Why doesn't my charge pump work with 1.5v supply?
With regular diodes, the forward volt drop is generally said to be 0.7 volts and this eats into the peak-to-peak voltage made by your oscillator so, the negative clamping voltage below 1.5 volts (brought about by D1) is more likely to be 0.8 volts and therefore the peak voltage is 1.5 volts higher (due to the oscillator producing 1.5 volts p-p) at 2.3 volts. This is then rectified by another diode (D2) which also "loses" 0.7 volts in the process hence, the final rectified voltage is about 1.6 volts.

Of course, it's more likely that your oscillator is producing 1.3 or 1.4 volts p-p and this means your doubler is running out of steam big time. Try schottky diodes to get maybe 2.5 volts at the output (on very light loads).