A new technique for subsurface characterization of thin-film photovoltaic materials

Although the performance of many materials used in devices is determined by microscale and nanoscale structures and buried interfaces hidden up to a few micrometers below the material surface, most commonly used nanoscale characterization tools are only sensitive to phenomena either at the surface or just a few nanometers below it. As a step towards addressing the increasing need for subsurface characterization with nanoscale spatial resolution, researchers from the CNST and the University of Maryland have demonstrated a new approach for probing the interior of photovoltaic devices. The technique uses a scanning probe microscope to control a tapered optical fiber with a nanoscale aperture that transmits laser light in order to locally excite a subsurface volume of a photovoltaic device. The photocurrent generated by the cell is then measured using a low-noise amplifier. By varying the wavelength of the laser light in the optical fiber, the penetration depth of the light can be readily changed from approximately 100 nm to more than 3 μm, allowing different volumes of the device to be excited. However, deeper penetration results in lower spatial resolution. Therefore, for comparison, the researchers used a focused ion beam available to users in the CNST NanoFab to prepare a wedge-shaped device sample by carefully thinning down the absorber thickness at a small angle while ensuring that the device still functioned as a solar cell. This shape effectively brings some of the buried interfaces closer to the exposed surface so that these interfaces can be interrogated with higher spatial resolution using light with shallower penetration. The characterization technique was demonstrated on cadmium telluride (CdTe) solar cells, a common photovoltaic technology. These solar cells are based on 1 μm to 3 μm thick multilayer polycrystalline films with the size of the crystal grains comparable to or smaller than the device thickness. The cells have both intentional and unintentional variation in composition throughout the absorber layer and device interfaces, making them ideal for testing this technique. Currently, solar cells of this type operate at efficiencies well below the theoretical limit, and their inefficiency is believed to be due to the uncontrolled and often unknown effects of the microscale structure buried beneath the surfaces of the devices. According to CNST researcher Nikolai Zhitenev, the new technique is just an early step towards the development of a full set of quantitative nanoscale sub-surface measurement tools. While patterning the photovoltaic device into a wedge shape inevitably modifies its structure and performance, the measurements are highly reproducible for a variety of processing conditions. Through the development of new theoretical models explicitly incorporating the effects of the device modification, the new measurement approach can be developed into a powerful quantitative technique.