Scanning probe microscopy of solar cells: From inorganic thin films to organic photovoltaics

Scanning probe microscopy (SPM) has made significant contributions to our understanding of the sub-processes underlying photovoltaic action. These techniques allow local investigation of the electrical and optical properties of a material. Spatially resolved measurements of surface photovoltage and photocurrent have been particularly useful in understanding charge generation and separation. In thin-film inorganic solar cells, for example, Kelvin probe force microscopy (KPFM) has revealed that charge separation does not occur at a heterojunction as expected, but instead occurs at a homojunction buried ∼50 nm within the absorbing layer. In organic photovoltaics, submicron maps of photocurrent have contributed to the understanding of the interplay between processing conditions, blend morphology, and device performance. Such functional imaging distinguishes SPM from complementary structural characterization techniques. Our goal in this article is to provide the materials science community with an appreciation for the capabilities, considerations, and limitations associated with SPM studies of solar cell materials and devices. Highlighted techniques include scanning tunneling microscopy, photoconductive atomic force microscopy, near-field scanning optical microscopy, KPFM, and time-resolved electric force microscopy.