SUSTAINABILITY Improving the efficiency of SHJ solar cells

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SHJ stands for “Silicon Heterojunction” solar cells, which is an integration of semiconductor technology with solar energy. The article starts with explaining SHJ solar cells in brief and goes on to describe methods based on recent studies to improve their efficiency.

Silicon heterojunction solar cells (n-type) contain an n-type crystalline silicon wafer (c-si) with coatings of p-type and n-type thin-film amorphous silicon layers (a-si). The c-silicon acts as the absorbing layer. An SHJ solar cell is named so because it contains a junction of crystalline and amorphous silicon- two materials with dissimilar band gaps and chemical properties.

The layers form a p-i-n-i-n structure. There are two junctions: front heterojunction and rear heterojunction. The combination of n-type a-si and n-type c-si forms the front heterojunction. The combination of p-type a-si and n-type c-si forms the rear heterojunction. An additional Intrinsic a-si layer is added in both junctions to improve the performance.

Anti-reflection coating prevents the reflection of sun rays and the recombination of electron-hole pairs. A conductive layer of ITO (Indium Tin Oxide) offers enhanced electrical conductivity. In addition, a wide bandgap layer is deposited to achieve a high open-circuit voltage. A high open-circuit voltage can draw maximum power during unfavorable weather conditions.

SHJ solar cells are highly efficient in converting solar energy to electricity and have bifacial capabilities due to both front and rear heterojunctions. The effective design separates metal contacts from the absorbing portion. The SHJ manufacturing process is complex.

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Understanding the difference between n- and p-type semiconductors

Presently, SHJ technology holds only 2 % of the PV market share. However, it has a high growth potential for the upcoming years. However, SHJ solar cells are used in utility sectors, homes, and commercial spaces.

Improving the efficiency of SHJ solar cells

New research is refining SHJ solar cells, pushing their efficiency even higher. This section explores how scientists are optimizing materials and processes to unlock even greater power conversion.

Careful designing

Optimize the design through metal contacts, back reflector, and deposition parameters.

Using ARC

Using anti-reflection coatings reduces the amount of sunlight reflection from SHJ solar cells.

Passivation through intrinsic layer deposition

As mentioned in the SHJ solar cell structural explanation (above), it is critical to add an intrinsic amorphous silicon layer (i-a-si) in the front and rear junctions to improve efficiency. A recent study showcases that processing the i-a-si layer into a new type of layer “(i)a-Si:H” results in an improved efficiency of up to 23.71 %.

Experimental explanation
A PECVD (Plasma Enhanced Chemical Vapour Deposition) method and VHF (Very High Frequency) treatment were able to carry out amorphous silicon deposition to manipulate its microstructure and extract a new compound film. Through this technique, an undoped thin film of hydrogenated amorphous silicon, called (i)a-Si:H has been introduced to the SHJ solar cell to test levels of passivation.

In simple words, passivation means the prevention of electron-hole recombination in SHJ solar cells. Passivation results in the presence of more electrons and holes, leading to low defects and increased charge carrier concentration, efficiency, and protection. This new study presents that (i)a-Si:H layer offers better passivation capabilities that result in lesser losses and higher efficiency.

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The selected temperature range is from 140 to 200 degrees Celsius. Higher temperatures resulted in better passivation due to the denser nature of films. For lower temperatures, the HPT (Hydrogen Plasma Treatment) of (i)a-Si:H results in a porous film that showcases exceptional levels of passivation quality for the SHJ solar cell.

The lower temperatures in the 140 to 200 degrees Celsius range result in high open-circuit voltage but poor FF. The FF (Fill Factor) is a figure of merit for a solar cell to describe its efficiency with lower losses. FF is the ratio of maximum power output to the product of open-circuit voltage and short-circuit current.

A balance between open-circuit voltage and FF is achievable between 160 to 180 degrees Celsius. The efficiency recorded for 160 degrees Celsius is about 23.71 %. In addition, optimizing the p-layers of SHJ solar cells further enables FF to reach 83.3 % and an efficiency improvement of about 24.18 % for the same temperature.

Optimizing the contact layers

A recent study increased the efficiency of SHJ solar cells up to 26.81 % by improving back contacts. In all the contact layers, effective doping is critical to enable passivation, favorable band bending to collect charge carriers and reduce energy gaps for current flow.

Experimental explanation
Various deposition methods including the CO2 method and many pre-treatment and post-treatment procedures with adjustments were implemented to improve the efficiency and FF of SHJ solar cells.

In traditional SHJ solar cells, electrical back contacts are placed on the front. But in a back contact SHJ solar cell, the back contact is placed towards the rear side. Back contacts on the rear side reduce losses due to shading.

The hole-sensitive contact is at the rear side of the SHJ solar cell. The back contact is generally hole-selective. Such a contact offers no opposition to positive charge flow and blocks electron flow to facilitate charge separation, band bending, and passivation.

Traditionally, the back contacts doped with p-type a-si were less effective. In this study, back contacts were optimized through a combination of p-type doped nanocrystalline silicon and a transparent conductive oxide with low sheet resistance. The results showed a decreased contact resistivity.

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The contact resistivity of hole-selective contact is decreased to offer better transportation of charge carriers. In addition to efficiency, the FF increased up to 86.59 %.

Stop ignoring p-type SHJ cells

In a recent study, an efficiency of 26.56 % has been attained for “unconventional p-type” SHJ solar cells. The p-type wafer is a boron-doped CZ (Czochralski) wafer for SHJ solar cells. This study uses gallium as a dopant instead of boron to avoid boron-oxygen defects.

Optimizing the contact layers with n-type or p-type doped a-si:H layer improves carrier sensitivity and charge collection capabilities with reduced parasitic parameters. A method known as phosphorus diffusion gettering pretreatment has improved the efficiency of the commercial p-type SHJ solar cell through nanocrystalline silicon (nc-si:H).

References

onlinelibrary.wiley.com

www.sciencedirect.com

www.nature.com

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