Light-induced degradation (LID) of amorphous silicon (a-Si:H) solar cells due to the Staebler-Wronski effect has been widely discussed in literature. However, it has been most often discussed with respect to the degradation of the intrinsic absorber layer.

In the present study, LID of a-Si:H solar cells is studied with respect to the amorphous silicon carbide (p-(a-SiC:H)) layer that is part of the window layer of high efficiency solar cells. We have deposited solar cell series varying the p-(a-SiC:H) thickness and the substrate roughness of single junction solar cells in superstrate configuration. The solar cell design is state-of-the-art using low-pressure chemical vapor deposition zinc-oxide for front and back contacts that are in contact with p- and n-doped silicon oxide layers. Plasma-enhanced chemical vapor deposition (PECVD, 40 and 13 MHz) has been used for all silicon layers, using a cluster tool with dedicated chambers for p-doped, intrinsic, and n-doped layers.

During light soaking, a systematic open-circuit voltage (Voc) increase could be observed for thin p-layers, while Voc decreases for thick p-layers. This effect is more pronounced for rough than for smooth substrates: The critical p-(a-SiC:H) thickness, at which light soaking has no effect on the Voc, increases with increasing substrate roughness. These Voc changes have a strong impact on the conversion efficiency of the solar cells. First, the optimum p-(a-SiC:H) thickness depends on the substrate roughness. Second, highest stabilized cell efficiencies are obtained using thinner p-(a-SiC:H) layers than what is optimum in initial state. Different contributions of short-circuit current, fill factor, and Voc to LID of the conversion efficiency are discussed. All trends could be reproduced using different cell designs in three different PECVD systems.

To discriminate the effect of effective p-layer thickness on rough substrates, the nominal thicknesses are corrected by the effective surface as determined from AFM measurements.

Different mechanisms could lead to the observed Voc changes. These are investigated by bias light and bias voltage dependent EQE measurements and by analyzing the degradation /annealing kinetics of the solar cells. The changes are related to layer properties as measured by ellipsometry, photothermal deflection spectroscopy, and conductivity. Finally, we will briefly discuss our latest tandem and triple junction solar cells where we incorporated these a-Si:H cells as top cells.

Michael Stuckelberger

Ecole Polytechnique F&#233;d&#233;rale de Lausanne

Appearances

Ecole Polytechnique Fédérale de Lausanne