Yukiko Kamikawa

Effects of long-term heat-light soaking on Cu(In,Ga)Se2 solar cells with KF postdeposition treatment

The effects of long-term heat-light soaking on Cu(In,Ga)Se2 (CIGS) solar cells with KF postdeposition treatment (KF-PDT) have been investigated. CIGS solar cells with KF-PDT frequently deteriorate after storage in the dark because of decreasing hole concentrations in the CIGS layers. Although light soaking improves hole concentrations, the resulting increase in conversion efficiency is not significant. In contrast, we found that long-term heat-light soaking effectively increases conversion efficiency, since the saturation current density and ideality factor are clearly improved by passivating the recombination centers at CdS/CIGS interfaces.

Effects of Mo surface oxidation on Cu(In,Ga)Se2 solar cells fabricated by three-stage process with KF postdeposition treatment

The surface oxidation condition of the Mo back contact on a soda lime glass (SLG) substrate was varied by air annealing and chemical etching. Then, the evolution of a photovoltaic property was studied for Cu(In,Ga)Se2 (CIGS) solar cells grown by a three stage process with KF postdeposition treatment. Upon the removal of the oxidized layer from the Mo surface by chemical etching, the c-axis orientation of MoSe2 tended to be random, whereas the c-axis was perpendicular when the Mo surface was oxidized. An enhancement of the diffusion of Na and K from SLG to CIGS was observed upon removing the molybdenum oxide, which functions as a barrier to alkali-metal diffusion. The varied orientation of MoSe2 can also affect the alkali-metal diffusion kinetics. The open-circuit voltage (VOC) markedly increased after removing the oxidized layer from the Mo surface, mainly as a result of an increase in carrier density in CIGS.

Effects of RbF postdeposition treatment and heat-light soaking on the metastable acceptor activation of CuInSe2 thin film photovoltaic devices

Granular particles formed on a CuInSe2 (CIS) thin film surface with alkali-halide RbF post-deposition treatment (RbF-PDT) are found to concentrate preferably on other than smooth (112)-planes present on the CIS film surface. As a consequence, a large number of distinctive pores are selectively formed at relatively rough facets. Although the open circuit voltage and fill factor values of CIS solar cells improve with either RbF-PDT or heat-light soaking (HLS) treatment, the effects of these treatments are distinguishable. The ratio of carrier recombination rates at the interface (Ri) to that in the bulk (Rb), namely, Ri/Rb, decreases with RbF-PDT. HLS treatment performed on RbF-PDT CIS devices leads to a further decrease in the value of Ri/Rb, whereas the value of Ri/Rb of CIS devices fabricated without RbF-PDT increases with HLS treatment. Ternary CIS solar cells grown with RbF-PDT and HLS treatment reproducibly demonstrate enhanced photovoltaic efficiencies.Granular particles formed on a CuInSe2 (CIS) thin film surface with alkali-halide RbF post-deposition treatment (RbF-PDT) are found to concentrate preferably on other than smooth (112)-planes present on the CIS film surface. As a consequence, a large number of distinctive pores are selectively formed at relatively rough facets. Although the open circuit voltage and fill factor values of CIS solar cells improve with either RbF-PDT or heat-light soaking (HLS) treatment, the effects of these treatments are distinguishable. The ratio of carrier recombination rates at the interface (Ri) to that in the bulk (Rb), namely, Ri/Rb, decreases with RbF-PDT. HLS treatment performed on RbF-PDT CIS devices leads to a further decrease in the value of Ri/Rb, whereas the value of Ri/Rb of CIS devices fabricated without RbF-PDT increases with HLS treatment. Ternary CIS solar cells grown with RbF-PDT and HLS treatment reproducibly demonstrate enhanced photovoltaic efficiencies.

Degradation mechanism of Cu(In,Ga)Se2 solar cells induced by exposure to air

The degradation mechanism of unencapsulated Cu(In,Ga)Se2 (CIGS) solar cells upon exposure to air has been investigated. Exposure to air at room temperature slightly reduces the conversion efficiency of CIGS solar cells. However, this conversion efficiency decreases significantly under damp heat testing at 85 °C and a relative humidity of 85% for 15 h. The shunt resistance and conversion efficiency are completely recovered after removing the side edges of the CIGS solar cells by mechanical scribing. This result suggests that low-resistive layers are formed on the sidewalls of the solar cells during damp heat testing. In addition, alkaline solution etching has been confirmed to be an effective way of removing the low-resistive layers. The low-resistive layers on the sidewalls are identified to be molybdenum oxides and sodium molybdate by Auger electron spectroscopy. After etching the oxides on the sidewalls, the saturation current density and ideality factor are confirmed to be improved.

Effect of thermal annealing on the redistribution of alkali metals in Cu(In,Ga)Se2 solar cells on glass substrate

The precise control of alkali-metal concentrations in Cu(In,Ga)Se2 (CIGS) solar cells via post deposition treatment (PDT) has recently attracted attention. When PDT is performed at an elevated temperature, an accompanying annealing effect is expected. Here, we investigate how thermal annealing affects the redistribution of alkali metals in CIGS solar cells on glass substrates and the properties of the solar cells. In addition, we investigate the origin of non-homogeneous alkali-metal depth profiles that are typical of CIGS grown using a three-stage process. In particular, we use secondary-ion mass spectrometry measurements of the ion concentration as a function of distance from the CIGS surface to investigate the impact of thermal annealing on the distribution of alkali metals (Na, Ka, and Rb) and constituent elements (Ga and In) in the CIGS absorbers. We find that the depth profiles of the alkali metals strongly reflect the density of sites that tend to accommodate alkali metals, i.e., vacancies. Annealing at elevated temperature caused a redistribution of the alkali metals. The thermal-diffusion kinetics of alkali metals depends strongly on the species involved. We introduced low flux potassium fluoride (KF) to study a side effect of KF-PDT, i.e., Na removal from CIGS, separately from its predominant effects such as surface modification. When sufficient amounts of Na are supplied from the soda lime glass via annealing at an elevated temperature, the negative effect was not apparent. Conversely, when the Na supply was not sufficient, it caused a deterioration of the photovoltaic properties.The precise control of alkali-metal concentrations in Cu(In,Ga)Se2 (CIGS) solar cells via post deposition treatment (PDT) has recently attracted attention. When PDT is performed at an elevated temperature, an accompanying annealing effect is expected. Here, we investigate how thermal annealing affects the redistribution of alkali metals in CIGS solar cells on glass substrates and the properties of the solar cells. In addition, we investigate the origin of non-homogeneous alkali-metal depth profiles that are typical of CIGS grown using a three-stage process. In particular, we use secondary-ion mass spectrometry measurements of the ion concentration as a function of distance from the CIGS surface to investigate the impact of thermal annealing on the distribution of alkali metals (Na, Ka, and Rb) and constituent elements (Ga and In) in the CIGS absorbers. We find that the depth profiles of the alkali metals strongly reflect the density of sites that tend to accommodate alkali metals, i.e., vacancies. Anneali...