Intensity and decay time of a near-band-edge photoluminescence (PL) of a Cu(In,Ga)Se 2 (CIGS) thin film grown on a Mo/soda lime glass (SLG) substrate decreased with air exposure time, showing the degradation of the CIGS film surface. The dependence of Mo sputtering conditions on intensity and decay time of PL was studied in CIGS films deposited on Mo/SLG substrates. The Mo/SiO 2 (20 nm)/SLG substrate was used to control Na diffusion during CIGS deposition. Intensity and decay time of PL with time in air changed depending on the Mo-film sputtering temperature and pressure. Based on the secondary ion mass spectroscopy, the in-depth profile of Na in the CIGS film is correlated with those of H and O, which depended on the Mo-film sputtering condition and air exposure time. The degradation of the CIGS film in view of PL is discussed in terms of the Na close to the CIGS surface, which reacted with O 2 and H 2 O. CIGS solar cells were examined in relation to Mo sputtering conditions and the SiO 2 sodium barrier. The control of Na diffusion was found to be important for both reducing the rate of CIGS film surface degradation and the realization of a CIGS solar cell with a uniform PL intensity image.

Cu(In,Ga)Se2 (CIGS) thin films and solar cells were studied by photoacoustic spectroscopy (PAS) which is non-contact and non-destructive characterization of optical absorption properties. As preliminary experiment, the band gap of CIGS bulk alloy crystals were characterized by PAS, photoluminescence, and electroreflectance measurements, and the utilization of PAS was examined as the characterization of CIGS alloys. Photoacoustic (PA) spectra of CIGS thin films grown on a Mo/soda-lime glass (SLG) substrate showed the existence of the deep absorption band. The deep absorption band was observed only in PA spectra of CIGS/Mo/SLG films grown by three-stage method. The microphone-based PAS method detected the signal from the deep interface in CIGS/Mo/SLG structure sensitively. On the other hand, the PA spectrum of CdS/CIGS/Mo/SLG obtained by the transparent piezoelectric photo-thermal (Tr-PPT) method showed the clear absorption edge. Therefore, microphone-based PAS and Tr-PPT measurements can give complementary absorption properties of CIGS films.

The photo‐acoustic spectroscopy (PAS) using a transparent piezoelectric photo‐thermal (Tr‐PPT) method was carried out on Cu(In,Ga)Se2 (CIGS) thin films (both CIGS/Mo/SLG and CdS/CIGS/Mo/SLG) and solar cells (ZnO/CdS/CIGS/Mo/SLG). Using the Tr‐PPT method, the high background absorption in the below gap region observed in both a microphone and a conventional transducer PAS spectra was strongly reduced. This high background absorption came from the CIGS/Mo interface. This result proves that the Tr‐PPT PAS is the surface sensitive method. In the below‐band region, a bell‐shape deep absorption band has been observed at 0.76 eV, in which a full‐width at the half‐maximum value was 70‐120 meV. This deep absorption band was observed for both CdS/CIGS/Mo/SLG and ZnO/CdS/CIGS/Mo/SLG structures. The peak energy of the absorption band was independent of the alloy composition for 0.25≤Ga/III≤0.58. Intensity of the PA signal was negatively correlated to the Na concentration at the CIGS film surface. The origin of the 0.76 eV peak is discussed with relation to native defects such as a Cu‐vacancy‐related defect (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)