‘n-type Solar Cell Technology,’
Center of Change in Solar Modules
Nov, 2021

 n-type, Leader of Future Solar Cell Market 

In 2017, the Korean government announced the Renewable Energy 3020 Implementation Plan. The plan sets a goal to expand installed capacity to 30.8 GW and boost the share of renewable energy to 20 percent by 2030. Related plans - the 3rd Five-Year Plan for Green Growth, the 3rd Basic Plan on Electricity Demand and Supply, the 4th Energy Development Plan, and Korean New Deal Policy – were announced in conjunction with the policy on the energy market and carbon conversion in 2019. According to the 5th Basic Plan on Renewable Energy Technology Development and Supply, the aim is to reduce greenhouse gas (GHG) emissions by 69 million tCO₂, which is equivalent to planting about six billion trees, by supplying renewable energy by 2034. Demand for diversifying PV plant locations to overcome the limitations of existing technology is growing as the mid-and long-term strategy has been recently established. In this respect, demand for high-efficiency and high-capacity solar power cells and modules has increased, while transitioning from the existing p-type crystalline silicon solar cells to highly efficient n-type silicon solar cells.

Most of the p-type solar cells in production have a PERC (passivated emitter and rear contract) structure with average efficiency of around 22.5 percent. PERC brought improvements in both price and efficiency, and dramatically changed the PV market.  

* PERC (passivated emitter and rear contact): is a technology that boosts generating efficiency with an extra layer on the backside of a solar cell, allowing more sunlight to be reflected into the cell. 

< Wafer Type Market Share Forecast by (Left) ITRPV and (Right) PV-tech, Source: Verband Deutscher Maschinen- und Anlagenbau (VDMA) and PV Tech Research>

Although the market share forecasting data that ITRPV (International Technology Roadmap) and PV-tech announced predicted that the n-type market would grow while the p-PERC cell market would stabilize, a report published by PV-tech in 2021 forecasted that the n-type market will start to grow from 2024 and completely replace p-type by 2028.

Solar cell structures can be categorized into three types: n-type single-crystalline HJT (Heterojunction), IBC (Interdigitated Back Contact), and TOPCon (Tunnel Oxide Passivated Contact). The latest module trend expects that the market growth will be centered on HJT and TOPCon solar cells. 

<p- and n-type Solar Cell Efficiency Increase Trend, Source: Korea Institute of Energy Research>

 Journey of Silicon Solar Cells, From Aerospace To Key Power Source of Today’s Renewable Energy 

Forty years ago, early commercial silicon solar cells were used mostly for space power application and satellites. Studies on early solar cells conducted by Bell Laboratories, an American cable and telecommunication research lab, were mostly related to investigating problems affecting space solar cell designs and wafer types. 
The study results showed that all sorts of radiation from space noticeably degraded the *minority carrier lifetime of wafers, and this has become a significant performance indicator in wafer competitiveness to date. When minority carrier lifetime in a wafer is shortened, accordingly, the diffusion length of electron and **hole decreases, as does the carrier collection probability. This means a decrease in solar cell efficiency. As the performance of the solar cell has improved, usage has also changed from aerospace to onshore power generation, and now it has become the core power source for renewable energy.

1) Reasons the p-type Wafer Was At the Center of Attention

<Characteristics of n- and p-Type Wafer Solar Cell with High Energy Radiation, Source: Solar Cells and Their Mounting>

During the high energy radiation test to study space solar cell performance, the n-type wafer demonstrated excellent properties in BOL (beginning of lifetime), but the p-type wafer exhibited better properties in EOL (end of lifetime). This is why the p-type drew more attention, considering the long-time use of solar cells. If the effective lifetime is the same, mobility of minority carriers becomes more important. Comparing the mobility of each type’s minority carrier, the electron, which is the p-type’s minority carrier, is three times better than the hole, and this is the reason why making a p-type solar cell is determined to be advantageous. Accordingly, levels of understanding and technology readiness on p-type wafer solar cells has increased and that has led p-type to dominate solar cell manufacturing infrastructure and expanded its application to onshore power generation.

