Transition Metal Oxides as Selective Carrier Transport Layers in Silicon Heterojunction Solar Cells

Silicon heterojunction (SHJ) technology is gaining increasing attention due to its lowtemperature and simple fabrication process. Intrinsic hydrogenated amorphous silicon
((i)a-Si:H) serves as a passivation layer, providing excellent chemical passivation, while
doped a-Si:H offers good field passivation and selective contact. Thanks to these features,
SHJ has achieved a high conversion efficiency of 27.09%, approaching the theoretical
limit for silicon-based solar cells. However, achieving excellent passivation performance
results in optical and electrical losses, as the band gap of doped a-Si:H causes parasitic
absorption. One strategy to address this issue is to replace the doped layer with transition
metal oxides (TMOs), which can enable selective contact due to their high or low work
functions. Few researchers have integrated TMOs in silicon-based solar cells to replace
silicon-based doped thin films as selective contact layers. Wide-bandgap TMOs improve
the optical performance of the cells. However, interface reaction between TMOs and the
silicon substrate becomes an issue limiting the electronic properties of the device.

In this research, we first present three different interface engineering methods: no
plasma treatment (noPT), plasma treatment (PT), and plasma treatment with boron
(PTB). We applied these methods to SHJ solar cells with MoOx (2.9 < x < 3) as the hole
transport layer (HTL).MoOx thin-film is deposited by thermal evaporation. The methods
were implemented at theMoOx/(i)a-Si:H interface. Additionally, to address sustainability
concerns related to indium consumption and fully exploit the optical advantages of
MoOx, we propose bifacial SHJ solar cells withMoOx as the HTL to reduce the thickness
indium doped tin oxide (ITO) films. Furthermore, to test the capability of these interface
engineering methods with other TMO materials, we usedWOx (2.9 < x < 3) and V2Ox (2.9
< x < 3) as the hole transport films in SHJ solar cells. The TMO thin-films are deposited by
thermal evaporation. The specific results are summarized as follows.

Chapter 3 explores using MoOx as a HTL to address these issues. By tailoring the
interface betweenMoOx and (i)a-Si:H using interface engineeringmethods, the oxygen
content in MoOx layers has been successfully optimized. The PTB method reduces
the formation of SiOx layer resulting in improved electronic properties and low contact
resistivity. PTB treated samples showed the best performance, with high open-circuit
voltage (VOC) and fill factor (FF). This approach achieved a certified conversion efficiency
of 23.83% with an ultra-thinMoOx layer of 1.7 nm. Notably, a short-circuit current density
(JSC) above 40 mA/cm² has been achieved.

Chapter 4 explores reducing indium consumption by optimizing n-contacts as electron
collector layer andMoOx as a hole collector layer. Bifacial SHJ solar cells withMoOx
as the HTL and various electron transport layers (ETL) were fabricated and optimized
using optical simulations. The results showed that bilayer ((n)nc-Si:H/a-Si:H) and trilayer
((n)nc-SiOx:H/nc-Si:H/a-Si:H) better than monolayer (a-Si:H) in electronic and optical
performance. The use of ultra-thin transparent conductive oxide (TCO) layers combined
withMgF2 as double layer antireflection coatings (DLARC) significantly reduced TCO consumption whilemaintaining high performance. Devices exhibiting 10-nm thick indium
tungsten oxide (IWO) on both side and bilayer n-contact achieved certified efficiencies
of 21.66% and 20.66% when measured from theMoOx and n-contact side, respectively.
This device realizes 90% TCO-reduction. The best-performing bifacial cells exhibited
conversion efficiencies of 23.25% and 22.75% measuring from front and back side, respectively.
The bifaciality factor of the champion device is 0.96. It demonstrates over 67%
reduction in TCO usage compared to traditional SHJ solar cells. This study successfully
shows that reducing TCO thickness and optimizing the interface withMoOx can lead to
high-efficiency bifacial SHJ solar cells, contributing to sustainability challenges related to
indium consumption.

Chapter 5 investigates the application of TMOs likeWOx, and V2Ox as HTLs combined
with interface engineering methods in SHJ solar cells. The study aims to investigate the
applicability of interface engineering methods to other TMOs. X-ray photoelectron
spectroscopy (XPS) is employed to measure oxygen content and defects within TMO films.
The XPS results demonstrate that PTB resulted in higher oxygen content and fewer oxygen
vacancies. The optimal WOx thickness was found to be 2 nm, achieving a champion
cell with 23.30% efficiency with PTB and improved FF of over 80%. Similarly, with PTB
method, the highest efficiency of 22.04% has been realized with 3-nmthick V2Ox layer.
The PTB method effectively controlled interface reactions, leading to better electronic
properties of TMOs. The study concludes that the PTB method offers suitable interface
conditions for depositing TMOs, enhancing SHJ solar cell performance.

Our findings in this study may provide valuable insights into applying TMOs in SHJ
solar cells to achieve high performance. With optimized interface engineeringmethods,
we can significantly reduce the optimal thickness of TMOs and achieve world-record
efficiency. Furthermore, by applying TMOs in bifacial SHJ solar cells, we can decrease the
thickness of indium-based TCOs, realizing both high efficiency and high bifaciality factor.
This approach makes it possible to develop sustainable and efficient solar cells.