Experimental methods
Films Deposition and Characterization: n-type c-Si wafers (100) with a
thickness of 170 µm and resistivity of 1–3 Ω cm were cleaned by the
Radio Corporation of America (RCA) procedure and then dipped in HF
(1-2%) solution. Prior to deposition of ZnTiO3, thin
SiO2 passivation layers were grown by thermal treatment
under 300 °C for 30 min in air atmosphere. Following this,
ZnTiO3 films were thermally evaporated at the rate of
0.1 Å s-1 from high-purity (99.9%)
ZnTiO3 powder, the thickness of ZnTiO3were monitored using a crystal oscillator, and then metallized with a
thermally evaporated Al electrode. Contact resistivity
(ρ c) of
c-Si/SiO2/ZnTiO3/Al annealed at
different temperatures was measured using the transfer length method
(TLM), and the TLM pattern has the length of 20 mm and the pad spacing
of 0.5, 1, 1.5, 2, 3, and 4 mm, respectively. The thickness of
ZnTiO3 was varied and the thickness of Al was 150 nm.
The ρ c was extracted from current-voltage
(I –V ) curves obtained from a Keithley 2400 source meter.
To examine the thermal stability of the contacts, the samples were
placed on a hot plate at different temperatures for 30 minutes in air
ambient. As a comparison, a TiO2 film with a thickness
of 4 nm was also deposited as an electron-selective layer by atomic
layer deposition (ALD), using titanium tetrakis (dimethylamide), and
H2O as precursors at 150 °C, and the sample was also
annealed under the same condition. Two ZnTiO3 samples
were fabricated for the chemical composition and work function
(WF ) measurement by X-ray photoelectron spectroscopy (XPS)
(ThermoFischer, ESCALAB Xi+) and ultraviolet photoelectron spectroscopy
(UPS) (Kratos AXIS Ultra DLD). They were prepared as following: 50 nm
thick ZnTiO3 layer and 100 nm thick Al were deposited on
n-type polished c-Si wafers sequentially. Only one sample was annealed
on a hot plate at 300 °C and the other one did not receive annealing.
Then the Al layers of the two samples were removed using a dilute
hydrochloric acid solution at room temperature, and then were rinsed in
deionized water. Prior to XPS and UPS measurements, the samples were
further etched by Ar ions with the etching depth of 5 nm.
Solar Cells Fabrication and Characterization: The c-Si wafers were
n-type with a thickness of 170 µm and resistivity of 1–3 Ω cm. After
texturing to form an array of random pyramids in KOH solution, standard
RCA cleaning and dilute HF dip were performed. Subsequently,
p+ emitter was formed by boron diffusion. Then the
front surfaces were passivated with
Al2O3/SiNx stack layers.
The front Ag grids were fabricated by screen-printing paste and firing.
For the electron-selective contacts, the rear surfaces of the wafers
were cleaned by diluted HF (1-2%) solution and rinsed with deionized
water. Then a SiO2 layer was thermally grown on the rear
surface at 300 °C for 30 min in air atmosphere, following with thermal
evaporation of ZnTiO3/Al. To examine the thermal
stability of the devices, the post-annealing treatment was performed on
a hot plate under different temperatures for 30 minutes in air ambient.
The current density-voltage (J -V ) characteristics of the
solar cells were investigated under the illumination of AM1.5G (100 mW
cm-2, 25 ℃) using Newport 92250 A-1000 solar cellI -V tester and Keithley 2400 source meter.
The cross-sectional analysis of the electron-selective contacts was
prepared using the focused ion beam (FIB) lift-out technique by
Thermalfisher scois 2 FIB system. Then high-angle-annular dark-field
scanning transmission electron microscopy (HAADF-STEM) images,
energy-dispersive X-Ray spectroscopy (EDX) mapping and line scan were
used to characterize the
c-Si/SiO2/ZnTiO3/Al contacts before and
after annealing using Thermalfisher Titan Themis Z system. For the
passivation effect measurements, external quantum efficiency (EQE)
spectra, photoluminescence (PL) mapping and effective minority carrier
lifetimes
(τ eff)
were examined. The EQE of the solar cells with different post-annealing
temperatures were tested via Crowntech QTEST HIFINITY 5. PL images of
both an as-fabricated solar cell and a 300 °C post-annealed solar cell
were obtained from a FL-B01 system with an 808 nm Laser injection.
