No abstract

No abstract

We developed a fabrication technique of very thin silicon nanowall structures. The minimum width of the fabricated silicon nanowall structures was about 3 nm. This thinnest region of the silicon nanowall structures was investigated by using cathode luminescence and ultraviolet photoelectron spectroscopy (UPS). The UPS measurements revealed that the density of states (DOS) of the thinnest region showed a stepwise shape which is completely different from that of the bulk Si. Theoretical analysis clearly demonstrated that this change of the DOS shape was due to the quantum size effect.

To improve the conversion efficiency of Si solar cells, we have developed a thin Si wafer-based solar cell that uses a rib structure. The open-circuit voltage of a solar cell is known to increase with deceasing wafer thickness if the cell is adequately passivated. However, it is not easy to handle very thin wafers because they are brittle and are subject to warpage. We fabricated a lattice-shaped rib structure on the rear side of a thin Si wafer to improve the wafer’s strength. A silicon nitride film was deposited on the Si wafer surface and patterned to form a mask to fabricate the lattice-shaped rib, and the wafer was then etched using KOH to reduce the thickness of the active area, except for the rib region. Using this structure in a Si heterojunction cell, we demonstrated that a high open-circuit voltage (VOC) could be obtained by thinning the wafer without sacrificing its strength. A wafer with thickness of 30 μm was prepared easily using this structure. We then fabricated Si heterojunction solar cells using these rib wafers, and measured their implied VOC as a function of wafer thickness. The measured values were compared with device simulation results, and we found that the measured VOC agrees well with the simulated results. To optimize the rib and cell design, we also performed device simulations using various wafer thicknesses and rib dimensions.

No abstract

We applied an epitaxial n + -type Si emitter layer grown on a p-type Si substrate by our environmentally-light-load sputter epitaxy method using RF magnetron sputtering without dopant activation annealing for a Si solar cell. We also applied low-temperature cleaning of the substrate with a hydrogen-fluoride treatment at room temperature prior to the emitter layer growth instead of the conventionally used high-temperature thermal cleaning under vacuum condition. In addition, by our sputter epitaxy method, we determined the optimum temperature for the emitter growth. An emitter layer with good crystallinity is obtained, and the solar cell, formed with an emitter layer grown at the optimum growth temperature of 410 °C, exhibits an energy conversion efficiency of 12.3% in 100% aperture ratio equivalent without a texture or an antireflection coat. By the above lowtemperature processes, a solar cell can be fabricated with process temperatures below 410 °C, which exhibits low temperature processes.

Passivation films or antireflection coatings are generally prepared using costly vacuum or high-temperature processes. Thus, we report the preparation of TiOx–SiOx composite films by novel spin coatable solutions for the synthesis of low-cost passivation coating materials. The desired films were formed by varying the mixing ratios of TiOx and SiOx, and the resulting films exhibited excellent surface passivation properties. For the p-type wafer, an optimal effective surface recombination velocity (Seff) of 93 cm/s was achieved at , while a surface recombination current density (J0s) of 195 fA/cm2 was obtained. In contrast, for the n-type wafer, an Seff of 27 cm/s and a J0s of 38 fA/cm2 were achieved at . This excellent surface passivation effect could be attributed to the low interface state density and high positive fixed charge density. Furthermore, the thickness of the interfacial SiOx layer was determined to be important for obtaining the desired surface passivation effect.

No abstract