Crystalline Silicon PERC Solar Cell with Ozonized AlOx Passivation Layer on the Rear Side
Abstract
We present a method of ozonation to form the rear-side passivation layers of crystalline silicon PERC cells. In the method, a thin aluminum film was deposited on the back surface of a silicon wafer and then was oxidized into an aluminum oxide layer by gaseous ozone. Lifetimes of the wafers with such passivation layers proved to be increased with respect to those untreated, and the resultant PERC cells showed a performance improvement compared with standard cells with full back surface fields.
Introduction
A basic cell structure of crystalline silicon PERC (passivated emitter and rear cell) cells commonly fabricated by industry is shown in Figure 1 [1], where silver electrodes are screen printed on the front surface of a p-type textured wafer with an antireflection coating (ARC) and a diffused N layer, while local contacts are formed by fired aluminum paste at the laser-ablated parts of the back surface with a stack of AlOx/SiNx. A local BSF (back surface field) is formed on the rear contact to facilitate the collection of holes, and the thin AlOx (arising from Al2O3) layer contributing field-effect passivation will eject electrons and thus reduce recombination of electrons and holes near the rear side of the wafer [2].
The Al2O3 layer with an appropriate thickness can be deposited on a p-type silicon wafer by a PECVD [3, 4] or an ALD [5, 6] technique. The resultant passivation layer AlOx is formed at the interface of Si/Al2O3 after the wafer is annealed at a proper temperature and produces negative charges with a density that is several times as high as 10 12 cm.2. In between silicon and AlOx, there exists a layer of SiOx in a tetrahedral geometry [7, 8]. The AlOx near the SiOx also has a tetrahedral geometry, rendering the insufficiency of aluminum atoms and henceforth negative charges. Nowadays, industrial PERC cells with such passivation layers have achieved a conversion efficiency of ~21% or even higher [9, 10]. Deposition of an Al2O3 layer by either PECVD or ALD, however, is not a cost-effective way because the precursor trimethylaluminum and a vacuum-processing facility are both required. Many different methods for the formation of Al2O3 layers for the purpose of passivation have been developed without using vacuum-chamber-equipped facilities.
A good passivation layer was obtained by using a technique of reactive sputtering without using trimethylaluminum, resulting in PERC cells that showed a significant improvement in efficiency compared with standard full BSF cells [11]. A printable aluminum oxide paste was demonstrated to support an efficiency of 20.1%, which could be easily integrated into an existing production line and cause a reduction of additional cost for equipment [12]. The researchers of [13] reported a high-level passivation with an Al2O3 layer synthesized on a p-type crystalline silicon wafer by a sol-gel method. The inventors of [14] proposed spraying methanol solution containing aluminum elements (or specifically, containing aluminum acetylacetonate) on the surface of a silicon wafer for forming a passivation layer. This idea could result in a tremendous reduction in manufacturing cost, however, leaving unsolved a problem of uneven thicknesses of the passivation layers from sample to sample. In this study, we present a different method for forming an Al2O3 layer for the back surface passivation of a PERC cell. Although vacuum-supported equipment is employed in the method, Al2O3 layers with a uniform thickness can be formed without trimethylaluminum used. In this new method, a thin aluminum film was first deposited on the back surface of a phosphorus-diffused silicon wafer and then oxidized into an aluminum oxide layer by gaseous ozone. Such an aluminum oxide layer proved to produce a good level of passivation after annealed at a proper temperature. Section 2 reveals the formation of such a passivation layer and briefs the fabrication process of PERC cells in this study. Experimental results for minority carrier lifetime and cell performance measurements are presented in Section 3, followed by a conclusion section.
Experiments
2.1. Al2O3 Layer Formed by Ozonation Method
Pseudosquare- (100-) oriented 200 μm thick diamond-wire-sawn single-crystalline silicon wafers in the dimensions of
were cut into smaller pieces with a size of
For lifetime measurement, these wafers were treated to form an Al2O3 layer on, respectively, their front and back surfaces. To form the Al2O3 layer, we first used an e-gun evaporator to deposit a 3 nm thick aluminum film on the two surfaces of the wafer. The panel setting for this film thickness was 3 nm, which, however, was believed to be smaller than the true value of thickness, as we will see later. The as-deposited wafers were then put in a beaker with a piece of saran wrap covering its top for hermetic seal while allowing an ozone gas supplier to constantly feed the beaker with gaseous ozone. After a period of time, the aluminum films were transformed into Al2O3 films, followed then by an annealing process. Such a treatment for Al2O3 formation was also applied to commercial blue wafers as well as textured wafers, the latter of which had a surface morphology of inverted pyramid-like structure (see Figure 2) that was formed by using our proprietary method. Here, the commercial blue wafers refer to the wafers processed in an industrial production line up to the step of deposition of PECVD ARC layers. Al2O3 films were formed only on the back surfaces of these wafers, and so we could observe the improvement in lifetime with respect to untreated commercial blue wafers.
Surface morphologies measured at two positions of the wafer used for PERC cell fabrication in the study.
Figure 3 shows the stack of Al2O3/SiO2/Si for a sample that was originally coated with an aluminum film and was subsequently ozone-treated and annealed at 600°C for 90 seconds. It can be seen that the thicknesses of Al2O3 and SiO2 read, respectively, 7.64 nm and 3.15 nm. In the following, we will show that this annealing condition gave rise to the best passivation effect. After oxidized, a 3 nm thick aluminum layer was supposed to become about 5 nm thick. The Al2O3 thickness obtained was not consistent with the expected for the possible reason of unreliable aluminum thickness at only several nanometers achieved by using e-gun evaporation.
A depth profile showing atomic compositions obtained by X-ray photoelectron spectroscopy is given in Figure 4 for a sample with the stack of Al2O3/SiO2/Si on a silicon substrate after the ozonation method was applied on the aluminum metal film, and the sample was annealed at 600°C for 90 seconds. Obviously, Al2O3 was formed at etch time less than ~100 seconds. After the etch time of 100 seconds, AlOx with
can be seen. For example, at the etch times between 125 and 150 seconds, is about 1.7. However, we are aware that the depth profile does not appear to be as steep as it should be to show the respective layers on the silicon substrate. This is because the atomic composition at each etch time is taken as an average quantity over the atomic compositions obtained at neighboring etch times for the XPS instrument we used here. On the other hand, EDS analysis (Figure 5) reveals the compositions of Si, Al, O, and Pt (platinum) atoms, where Pt atoms were detected because the sample was covered with platinum metal for measurement. The oxygen/Si ratio appeared to be about 2 : 1 at the positions of 40-43 nm, indicating a layer of SiO2 there. For the region of 43-45 nm, the oxide was much like SiO4, indicating a tetrahedral geometry. Then away from the SiO4 layer, i.e., from the position of ~45 nm, the number of Al atoms increased and supported the existence of AlOx, where. with Al vacancy at the positions of 44-46 nm, and approach Al2O3 at farther positions, i.e., the positions of 46-53 nm. After the point of 53 nm, a rapidly growing number of Pt atoms were detected. The structural transition from SiOx to Al2O3 over the interface region was consistent with the remark in [7].
