Hom Nath Luitel

A broadband-sensitive upconverter La(Ga0.5Sc0.5)O3:Er,Ni,Nb for crystalline silicon solar cells

We have developed an upconverter that significantly broadens the sensitive range, to overcome the shortcoming that conventional Er3+-doped upconverters used for crystalline silicon solar cells can utilize only a small portion of the solar spectrum at around 1.55 μm. We have designed the combination of the sensitizers and host material to utilize photons not absorbed by silicon or Er3+ ions. Ni2+ ions have been selected as the sensitizers that absorb photons in the wavelength range between the silicon absorption edge (1.1 μm) and the Er3+ absorption band and transfer the energies to the Er3+ emitters, with La(Ga,Sc)O3 as the host material. The Ga to Sc ratio has been optimized to tune the location of the Ni2+ absorption band for sufficient energy transfer. Co-doping with Nb5+ ions is needed for charge balance to introduce divalent Ni2+ ions into the trivalent Ga3+ and Sc3+ sites. In addition to 1.45–1.58 μm photons directly absorbed by the Er3+ ions, we have demonstrated upconversion of 1.1–1.35 μm photons...

Broadband-sensitive Ni2+–Er3+ based upconverters for crystalline silicon solar cells

We have developed Ni2+, Er3+ codoped CaZrO3 broadband-sensitive upconverters that significantly broaden the sensitive range, and hence overcome the shortcomings of conventional Er3+ doped upconverters used for crystalline silicon (c-Si) solar cells that utilize only a small fraction of the solar spectrum around 1550 nm. We have designed the combination of sensitizers and host material to utilize photons that are not absorbed by c-Si itself or Er3+ ions. Six coordinated Ni2+ ions substituted at the Zr4+ sites absorb (1060–1450) nm photons and transfer the energies to the Er3+ ions, and the Er3+ upconverts at 980 nm. Co-doping with monovalent charge compensators such as Li+ for high Er3+ solubilisation at the Ca2+ sites and multivalent ions (Nb5+) for stabilization of Ni2+ at the Zr4+ sites is essential. In addition to 1450–1600 nm (≈2 × 1020 m−2 s−1) photons directly absorbed by the Er3+ ions, we have demonstrated upconversion of 1060–1450 nm (≈6 × 1020 m−2 s−1) photons in the Ni2+ absorption band to 980 nm photons using the CaZrO3:Ni2+,Er3+ upconverters. Compared with the current density gain of present c-Si solar cells (∼40 mA cm−2), the upconverted photons could increase this by ∼7.3 mA cm−2, which is about 18% improvement. This architecture for broadband-sensitive upconversion may pave a new direction for the improvement in efficiency of the present c-Si solar cells to surpass the limiting conversion efficiency of single-junction solar cells.

Energy transfer between Ni2+ sensitizers and Er3+ emitters in broadband-sensitive upconverters La(Ga,Sc,In)O3:Er,Ni,Nb

We have analyzed broadband-sensitive upconversion from 1.1–1.6 μm to 0.98 μm in La(Ga,Sc,In)O3 doped with Er, Ni, and Nb, which could significantly boost the conversion efficiency of crystalline silicon solar cells, in particular, energy transfer from the Ni2+ sensitizers to the Er3+ emitters and back transfer from the Er3+ to the Ni2+. We have compared these processes and the resultant upconversion emission intensities depending on the host material compositions. With increasing the bond length between the Ni2+ and surrounding oxygen ions, the Ni2+ emission band located at around 1.2–1.6 μm red-shifts and hence overlaps more significantly with the Er3+ absorption band ranging from 1.45 μm to 1.6 μm, resulting in more rapid energy transfer from the Ni2+ to the Er3+. However, back energy transfer from the Er3+ to the Ni2+ deteriorates the performance more considerably, because of more significant overlap between the Er3+ emission band and Ni2+ absorption band. This trade-off relationship strongly affects t...

Broadband-sensitive cooperative upconversion emission of La(Ga0.5Sc0.5)O3:Er,Ni,Nb

We systematically conducted time-resolved photoluminescence measurements on La(Ga0.5Sc0.5)O3:Er,Ni,Nb to elucidate the dominant mechanism of Ni2+-sensitized Er3+ upconversion emission at approximately 0.98 µm under 1.0–1.45-µm excitation, which could significantly improve the conversion efficiency of crystalline silicon solar cells. After detailed analysis using rate equations describing the energy transfer involved in the Ni2+ and Er3+ and upconversion, we concluded that the upconversion emission is dominated by the excitation of two Er3+ emitters due to the resonance energy transfer from two Ni2+ sensitizers, followed by further excitation to the initial state of the upconversion emission caused by another energy transfer between the two first-excited Er3+, i.e., cooperative upconversion.

