Nano-cathodoluminescence reveals the spectral properties of individual InGaN quantum wells in high efficiency light emitting diodes. We observe a variation in the emission wavelength of each quantum well, in correlation with the Si dopant concentration in the quantum barriers. This is reproduced by band profile simulations which reveal the reduction of the Stark shift in the quantum wells by Si doping. We demonstrate nano-cathodoluminescence is a powerful technique to optimize doping in optoelectronic devices.