The Staebler and Wronski light soaking effect [1-3] in hydrogenated amorphous silicon (a-Si:H) results as a worsening of solar cell performances under light soaking which results in a substantial loss of cell power conversion efficiency compared to time zero performance. In this work, comparing the behavior of single a-Si:H p-i-n devices with that of tandem a-Si:H / microcrystalline Si solar cells, we report on the performance improvement of the solar cells under prolonged exposure to reverse bias DC electric fields and illumination. In the paper we discuss about the radical differences between the forward and reverse bias stress and on the spectroscopic signature of the defects responsible for the instability / improvement effect. In addition, we determine the range of wavelengths where the photons are effective to improve the lifetime and cell efficiency. Finally, examples of reverse bias practical use in solar cell …