Effective carrier lifetime of solar cells with different post-annealing
temperatures was extracted by Suns-V OCmeasurements using Sinton Instruments WCT-120.
Results and Discussion
The contact property of ZnTiO3 on n-type c-Si was
investigated by measuring the contact resistivityρ c of
c-Si/SiO2/ZnTiO3/Al stacked layers,
using the TLM model, as shown in
Figure
1a.
Note
that the contact resistivity is one of the most important parameters to
evaluate the carrier selectivity. Figure 1b shows the dependence ofρ con
annealing
temperature after metallization. The thicknesses of
ZnTiO3 are 1, 3 and 5 nm, respectively. It can be seen
that all of the c-Si/SiO2/ZnTiO3/Al
contacts possess ultralow 𝜌c (less than 5 mΩ
cm2) before annealing. The 𝜌c of the
c-Si/SiO2/ZnTiO3/Al contacts with
ZnTiO3 thickness of 1 nm is unchanged when the
post-annealing temperature is ≤ 250 °C, and then it starts to increase
slightly, and further increases sharply when the annealing temperature
is > 350 °C. For the electron-selective contact with
ZnTiO3 thickness of 3 nm, its 𝜌c shows a
slight increase until the annealing temperature of 350°C and increases
drastically when the annealing temperature is > 400 °C. For
the electron-selective contact with ZnTiO3 thickness of
5 nm, the 𝜌c decreases with annealing temperature (≤ 350
°C) and starts to increase sharply at the temperature of 400 °C.
Obviously, the SiO2/ZnTiO3/Al stacks can
form highly thermal stable ohmic contacts with n-type c-Si when the
temperature is ≤ 350 °C. Moreover, the thicker ZnTiO3,
the better heat resistance. Figure 1c
compares
the thermal stability of Ohmic contacts between our
ZnTiO3-based stacks and other well-known Ohmic
contacts.18, 30, 39 LiF- and
SiO2-based contacts are stable within a relatively small
annealing temperature range (<200 °C).
TiO2-based contacts have improved thermal stability. In
contrast, ZnTiO3-based contacts have the best thermal
stability and possess the lowest 𝜌c over the largest
annealing temperature range.
The influence of post-metallization annealing temperature on the
ZnTiO3-based layers was also investigated in device
level. The solar cell structure is depicted in Figure 1d.
SiO2/ZnTiO3 stacks are utilized as
passivating, electron-selective contacts. The thickness of
ZnTiO3 is between 1 and 5 nm. The solar cells were
annealed on a hotplate in air ambient for 20 min with temperature range
from 25-400
°C.
The corresponding cell parameters (V OC,J SC, FF and PCE) at each annealing
temperature step are presented in Figure 1e-h. As can be seen, theV OCimproves from 624mV to 640 mV as the annealing temperature increases
from 25 °C to 250 °C for the ZnTiO3 thickness of 1 nm.
When the annealing temperature is further increased, theV OC drops obviously. For the solar cells with
thicker ZnTiO3 (3
nm, 5 nm), although their initial V OC (about 600
mV) are lower than that of the cells with 1 nm ZnTiO3,
they are improved more drastically, and reach around 650 mV. Moreover,
it is obvious that the thicker the ZnTiO3 film, the
higher the optimal annealing temperature. Similar behaviors can also be
observed from J SC. The FF of cells with
the ZnTiO3 thickness of 1 nm and 3 nm have negligible
change within the annealing temperature of 350 °C, while the FFof the cells with 5-nm-thick ZnTiO3 is increased by 2%
in absolute value when the annealing temperature increases from 25 °C to
300 °C. Therefore, the FF results are consistent with the contact
resistivity results. As the comprehensive result ofV OC, J SC and FF ,
the PCE of the ZnTiO3/c-Si heterojunction solar cells
increase remarkably with post-annealing temperature. The optimal
temperature is either 250 or 300 °C depending on the thickness of
ZnTiO3.