BY ARABINDA DAS and JUN LU, TechInsights, Ottawa, ON
Last year was a great year for photovoltaic (PV) technology. According to Renewable Energy World magazine, since April 2016, 21 MW of solar PV mini-grids were announced in emerging markets [1]. The exact numbers of installed solar grids for 2016 has not been published yet but looking at the data for 2015, the PV industry is growing, helped by the $/watt for solar panels continuing to drop. The $/watt is obtained by taking the ratio of total cost of manufacturing and the number of watts generated. According to the Photovoltaic Magazine, the PV market continued to grow worldwide in 2015. The magazine also makes reference to the newly published report by the International Energy Agency Photovoltaic Power System (IEA PVPS) programme’s “Snapshot of Global Photovoltaic Markets 2015,” which also states that the total capacity around the globe has crossed the 200 GW benchmark and is continuing to grow [2]. This milestone of 200 GW in installed systems is a remarkable achievement and makes us think of the amazing journey of PV technology. The technology was born in Bell Labs, around 1954, with a solar cell efficiency of just 4% [3]. By the end of the 20th century, the overall solar cell efficiency was close to 11% and the worldwide installed capacity of PV was only 1 GW [3]. Today, seventeen years later, it has soared to 200 GW, with single junction cells having efficiencies around 20% [2].
Si-based solar cells
To celebrate this important milestone, we put TechInsights’ analysis and technical databases to work to investigate the structure of solar cells of two leading manufacturers and compare them to earlier technologies. We chose to analyze Si-based solar cells only, as they represent over 85% of the global market. According to the 2016 IHS Markit report, the top three PV module suppliers in the world are Trina Solar, SunPower, and First Solar [4]. We procured panels from Trina Solar, a Chinese based company, and SunPower, an American company, and carried out a structural analysis of these panels. These analyses helped us take a snapshot of current PV technology. We compared these two types of panels with an older panel from our database. This panel is about eight years old and was made by Kaneka (Japan). We will provide an overview of each panel and their underlying structure.
Table 1 consolidates some of the important param- eters of the three panels. The SunPower panel is based on monocrystalline silicon and the Trina solar panels are based on polycrystalline silicon. The older Kaneka panel is based on amorphous Si thin film technology. The panel from Kaneka is an earlier product; their recent products are made using hybrid technology, a combination of amorphous films and polycrystalline substrates, The Kaneka panel complements very well the other two products which are based on Si crystalline wafers. The technology to fabricate the solar cells (thin film, multi-crystalline or mono-crystalline) has a direct impact on the efficiency of the cells and on their electrical parameters like the open circuit voltage (Voc) and the short circuit current (Isc), as can be seen in Table 1. This table also shows that the Kaneka thin-film based panel has the lowest nominal power among the three. The ratio of nominal power to the light power that is received by the PV panel is indicative of its efficiency. It can be seen also that Kaneka’s thin film panel has the highest open circuit voltage which is the maximum voltage available from the solar cell without any load connected to it.
Table 1 indicates that SunPower is the only one among the three that uses an n-type substrate and has the highest solar efficiency. SunPower has the lowest weight per meter-square of all the panels assessed (9.3kg).
Unlike SunPower panels, most installed Si solar panels employ a p-type substrate, even though the first silicon-based solar cells developed at Bell Labs were based on n-type Si substrates [3]. Researchers J. Libal and R. Kopecek posit that the industry transitioned to p-type substrates because the initial usage of solar cells was in space applications and p-type wafers demonstrated less degradation in the presence of cosmic rays. They suggest that for terrestrial applications there is growing evidence that n-type based solar panels are preferred over p-type based panels [5]. The reasons for choosing n-type Si substrates rather than p-type substrates are because the former are less sensitive to metallic impurities and thus are less expensive to fabricate. In general, the minority carrier diffusion lengths in n-type substrates are higher than p-type Si substrates. Also, n-type Si substrates can withstand higher processing temperatures than p-type substrates, which are prone to boron diffusion. According to the International Technology Roadmap for Photovoltaic (ITRPV), n-type based substrates will increase in prevalence and may eventually replace the p-type monocrystalline Si cells [6].
Thin film based solar panels are very different from monocrystalline Si cells. Thin film cells have the lowest efficiency and yet they too have a role to play in the PV industry. They are the most versatile; they can be coated on different substrates such as glass, plastic or even flexible substrates. The other big advantage of amorphous solar films is that they can be manufac- tured in a range of shapes, even non-polygonal shapes, thus they can be used in various applications. Also, thin film solar panels are not affected by high temper- atures, unlike crystalline solar panels. Thin film based panels made from amorphous Si are more effective for wavelengths between 400 nm to 700 nm, which is also the sensitive spectrum of the human eye; thus they can be used as light sensors [7]. Usually, thin film panels are almost half the price of monocrystalline panels. Amorphous silicon solar cells only require 1% of the silicon used in crystalline silicon solar cells [7].
