The Principle of HJT Solar Cells

HJT solar cells work similarly to other photovoltaic modules in terms of the photovoltaic effect, the main difference in this technology is the use of a three-layer absorbing material that combines thin film and conventional photovoltaic technology. The process involves connecting a load to the module's terminals, where photons are converted to electrical energy and produce a current that flows through the load.

To generate electricity, photons hit the P-N junction absorber and excite electrons, which move to the conduction band and produce electron-hole (e-h).

The excited electrons are collected by the terminals connected to the P-doped layer, generating a current that flows through the load.

After flowing through the load, the electrons flow back to the back contact of the cell and recombine with the holes, ending that particular e-h. This happens continuously as the module generates power.

A phenomenon called surface compounding occurs in standard c-Si PV modules, which limits their efficiency. In this process, excited electrons are paired with holes on the surface of the material, causing them to recombine without the electrons being collected and flowing as current.

To reduce surface compounding, HJT solar cell use a passivated semiconductor film to separate the highly recombinant active (ohmic) contacts from the wafer-based layer, which has a wider band gap layer made of a-Si:H. This buffer layer allows the charge to trickle slowly enough to generate high voltages, yet fast enough to avoid compounding before collecting electrons, thereby increasing the efficiency of the heterojunction cell.

During the light absorption process, all three semiconductor layers will absorb photons and convert them into electrical energy.

The first photons that arrive will be absorbed by the external a-Si:H layer, converting them into electrical energy. However, the majority of the photons are converted by the c-Si layer, which has the highest solar energy conversion efficiency of any material in the cell. The remaining photons are ultimately converted by the a-Si:H layer on the back side of the module. This three-step process is the reason why single-sided heterojunction solar cells achieve solar efficiencies of up to 26.7%.