Improving carrier transport in Cu₂O thin films by rapid thermal annealing

Abstract

Cuprous oxide (Cu<sub>2</sub>O) is a promising material for large scale photovoltaic applications. The efficiencies of thin film structures are, however, currently lower than those for structures based on Cu<sub>2</sub>O sheets, possibly due to their poorer transport properties. This study shows that post-deposition rapid thermal annealing (RTA) of Cu<sub>2</sub>O films is an effective approach for improving carrier transport in films prepared by reactive magnetron sputtering. The as-deposited Cu<sub>2</sub>O films were poly-crystalline, p-type, with weak near band edge (NBE) emission in photoluminescence spectra, a grain size of ~100 nm and a hole mobility of 2 - 18 cm<sup>2</sup>/Vs. Subsequent RTA (3 min) at a pressure of 50 Pa and temperatures of 600 - 1000 °C enhanced the NBE by 2-3 orders of magnitude, evidencing improved crystalline quality and reduction of non-radiative carrier recombination. Both grain size and hole mobility were increased considerably upon RTA, reaching values above 1µm and up to 58 cm<sup>2</sup>/Vs, respectively, for films annealed at 900 - 1000 °C. These films also exhibited a resistivity of ~50 - 200 Ω cm, a hole concentration of ~ 10<sup>15</sup> cm<sup>-3</sup> at room temperature, and a transmittance above 80.

Metal Oxide Thin-Film Heterojunctions for Photovoltaic Applications

“…These values are comparable to those reported previously for sputter-deposited polycrystalline Cu 2 O thin films on quartz [14,15], suggesting that the annealed Cu 2 O thin films are well suited for photovoltaic applications. The increase in carrier mobility after annealing can, at least partly, be attributed to the increase in grain size and reduced grain-boundary scattering [16,17].…”

“…One of the main advantages of the magnetron sputtering technique, which has been used to synthesize the metal oxide thin films in this work, is that it provides good control of the stoichiometry of the films by varying the process parameters, such as substrate temperature, gas mass flows, and target power density, e.g., in order to obtain Cu-rich or O-rich phases for Cu 2 O [12,16]. This aspect, combined with the ability to control the electrical properties of the sputter-deposited thin films by rapid thermal annealing [17], suggests that it can indeed be possible to modify the band alignment at the interface to improve the electrical characteristics of the AZO/Cu 2 O heterojunction. The impact of hydrogen ion-implantation and subsequent thermal annealing of the Cu 2 O layer on the electrical properties of the AZO/Cu 2 O heterojunction will be the subject of further investigation.…”

Rydberg excitons in Cu2O microcrystals grown on a silicon platform

“…refs. [16][17][18] ), our results demonstrate a unique combination of excellent optical material quality, sitecontrolled growth and suitability for photonic circuit integration, which holds great promise for enabling integrated devices with novel functionalities based on excitons in Cu 2 O. In the following, we will focus on photoluminescence spectroscopy of single Cu 2 O microcrystals at milli-Kelvin temperatures to probe their fundamental excitonic properties.…”

“…Apart from quantum optics, recent reports have reinforced the significant potential of Cu 2 O as a lowcost, non-toxic material in areas such as photocatalysis 8 , solar water splitting 9 , and solar cell devices 10 showing promising photovoltaic efficiencies and marked excitonic effects 11 . Various methods have been reported for the growth of Cu 2 O thin films and single crystals 12 , in particular molecular beam epitaxy 13,14 , magnetron sputtering 15,16 in combination with thermal annealing 17 , electrodeposition 18 , thermal oxidation 19 , and the floating zone method 20 . However, state-of-the-art quantum optics experiments still focus on natural bulk crystals originating from mines, clearly underlining that significant progress in Cu 2 O growth is required to surpass inherent limitations of natural samples.…”

Rapid Vapor-Phase Deposition of High-Mobility p-Type Buffer Layers on Perovskite Photovoltaics for Efficient Semitransparent Devices

“…Cu2O in direct contact with CH3NH3PbI3 as a hole transport layer beneath the perovskite does not accelerate degradation 35 , unlike MoOx 26,27 . Several options exist to grow thin films of Cu2O, such as spin coating Cu2O nanoparticles, sputtering 36 , thermal evaporation 37 , pulsed laser deposition 34 , atomic layer deposition (ALD) 38 or atmospheric pressure chemical vapour deposition (AP-CVD) [39][40][41] . Due to the limited thermal stability of perovskites the deposition method must be able to achieve the film requirements (high mobility, high transmittance, pinhole-free) at low temperatures 28,42 .…”