A polymer vertical transistor with an on/off current ratio higher than 104 is demonstrated. The proposed space-charge limited transistor (SCLT) uses a metal-grid base containing high-density submicrometre openings to modulate the vertical space-charge-limited current (SCLC). The key to obtaining a high on/off current ratio is to reduce the leakage current of SCLT. In this paper, an improved device structure that isolates the grid metal by using both top and bottom insulating layers is demonstrated. Then, with an identical proposed structure, the geometric design is also found to significantly influence the on/off ratio over 3 orders of magnitude. The competition between the SCLC and the grid to collector leakage current is analysed. Finally, the influence of tetrafluoro-tetracyano-quinodimethane doping on the transistor characteristics is investigated. The results are important for the design of polymer vertical transistors with high on/off ratios.