Photovoltaics are a unique class of optoelectronic materials that convert photons of light into an electrical current. This photovoltaic effect occurs when the substrate of the cell is exposed to wavelengths of light within the passband of its p-n junctions. Typically, light beyond the visible spectrum (380–700nm) is rejected at the surface interface, which limits the efficiency of modern solar cell technology. Thin film characterization of solar cells is partly focussed on predicting the performance of this absorption–conversion process, which is defined as quantum efficiency (QE).
QE is expressed as a ratio of the number of incident photons to converted charge carriers. This phenomenon is determined by several critical factors, including: the chemical composition of the p-n junctions; the thermal stability of the photovoltaic’s thin film structure; and the performance of anti-reflective coatings on the cell substrate. Despite significant research and development into multi-junction photovoltaics and new deposition methods, modern solar cells perform up to maximum QEs of 20–30%.
Emerging photovoltaics represent significantly improved performance over established single- and multi-junction cells. However, thin film characterization of complex and innovative nanostructures requires high-resolution imaging technologies capable of comprehensively assessing multiple properties of devices in their entirety. This includes metrological profiling of the substrate; conductivity and permittivity of the electrical junctions; and the overarching thermal properties of the photovoltaic cell.
Atomic force microscopy (AFM) is a complementary imaging method for determining the performance of solar cells based on their nanoscale photoresponses. It is one of the leading technologies for thin film characterization in photovoltaic development and manufacturing, owing to its manifold capabilities.
Firstly, it enables high-resolution surface mapping of thin film structures in three dimensions providing rich data regarding the device’s microstructure. This is critical for thin film characterization focusing on the relationship between crystal size and photovoltaic response. Among the most exciting materials in modern solar cell engineering are perovskite quantum dots, which are nanoscale crystals capable of converting wavelengths of light based on their chemical composition and crystal size.
Broadening the particle size distribution of perovskite photovoltaics is associated with a commensurate increase in QE, but fully understanding how this granular structure affects photovoltaic responses requires a thorough analysis of the multilayer metrology. Characterization of the perovskite solar cell’s surface topography and roughness can help eliminate manufacturing issues to optimize the development of innovative solar cell arrangements.
The second benefit of AFM thin film characterization in photovoltaic applications is the possibility of measuring both electrical and functional responses directly. Photoconductive AFM is performed under full illumination to determine the current conversion at localities in thin film structures and measure variations in performance across the full device.
These combined capacities make AFM the ideal solution for thin film characterization and surface coatings, supporting the development of novel photovoltaics with enhanced QEs.
Asylum Research is one of the leading authorities in AFM analysis of complex thin film structures and coatings. We have developed a range of systems suitable for measuring the metrological properties of solar cells, including surface roughness and uniformity. Our AFM technologies can also provide reliable quantification of the electrical properties of thin film structures.