Mo Back Contact for Flexible Polyimide Substrate Cu(In, Ga)Se₂ Thin-Film Solar Cells

A dc magnetic sputtering process is applied to growth of a Mo back contact layer onto the flexible polyimide (PI) and rigid soda-lime glass (SLG) substrates. The structural and electrical properties of the Mo layer coated on the two kinds of substrates are investigated by x-ray diffraction (XRD) and Hall effect measurements. The results show that the Mo layer on SLG indicate more better crystal quality and lower resistivity than that on the PI sheets. In contrast to the SLG substrate, the resistivity of the Mo layer on PI is increased by the vacuum annealing process at the substrate temperature of 450°C under Se atmosphere, which is attributed to the cracked Mo layer induced by the mismatch of the coefficient of thermal expansion between PI and Mo material. The Cu(In,Ga)Se₂ (CIGS) solar cells based on the PI and SLG substrates show the best conversion efficiencies of 8.16% and 10.98% (active area, 0.2cm²), respectively. The cell efficiency of flexible CIGS solar cells on PI is limited by its relatively lower fill factor caused by the Mo back contact.