When acquiring new solar panels, customers consider aspects like power output, efficiency, aesthetics, and even solar cell technology like Interdigitated Back Contact (IBC) or Passivated Emitter and Rear Contact (PERC), but few pay attention to the inner layers of the cell that constitutes an N-type or P-type solar panel.
The aforementioned aspects are quite important, but choosing a photovoltaic (PV) module featuring a P-type solar cell or an N-type solar cell, can make the difference in the performance and lifespan of the module. In this article, we will explain to you the structure of both types of solar cells, how they work, the differences and advantages of N-type and P-type solar panels, and other interesting details.
Table of Contents
Overview: Inner structure of solar panels and how they work
The most knowledgeable photovoltaic enthusiast might know a thing or two about the structural design and operation of solar cells, including facts like their structure, materials, and others.
While this is the case, it is always important to go through an overview of the subject before diving into the structural differences that make a P-type solar panel and an N-type solar panel.
Materials and structure of a solar cell
The materials and structure of a solar cell, vary slightly depending on the technology used to manufacture the cell. Traditional cells feature Aluminum Back Surface Field (Al-BSF), but there are newer technologies in the market including PERC, IBC, and bifacial technology. Understanding the traditional technology provides you with enough background to understand the differences between P-type and N-type solar cells.
Structure of Al-BSF c-Si Solar Cell | Source: Adapted from SinoVoltaics
Solar cells are structured with a P-N junction, featuring a P-type crystalline silicon (c-Si) wafer with additional holes (positively charged) and an N-type c-Si wafer with additional electrons (negatively charged). The order for the P-type and the N-type wafer varies, with the upper and thinner layer being the emitter, and the lower and thicker layer being the bulk region.
P-type c-Si wafers are made by doping high-purity c-Si with boron, which is a material featuring fewer electrons, producing positively charged wafers. Similarly, an N-type c-Si wafer is doped with phosphorous, which is a material featuring additional free electrons and therefore negatively charges the wafer. These layers placed one on top of another, create an internal electric field.
Al-BSF solar cells feature an Aluminum Back Surface Field and a full area of aluminum for the rear contacts and silver strips made from printed silver paste for the front contacts. A dielectric passivation layer is also coated on top of the emitter to prevent corrosion in the absorber layer of the cell.
How do solar cells generate power? Understanding the photovoltaic effect
After understanding how traditional solar cells are made, it is important to understand how they work. Solar cells produce power under the photovoltaic effect. This is a phenomenon where photons reaching the cell, excite electrons in the N-type semiconductor layer, ejecting them from the absorber layer as they form an electron-hole pair.
Photovoltaic Effect | Source: EIA
Excited electrons are collected at the negative terminal of the solar cell, flowing through the closed circuit, and finally returning through the positive terminal to recombine with a hole, ending that particular electron-hole pair. As electrons flow through the closed circuit, they generate an electric current that powers the load.
The explained photovoltaic effect is responsible for generating solar power in photovoltaic technology. This is a constant process happening as solar panels are exposed to the right amount of solar radiation.
N-type vs. P-type solar panels: What’s the difference and what’s better for you?
Most P-type and N-type solar cells are the same, featuring slight and very subtle manufacturing differences for N-type and P-type solar panels. In this section, you will learn about the difference between these two, why P-type solar panels became the norm in the industry and the advantages of N-type solar panels.
Structure difference between P-type & N-type solar panels
In the overview section, we explained that the absorber layer of the solar cell, features an N-type and a P-type c-Si wafer, with a varying order for the layers. One of the layers is called the bulk region and it is thicker than the emitter, which is placed on top of the bulk region. The variation in which wafers are placed is what makes the solar cell to be an N-type solar cell or a P-type solar cell.
Structure of the P-type (left) and N-type (right) solar cell | Source: Gangopadhyay, U. et al
- P-type solar panels are the most commonly sold and popular type of modules in the market. A P-type solar cell is manufactured by using a positively doped (P-type) bulk c-Si region, with a doping density of 1016cm-3 and a thickness of 200μm. The emitter layer for the cell is negatively doped (N-type), featuring a doping density of 1019cm-3 and a thickness of 0.5μm.
