A year after the then world-record efficiency of a perovskite-silicon tandem solar cell, researchers from EPFL and CSEM published a paper outlining the technical characteristics of the device and how this result was made possible. The key was to control the crystallization process of the perovskite by using an additive in the processing sequence.
However, the two scientific institutions did not release many details about the cell technology and how the new record was made possible. Now, more than a year later, they presented the cell and related manufacturing processes in a paper, “Interface passivation for 31.25% efficient perovskite/silicon tandem solar cells,” published last week Science.
“Our team took a pioneering approach by designing a tandem solar cell in which a perovskite layer is formally coated on top of a silicon-based cell,” said Xin-Yu Chin, lead author of the study. pv magazine. “The silicon-based cell featured micrometric pyramids, an industry-standard modification that improves its output of light current.“
One of the main challenges of perovskite/silicon tandem cells is the recombination losses on the top surface of the perovskite, which interfaces with the electron-selective contact. Recombination is the process by which photogenerated charge carriers—electrons and holes—recombine before they can be collected and used to generate electricity, resulting in efficiency losses. “To solve this problem, we included an additive in the processing sequence, which turned out to be important in controlling the crystallization process of the perovskite,” explained Chin. “This step effectively passivates the interface, effectively reducing recombination losses that degrade overall cell performance.”
The researchers used a phosphonic acid known as 2,3,4,5,6-pentafluorobenzylphosphonic acid (FBPAc) to passivate the perovskite absorber and another phosphonic acid, called methyl substtuted carbazole (Me-4PACz) to obtain passivated interfacial defects in the hole transport layer (HTL).
The cell is based on a glass and indium tin oxide (ITO) substrate, a Me-4PACz HTL, an FABr:FAI perovskite absorber with an energy bandgap of 1.70 eV, a buckminsterfullerene (C60) electron transport layer, a bathocuproine (BCP) buffer layer, and a copper-based top electrode ( Cu).
Tested under normal lighting conditions, the device showed an efficiency of 31.25%, a breakdown voltage of 1.91V, a short-circuit current of 20.47mA/cm2 and a duty cycle of 79.8%, all of which were certified by the USA. Department of Energy’s National Renewable Energy Laboratory (NREL).
“The use of Me-4PACz reduces the voltage drops at the perovskite/HTL interface, while the inclusion of FBPAc in the perovskite deposition sequence reduces the voltage drops at the perovskite/C 60 ETL interface and leads to more favorable perovskite microstructures with larger areas. The researchers highlighted and added that The outcome also suggests that the technology is ready to advance to the next stage of development, which requires a focused focus on stability and scalability.
“It is likely that the technology will take another 5-10 years to enter the market. Current industrial solutions are already applicable to all thin film materials used in tandem solar cells, as demonstrated by Oxford PV’s recent results,” Chin added. “The primary concern that the scientific community must address is the stability of the perovskite material. Can these materials be made stable enough to last more than 20 years in practical applications? This is a key question that will determine the commercial success and impact of this technology.”