Experimental Evidence of Parasitic Shunting in Silicon Nitride Rear Surface Passivated Solar Cells

摘要

Many solar cells incorporating SiN_x films as a rear surface passivation scheme have not reached the same high level of cell performance as solar cells incorporating high-temperature-grown silicon dioxide films as a rear surface passivation. In this paper it is shown by direct comparison of solar cells incorporating the two rear surface passivation schemes, that the performance loss is mainly due to a lower short-circuit current while the open-circuit voltage is equally high. With a solar cell test structure that features a separation of the rear metal contacts from the passivating SiN_x films, the loss in short-circuit current can be reduced drastically. Besides a lower short- circuit current, dark I-V curves of SiN_x rear surface passivated solar cells exhibit distinct shoulders. The results are explained by parasitic shunting of the induced floating junction (FJ) underneath the SiN_x films with the rear metal contacts. The floating junction is caused by the high density of fixed positive charges in the SiN_x films. Other two-dimensional effects arising from the injection level dependent SRV of the Si/SiN_x interfaces are discussed as well, but, are found to be of minor importance. Pinholes in the SiN_x films and optical effects due to a different internal rear surface reflectance can be excluded as a major cause for the performance loss of the SiN_x rear surface passivated cells.