To reduce the thickness of CuIn1-xGaxSe2 (CIGSe) solar cells, light management concepts must be used. One concept is the combination of an Al/indium1-tin1-oxide (ITO) reflector/contact and an optical scattering element at the back side of the device. Herein, nanostructured substrates, obtained by laser interference lithography and lift-off process, are covered with an Al/ITO back contact. Structure dimensions are a pitch of 1.96??m and heights of 100–700?nm. Solar cells with 600?nm CIGSe absorber having this integrated structured back reflector exhibit an increase in JSC of up to 3.2?mA?cm?2 compared with the Mo reference sample of identical thickness. To understand this optical gain, 3D finite-difference time-domain (FDTD) simulations are performed. To this end, a model is built to simulate structure formation. It is shown that locally enhanced optical absorption can be achieved by the integrated structured back reflector. It is also shown that parasitic absorption in the back ITO diffusion barrier needs to be minimized.