We investigated the influence of V-pits on the turn-on voltage of GaN-based high periodicity multiple quantum wells (MQWs) solar cells with different thickness of the p-GaN layer. Experimental current-voltage characteristics indicate that the sample with the thinnest p-GaN layer presents an early turn-on, which is not present for samples with a thicker p-GaN layer. Focusing on the V-defects analysis, through scanning electron microscopy (SEM) we found no difference in the density and dimensions of V-pits between sample with different p-GaN thickness. Through TCAD Synopsys Sentaurus simulations, the main non-illuminated current-voltage characteristics are reproduced considering V-defects. The results indicate that V-pits play a dominant role in current conduction, especially for the devices with the thinnest p-GaN layer due to the insufficient V-pit planarization. For such devices V-pits penetrate the junctions, and locally put the MQWs region in closer connection to the p-side contact, resulting in the formation of localized short circuit paths. Finally, a Gaussian distribution of V-pits dimensions and depth is considered to reach a good matching of experimental data. Based on combined electrical analysis, microscopy investigation and 2D simulations, results provide insight on the role of V-pits on the electrical performance of GaN-based MQWs solar cells. The outcome of this work will be useful for the design of future high-periodicity quantum wells devices, ensuring the desired turn-on voltage and showing the existence of a trade-off between the need of a thin p-GaN (to increase short-wavelength efficiency) and a thicker p-GaN, to avoid insufficient V-pit planarization.
GaN-based multi-quantum wells solar cells could be breakthrough devices for extreme applications like space environment and harsh photovoltaics. We performed a forward-bias stress on samples with 30 quantum wells with 15% indium content to better understand degradation mechanisms. DUTs were characterized by means of dark and illuminated IV, CV and Steady-State Photocapacitance measurements. First, we performed a step-stress experiment, by increasing current in 30 minutes steps up to 500 mA, when the device failed. We observed a variation in series resistance, a decrease in shunt resistance and a strong decrease in EQE, conversion efficiency and open-circuit voltage, especially at low excitation intensities. CV measurements showed an increase and then a decrease in free charge density in the device, that was correlated to the variation in trap-states density evaluated by SSPC. Based on the results of this step-stress experiment, we carried out a 100 hours constant-current stress at 40 mA. This stress showed a moderate decrease in series resistance and a decrease in EQE and conversion efficiency. An increase in charge density was observed and correlated with the decrease in conversion efficiency. The degradation of the device was related to the generation of defects, that may create a nearmidgap states, detected by SSPC measurement, and a shallow donor state that generates a change in free carrier density. These defects possibly migrate through the devices, as they are detected at different times by SSPC and CV measurements.
We investigate the degradation of high-periodicity GaN-based InGaN-GaN multiple quantum wells (MQWs) solar cells submitted to stress under high excitation intensity and high temperature; stress conditions are chosen to investigate cell behavior in a harsh scenario, such as wireless power transfer systems, space applications and concentrator harvesting systems. By examining the decrease in the short-circuit current and electroluminescence of these devices and the increase in the forward current at low bias, we suggest the presence of a thermally-activated diffusion process of impurities from the p-side of the device toward the active region. This process favors the increase in the Shockley-Read- Hall (SRH) recombination rate. By employing the van Opdorp and t’Hooft model, we analyzed the time-variation of non-radiative Shockley-Read-Hall lifetime during aging, extracting the diffusion coefficient of the defect involved in the degradation; we also extracted the related activation energy by an appropriate fitting of the degradation kinetics according to Fick’s second law of diffusion. The obtained values suggest that degradation originates from the diffusion of hydrogen, whose severity depends also on the thickness of the p-GaN layer of these devices. The proposed analysis methods and the obtained results are useful for understanding the physics of multiple quantum wells (MQWs) solar cells during degradation. The results can be used to increase the performance and reliability in novel applications where these devices are proposed, such us additional layer in multi-junction (MJ) solar cells, and the application in harsh environments.
GaN-based solar cells are promising devices for application in space environment, concentrator solar systems and wireless power transmission. Thus, it is essential to understand their degradation kinetics when submitted to high-temperature, high-intensity stress. We submitted GaN-based multiple-quantum-well solar cells with AlGaN electron-blocking-layer to two step-stress experiments at 35 °C and 175 °C in short-circuit condition under 405 nm monochromatic excitation by increasing optical power from 47 W/cm2 to 375 W/cm2. We found almost no degradation in the dark-IV, light-IV, electroluminescence and photocurrent characteristics after low-temperature stress, whereas the degradation after hightemperature stress was significant: we observed a lowering in power-conversion efficiency, a decrease in open-circuit voltage and an increase in low forward bias current. We then submitted the device to several constant power stress at 180 W/cm2 for 100 hours at 95 °C, 135 °C and 175 °C. We found that, by increasing the temperature, the short-circuit current during the stress decreased of 7%, 9% and 12.5% respectively. Dark IV characteristics showed an increase in low-forward bias current stronger at 175 °C. We also found a higher decrease in open-circuit voltage, external quantum efficiency, power conversion efficiency and electroluminescence with higher stress temperature. The causes of degradation are possibly diffusion mechanisms, which increase defect density in the p-GaN bulk region and/or in the GaN barrier region, promoting trap-assisted tunneling mechanisms, leading to the decrease in open-circuit voltage, and non-radiative recombination mechanisms, that cause the drop in quantum efficiency.
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