2.2. PERC Cell Fabrication
Here, we used diamond-wire-sawn single-crystalline silicon wafers for the study of PERC cells. These wafers were textured to have an inverted-pyramid-like structure on two sides and were then phosphorus diffused to form an
layer on the front side. After an Al2O3 layer was formed on the rear side of each diffused wafer with the size of. followed by annealing at 600°C for 90 seconds. Such an annealing condition was found to achieve the best minority carrier lifetime for bare wafers that were coated with Al2O3 on both sides and for diffused wafers coated with SiNx ARC on the front side and Al2O3 on the back side. Then, a 100 nm thick SiNx layer was deposited on the Al2O3 layer by PECVD, resulting in a stack of Al2O3/SiNx on the rear side. A photolithographic process was subsequently employed to form a pattern of line-shaped openings on the rear side. Then, a SiNx layer with a thickness of 100 nm was deposited by PECVD to form an ARC layer on the front side. Aluminum paste and silver paste were subsequently screen printed on the rear side and the front side, respectively, followed by cofiring in a conveyor belt furnace.
Experimental Results
3.1. Lifetime Measurement
First, we measured the lifetimes of bare wafers without Al2O3 formed yet on both the front and the rear sides. Figure 6 shows the lifetime measurement results by using the quasi-steady-state photoconductance technique with Sinton WCT-120, at the minority carrier density 10 15 cm.3. The bare wafers in the dimensions of were cleaned by supersonic acetone and were then SC-1 cleaned, followed by a saw damage removal process with a mixture of CH3COOH/HF/HNO3. Lifetimes of these wafers were measured and then were measured again after Al2O3 layers were formed by the ozonation method on both the front and the rear sides. Figure 6 shows the lifetimes for these ozone-treated bare wafers annealed at 600°C (blank squares) and 700°C (blank triangles), respectively, for various annealing time periods. The filled squares and filled triangles in the figure represent the lifetimes of the as-cleaned wafers that were not annealed and were measured here for reference only. The as-annealed wafers with Al2O3 layers marked by the blank squares and blank triangles should be compared with the as-cleaned wafers marked by the filled squares and filled triangles, respectively. It can be seen that the lifetime of the ozone-treated wafer annealed at the temperature of 600°C for an annealing time period of 90 seconds could improve by 3.9 times (from 2.6 μs to 10.15 μs) with respect to that of the corresponding as-cleaned wafer. On the other hand, the lifetime improvement for 700°C annealed ozone-treated wafer was 3.3 times at maximum (for an annealing time period of 60 seconds) with respect to that of the corresponding as-cleaned wafer. We have also measured the lifetimes of commercial blue wafers with and without an Al2O3 layer on the rear side. Figure 7 shows the lifetimes measured at 600°C and 700°C, respectively, for various annealing time periods. Again, we can see that the time period of 90 seconds was the best annealing time period for 600°C annealing, while the time period of 60 seconds was the best annealing time period for 700°C annealing, and that 600°C annealing supported a better lifetime than 700°C annealing. Likewise, the as-annealed wafers with Al2O3 layers are marked by the blank squares (for 600°C annealing) and blank triangles (for 700°C annealing), while the commercial blue wafers without Al2O3 layers (denoted by as-cleaned blue wafers) are marked by the filled squares and filled triangles, respectively. These as-cleaned blue wafers were not annealed and are shown only for comparison with the as-annealed blue wafers. The as-annealed blue wafers with Al2O3 layers marked by the blank squares and blank triangles should be compared with the as-cleaned blue wafers marked by the filled squares and filled triangles, respectively.
IBC Solar Cells: Definition, Benefits, vs. Similar Techs
The solar industry’s road for solar panels with a higher power is paved with different solar cell technologies that attempt to reduce power losses, increase efficiencies, and reduce production costs for photovoltaic (PV) modules. One of the most innovative methods to have proven higher efficiencies using crystalline silicon (c-Si) cells is the Interdigitated Back Contact (IBC) solar cell technology.
IBC solar cell technology has proven to be superior to traditional Aluminum Back Surface Field (Al-BSF) options, but it has the downside of having a more expensive and complex manufacturing process. In this article, we explain everything about IBC technology, including the components, structure for IBC solar cells, operating principle, and even compare IBC against other PV technologies.
What is an IBC solar cell and how does it work?
IBC solar cell technology restructures components in the solar cell and includes additional ones to increase efficiency for the cell, and provide additional benefits. In this section, we explain the materials and the structure of IBC solar cells, and we explain the operating principle for the technology.
Materials components of the IBC solar cell
The main component featured in most IBC solar cells is a c-Si wafer that acts as the n-type wafer absorber layer, but p-type wafers are also used. Monocrystalline silicon (mono c-Si) is the most common option due to its higher efficiency, but polycrystalline silicon (poly c-Si) can also be used.
An anti-reflective and passivation layer is placed on one of the two sides of the c-Si wafer, being manufactured with a thin layer of silicon dioxide (SiO2) placed through a thermal oxidation process. Materials like Silicon Nitride (SiNx) or Boron Nitride (BNx) are also suitable.
For IBC solar cells to relocate frontal contacts at the rear side of the cell, they require interspersed or interdigitated layers of n and p emitters called the diffusion layer. To create it, layers of the n-type wafer are doped with boron through masked diffusion, masked ion-implantation, or laser doping, creating the p-type (p ) digitation, while the n-type layers stay intact (n ).
Metal contacts are also placed by laser ablation or wet chemical deposition, using regular metals like silver, nickel, or copper for the contacts of the IBC solar cell.
This is one of the most popular approaches for manufacturing IBC solar cells, but there are different approaches available (Figure 1), which might require different materials for manufacturing the diffusion layer.
Structure of the IBC solar cell
Manufacturing IBC solar cell can be quite complex considering the creation of the diffusion layer, but understanding its structure is relatively simple.
The main layer for the IBC solar cell is the n-type or p-type c-Si wafer functioning as the absorber layer. This layer is manufactured by doping a c-Si layer with boron or phosphorous, to create a p-type or n-type doped wafer. Then, an anti-reflective and passivation coat usually made out of SiO2 is placed on one or two sides of the solar cell (Figure 2).
The major structural design modification for the IBC solar cells is the inclusion of a diffusion layer, which features interdigitated n-type and p-type layers allowing for the installation of rear side metal contacts (Figures 2 3).
Finally, every metal contact for the IBC solar cell is placed in the back of the cell, leaving the front of the cell entirely free from shading materials. This also allows for installing contacts in a wider area, causing series resistance for the cells to be lowered.
Working principle of the IBC solar cell
IBC solar cells generate solar power under the photovoltaic effect as Al-BSF solar cells do. The load is connected between positive and negative terminals of the IBC solar panel, with photons being converted into electricity, creating solar power to energize the load.
Alike traditional solar cells, photons impact the IBC solar cell absorber layer, exciting electrons and creating an electron-hole (e-h) pair. Since IBC solar panels do not feature frontal metal contacts that shade the cells, these solar cells have a higher area of conversion for photons to impact.
The e-h pair formed at the front of the IBC solar cell is then collected by a p-type interdigitated layer at the back. Collected electron flows from p metal contacts to the load, generating electricity, and then going back to the IBC solar cell through the n metal contact, ending that particular e-h pair.
