(Invited) Operando Measurement of Energy Band Alignment and Built-in Potential in Thin-Film Photovoltaic Devices

The energy band alignment in thin-film or low-dimensional devices is extremely important for charge separation, carrier transport and collection, as well as undesirable recombination. However, experimental determination of the energy level alignment and the quantitative measurement of related parameters, such as the built-in potential (V_bi), have been difficult to realise, especially under typical operating conditions for the devices. Indirect methods, such as Mott-Schottky analysis, electroabsorption spectroscopy, and dark current density-voltage (J-V) curve fitting, have been applied to measure the V_biin solar cells, but the quantitative information has been inaccurate or inconsistent.

Recently, SKPM has been used to image cross sections of solar cells to probe the SP depth profile across multiple layers. The vacuum level (VL) in an energy band structure is defined as the energy of an electron resting right at the surface of a material; thus, the VL alignment within a device can be obtained by multiplying the SP depth profile by a constant, i.e., the electron charge. In principle, this approach offers the advantage of being able to directly visualise the energy band alignment across the device. However, a major obstacle has limited the application of this approach in solar cell research: quantitatively accurate measurements and interpretations have yet to be realised in cross-sectional SKPM studies. Critical parameters for the operating mechanism of solar cells, such as the open-circuit voltage (V_oc) and V_bi, are often found to be much smaller than expected from J-V characterisation. Overall, the results obtained from cross-sectional SKPM measurements are often uncorrelated with the device performance and J-Vcharacteristics and are thus not applicable for understanding practical devices in their actual operating states.

In this presentation, we address the challenges in quantitative SKPM potentiometry in low-dimensional devices using organic photovoltaic (OPV) cells as model systems. Cross sections of OPV devices are exposed by using ion beam milling; and the SP depth profiles of the OPV device under operating conditions are directly visualised by SKPM imaging on the cross section. The tip/cantilever convolution effect due to finite tip size and cantilever beam cross-talk is identified as the source of the systematic underestimate of SP differences in cross-sectional SKPM measurements. A bias compensation method is developed to obtain quantitative measurements of internal potential difference parameters, such as V_oc, V_td and V_bi, in OPV devices. This method is generally applicable to other thin-film and low-dimensional devices.