Atomic force microscopy (AFM) is a versatile tool for mapping the surface of a material. Aside from the conventional AFM techniques that most scientists are accustomed to, there are also a series of lesser-known AFM variations. One such variation is photoconductive AFM (pc-AFM), which has been specifically designed using basic principles of AFM to measure the photoconductivity of a surface. In this article, we look at how pc-AFM works and how it has found its niche in helping to determine the nano-electrical properties of photovoltaic (solar) cells.
What is Photoconductive AFM (pc-AFM)?
As the name suggests, photoconductive AFM (pc-AFM) is a variation of AFM that has been specifically designed to measure the photoconductivity of a surface. Compared to other types of high-resolution microscopy, it is a relatively new technique. Given that there are limited applications for measuring photoconductivity, it is of no surprise that pc-AFM is more widely used in the analysis of solar cell devices and materials.
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Out of all the many types of solar cells, pc-AFM has found the most use with organic solar cells (otherwise known as organic photovoltaic cells, or OPVs) as it can be used with softer materials. This form of AFM is known for having a high image resolution and can characterize electrical properties at the nanoscale. Even though it could be seen as a somewhat niche market, the photovoltaics (PV) market is one of the most rapidly advancing renewable energy sectors, so it is perhaps a technique that will find more and more use as photovoltaic technology develops. Additionally, given that various nanomaterials are becoming more commonplace in solar cells, and most photovoltaic junctions being produced nowadays benefit from the use of nanomaterials, a technique that can accurately measure nanoscale properties will only gain more traction as the field progresses.
How pc-AFM Works
Pc-AFM works in a very similar fashion to standard AFM instruments and conductive AFM (C-AFM). The main difference between pc-AFM and AFM is that pc-AFM uses an inverted microscope objective as the source of illumination. Metal-coated silicon tips are always used with pc-AFM, as they enable a current to be effectively transmitted. As with any AFM technique, locating where the photocurrents are present on the surface of a material is done via the deflection of a laser beam off the back side of the cantilever and onto a position-sensitive photodiode (PSPD).
Light in the visible region of the electromagnetic spectrum is emitted from underneath the sample. This light often passes through an indium tin oxide (ITO) layer before reaching the sample. As the tip scans the surface, a current is passed through it and this becomes the anode. The ITO layer underneath the sample subsequently becomes the cathode, and this creates a complete electrical circuit. As the tip scans the surface, it measures the photocurrent generated by the material (which is photo-responsive). Because the measurements can be deduced at the nanoscale, the performance of the bulk solar cell can be generated by correlating these measurements.
This approach can also be used to see how the separation of donor and acceptor molecules occurs in the photoactive layer of the cell. In these circumstances, the holes are collected by the conductive AFM tip and the electrons migrate to the ITO cathode. This process becomes reversed when an electrical bias is applied at a value below the open-circuit voltage of the material, and the electrons move to the tip and the holes move to the ITO layer. The photocurrent images at both positive and negative electrical biases can then be used to determine how the donor and acceptor molecules behave.
Information That Can Be Gathered
Even though its overall application sector is niche; within the field of PVs, pc-AFM can be a powerful tool for deducing a significant amount of properties exhibited by a solar cell. As mentioned, the properties can be deduced down to the nanoscale level, and examples of the information that can be obtained using pc-AFM include: the mapping of photocurrents (including variations within the photocurrent); mapping the photoactive regions of a material; deducing the film morphology (and the differences between films) of a material; identifying the donor-acceptor domains within a junction; determining the current density-voltage plots; determining the quantum efficiency and power conversion efficiencies of a device; and identifying the concentration of charge carriers within a material or device.
Advantages Over Other Methods
There have been many methods throughout the years, prior to the creation of pc-AFM, that have been tried and tested to measure the localized electronic properties of PVs. Some of these include well-known techniques such as conventional AFM methods, transmission electron microscopy (TEM) and scanning transmission X-ray microscopy (STXM). While these methods can effectively map the topography of the surface, they have been ineffective in measuring the electrical capabilities of PVs. Many other types of AFM, such as conductive AFM (C-AFM), scanning Kelvin probe microscopy (SKPM) and Electrostatic force microscopy (EFM) can be used to deduce some of the electrical properties of PVs, but none of them have the scope that pc-AFM has when it comes to these specific devices and materials. The main area that sets pc-AFM apart from the other AFM variants, at least where PVs are concerned, is the ability to map photocurrents, photoactive regions of a material, photovoltaic devices and to deduce features that are below the surface.
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