1 Introduction
In recent decades, silicon heterojunction (SHJ) solar cells based on intrinsic amorphous silicon passivation layers and doped amorphous or microcrystalline silicon carrier selective layers have attracted more and more attention due to their advantages of high efficiency, weak light attenuation and low temperature coefficient.1-2The SHJ technology has succeeded in bringing the conversion efficiency of monocrystalline silicon solar cells to over 26 %.3Due to the relatively low carrier diffusion length of doped amorphous or microcrystalline silicon films, transparent conductive oxide (TCO) electrodes are essential for achieving efficient lateral carrier collection in the SHJ solar cells. High quality TCO layers with good optical and electrical properties are crucial for SHJ solar cells to realize high fill factor (FF) and high short-circuit current density (Jsc). Indium tin oxide (ITO) thin film is widely used in the industrial production of SHJ solar cells because of its high electrical conductivity. So far, in many reported high-efficiency SHJ solar cells, the ITO films were utilized as the front or bifacial transparent conductive electrodes.4-5 Different deposition techniques, including direct current (DC) or radio frequency (RF) sputtering,6 thermal7 or electron beam evaporation,8 plasma-enhanced reactive thermal evaporation,9 and pulsed laser deposition,10 etc., have been successfully employed to deposit ITO films with high conductivity. However, the Hall mobility of the ITO films prepared by these methods mentioned above is typically below 50 cm2V-1s-1. The high conductivity characteristic of ITO mainly benefits from the high carrier concentration (usually in the order of 1021cm-3) which causes serious parasitic absorption in the near-infrared (NIR) region and limits the short-circuit current density of the SHJ solar cells.11 To explore TCO films with both high transmittance and high mobility, so as to improve the Jsc to catch up with the Tunnel Oxide Passivated Contact (TOPCon) solar cells, is one of the main research directions of the high efficiency SHJ solar cells.
According to the literature, the relatively low mobility of ITO is mainly related to the low doping efficiency of tin.12Substitutional doping of Sn4+ with smaller ionic radius may cause lattice distortion in ITO and, as a less efficient dopant,13 relatively high doping concentration of tin impurities is required to achieve low resistivity.14To obtain TCO films with high mobility and low carrier concentration, various metal elements with higher ionic valence or more matched ionic radius with In3+, such as W,15Mo,16 Ce17 and Hf,18 etc., were proposed to replace Sn as the dopant elements of indium oxide films to reduce the impurity concentration without compromising film conductivity. Among them, indium oxide doped with Hf (IHfO) presents excellent opto-electronic properties and great application potential. A Hall mobility of 79.6 cm2V-1s-1, a carrier concentration of 5.04×1020cm-3 and a resistivity of 3.76×10-4Ω·cm were achieved on IHfO thin film grown by magnetron sputtering technique.19 The IHfO films have been successfully applied on amorphous silicon germanium thin film solar cells and SHJ solar cells and obtained conversion efficiencies of 8.3% and 19.08%, respectively.18-19
In recent years, reactive plasma deposition (RPD) technology, also named as hollow cathode discharge deposition, has been used to prepare TCO films and has received wide attention of the researchers in the field of heterojunction solar cells.20 During the RPD process, the substrate is not directly exposed to the plasma and the damage related to the ion bombardment is greatly reduced.20Thereby, RPD is considered to be a suitable technique for the devices such as heterojunction solar cells with extremely high-quality interface requirements. Furthermore, RPD is actually an ion-plating method working in the arc discharge plasma regime with low-voltage and high-current. The high ionization yield is conducive to promote the gas phase reaction and improve the opto-electronic properties of the grown TCO films. Previously, some TCO films including ZnO,21 Ce-doped In2O3 (ICO),22 W-doped In2O3 (IWO)23 and W and Ti co-doped In2O3(IWTO)24 have been successfully grown by RPD and have exhibited excellent opto-electronic properties. However, more research work is still needed in IHfO films prepared using RPD technique and application of IHfO films in high efficiency SHJ solar cells is also expected.
In this work, hydrogen doped IHfO thin films (IHfO:H) were prepared by RPD technique. The effects of hydrogen concentration and post-annealing temperature on the crystallinity, opto-electronic properties, and valence states of the IHfO:H thin films were investigated. Under the conditions of H2 concentration of 0.8 % and post-annealing temperature of 200 °C, the IHfO:H films with the best comprehensive properties were obtained. The IHfO:H thin film prepared under the optimized condition was applied to the bifacial SHJ solar cell as the front TCO layer and a conversion efficiency of more than 25 % has been achieved.