IBC solar cells vs. Traditional cells
After understanding more about IBC solar cells, it is important to compare them to the well-known traditional Al-BSF technology. In this section, we compare both options considering different aspects.
| Interdigitated Back Contact (IBC) | ||
| 25-30 years | ||
1 Considering a cost of 0.274€/W at 1.10/€
One structural problem that IBC solar cells improve from the design of traditional Al-BSF cells, is removing the front metal contact at the cell. This provides two advantages for IBC solar cell technology: reduced shading by locating metal contacts at the rear side of the cell and increasing power density by allowing installation of solar cells without space in between on the IBC solar panel.
Due to the improvements in IBC solar cells, IBC technology has achieved a recorded efficiency of 26.7%, which is 1.3% more than traditional technologies. IBC solar cell technology does not stop there, since researchers expect to achieve an efficiency of 29.1% for IBC solar cells.
IBC solar cell technology improves the temperature coefficient from.0.387%/ºC to.0.446%/ºC for traditional options, down to.0.29%/ºC. As a result, an IBC solar panel can deliver a better performance in hot climate installation.
While IBC solar cell had a high production cost and features a complex manufacturing process, the cost for this technology has been reduced to 0.30/W. With higher efficiency and only a slightly higher price, IBC solar cell technology is a compelling option for residential and industrial applications, which could cause IBC technology to take control of around 35% of the market share by 2025.
While Al-BSF and IBC solar panels can be used for residential and industrial applications, IBC solar cell technology has the upper hand in CPV applications. This is caused by IBC solar panels having a lower series resistance, higher bulk lifetime, and lower surface recombination, making it ideal for these applications with increased solar concentration which provides several interesting advantages.
IBC solar cells vs. PERC cells
Passivated Emitter and Rear Contact (PERC) and IBC solar panels share interesting design improvements from Al-BSF technology. Both technologies share higher efficiencies, better temperature coefficients, and larger areas for photon absorption.
PERC and IBC technologies share the reduction of the surface area occupied by the busbars or metal contacts, delivering similar advantages. While PERC technology only reduces the busbars, IBC solar panel technology eliminates it, further increasing the effective surface area for photon absorption.
IBC technology surpasses PERC technology in its efficiency, due to PERC technology only achieving an efficiency of 25.4%, while IBC solar panel technology achieved recorded efficiencies of 26.7%.
The major point in favor of PERC against IBC solar cell technology is that IBC technology is more expensive to manufacture than PERC technology.
While both technologies have their differences, they are an improvement from traditional Al-BSF options, and the major point in favor of both is that they can be combined. This opens a way for the creation of PERC-IBC solar panels, featuring additional advantages against traditional technologies.
Roundup: The benefits of IBC solar cells
IBC solar panels have many benefits that make them outstand from traditional Al-BSF technology and others. In this section, we round up the benefits of IBC solar cell technology.
Reduced losses by shading
IBC solar cell restructuration places frontal metal contact on the rear side of the cell, eliminating shade caused by the busbars. By doing this, IBC solar cell increases the photon effective absorption which results in reduced power losses and several other benefits.
Reduced series resistance
IBC solar cells lower the series resistance at the cell from traditional Al-BSF cells, by being able to place larger metal contacts at the rear side of the cell, becoming a key factor for CPV applications.
Increased power output per square meter
With an increased efficiency for IBC solar cells, an IBC solar panel can be manufactured without space between cells, further increasing the power output per square meter for a single module. This makes IBC solar cell technology more compelling for applications with limited space.
Independent optical/electrical optimizations
Since IBC solar cells relocate metal contacts at the back, the optical and electrical optimizations for the cell are decoupled, making each optimization completely independent from the other, making it easier for researchers to improve one or the other separately.
Who manufactures IBC panels?
IBC solar panels are manufactured by a few companies in the US, with the two most popular ones being SunPower and Trina Solar.
SunPower: Maxeon® solar panels
SunPower is a solar company manufacturing solar panels in the US for more than 35 years. This company delivered the first commercial IBC solar panels to the US, producing high-quality modules with excellent performance, with their Maxeon® solar panels.
Maxeon solar panels achieved one of the highest efficiencies for PV modules in the market. These modules feature a copper substrate that increases strength and resistance to corrosion, featuring high-quality silicon layers for the solar cells that produce 60% more power than other technologies in the market.
Trina Solar
Trina Solar has provided some of the most cost-effective solar solutions in the US solar market for around 25 years, ideal for residential, commercial, and utility-scale applications. The company focuses on improving PV technology, known for setting a new record for mono c-Si IBC solar cells in 2018.
This company is one of the largest IBC solar panel producers in the US. Trina Solar has shipped over 80GW in solar panels worldwide and performed grid-tied installations for over 5.5GW in the US.
Industrial Silicon Solar Cells
The chapter will introduce industrial silicon solar cell manufacturing technologies with its current status. Commercial p-type and high efficiency n-type solar cell structures will be discussed and compared so that the reader can get a head-start in industrial solar cells. A brief over-view of various process steps from texturing to screen-printed metallization is presented. Texturing processes for mono-crystalline and multi-crystalline silicon wafers have been reviewed with the latest processes. An over-view of the thermal processes of diffusion and anti-reflective coating deposition has been presented. The well-established screen-printing process for solar cell metallization is introduced with the fast-firing step for sintering of the contacts. I-V testing of solar cells with various parameters for solar cell characterization is introduced. Latest developments in various processes and equipment manufacturing are also discussed along with the expected future trends.
Keywords
- silicon
- solar cells
- manufacturing
- multi-crystalline
- mono-crystalline
- texturing
Author Information
Sukumar Madugula Reddy
Address all correspondence to: mehul.c.raval@iitb.ac.in
Introduction
Photovoltaics are an important renewable energy source which has grown rapidly from 8 GW in 2007 to 400 GW in 2017 [1]. Along with the increasing demand, the PV system costing has also dropped significantly from 35.7 /Wp in 1980 to 0.34 /Wp in 2017 accelerating its adoption [2]. Silicon (Si) which is an important material of the microelectronics industry has also been the widely used bulk material of solar cells since the 1950s with a market share of 90% [2]. The chapter will introduce the typical steps for manufacturing commercial silicon solar cells. A brief history of solar cells and over-view of the type of silicon substrates along with the different solar cell architecture will be introduced in Sections 2 and 3. Subsequently, the wet-chemistry and high temperature steps used in fabrication will be described in Sections 4 and 5. Section 6 will discuss about the metallization process along with typical characterization parameters for commercial solar cells. Finally, future roadmap and expected trends will be discussed in the concluding section.
Evolution of solar cells
The ‘photovoltaic effect’ literally means generation of a voltage upon exposure to light. The phenomenon was first observed by the French physicist Edmund Becquerel on an electrochemical cell in 1839, while it was observed by British scientists W.G. Adams and R.E. Day on a solid-state device made of selenium in 1876 [3]. From the 1950s onwards, there was Rapid progress in the performance of commercial solar cells from 23% [2] and silicon has been the ‘work-horse’ of the photovoltaic industry since then. The evolution of silicon solar cells is shown in Figure 1.
