At the very least, in general he articles available for free online apparently are better cited, see the link below.

http://ivyspring.com/steveLawrence/SteveLawrence.htm

Hi Amin, the phenomenon you refer to is called the "citation advantage". I did a search on Google Scholar in the hope of finding that definitive article but as I think you hint at in your question, this is a contested space.

My thought is, is this the right question to ask? Citations should inform the paper they are included in, be relevant and included after a lengthy process of thought, discussion, editing and refinement. The form of access should not be the first question asked of a paper, if only because anyone working in a research or university context should be able to source any paper through their library and their national and international networks.

The real drivers are: 1) Mandated Open Access publication from research funders; 1a) Mandated Open Access to participate in research assessment and research management exercises;  2) mandated Open Access publication from host or home institutions ; 3) Visibility and promotion of research - you may not be cited more but the chances are more people will download your article; 4) linking with social media - by that I mean you can talk about a paper easily as anyone who sees your social media output can click and see it immediately; 5) (easier) access to alt metrics measures of use.

BW Matt

Dear Artur and Matt, 

I appreciate you for your answers.

Note that in the link given by Arthur, the statistics have been performed on publications related to computer science and that as said in this field conference articles are often more prestigious than journal articles. But i don't think it can be automatically generalized to other fields. Therefore I agree with matt; with free online papers, "you may not be cited more but the chances are more people will download your article" !!!