Charge-carrier-concentration inhomogeneities in alkali-treated Cu(In,Ga)Se₂ revealed by conductive atomic force microscopy tomography

Photovoltaic power conversion using polycrystalline light-absorbing semiconductors enables low-cost electricity generation. Cu(In,Ga)Se₂ (CIGS) are among the best performing thin-film solar cells with notable recent improvements upon an alkali-fluoride (AlkF) post-deposition treatment (PDT). Here we show that the success of this treatment can be hampered by spatial inhomogeneities in the conductivity. We apply an emerging conductive atomic force microscopy (C-AFM) tomography technique and obtain three-dimensional conductivity maps, enabling imaging of the carrier concentration grain by grain on the submicrometre scale. We find that a solar cell with KF PDT shows a stronger inhomogeneity of charge-carrier concentration, while RbF and CsF lead to narrow distributions at higher charge-carrier concentrations. The CIGS charge-carrier concentration and its homogeneity influence directly the open-circuit voltage of solar cells, thereby impacting device performance. Our insights support the development of higher efficiency thin-film photovoltaics through optimized AlkF PDTs. Moreover, the C-AFM tomography method is widely applicable to energy materials.