How can I determine the alloy in a Peltier module?
You asked a similar question yesterday about finding the Seebeck coefficient. In that question you gave the following equation:

$$V = aT + RI$$

Now look at this graph you just provided:



If you can make one assumption about the device, namely that its resistance doesn't change significantly as the temperature delta increases then you can relatively easily figure out the Seebeck coefficient.

Assuming the resistance doesn't change and the current is held constant the change in voltage is entirely due to the change in temperature, which you know from the graph, and the Seebeck coefficient. 

For instance, at zero degrees temperature delta the voltage total is about 2.0V. Because the Seebeck term is cancelled, due to the temperature delta being zero, RI is about 2.0V (given a current around 0.45 amps). If you draw a straight line up from that, meaning RI does not change, with a temperature delta of 40°C (313.15 Kelvin) the voltage total is right about 3.9V. 

With a little math we can solve the rest:

$$3.9[V] = a[V/K]*313.15[K] + 2.0[V]$$

What does a, the Seebeck coefficient, equal? 6067!? Seems high right? Look in the upper corner of the graph, this device has 127 couples stacked on top of eachother. So divide the result to get the coefficient per couple.

$$6067\div127 \approx 47$$

Which material has the Seebeck coefficient of around 47? Antimony. Though it's not likely it's made of pure antimony. But you were just looking for the Seebeck coefficient anyway, so that xy problem is solved.