Mika Kambe

TiO2-Coated Transparent Conductive Oxide (SnO2:F) Films Prepared by Atmospheric Pressure Chemical Vapor Deposition with High Durability against Atomic Hydrogen

The durability of textured transparent conductive oxide (TCO) thin films against atomic hydrogen was investigated. An ultrathin TiO2 layer of 2 nm thickness was deposited on textured fluorine-doped tin oxide (SnO2:F) films, successively by atmospheric pressure chemical vapor deposition (AP-CVD). TCO films with a TiO2 layer showed a higher optical transmittance and a lower resistivity after exposure to atomic hydrogen excited by very high frequency (VHF) plasma, while TCO films without a TiO2 layer showed a lower optical transmittance and a higher resistivity after the exposure. These TCO films were characterized by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) before and after the exposure to atomic hydrogen.

Fabrication of A-Si:H Solar cells on high haze SnO 2 :F thin films

In this work we report on the properties of amorphous silicon (a-Si:H) p-i-n solar cells fabricated on glass substrates covered with extremely high haze fluorinedoped tin oxide (SnO2:F) transparent conductive oxide (TCO) thin films, HU-TCO. The HU-TCO shows high haze value thorough the whole optical region where a-Si:H and microcrystalline silicon (μc-Si:H) solar cells are sensitive. We demonstrate here that open-circuit voltage (Voc) and fill factor (FF) of a-Si:H solar cells fabricated on HU-TCOs does not decrease significantly compared to the cells on conventional pyramidal texture TCOs. We also demonstrate here that Voc and FF of a-Si:H solar cells depend on not only surface morphology of TCOs, but also strongly depend on thickness and deposition conditions of the solar cells.

Chemically strengthened cover glass for preventing potential induced degradation of crystalline silicon solar cells

It is shown that Potential Induced Degradation (PID) can be prevented by using chemically strengthened glass as module cover glass in this paper. While several factors are considered to be the causes of PID, migration of Na ions from module cover glass has been pointed out as a principal factor. It has been known that photovoltaic modules with proper materials of encapsulant, antireflection coating layer on Si wafer, and/or appropriate system conditions do not show PID. Without such proper measures, photovoltaic modules show PID under tough conditions, high temperature, high humidity, and/or high bias voltage. Chemically strengthened glass is known to have lower Na concentration in its surface compared to conventional thermally strengthened glass. Small modules containing chemically strengthened glass were prepared and operated in an accelerated PID test. Negative voltage bias of 1000 V was applied continuously to the cells in the modules by connecting shorted leads during the accelerated PID test. The small modules with chemically strengthened glass show the same maximum power under 1000 W/m2 illumination after those accelerated PID tests with high temperature and high bias voltage as before, even though no other measures were made to the modules for preventing PID. In other words, only replacing cover glass with chemically strengthened glass prevents Na-related PID. Chemically strengthened glass as photovoltaic module cover glass is now commercially available, as “Leoflex™” from Asahi Glass. Composition of the Leoflex™ glass is specially designed for good chemical-strengthening characteristics.

Improved light-trapping effect in a-Si:H / μC-Si:H tandem solar cells by using high haze SnO 2 :F thin films

In this paper we report on the fabrication of amorphous silicon (a-Si:H) / microcrystalline silicon (μc- Si:H) p-i-n tandem solar cells on high haze fluorine-doped tin oxide (SnO2:F) transparent conductive oxide (TCO) thin films, type-HU. The HU-TCO has double texture morphology on its surface and shows high haze value through the whole optical region where the tandem solar cells are sensitive. We demonstrate here that light-trapping effect is improved by using HU-TCO. HU-TCO provided higher short-circuit current than conventional pyramidal TCO when i-layer thicknesses of a-Si:H and μc- Si:H of tandem solar cell were 0.35 and 1.5 µm, respectively. Quantum efficiency of bottom μc-Si:H cell was improved by 1 mA/cm2 by using HU-TCO.

Relation of TCO surface texture shape to a-Si/μc-Si tandem cell performance on W-texture SnO 2 :F TCO substrates

Double-texture (W-texture) TCO, which was named “HU-TCO”, has a combination of 300–500nm height and over 1μm width hills which are covered with submicron-size pyramidal texture that leads to high haze value in a wide wavelength range. We studied the relation between the bottom current of the a-Si:H/μc-Si:H tandem solar cells and the TCO texture indexes, such as the haze ratio at 800nm, and light scattering angle distribution. There is a positive correlation between bottom current and haze ratio at 800nm; however it was observed that the bottom current hit a peak at very high haze value. We modified amorphous p-layer of top cell and confirmed that the player configuration strongly affected Voc. Appropriate player configuration not only affected the Voc but also further increased the bottom current on the HU-TCO with high hills.