14 Feb 2021

Reflective back contacts for ultrathin CIGSe solar cells

by | posted in: News | 0

We have designed ohmic and reflective back contacts (RBC) for ultrathin CIGSe solar cells. They are made of multilayer stacks compatible with the direct deposition of CIGS at 500°C and above. Diffusion mechanisms and reactions at each interface are carefully analyzed. The best ultrathin CIGS solar cell on RBC exhibits an efficiency of 13.5% (+1.0% as compared to our Mo reference) with a short-circuit current density of 28.9 mA/cm2 (+2.6 mA/cm2) enabled by double-pass absorption in the 510 nm thick CIGS absorber.

RBC are easy to fabricate and could benefit other photovoltaic devices that require highly reflective and conductive contacts subject to high temperature processes. These results were published in the January issue of Progress in Photovoltaics.

Scanning electron microscopy (SEM) cross-section images of ultrathin CIGS layers coevaporated on a reflective back contact (RBC), and EQE of ultrathin CIGS solar cells with RBC and conventional Mo back contacts.

Cu(In,Ga)Se2-based (CIGS) solar cells are one of the most promising thin-film photovoltaic technologies with recent record efficiencies above 23% and thicknesses between 2 and 3 µm. Thinning down the absorber to 500 nm or less is a promising way to maintain low costs for thin-film solar cells and modules. However, ultrathin CIGS solar cells suffer from the low reflectivity and high surface recombination velocity of conventional Mo back contacts. Various back contact architectures have been investigated in the literature in order to enhance absorption in ultrathin CIGS layers. However, solutions that fulfill all the requirements are still missing.

In this work, we have investigated RBC made of a reflective silver mirror encapsulated in ZnO:Al layers, and of a top layer of In2O3:Sn (ITO) as a back contact with CIGS. In this work, the CIGS/RBC interface was thoroughly investigated by transmission electron microscopy in scanning mode (STEM) coupled with energy dispersive X-ray (EDX) spectroscopy. The CIGS composition grading close to its back interface was examined together with the diffusion and chemical reaction of elements from each layer. The best ultrathin CIGS solar cell on RBC exhibits an efficiency of 13.5% (+1.0% as compared to our Mo reference) with a short-circuit current density of 28.9 mA/cm2 (+2.6 mA/cm2) enabled by double-pass absorption in the 510 nm thick CIGS absorber.

These results pave the way toward the implementation of novel light trapping architectures in ultrathin CIGS solar cells, and could benefit other thin-film photovoltaic devices that require highly reflective and conductive contacts subject to high temperature processes.

These results were obtained in the framework of the ARCIGS-M H2020 european project (coord. Marika Edoff), with the strong involvement of Louis Gouillart during his PhD work, in close collaboration with UMR IPVF and Uppsala University.

References: