Amorphous silicon p-i-n diodes with intrinsic layer thickness exceeding 50 microns have been produced. As a probe of the carrier collection, reverse bias collection efficiency measurements using weakly absorbed light were performed. The results have shown for the first time that amorphous silicon diodes of such thickness can be fully depleted. By analyzing the shape of the collection efficiency curve, profiles of the light induced space charge within the intrinsic layer are obtained. The space charge density in thick devices is found to vary strongly with position in the intrinsic layer as well as with excitation intensity.