To underscore the effects of boron doping on the photovoltaic characteristics of a-Si:H based pi-n solar cells we used the in-situ Kelvin probe technique to determine the evolution of the contact potential at the SnO2/p interface in two series of solar cells: one with a p(a-Si:H) layer and another with a p(a-SiC:H) layer plus a buffer a-SiC:H layer. In each series, the flow of diborane was varied between 0.5 and 7 seem. The in-situ Kelvin probe measurements on thick layers indicate that the doping efficiency is higher in a-Si:H than in a-SiC:H layers. In the case of cells with a p(a-Si:H) layer we observe an optimum in V-oc as a function of the diborane flow rate. Below this optimum the increase in V-oc is correlated to the increase of the contact potential measured by the Kelvin probe. Above the optimum value of the diborane flow, the further increase of the contact potential contrasts with the decrease of V-oc. Moreover, we found that V-oc of the cells with a p(a-SiC:H) is independent of the diborane how rate. This unexpected behaviour is interpreted in terms of the diffusion of impurities and changes in the p-layer induced by boron atoms; it is supported by secondary-ion mass spectrometry and in-situ spectroscopic ellipsometry measurements.