Experimental evidence of parasitic shunting in silicon nitride rear surface passivated solar cells

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.