Abstract
The non-toxic and wide bandgap material TiO2 is explored as an n-type buffer layer on p-type Cu(In,Ga)Se2 (CIGS) absorber layer for thin film solar cells. The amorphous TiO2 thin film deposited by atomic layer deposition process at low temperatures shows conformal coverage on the CIGS absorber layer. Solar cells from non-vacuum deposited CIGS absorbers with TiO2 buffer layer result in a high short-circuit current density of 38.9 mA/cm2 as compared to 36.9 mA/cm2 measured in the reference cell with CdS buffer layer, without compromising open-circuit voltage. The significant photocurrent gain, mainly in the UV part of the spectrum, can be attributed to the low parasitic absorption loss in the ultrathin TiO2 layer (~10 nm) with a larger bandgap of 3.4 eV compared to 2.4 eV of the traditionally used CdS. Overall the solar cell conversion efficiency was improved from 9.5% to 9.9% by substituting the CdS by TiO2 on an active cell area of 10.5 mm2. Optimized TiO2/CIGS solar cells show excellent long-term stability. The results imply that TiO2 is a promising buffer layer material for CIGS solar cells, avoiding the toxic CdS buffer layer with added performance advantage.
Highlights
Our results demonstrate that TiO2 is a promising candidate to successfully substitute the toxic CdS buffer layer
TiO2 thin films were deposited on printed nanoparticle based CIGS absorber layers by using the atomic layer deposition (ALD) technique
As n-type buffer layer, either CdS or TiO2 is deposited by chemical bath deposition or ALD, respectively, followed by sputtering of the transparent electron contact indium tin oxide (ITO)
Summary
To optimize the device performance, the influence of the TiO2 deposition temperature and thickness on Voc and short-circuit current density (Jsc) were investigated (Fig. 3(a,b), respectively). A further temperature increase leads to both Voc and Jsc drop to as low as 385 mV and 34.7 mA/ cm[2] at 180 °C, respectively As it was found earlier by Yin et al.[21] the TiO2 film deposited at 120 °C is in an amorphous phase and shows a smooth morphology. We conclude that the ultrathin amorphous TiO2 layer is a promising candidate for the application in high efficiency CIGS thin film solar cells to further boost their performance
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