Hajime Shibata

In this study, the origins of efficiency gains in Cu(In,Ga)Se₂(CIGS) solar cells are investigated by introducing an Al₂O₃passivation layer in terms of the oxidation condition of Mo back contact, alkali‐metal diffusion, minority carrier lifetimes (τ), and charge conditions. The study reveals that introduction of an Al₂O₃back‐contact passivation layer into solar cells yields multiple impacts. Al₂O₃deposition enhances the oxidation of the Mo back contacts, increasing Na solubility in Mo and Na diffusion from Mo into the CIGS layer, thereby modifying the metastable properties of CIGS. The charge condition at the CIGS/Al₂O₃interface is not fixed negative charge but variable, dependent on whether electrons or holes are supplied. During solar cell operation, the interfacial charge condition is expected to be neutral or positive for Al₂O₃grown using plasma or thermal atomic layer deposition techniques, respectively. Moreover, the mechanical peeling off of CIGS from Mo back contact enhancedτin a similar way as with the insertion of Al₂O₃. Based on this study, the enhancement of alkali metal supply and the removal of direct contact of CIGS to the metal contact (Mo) can play crucial roles in improving the performance of CIGS solar cell.

We investigated the photo-stability of PBDB-T, which is a widely used donor material for organic solar cells (OSCs), in OSCs. The OSCs based on PBDB-T did not show good stability, which can be originated in the dedoping of PEDOT:PSS by PBDB-T.

After our observation of "quasi-equilibrium" in charge-state evolution for 2.0 MeV/u sulfur ions after carbon foil penetration, we have extended the initial ions to 1.0 and 2.0 MeV/u carbon ions. A significant quasi-equilibrium was observed for 1.0 MeV/u C+C collisions, where the equilibrium mean charge-state was 4.9, while for 2.0 MeV/u C+C collisions, where the equilibrium mean charge-states was 5.6, was a weak quasi-equilibrium.

Polycrystalline W and H codoped In₂O₃ (In₂O₃:W,H) transparent conducting oxide (TCO) layers with high electron mobility were found to enhance both the short‐circuit current density (J_sc) and fill factor (FF) of Cu(In,Ga)Se₂ (CIGS) modules with glass/Mo/CIGS/CdS/ZnO/TCO structures. An In₂O₃:W,H TCO layer was formed by reactive plasma deposition without substrate heating, followed by postannealing at 150°C. A phase transition from amorphous to polycrystalline was confirmed during postannealing, and a layer with micrometer‐scale lateral grain sizes evenly covered the rough ZnO surface generated by the naturally textured CIGS. The crystallized In₂O₃:W,H film showed high electron mobility (apprixmately 80 cm² V⁻¹ s⁻¹) at a carrier density of approximately 2 to 3 × 10²⁰ cm⁻³. Therefore, the In₂O₃:W,H layer achieved lower sheet resistance and lower free carrier absorption than a ZnO:Al layer, which is commonly used in CIGS modules. The CIGS mini‐module employing the In₂O₃:W,H layer with a designated area of 3.792 cm² exhibited a higher efficiency (η) of 20.93% (V_oc = 3.081 V [translated value per subcell: 0.770 V], short‐circuit current = 33.21 mA [J_sc = 35.03 mA cm⁻²], FF = 0.775) with improved J_sc and FF, when compared with a mini‐module employing a ZnO:Al layer. The results indicate that polycrystalline In₂O₃‐based TCOs with high electron mobility are suitable for CIGS solar cells and modules.

To identify the common degradation mechanism(s) between field-aged photovoltaic (PV) modules and PV cells/modules exposed to artificial corrosive stress, the ac impedance associated with the corrosion in the front electrodes was analyzed in the individual PV cells laminated within the PV modules, fielded for 21 years. By evaluating the ac impedance together with the PV performance for these PV cells, we found that the short-circuit current (I _sc ) remarkably decreased, depending on the expression and elevation of the ac impedance relevant to the front-electrode corrosion. This I _sc -reduction can be explained by the formation of rectifying contacts to restrict the current conduction between the front electrodes and the associated silicon wafer. Since the I _sc -reduction was primarily observed in the PV cells with extensive loss of fill factor, we suggest that the power-loss is induced by the gap formation underneath the front electrodes, followed by the emergence of rectifying contacts, even in the PV cells/modules degraded in field. The ac impedance parameters of the PV cells/modules could be a valuable predictor of the service lifetime of PV modules, as our findings in this study mean the equivalency of the failure attained by extended damp-heat testing with that observed in the field.