We have developed highly crystallized n-type microcrystalline Si layers as window layers for rear emitter Si heterojunction solar cells. We introduce a seed layer between an n-type microcrystalline Si layer and an intrinsic amorphous Si layer to improve the crystallinity of the n-type microcrystalline Si layer. By using this stacked layer instead of an n-type amorphous Si layer, the contact resistance between the n-type thin layer and In2O3:H is reduced without Al-doped ZnO. As a result, we obtain a high short-circuit current and a high fill factor simultaneously, and achieve a solar cell efficiency of 23.43%.

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In this paper, a structural health monitoring (SHM) methodology that can detect and characterize local structural damages in early stage is developed, by merging the concepts of two existing SHM principles, a piezoelectric impedance-based methodology and a nonlinear wave modulation spectroscopy. The presented SHM system mainly consists of a piezoelectric element bonded on the structural surface, a high-frequency harmonic voltage source, and a current detector. When the structure is subjected to a dynamic load at low-frequencies, it vibrates, and the scattering conditions for the high-frequency elastic waves in the vicinity of the inherent damages will change in synchronization with the structural vibration. This nonlinear effects of vibro-acoustic interaction between the low-frequency vibration and the high-frequency wave field causes the change in the driving-point impedance at the high frequency range, which can significantly modulate the coupled electro-mechanical impedance (or admittance) of the piezoelectric element. Therefore, if the piezoelectric element is driven by a fixed amplitude high-frequency harmonic voltage source, the nonlinear modulation of the coupled admittance can be observed as the amplitude and phase modulation of the current flowing through the piezoelectric element. A simplified modeling study of the above-mentioned nonlinear piezoelectric impedance modulation successfully leads to a damage evaluation index that assesses the intensity of the modulation of the modal stiffness. Experiments using a cracked beam are conducted to see how the impedance modulation can be observed and to examine the performance of the proposed method.

In this paper, the applicability of a nonlinear wave modulation-based crack monitoring methodology has been experimentally investigated. Experiments using a beam specimen with a low-cycle fatigue crack have been conducted for the purpose of preliminary study, in which two PZT patches attached on the beam have been used as the transducer of high frequency probe wave. When the specimen is subjected to a harmonic loading at low frequencies, it vibrates, and the presence of the crack introduces a nonlinear effect to the vibro-acoustic dynamics resulting an interaction between the low frequency structural vibration and the high frequency probe wave. This nonlinearity is observed as the amplitude and phase modulation of the received probe wave synchronous with the structural vibration. To investigate the relationship between the modulations, the structural vibration and the damage extent, the collected signal at the receiver PZT has been separated into low frequency and high frequency components, the former has been used to obtain the information about the structural vibration, while the latter has been demodulated in amplitude and phase. The demodulated waveforms have been examined as a potential indicator of the crack extent, especially focusing on their higher harmonics. Then, a "modulation surface" constructed from the modulated envelopes and the low frequency components has been proposed, which could provide more detailed view of the nonlinear wave modulation effects induced by the crack development. Several candidates for a damage indicator based on the modulation surface have been presented to demonstrate the usefulness of the modulation surface as a sensitive and promising feature relevant to the damage extent.