That's the general conclusion of a 2007 study:
It has been argued that greater transparency is needed to reduce corruption. One way of increasing transparency is through the adoption of Freedom of Information (FOI) laws. This paper uses the introduction of FOI laws as a natural experiment to determine their effect on corruption. Using a sample of democratic countries and two different corruption indices, I find that countries that adopted FOI laws saw an increase in corruption. Results are robust throughout different specifications. Moreover, I find that countries with plurality systems potentially experienced a decrease in corruption following the adoption of FOI legislation. Having a parliamentary system, however, had no impact on the effect of the reform.
It seems somewhat contradictory that FOI laws have different effects depending on the flavor of the voting system. Perhaps there are other confounders. Is there more research on this topic of FOI laws and their effect on corruption? Has some kind of consensus been reached in this field, or are published conclusions still somewhat contradictory, even more recently?