Silicon solar cells are the backbone of the photovoltaic industry. Champion solar cell efficiencies up to 26.8% have recently been reported, while efficiencies in high volume production now routinely exceed 24%. These incredible achievements, which even experts in the field would have considered fanciful a few years ago, rely significantly on the availability and systematic application of sophisticated characterization and inspection methods and systems. This lecture reviews some of the most common characterization techniques for solar cells and modules, and their application in R&D, in production and in the field.
Speakers / Authors: Ron Sinton | Sinton Instruments & Thorsten Trupke | University of New South Wales
The IV curve represents the most important electrical characteristic of a solar cell or module, containing rich information about device performance, including power output and conversion efficiency. The detailed analysis of IV data can provide important additional information about performance limiting loss mechanisms.
In the first part of the tutorial, we will review the fundamental mechanism underlying the exponential shape of the IV curve, namely the dependence of total recombination within the volume of a device on the separation of the quasi-Fermi levels (essentially an implied voltage). Importantly, this mechanism is already present in an absorber without any specific solar cell device structure. This formulation of solar cell operation enables data at any step in the manufacturing process, from ingot to fielded system, to be described as an effective IV-curve. This enables fundamental device physics to be described in universally understood terminology from end-to-end for PV manufacturing. Complementary techniques for lifetime measurements, Suns-Voc and so-called Suns-Photoluminescence experiments will be used as experimental evidence for this concept. In the following section an example of an IV curve analysis will be presented, including a detailed energy loss analysis. Technical aspects of illuminated IV measurements, including some limitations associated with performing these measurements at ever increasing throughput in high volume production will be discussed in this context. To achieve TWs of solar cell production capacity, the use of Ag is being dramatically reduced to the extent that conventional IV cell sorting prior to use in modules is very difficult. Can cells be binned and sorted using PL and EL techniques? Progress on this concept will be shown.
The second part of the tutorial will focus on the characterization of crystalline silicon modules. A few interesting aspects of module design and testing will be discussed. We will also review the application of photoluminescence (PL) imaging to modules, with specific emphasis on benefits of line scan PL imaging. Given the generally very low luminescence yield from c-Si devices, PL imaging must traditionally be carried out in a laboratory environment, using specialized light tight enclosures, to avoid artefacts from stray light. In the last section of the tutorial, we will review recent work on the application of PL imaging in full sunlight to silicon modules that are installed and operating in the field.
