2018
In this optical study, we looked at a key challenge for ultra-thin CIGS solar cells: how to reduce material use without giving away too much light. CIGS is a high-performing thin-film photovoltaic material, but thinning its absorber to below one micrometre makes it harder to capture light and increases the importance of losses at the rear of the device. The molybdenum back contact is especially relevant because it is not strongly reflective and can absorb long-wavelength light that would be more useful in the active CIGS layer.
We used calibrated three-dimensional optical modelling to investigate the molybdenum/CIGS interface. The modelling linked losses there to plasmonic waves, then let us test a way of reducing them: putting a dielectric spacer between the metal and the semiconductor. The aim was to change the optical conditions at the rear so that more long-wavelength light returns to the absorber instead of disappearing into molybdenum.
We first tested an ideal, lossless spacer with different refractive indices and thicknesses. A low refractive index at the right thickness could reduce molybdenum absorption and improve rear reflection. We then moved to a realistic two-layer design: magnesium fluoride on the molybdenum side and ultrathin aluminium oxide next to CIGS. Magnesium fluoride provides the desired low refractive index, while aluminium oxide can passivate the rear surface and may support a higher open-circuit voltage.
To keep the work grounded, we calibrated the model against fabricated TNO cells, comparing simulated absorption and reflection with measured external quantum efficiency and reflectance. We also optimised local point contacts, which are needed to collect carriers through a passivating dielectric layer.
For a 750-nanometre CIGS absorber, the combined spacer and point-contact design increased implied photocurrent density by ten percent between 700 and 1,150 nanometres compared with the reference cell. The gain comes from removing plasmonic resonances and strengthening the electric field near the bottom of the CIGS layer. Our work shows how careful interface engineering can make thin-film photovoltaics more material-efficient without sacrificing optical performance.