Multi-crystalline (MC) solar panels are also cheaper than monocrystalline solar panels. MC panels are made by melting raw silicon and confining them into square molds, where they are cooled. This MC-Si process does not require the expensive Czochralski process. In the early days, the cost of fabrication of MC-Si panels was higher than thin film based panels. Now, due to the major advances in fabrication technologies, these panels often have the best $/ watt, which represent the ratio of cost to manufacture to energy output [8]. It is difficult to compare $/watt directly from different manufacturers and different types of solar panels as the technology is manufacturing is changing rapidly and often the most recent products of a manufacturer are not compared. A more sensible factor of comparison would be the ratio of total kilowatt-hours the system generates in its lifetime divided by the cost per square unit of the panel. To make a detailed estimation even the installation cost and tolerance to shade, overall reliability must be included in the calculations, which is beyond the scope of this article.
Solar panel overview
FIGURE 1 shows the panel from Kaneka. It indicates that the Kaneka solar panel cells are long strips that run across the whole length of the panel. The color of the panels is a shade of purple. The Kaneka Solar which is amorphous Si-based, has a very uniform color. The inherent structure of amorphous Si-films has many structural defects because they are not crystalline and thus are tolerant to other defects like impurities during manufacturing, unlike crystalline based panels [7]. The color of the thin film panels is strongly thickness dependent because thickness affects the light absorption. A solar cell’s outward appearance can range from blue to black and is dependent on the absorption and reflectivity of their surface. Ideally, if the cell absorbs all the light impinging on the surface it should be black. FIGURE 2 shows the panels from Trina solar and Sunpower. The Trina Solar panel has a blueish color and each cell is perfectly square. The SunPower SPR-X20- 250-BLK solar cell has a uniform blackish color. The spacing between the cells, the interconnect resis- tance, the top contacts and the materials used for the connections affect the overall performance of the panel. All three manufacturers connect their cells within a PV module and PV modules within an array in a series configuration.
Table 2 summarizes the cell dimensions for the three manufacturers. Kaneka panels have the narrowest space (0.55mm) between the cells. The Trina solar panel has a 3 mm wide gap and a 5 mm gap, between two adjacent solar cells, in the horizontal and vertical direction respectively. These gaps are used for bus electrodes. In the SunPower solar panel, the metal grid is placed on the back surface eliminating metal finger width as a layout constraint. This design significantly reduces the finger resistance and improves the series resistance.
For all panels, interconnects are made between the cells. The metallization and interconnects between the cells is a field of technology on its own. There are various techniques like lithography, laser grooving and printed contacts and these details are discussed more in detail elsewhere [9, 10, 11].
Solar panel cross-sections
In this section, we look into the layers deposited on the substrates. Cross-sectioning these big panels is not a trivial feat. These panels are covered with tempered glass and shatter during sawing and cross-sectioning. To extract a small rectangular piece requires patience and involves sawing and grinding processes. In most cases, the glass was removed before doing the cross-section. FIGURE 3 illustrates two SEM cross-sectional images and one schematic drawing. The SEM cross-sectional images show the top and bottom part of the Kaneka solar cell. In figure 3(a), the active layers comprise indium- tin- oxide, an amorphous silicon layer capped with zinc oxide, silver and a very thin layer of Ni-Al. On top of the Ni-Al film, solder is deposited. Ni-Al provides better adhesion to solder. Two electrical contacts are made between the cells, one to the indium-tin-oxide for the back contact and the other to the Ni-Al layer. Figure 3(b) exposes the layers under the glass substrate. The rear surface of the glass substrate is covered by a soft material such as EVA (ethyl-vinyl-acetate), which in turn is covered by a rear Polyvinyl Fluoride (PVF) layer called the backsheet (Tedlar or similar). EVA is also used on the top surface (figure 3(a)). The usage of these layers is standard practice in the PV industry. The main function of these layers is that they are impervious to moisture and are stable under prolonged exposure to sunlight. On the front side, EVA also helps to reduce reflection and provides good adhesion between the top glass and the solar panels. Figure 3(c) shows the complete stack in the Kaneka solar cell.
FIGURE 4 presents the stack of materials on the multi- crystalline substrate of the Trina Solar panel. The substrate is p-type and has a very thin phosphorous doped region near the top surface. This n-doped region forms the PN junction. A silicon nitride anti-reflective coating layer is deposited on top of the substrate and in designated areas the passivation is opened and silver is deposited to make electrical contact to the n-doped regions. At the bottom of the multi-crystalline substrate, there is also a thin region of high p-doping concentration and this forms the back surface field layer. This solar cell module is fabricated using passivated emitter and full metal back-surface-field (BSF) technology. BSF technology is implemented to mitigate rear surface recombination and this is done by doping heavily at the rear surface of the substrate. This high doping concentration keeps minority carriers (electrons) away from the rear contact because the interface between the high and low doped areas of same conductivity acts like a diode and restricts the flow of the minority carriers to the rear surface. Passivated emitters in the front side and BSF layer on the rear side improve the efficiency of the cells. Figure 3(b) is the schematic repre- sentation of the cell without the EVA and PVF layers.
