Reducing VOC losses in bifacial (Ag, Cu)(In, Ga)Se2 solar cells

Bifacial solar cells are ideally suited to a wide range of application cases, owing to the superior yearly energy yield due to conversion of stray light reflected from the ground. To open these applications to the copper indium gallium selenide (CIGS) technology, we investigate the loss in performance and open-circuit voltage of bifacial CIGS solar cells to their counterparts on traditional opaque back contacts. Surprisingly, the absorber front interface and the cell performance are found to be very sensitive to the specifics of the rinsing step prior to the deposition of the buffer layer, in stark contrast to absorbers grown on metal Mo back contact. Comparison of photoluminescence quantum yield (PLQY), radiative voltage losses and cell V OC revealed that absorber rinsing step with ammonia solution leads to substantial voltage losses, partly mitigated by substituting the rinsing solution by DI water. These losses, attributed to non-radiative recombination at the absorber/buffer interface, were partially recovered through heat-light soaking treatments, yielding cell with voltage approaching the initial potential of the bare absorbers. We demonstrate a bifacial solar cell with an in-house front illumination efficiency of 19.9%, with improved back contact transparency as compared to the current state of the art with similar performance.