Each of the steps in the fabrication of a photovoltaic panel is carried out under very different conditions and requires the use of different characterization tools, depending on the parameter to be measured, the environmental conditions, and the geometry of the problem. For example, the techniques and toolset used to control or characterize a fabrication line based on glass substrates (batch process) are different than those used for flexible substrates in a roll-to-roll configuration, where the deposition is continuous over several kilometers of roll.
Metrology tools can be used after a single layer or complete device is finished, and can be also be used during the deposition process to monitor various parameters providing real time information to control the process.
The benefits of metrology used in a production environment can be summarized into two main categories: production cost reduction and quality assurance.
The determination of key parameters at different stages of a solar panel manufacture and the ability to maintain their values between pre-established limits is crucial to assure the end-user that the finished photovoltaic panel will provide the promised output power.
The use of metrology tools allows for a rational utilization of resources during fabrication and quality control after the product is finished.
Determination of layer composition, thickness and uniformity as the material is deposited may be used, through a feedback mechanism, to control the deposition “knobs” (power to material sources, speed of the substrate under the sources, temperature of the sources, etc.) to maintain the values within the acceptable manufacturing limits that will result in an efficient device. A lack of these metrology-based controls mean that if the parameters drift away from the optimal values, and outside the acceptable limits, the time, materials and energy used for the process will be wasted.
After the solar cells/modules are fabricated, metrology is needed in order to characterize their performance. By measuring the current and voltage characteristics obtained under illumination, it is possible to determine the maximum power the devices can provide under different levels of illumination, and how efficiently they convert sunlight into electricity. By measuring the current generated when filtering the light, it is possible to determine which parts of the solar spectrum are better utilized, and if particular layers of the device are performing in a sub or super standard way.
Of course, metrology is integral to the whole scientific research and development process, where the use of different techniques and correlation of results allows for an understanding of the influence of deposition parameters on the properties of the finished layers, and the subsequent implementation of modifications in the fabrication process that allow for improved solar panels with higher efficiency, longer lifetime and better performance.
Establishing standard performance measurements to fairly compare different modules and PV types
Establishing in-line metrology for thickness and composition control for stack layers