FIGURE 5 shows an optical cross-section of the SunPower cell. Figure 5(a) shows that SunPower employs a backside junction technology with interdigitated backside p-emitter and n-base metal. This means that both the contact’s n and p-electrodes are at the bottom of the substrate and are placed in in an alternating manner. Having all the metal contacts on the rear side has two big advantages:
1. Metallic contacts are reflective and occupy space that can be used to collect more sunlight; transferring these contacts to the rear side improves the cell efficiency and also leaves the front surface with a uniformly black color, which is more aesthetic for the home users.
2. It reduces bulk recombination. The mono-crystalline substrate is only 120 μm thick. It is designed so that the carrier is generated close to the junction. The substrate is n-type and p-electrodes are formed by localized doping on the bottom part of the substrate.
Figure 5(b) illustrates the general structure of the cell.
FIGURE 6 depicts a SEM cross-section of the metal fingers that connect to the interdigitated electrodes. The pitch between the metal fingers is 920 um and repeats over the entire back surface of the panel.
All three manufacturers employ some sort of surface texturing along with anti-reflective coatings to reduce reflection but SunPower uses the most advanced technology for surface texturing. FIGURE 7 illustrates a SEM topographical image of the front surface texture of the monocrystalline substrate having pyramids, which are etched into the silicon surface. These faceted surfaces increase the probability of reflected light entering back to the surface of the substrate. A similar concept is also applied to the back surface.
The future is sunny and bright
Of the three panels we analyzed, SunPower solar panels employ the most advanced technologies and they illustrate how the solar cell has evolved over the ages. It started from a simple PN junction, then passivated emitters were intro- duced along with local back-surface-field (BSF) technology, which came to be known as Passivated-Emitter with Rear Locally (PERL) diffused technology. In contrast, today the most advanced technology is interdigitated back contacts along with passivated contacts.
In addition to these advances, there is great progress in tandem cells and multi-junctions to capture the different wavelength regions of the sun’s rays. A recent article in IEEE spectrum magazine presented the state of art of record-breaking PV cells made with different techniques such as thin film, crystalline Si, single junction, multi-junction cells. PV cells especially the multi-junction cells, have now crossed the 50% efficiency barrier [12]. Similarly, a publication from the alterenergy.org has collected all the major advances made in PV technology and discusses concepts like colloidal quantum dots and GaAs for cell technology, along with new applications [13]. Today, we regularly read about new materials (like perovskites) and come across new techniques that improve solar panel efficiencies, including new manufacturing methods to reduce the overall cost of fabrication. Moreover, PV cells are used in an innovative manner. The installation of PV panels is no more restricted to isolated rooftops or solar farm. An article in the Guardian made a reference to a solar panel road in Normandy, France [14]. At TechInsights, we will continue to keep an eye on emerging solar cell technologies.
The efforts emerging from various organizations all over the world are very encouraging. There are indeed many challenges for renewable energy to overcome before fiscal parity with fossil fuels is achieved; particularly for PV energy. Nevertheless, there is an increased focus on climate change issues. This has resulted in a significant amount of resources being allotted to PV technology in many countries, especially in developing countries such as China, India, and Brazil [1, 2]. This optimistic scenario reminds us of the song “I Can See Clearly Now” by the 1970s American singer Johnny Nash, where the refrain runs optimistically, “It’s gonna be a bright, bright sun-shiny day.”
References
1. http://www.renewableenergyworld.com/articles/2017/01/21-mw- of-solar-pv-for-emerging-market-community-mini-grids-announced- since-april.html;
2. http://www.pv-magazine.com/news/details/beitrag/iea-pvps— installed-pv-capacity-at-227-gw-worldwide_100024068/#ixzz4MB1 a44hq
3. The history of solar: https://www1.eere.energy.gov/solar/pdfs/solar_ timeline.pdf
4. http://news.ihsmarkit.com/press-release/technology/ihs-markit- names-trina-solar-sunpower-first-solar-hanwha-q-cells-and-jinko-
5. www.pv-tech.org/guest…/n_type_silicon_solar_cell_technology_ ready_for_take_off
6. http://www.itrpv.net/; http://www.itrpv.net/Reports/Downloads/2016/ 7. http://www.solar-facts-and-advice.com/amorphous-silicon.html
8. http://energyinformative.org/solar-cell-comparison-chart-mono-
polycrystalline-thin-film/
9. RP_0706-14839-O-4CS-11Kaneka
10. RP_0616-41931-O-5SA-100_Trina
11. RP_0716-42662-O-5SA-100_SunPower
12. http://spectrum.ieee.org/green-tech/solar/what-makes-a-good-pv-
technology
13. http://www.altenergy.org/renewables/solar/latest-solar-technology.
html
14. https://www.theguardian.com/environment/2016/dec/22/solar-panel-
road-tourouvre-au-perche-normandy