Bifacial monocrystalline PERC PV modules are buzzwords in the current PV market, but what does the future look like for solar PV modules? Solar cell technology is constantly changing as PV module manufacturers aim to create higher efficiency modules and gain market share. Characteristics including the size of the solar PV cell, thickness of the wafer, doping agent, and the technology utilized in the solar cell are all varied by PV manufacturers when creating novel modules. That said, manufacturers also try to maintain as much proven technology as possible, and phase new technologies in one or two changes at a time. This limits the uncertainty in issues that may arise in a new technology and creates some comfort for investors in the PV module design.
Module Basics
There are two main types of modules available in the industry, crystalline silicon and thin film. Crystalline silicon includes mono-silicon and polysilicon and all variations therein while thin film includes cadmium-telluride and copper-indium-gallium-selenide among others.
Crystalline silicon modules are produced from purified pieces of silicon called “ingots”. The ingots are cut into thin “wafers” and post processed into PV cells. The cells are then joined together through various methods that includes printing conductive metal lines on the cell surface and then soldering to join the cells electrically, before being laminated between an encapsulant and glass. Thin film modules are produced by coating large sheets of glass with materials that can absorb sunlight and convert it to electricity. Cadmium Telluride (CdTe) is the most common thin film module technology. CdTe modules are most commonly deposited directly onto glass through a series of vacuum deposition stages where the materials are sublimated onto the base substrate. The materials are then cured in a furnace, post processed, laser scribed into series PV cells, and then laminated together with other functional layers of the module.
Evolution of Modules
The types of modules most prevalent in the current market include mono-crystalline and poly-crystalline silicon aluminum back surface field (“Al-BSF”), mono-crystalline silicon Passivated Emitter and Rear Cell (PERC), and cadmium telluride (CdTe) thin film. Additionally, many of the largest module manufacturers are focused on releasing products that employ a tunnel oxide passivated contact (TOPCon) structure, intended to improve open circuit voltage, fill factor, and overall efficiency.
Size and capacity have also continued to increase over the years. While a larger footprint and higher power rating tend to lead to lower installed costs, both introduce challenges as well. Higher power ratings of modules drive higher power totals of strings, higher capacity of inverters, and more power flowing through the smaller components of the collection system, which can negatively affect reliability and drive higher costs in controlling safety. Larger module sizes introduce a larger surface area to interact with environmental forces like wind and snow load. They also drive design changes in racking and trackers in order to accommodate larger surface areas and a higher module weight.
Here we will discuss changes to PV module technology that are expected to emerge in the next five years, based on the International Technology Roadmap for Photovoltaic (ITRPV) 2022 results. IRTPV generally illustrates trends in the PV market, and changes expected in solar PV cell technology.
Silicon Doping
Many different chemicals are added to the silicon substrate that PV cells are based on to achieve a variety of different outcomes. These chemicals were initially added to silicon as impurities in order to create the p-n junction that powers a solar cell. Current research and development of doping agents aims to improve cell efficiency and reliability. The most common novel doping agent is gallium, which has shown promising results for decreasing Light Induced Degradation (LID) and Light and Elevated Temperature Induced Degradation (LeTID) in crystalline silicon solar cells. Crystalline silicon solar cells experience LID in the first few hours to weeks of operation and it results in 1-3% reduction in the performance of the cell. LeTID is a longer-term mechanism that has been shown to lead to degradation in the 1-10% range. It is currently mitigated by careful control of the solar cell manufacturing process. While gallium doping shows promise in reducing these degradation mechanisms, limited deployment in the field carries operational uncertainty until more operating years are completed.
Silicon N-Type Cell Technology
The silicon utilized to form an ingot is doped with various elements to create an electric potential. P-type PV cells are doped with an element, often boron, that has an excess of electrons, causing the PV cell to want to lose an electron. N-type is the opposite and is doped with an element that needs additional electrons, often phosphorus, thereby attracting them. Based on the ITRPV roadmap, n-type modules are expected to surpass the p-type wafer material world market share around 2029. N-type technology has typically resulted in a more costly manufacturing process; however, new advances, including n-type TOPCon cell designs, have made this technology more comparable to the cost of p-type technology. N-Type PV cells have several operational advantages to p-type as well, including increased cell efficiency, better performance at nominal operating temperatures, and mitigation of LID and LeTID. However, as with gallium doping, n-type technology has limited deployment in the field and carries operational uncertainty due to novel degradation mechanisms. For example, researchers are currently investigating the susceptibility of n-type cells to UV induced degradation (“UVID”) which has been shown to cause both front and back side power loss in some n-type cell designs.
Thin Film Cell Technology
CdTe is a direct bandgap semiconductor and well matched to the solar spectrum, which makes it an ideal material for PV cell applications. Recent innovations introduced in CdTe modules include higher commercial efficiencies, larger platform modules, and most recently, bifacial modules. Upcoming innovations include the reduction of copper in an effort to reduce year to year power degradation. As new innovations have been introduced into commercial products, some unexpected operational issues are also created, which drive iterative revisions to new products.
Other types of thin film PV technologies include amorphous silicon, which is most commonly used for small, low power applications like consumer electronics; copper-indium-gallium-selenide, or CIGS, which is vapor deposited onto a substrate similar to CdTe but is less common due to high manufacturing costs and issues with cell stability; and perovskites, which are discussed in further detail below.
Perovskites
Perovskite materials have been identified as a less expensive alternative to silicon as a base substrate material for PV. In research and development environments, perovskites have shown equal, and sometimes greater, efficiencies when compared to silicon-based cells. Early development has been based on perovskites applied to a glass substrate as a thin film application. New research has explored applying perovskites to layers of metal and dielectric substrates as well as silicon substrates, which has increased efficiency of the cell.
Thus far, perovskites have proven to degrade very quickly which has hindered their introduction as a commercial product. Recently however several manufacturers and consortiums have launched pilot manufacturing lines of perovskite on silicon tandem solar cells with plans to offer commercial products as soon as 2023. Perovskite technology is relatively new and the long-term risks associated with it are still under evaluation.
How Natural Power can help
Natural Power has a team of engineers with experience and expertise assessing photovoltaic research, development and commercially available technology. Our experience includes reviewing new platforms, designs, and manufacturing. Reach out if you would like more information on how we can help you evaluate your modules.
Author: Kim Clark, Senior project engineer, Natural Power