Spacer Engineering Enables Fine‐Tuned Thin Film Microstructure and Efficient Charge Transport for Ultrasensitive 2D Perovskite‐Based Heterojunction Phototransistors and Optoelectronic Synapses

2D Ruddlesden–Popper phase layered perovskites (RPLPs) hold great promise for optoelectronic applications. In this study, a series of high‐performance heterojunction phototransistors (HPTs) based on RPLPs with different organic spacer cations (namely butylammonium (BA+), cyclohexylammonium (CyHA+), phenethylammonium (PEA+), p‐fluorophenylethylammonium (p‐F‐PEA+), and 2‐thiophenethylammonium (2‐ThEA+)) are fabricated successfully, in which high‐mobility organic semiconductor 2,7‐dioctyl[1]benzothieno[3,2‐b]benzothiophene is adopted to form type II heterojunction channels with RPLPs. The 2‐ThEA+‐RPLP‐based HPTs show the highest photosensitivity of 3.18 × 107 and the best detectivity of 9.00 × 1018 Jones, while the p‐F‐PEA+‐RPLP‐based ones exhibit the highest photoresponsivity of 5.51 × 106 A W−1 and external quantum efficiency of 1.32 × 109%, all of which are among the highest reported values to date. These heterojunction systems also mimicked several optically controllable fundamental characteristics of biological synapses, including excitatory postsynaptic current, paired‐pulse facilitation, and the transition from short‐term memory to long‐term memory states. The device based on 2‐ThEA+‐RPLP film shows an ultra‐high PPF index of 234%. Moreover, spacer engineering brought fine‐tuned thin film microstructures and efficient charge transport/transfer, which contributes to the superior photodetection performance and synaptic functions of these RPLP‐based HPTs. In‐depth structure‐property correlations between the organic spacer cations/RPLPs and thin film microstructure/device performance are systematically investigated. A series of heterojunction phototransistors (HPTs) based on Ruddlesden–Popper phase layered perovskites (RPLPs) with different organic spacers are developed. The 2‐thiophenethylammonium‐RPLP‐ and p‐fluorophenylethylammonium‐RPLP‐based HPTs show ultrasensitive performance with ultra‐high photoresponsivity. These HPTs work well as optoelectronic synapses. Systematically studies on the structure‐property relationship of organic spacers/RPLPs provides valuable guidance for the further development of perovskite materials and their optoelectronic applications.