We report our progress in the optimization of Ag/ZnO back reflectors (BR) for a-Si:H and nc-Si:H solar
cells. Theoretically, a BR with a smooth metal surface and a textured dielectric surface would be more desirable. A
smooth metal/dielectric interface reduces the plasmonic resonance loss and parasitic losses due to light trapped in
sharp angles; a textured dielectric/semiconductor interface provides scattering for light trapping. In order to obtain
sufficient light scattering at the ZnO/silicon interface, a highly textured ZnO layer is normally used. However, a
highly textured ZnO surface causes deterioration of nc-Si:H material quality. In addition, to make a highly textured
ZnO surface, a thick ZnO layer is needed, which could introduce additional absorption in the bulk ZnO layer and
reduce the photocurrent density. Therefore, Ag/ZnO BR structures for nc-Si:H solar cells needs to be optimized
experimentally. In this study, we found that an optimized Ag/ZnO BR for nc-Si:H solar cells is constructed with
textured Ag and thin ZnO layers. Although a textured Ag layer might cause certain losses resulting from plasmonic
absorption, the enhanced light scattering by a moderately textured Ag layer makes it possible to use a thin ZnO
layer, where the absorption in the ZnO layer is low. With such a BR, we achieved a short-circuit current density of
over 29 mA/cm2 from a nc-Si:H single-junction solar cell. Using the high performance nc-Si:H cell in an a-Si:H/nc-
Si:H/nc-Si:H triple-junction structure, we achieved an initial active-area efficiency of 14.5% with a total current
density exceeding 30 mA/cm2.
Transport properties are very important for solar cells. The efficiency of solar cells is determined by the
competition of carrier collection and recombination. The most important parameter is the carrier mobility-lifetime
product. However, methods commonly used for measuring transport parameters require specially designed samples.
The results are often not easily correlated to solar cell performance. In this paper, we present our studies of
extraction of material properties from conventional current-voltage characteristics and quantum efficiency curves.
First, we carried out analyses of shunt resistance as a function of the light intensity. For solar cells with no clear
parasitic shunt resistance, the shunt resistance is inversely proportional to the short-circuit current, and its
proportionality coefficient is related to the effective carrier mobility-lifetime product. For an a-Si:H solar cell made
under an optimized condition with high hydrogen dilution, the effective mobility-lifetime product was estimated to
be 1.2x10-8 cm2/V. For a-SiGe:H solar cells, the effective mobility-lifetime product depends on Ge content. For
optimized a-SiGe:H bottom cells used in high efficiency a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structures, their
values are ~5.0x10-9 cm2/V. For high efficiency nc-Si:H solar cells, the effective mobility-lifetime product is
~5.0x10-7 cm2/V. Second, we measured the quantum efficiency as a function of electrical bias and developed an
analytical model to deduce the effective mobility-lifetime product. The results obtained from the second method are
consistent with the values from the first method. We will present detailed analyses and interpretations of the
transport parameters and their correlation to solar cell performance.
Conference Committee Involvement (4)
Thin Film Solar Technology IV
12 August 2012 | San Diego, California, United States
Thin Film Solar Technology III
21 August 2011 | San Diego, California, United States
Thin Film Solar Technology II
1 August 2010 | San Diego, California, United States
Thin Film Solar Technology
2 August 2009 | San Diego, California, United States
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