**1. Introduction**

Neuromorphic engineering is an emerging technological area, which aims at mimicking the biological functionalities of neurons, synapses, or a whole brain by various electronic materials and devices [1–6]. Recently, the use of organic electronics in neuromorphic systems has gained tremendous attention, thanks to its capacity to expand the technological scope of such systems by creating unconventional interfaces such as direct neuroprotheses and robotic sensory bridges [7–10]. There are many possible routes to organic-based neuromorphic architecture, including electrochemical [11,12], memristive [13], and field-effect approaches [14–16]. Among them, organic field-effect transistor (OFET)-based synaptic devices are a particularly promising element, considering the possibility of a fully solid-state, flexible neuromorphic chip that leverages the versatility of OFETs in constructing various circuit building blocks [17–20]. Despite the rapidly growing technological viability of OFET synapses, there is still a lack of understanding on fundamental phenomena prevailing at the single-device level, which acts as a current bottleneck for the development of organic-based complex neuromorphic hardware systems. We recognize this issue, and present here a detailed analysis of one specific neuromorphic functionality, namely the short-term plasticity (STP) in flexible OFET synaptic devices. By combining experimental measurements and numerical modeling, systematic understanding of the voltage-dependent transmission behavior at the synaptic junction is obtained. By increasing the input-spike voltage magnitude, slowing down of both charging and discharging is observed, as the floating carrier reservoir turns electrostatically populated. The detailed analysis from this study builds a solid foundation for advanced models and the realization of flexible organic neuromorphic circuitries.
