Trimming optical losses in tandem perovskite cells

Researchers at Spain’s Charles III University of Madrid claim to have significantly reduced optical losses in a monolithic, nano-structured perovskite silicon tandem solar cell by using a new design.

Such two-terminal tandem cell devices are said to offer high conversion efficiency, due to a large number of layers, but to also suffer significant optical losses because of the high number of interfaces.

“Our proposed design increases the performance of both sub-cells by managing light towards the active layer, as well as by minimizing reflection losses in the interfaces,” the scientists wrote in the paper A monolithic nanostructured-perovskite/silicon tandem solar cell: feasibility of light management through geometry and materials selection, published in Nature.

Current-matching

The Madrid team said current-matching between the sub-cells was crucial to reducing optical losses, and in particular reflection losses, which would otherwise be exhibited during the extraction of photo-generated charges.

“The simplified design of the solar cell comprises the following layers from bottom to top: silver contact (300nm)/crystalline silicon (c-Si, 200µm)/indium tin oxide (ITO, 44nm) and a nano-structure sandwiched between the electron transport layer (ETL) and the hole transport layer (HTL) with a width regulated by a dielectric spacer (DS),” the paper noted.

For the simulation, the researchers used the methylammonium lead iodide (MAPbI₃) perovskite, one of the most commonly applied in solar energy research, to fill the gaps between the numerous cell layers. The analysis – which is said to have explored the geometrical dimensions and materials for the electron transport layer, the hole transport layer and the dielectric spacer – meant the researchers could discover the value ranges of refractive indices which could enable the minimization of optical losses.

Difficulty

The Madrid team admitted their cell architecture may be particularly difficult to scale up to mass production but pointed out, according to the literature it would not be unfeasible, as more complex structures of a similar kind have been fabricated.

“We obtain a reduction in the optical losses, in particular they are more than 33% lower than those of a planar cell, mainly due to a reduction of the reflectivity in the device,” the scientists wrote. “An evident reduction of the losses using the nano-structure appears, achieving almost a reduction of 50% in the best case.”

The Spanish team claim to have also demonstrated a wide range of dimensions of the layers which could allow current-matching between them while maintaining good optical performance.

In July 2018, Belgian research institute Imec announced it had achieved 27.1% efficiency in a perovskite/silicon tandem cell by reducing interconnection losses.