Nanographene-Based Heterojunctions for High-Performance Organic Phototransistor Memory Devices.

Organic phototransistors can enable many important applications such as nonvolatile memory, artificial synapses, and photodetectors in next-generation optical communication and wearable electronics. However, it is still a challenge to achieve a big memory window (threshold voltage response ∆Vth ) for phototransistors. Here, a nanographene-based heterojunction phototransistor memory with large ∆Vth responses is reported. Exposure to low intensity light (25.7 µW cm-2 ) for 1 s yields a memory window of 35 V, and the threshold voltage shift is found to be larger than 140 V under continuous light illumination. The device exhibits both good photosensitivity (3.6 × 105 ) and memory properties including long retention time (>1.5 × 105  s), large hysteresis (45.35 V), and high endurance for voltage-erasing and light-programming. These findings demonstrate the high application potential of nanographenes in the field of optoelectronics. In addition, the working principle of these hybrid nanographene-organic structured heterojunction phototransistor memory devices is described which provides new insight into the design of high-performance organic phototransistor devices.

Persistent Ion Accumulation at Interfaces Improves the Performance of Perovskite Solar Cells.

The mixed ionic-electronic nature of lead halide perovskites makes their performance in solar cells complex in nature. Ion migration is often associated with negative impacts-such as hysteresis or device degradation-leading to significant efforts to suppress ionic movement in perovskite solar cells. In this work, we demonstrate that ion trapping at the perovskite/electron transport layer interface induces band bending, thus increasing the built-in potential and open-circuit voltage of the device. Quantum chemical calculations reveal that iodine interstitials are stabilized at that interface, effectively trapping them at a remarkably high density of ∼1021 cm-3 which causes the band bending. Despite the presence of this high density of ionic defects, the electronic structure calculations show no sub-band-gap states (electronic traps) are formed due to a pronounced perovskite lattice reorganization. Our work demonstrates that ionic traps can have a positive impact on device performance of perovskite solar cells.

Oxygen-induced degradation in AgBiS2 nanocrystal solar cells.

AgBiS2 nanocrystal solar cells are among the most sustainable emerging photovoltaic technologies. Their environmentally-friendly composition and low energy consumption during fabrication make them particularly attractive for future applications. However, much remains unknown about the stability of these devices, in particular under operational conditions. In this study, we explore the effects of oxygen and light on the stability of AgBiS2 nanocrystal solar cells and identify its dependence on the charge extraction layers. Normally, the rate of oxygen-induced degradation of nanocrystals is related to their ligands, which determine the access sites by steric hindrance. We demonstrate that the ligands, commonly used in AgBiS2 solar cells, also play a crucial chemical role in the oxidation process. Specifically, we show that the tetramethylammonium iodide ligands enable their oxidation, leading to the formation of bismuth oxide and silver sulphide. Additionally, the rate of oxidation is impacted by the presence of water, often present at the surface of the ZnO electron extraction layer. Moreover, the degradation of the organic hole extraction layer also impacts the overall device stability and the materials' photophysics. The understanding of these degradation processes is necessary for the development of mitigation strategies for future generations of more stable AgBiS2 nanocrystal solar cells.

Ruddlesden-Popper-Phase Hybrid Halide Perovskite/Small-Molecule Organic Blend Memory Transistors.

Controlling the morphology of metal halide perovskite layers during processing is critical for the manufacturing of optoelectronics. Here, a strategy to control the microstructure of solution-processed layered Ruddlesden-Popper-phase perovskite films based on phenethylammonium lead bromide ((PEA)2 PbBr4 ) is reported. The method relies on the addition of the organic semiconductor 2,7-dioctyl[1]benzothieno[3,2-b]benzothiophene (C8 -BTBT) into the perovskite formulation, where it facilitates the formation of large, near-single-crystalline-quality platelet-like (PEA)2 PbBr4 domains overlaid by a ≈5-nm-thin C8 -BTBT layer. Transistors with (PEA)2 PbBr4 /C8 -BTBT channels exhibit an unexpectedly large hysteresis window between forward and return bias sweeps. Material and device analysis combined with theoretical calculations suggest that the C8 -BTBT-rich phase acts as the hole-transporting channel, while the quantum wells in (PEA)2 PbBr4 act as the charge storage element where carriers from the channel are injected, stored, or extracted via tunneling. When tested as a non-volatile memory, the devices exhibit a record memory window (>180 V), a high erase/write channel current ratio (104 ), good data retention, and high endurance (>104 cycles). The results here highlight a new memory device concept for application in large-area electronics, while the growth technique can potentially be exploited for the development of other optoelectronic devices including solar cells, photodetectors, and light-emitting diodes.

Enhancing the Open-Circuit Voltage of Perovskite Solar Cells by Embedding Molecular Dipoles within Their Hole-Blocking Layer.

Engineering the energetics of perovskite photovoltaic devices through deliberate introduction of dipoles to control the built-in potential of the devices offers an opportunity to enhance their performance without the need to modify the active layer itself. In this work, we demonstrate how the incorporation of molecular dipoles into the bathocuproine (BCP) hole-blocking layer of inverted perovskite solar cells improves the device open-circuit voltage (VOC) and, consequently, their performance. We explore a series of four thiaazulenic derivatives that exhibit increasing dipole moments and demonstrate that these molecules can be introduced into the solution-processed BCP layer to effectively increase the built-in potential within the device without altering any of the other device layers. As a result, the VOC of the devices is enhanced by up to 130 mV, with larger dipoles resulting in higher VOC. To investigate the limitations of this approach, we employ numerical device simulations that demonstrate that the highest dipole derivatives used in this work eliminate all limitations on the VOC stemming from the built-in potential of the device.