Yongbo Yuan

Irreversible light-soaking effect of perovskite solar cells caused by light-induced oxygen vacancies in titanium oxide

An irreversible light-soaking effect was disclosed in perovskite solar cells using TiO2 as an electron transporting layer. The power conversion efficiency of a fresh device was improved more by twice after light soaking for 15 min and then remained 70% even though the device was recovered in the dark for 4 days. The buried mechanism was explored by shedding light on the interaction between light and titanium dioxide. Oxygen vacancies in TiO2 were found to be increased by light-soaking, especially for wavelengths shorter than 400 nm. Such vacancies enhanced the N-type doping in the semiconductor, which not only increased the conductivity of the titania film but also accelerated the charge extraction rate between perovskite crystallites and titania, and finally contributed to upgraded power conversion efficiency.

Seesaw-like polarized transmission behavior of silver nanowire arrays aligned by off-center spin-coating

Straight silver nanowires were synthesized by accelerated oxidization and then aligned into ordered arrays by off-center spin-coating. Seesaw-like behavior was observed in the polarized transmission spectra of the arrays. With the increment of polarization angle (θP, defined as the angle between axis of nanowires and direction of electric field of light), transmission changed repeatedly with a period of 180°, but it moved to opposite directions between the two regions separated by supporting points locating at 494 nm. The behavior is ascribed to the competition between the extinction behaviors of the two modes of surface plasma polaritons on silver nanowires. One is the longitudinal mode which is excited by long wavelengths and tuned by function of cos2( θ p) and the other is the transverse mode that is excited by short wavelengths and tuned by function of sin2( θ p). Simulation was performed based on the finite-difference time domain method. The effect of the nanowire diameter and length (aspect ratio) on the position of the supporting point was studied. As nanowire width increased from 20 nm to 350 nm, the supporting point moved from 400 to 500 nm. While it changed slightly when the nanowire length increased from 3 μm to infinitely long (width fixed at 260 nm). In current study, the position of the supporting point is mainly determined by the nanowire width.

Dependence of power conversion properties of perovskite solar cells on operating temperature

Power conversion properties of perovskite solar cells are studied in the temperature range of 310 K to 240 K (and recovering back). As the temperature lowers down, the fill factor (FF) decreases while the open circuit voltage (VOC) increases in the case of reverse scans (scanning from positive voltages to negative ones). The decreased FF is ascribed to the increased resistance of charge transport materials (both TiO2 and Spiro-OMeTAD) as well as the increased interfacial charge transfer resistance, while the increased VOC is due to retarded recombination which is revealed by the transient photovoltage decay measurement. Hysteresis appears in the current-voltage curves, but it shrinks with temperature decreasing and even vanishes as the temperature becomes lower than 270 K. Mott-Schottky capacitance analysis shows that ion migration exists in the device, especially for temperatures >270 K. The “S shape” current-voltage characteristic is observed at lowered temperatures, which is caused by retarded charge extraction across the interface between the active layer and charge-transport materials. Similar power conversion properties are observed when elevating the temperature from 240 K to 310 K; thus, the temperature-sensitive behavior is reversible. The observed behavior is compared with silicon solar cells. The study shows that lowering the temperature is harmful to the charge extraction processes of perovskite solar cells. Highly conductive charge-transport materials are needed for the devices to operate in a colder environment.Power conversion properties of perovskite solar cells are studied in the temperature range of 310 K to 240 K (and recovering back). As the temperature lowers down, the fill factor (FF) decreases while the open circuit voltage (VOC) increases in the case of reverse scans (scanning from positive voltages to negative ones). The decreased FF is ascribed to the increased resistance of charge transport materials (both TiO2 and Spiro-OMeTAD) as well as the increased interfacial charge transfer resistance, while the increased VOC is due to retarded recombination which is revealed by the transient photovoltage decay measurement. Hysteresis appears in the current-voltage curves, but it shrinks with temperature decreasing and even vanishes as the temperature becomes lower than 270 K. Mott-Schottky capacitance analysis shows that ion migration exists in the device, especially for temperatures >270 K. The “S shape” current-voltage characteristic is observed at lowered temperatures, which is caused by retarded charge e...

Flexible Neuromorphic Architectures Based on Self-Supported Multiterminal Organic Transistors

Because of the fast expansion of artificial intelligence, development and applications of neuromorphic systems attract extensive interest. In this paper, a highly interconnected neuromorphic architecture (HINA) based on flexible self-supported multiterminal organic transistors is proposed. Au electrodes, poly(3-hexylthiophene) active channels, and ion-conducting membranes were combined to fabricate organic neuromorphic devices. Especially, freestanding ion-conducting membranes were used as gate dielectrics as well as support substrates. Basic neuromorphic behavior and four forms of spike-timing-dependent plasticity were emulated. The fabricated neuromorphic device showed excellent electrical stability and mechanical flexibility after 1000 bends. Most importantly, the device structure is interconnected in a way similar to the neural architecture of the human brain and realizes not only the structure of the multigate but also characteristics of the global gate. Dynamic processes of memorizing and forgetting were incorporated into the global gate matrix simulation. Pavlovs learning rule was also simulated by taking advantage of the multigate array. Realization of HINAs would open a new path for flexible and sophisticated neural networks.