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Solar Panel Efficiency
Solar panel efficiency is a measure of the amount of sunlight (irradiation) that falls on the surface of a solar panel and is converted into electricity. Due to the many advances in photovoltaic technology over recent years, the average panel conversion efficiency has increased from 15% to well over 22%. This large jump in efficiency resulted in the power rating of a standard-size panel increasing from 250W to over 420W.
As explained below, solar panel efficiency is determined by two main factors; the photovoltaic (PV) cell efficiency, based on the cell design and silicon type, and the total panel efficiency, based on the cell layout, configuration and panel size. Increasing the panel size can also increase efficiency due to creating a larger surface area to capture sunlight, with the most powerful solar panels now achieving up to 700W power ratings.
Cell efficiency is determined by the cell structure and type of substrate used, which is generally either P-type or N-type silicon. Cell efficiency is calculated by what is known as the fill factor (FF), which is the maximum conversion efficiency of a PV cell at the optimum operating voltage and current. Note cell efficiency should not be confused with panel efficiency. The panel efficiency is always lower due to the internal cell gaps and frame structure included in the panel area. See further details below.
The cell design plays a significant role in panel efficiency. Key features include the silicon type, busbar configuration, junction and passivation type (PERC). Panels built using Back-contact (IBC) cells are currently the most efficient (up to 23.8%) due to the high purity N-type silicon substrate and no losses from busbar shading. However, panels developed using the latest N-Type TOPcon, and advanced heterojunction (HJT) cells have achieved efficiency levels well above 22%. Ultra-high efficiency Tandem Perovskite cells are still in development but are expected to become commercially viable within the next two years. For a deeper technical insight, Progress in Photovoltaics publishes listings of the latest photovoltaic cell technologies twice a year.
Solar panel efficiency is measured under standard test conditions (STC) based on a cell temperature of 25°C, solar irradiance of 1000W/m2 and Air Mass of 1.5. The efficiency (%) of a panel is effectively calculated by dividing the maximum power rating, or Pmax (W) at STC, by the total panel area measured in square meters.
Overall panel efficiency can be influenced by many factors, including; temperature, irradiance level, cell type, and interconnection of the cells. Surprisingly, even the colour of the protective backsheet can affect efficiency. A black backsheet might look more aesthetically pleasing, but it absorbs more heat resulting in higher cell temperature, which increases resistance, this in turn slightly reduces total conversion efficiency.
Panels built using advanced ‘Interdigitated back contact’ or IBC cells are the most efficient, followed by heterojunction (HJT) cells, TOPcon cells, half-cut and multi-busbar monocrystalline PERC cells, shingled cells and finally 60-cell (4-5 busbar) mono cells. 60-cell poly or multicrystalline panels are generally the least efficient and equally the lowest cost panels.
Top 10 most efficient solar panels
The last two years have seen a surge in manufacturers releasing more efficient solar panels based on high-performance N-type HJT, TOPcon and Back-contact (IBC) cells. SunPower Maxeon panels led the industry for over a decade, but for the first time, lesser-known manufacturer Aiko Solar released the Black Hole series panels with an incredible 23.6% module conversion efficiency using a unique new ABC (All Back Contact) cell technology. Recom Tech also announced a next-generation Black Tiger series claimed to achieve 23.6% efficiency using a new TOPcon Back-contact cell architecture. LONGi Solar was only the second manufacturer to develop a module efficiency level of 22.8% with the new Hi-Mo 6 Scientists series. The Hi-Mo 6 series is based on a new hybrid IBC cell design, which LONGi calls HPBC. Canadian Solar has also revealed a new-generation Hi Hero module built using HJT cells, which is on par with the efficiency level of the renowned Maxeon series.
Other leading panels include those from Jinko, REC, and Risen, featuring N-type HJT and TOPcon cells. High-performance panels from SPIC and Belinus using IBC cells have also closed the gap, plus new panels featuring multi-busbar (MBB) half-cut N-type TOPCon cells from JA Solar, Jolywood and Qcells and most leading manufacturers have helped boost panel efficiency above 22%.
|Black Hole series
|Hi-Mo 6 Scientist
|Hi Hero HJT
|Tiger NEO N-Type
|Alpha Pure R
Updated June 2023. Residential size panels. 54 to 66 cells (108-HC, 120-HC or 132-HC) and 96/104 cell formats. Does not include commercial panels greater than 2.0m in length.
Below is the latest Clean Energy Reviews downloadable chart of the most efficient residential solar panels for 2023, with PV cell technology details added for comparison.
Why efficiency matters
The term efficiency is thrown around a lot but a slightly more efficient panel doesn’t always equate to a better quality panel. Many people consider efficiency to be the most important criteria when selecting a solar panel, but what matters most is the manufacturing quality which is related to real world performance, reliability, manufacturers service, and warranty conditions. Read more about selecting the best quality solar panels here.
In environmental terms, increased efficiency generally means a solar panel will pay back the embodied energy (energy used to extract the raw materials and manufacture the solar panel) in less time. Based on detailed lifecycle analysis, most silicon-based solar panels already repay the embodied energy within two years, depending on the location. However, as panel efficiency has increased beyond 20%, payback time has reduced to less than 1.5 years in many locations. Increased efficiency also means a solar system will generate more electricity over the average 20 year life of a solar panel and repay the upfront cost sooner, meaning the return on investment (ROI) will be improved further.
