I am depositing molybdenum on soda lime glass to be used as back contact for solar cell(CZTS) application using sputtering (magnetron). I have deposited ~500nm film at various temperatures and pressures. The problem regarding to resistivity and adhesion is solved but I am not able to get rid of pinholes. What impact it makes on cell performance? If it is permissible then in how much percentage?
Can anyone suggest me any option or technique to get rid of the problem.
May be you could try depositing the film in two steps.. For eg, deposit first one of ~ 300nm, it might have pinholes, and then deposit another of ~200nm on top of it.. This might fill in the pinholes..
what will be the difference between 200nm and 300nm layers? if both are identical then it will be 500nm at a whole!!
What I am suggesting is that you could deposit a 500 nm layer in two steps. The second deposition step will fill in the pin holes in the first layer so the final 500 nm film might not have any pin hole.
I am depositing film in three layers 1) high pressure 2) intermediate pressure and 3) low pressure at constant power. So obviously the three layers will have different deposition rate and thickness. But then too I am getting holes.
Note: I have deposited individual films of high, intermediate and low pressure also. Though pin holes were observed in all films.
I usually deposit Molybdenum by d.c. magnetron sputtering in a bi-layer configuration at room temperature. The first Mo film is quite thin, being only 30 nm thick and it is deposited in an Ar flow of 45 sccm, which corresponds to a pressure of 4.5 x 10-1 Pa. The Ar flow is then decreased to 15 sccm, which corresponds to a pressure of 1.5 x 10-1 Pa and a Mo film, approximately 500 nm thick, is deposited. A bi-layer deposition is needed in order to enhance the adhesion of the Mo film to the substrate and to avoid morphological defects on the surface of the film; in fact, the second layer is very compact and dense with a good surface smoothness without pits and pinholes.
@Alessio Bosio: I am too depositing at same parameters but though pinholes are found in film. I am also getting film of 500nm thick, highly adhesive and resistivity of order 10-5 ohm.cm but with pinholes. Is the base pressure or gas impurity, the culprit behind the pinholes?
I don't think that the base pressure can cause problems, since in many laboratories excellent films are obtained under these conditions. Instead, I believe that the sputtering conditions are very important. Can you tell me the sputtering parameters used for the deposition? What are the dimension of the Mo target and what is the target-substrate distance? What is the purity (inside the sputtering chamber) of the Ar gas? What is the purity of the Ar in the cylinder? How long is the Ar gas pipe line? Is a steinless steel pipe line or a polymeric one?
About the impact on cell performance - normally pinholes can act as shunts with low parallel resistance but since the Mo is a rear electrode it's probably not that problematic. It goes on the insulating glass so a connection between the CZTS and glass through a pinhole will do very little electrically, and furthermore, a shunt is a low resistance pathway - how will a hole be more conductive than a pure metal? So from that viewpoint you are ok.
I think it would only be a problem if the CZTS does not cover the pinholes but maintains the same pinholes, and if those are then filled by the CdS because a contact between CdS and Mo WILL decrease cell performance. I'm assuming you use the cell structure in http://www.solarnenergy.com/contents_img/100329.jpg.
@Manuel: Yes, I have used same structure.
and in order to protect cell from shunting is it possible to add some insulating layer from which minority charge carriers can tunnel. I think it may hinder cell potential. I need your suggestion on it.
The answers of Manuel and Chao are okay and may help in solving your problem. Pinholes are very unwanted in thin films especially when remain uncovered with CZTS as Manuel hinted.
The main cause is the lack of substrate cleaning where dust particles, residual of surface contamination on the substrate when heated inside the deposition chamber can evaporate and leave their place vacant. So, you have to clean the substrate very elaborately and preserve it clean during the transport to bell jar for evaporation.
Best wishes
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