Ambipolar Charge Storage in Type‐I Core/Shell Semiconductor Quantum Dots toward Optoelectronic Transistor‐Based Memories

Abstract

Efficient charge storage media play a pivotal role in transistor‐based memories and thus are under intense research. In this work, the charge storage ability of type‐I InP/ZnS core/shell quantum dots is well revealed through studying a pentacene‐based organic transistor with the quantum dots (QDs) integrated. The quantum well‐like energy band structure enables the QDs to directly confine either holes or electrons in the core, signifying a dielectric layer‐free nonvolatile memory. Especially, the QDs in this device can be charged by electrons using light illumination as the exclusive method. The electron charging process is ascribed to the photoexcitation process in the InP‐core and the hot holes induced. The QDs layer demonstrates an electron storage density of ≈5.0 × 1011 cm−2 and a hole storage density of ≈6.4 × 1011 cm−2. Resultingly, the output device shows a fast response speed to gate voltage (10 µs), large memory window (42 V), good retention (>4.0 × 104 s), and reliable endurance. This work suggests that the core/shell quantum dot as a kind of charge storage medium is of great promise for optoelectronic memories.

Fully Printed Optoelectronic Synaptic Transistors Based on Quantum Dot–Metal Oxide Semiconductor Heterojunctions

“…Additionally, the low-efficiency electron transport in InP/ZnSe QDs resulted in a negligible photoexcitation current in base QD TFTs. After printing the InP/ZnSe QD layer over the SnO 2 layer, as shown in Figure S4e, the photoexcited electron–hole pairs were effectively separated near the heterojunction, and electrons could be effectively transferred from the CBM of InP/ZnSe QDs (−3.4 eV) to the CBM of the SnO 2 layer, except for a few electrons trapped at the interface between the InP/ZnSe QDs and SnO 2 layers. ,, Then, combining the electrons of the ionized oxygen vacancies in SnO 2 causes a significantly enhanced EPSC. Subsequently, when the optical pulse ceased, the photoinduced high channel conductance decayed slowly through the recombination of V O 1+ and V O 2 + in the SnO 2 film owing to the PPC effect and the slow detrapping of electrons at the heterointerface (Figure S4f).…”

“…Owing to the rational energy band alignment, 30 photogenerated electrons and holes in InP/ZnSe QDs can be effectively separated at the heterojunction interface, and then the photoelectrons are transferred to the n-type SnO 2 channel, resulting in highly boosted optoelectronic conductance. 25,31,32 Meanwhile, the distribution of oxygen vacancy (V O ) defects in the SnO 2 film delays electron recombination and enhances PPC behavior and synaptic plasticity. 17,33 Consequently, the fully printed QD/SnO 2 thin-film transistors (TFTs) exhibit superior optoelectronic synaptic responses, including excitatory postsynaptic current (EPSC), short/longterm plasticity (STP/LTP), and paired-pulse facilitation (PPF), with low operating voltage (V ds ) of 0.1 V, and low energy consumption of ∼5.6 pJ per event.…”

“…Eco-friendly InP/ZnSe QDs were used as the long-wavelength (∼520 nm) photoabsorbing layer, and SnO 2 acted as the high-speed carrier transport channel layer. Owing to the rational energy band alignment, photogenerated electrons and holes in InP/ZnSe QDs can be effectively separated at the heterojunction interface, and then the photoelectrons are transferred to the n-type SnO 2 channel, resulting in highly boosted optoelectronic conductance. ,, Meanwhile, the distribution of oxygen vacancy (V O ) defects in the SnO 2 film delays electron recombination and enhances PPC behavior and synaptic plasticity. , Consequently, the fully printed QD/SnO 2 thin-film transistors (TFTs) exhibit superior optoelectronic synaptic responses, including excitatory postsynaptic current (EPSC), short/long-term plasticity (STP/LTP), and paired-pulse facilitation (PPF), with low operating voltage ( V ds ) of 0.1 V, and low energy consumption of ∼5.6 pJ per event. Furthermore, an artificial vision system was constructed by modulating the optoelectronic synaptic plasticity of the QD/SnO 2 transistors, illustrating a significantly improved accuracy of 91% in image recognition, compared to that of bare SnO 2 based counterparts (58%).…”

Retina‐Inspired Color‐Cognitive Learning via Chromatically Controllable Mixed Quantum Dot Synaptic Transistor Arrays

“…[9][10][11][12][13][14] In particular, colloidal QDs have become a promising candidate for photosensing and memorization because of their unique optoelectronic properties such as high photoabsorption, large bandgap tunability, facile ligand exchange-based surface engineering, nonvolatile charge storage capability, as well as low temperature solution processability. [15][16][17][18] Also, QD-based phototransistors have recently drawn special attention with versatile photosynaptic memory behaviors, photoelectrical modulation functions for human-like cognition and visual adaptation, and neuromorphic image processing. [9,[19][20][21][22] Artificial photonic synapses are emerging as a promising implementation to emulate the human visual cognitive system by consolidating a series of processes for sensing and memorizing visual information into one system.…”

An Irradiance‐Adaptable Near‐Infrared Vertical Heterojunction Phototransistor

“…This point can be corroborated by the huge hysteresis of the cyclic transfer curves of the vertical device, as shown in Figure S5 (Supporting Information). [26] Therefore, the photoinduced electrons would get trapped in the PbS QDs film and electrostatically repel the injection of electrons from the graphene, leading to the reduction of the electron density in the conduction band. [27] The conductance is lowered as a result.…”