Longer life and lower degradation
Solar panel efficiency generally indicates performance, especially as most high-efficiency panels use higher-grade N-type silicon cells with an improved temperature coefficient and lower power degradation over time. efficient panels using N-type cells benefit from a lower rate of light-induced degradation or LID, which is as low as 0.25% of power loss per year. When calculated over the panel’s 25 to 30 year life, many of these high-efficiency panels are guaranteed to still generate 90% or more of the original rated capacity, depending on the manufacturer’s warranty details. Due to the higher purity composition, N-type cells offer higher performance by having a greater tolerance to impurities and lower defects, increasing overall efficiency.
Area Vs Efficiency
Efficiency does make a big difference in the amount of roof area required. Higher efficiency panels generate more energy per square meter and thus require less overall area. This is perfect for rooftops with limited space and can also allow larger capacity systems to be fitted to any roof. For example, 12 x higher efficiency 400W solar panels, with a 21.8% conversion efficiency, will provide around 1200W (1.2kW) more total solar capacity than the same number of similar size 300W panels with a lower 17.5% efficiency.
- 12 x 300W panels at 17.5% efficiency = 3,600 W
- 12 x 400W panels at 21.8% efficiency = 4,800 W
In real-world use, solar panel operating efficiency is dependent on many external factors. Depending on the local environmental conditions these various factors can reduce panel efficiency and overall system performance. The main factors which affect solar panel efficiency are listed below:
The factors which have the most significant impact on panel efficiency in real-world use are irradiance, shading, orientation and temperature.
The level of solar irradiance, also referred to as solar radiation, is measured in watts per square meter (W/m2) and is influenced by atmospheric conditions such as clouds smog, latitude and time of year. The average solar irradiance just outside the Earth’s atmosphere is around 1360 W/m2, while the solar irradiance at ground level, averaged throughout the year, is roughly 1000W/m2, hence why this is the official figure used under standard test conditions (STC) to determine the solar panel efficiency and power ratings. However, solar irradiance can be as high as 1200W/m2 in some locations during the middle of summer when the sun is directly overhead. In contrast, solar irradiance can fall well below 500W/m2 on a sunny day in winter or in smoggy conditions.
Naturally, if a panel is fully shaded, the power output will be very low, but partial shading can also have a big impact, not only on panel efficiency but total system efficiency. For example, slight shading over several cells on a single panel can reduce power output by 50% or more, which in turn can reduce the entire string power by a similar amount since most panels are connected in series and shading one panel affects the whole string. Therefore it is very important to try to reduce or eliminate shading if possible. Luckily there are add-on devices known as optimisers and micro-inverters, which can reduce the negative effect of shading, especially when only a small number of panels are shaded. Using shorter strings in parallel can also help reduce the effect of shading, as the shaded panels in one string will not reduce the current output of parallel unshaded strings.
Efficiency Vs temperature
The power rating of a solar panel, measured in Watts (W), is calculated under Standard Test Conditions (STC) at a cell temperature of 25°C and an irradiance level of 1000W/m2. However, in real-world use, cell temperature generally rises well above 25°C, depending on the ambient air temperature, wind speed, time of day and amount of solar irradiance (W/m2). During sunny weather, the internal cell temperature is typically 20-30°C higher than the ambient air temperature, which equates to approximately 8-15% reduction in total power output. depending on the type of solar cell and its temperature coefficient. To provide an average real-world estimate of solar panel performance, most manufacturers will also specify the power rating under NOCT conditions or the Nominal Operating Cell Temperature. NOCT performance is typically specified at a cell temperature of 45°C and a lower solar irradiance level of 800W/m2, which attempts to approximate the average real-world operating conditions of a solar panel.
Conversely, extremely cold temperatures can result in an increase in power generation above the nameplate rating as the PV cell voltage increases at lower temperatures below STC (25°C). Solar panels can exceed the panel power rating (Pmax) for short periods of time during very cold weather. This often occurs when full sunlight breaks through after a period of cloudy weather.
The Power Temperature Coefficient
Cell temperatures above or below STC will either reduce or increase the power output by a specific amount for every degree above or below 25°C. This is known as the power temperature coefficient which is measured in %/°C. Monocrystalline panels have an average temperature coefficient of.0.38% /°C, while polycrystalline panels are slightly higher at.0.40% /°C. Monocrystalline IBC cells have a much better (lower) temperature coefficient of around.0.30%/°C while the best performing cells at high temperatures are HJT (heterojunction) cells which are as low as.0.25% /°C.
Temperature coefficient comparison
The power temperature coefficient is measured in % per °C. Lower is more efficient
- Polycrystalline P-Type cells. 0.39 to 0.43 % /°C
- Monocrystalline P-Type cells. 0.35 to 0.40 % /°C
- Monocrystalline N-type TOPcon. 0.29 to 0.32 % /°C
- Monocrystalline N-Type IBC cells. 0.28 to 0.31 % /°C
- Monocrystalline N-Type HJT cells. 0.25 to 0.27 % /°C
The chart below highlights the difference in power loss between panels using different PV cell types. N-type heterojunction (HJT), TOPcon and IBC cells show far lower power loss at elevated temperatures compared to traditional poly and monocrystalline P-Type cells.
