NOVELTY - The apparatus comprises a vertically arranged treatment chamber (3) with an opening, through which the treatment liquid flows into the treatment chamber and fills the treatment chamber, a gap, which is formed between an overflow edge of the treatment chamber and the underside of a flat material, a unit for conveying the treatment liquid into the treatment chamber, a container (14) for collecting the treatment liquid flowing from the treatment chamber through the gap, and a centrally or ring-shapedly arranged unit for feeding the treatment liquid at or over the bottom of the treatment chamber.    USE - Apparatus for one-sided wet-chemical and/or electrolytic treatment or for cleaning of a completely flat or structured underside of a flat material with a treatment liquid, where the flat material is a fracture-sensitive material such as silicon solar cells, wafers, hybrids, circuit boards and compact disks.    ADVANTAGE - The device is easy to handle, is capable of rapidly and safely treating or cleaning the flat material without the risk of breakage or rupture, and decreases the cost and effort in the case of change in shape of the material.    DETAILED DESCRIPTION - The apparatus comprises a vertically arranged treatment chamber (3) with an opening, through which the treatment liquid flows into the treatment chamber and fills the treatment chamber, a gap, which is formed between an overflow edge of the treatment chamber and the underside of a flat material, a unit for conveying the treatment liquid into the treatment chamber, a container (14) for collecting the treatment liquid flowing from the treatment chamber through the gap, a centrally or ring-shapedly arranged unit for feeding the treatment liquid at or over the bottom of the treatment chamber and/or a symmetrical unit disposed inside the treatment chamber to direct the flow of the treatment liquid in the vicinity of the material that is radially-symmetric from the vertical middle axis to all sides of the treatment chamber, and a support (2). The overflow edge that serves as an outlet for the treatment liquid is arranged at an upper edge of the treatment chamber. The gap height is exactly similar along the total circumference of the treatment chamber in relation to the overflow edge. The overflow edge and the gap run exactly horizontal along the total circumference of the treatment chamber. The material is displaced without any horizontally acting force. The treatment chamber has a circular, quadratic or rectangular cross-section with or without truncated corners, so that the treatment material is congruently positioned with the treatment chamber. The apparatus consists of electrically conducting or non-conducting materials. The material placed on the electrically conducting or non-conducting materials is kept in an adequate distance to the overflow edge, so that it is contacted by the treatment liquid. A support is adjustable in the height. An INDEPENDENT CLAIM is included for a method for one-sided wet-chemical and/or electrolytic treatment or for cleaning of completely flat or structured underside of a flat material e.g. wafer.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of an arrangement for electrolytically metallizing silicon solar cells with illuminated underside.    Support (2)    Treatment chamber (3)    Pump (5)    Collection container (14)    Anode. (21)