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Low-voltage operation in accumulation mode organic electrochemical transistors (OECTs) is essential for biosensing applications and for potential use with low-voltage portable power supplies. Here, we employ a small molecule additive, dodecylbenzenesulfonate (DBSA), by adding it to the electrolyte in OECTs to improve the device performance. We find that DBSA lowers the operation voltage, increases the ON current, and increases the transconductance of the device. Such improvements are found for a range of p-type polymers including P3HT, PBTTT and DPPT-TT which have different electronic and structural properties. To investigate the device operational mechanism modulated by DBSA, we directly probe the molecular structure changes of three polymers upon charge injection (i.e. polaron formation) and correlate them to polaron density and OECT performance. We find that the electrolyte mixture (containing DBSA) enhances the electrochemical doping of the polymer by lowering the onset of oxidation and allowing the generation of a higher polaron density. For example, for P3HT the VON value decreases to 0.05 V, the ON current increases by ∼3 times, and the transconductance (gm) increases to 4 mS, which is, to the best of our knowledge, the highest transconductance of P3HT OECT reported. These results demonstrate a simple, but effective way of using a small molecule additive, such as DBSA, and a possibility to utilise otherwise unsuitable polymers with deep highest occupied molecular orbital (HOMO) levels, for low-power accumulation mode OECTs.