A New View of Microcrystalline Silicon: The Role of Plasma Processing in Achieving a Dense and Stable Absorber Material for Photovoltaic Applications

Abstract

To further lower production costs and increase conversion efficiency of thin‐film silicon solar modules, challenges are the deposition of high‐quality microcrystalline silicon (μc‐Si:H) at an increased rate and on textured substrates that guarantee efficient light trapping. A qualitative model that explains how plasma processes act on the properties of μc‐Si:H and on the related solar cell performance is presented, evidencing the growth of two different material phases. The first phase, which gives signature for bulk defect density, can be obtained at high quality over a wide range of plasma process parameters and dominates cell performance on flat substrates. The second phase, which consists of nanoporous 2D regions, typically appears when the material is grown on substrates with inappropriate roughness, and alters or even dominates the electrical performance of the device. The formation of this second material phase is shown to be highly sensitive to deposition conditions and substrate geometry, especially at high deposition rates. This porous material phase is more prone to the incorporation of contaminants present in the plasma during film deposition and is reported to lead to solar cells with instabilities with respect to humidity exposure and post‐deposition oxidation. It is demonstrated how defective zones influence can be mitigated by the choice of suitable plasma processes and silicon sub‐oxide doped layers, for reaching high efficiency stable thin film silicon solar cells.

Substrate and p-layer effects on polymorphous silicon solar cells

“…Since the growth of both μc-SiO:H and pm-Si:H requires a hydrogen rich plasma, an a-SiC:H p-layer was used in the case of Table 1, which did show a strong dependence on the TCO and p-layer type, thus suggesting that the surface roughness as well as the TCO/p-layer and p/i interface regions play very important roles in determining the overall cell performance. Similar results have been reported for μc-Si:H p-i-n devices and it was also demonstrated that the FTPS method is sensitive to bulk material properties but not to local shunts induced by TCO morphology [28].…”

Thin-film silicon triple-junction solar cell with 12.5% stable efficiency on innovative flat light-scattering substrate

“…18 Small amounts of carbon dioxide (CO 2 ) were added to the precursor gases of the n-and p-doped layers to incorporate oxygen during layer growth. These silicon oxide (SiO x ) doped layers are known to have two predominant effects in p-i-n solar cells: first, the incorporation of these layers leads to cells with V oc and FF values that are more tolerant to rough substrates and, second, these layers are more transparent than standard doped layers and therefore lead to less parasitic absorption.…”

“…24,25 An extended study of the material quality as a function of the growth conditions and oxygen content of the doped layers of p-i-n solar cells can be found in Ref. 18. The front contact was a 2.5-lm-thick borondoped LP-CVD ZnO layer.…”

“…This, in turn, avoids losses in FF and V oc which are often observed for high crystallinity or high growth rates. 18 Other routes for process time and layer thicknesses reduction could be the incorporation of an a-SiGe:H middle cell. 28,30 Finally, reflection and absorption losses due to the front contact can still be improved, either by reducing doping in the LP-CVD ZnO:B used in this contribution or using an ITO front contact coupled with double antireflection layers.…”

“…[3][4][5][6][7][8][9][10][11][12][13][14][15] However, most of the time, a rough surface induces structural defects in the device grown on top of it, leading to reduced opencircuit voltage (V oc ) and fill factor (FF). 12,[14][15][16][17][18] Therefore, a compromise between roughness for efficient light scattering (high J sc ) and smoothness for high V oc and FF has to be made. A different approach is to decouple the growth interface from the scattering interface.…”