Solar cell thin film analyzers are used to facilitate research and development (R&D) into novel photovoltaic architectures by monitoring the processing conditions for epitaxial deposition of photovoltaic materials. These growth methods cover a range of advanced techniques, such as plasma-enhanced chemical vapor deposition (PECVD) and radio frequency sputtering, which are used to generate a stack of p-n photovoltaic junctions on a suitable substrate material.
This blog post will explore the importance of solar cell thin film analyzers in more detail while highlighting some of the instruments available from Hiden Analytical for R&D and quality control (QC) in thin film photovoltaic manufacturing.
Solar Cell Thin Film Analyzers: Improving on Conventional Technology
Heterojunction photovoltaics comprised of crystalline silicon (c-Si) remain the prime technology for manufacturing solar cells due to their high conversion efficiency. They are typically engineered by epitaxial growth of multiple layers of p- and n-type silicon to create a thick film structure up to a maximum 200 micrometers (μm) thick. This creates a single p-n junction of alternating silicon layers treated with different dopant atoms, usually phosphorous (P) and boron (B). This junction is tuned to a specific bandgap, resulting in low out-of-band attenuation and a theoretical maximum efficiency of up to 30%.
Solar cell thin film analyzers have comprehensively improved the quality of silicon semiconductors by optimizing doping, epitaxy, and etching processes. This has been achieved through residual gas analysis (RGA), surface depth profiling, and plasma analysis using Hiden mass spectrometers.
Alongside enhancing the performance of existing technologies, mass spectrometry and solar cell thin film analyzers are widely used for epitaxy process control and research and development into novel photovoltaic materials and solar cell architectures.
Thin Film and Multijunction Solar Cells
Thin film solar cells are envisaged as the successor technology to last generation c-Si photovoltaics. They are engineered by depositing multiple layers of semiconducting materials, such as cadmium telluride (CdTe), onto a functional substrate without exceeding a typical film thickness of 10 micrometers (μm). These films are then sandwiched between an anode and a cathode to facilitate the absorption and conversion of photons into a voltage.
Research into cadmium telluride as a potential semiconductor for solar cell thin films stems back decades, owing to its optimal band gap of approximately 1.5 eV at temperatures of 0 – 300K. This is the optimal range for photons distributed throughout the wavelength spectrum of sunlight, enabling solar conversion efficiencies of up to 20%. Alternative photovoltaic materials for thin film solar cells include gallium (Ga), indium phosphide (InP), and germanium (Ge), but cadmium remains the most commercially viable owing to its good quantum efficiency and comparatively low cost.
Research into solar cell thin films is primarily focused on improving the conversion efficiency to achieve a faster energy payback. This is being explored through novel material characterization to determine the viability of different materials arranged in multijunction, or tandem solar cell structures.
Hiden Analytical’s Solar Cell Thin Film Analyzers
There are numerous suitable solar cell thin film analyzers available from Hiden Analytical, but the two that have been used most effectively for photovoltaic R&D are the SIMS Workstation and the EQP plasma diagnostics tool.
Using secondary ion mass spectrometry (SIMS), it has been possible to determine the efficacy of novel superstrate configuration methods for hetero- and multi-junction photovoltaics comprising cadmium telluride and cadmium sulfide on flexible molybdenum (Mo) foils. This has been used to illuminate some of the challenges and prospects of developing novel solar cells manufactured through RF sputtering.
The EQP solar cell thin film analyzer has also been used as part of a novel sputter-plasma diagnostic tool for investigating new solar cell materials such as zinc sulfide (ZnS). This is equipped with software-controlled ion extraction optics capable of scanning sample chambers at 0.05 eV increments for ultra-precise analysis of positive and negative ions, neutrals, and radicals.