Question: I have thermoelectric modules and set it in series, then I connected to a DC to DC step up (CN6009) to enhance the voltage. Then I connected to my phone with usb charger to try charge with it, but the voltage suddenly drops. Should I change the step up or increase the amount of the module?

Answer: 
You've got a couple of things going on that are causing your problems:

1. The output resistance of [TEG modules](https://en.wikipedia.org/wiki/Thermoelectric_generator#Practical_limitations) is fairly high. They act like a battery with a big resistor in series.
2. By boosting the voltage, you increase the needed current. Say you need 5V at 1A to charge your phone, and your TEG can deliver 2.5V. Your TEG will have to provide 2.5V at 2A in order to deliver the 5 watts of power your charger needs.

Those two combine to cause your voltage drop.

1. Voltage **always** drops when current flows through a resistor. The TEG doesn't have a seperate resistor in it. Its construction involves materials and connections that raise the electrical resistance. They **must** be made that way to work, it isn't an artificial limit.
2. You have the TEGs in series, so the internal resistance adds up. If you have, say, three TEGs in series then you have three times the resistance.
3. Boosting the voltage increases the current draw from the TEG, and makes the voltage drop worse.

The problem boils down to your charger needing more power than the TEGs can provide.

* You can try putting more TEGs in parallel.
* You can try using a more efficient boost converter.
* You could use larger TEGs.
* You could put the TEGs in parallel and charge a low voltage battery, then charge your phone from the low voltage battery using a boost converter.

That last solution means you charge a low voltage battery slowly with your TEGs. When that battery is fully charged, you use it with a boost converter to quickly charge your phone.
I think that peltier devices act as current sources up to some maximum voltage. If you demand more current than the devices supply, the voltage will drop. Measure the current produced by one device and then calculate the number of peltier devices you need to deliver the target current into your boost converter.