Back contact buffer layer of WO₃ nanosheets in thin-film CdTe solar cell

Benefiting from the controlled hydrothermal synthesis of tungsten trioxide (WO₃) nanosheets (NSs), a unique thin-film back contact was designed for the CdTe solar cells. The hydrothermally prepared WO₃ NSs was spin-coated as the thin-film buffer layer on the back contact of CdTe thin-film solar cells to replace traditional Cu contact. The morphology, phase crystallization, chemical compositions, and the intimate junction contact were confirmed by SEM, TEM, XRD, Raman, and XPS characterizations. The interpretation of the quantitative band alignment carried out on an X-ray photoelectron spectroscopy (XPS) showed that the WO₃/CdTe energy barrier at the interface was substantially lower. The offset of the valence band (∆E_VBO) and conduction band (∆E_CBO) at WO₃/CdTe heterojunction were found 0.69 and 1.86 eV respectively. Moreover, by combining WO₃ as a high work-function buffer layer with various metals contacts such as Au, WO₃/Au, Cu/WO₃/Au, and the conventional Cu/Au metal layer, low resistive ohmic back contacts for CdTe thin-film solar cells were developed and analyzed. Interestingly, the CdTe thin-film solar cells fabricated with WO₃ NSs as a buffer layer between CdTe/Cu (3 nm) and metallic Au electrodes demonstrated the best performance with the conversion efficiency of 13.0%. To the best of our knowledge, so far this is the first report on WO₃ NSs that opened a new paradigm to use WO₃ nanostructures as a buffer layer in the thin-film CdTe solar cell to get improved thermal stability and device efficiency in the harsh environment.