A solar concentrator module comprises: a tandem solar cell including an upper diffused junction n-type GaAs wafer solar cell, and a lower diffused junction n-type GaSb wafer solar cell; the GaAs cell has a graded emitter conductivity type modifier concn. effective to preserve emitter carrier lifetime and to maintain a low series resistance and a high light generated current, and a junction depth sufficient to provide high light generated current; the GaSb cell receives light passing through the GaAs cell and has at an upper surface a first light, relatively shallow, relatively uniform diffusion of conductivity-type modifier under the grid lines to maintian a low series resistance. The GaAs cell has a conductivity-type modifier concn. 10 power (18) cm3-10 power (19) cm3 in a first region extending 0.1-0.5 micron from the upper surface thereof, followed by an emitter junction where the doping concn. decreases from between 10 power (-18) to 10 power (17) cm3 at a distance between 0.6-10 micron from the surface. The first light diffusion on the GaSb cell is homogeneous toprovide a junction at the shallow depth, and the heavy diffusion is patterned and provides a junction at a greater diffusion depth only under the grid lines. Further disclosed a method of making diffused junction III-V wafer e.g. Zn diffused GaAs; a method for producing a front side grid electrode on a III-V wafer, e.g. GaSb, having an emitter with a texture etched surface.    The GaSb cell has a texture etch on the upper surface that is effective to lower reflection of incident solar radiation leading to increased currents. The etch being positioned between the gride lines and having a depth larger than the wavelength to which GaSb is photoresponsive and smaller than the minority carrier diffusion length.    USE/ADVANTAGE -   Improved methods of making GaAs and GaSb solar cells. Particularly Zn diffusion procedures applicable for large scale mfr. of tandem cells chaving a high energy conversion efficiency.