What is a heterojunction solar panel?
The assembly method of heterojunction solar panels is similar to that of standard homojunction modules, but the uniqueness of this technology lies in the solar cells themselves. To understand this technology, we provide you with an in-depth analysis of the materials, structure, manufacturing, and classification of heterojunction panels.
Materials required for manufacturing heterojunction solar cells
Heterojunction batteries use three important materials:
Crystalline silicon (c-Si)
Amorphous silicon (a-Si)
Indium tin oxide (ITO)
Crystal silicon is often used to manufacture standard homogeneous junction solar cells, as seen in traditional panels. There are two types of c-Si, polycrystalline silicon and monocrystalline silicon, but monocrystalline silicon is the only one considered for use in HJT solar cells because it has higher purity and is therefore more efficient.
Amorphous silicon is used in thin-film photovoltaic technology and is the second most important material for manufacturing heterojunction solar cells.
Although a-Si itself has density defects, the application of hydrogenation technology solves these defects and produces hydrogenated amorphous silicon (a-Si: H), which is more easily doped and has a wider bandgap, making it more suitable for manufacturing HJT batteries.
Indium tin oxide is the preferred material for transparent conductive oxide (TCO) layers in heterojunction solar cells, but researchers are studying the use of indium free materials to reduce the cost of this layer. The reflectivity and conductivity of ITO make it a better contact layer and outer layer for HJT solar cells.
The structure of heterojunction solar cells
The absorption layer of heterojunction solar cells includes a layer based on c-Si chips, placed between two thin intrinsic (i) a-Si: H layers, with doped a-Si: H layers placed at the top of each a-Si: H (i) layer. The number of TCO layers depends on whether the HJT battery is single-sided or double-sided, and the latter layer is a metal layer used as a conductor for single-sided heterojunction batteries.
Manufacturing of heterojunction solar cells
The manufacturing process of heterojunction solar cells involves several steps. These are:
Wafer processing
Wet chemical treatment
Core layer sedimentation
TCO deposition
Metallization
Crystal silicon processing includes cutting c-Si batteries with a diamond saw. Performing this process with extreme precision will produce high-quality c-Si layers, which will translate into higher efficiency.
During the wet chemical treatment process, organic and metal impurities are removed from the c-Si wafer. Wet chemical treatment usually uses two methods, one is the RCA method using concentrated sulfuric acid and hydrogen peroxide, and the other is a cost-effective alternative method using ozone based processes that can achieve similar results.
After wet chemical treatment, the deposition process using plasma enhanced chemical vapor deposition (PECVD) is applied to deposit amorphous silicon layers on both sides of the crystalline silicon substrate.
The second part of the deposition process uses physical vapor deposition (PVD) to apply ITO through sputtering, forming the TCO layer of heterojunction solar cells. Another process uses reactive plasma deposition (RPD) to apply the TCO layer, but this is a less popular choice.
The metalization process is different from conventional manufacturing processes because the hydrogen in a-Si: H limits the temperature to a maximum of 200-220 º C. Using specially treated low-temperature silver paste, the electrodes are placed on the battery through copper electroplating or screen printing processes.
Classification of heterojunction solar cells
Heterojunction solar cells can be classified into two types based on doping: n-type or p-type.
The most popular doping method uses n-type c-Si chips. These are doped with phosphorus, which provides them with additional electrons to make them negatively charged. These solar cells are not affected by boron oxygen, which reduces the purity and efficiency of the cells.
P-type solar cells are more suitable for space applications because they are more resistant to the radiation levels perceived in space. P-type c-Si chips doped with boron provide one less electron to the battery, making it positively charged.
How do heterojunction solar panels work?
The working principle of heterojunction solar panels under photovoltaic effect is similar to other photovoltaic modules, with the main difference being that this technology uses three-layer absorbing materials, combining thin films and traditional photovoltaic technology. This process involves connecting the load to the terminals of the module, converting photons into electrical energy and generating current that flows through the load.
In order to generate electricity, photons collide with the PN junction absorber and excite electrons, causing them to move to the conduction band and generate electron hole (eh) pairs.
The excited electrons are collected by terminals connected to the P-doped layer, generating a current flowing through the load.
After passing through the load, electrons flow back to the back contact of the battery and recombine with holes, ending a specific eh pair. As the module generates electricity, this situation continues to occur.
A phenomenon called surface recombination occurs in standard c-Si PV modules, which limits their efficiency. During this process, stimulated electrons pair with holes on the material surface, causing them to recombine, while electrons are not collected and flow in the form of current.
To reduce surface recombination, HJT batteries use a passive semiconductor film with a wider gap layer made of a-Si: H to separate highly composite active (Ohmic) contacts from chip based layers. This buffer layer makes the charge trickle slow enough to generate high voltage, but fast enough to avoid recombination before collecting electrons, thereby improving the efficiency of HJT batteries.
During the process of light absorption, all three semiconductor layers will absorb photons and convert them into electrical energy.
The first photon to arrive will be absorbed by the external a-Si: H layer, converting them into electrical energy.
However, most photons are converted by the c-Si layer, which has the highest solar energy conversion efficiency in battery materials. The remaining photons are ultimately converted by the a-Si: H layer on the back of the module. These three step processes are the reason why single-sided heterojunction solar cells achieve a solar energy efficiency of up to 26.7%.
