Comparison of FTPS Performed on Thin Films and Solar Cells

Fourier Transform Photocurrent Spectroscopy (FTPS) has been used to investigate on the absorption coefficient versus photon energy (α(hν)) spectroscopy of thin film semiconductors. We have developed an FTPS experiment to study the α(hν) spectrum of hydrogenated amorphous silicon (a-Si:H) thin films, to subsequently deduce information on the density of states in the sub bandgap region. In this work we studied whether the same α(hν) spectroscopy could be found both for coplanar a-Si:H films deposited on glass and for NIP diodes incorporating the same material as the intrinsic layer. The back and front contacts of the diodes are made of transparent conductive oxides, either ITO or ZnO. The results show that the Urbach tails of both coplanar and NIP diodes are very similar but that α at low photon energy is slightly lower for the diodes than for the coplanar samples. In addition, we noticed that the diode with ZnO contacts presents a rapid increase of α below 0.95 eV. Transmission-reflection measurements performed on the diodes show that the diode with ZnO contacts exhibits a weaker transmission in the same wavelength range where we observed an increase of α, though the reflection with both types of contacts remains the same, indicating a parasitic absorption of the back ZnO layer. These results show that FTPS measurements performed on diodes must be taken very cautiously and that one has to take into account the optical properties (e.g., absorption) of the contacts to properly deduce reliable α(hν) spectroscopy.