* Minority Carrier: The less abundant chargers become minority carriers. In n-type cells, they are electrons, whereas holes are the minority carriers in p-type cells.    
** Hole: An electric charge carrier with a positive charge

2) Growth of High-Performance n-type Wafer

As the application of solar cells moved to onshore power generation, BOL became more significant than EOL. On the ground, destruction or lifetime depression of elements caused by cosmic high energy radiation, including *x-ray and **gamma-ray, from space or the sun occurs much less compared to in space. This means that loss of durability due to high energy exposure should be considered less important because generation on the ground offers a better environment to maintain early wafer properties. In fact, silicon wafer solar cells showed only a slight drop in efficiency after long-time use for ground-based power generation from its initial use.

Wafer production requires a large amount of energy. Hence, wafers account for a fairly high proportion of solar cell production costs, while wafer slicing is getting thinner to extract a larger number of slices from a limited area. 

However, it is not just about price. It is because a better surface ***passivation led by technology advancement can increase collection probability according to the diffusion length of electrons (or holes) by using thin wafers, and passivation has become a method to increase solar cell efficiency. As the loss (rebonding) of electrons (or holes) on a wafer surface decreases, effective lifetime within silicon wafers became significant. Therefore, the use of n-type wafers, which have a longer initial effective lifetime, means increased efficiency. Considering the latest trends – large power output by reducing cost in a limited installed area - in solar power generation, increasing efficiency by changing wafers is an attractive option even apart from the issue in the process.

Solar cells undergo high-temperature exposure, including diffusion and electrode formation. The exposure results in shortening the effective lifetime of minority carriers by inducing activation of foreign substances in wafers. Most solar cell structures, except certain structures like heterojunction solar cells, are exposed to high temperatures of over 800℃. Therefore, it is important to choose an n-type wafer with high resistance to foreign substances in the solar cell efficiency competition. 

LID (light induced degradation) also can be an important issue. LID mainly occurs due to a reaction of boron and oxygen, foreign substances in p-type wafers, whereas n-type wafers are relatively free from LID. Hence, continuous initial power generation performance can be expected after installing solar panels. 

* X-ray: Electromagnetic waves with a wavelength of 0.01 – 10 nanometers. These have the shortest wavelength after gamma rays. 
** Gamma Ray: There is no fixed wavelength limit, but it is an electromagnetic wave with the shortest wavelength of 10-11 m or below, or with a frequency of 1019 Hz or above.
*** Passivation: Passivation is a technology to reduce the number of dangling bonds on the surface of silicon wafers using thin film and slow the speed of rebonding to increase the efficiency of solar cells.

 Present Status of n-type Solar Cells 

Along with the possibility of mass production of the TOPCon (Tunnel Oxide Passivated Contract) structure, n-type solar cells are receiving the most attention. The TOPCon solar cell structure presented by Fraunhofer ISE, a German research institute, uses thin tunnel oxide film and a doping polysilicon layer on the back of the cell. This back-structure dramatically reduces the rebonding speed on the back by improving the selectivity of majority carriers and minority carriers, and can, as a result, increase the open-circuit voltage of solar cells. In solar cells, it is important to separate and collect electrons and holes formed after irradiation. That is, by improving the selective collecting capability of majority and minority carriers, the open-circuit voltage and efficiency of solar cells are improved through the reduction of rebonding of electrons and holes. 

In terms of solar cell price per instantaneous peak power, n-type is about 1 - 1.3 times higher than p-type. However, the n-type has better efficiency compared to p-type and therefore its LCOE (Levelized Cost of Energy) could be equal or lower, depending on conditions. According to the latest data from PV Infolink, it is forecast that TOPCon solar cells and HJT will share the production capacity of n-type solar cells in 2025, and TOPCon solar cells are expected to be over 70 GW. 

 Leading Next-Generation Technology! Q CELLS Efforts for Quality Products 

Q CELLS, a global top-tier manufacturer of solar modules, announced that it is developing an n-type solar cell at a global R&D center in Thalheim (Saxony-Anhalt, Germany) with a EUR 3.5 million (KRW 48.3 billion) investment. Eventually, Q CELLS will manufacture and sell ‘Q.TRON,’ a n-type solar module. It is expected that the company will continue to introduce leading PV technology to the market based on bold investment and technological excellence. 

Q CELLS will make huge contributions to the successful execution of the Renewable Energy 3020 Implementation Plan and the 2050 Carbon Neutral Strategy with its development of the high energy n-type PV module. 

#Energy insight
#Solar Modules
#n-type Solar Cell Technology
#Silicon solar cells
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