Progressive current degradation and breakdown behavior in GaN LEDs under high reverse bias stress

The progressive current degradation and breakdown behaviors of GaN-based light emitting diodes under high reversebias stress are studied by combining the electrical, optical, and surface morphology characterizations. The current features a typical “soft breakdown” behavior, which is linearly correlated to an increase of the accumulative number of electroluminescence spots. The time-to-failure for each failure site approximately obeys a Weibull distribution with slopes of about 0.67 and 4.09 at the infant and wear-out periods, respectively. After breakdown, visible craters can be observed at the device surface as a result of transient electrostatic discharge. By performing focused ion beam cuts coupled with scan electron microscope, we observed a local current shunt path in the surface layer, caused by the rapid microstructure deterioration due to significant current heating effect, consistent well with the optical beam induced resistance change observations.

A generalized Norde plot for reverse biased Schottky contacts

When a metal makes intimate contact with a semiconductor material, a Schottky barrier may be created. The Schottky contact has many important applications in the integrated circuit （IC） electronics field. The parameters of such contacts can be determined from their current-voltage （I-V） characteristics. The literature contains many proposals for extracting the contact parameters using graphical methods. However, such methods are generally applicable only to contacts with a forward bias, whereas many Schottky contacts actually operate un- der a reverse bias. Accordingly, the present study proposed a generalized reverse current-voltage （I-V） plot which enables the series resis- tance, barrier height, and ideality factor of a reverse biased Schottky contact to be extracted from a single set of I-V measurements. A theo- retical derivation of the proposed approach was presented and a series of validation tests were then performed. The results show that the pro- posed method is capable of extracting reliable estimates of the contact parameters even in the presence of experimental noise.

Double Layer Composite Electrode Strategy for Efficient Perovskite Solar Cells with Excellent Reverse-Bias Stability

Perovskite solar cells(PSCs)have become the represent-atives of next generation of photovoltaics;nevertheless,their stability is insufficient for large scale deployment,particularly the reverse bias stability.Here,we propose a transparent conducting oxide(TCO)and low-cost metal composite electrode to improve the stability of PSCs without sacrificing the efficiency.The TCO can block ion migrations and chemical reactions between the metal and perovskite,while the metal greatly enhances the conductivity of the composite electrode.As a result,composite electrode-PSCs achieved a power conversion efficiency(PCE)of 23.7%(certified 23.2%)and exhibited excellent stability,maintaining 95%of the initial PCE when applying a reverse bias of 4.0 V for 60 s and over 92%of the initial PCE after 1000 h continuous light soaking.This composite electrode strategy can be extended to different combinations of TCOs and metals.It opens a new avenue for improving the stability of PSCs.

Sparkling hot spots in perovskite solar cells under reverse bias

Perovskite solar cells(PSCs)are attracting much attention and are on the way to commercialization.However,some modules are subject to reverse bias in actual fields,so it is meaningful to investigate the reverse-bias behav-ior of PSCs.Herein,an in-situ temperature and current measurement technique was developed.Intriguingly,some hot spots were observed and then quickly disappeared in the reverse biased PSCs,along with the simultaneous increase in the current and local temperature.Also,the potential mechanism has been revealed and analyzed.An abnormal bulge in the perovskite film was found at the hot spot.Accordingly,the appearance and disappearance of hot spots were perfectly explained by band bending and tunneling current caused by ion accumulation.Addi-tionally,statistical analysis suggested that sparkling hot spots were related to reverse voltage and efficiencies of PSCs.The research provides a great significance for the study of PSCs under reverse bias.

Experimental research on the relationship between bypass diode configuration of photovoltaic module and hot spot generation

A hot spot is a reliability problem in photovoltaic(PV) modules where a mismatched or shaded cell heats up significantly and degrades the PV module output power performance. High PV cell temperature due to a hot spot can damage the cell encapsulate and lead to second breakdown, which both cause permanent damage to the PV module. In present systems, bypass diodes are used to mitigate the hot spot problem. In this work, five commercial polysilicon P V modules configured with different numbers of bypass diodes are used to study the influence of bypass diodes on the reverse bias voltage of a shaded cell and the resulting hot spot phenomenon. The reverse bias voltage of the shaded cell, and the hot spot probability and severity decrease as the number of bypass diodes increases. Negative terminal voltage of a shaded cell accompanied by a switched-off bypass diode are the necessary condition for hot spot generation. In an extreme case where each cell has an individual bypass diode in a P V module, it still cannot avoid the hazards of a hot spot under the shading areas of 5-7 cm2, but the probability of a hot spot is reduced to a minimum of 0.41%.

Spin-dependent tunneling of light and heavy holes with electric and magnetic fields

The spin-dependent tunneling of light holes and heavy holes was analysed in a symmetrical heterostructure with externally applied electric and magnetic fields. The effects of the applied bias voltage, magnetic field and reverse bias were discussed for the polarization efficiency of light holes and heavy holes. The current density of spin-up and spin-down light holes increases as the bias voltage increases and reaches the saturation, whereas the current density of spin-up heavy holes is almost negligible. The applied bias voltage and the magnetic field highly influence the energy of resonance polarization, polarization efficiency, and the current density of heavy holes more than for the light holes.