Power Vs Temperature chart notes:
- STC = Standard test conditions. 25°C (77°F)
- NOCT = Nominal operating cell temperature. 45°C (113°F)
- (^) High cell temp = Typical cell temperature during hot summer weather. 65°C (149°F)
- (#) Maximum operating temp = Maximum panel operating temperature during extremely high temperatures mounted on a dark coloured rooftop. 85°C (185°F)
Cell temperature is generally 20°C higher than the ambient air temperature which equates to a 5-8% reduction in power output at NOCT. However, cell temperature can rise as high as 85°C when mounted on a dark coloured rooftop during very hot 45°C, windless days which is generally considered the maximum operating temperature of a solar panel.
most efficient solar Cells
The most efficient solar panels on the market generally use either N-type (IBC) monocrystalline silicon cells or other highly efficient N-type variations, including heterojunction (HJT) and TOPcon cells. Most manufacturers traditionally used the standard and lower-cost P-type mono-PERC cells; however, many large-volume manufacturers, including JinkoSolar, JA Solar, Longi Solar, Canadian Solar and Trina Solar, are now rapidly shifting to more efficient N-type cells using HJT or TOPcon cell designs.
Efficiency of panels using different cell types
- Polycrystalline. 15 to 18%
- Monocrystalline. 16.5 to 19%
- Polycrystalline PERC. 17 to 19.5%
- Monocrystalline PERC. 17.5 to 20%
- Monocrystalline N-type. 19 to 20.5%
- Monocrystalline N-type TOPcon. 21 to 22.6%
- Monocrystalline N-type HJT. 21.2 to 22.8%
- Monocrystalline N-type IBC. 21.5 to 23.6%
Several new variations of Interdigitated Back Contact (IBC) cell architectures have emerged, of which the exact cell construction has not been fully disclosed. This includes LONGi Solar’s Hybrid Passivated Back Contact (HPBC) technology and Aiko Solar’s ABC (All Back Contact) cell technology.
Cost Vs Efficiency
All manufacturers produce a range of panels with different efficiency ratings depending on the silicon type used and whether they incorporate PERC, multi busbar or other cell technologies. Very efficient panels above 21% featuring N-type cells are generally much more expensive, so if cost is a major limitation it would be better suited to locations with limited mounting space, otherwise, you can pay a premium for the same power capacity which could be achieved by using 1 or 2 additional panels. However, high-efficiency panels using N-type cells will almost always outperform and outlast panels using P-type cells due to the lower rate of light-induced degradation or LID, so the extra cost is usually worth it in the long term.
For Example, a high-efficiency 400W panel could cost 350 or more while a common 370W panel will typically cost closer to 185. This equates to roughly 0.50 per watt compared to 0.90 per watt. Although in the case of the leading manufacturers such as Sunpower, Panasonic and REC, the more expensive panels deliver higher performance with lower degradation rates and generally come with a longer manufacturer or product warranty period, so it’s often a wise investment.
Panel Size Vs Efficiency
Panel efficiency is calculated by the power rating divided by the total panel area, so just having a larger size panel does not always equate to higher efficiency. However, larger panels using larger size cells increases the cell surface area which does boost overall efficiency.
Most common residential panels still use the standard 6” (156mm) square 60-cell panels while commercial systems use the larger format 72 cell panels. However, as explained below, a new industry trend emerged in 2020 towards much larger panel sizes built around new larger size cells which increased panel efficiency and boosted power output up to an impressive 600W.
Common Solar panel sizes
- 60 cell panel (120 HC) : Approx width 0.98m x length 1.65m
- 72 cell panel (144 HC) : Approx width 1.0m x length 2.0m
- 96/104 cell panel: Approx width 1.05m x length 1.60m
- 66 cell panel (132 HC). Approx width 1.10m x length 1.80m
- 78 cell panel (156 HC): Approx width 1.30m x length 2.4m
A standard size 60-cell (1m x 1.65m) panel with 18-20% efficiency typically has a power rating of 300-330 Watts, whereas a panel using higher efficiency cells, of the same size, can produce up to 370W. As previously explained, the most efficient standard-size panels use high-performance N-type IBC or Interdigitated Back Contact cells which can achieve up to 22.8% panel efficiency and generate an impressive 390 to 440 Watts.
Popular half-cut or split cell modules have double the number of cells with roughly the same panel size. A panel with 60 cells in a half-cell format is doubled to 120 cells, and 72 cells in a half-cell format have 144 cells. The half-cut cell configuration is slightly more efficient as the panel voltage is the same but the current is split between the two halves. Due to the lower current, half-cut panels have lower resistive losses resulting in increased efficiency and a lower temperature co-efficient which also helps boost operating efficiency.
New Larger cells and high power 600W panels
To decrease manufacturing costs, gain efficiency and increase power, solar panel manufacturers have moved away from the standard 156mm (6”) square cell wafer size in favour of larger wafer sizes. There are a variety of various cell sizes now available with the most popular being 166mm, 182mm and 210mm. The larger cells combined with new larger panel formats have enabled manufacturers to develop extremely powerful solar panels with ratings up to 700W. Larger cell sizes have a greater surface area and when combined with the latest cell technologies such as multi-busbar (MBB), TOPcon and tiling ribbon, can boost panel efficiency well above 22%.
Enel North America to build 3 GW solar module manufacturing facility in U.S.
The proposed facility will be Enel’s second global PV manufacturing facility after Catania, Sicily, and once completed will be the largest U.S. PV module manufacturing facility.
Enel’s 3Sun Gigafactory in Catania, Italy.
Image: Enel North America
Enel North America affiliate 3Sun USA is scouring the U.S. for locations to build a 3 GW bifacial solar module and cell manufacturing facility with plans to scale up production at the facility to 6 GW per year.
Construction at the yet undisclosed location is to begin in the first half of 2023, with production anticipated by late 2024. The facility is expected to create up to 1,500 new full-time jobs and supports the domestic solar PV supply chain.
