Betavoltaic battery based on reduced-Graphene-Oxide/Si heterojunction

Highlights

• We present a new beta voltaic cell based on reduced Graphene Oxide (rGO)/Si heterojunction.

• The cell shows a high conversion efficiency of 3.9% under exposure of beta radioisotope Ni⁶³.

• The open circuit voltage and short circuit current of the cell are 34 mV 0.41 uA/cm² respectively.

• In our beta cell, the generated carriers can be collected in Graphene in addition to Si via ununiformed finger electrodes.

Abstract

This paper presents a new beta converter cell based on reduced graphene oxide (rGO)/Si heterojunction suitable for betavoltaic batteries. The potential barrier created in the rGO/Si interface induces an internal electric field in the Si substrate. This internal electric field can be used for separating the beta-generated electron-hole pairs in Si. Ni^63, with the activity of 5mci is used as the beta radioisotope source. The current-voltage characteristic of the cell is measured with and without radioisotope radiations. The circuit model of the junction is also presented. The cell shows an open circuit voltage of 34 mV, a short circuit current of 0.41 μA/cm^2, and high conversion efficiency of 3.9% under exposure of beta radioisotope Ni⁶³. The presented structure, additionally, benefits from the capability of graphene in the absorption of beta particles and carrier separation via ununiformed finger shape of front electrodes.

Introduction

Betavoltaic batteries are known as long lifetime, reliable, and constant energy sources have been attracted researchers' attention since the early 1950's [1]. Rappaport was the first who reported the energy conversion of a semiconductor-based beta cell [2]. In a Betavoltaic cell, the beta particles are absorbed in a semiconductor material and result in the generation of the electron-hole pairs. These beta-generated pairs are then separated by means of an internal electric field in the depletion region of a semiconductor junction. The separated carriers then flow in the external circuit and produce electricity [3].

To increase the efficiency of the energy conversion in the beta cells, different junction structures, and different materials have been proposed to be used in the betavoltaic batteries. The structure of the semiconductor junction can be either a p-n [[4], [5], [6], [7], [8], [9]], p-i-n [[10], [11], [12], [13]], or Schottky junction [14] that each one has its advantages and challenges. Due to the simplicity of the Schottky structures and the less susceptibility to radiation damage, this kind of junction structure has been attracted more attention by the research groups. It is also more expedient to apply the Schottky barrier in betavoltaic elements due to β-particles small penetration depth in semiconductors [15]. On the other hand, it is investigated that the large bandgap materials are good candidates for being used in radioisotope batteries [16]. The use of large bandgap materials like Diamond [17,18], GaN [[19], [20], [21]], and SiC [[22], [23], [24]] in Schottky junction structures has improved the characteristics of the beta cells. Other optimizing methods, like implementing different electrodes, are also suggested in Refs. [25,26]. It has been shown that the maximum efficiency for the Si-based beta cells is 13.7% in simulation and 1.75% (mostly below 1%) in fabricated samples [27]. These extremes for the fabricated GaN and SiC beta cells are 1.13% and 1.99%, respectively [27].

Recently, the use of nanomaterials and nanostructures in betavoltaic cells has resulted in batteries with better electrical features. The use of carbon nanotubes (CNT) in junction with thin-film Si has shown the betavoltaic characteristics [28]. It is demonstrated that the porous Si p-n diodes can convert the radioisotope energy to electrical energy with an efficiency of 0.22% [29]. The use of nanomaterials in the more complicated structures gives rise to cell characteristics. For example, the structure of the Au/SWCNT/Ti Schottky beta cell shows efficiency of 5.2% [30]. It is also demonstrated that the beta cell, in the form of TiO₂ nanotube arrays, which have been coated with graphene, has an efficiency of 3.9% [31]. Although the suggested devices have their own advantages, they suffer from such limitations as a complication in the fabrication process. In spite of many efforts conducted and different techniques presented in this area, there is still the requirement for an efficient and simple structure device as the betavoltaic cell.

Graphene is a semi-metal nanomaterial with light-like energy dispersion relation. The radiations (light or particles) can generate the electron-hole pairs in graphene. If these photo or beta generated electron-hole pairs are trapped by an internal electric field, they can contribute to the short circuit current of the device and increase the efficiency of the cell. Graphene, regardless of the production method, has good Schottky contact with Si [[32], [33], [34]]. Due to the energy band bending of Si in Schottky junction with graphene, an internal electric field is usually induced in Si substrate at the interface with graphene. But there is no internal field in graphene in regular Schottky junction with Si. In these structures, graphene acts only as of the carrier collector and, specifically, the transparent electrode in photodetectors and solar cells [[32], [33], [34]]. In this work, we demonstrate a beta cell consists of reduced-Graphene-Oxide (rGO)/Silicon Schottky junction, which benefits from its simple structure and low-cost manufacturing technology and acceptable efficiency in comparison with other works. Using silicon, which is a versatile material in the betavoltaic battery due to the mature processing technology [[35], [36], [37]] in combination with graphene as a novel nanomaterial with extraordinary characteristics [[38], [39], [40], [41]] would be an acceptable structure of future betavoltaic microbatteries.

In our presented rGO/Si heterojunction, we have used an asymmetric finger-shape front contact, which induces an internal electric field in the rGO layer. By means of the two different metals with different work functions as the front electrode, an asymmetric electric field profile is caused on the rGO layer, which traps the beta-generated carriers in the rGO layer. It should be noted that the idea of using two unbalanced electrodes for inducing an internal electric field in the rGO sheet was previously presented in photodetectors and solar cells [42,43]. Here, we have applied this idea along with the Schottky junction between rGO and Si to trap both beta-generated carriers in Si and rGO layer. Additionally, it is worth noting that our presented rGO/Si heterojunction also benefits from the overall advantages of graphene/Si Schottky junctions like large effective detection area and fast collecting behavior [32]. These factors can increase the efficiency of the photodetectors [32], solar cells [32,34], and the presented betavoltaic cell.

Our structure is based on the reduced graphene oxide synthesized by modified Hummer's method and characterized by Transmission Electron Microscopy (TEM) images and Raman spectroscopy before and after the reduction process. Ni⁶³ is used as the beta-emitting radioisotope due to its higher half-life. The beta particles or electrons, emitted from Ni⁶³, have the average energy of 16.7 keV and highest energy of 67 keV, which is less than 200–250 keV, the threshold energy to cause substantial permanent damage to the silicon crystalline matrix. The current-voltage characteristic curve of the fabricated device is measured and investigated in detail. The fabricated beta cell has a short circuit current of 0.41 μA/cm², the open circuit voltage of 34 mV, FF of 28%, and an efficiency of 3.9%.