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.