The systematic variations in the structural, optical, and electrical properties of polycrystalline $\text{Cu}\left(\text{In},\text{Ga}\right){\text{Se}}_2$ (CIGS) thin films with Na doping level were investigated. Precise control of the Na concentration in CIGS films was demonstrated using alkali-silicate glass thin layers of various thicknesses deposited on substrates prior to CIGS growth. The CIGS grain size was observed to decrease with increasing Na concentration, although the surface morphology became smoother and exhibited a stronger (112) texture, which has been demonstrated consequence of larger grain size. The Ga composition gradient in the CIGS films was found to become large due to the presence of Na during growth, which in turn led to a decrease in the nominal band gap energy. Variations in the photoluminescence spectra and electrical properties suggested that the formation of an acceptor energy state, which may originate from ${\text{O}}_{\text{Se}}$ point defects, was enhanced in the presence of Na. This result suggests that not only Na, but also the presence of O in combination with Na contributes to the compensation of point defects and enhances $p$ -type conductivity in CIGS films.

We present evidence for the production of photocurrent due to two-photon excitation in an intermediate band solar cell structure. The structure consists of an n-GaNAs intermediate band layer sandwiched between a p-AlGaAs emitter and an n-AlGaAs barrier layer with suitable doping level to block electron escaping from the intermediate band to the bottom n-GaAs substrate. Multi-band transitions observed in two-photon excitation experiments are explained using photo-modulated reflectance spectrum, and further support for intermediate band solar cell operation of this structure is given by current-voltage measurements.