“Recent policy tailwinds from the Inflation Reduction Act have served as a catalyst for our solar manufacturing ambitions in the US, ushering in a new era of made-in-America energy,” said Enrico Viale, head of Enel North America.
The proposed facility will be Enel’s second global PV manufacturing facility. The company previously announced the expansion of its 3Sun gigafactory in Catania, Sicily, increasing production capacity from 200 MW to 3 GW.
For the U.S. facility Enel intends to produce the same type of modules as its Sicily plant is producing, the bifacial heterojunction (B-HJT) PV cells, which capture more sunlight as the cells can respond to light on both front and rear surfaces.
3Sun’s B-HJT PV cells have already produced at high-efficiency levels. In February 2020 Enel announced that the cells carried a 24.63% efficiency, setting a record established by the Institute for Solar Energy Research in Hamelin, Germany.
Currently fewer than five large-scale solar gigafactories (over 1 GW) are operating in the U.S., while annual PV installations are projected to grow from 16 GW to 41 GW in 2025, according to Wood Mackenzie.
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Michael joined pv magazine USA in November 2022. He has been reporting on the United States solar, distributed energy, power and utility markets since 2014 and previously reported on MA, private equity and energy technology start-up companies for corporate finance news publications.
From 11% Thin Film to 23% Heterojunction Technology (HJT) PV Cell: Research, Development and Implementation Related 1600 × 1000 mm2 PV Modules in Industrial Production
Plasma-enhanced chemical vapor deposition (PECVD) developed for thin film (TF) Si:H-based materials resulted in large area thin film PV cells on glass and flexible substrates. However, these TF cells demonstrate low power conversion efficiency PCE = 11% for double and PCE = 13% for triple junction cells below predicted PCE ≈ 24%. PV cells on crystalline silicon (c-Si) provide PCE ≈ 17–19%. Cost of c-Si PV cells lowered continuously due to reducing price of silicon wafers and enlarging their size. Two factors stimulated a combination of PECVD films and c-Si devices: (a) compatibility of the technologies and (b) possibility for variation of electronic properties in PECVD materials. The latter results in additional build-in electric fields improving charge collection and harvesting solar spectrum. We describe a transformation of PECVD TF solar cell technology for 11% efficiency modules to heterojunction technology (HJT) c-Si modules with 23% efficiency. HJT PV structure comprises c-Si wafer with additional junctions created by PECVD deposited layers allowing development of single wafer PV cells with PCE ≈ 24% and the size limited by wafer (15.6 x 15.6 cm2). The chapter starts with background in PECVD and c-Si PV cells. Then, in Section 2, we describe electronic properties of PECVD materials in HJT PV structures. Section 3 deals with structure and fabrication process for HJT devices. In Section 4, we present and discuss performance characteristics of the devices. Section 5 describes implementation of the developed HJT module (1600 x 1000 mm2) based on HJT single wafer cells in industry with presentation and discussion of characteristics related to industrial production. Finally, Section 6 presents the outlook and summary of the chapter.
- photovoltaic solar cells
- HJT silicon solar cells
- solar cell modules
- plasma deposition
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Market of PV devices shows continuous increase, for example, for only 1 year, that is, 2016–2017, it has grown from 76 GW to about 100 GW (by more than 30%) . PV devices based on silicon dominate in the market (90%). Interdigitated back contact (IBC) cells on monocrystalline n-type silicon demonstrate mass production efficiency, PCE = 23% (2016) with prognosis to rise to PCE = 27% by 2027 . Fabrication of these devices is complicated because of multiple deposition and etching steps required to form both p- and n-doped contact areas on rear surface of the cells. over this fabrication is based on conventional crystalline silicon technology including high temperature processes. Alternative and relatively simple approach to get high efficiency defined as heterojunction technology (HJT) includes deposition of thin layers by plasma-enhanced chemical vapor deposition (PECVD) conducted at low temperature. PECVD technique provides a wide range of possibilities for material engineering with variation of structure, electronic properties and doping of the films. These films can be used for surface passivation and for creation of additional built in electric field at interfaces with silicon. HJT solar cells exhibit PCE = 22% (2016) with prognosis for rise to PCE = 24% in 2027 . Furthermore, a combination of HJT and back contact technology will allow to overcome PCE values predicted for conventional IBC cells made with diffusion approach as it is confirmed by a world record PCE values above 26% reported for such cells .
PECVD is a rather mature industrial technology exploited to fabricate both PV modules on both glass substrate with dimensions up to 2200 × 2600 mm 2 and flexible plastic or metal foil substrates. The best developed PECVD PV structures provide efficiency, that is, PCE = 11% for “micromorph” two junction tandem  and PCE = 13% for triple tandem on stainless steel foil . These values are less than those theoretically predicted PCE = 24%. Therefore, PECVD PV solar cell modules on glass are not able to compete with those based on crystalline silicon technology for terrestrial applications, though they occupy a segment of flexible solar cells in PV market. Advantages of PECVD technology for material engineering together with compatibility of this technique with c-Si technology made promising implementation of PECVD materials in c-Si PV technology resulting in development of HJT solar cells. The latter is attractive because of PECVD is a low-temperature process and also because of its performance demonstrated.
This chapter describes our experience in research and development of HJT solar cells and modules based on our previous background in fabrication of “micromorph” modules; implementation of HJT modules consisted of 60 cells in industrial production is also discussed.
