Yuki Nishi

High-Efficiency Oxide Solar Cells with ZnO/Cu2O Heterojunction Fabricated on Thermally Oxidized Cu2O Sheets

High conversion efficiencies were achieved in low cost n–p heterojunction oxide solar cells with an Al-doped ZnO (AZO)/non-doped ZnO (ZO)/Cu2O structure. This achievement was made possible by the formation of an n-ZO thin-film layer, prepared with an appropriate thickness by low damage deposition, on high quality Cu2O sheets produced by the thermal oxidization of copper sheets: n-ZO thin film optimal thickness ranges from 30 to 50 nm. Photovoltaic characteristics such as an open circuit voltage of 0.69 V, a fill factor of 0.55 and a conversion efficiency of 3.83% were attained under simulated AM1.5G solar illumination.

High-Efficiency Cu2O-Based Heterojunction Solar Cells Fabricated Using a Ga2O3 Thin Film as N-Type Layer

High-efficiency heterojunction solar cells consisting of a nondoped Ga2O3 thin film as an n-type semiconductor layer and a p-type Cu2O sheet as the active layer as well as the substrate, prepared by thermally oxidizing a Cu sheet, are demonstrated. The use of an n-type Ga2O3 thin film can greatly improve the performance of n-Ga2O3/p-Cu2O heterojunction solar cells. The highest efficiency of 5.38% was obtained in an Al-doped ZnO/Ga2O3/Cu2O heterojunction solar cell fabricated with an n-Ga2O3 thin-film layer prepared at room temperature with a thickness of 75 nm by a pulsed laser deposition method.

Heterojunction solar cell with 6% efficiency based on an n-type aluminum–gallium–oxide thin film and p-type sodium-doped Cu2O sheet

In this paper, we describe efforts to enhance the efficiency of Cu2O-based heterojunction solar cells fabricated with an aluminum–gallium–oxide (Al–Ga–O) thin film as the n-type layer and a p-type sodium (Na)-doped Cu2O (Cu2O:Na) sheet prepared by thermally oxidizing copper sheets. The optimal Al content [X; Al/(Ga + Al) atomic ratio] of an AlX–Ga1−X–O thin-film n-type layer was found to be approximately 2.5 at. %. The optimized resistivity was approximately 15 Ω cm for n-type AlX–Ga1−X–O/p-type Cu2O:Na heterojunction solar cells. A MgF2/AZO/Al0.025–Ga0.975–O/Cu2O:Na heterojunction solar cell with 6.1% efficiency was fabricated using a 60-nm-thick n-type oxide thin-film layer and a 0.2-mm-thick Cu2O:Na sheet with the optimized resistivity.

Efficiency enhancement using a Zn1− x Ge x -O thin film as an n-type window layer in Cu2O-based heterojunction solar cells

Efficiency enhancement was achieved in Cu2O-based heterojunction solar cells fabricated with a zinc–germanium-oxide (Zn1− x Ge x -O) thin film as the n-type window layer and a p-type Na-doped Cu2O (Cu2O:Na) sheet prepared by thermally oxidizing Cu sheets. The Ge content (x) dependence of the obtained photovoltaic properties of the heterojunction solar cells is mainly explained by the conduction band discontinuity that results from the electron affinity difference between Zn1− x Ge x -O and Cu2O:Na. The optimal value of x in Zn1− x Ge x -O thin films prepared by pulsed laser deposition was observed to be 0.62. An efficiency of 8.1% was obtained in a MgF2/Al-doped ZnO/Zn0.38Ge0.62-O/Cu2O:Na heterojunction solar cell.

Effect of inserting a thin buffer layer on the efficiency in n-ZnO/p-Cu2O heterojunction solar cells

Transparent conducting Al-doped ZnO (AZO)/nondoped ZnO/Cu2O heterojunction solar cells were fabricated by inserting a thin nondoped ZnO film as a buffer layer between an n+-AZO thin film and a p-Cu2O sheet prepared by thermally oxidizing a Cu sheet. The effect of inserting the buffer layer on the obtainable photovoltaic properties in n+-AZO/n-ZnO/p-Cu2O heterojunction solar cells was investigated to improve the conversion efficiency. An improvement of photovoltaic properties was obtained by optimizing the thickness as well as the deposition conditions in the nondoped ZnO thin-film buffer layer. The obtained improvement of photovoltaic properties may be attributable mainly to an increase of the barrier height formed in the np junction, resulting from the inserted buffer layer functioning as an n-type ZnO layer as well as an enhancement of carrier lifetimes near the interface between the nondoped ZnO thin-film buffer layer and the Cu2O. A high efficiency of 4.08% was obtained in an AZO/nondoped ZnO/Cu2O het...

