Fabrication of p-n heterostructure ZnO/Si moth-eye structures: Antireflection, enhanced charge separation and photocatalytic properties

Highlights

• Preparation of p-n heterostructure ZnO/Si moth-eye structures by multistep method.

• Determined the optimum growth process to form ZnO/Si heterojunction.

• The composite structures have antireflection performance and the super-hydrophilicity.

• The composite structures have high efficiency photocatalytic properties.

Abstract

The pyramidal silicon substrate is formed by wet etching, then ZnO nanorods are grown on the surface of the pyramidal microstructure by a hydrothermal method to form a moth-eye composite heterostructure. The composite heterostructure of this material determines its excellent anti-reflection properties and ability to absorb light from all angles. In addition, due to the effective heterojunction binding area, the composite micro/nano structure has excellent photoelectric conversion performance. Its surface structure and the large specific surface area gives the material super hydrophilicity, excellent gas sensing characteristic, and photocatalytic properties. Based on the above characteristics, the micro/nano heterostructure can be used in solar cells, sensors, light-emitting devices, and photocatalytic fields.

Introduction

ZnO is a type of standard direct gap semiconductor with wide bandgap (Eg = 3. 37 eV) at room temperature, and it has a high exciton binding energy of 60 meV [1]. Due various advantages, such as a proper band gap, high chemical stability and thermal stability, high photosensitivity, high photocatalytic activity, rich and controllable morphology, non-toxicity, pollution-free and low cost, etc., ZnO has been widely used since the 1990s [2]. For example, ZnO has a high transmissivity in the visible light range (400–800 nm) due to the band-gap of 3. 37 eV at room temperature, which is larger than the photon energy of visible light of 3. 10 eV. At room temperature, stimulated radiation of ZnO can occur at a lower threshold due to its high exciton binding energy, therefore ZnO is an ideal ultraviolet light emitting material. In recent years, because of its unique physical properties and photoelectric properties, the one-dimensional structure of ZnO nanomaterial has received widespread attention and has made great progress; for example, ZnO one-dimensional nanorods array has better optical properties and electronic transport performance than ZnO thin film [3], [4], [5]. When N-type ZnO is combined with P-type semiconductor, ZnO serves not only as an anti-reflection photonic window but also as an N-type semiconductor for generating carriers and providing depletion layer and built-in field. During this time, heterojunction will effectively improve the separation efficiency of holes and electrons, and broaden the absorption range of visible light, which is of great significance for the design and preparation of optoelectronic devices [6]. Due the large specific surface area of ZnO one-dimensional nanomaterial, the adsorption of gas by ZnO leads to a change of conductivity and shows a high sensitivity to gas, which can be used as a quality gas sensor; the increase in specific surface area helps to improve the photocatalytic degradation rate of photocatalytic reaction of organic compounds, and other reports have shown that ZnO shows superior photocatalytic activity and quantum yield than Titanium Dioxide when degrading toxic organic pollutants [7]. There are several technologies to prepare ZnO nanomaterial, such as magnetron sputtering [8], chemical vapor deposition (CVD) [9], hydrothermal synthesis [10], sol-gel method [11], pyrolysis process [12] and electrochemical deposition [13]. Hydrothermal method is an effective method for preparing large-scale ZnO nanorods array; it operates under mild reaction conditions without any contamination [14], [15].

As a type of narrow bandgap semiconductor material, silicon (Eg = 1. 12 eV) is the most widely used semiconductor material in the field of photoelectric conversion. Nevertheless, the high refractive index of silicon determines that more than 30% of incident light will be reflected [16]. Many previous studies showed that, in order to reduce the reflection on the silicon surface, to improve the absorption and utilization of light (i.e. increase the photoelectric conversion efficiency), and to increase the absorption of photons, the silicon surface can be processed in many ways to form an effective light-trapping structure to reduce the reflection of incident light. For example, maskless reactive ion etching (RIE) [17], mask-based reactive ion etching using self-organization techniques [18], Ag-assisted chemical corrosion [19], [20], laser etching [21], laser interference lithography [22], wet etching [23], [24], [25]. In this experiment, the monocrystalline silicon is wet-etched, and a pyramid structure with a relatively uniform topography size can be formed from the anisotropic etching of different crystal face, which can reflect the light many times to form an effective light-trapping structure. However, the above method only improves the absorptivity of incident light but fails to improve the utilization ratio of incident light. The photoelectric conversion performance of the composite structure prepared in this experiment for the moth-eye material is far superior to that of the single-crystal silicon material.

Here, we combined wet etching (NaOH) and then deposited the seed layer by magnetron sputtering. On the basis of the seed layer, the ZnO nanorods are hydrothermally grown uniformly on the pyramidal silicon substrate. Based on their excellent properties, a bionic coating with artificial moth-eye composite micro/nano structure is prepared. The material structure results in an excellent graded index of refraction and the ability to absorb the light incident from all angles, enhancing the antireflection performance of the material and broadening the absorption range of the wavelength of the incident light. Compared with two-dimensional planar silicon-based growth nanorods array, ZnO/Si heterojunction is formed and the effective bonding area of heterojunction is increased, the effective area of the built-in electric field is increased, and the photocarriers separation efficiency is improved and photocarriers recombination is inhibited, contributing to the excellent photoelectric conversion properties of this composite micro/nano structure. At the same time, the surface of the composite micro/nano structure and its huge specific surface area determines the super-hydrophilicity, fantastic gas sensitivity and photocatalytic properties of the material. Based on the above characteristics, this structure can be used in solar cells, sensors, photocatalytic materials, photoluminescence and electroluminescence and other fields.