Configurations and fabrication of HJT devices
In this section, we describe basic configurations of solar cells based on crystalline materials. Figure 1 shows cross-section diagrams for crystalline silicon solar cell (a) fabricated by standard diffusion processes with typical efficiency of 17–19%, PECVD thin silicon film “tandem” structure (c) comprising two p-i-n junctions with efficiency of 9–11% and HJT silicon-based solar cell incorporating some PECVD films.
n-Type c-Si is a conventional material for HJT cells nowadays, although HJT solar cells based on p-type silicon with efficiency above 20% have been also reported, for example, see Ref. .
Despite using floating zone (FZ), c-Si for record cells and reasonable parameters obtained on high-quality multicrystalline wafers manufactured with direct solidification technique , crystalline silicon made by Czochralski (CZ) technique is conventionally used for HJT cells’ mass production. In this study, 6” CZ Si pseudo square n-doped wafers with typical resistivity in the range from 1 to 5 Ohm⋅cm were used. The wafers were sliced with diamond wire technology from ingots with low impurity level providing bulk lifetime of minority carriers τ 1 ms measured by transient photoconductance technique on ingots.
It is worthy of some Комментарии и мнения владельцев in terms of different HJT configurations. Frontal side of solar cell is determined as that for penetration of incident light, and opposite side is determined as rare (or back) side. There is also not well-justified term “emitter” which nevertheless is widely used in the literature, it is referred to the position of p(or p ) layer. Two configurations of HJT cells are possible: with frontal emitter meaning p-layer position on frontal side and rare (back) emitter meaning p-layer on rear side. For industrial production, to our mind rear emitter is preferable because of higher contribution of the wafer in lateral conductivity resulting in lower requirements for contact grid (lines may be narrower and separated by longer distance) and consequently, reducing shadow losses. In addition, employing n-layer made of nanocrystalline silicon (PECVD nc-Si) on frontal side results in reducing absorption losses from frontal side. However, lower holes diffusion length and nonuniform absorption of the incident light inside of c-Si wafer resulting in much higher carrier generation rate at front interface lead to record efficiencies on laboratory cells with frontal emitter configuration . As seen in Figure 1(b) and (c), the structures comprise different PECVD films such as un-doped (e.g., amorphous silicon a-Si:H, microcrystalline mk-Si:H), p-doped (e.g. p-a-SiC:H, p-mk-Si:H) and n-doped (e.g. n-a-Si:H, n-mk-Si:H). Electronic properties of these films are discussed in Section 3 and film deposition in Section 5.
Here, we would like to provide some Комментарии и мнения владельцев on functions of these films in device structures. Historically, first c-Si solar cells contained p- and/or n-doped layers prepared with diffusion, and they contacted to metals. They are characterized by significant losses at interfaces due to several processes, for example, free carrier absorption, surface recombination, and so on, and efficiency achieved is 17–19%. Further progress is related to the development of significantly more complex structures such as passivated emitter rear locally diffused (PERL)  and passivated emitter and rear cell (PERC) solar cells . Then PV structures prepared by heterojunction technology called HJT solar cells_ have been developed . In these HJT structures crystalline silicon surfaces is passivated by PECVD films which also create heterojunctions providing additional built-in electric fields, reduce surface recombination and back diffusion of photocarriers and serve as anti-epitaxial buffer.
Electronic properties of PECVD materials used in HJT solar cells
Let us consider some principal electronic properties of PECVD films used in HJT solar cells. They are listed in Table 1 that contains some electrical and optical characteristics of the films. It is seen that properties of PECVD films differ significantly from those in crystalline materials comprising the same atoms. For example, optical gap for c-Si Eg = 1.1 eV and for amorphous silicon a-Si:H Eg = 1.62–1.65 eV, there is also a difference in activation energy of conductivity. These films can be also doped in n- and p-type though with less efficiency of doping compared to crystalline one. Difference of these characteristics provides possibility creation of heterojunctions with crystalline silicon resulting in local built-in electric fields. Conventionally, PECVD films are deposited from hydride gases such as silane (SiH4), methane (CH4), and germane (GeH4), which are often supplied as a mixture with hydrogen. Thus, glow discharge during film growth contains significant amount of hydrogen in the form of molecules, atoms and ions. The latter two are very active chemically promoting passivation of substrate surface, which is of principle importance during fabrication of HJT solar cells.
Electronic properties of PECVD films incorporated in HJT solar cells.
E 04 values are presented.
There are various techniques to grow thin device quality films, for example, atomic layer deposition (ALD), hot wire (HW) deposition, inductively coupled plasma (ICP), direct current (DC), low frequency (LF), radio frequency (RF), very high frequency (VHF), microwave plasma in capacitance type reactors, and so on. Comparison of these techniques is out of the scope of the chapter; therefore, we only notice that the industry is mostly employed with RF and VHF PECVD systems. The latter type is used for fabrication of HJT solar cells in this chapter.
Nanometer-scale thicknesses of the PECVD films in HJT structures are really a challenge in material engineering and electronic characterization of such films. Conventionally, at initial stage, material of each film and its electronic properties are optimized by preparing the samples on appropriate substrate such as glass or silicon. Thickness of the film at this stage is more than 100 nm, while in HJT structures, we need 5–20 nm thickness. Therefore, questions arise: is it possible to characterize such thin films? Is it possible to apply electronic characteristics measured in thicker films in device design? Up to now, there are contradictory data reported. Some researchers have observed changes in electronic properties with thickness , while others have revealed such behavior. Here, we present some data obtained in thin films by attenuated total reflection infrared spectroscopy (ATR IR) spectroscopy technique allowing characterization of rather thin films. To our mind, both spectral ellipsometry and ATR IR are widely used technique for thin film characterization. ATR IR spectroscopy allows measurements of the films with thickness less than 20 nm, that is, in the range of thickness of the films in HJT device structures. Figure 2 shows IR spectra (measured by transmission on silicon substrate and ATR IR technique) of the films deposited on different substrate (glass and silicon) in the same run. The peaks for both curves are located at the value of k = 2000 cm −1 suggesting practically the same hydrogen bonding structure in the samples.
