The present status and prospects for further development of polycrystalline or amorphous transparent conducting oxide (TCO) semiconductors used for practical thin-film transparent electrode applications are presented in this paper. The important TCO semiconductors are impurity-doped ZnO, In2O3 and SnO2 as well as multicomponent oxides consisting of combinations of ZnO, In2O3 and SnO2, including some ternary compounds existing in their systems. Development of these and other TCO semiconductors is important because the expanding need for transparent electrodes for optoelectronic device applications is jeopardizing the availability of indium-tin-oxide (ITO), whose main constituent, indium, is a very expensive and scarce material. Al- and Ga-doped ZnO (AZO and GZO) semiconductors are promising as alternatives to ITO for thin-film transparent electrode applications. In particular, AZO thin films, with a low resistivity of the order of 10−5 Ω cm and source materials that are inexpensive and non-toxic, are the best candidates. However, further development of the deposition techniques, such as magnetron sputtering or vacuum arc plasma evaporation, as well as of the targets is required to enable the preparation of AZO and GZO films on large area substrates with a high deposition rate.

https://iopscience.iop.org/article/10.1088/0268-1242/20/4/004/pdf

Transparent conducting oxide semiconductors for transparent electrodes

https://chemistry.beloit.edu/classes/nanotech/ZnO/mattoday10_05_40.pdf

ZnO - nanostructures, defects, and devices

Copper-oxide compound semiconductors provide a unique possibility to tune the optical and electronic properties from insulating to metallic conduction, from bandgap energies of 2.1 eV to the infrared at 1.40 eV, i.e., right into the middle of the efficiency maximum for solar-cell applications. Three distinctly different phases, Cu2O, Cu4O3, and CuO, of this binary semiconductor can be prepared by thin-film deposition techniques, which differ in the oxidation state of copper. Their material properties as far as they are known by experiment or predicted by theory are reviewed. They are supplemented by new experimental results from thin-film growth and characterization, both will be critically discussed and summarized. With respect to devices the focus is on solar-cell performances based on Cu2O. It is demonstrated by photoelectron spectroscopy (XPS) that the heterojunction system p-Cu2O/n-AlGaN is much more promising for the application as efficient solar cells than that of p-Cu2O/n-ZnO heterojunction devices that have been favored up to now.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/pssb.201248128

Binary copper oxide semiconductors: From materials towards devices

We report on the fabrication of heterojunction solar cells made by deposition of transparent conducting oxide (TCO) films on Cu2O substrates. The TCO films have been grown by ion beam sputtering on good quality Cu2O sheets prepared by oxidizing copper at a high temperature. The best solar cell has reached an open-circuit voltage of 0.595V, a short-circuit current density of 6.78mA∕cm2, a fill factor of 50%, and a conversion efficiency of 2% under simulated AM1.5G illumination, which is the highest efficiency value reported for this kind of heterojunction devices. These devices represent a good starting point for the development of very low cost solar cells.

Heterojunction solar cell with 2% efficiency based on a Cu2O substrate

Although ZnO and ZnS are abundant, stable, and environmentally benign, their band gap energies (3.44, 3.72 eV, respectively) are too large for optimal photovoltaic efficiency. By using band-corrected pseudopotential density functional theory calculations, we study how the band gap, optical absorption, and carrier localization can be controlled by forming quantum-well-like and nanowire-based heterostructures of ZnO/ZnS and ZnO/ZnTe. In the case of ZnO/ZnS core/shell nanowires, which can be synthesized using existing methods, we obtain a band gap of 2.07 eV, which corresponds to a Shockley−Quiesser efficiency limit of 23%. On the basis of these nanowire results, we propose that ZnO/ZnS core/shell nanowires can be used as photovoltaic devices with organic polymer semiconductors as p-channel contacts.

/pdf/optical-properties-of-zno-zns-and-zno-znte-heterostructures-1hq03qg10o.pdf

Optical properties of ZnO/ZnS and ZnO/ZnTe heterostructures for photovoltaic applications.

Effect of ZnO film deposition methods on the photovoltaic properties of ZnO–Cu2O heterojunction devices

The preparation of low resistivity polycrystalline Al-doped ZnO (AZO) thin films on glass substrates by a pulsed laser deposition method is described, including a description of conditions necessary to obtain low resistivity on the order of 1×10−4 Ω cm. The resistivity of transparent conducting AZO films obtained was affected by both the target sintering conditions and the deposition conditions. A decrease in carrier concentration associated with a increase in deposition temperature is ascribed to an increase of oxygen content in the films, resulting from enhancement of oxidation on the substrate surface. An increase in Hall mobility associated with increases of either the deposition temperature or film thickness is attributed to imperfections of the ZnO lattice, as evidenced by improvement of the crystallinity such as an increase of crystallite size. It was found that the mobility is also greatly influenced by a combination of ionized impurity scattering and grain boundary scattering or dislocation scatt...

Kazuhiko Ihara

Papers

Effect of ZnO film deposition methods on the photovoltaic properties of ZnO–Cu2O heterojunction devices

Low resistivity polycrystalline ZnO:Al thin films prepared by pulsed laser deposition