NOVELTY - A thin film photovoltaic back contact material stack comprises: first layer, bulk current transport layer (406), and ohmic contact layer (414).    USE - As thin film photovoltaic back contact material stack (claimed) for thin film solar cells such as copper indium gallium (sulfide) selenide (CIGS) solar cells, cadmium telluride (CdTe) solar cells, and copper zinc tin (sulfide) selenide (CZTS) solar cells.    ADVANTAGE - The thin film photovoltaic back contact material stack provides back contact for thin film solar cells to improve efficiency of thin film photovoltaic (TFPV) solar cells, at low cost and with ease of manufacturing. In thin film photovoltaic back contact material stack, adhesion-promoter and diffusion barrier layers are formed to ensure good adhesion to substrate and to control diffusion of components such as Na out of substrate or to control diffusion of components from material stacks into substrate. Inexpensive, high conductivity materials are used as primary bulk current transport layers. Stress management and diffusion barrier layers are used throughout back contact material stack to control stress within material stack and to prevent materials from current transport layers from diffusing into absorber and buffer layers formed above back contact material stack. High reflectivity layers are used to improve light absorption by the TFPV solar cell. Interface layers are used to ensure good ohmic contact between back contact material stack and absorber layer.    DETAILED DESCRIPTION - A thin film photovoltaic back contact material stack comprises: first layer, bulk current transport layer (406), and ohmic contact layer (414). The first layer comprises one of metal, metal alloy, metal oxide, metal nitride, or metal oxy-nitride. The bulk current transport layer comprises one of metal, metal alloy, conductive metal oxide, conductive metal nitride, or conductive metal silicide. The thin film photovoltaic back contact material stack further comprises optical reflector layer (410) positioned between bulk current transport layer and ohmic contact layer, where optical reflector layer is further positioned between two diffusion barrier layers (408, 412); or diffusion barrier layer between bulk current transport layer and ohmic contact layer. The optical reflector layer comprises one of metal, metal alloy, metal oxide, metal nitride, or metal silicide. The diffusion barrier layers including optical reflector layer comprises one of metal oxide, metal nitride, metal oxy-nitride, or metal silicide. The diffusion barrier layer between bulk current transport layer and ohmic contact layer comprises one of metal, metal alloy, conductive metal oxide, conductive metal nitride, or conductive metal silicide.    DESCRIPTION OF DRAWING(S) - The figure shows schematic diagram of the thin film photovoltaic (TFPV) back contact stack.    Substrate (402)    Diffusion barrier layer (404, 408, 412)    Bulk current transport layer (406)    Optical reflector layer (410)    Ohmic contact layer (414)