. Optical and electrical information can be obtained in a non-contact fashion and resolved in time with sub-picosecond resolution. This versatility makes TRTS particularly suited to interrogate physical processes on nano-structured semiconductors, which nowadays constitutes the research forefront towards high efficiency and low cost PV devices. 

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