You will need a return Tajik visa, a letter from a western organisation accrediting you, and a Tajik government pass (obtainable from the Tajik ministry of foreign affairs).
Kish, an island off the Iranian coast, is a popular destination for so-called visa runs - leaving the country you are working in, for a short time, so you can get a new visa on return.
The compelling argument is that out of 50, 000 US graduate school and PhD graduates annually, 17, 000 return back to their homeland because no appropriate visa option exists for them to stay in the US to run a startup or take on engineering jobs.