The first silicon solar cells demonstrated by Russell Ohl of Bell Laboratories during 1940s were based on natural junctions formed from impurity segregation during the recrystallization process [3]. The cells had an efficiency of
During the 1950s, there was Rapid development in the high-temperature diffusion process for dopants in silicon. Person, Fuller and Chaplin of Bell Laboratories demonstrated a 4.5% efficient solar cell with lithium-based doping, which improved to 6% with boron diffusion. The solar cell had a ‘wrap-up’ around structure (Figure 1(b)) with both contacts on back side to avoid shading losses, but led to higher resistive losses due to the wrap-around structure. By 1960, the cell structure evolved to as shown in Figure 1(c). Since the application was for space explorations, high resistivity substrate of 10 Ω cm was used to have maximum radiation resistance. Vacuum evaporated contacts were used on both sides, while a silicon monoxide coating was used as an anti-reflective coating (ARC) on the front-side (FS) [3].
In early 1970s it was found that having sintered aluminum on the rear-side improved the cell performance by forming a heavily doped interface known as the ‘back-surface field (Al-BSF)’ and gettering of the impurities [3]. The Al-BSF reduces recombination of the carriers on the rear-side and hence improves the voltage and the long-wavelength spectral response. Implementation of finer and closely spaced fingers reduced the requirement on the junction doping and eliminated the dead layer. An ARC of titanium dioxide (TiOx) was used and its thickness was selected to reduce the reflection for shorter wavelengths and gave a violet appearance to the solar cells. Further improvement was made by texturing the wafers using anisotropic etching of (100) wafers to expose the (111) surfaces. The texturing led to improved light-trapping and gave the cells a dark velvet appearance. The improved cell architecture is shown in Figure 1(d). In 1976, Rittner and Arndt demonstrated terrestrial solar cells with efficiencies approaching 17% [3].
The passivated emitter solar cell (PESC) achieved a milestone of 20% efficiency in 1984–1986. The metal/silicon contact area was only 0.3% in PESC cells, while a double layer ARC of ZnS/MgF2 was used in both cell structures. In 1994, passivated emitter rear locally diffused (PERL) cell with an efficiency of 24% were demonstrated [3]. As compared to the PESC cell, the PERL cell had inverted pyramids on FS for better light-trapping and oxide-based passivation on both sides. Oxide passivation layer on the rear-side also improved the internal reflectance of the long wavelength and hence the spectrum response.
In addition to the evolving solar cell architectures, there has also been continuous development in the manufacturing domain in terms of increased throughput, improved process-steps and reduced costs. A brief over-view of the manufacturing of Si substrates and various types of solar cells is given in the next section.
Commercial silicon solar cell technologies
Si is the second most abundant material on earth after oxygen and has been widely used in the semiconductor industry. Metallurgical grade silicon (Mg-Si) of 98% purity is obtained by heating quartz (SiO2) with carbon at high temperatures of 1,500-2,000 [4]. Mg-Si is further purified to obtain solar grade silicon chunks of 99.99% purity. The refined solar grade Si chunks are then processed further to obtain mono-crystalline and multi-crystalline forms of Si ingots, which are a large mass of silicon. In mono-crystalline Si, the atoms are arranged in the same crystal orientation throughout the material. For solar cells, (100) orientation is preferred as it can be easily textured to reduce the surface reflection [5]. Multi-crystalline Si, as the name suggest has multiple grains of Si material with different orientations, unlike the mono-crystalline substrates. Mono-crystalline material have higher minority carrier lifetime compared to multi-crystalline Si and hence higher solar cell efficiencies for a given solar cell technology.
The Czochralski (Cz) method for making mono-crystalline Si ingots is illustrated in Figure 2(a). High purity molten silicon with dopant is maintained above the melting point and then a seed crystal is pulled at a very slow rate to obtain an ingot of as large as 300 mm in diameter and 2 m in length [6]. The molten silicon can be doped with either p-type or n-type dopants to obtain the specific type of mono-crystalline Si ingot of up to 200 kg [2]. Wafers sawn from the ingots have circular edges and hence the shape is called a ‘psuedo square’. Multi-crystalline silicon ingots are made by melting high purity Si and crystallizing them in a large crucible by directional solidification process [7] as demonstrated in Figure 2(b). The process does not have a reference crystal orientation like the Cz process and hence forms silicon material of different orientations. Currently the multi-crystalline Si ingots weigh 800 kg [2] which are then cut into bricks and wafers are sawn further. Current size of mono-crystalline and multi-crystalline wafers for solar cell fabrication is 6 inch × 6 inch. The area of the mono-crystalline wafers will be little less due to the pseudo-square shape. The most widely used base material for making solar cells is boron doped p-type Si substrates. N-type Si substrates for also used for making high efficiency solar cells, but have additional technical challenges like obtaining uniform doping along the ingot compared to p-type substrates.
A broad classification of different types of solar cells along with efficiency ranges is shown in Figure 3. The standard aluminum back-surface field (Al-BSF) technology is one of the most common solar cell technology given its relatively simple manufacturing process. It is based on full rear-side (RS) Al deposition by screen-printing process and formation of a p BSF which helps repel the electrons from the rear-side of p-type substrate and improve the cell performance. The manufacturing flow for Al-BSF solar cells is shown in Figure 4. The standard design of commercial solar cells is with grid-pattern FS and full area RS contacts.
The passivated emitter rear contact (PERC) solar cell improves on the Al-BSF architecture by addition of rear-side passivation layer to improve rear-side passivation and internal reflection. Aluminum-oxide is a suitable material for RS passivation with average solar cell efficiencies nearing 21% obtained in production [8]. An existing Al-BSF solar cell line can be upgraded to PERC process by two additional tools (RS passivation layer deposition and laser for localized contact opening on the RS).
The remaining three cell architectures are mainly higher efficiency technologies based on n-type Si substrates. The a-Si heterojunction solar cell has a-Si layers on the FS and RS of n-type Si substrate to form ‘heterojunctions’ unlike the conventional high temperature diffusion-based p-n junction. Such technology allows processing at lower temperatures, but is very sensitive to the quality of the surface interfaces. a-Si-based heterojunction solar cell was commercially manufactured by Sanyo Electric, which is now taken over by Panasonic [9]. In the interdigitated back contact (IBC) solar cell design, both contacts are present on the rear-side eliminating the FS contact shading losses. Typically for IBC solar cells, the junction will also be located on the rear-side. One of the early manufacturers of the high efficiency n-type IBC solar cell is SunPower Corporation [10]. Bifacial cells, as the name suggests can capture light from both sides of the solar cells. This entails that the rear-side also has a grid-pattern contacts to enable light collection. An example of the bifacial technology is the BiSON solar cell developed and commercialized by ISC, Konstanz [11]. It should be noted that the indicated classification is not an exhaustive list of various other types of solar cell architectures which are in RD phase, close to commercialization or already being manufactured. The subsequent sections will give an over-view of the process steps for manufacturing of Al-BSF solar cells.
Wet-chemistry processes for solar cell fabrication
Wet-chemistry-based treatment is an important step in solar cell processing for saw damage removal (SDR) for the as-cut wafers, texturing of the surface to increase the absorption of incoming solar radiation and edge isolation after the diffusion process. As discussed in the previous section, there are mainly mono-crystalline and multi-crystalline silicon wafers used for fabrication of solar cells. The wet-chemistry-based processing for the respective types of wafers will be discussed ahead.
