Over 19% Efficiency Scope For IBC CIGS Cells

An optimal geometry and engineered bandgap grading can boost the power conversion efficiency of a new interdigitated back-contacted (IBC) structure up to 17%. Further, with a reasonably low defect density in the absorber layer, the efficiencies can go up as high as 19%. Additionally, with a reasonably low defect density in the absorber layer, efficiencies can as well hit 19.7%.

These claims are made by the Netherlands’ based Delft University of Technology researchers in their research work titled Interdigitated back-contacted structure: A different approach towards high-efficiency ultrathin copper indium gallium (di)selenide solar cells. The work has been published by Progress in Photovoltaics.

Copper indium gallium (di)selenide (CIGS) is a good absorber material for highly efficient thin-film solar cell applications thanks to high absorption coefficient and tuneable bandgap. But this material suffers due to optical losses and other performance deteriorations, the researchers claim, especially in the front/back-contacted (FBC) structure, these suffer optical losses due to parasitic absorption of the top layers. The optical performance of flexible CIGS solar cells reduces further due to the metallic grid causing an additional optical shading.

The researchers see an IBC solar cell structure having a submicron thickness of 673 nm as helping avoid these optical losses and boosting the efficiency of a CIGS solar cell by preventing parasitic absorption of the front layers. They modeled this solar cell architecture using a 2-D opto-electrical simulations for both the FBC and IBC solar cell structures.

They deployed FBC cells fabricated in Solliance Solar Research Institute for their research.

The team placed a silver reflector at the rear side to reflect the photons from the rear side for a second absorption chance. A gallium-doped zinc-oxide (GZO) type-a was chosen for its high thermal stability and low free carrier absorption. They found at the front side, the presence of negative fixed charges induced an electric field, preventing the accumulation of electrons at the CIGS/Al₂O₃ interface (field effect passivation). Research showed wider the transparent conductive oxide (TCO), the less the electrical shading and the higher efficiency.

“We showed how an IBC structure with optimal bandgap grading and high absorber quality can help us achieve high efficiencies with submicron CIGS layers. Indeed, better optical performance is still possible by, for example, high aspect ratio ARC.” Researchers claim. “Although the proposed structure needs to answer many fabrication challenges, including potentially costly patterning steps, it can pave the way towards high‐efficiency thin‐film CIGS solar cells and their deployment in three‐ and four‐terminal tandem devices.”