Takashi Koida

Hydrogen-doped In2O3 transparent conducting oxide films prepared by solid-phase crystallization method

We have characterized amorphous to crystalline transformation of hydrogen (H)-doped In2O3 (In2O3:H) films by transmission electron microscopy, thermal desorption spectroscopy, spectroscopic ellipsometry, and Hall measurements. The In2O3:H films that show a mixed-phase structure embedded with small density of crystalline grains in a large volume fraction of amorphous phase have been fabricated at room temperature by the sputtering of an In2O3 ceramic target with introduction of H2O vapor, and the films have been postannealed in vacuum to crystallize the amorphous phase. With increasing annealing temperature up to 200u2009°C, the film shows a large increase in Hall mobility (μHall) from 42 to 110u2002cm2/Vu2009s and a decrease in carrier density (NHall) from 4.6×1020 to 2.1×1020u2002cm−3 with slight decrease in resistivity. The change in μHall and NHall with annealing temperature is strongly correlated with the volume fractions of the amorphous and crystalline phases in the films. Analyses of dielectric functions of the fi...

High electron mobility of indium oxide grown on yttria-stabilized zirconia

In2O3 heteroepitaxial layers of improved surface morphology and mobility were obtained by pulsed-laser-deposition method supplying proper oxygen to indium ratio on the growing surface, combined with the suppression of thermal decomposition of In2O3 layers. In situ monitoring of reflection high-energy electron diffraction patterns and ex situ monitoring of growth rate of the epilayers grown under different oxygen pressures and growth temperatures revealed that thermal decomposition occurred during the high-temperature growth and the use of significant oxygen pressure could suppress decomposition allowing for an increase in obtainable growth temperatures. As a result of the decomposition control and growth optimization, undoped In2O3 epilayers exhibited atomically flat surfaces, improved Hall mobility of 110cm2∕Vs, and carrier density of 6.6×1018cm−3. The influences of growth and postgrowth conditions on the electrical properties of epilayers were investigated based on charged and neutral impurity scatterin...

High-efficiency microcrystalline silicon solar cells on honeycomb textured substrates grown with high-rate VHF plasma-enhanced chemical vapor deposition

The potential of high-rate growth of high-quality microcrystalline silicon (µc-Si:H) films for solar cell applications is investigated by very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) under a high-pressure SiH4 depletion condition. It is found that the morphology of textured substrates plays an important role in not only light trapping but also µc-Si:H film growth. A high conversion efficiency of 11.1% is attained in a substrate-type µc-Si:H cell on a substrate with honeycomb textures, which has rounded concaves in a honeycomb arrangement with an appropriate period. It is also clarified that ZnO:B films grown by metal organic chemical vapor deposition (MOCVD) are beneficial in terms of carrier collection compared with the standard In2O3:Sn (ITO) film grown by sputtering. On the basis of these findings, a new world-record µc-Si:H cell with a certified conversion efficiency of 11.8% is developed with a relatively high deposition rate of 1 nm/s.

Microcrystalline Silicon Solar Cells with 10.5% Efficiency Realized by Improved Photon Absorption via Periodic Textures and Highly Transparent Conductive Oxide

We developed advanced light management techniques and applied them to single-junction microcrystalline silicon solar cells to improve their current density and conversion efficiency. A high short-circuit current density of 30.8 mA/cm2 is attained in a 3-µm-thick cell aided by the superior light-trapping effect of periodically textured back reflectors, and by the reduced absorption loss from the high-mobility transparent conductive oxide films. In addition, an unprecedented efficiency of 10.5% is independently confirmed in a 1.8-µm-thick cell with the same structure. These results indicate the importance of light management for further efficiency improvements in thin-film silicon solar cells.

High-mobility transparent conductive Zr-doped In2O3

Optical and electric properties in Zr-doped In2O3 epitaxial layers were systematically investigated. The films at Zr concentrations of 0.3–0.5at.% are found to be superior transparent conductive oxides in transparency in near infrared wavelength region and Hall mobility compared to Sn-doped In2O3. Maximum mobilities are over 100cm2∕Vs and corresponding carrier densities are approximately 1×1020cm−3. From the relationship between the values of Hall mobility and carrier concentration of the epilayers, a number and/or effects of multicharged and neutral scattering centers of electrons seem to be reduced.

High-efficiency amorphous silicon solar cells: Impact of deposition rate on metastability

Hydrogenated amorphous silicon (a-Si:H) films, used for light absorbers of p-i-n solar cells, were deposited at various deposition rates (Rd) ranging over two orders of magnitude (Rdu2009∼u20092u2009×u200910−3–3 ×u200910−1u2009nm/s) by using diode and triode plasma-enhanced chemical vapor deposition (PECVD). The impact of varying Rd on the light-soaking stability of the solar cells has been investigated. Although a reduction of Rd mitigates the light-induced degradation in the typical range of Rd (>10−1u2009nm/s), it remains present even in the very low Rd (<10−2u2009nm/s), indicating that the metastable effect persists in a-Si:H regardless of Rd. The best performing cell, whose a-Si:H absorber is characterized by low amount of metastable defect and high bandgap, can be obtained at Rd of ∼1–3u2009×u200910−2u2009nm/s by triode PECVD. By applying such a-Si:H in the improved p-i-n devices, we demonstrate two record independently confirmed stabilized efficiencies of 10.22% for single-junction and 12.69% for a-Si:H/hydrogenated microcrystalline silicon ...

Comparative studies of transparent conductive Ti-, Zr-, and sn-doped In2O3 using a combinatorial approach

We report on comparative studies of transparent conductive Ti-, Zr-, and Sn-doped In2O3 using a combinatorial approach. In2−2xMe2xO3 (Me:Ti, Zr, Sn) composition-spread epilayers (0≤x≤0.1) were fabricated on yttria-stabilized zirconia substrates using the combinatorial pulsed laser deposition technique, and structural, optical, and electrical properties for each composition were systematically investigated. In2−2xTi2xO3 (0.003≤x<0.01) and In2−2xZr2xO3 (0.003≤x<0.05) exhibited superior transparency in the near infrared wavelength region compared to In2−2xSn2xO3 without compromising the conductivity. The results are discussed in terms of scattering centers of electrons from temperature dependence of Hall mobility and the relationship between the values of the room temperature Hall mobility and carrier concentration.

Improved near-infrared transparency in sputtered In2O3-based transparent conductive oxide thin films by Zr-doping

Transparent conductive Zr-doped In2O3 (In2−2xZr2xO3) films were deposited on glasses by sputtering method. High mobility of over 80u2009cm2∕Vu2009s was achieved under a carrier density of 1.3−2.9×1020u2009cm−3 at Zr concentrations (x) of 0.014–0.022, and the film at x=0.022 showed the lowest resistivity of 2.6×10−4u2009Ωu2009cm. Reflecting the high mobility and the low carrier density, the transparency extended from the visible to the near-infrared (NIR) wavelength region with reduced magnitude of the free-carrier absorption. The results indicate that Zr-doped In2O3 films have a performance advantage for applications that require high conductivity and transparency in NIR wavelength region.

High-efficiency thin-film silicon solar cells realized by integrating stable a-Si:H absorbers into improved device design

We report that thin-film silicon solar cells exhibiting high stabilized efficiencies can be obtained by depositing hydrogenated amorphous silicon (a-Si:H) absorbers using triode-type plasma-enhanced chemical vapor deposition. The improved light-soaking stability and performance of solar cells are also realized by optimizing the device design, such as p and p–i buffer layers. As a result, we attain independently confirmed stabilized efficiencies of 10.1–10.2% for a-Si:H single-junction solar cells (absorber thickness: ti = 220–310 nm) and 12.69% for an a-Si:H (ti = 350 nm)/hydrogenated microcrystalline silicon (µc-Si:H) tandem solar cell fabricated using textured SnO2 and ZnO substrates, respectively. The relative efficiency degradations of these solar cells are ~10 and 3%, respectively, under 1 sun illumination at 50 °C for 1000 h.

Multi Junction Solar Cells Stacked with Transparent and Conductive Adhesive

We propose a polyimide transparent adhesive layer dispersed with In2O3–SnO2 (ITO) conductive particles (polyimide-ITO) to be used in the mechanical stacking of solar cells. A 20-µm-thick polyimide-ITO layer had a high transmissivity from 78 to 80% for wavelengths ranging from 500 to 1000 nm and a low connecting resistivity of 2.3 Ω cm2 at minimum. The fabrication of stacked cell consisting of a top hydrogenated amorphous silicon (a-Si:H) p–i–n cell and a bottom hetero-junction with an intrinsic thin-layer (HIT)-type silicon cell was demonstrated using an intermediate polyimide-ITO layer. A high open circuit voltage of 1.34 V was experimentally obtained. Simultaneous electric power generation from the top and bottom solar cells was achieved.