code: def get_carrier_concentration(self):
          
  
          return {temp: [1e24 * i / self.vol for i in self._carrier_conc[temp]]
                  for temp in self._carrier_conc}
docs: gives the carrier concentration (in cm^-3)
  
          Returns
              a dictionary {temp:[]} with an array of carrier concentration
              (in cm^-3) at each temperature
              The array relates to each step of electron chemical potential