Imaging interfacial layers and internal fields in nanocrystalline junctions

Nanotechnology will likely play a large role in developing future-generation solar photoconversion concepts. Thus, improved resolution or new techniques with the ability to characterize electronic properties of exceptionally small features could greatly aid device design. For example, photovoltaic devices with conductive films of colloidally synthesized PbSe quantum dots (QDs) possess external quantum efficiencies in the blue region of the solar spectrum greater than 100% due to multiple exciton generation (MEG) (where a high-energy photon can produce multiple electron-hole pairs) (1). This greatly motivates continued research on this type of solar cell that has the potential to achieve >40% power conversion efficiency (2). The state-of-the-art (highest overall efficiency) optimized structure for lead chalcogenide QD solar cells uses a variety of interfacial layers that play an important role in the device functionality. The p-n heterojunction (3) model is often used to describe the operation of QD solar cells despite the complex electronic structure of a disordered array of QDs acting as a macroscopic thin-film semiconductor (4). Advancements in device efficiency could follow better understanding of energetics along interfaces, throughout coupled films, and within individual nanostructures. Atomic force microscopy (AFM) techniques offer exceptional spatial resolution that can resolve such properties within devices and individual structures. Scanning Kelvin probe microscopy (SKPM) is one such technique that can accomplish these goals. We have correlated the contact potential difference between a conductive AFM tip and the layers within an operating colloidal QD solar cell with device cross-section exposed. SKPM can also be used on isolated nanostructures to visualize regions of localized band bending and space charge.