- N-type solar panels are an alternative with rising popularity due to their several advantages over the P-type solar panel. The N-type solar cell features a negatively doped (N-type) bulk c-Si region with a 200μm thickness and doping density of 1016cm-3, while the emitter layer is positively doped (P-type) featuring a density of 1019cm-3 and thickness of 0.5μm.
To summarize, the main aspect that makes P-type and N-type solar cells different is the doping used for the bulk region and for the emitter. When phosphorous is used to negatively dope the bulk region this creates an N-type solar cell, meanwhile when boron is used to positively dope the crystalline silicon in the bulk region, this makes a P-type solar panel.
How did P-type solar panels become the norm in the solar industry?
When photovoltaics were been researched back in the 50s, manufacturing costs were extremely high, but this was not a limitation for space applications requiring a viable power source in space, where there were no other ways to generate power for a spacecraft like the Vanguard 1, the first satellite featuring solar panels in space.
As space applications became a priority, P-type solar panels featuring a high resistance to radiation and degradation in space, became an interest. A large number of resources were used in space photovoltaic application technology back in the 50s. The solar industry just kept the momentum going, using this well-researched technology by lowering prices to produce better P-type solar panels for terrestrial applications. This is why this technology became the norm for the industry.
Advantages and rising popularity of N-type solar panels
Even though P-type solar panels have been the norm for years, this technology has its flaws, especially for terrestrial applications. Boron is used for doping P-type solar panels, but they cause a problem known as a boron-oxygen defect (not a problem in space where there is no oxygen). This defect produces a high amount of Light-Induced Degradation (LID) in P-type solar panels, reducing their performance by up to 10% in some cases.
N-type solar panels doped with phosphorous instead of boron, are completely immune to the boron-oxygen defect that would otherwise reduce its performance. Since there is no LID, there is no fast-paced performance degradation as with P-type solar panels. N-type solar panels also feature a higher conversion efficiency over their lifespan, turning them into a better investment.
Benefits & advantages of N-type and P-type solar panels
Understanding structural differences between N-type and P-type solar panels can shine some light on the benefits and advantages of each technology. To further explain these, we have compared N-type vs. P-type solar panels in the table below.
Warranties vary between manufacturers
The main advantage of N-type vs. P-type solar panels is the lack of a boron-oxygen defect reducing the performance of the module by up to 10% in just a few weeks, which is caused by the LID. N-type solar panels are immune to this phenomenon and only suffer from regular degradation over the years.
Since holes are minority carriers in bulk regions for N-type solar panels, there is less of a window for the recombination process to occur, causing modules to feature higher efficiencies. N-type solar panels currently have achieved an efficiency of 25.7% and have the potential to keep on increasing, while P-type solar panels have only achieved an efficiency of 23.6%.
Manufacturing costs represent one of the few disadvantages of N-type solar panels. P-type solar panels are a highly developed technology with matured mass manufacturing, which reduces its cost. Manufacturing processes for N-type solar panels are quite similar, but some different steps in the manufacturing process cause a higher cost for these modules in the present.
Last but not least, the warranty for N-type solar panels truly proves that the technology is better and can be backed up for a longer time by the manufacturer. These modules feature a 20-year warranty for the product and a 30-year warranty for the performance, while many P-type solar panels are only backed with a 12-year warranty for the product and a 25-year warranty for the performance.
N-type solar panels: Present and future
The N-type solar panel is a highly valuable technology that is becoming widely popular in the present. The development of this technology will most likely keep on growing in the near and distant future.
The conversion efficiency of N-type solar panels is one of the features where this technology is growing, having its record broken three times in less than a year. The first time was Jinko Solar which broke the record in the last quarter of 2021, featuring an efficiency of 25.4%, the second one was Trina Solar in the first quarter of 2022 with an efficiency of 25.5%, and the third one was Jinko Solar again in the second quarter of 2022, setting a record efficiency of 25.7%.
Solar cell wafer materials market share | Source: ITRPV (2021)
As N-type solar panels become a more attractive option, their market share will keep on growing in the future. In 2021, the market share for N-type solar panels oscillated between 5% and 10%, with a growing expectancy to around 20% in 2022. As popularity keeps on rising, N-type solar panels will hold more than 70% of the market by 2032, probably leaving P-type modules with less than 30% of the market.