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When deciding if you should perform 1D, 2D or 3D, simulations, consider the dimensionality of your problem. For example if you consider a solar cell, it is only a few micros thick, and there is rapid variation in the structure, charge densities, mobilities, and doping as a function of depth (y). However, the structure will not vary very quickly in the lateral (xz) plane. Therefore, in general to capture all interesting effects present within a solar cell one only needs a 1D model. If one now considers OFETs, there is both vertical an lateral current flow, therefore one can not get away with a 1D model any more, as one must simulate both vertical current flow, and current between the source and the drain, thus one needs a 2D simulation. As the number of dimensions increases, computation speed will decrease, therefore my general advice is to use the minimum number of dimensions possible to solve your problem.