Abstract
Amorphous tin-gallium oxide (a-SGO) grown with atomic layer deposition was evaluated as a buffer layer in (Ag,Cu)(In,Ga)Se2 thin-film solar cells in search for a new material that is compatible with a variety of absorber band gaps. Hard and soft X-ray photoelectron spectroscopy on absorber/a-SGO stacks combined with J–V characterization of solar cells that were fabricated, showed that the conduction band alignment at the absorber/a-SGO interface can be tuned by varying the cation composition and/or growth temperature. Here, the surface band gap was 1.1 eV for the absorber. However, optical band gap data for a-SGO indicate that a suitable conduction band alignment can most likely be achieved even for wider absorber band gaps relevant for tandem top cells. A best efficiency of 17.0% was achieved for (Ag,Cu)(In,Ga)Se2/a-SGO devices, compared to η = 18.6% for the best corresponding CdS reference. Lower fill factor and open-circuit voltage values were responsible for lower cell efficiencies. The reduced fill factor is explained by a larger series resistance, seemingly related to interface properties, which are yet to be optimized. Some layer constellations resulted in degradation in fill factor during light soaking as well. This may partly be explained by light-induced changes in the electrical properties of a-SGO, according to analysis of Al/SGO/n-Si metal-oxide-semiconductor capacitors that were fabricated and characterized with J–V and C–V. Moreover, the introduction of a 1 nm thick Ga2O3 interlayer between the absorber and a-SGO improved the open-circuit voltage, which further indicates that the absorber/a-SGO interface can be improved.
Highlights
Thin film solar cells are attractive for photovoltaic power generation
We introduced atomic layer deposition (ALD) Amorphous tin-gallium oxide (a-SGO) as a buffer layer in RbF postdeposition treated (Ag,Cu)(In,Ga)Se2 (ACIGS-RbF) solar cells and eval uated its potential as a new buffer layer material
Conformal and homogeneous a-SGO buffer layers were successfully grown on ACIGS absorber surfaces by ALD
Summary
Thin film solar cells are attractive for photovoltaic power generation. Partly because of a potentially lowered production cost, and due to the compatibility with flexible lightweight substrates and architectural integration. A few different buffer layers with variable conduction band minimum (CBM) energies have been evaluated for use in CIGSbased solar cells [3]. One potential drawback of Zn1-xSnxOy is that its band gap is sensitive to several ALD process parameters, such as growth temperature and flow conditions. This is possibly correlated with microstructural variations observed in these films [8]. Aiming to develop a new buffer-layer material with a highly variable band gap and new interface chemistry, we recently developed an ALD process for amorphous Sn1-xGaxOy (a-SGO) [9]. The effect of light soaking on the electrical properties of a-SGO was investigated by fabricating Al/a-SGO/n-Si metal-oxide-semiconductor capacitor (MOS CAP) structures, which were characterized by means of J–V and capacitance–voltage (C–V) measurements
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