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The thermal stress and mechanical induced reliability issues of the Mo layer deposited on polyimide (PI) flexible substrate are studied. We found that the CIGS film cracks after one-hour 400°C selenization process. And the CIGS crack was initiated from the cracking of the Mo film and it can be improved by a Mo annealing process before the precursor deposition. Lower the ramping rate of the selenization temperature and deposit an additional Mo film on the backside of PI substrate can help to improve substrate curving. However, without the Mo annealing, it provides very little improvement on Mo cracking. The annealing improves the Mo cracking resistance. Adding Ag or Al into Mo film reduces film's resistance and improves “failure diameter” down to curvature of 10 mm in tensile stressing tests.