Figure
1. (a) Schematic diagram of the TLM test structure; (b) Dependence of
the ρ c on ZnTiO3 thickness and
annealing temperature after metallization; (c) ρ cof ZnTiO3-based contacts and other well-known Ohmic
contacts as a function of annealing temperature; (d) Schematic of the
c-Si solar cell. SiO2/ZnTiO3/Al is
deposited as the electron-selective contacts; (e-h)V OC, J SC, FF and
PCE of the c-Si solar cells with different thickness of
ZnTiO3 under different annealing temperatures in air
ambient for 30 min; (i) The normalized PCE of different dopant-free
electron-selective contacts as a function of annealing temperature.
Figure 1(i) shows the thermal stability of c-Si solar cells with various
dopant-free electron-selective contacts, including the
ZnTiO3-based contacts in this work. The PCE at each
temperature step are normalized by their initial value (before
annealing). The solar cells with
TiO2/Al,a-Si/LiFx/Al or
a-Si/LiFx/Ti/Al as electron-selective contacts suffers
relatively severe degradation.30 The cells with
Al-doped
TiO2(ATO)/LiFx/Al,17a-Si/CaAcac/Al,28 TiN/Al23 or
ZnSe41 contacts present greatly improved stability.
Uniquely, the solar cells with ZnTiO3-based contacts
show increased PCE with increasing post-metallization annealing
temperature up to 300 °C. Hence unlike other dopant-free contacts, the
ZnTiO3-based contacts embrace thermal treatment, which
is usually unavoidable in device fabrication. Moreover, it is worth
pointing out that the annealing time at each temperature is generally
within 5 minutes for those works in literature, but the annealing time
is 30 minutes in this work. This means that the
ZnTiO3-based electron-selective contacts is compatible
with the silicon heterojunction (SHJ) production lines (the curing
temperature after screen-printing electrodes is between 200-300 °C and
the time is 20-30 minutes).
Besides J -V characteristics, the EQE spectra were also
examined
at
each annealing step, as shown in Figure 2a. The results show that the
EQE gradually increases at long wavelengths (700-1100 nm)
from
25-300
°C,
which soundly explains the improvement of J SC. In
addition, considering that the improvement of EQE only happens in long
wavelength and the fact that the optical reflectance is unchanged (see
Figure S1), we can conclude that the electron selectivity of
ZnTiO3-based contacts is enhanced with increasing
post-metallization annealing temperature (within 300 °C).
𝜌c and interface passivation are the two factors
affecting carrier selectivity. Because 𝜌c is nearly
unchanged, at least which is true for the ZnTiO3thickness not greater than 3nm, thus the enhanced performance
(V OC, J SC and PCE) of the
solar cells mainly originates from the improved interface passivation at
the back side. This conclusion is further examined by PL mapping andτ eff characterization. Figure 2b compares the PL
mappings of an as-fabricated solar cell and a 300 °C post-annealed solar
cell. The PL intensity of the annealed solar cell is much stronger than
that of the as-fabricated one, indicating the greatly reduced
nonradiative carrier recombination. Surface passivation can also be
evaluated by τ eff, which is often obtained by
utilizing photoconductance decay (PCD) method. However, this method is
unsuitable to characterize our samples because Al capping layer plays an
important role in the ZnTiO3-based electron-selective
contacts. Recently, Liang also found that fully metalized samples are
not allowed the minority carrier lifetime to be measured with the PCD
technique.43 Therefore, here we characterizeτ eff in final device level by Suns-V oc
measurement mode.44 Figure 2c shows that theτ eff increases with post-annealing temperature
(within 300 °C), again demonstrating that the passivation effect is
improved by post-annealing treatment. Benefiting from the enhanced
passivation effect in rear side, the optimal ZnTiO3/c-Si
heterojunction solar cell, which is annealed at 300 °C, possesses a PCE
of 22.0% with a V OC of 662 mV,J SC of 40.1 mA cm−2 andFF of 82.7%, as shown in Figure 2d.
The
PCE is impressive and is comparable with that of conventional c-Si solar
cells in current PV industry. Moreover, it is worth mentioning that
capital intensive equipment such as PECVD that used for growing a-Si:H
films can be avoided.