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

Many solar cells incorporating SiNx 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 SiNx films, the loss in short-circuit current can be reduced drastically. Besides a lower short- circuit current, dark I-V curves of SiNx rear surface passivated solar cells exhibit distinct shoulders. The results are explained by parasitic shunting of the induced floating junction (FJ) underneath the SiNx films with the rear metal contacts. The floating junction is caused by the high density of fixed positive charges in the SiNx, films. Other two-dimensional effects arising from the injection level dependent SRV of the Si/SiNx interfaces are discussed as well, but, are found to be of minor importance. Pinholes in the SiNx 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 SiNx rear surface passivated cells. Copyright (C) 2002 John Wiley Sons, Ltd.