Abstract
Author(s): Werner, F; Veith-Wolf, B; Spindler, C; Barget, MR; Babbe, F; Guillot, J; Schmidt, J; Siebentritt, S | Abstract: Copper-indium-gallium-diselenide (CIGS) thin-film solar cells suffer from high recombination losses at the back contact and parasitic absorption in the front-contact layers. Dielectric passivation layers overcome these limitations and enable an efficient control over interface recombination, which becomes increasingly relevant as thin-film solar cells increase in efficiency and become thinner to reduce the consumption of precious resources. We present the optoelectronic and chemical interface properties of oxide-based passivation layers deposited by atomic layer deposition on CIGS. A suitable postdeposition annealing removes detrimental interface defects and leads to restructuring and oxidation of the CIGS surface. The optoelectronic interface properties are very similar for different passivation approaches, demonstrating that an efficient suppression of interface states is possible independent of the metal used in the passivating oxide. If aluminum oxide (Al2O3) is used as the passivation layer we confirm an additional field-effect passivation due to interface charges, resulting in an efficient interface passivation superior to that of a state-of-the-art cadmium-sulfide (CdS) buffer layer. Based on this chemical interface model we present a full-area rear-interface passivation layer without any contact patterning, resulting in a 1% absolute efficiency gain compared to a standard molybdenum back contact.
Highlights
Photovoltaic (PV) technologies directly convert sunlight into electricity and form a central part of a stable and clean energy supply based on renewable energy sources
Our findings provide key perspectives to interface passivation in CIGS solar cells: 1. It is mostly the oxygen that provides a large part of the beneficial effect of Al2O3—or other oxide-based dielectric passivation layers—meaning that the actual material used for the passivation layer is less relevant
We compare this data to the quasi-Fermi-level splitting (QFLS) deficit of CIGS passivated with a thin layer of amorphous TiO2 deposited by thermal atomic layer deposition [24] (ALD) (30 ALD cycles, approximately 2 nm, same annealing conditions as Al2O3), which results in a passivation quality comparable to CdS
Summary
Photovoltaic (PV) technologies directly convert sunlight into electricity and form a central part of a stable and clean energy supply based on renewable energy sources. The excellent passivation quality of Al2O3 on c-Si, for example, is due to a high negative fixed-charge density near the c-Si/Al2O3 interface, which results in a depletion of electrons from the passivated interface (“field-effect passivation”) and a low interface defect density (“chemical passivation”) [26,34] These beneficial properties depend strongly on interface chemistry, which is very different between CIGS and Si. insulating dielectric passivation layers in polycrystalline CIGS devices require large-area patterning on a scale of a few tens of nanometers [35,36,37] to open conductive pathways, a factor of at least 1000 smaller than Si-PV, necessitating alternative approaches for industrial exploitation. Contact patterning is not required, and full-area passivation layers provide an efficiency boost with a very simple device process
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