Effect of Ti compositions for efficiency enhancement of CaTiO3:Er3+,Ni2+ broadband-sensitive upconverters

Improving the efficiency of upconversion (UC) materials is a hot topic in recent days due to the important applications of UC materials in photovoltaics, photonics devices, photocatalysts, sensors, biological imaging, and therapeutics. Recently, we have reported a broadband-sensitive UC emission in Ni2+, Er3+-codoped perovskites. However, the applications of these perovskites are limited due to their low conversion efficiency. Herein, we realized highly improved UC efficiency in the CaTiO3:Er3+,Ni2+ upconverter as compared to those of the previously reported CaZrO3 and La(Ga,Sc)O3 upconverters. Ti composition plays important roles in stabilizing divalent nickel (Ni2+) in an octahedral coordination, which is the key point for sensitization to Er3+ emitters. Furthermore, oxygen vacancies and consequently tetrahedral Ni2+ ions, which kill the luminescence, are suppressed, and as a result, the UC emission intensity is dramatically increased. The 0.1 mole Ti-deficient sample with the (Ca0.8Er0.10Li0.10)(Ti0.894Ni0.002Nb0.004)O2.8 composition exhibited the most intense broadband-sensitive UC emission, which was 264-fold stronger than that of the stoichiometric sample and more than 12 folds as compared to that of the previously reported CaZrO3:Er,Ni and La(Ga,Sc)O3 upconverters. The highest UC quantum yield of ∼2.53% was realized in the optimized CaTiO3:Er3+,Ni2+ upconverter under 1490 nm laser excitation of ∼1000 W m−2.

Broadband-sensitive upconverters co-doped with Er3+ and Ni2+ for crystalline silicon solar cells

We have demonstrated broadband sensitization of Er3+-doped upconverters coupled with crystalline silicon (c-Si) solar cells by introducing Ni2+ co-dopants into ABO3-type perovskite host materials such as La(Ga,Sc,In)O3 and CaZrO3. The Ni2+ sensitizers absorb 1.1−1.45 μm photons, which are not absorbed by either c-Si or Er3+, and transfer the energies to the Er3+ emitters. Thus, 1.1−1.45 μm photons are also upconverted to 0.98 μm photons, in addition to 1.45−1.6 μm photons that are directly absorbed by the Er3+. To compensate the charge imbalance caused by introducing divalent Ni2+ ions into the trivalent (Ga3+, Sc3+, and In3+) and tetravalent (Zr4+) sites, Nb5+ co-dopants were incorporated. Similarly, codoping with monovalent ions (Li+, Na+, K+) notably enhanced the upconversion emission when the Ca2+ sites were substituted with the Er3+ ions. These broadband-sensitive upconverters overcome the shortcoming of conventional Er3+- doped upconverters that only a small portion of the solar spectrum at around 1.55 μm is utilized. If all the photons in the Er3+ absorption band ranging from 1.45 μm to 1.6 μm were perfectly upconverted, the improvement in the short-circuit current density (JSC) would be 1.9 mA/cm2 under the AM1.5G 1 sun solar illumination. The additional improvement for the broadband-sensitive upconverters developed here could be as high as 4.1 mA/cm2 by utilizing 1.1−1.45 μm photons, thus totally 6.1 mA/cm2. This corresponds to a significant gain in conversion efficiency (η) by 3.8% for c-Si solar cells with JSC = 40 mA/cm2 and η = 25%. The architecture of the broadband sensitization opens the door toward the concept of the third-generation solar cells with high conversion efficiency and low cost.

Concept of the solar-pumped laser-photovoltaics combined system and its application to laser beam power feeding to electric vehicles

We have developed a compact solar-pumped laser (µSPL) employing an off-axis parabolic mirror with an aperture of 76.2 mm diameter and an yttrium aluminum garnet (YAG) ceramic rod of 1 mm × 10 mm doped with 1% Nd and 0.1% Cr as a laser medium. The laser oscillation wavelength of 1.06 µm, just below the optical absorption edge of Si cells, is suitable for photoelectric conversion with minimal thermal loss. The concept of laser beam power feeding to an electric vehicle equipped with a photovoltaic panel on the roof was proposed by Ueda in 2010, in which the electricity generated by solar panels over the road is utilized to drive a semiconductor laser located on each traffic signal along the road. By substituting this solar-electricity-driven semiconductor laser with a solar-pumped laser, the energy loss of over 50% in converting the solar electricity to a laser beam can be eliminated. The overall feasibility of this system in an urban area such as Tokyo was investigated.