This paper reports a systematic investigation of the growth mechanism of zinc phthalocyanine, a p-type semi-conductor, on oriented polymer substrates of bisphenol A polycarbonate (PC). It focuses on the impact of the substrate temperature on the polymorphism and the in-plane orientation of the ZnPc nanocrystals. The study combines transmission electron microscopy, X-ray diffraction, atomic force microscopy, UV-vis absorption spectroscopy, and force field-based molecular calculations. Large areas of oriented PC substrates are prepared by a simple process that combines mechanical rubbing and solvent-induced crystallization. The PC substrates show a smooth semicrystalline morphology with a preferential (a,c) surface of crystalline lamellae with a high in-plane orientation of the polymer chains. These substrates induce a unidirectional orientation of ZnPc nanocrystals with a preferred contact plane. The selection of the preferred ZnPc in-plane growth direction relies on a "self-amplified" mechanism whereby a fraction of ZnPc nanocrystals oriented by the polycarbonate substrate enforces the orientation of neighbouring domains during film growth. Regarding polymorphism, two domains of substrate temperature (T-s) are evidenced. For 33 degrees C <= T-s = 115 degrees C, ZnPc nanocrystals grow exclusively in the alpha form. The determined crystal structure of alpha-ZnPc (a = 1.23 nm, b = 0.38 nm, c = 1.28 nm and gamma = 96 +/- 1 degrees, Z = 1) is isomorphous to the recently refined structures of CuPc and CoPc. For 115 degrees C <= T-s <= 200 degrees C, ZnPc films consist of both alpha and beta nanocrystals with a gradual increase of the proportion of beta-form with increasing T-s. The onset of the beta polymorph growth coincides with a marked change in the nanocrystal size. The beta phase appears when the ZnPc nanocrystals reach some critical dimensions which can be estimated from the T-s-dependence of the nanocrystal size. The relative stability of the alpha and beta polymorphs is explained by the balance of the bulk and the surface energy of the nanocrystals. A simple model is developed to predict and force-field based molecular simulations are used to estimate the critical dimensions of the ZnPc nanocrystals.