Figure 3 presents ATR IR spectra around k = 2000 cm −1 (Si-H stretching mode) for the intrinsic and doped samples with different thicknesses. One can see in the figure that using ATR IR absorption spectra of a-Si:H film, it is possible to observe Si-H stretching mode in the films with thickness less than 20 nm; therefore, this technique can be effectively used for optimization of the films with thickness required for HJT cells. It is clearly seen that all the curves are well centered at k = 2000 cm 1. No detectable changes have been observed with thickness.
This is revealed even more clearly in Figure 4, where the spectra are normalized at maximum value for both intrinsic and p-doped layer. Thus, we have demonstrated some evidence for negligible effect of thickness on microstructure and consequently on electronic properties.
In other words, some basic material optimization can be performed by optical measurements with ellipsometry or ATR IR spectroscopy in the films deposited on some acceptable substrates (e.g., on glass), which is of principal importance for optimization of uniformity and electronic properties in ultrathin films deposited in mass production PECVD systems.
After the basic optimization, the characteristics obtained can be extrapolated to the thicknesses of the films in device structures. However, such optimization is only initial stage because always growth conditions for the films even in the same run (with fixed operator deposition parameters) are different for the film deposited on glass from those when the film is deposited on stack of previously deposited films. Therefore, it should be noted that final optimization of the films is performed in concrete device structures for concrete film inserted between other materials. Figures of merit for such optimization are performance characteristics of the device.
Performance characteristics of HJT solar cells (single wafer devices)
Performance characteristics of both solar cells and modules allow obtaining finally power conversion efficiency (PCE) of solar energy into electric energy, to see harvesting photons of different energy of the Sun spectrum and to get insight into technological issues. In this section, we describe and discuss these characteristics for single wafer HJT solar cells.
PCE values are conventionally determined from current–voltage J(U) characteristics measured with illumination of solar simulator providing incident light intensity Iinc = 1000 W/m 2 and AM 1.5 conditions. An example of J(U) characteristic is shown in Figure 7, and the characteristics calculated from this J(U) curve are given in the insert. PCE is defined by the following equation:
where Jsc is the short circuit current density determined at U = 0, Uoc is the open circuit voltage determined at J = 0, Iinc is the incident light intensity and FF is the fill factor.
Progress in efficiency for single wafer HJT solar cells and some images of the samples as a function of RD time in RDC TF TE are presented in Figure 5.
Starting in 2014, with small area prototypes RDC TF TE in 2017 achieved PCE = 22.8% for solar cell with 156 × 156 mm 2 dimensions (corresponding area S = 244 cm 2 ) in 2017. This cell is the base for large area modules with dimensions 1600 × 1000 mm 2. Fabrication and characteristics of this module are presented and discussed in Section 5.
In order to improve light trapping in the device structures, Si wafers were textured using isopropanol alcohol (IPA) free alkaline process. Despite diamond wire, sliced wafer can be successfully textured from as cut state without surface damage etch (SDE) step, we used SDE because it facilitated uniform texturing, improved process stability and reduced production costs by lower consumption of surfactants providing anisotropic Si etching along direction. As a consequence, textured wafers have pyramidal surface topology with size and distribution of pyramids controlled by parameters of etching process. An example of AFM image of textured wafer is shown in Figure 6. One can see in the figure that surface of textured wafer is uniformly covered by pyramids with average height about 1.5 mkm.
Special attention has been paid to final cleaning of wafers surface from organic and metal impurities. We used several cleaning steps followed by final HF-dip and hot nitrogen drying procedure.
Current-voltage characteristic of our best cell measured under standard test conditions (STC) is shown in Figure 7 for single wafer HJT solar cell (156 × 156 mm 2 ). This cell exhibits such parameters as efficiency PCE = 23.04%, Uoc = 735 mV, Isc = 9.45 A and FF = 81.0%.
Spectral characteristics for different c-Si solar cells, including single wafer HJT solar cell, are presented in Figure 8. Comparing spectral curves in Figure 8, we can see that PERC solar cell has higher response in short wavelength range (λ 500 nm) and lower response in long wavelength region (λ 900 nm) than HJT device resulting in a small difference about 2% in integral current. Multicrystalline solar cell made by the BSF technology currently dominating in PV market has a little bit better response for λ 350 nm and worse response for λ 900 nm when compared to that for HJT device resulting in lower value of integral current. Both PERC and HJT silicon solar cells provide high and close values of integral short circuit currents.
Average values of characteristics are presented in Table 2.
|9.45 ± 0.01
|0.73 ± 0.01
|5.52 ± 0.01
|0.63 ± 0.01
|8.80 ± 0.01
|79.9 ± 0.1
|22.6 ± 0.1
|“Kaneka” 2017 
Performance characteristics of HJT single wafer (156 × 156 mm 2 ) HJT solar cells.
Calculation based on data in : area S = 180 cm 2. density of short circuit current Jsc = 42.5 mA/cm 2.
