The silicon vertical multi-junction （VMJ） solar cell has a good potential in high concentration, but it requires high quality front and back surface passivation layers to keep its high efficiency. We try to add dopants into the front and back surfaces of the VMJ cell to release this strict requirement in this work. The effects of recombination velocities, doping types and doping pro- files of front and back surfaces on the performance of the P-type VMJ cell were calculated under 1 sun and 1000 suns. The 2D numerical simulation tool TCAD software was used. The performance of the VMJ cell without front and back surface dopants was also calculated for comparison. It was found that the requirement of high quality front and back surface passivation layers could be released remarkably by adding either N-type or W-type front and back surface dopants. For the two types of front surface dopants, the highest efficiencies of the cells were got by light dopant; for the two types of back surface dopants, the doping type and profile affected little on the performance of the cell in our calculation range. It was also found that the series resistance of the VMJ cell with N-type front surface dopant was decreased by the 2D effect of front surface emitter. The VMJ cell with W-type front surface dopant had the highest efficiency under 1000 suns and the VMJ cell with N-type front surface dopant had the highest efficiency under 1 sun in our calculation range.
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%.
Finger interruptions are common problems in screen printed solar cells, resulting in poor performance in efficiency because of high effective series resistance. Electroluminescence（EL） imaging is typically used to identify interrupted fingers. In this paper, we demonstrate an alternative method based on photoluminescence（PL） imaging to identify local series resistance defects, with a particular focus on finger interruptions. Ability to detect finger interruptions by using PL imaging under current extraction is analyzed and verified. The influences of external bias control and illumination intensity on PL images are then studied in detail. Finally, in comparison with EL imaging, the using of PL imaging to identify finger interruptions possesses the prominent advantages： in PL images, regions affected by interrupted fingers show higher luminescence intensity, while regions affected by recombination defects show lower luminescence intensity. This inverse signal contrast allows PL imaging to more accurately identify the defect types.
