Yasuhiro Aida

Degradation and passivation of CuInSe2

The degradation of CuInSe2 absorbers in ambient air is observed by the decay of the quasi-Fermi level splitting under well defined illumination with time. The decay is faster and stronger in absorb ...

Current loss due to recombination in Cu-rich CuInSe2 solar cells

The absorbers in Cu(In,Ga)Se2 solar cells in general are Cu-poor. However, better transport properties and lower bulk recombination in “Cu-rich” material led us to develop “Cu-rich” CuInSe2 solar cells. We expect higher diffusion lengths and better carrier lifetimes for “Cu-rich” CuInSe2 solar cells, resulting in a higher short circuit current of “Cu-rich” solar cells, compared to Cu-poor ones. However, recent investigations show that the current is lower for absorbers grown under Cu-excess compared to Cu-poor absorbers. Therefore, this work investigates both “Cu-rich” and Cu-poor CuInSe2 absorbers, as well as their resulting cells, in order to understand why the “Cu-rich” CuInSe2 solar cells do not show the expected increase in current. While this contribution gives proof that “Cu-rich” based solar cells in fact do have better carrier collection properties, one limitation of “Cu-rich” devices is a very short space charge width associated with a higher doping level. We suggest tunneling enhanced recombination in the space charge region as the most likely cause of the loss in current. This work shows also that the high doping level of the “Cu-rich” film cannot be decreased by controlling the sodium supply.

Defect levels in the epitaxial and polycrystalline CuGaSe2 by photocurrent and capacitance methods

The defect levels in epitaxial and polycrystalline wide bandgap chalcopyrite CuGaSe2 with various stoichiometry deviations were investigated using modulated photocurrent spectroscopy. The results were analyzed as a function of light intensity and Fermi-level position. Comparison of the results from epitaxial and polycrystalline material distinguished levels belonging to intrinsic defects and their correlation with the material stoichiometry. We also compared the fingerprints of defect levels by MPC to the results derived from capacitance spectroscopy performed on Schottky diodes fabricated on both epitaxial and polycrystalline layers. This allowed us to attribute unambiguously levels observed in the capacitance response to bulk point defects. In the final conclusions we provide information on the electronic parameters of nine defect levels observed in CuGaSe2 and their correlation with the material stoichiometry. These results should help to identify intrinsic defects that are important for the photovolta...

Photoinduced current transient spectroscopy of defect levels in CuInSe2 and CuGaSe2 epitaxial and polycrystalline layers

Photoinduced current transient spectroscopy of defect levels in CuInSe2 and CuGaSe2 epitaxial and polycrystalline layers

Giant increase in piezoelectric coefficient of AlN by Mg-Nb simultaneous addition and multiple chemical states of Nb

Aluminum nitride (AlN) is one of piezoelectric materials, which are eagerly anticipated for use in microelectromechanical systems (MEMS) applications such as communication resonators, sensors, and energy harvesters. AlN is particularly excellent in generated voltage characteristics for the MEMS rather than oxide piezoelectric materials such as lead zirconium titanate Pb(Zr, Ti)O3. However, it is necessary to improve the piezoelectric properties of AlN in order to advance the performance of the MEMS. We dramatically increased the piezoelectric coefficient d33 of AlN films by simultaneously adding magnesium (Mg) and niobium (Nb). The d33 of Mg39.3Nb25.0Al35.7N is 22 pC/N, which is about four times that of AlN. The d33 is increased by Mg and Nb simultaneous addition, and is not increased by Mg or Nb single addition. Interestingly, the Nb has multiple chemical states, and which are influenced by the Mg concentration.