Characterization of semiconductors from photoconductivity techniques: uniform and polychromatic illumination
Résumé
In this chapter we present optoelectronic characterization techniques that can be applied to coplanar samples as well as solar devices. Using the photoconductive properties of the semiconductor, the carrier generation is achieved with several photon energies lower than the band gap width of the material under investigation. These techniques are the constant photocurrent method (CPM) and the Fourier transform photocurrent spectroscopy (FTPS). The CPM was developed to gain information on the optical absorption of a semiconductor in an energy range where the sensitivity of the classical UVvisible spectroscopy is too low for this technique to be applicable. First, we present the theoretical and experimental bases of the CPM. Then, we show that the optical absorption is linked to the density of states (DOS) in the gap of the material and we present some of the models that were used to extract this DOS from the CPM results. Finally, we underline some of the limits of the CPM technique. One the main drawback of the CPM is the time needed for the acquisition of the variations of the absorption coefficient as function of the photon energy. The FTPS was imagined to overcome this drawback and, indeed, the same spectrum can be obtained within a few seconds. We present the theoretical and experimental bases of the FTPS and show that this technique can be used as a systematic mean of characterization of materials and devices replacing advantageously the CPM. To illustrate the powerfulness of the FPTS we give experimental results obtained on a solar device and compare them to the spectrum obtained on the device absorber. We conclude by presenting some results obtained by FTPS on perovskites, a very promising material for the future development of solar energy.