The concentration dependence of amplified spontaneous emission (ASE) in optically pumped polystyrene (PS) films containing a variable concentration (between 2.5 and 100 wt%) of the luminescent and hole-transporting organic molecule N,N'-Bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) is reported. It is observed that both, the ASE threshold and linewidth above threshold, decrease with concentration up to 20 wt% doped films and then keeps a constant value up to concentrations of 100 wt% (neat films). Accordingly, photoluminescence (PL) efficiency increases with concentration up to 20 wt% and then saturates. Results also show that the position of ASE can be tuned between 413 and 421 nm by changing the concentration of TPD. These shifts are also observed in the PL emission and are presumably due to the increase of absorption, as well as polarity of the medium, with concentration. Measurements and modeling of the waveguiding characteristics of the films, show that the observed shifts are not due to cut-off thickness limitations. In addition, although the different confinement of the waveguide modes might be influencing the ASE performance, it seems to be determined mostly by the PL efficiency. Results show that the most efficient films are those doped with 20-30 wt% TPD. Finally, a study of the photostability of the samples is also included.