In polysilicon, electrical properties are strongly influenced by Al concentration (NAl). Al acts as an acceptor or as a recombinating agent within grains and it can electrically passivate grain boundaries, according to its site occupation or to its chemical environment. The influence of oxygen on the Al precipitation at internal interfaces and dislocations was analysed. The LBIC technique was applied in the mapping mode to characterize the electrical activity of grain boundaries and specifically of twin boundaries in relationship with co-segregation mechanism.