The long-time scale nonlinear morphological dynamics of the interface during the thin-film directional solidification of an SCN/acetone alloy have been simulated by an efficient phase field simulation. In addition to the agreement with classic theories, the mode transition (lambda(c)<----> lambda(c)/2) through periodic-chaotic-periodic solute pinch-off after a step change of the solidification speed is illustrated; lambda(c) is the critical wavelength. The tip-splitting, coarsening, and overshooting during mode transition are consistent with experimental observations. However, the transition time is up to 5D(L)/kV(2) making a real experiment at lower solidification speed very difficult to perform, where D-L is the acetone diffusivity, k the segregation coefficient, and V the solidification speed. The subcriticality of the primary bifurcation is further confirmed by simulation. (C) 2003 Elsevier B.V. All rights reserved.