Dispersion values for the measured characteristics indicate rather good reproducibility of electronic properties observed in the device structures. For comparison, record data on HJT single wafer solar cell reported by “Kaneka Corp,” Japan in 2017 . This company increased its previous record in 2015 from PCE =25.1% to PCE = 26.6% in 2017 and the predicted efficiency exceeded 27% soon. It is worth to note that cells with record characteristics require usually special design and materials which may not be compatible with mass production conditions and/or facilities; however, such high efficiency level of laboratory cells demonstrates definitely potential for further improvement of HJT technology.
Implementation of the developed single wafer HJT structures in 1600 × 1000 mm 2 modules in industrial production
PECVD films for previous optimization were deposited in RDC TF TE laboratory system from “Oerlikon Solar,” Switzerland, model Gen 5 KAI, photo is presented in Figure 9(a). Reactor of this system is similar to the industrial system “KAI MT R1.0 Modular PECVD System” installed for industrial fabrication of the large modules S = 1600 × 1000 mm 2. Photo of this system is shown in Figure 9(b). In both PECVD systems, capacitive glow discharge at frequency f = 40.68 MHz, is used, deposition temperature is about Td = 200 C, technological gases of semiconductor purity.
We shall skip description of technological process details and discuss only some specific issues related to HJT with crystalline silicon wafers large area module fabrication. Initially, these systems were employed for fabrication of thin film double junction tandem module on glass substrate (see diagram in Figure 1(c)). For HJT solar cell module, silicon wafers are placed on a special wafer carrier developed by RDC TF TE and then loaded into reactor. details (process sequence, equipment, project milestones, etc.) on process of conversion silicon thin film solar module to high efficiency HJT module production can be found in ref. .
An important issue for large area modules is uniformity of electronic properties of the films deposited on large area substrate (or the carrier with 60 wafers).
Map of thicknesses for a-Si:H films deposited on large area glass substrate is shown in Figure 10 with mean thickness value = 48.5 nm and deviation of thickness =7.1%. In other words, thickness of the films over entire reactor active area (1300 × 1000 mm 2 ) is better than 10%.
Figure 11 shows five-point average thicknesses and five-point bandgap values for the array of 5 × 5 silicon wafers, with mean values of thickness d =94.7 and deviation 3.4% and with mean value of optical gap g =1.72 eV and deviation g =HJT 1.1%. These characteristics meet uniformity requirement for fabrication of HJT cells on wafer distributed over entire reactor area with thin films incorporated in the device structure.
Performance characteristic of “Hevel” HJT solar cell module is shown in Figure 12. Comparison of this characteristic with that for HJT module reported by “Meyer-Burger” is presented in Table 3.
Comparison of performance characteristics of 1600 × 1000 mm 2 HJT modules consisted of 60 single wafer cells made by “Hevel Solar” and “Meyer Burger” .
Note: PCE indicated in table are effective values calculated with integral module area without subtraction area occupied by electrode stripes and elements of hermetization and assembling.
One can see in Table 3 that the values reported by “Hevel Solar” are still less than those of “Meyer Burger.” However, this difference partially comes from using full square wafers in MB modules, which results in reduced dead space area and corresponding gain in current value. It should be noted that it is difficult to perform correct comparison because of the difference in the form of silicon wafers (not always reported), in normalization over area taking into account substrate area occupied by contact grid or not, and so on resulting in some uncertainties when comparing the devices.
Interesting data on outdoor 1-year testing of “Hevel Solar” HJT modules can be found in Ref.  (Figure 12).
Summary and outlook
We have briefly described a successful transformation of technology for thin film solar cell modules (1000 × 1300 mm 2 ) with efficiency 11% to heterojunction technology (HJT) for c-Si solar cell modules (1000 × 1600 mm 2 ) with efficiency around 20% with employing the same essential equipment for PECVD materials. Now, the developed HJT modules are commercially produced by “Hevel Solar” (Russia) .
PECVD technique being principal in HJT module fabrication for both passivation and growth of semiconductor films is very versatile technology with high potential for further material engineering.
Well-known theoretical estimation of efficiency for one bandgap material c-Si gives value around PCE ≈ 30–34%, while record value achieved in 2017 by “Kaneka Corp.” (Japan) is about PCE ≈ 27%. Thus some potential still exists for PCE increase for one gap c-Si HJT solar cells, which can be realized by improving passivation, electrodes, improving short wavelength collection by frontal interface, and so on.
General road to increase conversion efficiency is related to multijunction (MJ) design and fabrication of PV structures comprising materials with different bandgaps adjusted for harvesting maximum of solar energy spectrum. This has been demonstrated by MJ solar cells with A3B5 semiconductors provided the highest reported values of PCE = 46% .
Therefore, MJ approach should be taken into account considering further development of HJT c-Si solar cells with efficiency above 34%.
Conflict of interest
Hereby the authors declare a lack of any known for them conflicts of interests.
|plasma-enhanced chemical vapor deposition
|attenuated total reflection infrared spectroscopy
|Fourier transform spectroscopy
|heterojunction technology (devices, solar cells)
|interdigitated back contact (solar cells)
|power conversion efficiency
|optical gap, eV
|photovoltaic (structures, solar cells)
|transparent conductive oxide
|indium tin oxide
|un-doped amorphous silicon
|p-doped amorphous silicon
|n-doped amorphous silicon
|c-Si, (p)c-Si, (n)
|c-Si un-doped crystalline silicon, p- and n-doped crystalline silicon
|un-doped microcrystalline silicon
|n-doped microcrystalline silicon
|p-doped microcrystalline silicon
|p-doped microcrystalline silicon carbide
|back surface field (solar cell)
|silicon fabricated by Czochralski technique
|passivated emitter (usually p-Si) and rear cell (silicon solar cell).
|passivated emitter rear locally diffused (silicon solar cell).
|passivated emitter rear totally diffused (solar cells).
|short circuit current density, mA/cm2.
|short circuit current, A
|open circuit voltage, V, mV
|effective shunt resistance, Ohm
|effective series resistance, Ohm
|standard test conditions, output performance conditions used by most manufactures AM 1.5, I = 1000 W/m2, T = 25°C
Heterojunction Solar Cells: A Detailed Guide
Unlike other products, choosing solar cells comes down to a essential features, such as price, warranty, performance, and, most importantly, efficiency. And when it comes to efficiency, there is a new player in town that retailers should know about—heterojunction cells (HJT cells).