4.1 Texturing of mono-crystalline silicon wafers
As indicated in Section 2, the development of solar cells started primarily with mono-crystalline wafers and hence employed well-established methods from the domain of microelectronics. Alkaline anisotropic etching based on KOH/NaOH is used for pyramidal texturing of mono-crystalline wafers. An as-cut mono-crystalline wafer has a weighted average reflectance of 30% (over wavelength of 300–1,200 nm) which reduces to 11–12% after the texturing process. Typical morphology of an alkaline textured surface is shown in Figure 5. The anisotropic etching solution etches the (100) surface of the wafers to expose the (111) faces which have a higher density of silicon atoms and hence a slower etch rate compared to the (100) faces. This results in formation of random pyramid structures which form an angle of 54.7° with respect to the wafer surface.
Typical parameters for the alkaline texturing process are shown in Table 1. It should be noted that the values of various parameters are indicative and are not to be taken as absolute as there are a variety of additive manufacturers in the market. Isopropyl alcohol (IPA) was initially used as an additive in the texturing solution, which is not involved in the etching reaction, but acts as a wetting agent to improve the homogeneity of texturing process by preventing the H2 bubbles (generated during the reaction) adhering to the silicon surface [12]. However by 2010, IPA was gradually replaced with alternative additives due to drawbacks like unstable concentration as the bath temperature is close to the boiling point of IPA (82.4°C), high costs, high consumption, health hazards and explosiveness [12]. Many groups have published development work to replace IPA with alternate additives to overcome the disadvantages of IPA, increase the process window and reduce the surface reflectance [12, 13, 14, 15, 16]. Additives also reduce the processing time to 100 runs.
| KOH (%) | 3 | |
| IPA (%) | 6 | — |
| Additive (%) | — | |
| Process temperature [°C] | 80 | 70–100 |
| Pyramid size [μm] | 5–12 | 2–7 |
| Process time [min] | 30–40 | 5–10 |
| Organic content [wt%] | 4–10 | |
| Boiling point [°C] | 83 | 100 |
| Bath lifetimes | 100 |
Table 1.
Process parameters for IPA-based and additive-based alkaline texturing of mono-crystalline wafers.
The texturing process of the mono-crystalline wafers is typically performed in a ‘batch’ which implies that the wafers are loaded in a carrier with slots to hold the wafers (100 slots in a carrier) and then the batch is processed sequentially in baths for texturing, cleaning, treatment steps to remove the organic residue and metal contamination and drying the processed wafers. The carriers are typically coated with PVDF which has very good resistance to various chemicals, abrasion and mechanical wear and tear. Typical carrier for mono-crystalline wafers handling is shown in Figure 6. The batch texturing tool has dedicated baths for each step with dosing tanks for chemicals used in the bath. The tool processes many carriers simultaneously and can reach a throughput of 6,000 wafers/h with processing of four carriers at the same time.
4.2 Texturing of multi-crystalline silicon wafers
Multi-crystalline wafers offer a cost advantage compared to the mono-crystalline wafers and hence have been more widely adopted. However, the alkaline chemistry used for texturing mono-crystalline wafers does not work well for multi-crystalline wafers due to the presence of different grain orientations. An alternative acidic chemistry based on HF and HNO3 was developed to remove the saw damage and texture the multi-crystalline wafers simultaneously [17, 18]. The acidic solution-based texturing operates at temperatures below room temperature and hence leads to reduced reaction gas emission, little heat generation, higher stability of the etching solution and better control of the etch rate [18]. A comparison of alkaline texturing and acidic texturing process for multi-crystalline wafers is shown in Figure 7.
The acidic texturing process of multi-crystalline wafer can be done in significantly reduced time compared to the alkaline texturing process and hence can be implemented in an ‘inline’ configuration where the wafers are passed through rollers immersed in the etching bath. A representative image of an inline process along with the typical acidic texturing process is shown in Figure 8. For a five lane configuration, the inline tool can have a throughput of up to 4,000 wafers/h. It is important to note that the wafer surface facing down in the etching solution is textured better than the top-side and is the ‘sunny-side’ for further processing. The acidic texturing process leads to formation of porous silicon on the textured surface which absorbs light and also increases the surface recombination [18]. Hence the porous silicon is removed using a dilute alkaline solution. Subsequently, an acidic clean (HF HCl) is performed to remove oxides and metal contamination from the wafer surfaces.
It is important to note that the acidic texturing process discussed above is suitable for the slurry-wire sawn (SWS) multi-crystalline wafers. In the past few years, diamond-wire sawing (DWS) process has replaced the slurry-wire-based cutting due to process and economic advantages [19]. The saw damage of the SWS multi-crystalline wafers is more than the DWS wafers, which have deep straight grooves and a much more smoother surface than the slurry-wire sawn wafers [19]. The saw damage for the SWS wafers plays an important role for initiating the texturing process, which does not occur for the DWS wafers.
Various methods have been proposed to texture DWS multi-crystalline wafers and are summarized in Table 2 [20]. By tuning the various methods, reflectance of close to 0% can be obtained and hence the term ‘black silicon’ has been used for the texturing process of DWS multi-crystalline wafers. RIE was the first method for making black silicon and uses sulfur hexaflouride (SF6) to react with Si and gases like Cl2 and O2 for passivating and limiting the reaction [20]. Recently, commercial multi PERC solar cells with average efficiency of 21.3% have been demonstrated with RIE-based texturing process [21]. However, since RIE is a vacuum-based process the throughput is low as compared to a typical inline process and also additional pre-processing and post-processing is required to remove the saw damage and damage due to ion-bombardment, respectively. A variant of the RIE method which does not require vacuum or plasma has been implemented in a commercial tool [22].
| Reactive ion etching (RIE) | SF6/O2, SF6/Cl2/O2, SF6/O2/CH4 | None | None | 4.0 |
| Plasma immersion ion implantation (PIII) | SF6/O2 | None | None | 1.8 |
| Laser irradiation | CCl4, C2Cl3F3, SF6, Cl2, N2, air | None | None | 2.5 |
| Plasma etching | SF6 | Ag nano particles | None | 4.2 |
| Metal-assisted chemical etching (MACE) | AgNO3/HF/HNO3 | None | Ag, Au | 0.3 |
| Electrochemical etching | HF, EtOH,H2O | None | None |
Table 2.
Various methods for texturing diamond-wire sawn multi-crystalline wafers [20].
One of the approaches for texturing DWS multi-crystalline wafers is to upgrade the existing acidic texturing-based chemistry with additives [23, 24, 25]. Such an approach can potentially have a lower CoO compared to the MACE-based approach [23]. Reflectance of such an additive-based approach has been demonstrated to be similar to the conventional isotexturing solution with solar cell efficiency of 18.7% for the Al-BSF-based structure [24].
MACE-based texturing is similar to the conventional acidic etching method with an additional step of catalytic metal deposition. The process flow consists of SDR, catalyst metal deposition, chemical etching and post-treatment. Efficiencies of 19.2% have been obtained for commercial multi Al-BSF cells using batch-type MACE texturing process [26]. Inline-type MACE-based commercial tool has been demonstrated with the possibility to tune the reflectance in the range of 12–23% and obtain average efficiency for Al-BSF and PERC structure of 18.8 and 20.2%, respectively [27]. Representative images of textured surface based on MACE process are shown in Figure 9. The cost of ownership (CoO) of the inline MACE process is potentially lower compared to the batch-based MACE process with scope to reduce it further by recycling Ag from the texturing bath [27].
