FIGURE 4 Device performance with partial rear SrF2 films. (A) A sketch of the solar cells applying SrF2 electron-selective contacts. (B-E) Voc, FF, Jsc, and PCE of the device with various SrF2 thicknesses (nm). (F) Light J–V characteristics of the device under AM1.5 conditions.
To evaluate the optical properties of SrF2 electron selective contacts when applied on PERC structure solar cells. Figure 5A depicts the characterized EQE, reflectance of the highest efficiency device, and its relevance, with the measurements were taken between the finger electrode.
Obviously, a considerably high integration Jsc of the EQE of 42.30 mA∙cm-2 was achieved, surpassing the Jsc value of the counterpart solar cell which was 41.32 mA∙cm-2. These findings are consistent with the Jsc values obtained from the J-V characterization discussed earlier. The optical quality of the back side can be indicated by the internal quantum efficiency (IQE) for longer wavelengths (900 ~ 1200 nm). A higher IQE is observed at wavelengths between 500 nm and 1200 nm for cells with SrF2 interlayer than in the case of the cells only metalized by Al backside. This maybe indicates that SrF2 film increases the spectral response and light reflection at its interfaces to the Si bulk, demonstrating the high electron selectivity characteristics offered by the low WF SrF2/Al contacts.
The stability of PCE is an essential factor to consider when evaluating the practical application of a solar cell. The champion samples were surveyed and stored in an ambient atmosphere as shown in Figure 5B. The PCE can remain constant for the first 168 hours at 21.5%. Then, a tiny decline can be seen, but it is still close to 21% after 1000 hours. The PCE was maintained at 17% to 17.5% even after samples were exposed to the air for 5000 hours and 15000 hours. We identify several potential causes for this phenomenon: (i) the increasing contact resistivity in Figure 2b. and (ii) the advent of Al diffusion into the silicon substrate of the electron-selective contact layer can lead to a rectifying contact, as has been widely reported in the literature.36,42 The stability of this structure could be further enhanced after module encapsulation. The use of SrF2 as DF-ECSCs presents significant potential for mass manufacture due to its excellent PCE stability and high tolerance to variations in thickness during fabrication.