Perovskite-silicon tandem solar cells have practical efficiency potential of 39.5%

A group of researchers from Germany's Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) has estimated that the practical power conversion efficiency potential of perovskite-silicon tandem solar cells may reach up to 39.5%.

“The calculated practical efficiency potential of 39.5% for a perovskite silicon tandem device under standard measurement conditions (STC) can serve as an input for future research and development studies which are required for a better understanding of the full system prior to the commercialization of the promising tandem solar cell technology,” the research's corresponding author, Oussama Er-raji, told pv magazine.

“In this work, we extensively characterized a fully-textured perovskite silicon tandem solar cell to determine its performance losses,” said co-author Christoph Messmer. “By implementing the extracted device characteristics in a Sentaurus TCAD simulation model, we obtained a reproduction of the experimental device’s optical and electrical characteristics. Subsequently, by alleviating one step at a time the loss constraints, the simulation model identified the impact of each loss channel on the efficiency.”

Sentaurus CAD is a multidimensional simulator capable of simulating the electrical, thermal, and optical characteristics of silicon-based devices. It is also used to simulate the optoelectronic characteristics of semiconductor devices such as image sensors and PV cells.

In the study “Loss Analysis of Fully-Textured Perovskite Silicon Tandem Solar Cells: Characterization Methods and Simulation toward the Practical Efficiency Potential,” published in RRL Solar, the research team said it investigated voltage, current, and fill factor deficiencies in a fully-textured perovskite silicon tandem solar cell based on a 20 nm thick indium tin oxide (ITO) recombination layer placed between the top and bottom cells.

The subcell relied on a 630 nm thick perovskite absorber with an energy bandgap of 1.67 eV, a carbazole (2PACz) layer, a 20 nm thin tin oxide (SnO_x) buffer layer, an electron transport layer (ETL) based on a buckminsterfullerene (C60), an ITO window layer, a silver (Ag) metal contact, and an anti-reflective coating based on magnesium fluoride (MgF2). “The fully-textured perovskite silicon tandem solar cell delivers a stabilized 26.7% PCE when operated at a fixed voltage close to the maximum power point,” the researchers said.

The scientists conducted their comprehensive loss analysis on the tandem device considering high series resistance, current mismatch in the subcells, high nonradiative recombination losses, and band misalignment at the interface between the perovskite absorber and the ETL.

They identified the most significant loss mechanisms at the perovskite/C₆₀ interface (−4.6%), in the series resistance (−2.9%), and in light management (−2.1%). “By reducing the origins of these losses one step at a time, we calculated the efficiency improvements by cumulatively mitigating the characterized loss channels, ultimately reaching a practical efficiency potential of 39.5% for this device architecture,” they explained.

The researchers also noted that the 39.5% efficiency threshold may be exceeded only by changing cell architecture, eventually by replacing C₆₀ with a more transparent ETL and finding more transparent alternatives to the ITO layers. “Despite the fact that this analysis shows a quite idealized scenario when solving the device limitations, it is still useful to assess the ‘ideal' efficiency boost for each limitation,” they stressed.