FIGURE.5  (A) The EQE, reflectance, and IQE measurements of the solar cells. (B) The stability of the champion PCE device in an ambient atmosphere.
 
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 SrF2 layer. 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 ( = 1486.6 eV) and He I radiation ( = 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).
Cell Fabrication and Characterization.
Based on the n-type silicon substrates, the proof-of-concept 2 × 2 cm2 solar cells were fabricated. A p-type emitter was thermally diffused by a BBr3 source through a furnace after texturing in KOH solution and the Radio Corporation of America (RCA) standard cleaning procedure. After that, Al2O3/SiNx layers for passivation and antireflection were applied to the front side of the emitter deposited by atomic layer deposition (ALD) and plasma-enhanced chemical vapor deposition (PECVD), respectively. The back surface of the solar cell was also passivated by the SiNx layer. An array of 25 μm-diameter contact holes which account for 1% of the backside were opened with a picosecond laser. The front surface Ag metal fingers were formed by the screen-printing paste and annealing process. The solar cells were finished by a thermally evaporated Al (800 nm) or various thicknesses SrF2/Al stack layer on the backside.
The current-voltage (J-V) measurements were carried out on Solar Cell I-V Tester (VS-6821M) under standard test conditions (25℃, AM 1.5G, 1000W/m2). The illumination intensity was checked by the WPVS reference solar cell. A quantum efficiency-reflection (QE-R) spectral testing instrument from Enli-tech corporation was used to characterize the external quantum efficiency (EQE) and reflectivity.
 
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
 
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 61774173), and the Basic and Applied Basic Research Foundation of Guangdong Province (2022A1515011722).
Conflict of Interest
The authors declare no conflict of interest.
 
 
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