The surface photovoltage (SPV) measurement is a very suitable method to assess the minority carrier diffusion length in microcrystalline silicon (muc-Si:H). Surface states create space charge region, needed for the SPV technique and diffusion length evaluation, but additionally another space charge region, the bottom one, can appear. We present results of monitoring experimentally the bottom space charge region by SPV technique in thick layers of undoped muc-Si:H grown on different substrates. In layers grown on p(+) crystalline silicon substrate and a-Si:H sublayers the surface photovoltage as a function of the absorption coefficient is characterised by a distinct peak at low absorption. Mathematical modelling prescribes the experimental data to a photovoltage signal originating in the bottom interface muc-Si:H/substrate.