4.3 Wet-chemistry-based edge isolation
The emitter region in a solar cell is fabricated by a high temperature diffusion process (to be discussed in sections ahead). During the diffusion process, phosphor silicate glass (PSG) is deposited on the wafer which should be removed before deposition of the ARC layer. As depicted in Figure 10, after the diffusion step, the n-type region is also present on the edges and the rear-side of the wafer. The n-type layer on edges and the rear-side will short-circuit the emitter with the base substrate and hence it is important to etch these regions and isolate the emitter on the FS from the base substrate as depicted in Figure 10(c).
The edge isolation process can be performed in an inline manner similar to the texturing process discussed in the previous section. The exception in this case is that the chemical should etch only the rear-side and edges without interacting with the FS. A representative image of the edge isolation process is shown in Figure 11. It is important to note that the rollers are present only on the bottom-side to avoid any contact of the etching solution with the front-side. The subsequent steps after the RS etching are similar to those in the inline texturing machine.
Thermal processes for solar cell fabrication
High temperature processes form a vital part of solar cell fabrication. Examples of such processes are forming the p-n junction by diffusion, firing of screen-printed contacts, activating surface passivation layers or annealing process induced defects. The section glimpses the basic physics of emitter diffusion process and plasma enhanced chemical vapor deposition (PECVD).
5.1 Emitter diffusion
Emitter diffusion is one of the crucial thermal steps in the industrial solar cell fabrication. The n-type emitter of the crystalline p-type silicon solar cells is formed by phosphorus (P) diffusion. In the diffusion process, the Si wafers are sent in a furnace and exposed at 800–900°C to phosphoryl chloride (POCl3) and O2 which results in PSG deposition on the Si wafer surfaces. This step is called as pre-deposition, where the PSG [28] acts as source of phosphorus (P) dopants to diffuse into the Si wafer. The next step is drive-in, where the supply of dopant gases is disconnected and P from the PSG layer diffuses further into the Si wafer. Hannes et al. [29] illustrates for the optimum process feasibility for photovoltaic applications, three different effects have to be considered. Firstly, the in-diffusion of P from the PSG and its presence in electrically active and inactive states in the Si wafer, which increases Shockley-Read-Hall (SRH) recombination. Secondly, the gettering of impurities into the Si layer towards the PSG layer. Finally, the metal contact formation with the P-doped Si emitter draws out the generated power.
The diffusion process is quantified by sheet resistance which depends on the depth of p-n junction and P concentration profile. The sheet resistance has units of Ω/cm (commonly measured as Ω/□) and is measured using a four-point probe system. The definition of sheet resistance is illustrated in Eq. (1).
where R = resistance of a rectangular section (Ω); ρ = resistivity (Ω cm); l = length of the rectangular section (cm); A = area of the rectangular section (cm 2 ); W = width of the rectangular section (cm); D = depth of the rectangular section (cm) and ρsheet = resistance for given depth ( D ) when l = W (Ω/□).
The earlier values of emitter sheet resistance were 30–60 Ω/□ with p-n junction depths of 400 nm and high P surface concentration. With improvements in the front-side silver (Ag) contacting paste, the emitter sheet resistance is now in the range of 90–110 Ω/□ with junction depth of around 300 nm and lower P surface concentration. Shifting to larger sheet-resistance allows to capture more light in the UV and blue spectrum, while also reducing the surface recombination to improve the Voc. It should be noted that the diffusion process occurs on the FS (directly exposed to the gases) and also on the edges and RS. If the edge isolation process is not carried out (as discussed in Section 4.3), the emitter will be short-circuited with the substrate.
Figure 12 shows the POCl3 diffusion process in a closed quartz-tube system.POCl3 is a liquid source supplied to the process tube by bubbling it with a carrier gas N2. By mixing O 2 with the POCl3, there will be an epitaxial growth of PSG layer as indicated in Eq. (2) [30].
At the Si surface, 2 P 2 O 5 is reduced to elemental phosphorus during the drive-in step as shown in Eq. (3) [30].
Chlorine which is a by-product during the pre-deposition cleans the wafers and quartz-tube by forming complexes with metals. PSG is used as source for driving in the P atoms into Si surface. During the drive-in process, POCl3 is switched off and only O2 is added to build up a thin oxide layer beneath the PSG to enhance the diffusion of P atoms into Si surface.
- Loading zone (LZ)—area from where the wafers are loaded into the tube.
- Center loading zone (CLZ)—area between the loading zone and centre zone.
- Center zone (CZ)—center area of the tube.
- Center gas zone (CGZ)—area between the centre zone and gas zone.
- Gas zone (GZ)—area from where the gases move out through the exhaust.
Typically the temperatures of each heating zone are adjusted to obtain equal emitter sheet resistance for all wafers across the boat.
Environment of diffusion process should be very clean and hence quartz material is used for the tubes. Cleanliness of the tubes and loading-area maintenance also affects the process results. Since in gas-phase diffusion there is no residue in the tube, it results in a cleaner process. By half pitch loading in the low pressure (LP) conditions [31], the throughput can be increased. Commonly 1,000 wafers are loaded in a single tube and with five diffusion tubes in a batch-type diffusion system, a throughput of up to 3,800 wafers/h can be achieved for solar cell manufacturing.
An inline diffusion system where the wafers are transported on a belt with phosphoric acid as the source of P dopants was also used in commercial production [32]. However, compared to the inline process, the batch process is more clean, effective and efficient. For n-type solar cells or advanced solar cells concepts like PERT, the p-type batch diffusion is based on boron (B) dopant sources like boron tribromide (BBr3) [33, 34].
5.2 Anti-reflective coating (ARC) deposition
A bare Si surface reflects 30% of the light incident. As discussed in Section 4, the texturing process improves the light-capturing. It is desirable to reduce the reflectance further which is obtained by depositing an ARC layer. TiOx was one of the earliest material to be used as an ARC layer for solar cells, however since it could not provide adequate surface passivation it was eventually replaces by SiNx:H [37]. Thermally grown silicon oxide (SiO2) was also employed as the passivating material in the record breaking passivated emitter rear locally diffused (PERL) cells [37]. High thermal budget and long process time made SiO2-based passivation unsuitable for mass-production of solar cells [37]. A comprehensive review of various ARC and passivating material for solar cell applications is discussed in [37].
The plasma enhanced chemical vapour deposition (PECVD) process is suitable for depositing an ARC layer of SiNx:H which not only reduces the reflection but also passivates the front-side n-type emitter and the bulk thus improving the solar cell efficiency [36, 37]. A schematic of a batch-type PECVD system is shown in Figure 14. The wafers are loaded in a graphite boat with the front-sides facing each other. An RF plasma based on process gases ammonia (NH3) and silane (SiH4) operating at a temperature of 400–450°C deposit the hydrogenated SiNx:H layer as per Eq. (4) [35]. The hydrogen incorporated in the SiNx:H film diffuses into the bulk during the firing step (discussed in next section) and passivates the dangling bonds to improve the solar cell performance [36, 37].
