2.2. Contact Resistivity and Stability.
The contact resistivity (ρ c) of the
c-Si(n)/SrF2/Al stack is a key factor in evaluating its
electron selectivity ability. It was evaluated by the transfer length
method (TLM) in Figure 2, with a schematic of the test structure
inset.35 The Schottky barrier height ( >
0.7 eV) is caused by the potent Fermi-level pinning effects when an
aluminum electrode is in straight contact with the lightly doped n-Si
and behaves in a rectifying fashion. By contrast, Figure 2A exhibits
that the interposition of a nanoscale SrF2 thin film
between the c-Si(n) substrate and aluminum electrode considerably
enhances the contact presence and permits an Ohmic contact. The value ofρ c is calculated as 2 mΩ∙cm2from the 4 nm SrF2/Al stack, which is lower than the
great majority of other dopant-free structures in c-Si solar cells.
DF-ECSCs works in kinds of literature report that the minimumρ c can be achieved by EuFx(15.6 mΩ·cm2), YF3 (17.8
mΩ·cm2), MgOx (17.5
mΩ·cm2), CeF3 (10.96
mΩ·cm2), MgAcac (7.6 mΩ·cm2),
MgFx (35 mΩ·cm2),
TiO2(20 mΩ·cm2),
TaNx (42 mΩ·cm2) prepared by
thermal evaporation or by atomic layer deposition
(ALD).11,18-20,22,26,36,37 And the value ofρ c resembles the most typical material
LiFx (1
mΩ·cm2).7 However, lithium fluoride
is toxic and harmful to human health and the environment, and Li
resources and its compounds are in short supply on earth, which is not
conducive to large-scale applications. In contrast, SrF2is commonly used as an additive for toothpaste in humans’ daily life,
which is non-toxic to physical health and the environment. Hence, it has
more prospects for large-scale application. The lowρ c formed by n-Si/SrF2/Al may be
attributed to the lower WF of SrF2. The energy band of
c-Si(n) is forced to bend downwards further as Fermi depinning, which
makes it easier for an electron aggregation area to form, thereby
realizing the selective transmission of
electrons.22,38
It can be seen in Figure 2B that ρc remains in
Ohmic contact (< 42 mΩ·cm2) as the thickness
of SrF2 variates from 1.5, 4, 6, to 9 nm, indicating its
high tolerance for thickness thus providing a wide processing window. By
contrast, the most researched electron-selective structures,
n-Si/LiFx /Al and
n-Si/MgOx /Al have a narrow manufacturing window
(1.5 ± 0.5 nm). The contact resistance stability of these
c-Si/SrF2/Al stack samples under air storage was
investigated. Despite being exposed for 168 hours, theρc is still less than 45 mΩ·cm2for all thicknesses investigated. Moreover, theρc for SrF2 within 9 nm remains
below 140 mΩ·cm2 over one and a half years. The
results indicate that SrF2 has considerable potential as
a material for DF-ECSCs in c-Si solar cells, making it a promising
candidate for large-scale manufacturing.