HJT cells combine the power of thin film absorption and passivation properties with the benefits of crystalline silicon solar cells. The result is highly efficient solar cells with lower final energy costs. Manufacturers worldwide are beginning to deploy HJT cell technology in their products, making now a perfect time to learn more about HJT solar cells.
What is a heterojunction solar cell?
An HJT solar cell is made by placing a crystalline silicon cell between two layers of thin amorphous silicon films. Hence, it combines the benefits of two technologies—crystalline silicon solar cells and thin film solar cells. As a result, HJT solar cells allow for more energy generation.
Crystalline silicon (mono or polycrystalline) cells are the most common solar cells. They are made by cutting silicon crystal blocks into thin sheets to form individual cells. On the other hand, photovoltaic (PV) cells are amorphous thin film solar cells. They can be made using a wide array of materials, the most commonly used material being silicon. Nonetheless, amorphous silicon doesn’t have a regular crystalline structure like crystalline silicon. Instead, the silicon atoms exist in random order, and they can be easily deposited onto any surface.
When it comes to production, amorphous silicon is cheaper to manufacture than crystalline silicon, which has to be grown into blocks and cut into sheets. However, on the flip side, amorphous silicon is less efficient than crystalline silicon.
So, HJT solar cells are made by coating an n-type crystalline silicon wafer with amorphous silicon on both sides alongside conductive oxide (TCO). TCO absorbs the power generated by the cell, and all the layers of thin film solar absorb extra photons.
How do heterojunction solar cells increase efficiency?
Before we get into the technical details, let’s understand solar panel efficiency. The efficiency of a solar cell refers to the amount of light it can convert into electricity. So, a highly-efficient solar cell can convert more electricity from the same amount of light than a less efficient solar cell.
Since the advent of solar cells, manufacturers and researchers have been trying to develop solar technologies that can squeeze more electricity from the same amount of sunlight. This idea is how HJT solar cells were developed.
Typically, solar cells are partially opaque. So, they only capture part of the sunlight that hits it. The rest passes through the cell or bounces off the surface. But HJT solar cells are made using three layers of photovoltaic material. In short, the middle layer is monocrystalline silicon, while the top and bottom layers are amorphous thin-film silicon.
During the light-absorbing process, the first photon reaches the top amorphous silicon layer. Then, it captures some sunlight and passes the rest to the middle layer. The middle monocrystalline layer converts a majority of photons into electricity, and the remaining photons are given to the bottom layer, which grabs the sunlight that would otherwise bounce off.
No doubt, a minute amount of sunlight still passes through the HJT cell, but the amount is considerably lower than traditional solar cells. So, HJT solar cells generate more electricity from the same amount of sunlight. And due to the three-layer technology, HJT solar cells reach an efficiency of around 26.81%.
Advantages of the heterojunction solar cell
There are several reasons behind HJT solar technology’s rising popularity. First, HJT solar cells are more efficient than standard crystalline solar cells. Second, they have nearly 26.81% efficiency at the laboratory level, and there could be more in store.
In addition, technologies like PERC used to achieve higher levels of efficiency are often costly. For example, Maxeon cells manufactured by SunPower feature a thick block of copper behind every cell. While this can significantly improve efficiency, copper is a costly metal. In comparison, HJT solar cells use amorphous silicon, which is relatively inexpensive. Therefore, it can be manufactured at a lower cost.
Nonetheless, manufacturers develop various HJT solar panels with different efficiency ratings. So, it depends on the silicon type used and the incorporation of cell technologies, which affects the price. For example, a highly efficient HJT solar panel with an efficiency of 400W might cost 350, whereas a 370W panel might cost around 185. However, the more expensive panels deliver higher performance and longer life expectancy.
To give you a more realistic idea, imagine an HJT panel of 400W and an efficiency of 26.81% running for 20 years (6 hours a day). Throughout its service life, it will generate 4697.112 KW hours of electricity. On the other hand, a p-type monocrystalline silicon panel, with an efficiency of 24%, would only be able to generate 4204.800 KW hours of electricity during the same service period. Therefore, it means HJT batteries are more economical in the long run.
Lastly, HJT solar cells have low-temperature coefficients. A lower temperature coefficient translates to better performance at higher temperatures. HJT cells have temperature coefficients of around.0.3%. Further, higher temperatures don’t affect these cells, and they sustain less performance loss over their cycles than crystalline or amorphous silicon cells.
The cost-effectiveness and other benefits of HJT solar cells signify a drastic rise in this technology’s adoption in the future. After all, the HJT manufacturing process features four fewer steps than PERC technology.
Several companies have already embraced HJT technology, including Panasonic HIT panels, REC Alpha panels, and SolarTech Universal. Also, according to the ITRPV 2019 report, the HJT solar cells market share will rise from 12% in 2026 to 15% in 2029–making it an excellent time to embrace the technology.