To overcome operational stability issues of wide-bandgap metal halide perovskite cells, a team from City University of Hong Kong designed a series of “multifunctional” organic redox mediators based on anthraquinone, which were able to “selectively reduce iodine and oxidize metallic Pb0, while simultaneously passivating defects through tailored cationic substitution”.
Both the stability and performance of perovskite solar cells utilizing the organic redox mediators based on anthraquinone improved, according to the researchers. The work involved a structural analysis and density functional theory calculations on “redox mediator/perovskite interfacial models to gain insights into the roles of AQS derivatives in facilitating the overall redox reaction and passivating perovskite”, as well as to estimate their interactions.
“It fell mostly to what we predicted after our thorough investigation of the underlying mechanism for causing halide segregation in wide bandgap perovskites-based solar cells under the operation conditions,” Alex Jen Kwan-yue, the research's corresponding author, told pv magazine. “After we figured out a few possible reasons, we have tailor-designed suitable redox mediators that not only can stabilize the halide segregation, but also passivate the defects formed during the processing and operation. These molecular engineered molecules were proven to be very effective in serving the purpose.”
For the target perovskite solar cell devices, redox mediator anthraquinone derivatives were added to the perovskite precursor, specifically three compounds known as AQSH, AQSN and AQSP. The resulting power conversion efficiency was 19.58% with a “high” open-circuit voltage of 1.35 V, whereas the control device had a power conversion efficiency of 18.68%. The target outperformance was primarily ascribed to the enhanced open circuit voltage.
These devices retained 95% of their initial efficiency after 500 hours of operation at maximum power point. The target short-circuit current density was 17.52 mA/cm2, with a fill factor of 82.74%.
The team subsequently integrated the perovskite solar cell into a two-terminal monolithic perovskite-organic tandem device. The resulting tandem cell achieved a certified efficiency of 24.27%, an open-circuit voltage was 2.151 V, a short-circuit current density of 14.36 mA/cm2, and a fill factor of 81.65%. The results of long-term operational stability were “impressive,” noted the team.
The single-junction perovskite wide bandgap subcell was fabricated on glass substrates pre-patterned with indium tin oxide glass featuring a p-type self-assembled monolayer (SAM), a perovskite film, and a C60 layer. The perovskite precursor composition comprised primarily (Cs0.2FA0.8Pb(I0.6Br0.4)3) and MAPbCl3.
The organic subcell used in the tandem had an efficiency of 16.62%, an open-circuit voltage of 0.840 V, a short-circuit current density of 26.58 mA/cm2, and fill factor of 74.43%. It was a single junction molybdenum oxide on indium tin oxide in a p-i-n type configuration on glass.
“Among tandem solar cells, perovskite-organic variants stand out due to their solution–processibility, highly tunable physicochemical properties, and cost-effective raw materials,” said Jen Kwan-yue, explaining the choice of a perovskite-organic solar cell combination for the tandem device.“Additionally, their flexibility and transparency open up various innovative applications, such as wearable electronics and building or vehicle-integrated PVs. These compelling advantages have driven our focus on perovskite-organic tandem solar cells.”
Looking at upcoming research plans, Jen Kwan-yue said, “Our research group is dedicated to developing new redox mediators with enhanced redox capabilities, superior electrical properties, and efficient defect passivation. Concurrently, we are working on improving the performance of perovskite-organic tandem solar cells through delicate interface engineering and molecular design for low-bandgap non-fullerene acceptors.”
A spinoff company, HKTech Solar, will commercialize the technology. It has attracted “significant funding” from investors and the government to develop PV products, according to Jen Kwan-yue.
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