【Member Papers】High-open-circuit voltage diamond alpha-voltaic battery with interface reconstructed by amorphous gallium oxide

      Nuclear batteries, with their miniature size, ability to withstand extreme environments, long operational lifetime, and excellent environmental adaptability, exhibit broad application prospects in deep space exploration, ocean and polar environments, desert regions, and biomedical fields. Diamond, as an ultra-wide-bandgap semiconductor, possesses outstanding radiation resistance and an exceptionally wide bandgap, making it an ideal material for α-voltaic batteries due to its potential to establish a high built-in potential barrier and to suppress radiation-induced damage. However, addressing interface leakage currents and low open-circuit voltage caused by Shockley–Read–Hall (SRH) recombination at oxygen-terminated diamond Schottky junction interfaces remains a major challenge.

      In this work, we fabricated a nuclear battery in which the diamond Schottky contact interface is reconstructed by amorphous Gallium Oxide. Using a ²⁴¹Am α source with an activity of 8.85 μCi/cm², a high open-circuit voltage of 2.41 V was achieved. In addition, a short-circuit current density of 6.6 nA/cm² and a maximum output power density of 9.72 nW/cm² were obtained, resulting in an overall α-voltaic energy conversion efficiency of 3.7%. The process reproducibility and temperature stability of the nuclear battery were also experimentally verified.

      Further analysis of the interfacial energy band structure between diamond and amorphous Gallium Oxide revealed the relationship between carrier extraction and interface passivation. Based on an investigation of the chemical bonding states at the interface, variations in the concentrations of interfacial ketone and ester bonds were identified, along with their contributions to the uniformity of the electron potential barrier. On this basis, a new mechanism for amorphous Gallium Oxide–induced reconstruction of oxygen-terminated diamond interfaces is proposed. This work provides a new strategy for improving interfacial carrier transport in diamond and enhancing the open-circuit voltage of α-voltaic batteries.

      The related results were published in the journal Carbon, a leading journal in carbon materials science and technology, under the title “High-open-circuit voltage diamond alpha-voltaic battery with interface reconstructed by amorphous gallium oxide.” The first author of the paper is doctoral student Chuanlong Li from the School of Astronautics, Harbin Institute of Technology, and the corresponding author is Associate Professor Benjian Liu from the same institution.

Background

      With the expansion of exploration in extreme environments and the rapid development of micro-electromechanical systems (MEMS), the demand for long-lifetime, high–energy-density power sources is steadily increasing. Over the prolonged evolution of various power technologies—such as chemical batteries, photovoltaic devices, and supercapacitors—researchers have found that isotope batteries based on the radiovoltaic effect uniquely combine long service life, high energy density, miniature size, and excellent stability under extreme conditions. These advantages enable them to overcome many inherent limitations of conventional power sources, including the low energy density and poor low-temperature performance of chemical batteries, the environmental dependence of photovoltaic devices, and the fact that supercapacitors can store but not generate energy.

      As an emerging class of radiovoltaic isotope batteries, however, it must be acknowledged that their power density and energy conversion efficiency have not yet reached their theoretical limits. Addressing this gap involves multidisciplinary challenges, including advances in semiconductor device architecture and materials preparation.

      Diamond, owing to its wide bandgap of 5.45 eV at room temperature, high displacement energy, high critical electric field strength, high electron and hole mobilities, and exceptional thermal conductivity, is an ideal material for high-efficiency isotope batteries, power devices, and ultraviolet detectors. In particular, the combination of high displacement energy and wide bandgap endows diamond with outstanding tolerance to extreme environments such as high/low temperatures and intense radiation, making it one of the most promising materials for isotope battery applications. In principle, once challenges related to diamond doping, epitaxial growth, and precision processing are resolved, its performance could surpass current technologies by a qualitative margin. However, efficient n-type doping of diamond remains unresolved, and the widely adopted Schottky barrier structures still suffer from poor interfacial contact, barrier inhomogeneity, and high carrier recombination rates. These issues severely limit the open-circuit voltage and conversion efficiency of diamond-based Schottky isotope batteries, keeping their performance far below theoretical expectations.

      In this study, we propose a new strategy to enhance the open-circuit voltage of isotope batteries through amorphous Gallium Oxide (AGO)–induced reconstruction of oxygen-terminated diamond interfaces. By employing a staggered band alignment, non-equilibrium carriers at the Schottky contact interface can be selectively extracted and passivated, thereby reducing the recombination probability of non-equilibrium carriers at interfacial defect states. Enhanced carrier collection efficiency consequently leads to improved isotope battery performance.

      Further investigation of interfacial bonding reveals that, due to conjugation effects, carbon–oxygen atoms in ketone bonds cause electron cloud accumulation at the oxygen termination, forming a high potential barrier for electrons near the interface. This barrier hinders the transport of electrons collected near the cathode into the Schottky electrode material, while simultaneously attracting non-equilibrium holes and inducing significant recombination losses. In this work, gallium oxide thin films prepared by radio-frequency magnetron sputtering reconstruct the contact interface via the action of reduced gallium ions. This process reduces the density of ketone groups while introducing a higher proportion of ester groups, thereby mitigating electron cloud accumulation caused by electronegativity differences. As a result, the interfacial electron barrier is lowered, carrier transport across the interface is improved, and an open-circuit voltage exceeding 2.41 V is achieved for a single-junction diamond isotope battery, with the conversion efficiency increased to 3.7%. Reproducibility tests and temperature-dependent measurements further confirm the feasibility of the process and its thermal stability.

      Overall, this work provides a new approach for improving diamond Schottky contact interfaces and enhancing the performance of diamond-based isotope batteries.

Highlights

      1、An ultrathin amorphous Gallium Oxide dielectric layer was fabricated by RF magnetron sputtering, achieving a dark leakage current as low as 0.1 pA. Under ²⁴¹Am irradiation with an activity of 8.85 μCi/cm², the single-junction battery exhibits an open-circuit voltage of 2.41 V, a short-circuit current density of 6.6 nA/cm², a power density of 9.72 nW/cm², and a conversion efficiency of up to 3.7%.

      2、XPS and KPFM measurements were used to determine the band alignment at the interface between oxygen-terminated diamond and amorphous Gallium Oxide films, confirming the roles of electron extraction and hole passivation near the interface and reducing recombination losses of non-equilibrium carriers.

      3、Interfacial bonding analysis reveals the barrier effects of ketone and ester groups at oxygen-terminated diamond interfaces. By reducing electron delocalization, interfacial carrier transport is improved and recombination is suppressed, elucidating the mechanism by which interface states influence isotope battery performance.