Decoding Trap States in Working 2D Perovskite Multi‐Functional Devices

Abstract Hybrid organic–inorganic perovskites, such as 4‐fluorophenethylammonium lead iodide (F‐PEAI), is an emerging class of layered 2D materials of unique interest for a range of opto‐electronic applications, owing to their exceptional photophysical properties and enhanced environmental stability. Yet, the device performance is strongly governed by trap states, which remain challenging to characterise under realistic operating conditions. This is even more prominent in multi‐functional devices where, according to the user‐defined dominant charge carrier type and traps dynamic the system switches from a field effect transistor (FET), to a highly sensitive photodetectors, energy harvesting or a even synaptic device. Characterising trap states for each of these regimes is uniquely challenging as it requires methods capable of distinguishing different types of carriers and trapping centers within the stringent operating requirements, and ideally without compromising the functionalities of the device. Here, threshold voltage transient spectroscopy (TVTS) is introduced as a universal, non‐invasive method to probe trap states in fully processed 2D F‐PEAI single‐crystal field‐effect transistors that also function as high‐gain photodetectors. TVTS enables real‐time extraction of sub‐gap trap densities and energy distributions, with tunable sensitivity to emission or retrapping processes at any set temperature, providing insight into their real‐time influence on charge transport mechanisms. When applied to 2D F‐PEAI multi‐functional devices, TVTS reveals a transition in trap states and dynamics from deep majority‐carrier trapping (10 ¹³ cm ⁻² ) at cryogenic temperature to shallow trapping above 100K. Strong retrapping is observed to enhance minority‐carrier diffusion lengths ( 5 µm), yielding responsivities up to 120 A/W providing a pathway for enhancing the opto‐electronic device performance. These results establish TVTS as a powerful platform for in situ defect spectroscopy of multifunctional 2D perovskite devices.