Advanced Passivation Approaches for Thin Bifacial CIGSe Solar Cells

Bifacial Cu(In,Ga)Se2 (CIGSe) solar cells are an interesting new approach in thin-film photovoltaic technology, achieving a record efficiency of 19.7% with bifaciality exceeding 50% [1]. Further improvements in bifaciality and backside charge collection efficiency necessitate reducing the absorber layer thickness. However, by thinning the absorber, realizing the usual backside gallium grading becomes significantly challenging, requiring alternative strategies to mitigate recombination at the back contact. Our previous studies identified copper-doped GaOx as a highly effective hole-selective passivator for submicron monofacial CIGSe solar cells without gallium gradients [2]. Here, we explore the potential of extending this approach to bifacial configurations with transparent back contacts. Flat-gallium profile CIGSe absorbers were directly grown with thicknesses of 1.5 µm on indium tin oxide (ITO) and 1.1 µm on zirconium-doped indium oxide (IZrO) substrates. Photoluminescence (PL) analysis revealed that submicron-thick absorbers on ITO exhibited high PL quantum yield (YPL) (up to 2.1×10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup>), exceeding even the YPL of monofacial cells with opaque Mo back contact passivated by GaOx, obtaining 642 meV quasi-Fermi level splitting (QFLS), indicating effective passivation of the absorber backside. This passivation likely resulted from the formation of GaOx at the ITO/CIGSe interface that has been observed before. However, our bifacial cells with ITO back contacts displayed notable open-circuit voltage (VOC) deficits relative to QFLS of bifacial CIGSe solar cells on ITO, attributed to additional non-radiative recombination channels at the contacts, likely at the front interface. Additionally, we find very low PL emission from the transparent back side, which we demonstrate to be due to optical effects at the CIGSe/ITO interface. To address these limitations, IZrO was investigated as an alternative transparent back contact. Absorbers on IZrO exhibited minimal YPL loss from backside emission, indicating superior optical properties. Moreover, CIGSe solar cells on IZrO showed no additional VOC loss, and no blocking effect was observed, highlighting the excellent charge transport properties of the IZrO back contact. These results demonstrate that IZrO is a promising candidate for bifacial CIGSe devices, even under high-temperature CIGSe growth processes.