The performance of organic electrochemical transistors (OECTs) relies on the interaction between organic semiconductors and ions. Consequently, hydrophilic ethylene glycol (EG) side chains are incorporated into organic semiconductors to improve the channel's capacity to absorb ions. However, the EG substituted organic semiconductors tend to swell when immersed in aqueous electrolytes and exhibit microstructural changes induced by dopant ions. In our research, we introduce an alkyl spacer to create distance between the fullerene and EG chain. This approach is designed to reduce the negative effects of swelling and balance the ion and electron conduction. We conducted an analysis of OECTs using four fullerene derivatives: no alkyl spacer, butyl, hexyl, and octyl spacers. The OECTs based on fullerene derivatives with butyl and hexyl spacers exhibit enhanced transconductance (g_m=11.8 and 19.4 mS) compared to the ones without alkyl spacers. It has also been observed that the butyl and hexyl spacers lead to a more than tenfold increase in volumetric capacitance. Further increasing the alkyl spacer (octyl group) leads to no transistor behavior. Our study uncovers the relationship between alkyl spacers and the performance of OECTs based on fullerene derivatives. This will serve as a guideline for designing n-type small molecules for OECTs. Finally, we showcased the potential of utilizing OECTs based on these fullerene derivatives in cation sensing, which is promising for developing sweat sensors.