Perovskite Tandem May Be the Ideal Path

Tandem cell technology is an important innovation in the photovoltaic field. It achieves a photoelectric conversion efficiency that exceeds the theoretical efficiency limit of single-junction cells by stacking and combining two or more solar cell layers made of different materials.

The Working Principle of Tandem Cells

Since perovskite and silicon have different bandgaps, in order to make full use of the solar spectrum, perovskite solar cells can be used as the top cell to form a tandem solar cell with a silicon cell, that is, a perovskite/crystalline silicon tandem solar cell, and the structure is as shown in the figure. The working principle of the perovskite/crystalline silicon tandem solar cell is to use different bandgaps to absorb different solar spectra and improve the conversion efficiency of the cell. The perovskite cell and the silicon cell are stacked from the outside to the inside in the order of the energy gap from large to small, so that the short-wavelength light is absorbed by the outermost wide-bandgap perovskite solar cell, and the long-wavelength light can penetrate and be absorbed by the narrow-bandgap silicon solar cell. This makes it possible to convert light energy into electrical energy to the greatest extent, greatly improving the utilization rate of the solar spectrum, the performance and stability of the cell. When the bandgap of perovskite is 1.55 eV, it can absorb photons with a wavelength of less than 800 nm, while a silicon cell with a bandgap of 1.12 eV can absorb photons with a wavelength of less than 1100 nm. When the two form a tandem cell, their absorption spectra are complementary, which not only greatly improves the utilization rate of the solar spectrum but also reduces the preparation cost.

Two-terminal Tandem Perovskite Cells Are Expected to Be an Ideal Technology Path

The development path of perovskite cells is expected to transition from a single-layer structure to a four-terminal tandem, and the two-terminal tandem may become the ideal technology route in the future. From the perspective of the industrialization development path, in the early stage of the development of perovskite solar cells, the single-layer structure is dominant. After the single-junction cell technology matures, it is expected to gradually develop towards tandem cells, initially mainly with a four-terminal tandem structure, and then it is expected to gradually develop into a two-terminal tandem structure.

The four-terminal tandem structure is relatively simple, and there is no need to add new process equipment in the cell process. Traditional crystalline silicon manufacturers are more likely to accept it. It is expected that in the early stage of the development of tandem cells, it may transition with a four-terminal structure.

The two-terminal tandem structure has a higher process difficulty, but it can save packaging costs. At the same time, there is less parasitic absorption, higher efficiency, and great potential for commercial production. Once the process matures, two-terminal tandem cells are expected to become the ultimate technology route.

Figure: Single-layer, Two-terminal and Four-terminal Tandem Structures of Perovskite Cells

Perovskite/HJT Tandem May Be a Better Solution

The two-terminal tandem cell technology path can be divided into perovskite/TOPCon tandem and perovskite/HJT tandem.

The preparation of HJT cells is a low-temperature process, generally below 250°C, which is more suitable for the low-temperature preparation process of perovskite; while the temperature of the phosphorus diffusion process of TOPCon cells is about 750°C.

The surface of HJT cells is a TCO layer. When preparing tandem perovskite cells, perovskite can be directly stacked, and the process route is more compatible; while the surface of TOPCon cells is a silicon nitride layer. Silicon nitride is not conductive, and the silicon nitride layer needs to be removed or modified while ensuring the passivation effect.

In terms of conversion efficiency, HJT and perovskite are complementary in spectral absorption, enabling a higher conversion efficiency.

Comparison of TOPCon/HJT Perovskite Tandem Technology Solutions