The refractive index (RI) of the SiNx:H film is controlled by the ratio of SiH4/NH3 gas, while the thickness depends on the deposition duration. The SiNx:H-based ARC can minimize the reflection for a single wavelength and the wavelength-thickness is given by [38],
where t = thickness of the SiNx:H ARC layer, λ 0 = wavelength of incoming light and n 1 = refractive index of the SiNx:H layer.
Based on the relationship, the ARC is also called as a ‘quarter wavelength ARC’. For solar cells, the RI and thickness are selected to minimize the reflection at a wavelength of 600 nm as it is the peak of the solar spectrum. The thickness and RI of the ARC is selected to be the geometric mean of materials on either side, i.e., glass/air and Si. The typical thickness of the SiNx:H ARC is 80–85 nm with RI of 2.0–2.1 giving the solar cell a color of blue to violet blue. A representative image of textured multi-crystalline solar cell deposited with SiNx:H is shown in Figure 15(a), while the variation of SiNx:H color based on its thickness is shown in Figure 15(b). It is important to note that there is a dependence on the surface texture and ARC color for given deposition parameters. There is a variety of solar modules where the color of the solar cells is darker unlike the typical blue color. A typical ARC deposition stage in a solar cell manufacturing line consists of two PECVD systems, each with four tubes and a throughput of up to 3,500 wafers/h.
SiNx:H is not suitable for passivating p-type Si and hence dielectrics like Al2O3 are used for RS passivation for cell architecture like PERC cells [8] or for p-type emitters in n-type solar cells. For PERC solar cells, the Al2O3 passivating layer is capped by a SiNx:H to protect it from the Al-paste during the firing process and also serve as an internal reflector for the long wavelength light. Commercial PECVD and atomic layer deposition (ALD)-based systems are available for depositing Al2O3 with throughput of up to 4,800 wafers/h [39].
Metallization and solar cell characterization
6.1 Screen-printing-based metallization
The last processing step for solar cell fabrication is the FS and RS metallization to draw out the power with minimum resistive losses. Ag is a good contact material for the n-type emitter, while Al makes a very good contact with the p-type substrate. A combination of Ag/Al paste is used to print pads on the RS to facilitate interconnection of solar cells in a module. Screen-printing is a simple, fast and continuously evolving process for solar cell metallization.
A schematic representation of the screen-printing process is shown in Figure 16. The screens have an emulsion coated stainless steel mesh with openings as per the desired metallization pattern as illustrated in Figure 17(a). The metal paste is spread over the screen via the flood and the squeegee movement that deposits the paste on the solar cell based on the screen-pattern. Snap-off is the distance the screen and the solar cell. The squeegee pressure and the snap-off distance are the critical parameters that determine the paste lay down and geometry of the Ag FS fingers.
Typical paste lay down for Ag/Al RS pads, RS Al and FS Ag are 35–45 mg, 1.1–1.4 g and 100–120 mg, respectively for a 6 inch Al-BSF multi-crystalline solar cell. An illustrative Ag FS metallization pattern is shown in Figure 17(b). The Ag finger opening has reduced to below 30 μm, while application of 5 bus-bar is being increasingly adopted now. With such screen parameter and good paste lay down, consistent FF of 80% should be obtained for the Al-BSF solar cells with an optical shading loss of
6.2 Drying and fast firing of metallization pastes
The metallization pastes consist of metal powder, solvents and organic binders. In case of FS Ag paste, the paste also contains glass-frit while etches the SiNx:H layer and makes contact with the n-type emitter [41]. The metal pastes are dried after printing and finally they are sent through a fast-firing furnace for sintering and form the RS Al-BSF and FS Ag contact. An example of such a fast-firing furnace with the temperature profile is shown in Figure 18. The FS Ag finger sintering process is illustrated in Figure 19. When the solar cell passes through the fast-firing furnace, the organic binders are burnt, followed by melting of the glass frit and finally formation of Ag crystallites contacting the n-type emitter. The firing profile needs to be tuned based on the specific types of metallization pastes and emitter diffusion profile. As an example, the firing peak temperature could be low to not form a good ohmic contact on the FS, while a too high temperature can lead to diffusion of Ag through the junction and shunting of the p-n junction. Image of a complete multi-crystalline Al-BSF solar cell is shown in Figure 20.
6.3 Plating-based front-side metallization
The costing of various factors in solar cell processing have decreased over the years, while the contribution of front Ag is still the most significant [42]. Significant amount of work has been done to replace Ag by alternate metal like copper (Cu) which has a conductivity value of very close to that of Ag and also offers a potential significant cost advantage [43, 44]. Cu has high diffusivity and solubility in Si and hence a barrier-layer like nickel (Ni) is deposited on Si prior to Cu plating [42]. Light-induced plating (LIP) which is derived from conventional plating utilizes the photovoltaic effect of light to plate the desired metal and has many advantages compared to conventional plating [43, 44].
Ni-Cu-based front-side metallization requires an additional front-side ARC patterning step unlike the Ag paste-based metallization and in most cases also an additional Ni sintering step to reduce the contact resistance and have good adhesion of the metal stack [42]. Commercial DWS cut mc-Si solar cells based on Ni-Cu-Ag plated stack have been demonstrated with finger width of 22 μm, aspect-ratio of close to 0.5 and similar efficiency as that of reference screen-printed Ag-based solar cells [45].
Continuous improvement in the Ag FS pastes along with simplicity, reliability and high throughput of the screen-printing process has made it difficult for Ni-Cu-based metallization to compete with Ag-based FS metallization. However, high solar cell efficiency concepts like bifacial heterojunction solar cells, where Cu can be directly plated onto the transparent conducting oxide, the plating process is simplified and requires only a single tool [39]. Similarly, high efficiency concepts which require reduced amount of metal can achieve the same using plating-based metallization [42, 46].
6.4 I-V testing and characterization of solar cells
The final step is I-V testing of the complete solar-cells as per the standard test conditions (STC), i.e., AM 1.5G, 1000 W/m 2 with a Class AAA solar simulator. An example of FS probing of solar cell is shown in Figure 21. The typical parameters obtained from the I-V tester are indicated in Table 3. I-V testers have many characterization parameters which can be helpful for diagnosis of solar cell defects. Representative electroluminescence (EL) and thermal IR image of a solar cell with some defects are shown in Figures 22(a)–(c). An EL image of a good solar cell with uniform intensity is shown in Figure 22(a), while for a solar cell in which the FS fingers are not printed uniformly, a darker contrast can be seen in Figure 22(b). Figure 22(c) shows a thermal IR image of a solar cell with a localized shunt which has been formed during one of the processing steps. In the end, the solar cells are sorted in different efficiency bins based on the selected classification.
| Voc (V) | Good mc-Si Al-BSF solar cells have a value of 0.635 V |
| Isc (A) | Good mc-Si Al-BSF solar cells have a value of 9.0 A |
| FF (%) | Good mc-Si Al-BSF solar cells have a value of 80% |
| Efficiency (%) | Good mc-Si Al-BSF solar cells have a value of 18.6% |
| Vmpp (V) | Corresponding voltage at the maximum power point |
| Impp (A) | Corresponding current at the maximum power point |
| Rs (Ω) | Good mc-Si Al-BSF solar cells have a value of |
| Rsh (Ω) | Good mc-Si Al-BSF solar cells have a value of 100 Ω |
| Irev (A) | Reverse current at a voltage of −12 V should be |
| FS BB-BB resistance (Ω) | Resistance measured between the BB’s on the FS |
| RS BB-BB resistance (Ω) | Resistance measured between the BB’s on the RS |
Table 3.
