An apparatus for reactive thermal treatment of thin film photovoltaic devices includes a furnace tube including an inner wall extended from a first end to a second end. The apparatus further includes a gas supply device coupled to the second end and configured to fill one or more working gases into the furnace tube. Additionally, the apparatus includes a cover configured to seal the furnace tube at the first end and serve as a heat sink for the one or more working gases. Furthermore, the apparatus includes a fixture mechanically attached to the cover. The fixture is configured to load an array of substrates into the furnace tube as the cover seals the furnace tube. Moreover, the apparatus includes a crescent shaped baffle member disposed seamlessly at a lower portion of the inner wall for blocking a convection current of the one or more working gases cooled by the cover.