Impact of incorporating sodium into polycrystalline p-type Cu2O for heterojunction solar cell applications

The resistivity was controlled in the range of 103 to 10−2 Ω cm in polycrystalline p-type Cu2O sheets (incorporating sodium (Na)), which are suitable for Cu2O-based heterojunction solar cell applications. The Na-doped Cu2O sheets exhibited a hole concentration that ranged from 1013 to 1019 cm−3. In particular, a hole concentration of 1013–1016 cm−3 was obtained while maintaining a high Hall mobility above 100 cm2/V s, and, in addition, a degenerated semiconductor exhibiting metallic conduction was realized with a hole concentration above about 1 × 1019 cm−3. The mechanism associated with the Na doping can be explained by a copper vacancy produced due to charge compensation effects that result when a Na atom is incorporated at an interstitial site in the Cu2O lattice. For solar cell applications, the use of the Cu2O:Na sheet in a heterojunction solar cell successfully improved the obtained efficiency over that found in heterojunction solar cells fabricated using an undoped Cu2O sheet.

Cu2O-based solar cells using oxide semiconductors

We describe significant improvements of the photovoltaic properties that were achieved in Al-doped ZnO (AZO)/n-type oxide semiconductor/p-type Cu2O heterojunction solar cells fabricated using p-type Cu2O sheets prepared by thermally oxidizing Cu sheets. The multicomponent oxide thin film used as the n-type semiconductor layer was prepared with various chemical compositions on non-intentionally heated Cu2O sheets under various deposition conditions using a pulsed laser deposition method. In Cu2O-based heterojunction solar cells fabricated using various ternary compounds as the n-type oxide thin-film layer, the best photovoltaic performance was obtained with an n-ZnGa2O4 thin-film layer. In most of the Cu2O-based heterojunction solar cells using multicomponent oxides composed of combinations of various binary compounds, the obtained photovoltaic properties changed gradually as the chemical composition was varied. However, with the ZnO-MgO and Ga2O3-Al2O3 systems, higher conversion efficiencies (η) as well as a high open circuit voltage (Voc) were obtained by using a relatively small amount of MgO or Al2O3, e.g., (ZnO)0.91–(MgO)0.09 and (Ga2O3)0.975–(Al2O3)0.025, respectively. When Cu2O-based heterojunction solar cells were fabricated using Al2O3–Ga2O3–MgO–ZnO (AGMZO) multicomponent oxide thin films deposited with metal atomic ratios of 10, 60, 10 and 20 at.% for the Al, Ga, Mg and Zn, respectively, a high Voc of 0.98 V and an η of 4.82% were obtained. In addition, an enhanced η and an improved fill factor could be achieved in AZO/n-type multicomponent oxide/p-type Cu2O heterojunction solar cells fabricated using Na-doped Cu2O (Cu2O:Na) sheets that featured a resistivity controlled by optimizing the post-annealing temperature and duration. Consequently, an η of 6.25% and a Voc of 0.84 V were obtained in a MgF2/AZO/n-(Ga2O3–Al2O3)/p-Cu2O:Na heterojunction solar cell fabricated using a Cu2O:Na sheet with a resistivity of approximately 10 Ωcm and a (Ga0.975Al0.025)2O3 thin film with a thickness of approximately 60 nm. In addition, a Voc of 0.96 V and an η of 5.4% were obtained in a MgF2/AZO/n-AGMZO/p-Cu2O:Na heterojunction solar cell.

PL and EL characteristics in Bi- and rare earth-co-doped (La1-XGaX)2O3 phosphor thin films prepared by magnetron sputtering

Multicolor photoluminescence (PL) and electroluminescence (EL) were observed from newly developed Bi- and rare earth (RE)-co-doped (La_1-XGa_X)₂O₃ ((La_1-XGa_X)₂O₃:Bi,RE) phosphor thin films. (La_1-XGa_X)₂O₃:Bi,RE phosphor thin films were prepared by varying the Ga content (Ga/(La+Ga) atomic ratio) or the co-doped RE content (RE/(RE+La+Ga) atomic ratio) under co-doping Bi at a constant content (Bi/(Bi+La+Ga) atomic ratio) of 3 at.% using a combinatorial r.f. magnetron sputtering deposition method. High PL intensity was obtained in postannealed (La_0.9Ga_0.1)₂O₃:Bi,RE phosphor thin films prepared with a Ga content around 10 at.%; TFEL devices fabricated using the phosphor thin films exhibited high luminance. The obtained luminance intensities in EL and PL in the phosphor thin films prepared with various contents of co-doped RE, such as Dy, Er, Eu, Tb and Tm changed considerably as the kind and content of RE were varied. Color changes from blue and blue-green to various colors in PL and EL emissions, respectively, were obtained in postannealed (La_0.9Ga_0.1)₂O₃:Bi,RE phosphor thin films, i.e., films prepared by co-doping Bi at a constant content with various REs at varying levels of content. All the observed emission peaks in PL and EL from (La0_.9Ga_0.1)₂O₃:Bi,RE phosphor thin films were assigned to either the broad emission originating from the transition in Bi³⁺ or the visible emission peaks originating from the transition in the co-doped trivalent RE ion.