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One year after achieving what was at the time a world efficiency record for a perovskite-silicon tandem solar cell, scientists from EPFL and CSEM published an article showing the technical characteristics of the device and how it was made possible. that result.
The Swiss Center for Electronics and Microtechnology (CSEM) and the École Polytechnique Fédérale de Lausanne (EPFL) announced in June 2022 that they had achieved a power conversion efficiency of 31.25% for a tandem perovskite-silicon solar cell of 1 cm2, which according to them represented, at that time, a world record for a photovoltaic device of this type.
However, the two scientific institutions did not release many details about the cells technology or how the new record was made possible. Now, more than a year later, they present the cell and related manufacturing processes in the article " Interface passivation for 31.25%-efficient perovskite/silicon tandem solar cells." efficiency of 31.25%), published last week in Science .
“Our team took a pioneering approach by designing a tandem solar cell with a perovskite layer conformally coated on top of a silicon bottom cell,” Xin-Yu Chin, lead author of the research, told pv magazine . "The bottom silicon cell featured micrometer pyramids, an industry-standard modification that improves its photocurrent generation."
One of the main problems with perovskite/silicon tandem cells is the recombination losses that occur at the top surface of the perovskite, which interacts with selective electron contact. Recombination is a process in which photogenerated charge carriers - electrons and holes - recombine before they can be collected and used to produce electricity, causing efficiency losses. “To solve this problem, we incorporated an additive into the processing sequence, which was instrumental in regulating the perovskite crystallization process,” Chin explains. "This step efficiently passivated the interface, effectively mitigating recombination losses that hinder overall cell performance."
The scientists 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-substituted 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, an HTL of Me-4PACz, an absorber made of the FABr:FAI perovskite with an energy bandgap of 1.70 eV, a transport layer of buckminsterfullerene (C60) electrons, a bathocuproin (BCP) buffer layer, and a copper (Cu) top electrode.
Tested under standard lighting conditions, the device showed an efficiency of 31.25%, an open circuit voltage of 1.91 V, a short circuit current of 20.47 mA/cm2, and a fill factor of 79.8%. , all certified by the National Renewable Energy Laboratory (NREL) of the United States Department of Energy.
“The use of Me-4PACz reduces strain losses at the perovskite/HTL interface, while the inclusion of FBPAc in the perovskite deposition sequence reduces strain losses at the perovskite/C 60 ETL interface and leads to microstructures more favorable perovskite structures with larger domains,” the researchers noted, adding that the result also indicates that the technology is ready to move to the next phase of its development, which requires a central focus on stability and scalability aspects.
“It is likely that the technology will still need between 5 and 10 years to enter the market. Existing industrial solutions are already applicable to all thin-film materials used in the tandem solar cell, as exemplified by the recent results from Oxford PV,” added Chin. “The main concern for the scientific community to address is the stability of the perovskite material. Can these materials be stable enough to last more than 20 years in practical applications? This is the fundamental question that will determine the commercial success and impact of this technology.” |
