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

Abstract

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 © 2002 John Wiley & Sons, Ltd.

“Zero-charge” SiO2/Al2O3 stacks for the simultaneous passivation of n+ and p+ doped silicon surfaces by atomic layer deposition

“…Secondly, besides affecting the surface passivation, inversion layers potentially act as undesired conduction pathways to metal contacts. Such a pathway (sometimes referred to as parasitic shunting) affects the performance of solar cells via the fill-factor, short- circuit current and open-circuit voltage [16]. All of these unfavorable effects occur not only when n or n þ -type Si surfaces are passivated by the negative charge dielectric Al 2 O 3 , but they also take place when p or p þ type Si surfaces are passivated by a dielectric containing a positive charge density, such as SiN x .…”

Effective passivation of Si surfaces by plasma deposited SiOx/a-SiNx:H stacks

“…While generally providing a high level of passivation for p-type and ͑diffused͒ n-type Si surfaces, 1-5 a-SiN x :H films ͑SiN x , in brief͒ can lead to compromised solar cell performance when applied on the p-type Si base at the rear side. This is related to the presence of an inversion layer below the surface induced by the fixed positive charge density, Q f , present in ͑nearly͒ stoichiometric SiN x ͑with refractive index, n = ϳ 1.9-2.2͒, 5 which can lead to current flow between the rear metal contacts. This effect is known as parasitic shunting.…”

“…This effect is known as parasitic shunting. 5 The positive charges in SiN x originate from the so-called K centers, which are dangling bonds of Si atoms that are backbonded to three N atoms. [6][7][8] Therefore, Si-rich SiN x films generally exhibit a lower Q f than ͑nearly͒stoichiometric N-rich films.…”

Influence of a-Si:H deposition power on surface passivation property and thermal stability of a-Si:H/SiNx:H stacks

“…In this study, the MW power for a-Si:H deposition was varied from 100W to 300W with increments of 50W, covering the microwave power ranging from the level which can generate the plasma effectively to the level of dust being produced, while other parameters were kept constant. Immediately after the deposition of the underlying a-Si:H layer, the temperature of the chamber was increased to 400 • C, followed by the deposition of the capping SiNx:H layer on-top of the a-Si:H layer directly using precursor gases of SiH 4 and NH 3 , with all other parameters for the various samples kept constant.…”

“…The appeal of the a-Si:H is due primarily to its extremely low SRV and absence of parasitic shunting caused by positive-charge-induced inversion layers often encountered in silicon nitride passivated rear surfaces a E-mail: hua.li@student.unsw.edu.au when non-ideal metal contacts are used. 3 However the instability of a-Si:H passivating layers under certain thermal treatments such as required for metal firing processes for industrial solar cells has hindered the industrial application of a-Si:H. 4,5 The surface passivation by an a-Si:H layer has been reported to suffer significant degradation at temperatures higher than 300 • C, attributed to the out-effusion of the hydrogen from the a-Si:H passivating layer 6 to the ambient. Thus the critical issue for achieving and preserving excellent surface passivation on the rear surface with a-Si:H is to improve the thermal stability of the passivating layer when exposed to certain thermal processes such as required for annealing or firing processes which are necessary for cell manufacturing to form front or rear localized metal contacts through such a passivating layer.…”