Parameters for characterization of a solar cell obtained from I-V measurement.
What Does PERC in Solar Mean?
There is a lot of excitement in the solar industry about PERC technology. If you’ve heard the term, but aren’t completely sure what it means, you aren’t alone.
PERC stands for Passivated Emitter and Rear Cell technology. Depending on where you look, you may also see it referenced as Passive Emitter and Rear Contact.
Although the technology itself has existed since the mid-1980s, it only started to be utilized at the research level more recently. It took years for the general solar industry to reach the same efficiencies.
Now that advanced technology is available, knowing more about it can help you decide if it’s a good fit for your solar needs. In this article, we’ll explain everything you need to know about PERC technology as it relates to solar panels.
What is PERC in Solar?
A scientist in Australia named Martin Green and his team at the University of New South Wales developed PERC technology in the 1980s. This technology takes a traditional solar cell and adds an additional layer to the backside of the cell. This extra layer is termed the passivation layer.
So, how does a PERC solar cell work? The passivation layer acts as a mirror and reflects light that passes through the panel the first time. This gives the light a second chance to be absorbed by the solar cell and results in greater absorption of solar radiation.
For years manufacturers of solar cells directed all of their attention to the front side of a solar cell. PERC technology allows the backside of the solar cell to play an active role.
What Are the Benefits of PERC Solar Cells?
PERC solar cells are beneficial for a number of reasons. The main benefit is they achieve higher efficiency ratings than traditional solar cells. PERC technology enables the solar cell to absorb more light and this effectively boosts energy production. Although additional steps are needed in the manufacturing process, the efficiency gain makes it well worth it.
Another benefit of PERC solar cells is they grant you more flexibility. PERC technology works well in low-light and high temperatures. By having a greater energy density per square foot than traditional solar cells, you can utilize PERC solar cells to achieve your desired output with fewer panels. These attributes make solar possible in spaces or locations where it might otherwise seem a poor fit.
The ability to achieve the same output with fewer panels translates to cost savings. Being able to achieve your energy goals for less is always desirable. For some projects, this cost savings can mean the difference between going solar and just dreaming about it.
How Are PERC Solar Cells Different?
At first glance, you may not notice any obvious differences between a PERC solar cell and a typical photovoltaic solar cell. The construction of both cell types is similar and they both use incoming solar radiation to generate a flow of electrons.
The main difference between PERC solar cells and monocrystalline photovoltaic cells is the additional passivation layer on the backside of the solar cell. This layer is what increases the panel’s energy generation by boosting efficiency.
Wondering how does a PERC solar cell work to improve efficiency? Here are 3 ways the back surface passivation layer results in increased efficiency:
Reflects light back through solar cell
The back surface passivation layer acts as a mirror and reflects light that passes through the solar cell unabsorbed the first time. This gives the solar cell a second chance to absorb the light.
This second absorption attempt means more solar radiation will be directed to the cell. The very same cell is able to produce more energy which translates to greater efficiency.
Heat absorption is reduced
Another benefit of the passivation layer on PERC solar cells is that it reflects certain wavelengths of light. Solar cell silicon wafers cannot absorb light with wavelengths greater than 1180 nanometers.
This means that in a traditional cell, the higher-wavelength light waves get absorbed by the rear back layer of the solar cell. Absorption of the light here causes increased heat and as a byproduct reduced efficiency.
By contrast, a PERC solar cell’s back surface passivation layer is designed to reflect these higher-wavelength light waves so that they are not absorbed by the solar cell. In this way, heat energy in the solar cell is reduced and as a result, efficiency is increased.
Electron recombination is reduced
A third way the back surface passivation layer increases efficiency is that it reduces “electron recombination”. Electrons have a tendency to recombine and when they do, it blocks the free flow of electrons throughout the solar cell lowering efficiency. The added passivation layer in a PERC solar cell reduces electron recombination and this reduction boosts efficiency.
What is Mono PERC Solar?
Mono PERC solar is the most efficient solar available. When comparing monocrystalline vs. polycrystalline solar panels, monocrystalline panels have a higher efficiency rating. By adding PERC technology to this already superior solar panel, it’s possible to generate 6 to 12 percent more energy with mono PERC solar than with conventional solar panels.
One thing to note is that mono PERC solar panels are not the same as bifacial solar panels. Bifacial panels are made with dual-sided solar cells that are capable of absorbing sunlight on both sides of the panels.
How Are PERC Solar Cells Made?
Manufacturers make PERC solar cells by adding two additional steps to their solar cell process. Both of these steps occur on the back surface field (BSF) of the solar cell.
The first step is to apply the rear surface passivation film which is a dielectric passive layer. Once that has been applied, the rear surface passivation film is etched with chemicals or lasers to open tiny s in the layer. These s are what allow the panel to absorb more light.
The same equipment that is used for traditional solar cells can be employed for PERC solar cell production meaning manufacturing lines don’t need to invest in new equipment. This allows manufacturers to produce a superior product without any significant increase in manufacturing costs.
A conventional, or traditional solar cell consists of the following from front to rear:
- Screen-printed Silver paste front contact
- Anti-Reflective Coating (ARC)
- Silicon wafers that form the P-N junction
- Aluminum Back Surface Field (AI-BSF)
- Screen-printed Aluminum paste layer
From front to rear, a PERC solar cell is comprised of:
- Screen-printed Silver paste front contact
- Anti-Reflective Coating (ARC)
- Silicon wafers that form the P-N junction
- Local Aluminum Back Surface Field (Al-BSF)
- Dielectric passivation layer
- SiNx Capping Layer
- Screen-printed Aluminum paste layer
Solar technology continues to evolve and improve each year. PERC technology is a great example of how a small change to the existing design of solar panels makes a big impact.
N-Type vs. P-Type Solar Cells
Another advancement in solar panel cell technology has been the introduction of N-Type solar cells. Compared to traditional P-Type technology, solar panels with newer N-Type cells degrade less over the life of the panel.
What are N-Type Solar Cells?
N-type cells are doped with phosphorus, which has one more electron than silicon, making the cell negatively charged.
N-type cells are immune to boron-oxygen defects, and as a result, they are not affected by light-induced degradation (LID). As you might expect, these are positioned as a premium option because they degrade less over the life of the panel.
What are P-Type Solar Cells?
P-type cells are usually built with a silicon wafer doped with boron. Since boron has one less electron than silicon, it produces a positively charged cell.
P-type cells are affected by light-induced degradation, which causes an initial drop in output due to light exposure. This has historically been the most common treatment method for solar cells.
Questions?
You’ve taken the time and learned about PERC technology and what it means for solar. Feel free to reach out to us with any questions you may have. We’ll be happy to go over what technology seems like the right fit for your solar project.