3. Conclusion
The potential of SrF2 thin films as a dopant-free and
non-toxic electron-selective contact for silicon solar cells has been
explored. The n-Si/Al interface’s Fermi-level pinning effect was
alleviated by inserting a thermally evaporated SrF2layer. The ultra-low work function quality enhances the electron
selectivity and enables the contact resistivity to achieve 2
mΩ·cm2 for an adjusted 4 nm SrF2. In
comparison to other known electron-selective contact materials, the
SrF2/Al contact exhibits exceptional stability and
thickness tolerance. A dopant-free electron-selective contact consisting
of SrF2/Al layer was implemented in an n-type PERC solar
cell, which achieved a champion efficiency of 21.56%. Our research
findings confirm the outstanding electron-selective contact nature of
SrF2 for fabricating dopant-free silicon solar cells.
Experimental Section
Materials and Contact Characterization .
Commercially available n-type Czochralski (Cz) (resistivity
1~3 Ω∙cm) wafers were used as the substrate for all the
solar cells and testing samples fabrication. SrF2 thin
films were thermally evaporated in the background vacuum under
1×10-3 Pa. The deposition rate is 0.1
Å∙s-1 from 99.9% purity SrF2 powder.
The film thickness is monitored by a crystal oscillator and confirmed by
an ellipsometer. The composition of the SrF2 thin films
was measured by XPS and UPS. The XPS and UPS characterization was
carried out on Thermo Scientific Escalab 250Xi by using the Al Kα x-ray
source (hν = 1486.6 eV) and He I radiation (hν = 21.22
eV). Samples on the polished wafers were stored in the ultra-high vacuum
chamber (2×10-9 mbar) over light before measurement.
Surface-adventurous contamination C 1s = 284.8 eV was used to calibrate
the binding energy for the core level spectra. In order to acquire
accurate secondary electron cutoffs, samples were biased -10V after a
90-second Ar+ ions surface clean. The cross-section
images and elements distribution of the c-Si/SrF2/Al
interface were obtained using an HRTEM (JEOL JEM-2100F) combined with an
EDX line scanning. A Keithley 2400 source meter and the transfer-length
method (TLM) were used to test and extract the contact resistivity
between various thicknesses of SrF2 thin films on n-Si.
The measurements were tested in a dark environment. The
Suns-Voc characterization was tested by Sinton
Instruments. The asymmetric samples with SrF2 thins
films were thermally evaporated on Cz n-Si wafers (3~10
Ω∙cm).