To get rectification behavior from a junction it should be confirmed that the individual parts must be p-type and n-type. You should be confirm that (separately) before making junction, and then try to fabricate pn junction. After then if you observe straight line of IV curve then it is clear that a layer (p or n) has been out from the device during deposition. If you try to fabricate multi junction solar cell (CZTS/ZnS/ZnO) then you must be sure about the unique direction of the electric field in the depletion region. Two of those should be one kind semiconductor (say p-type) another should be opposite kind (n-type).

Hi,

for me it sounds like your devices are somehow shorted, probably related to the existence of shunt pathways throughout your absorber layer. This is further supported by the fact that you are able to measure a low resistance of only 30 Ohms between your bottom and top contact. Shunting of your device could be visualized with the help of lock-in thermography measurements (if such an equipment is available at your institution).

Does the location of the straight line I-V characteristic change between light and dark? If it doesn't (Isc = 0) then there is no junction and the fabrication needs to be altered.  If there is a Isc, then there may be a junction which suffers from a small shunt resistance or a large series resistance.   Usually shunts occur at the edges or the periphery of the active region of the device, so it may help to locate the shunt by scribing through just inside the edge to see if the shunt resistance changes.