An international research group led by the Università Degli Studi Di Pavia in Italy has built a perovskite solar cell based on a photo-ferroelectric perovskite interface that reportedly helps reduce open-circuit voltage losses, which are typical of perovskite PV devices.
“The work provides a new interface engineering design incorporating an ultrathin ferroelectric two-dimensional perovskite (2D) which sandwiches a perovskite bulk,” the research's lead author, Giulia Grancini, told pv magazine. “The electric field generated and maintained by external polarization promotes charge extraction while minimizing interfacial recombination.”
Ferroelectric materials attracted the interest of the scientific community working on solar PV in recent years as they have unique piezoelectric and optoelectronic properties. Ferroelectric PV has raised attention, in particular, for its unusual photovoltaic effect and controllability. In these devices, the photogenerated voltage is independent of the bandgap along the polarization direction. However, the small photogenerated current remains one of the challenges that need to be overcome.
In the study “Photo-ferroelectric perovskite interfaces for boosting VOC in efficient perovskite solar cells,” published in nature communications, the academics explained that using two-dimensional (2D) ferroelectric perovskites in PV devices is still in its early stage. Perovskite cells built with 2D hybrid materials are generally known for being more stable than conventional, 3D devices, due to the protection provided by the organic ligands, and exhibit large exciton binding energies.
“We design a photo-ferroelectric 2D/3D/2D perovskite junction by integrating a 2D ferroelectric perovskite single crystals in the perovskite bulk, as a tool to manipulate the perovskite interfacial electric field,” they explained. “Upon external polarization, the 2D ferroelectric layer polarizes, generating an electric field that adds to the original built-in electric field.”
The perovskite absorber consists of a 3D triple-cation perovskite bulk sandwiched between thin layers of two ferroelectric 2D perovskite layers. The selected 2D material is a ferroelectric fluorinated perovskite known as (4,4-DFPD)2PbI4, (where 4,4-DFPD is 4,4-difluoropiperidinium).
The solar cell was built with a substrate made of indium tin oxide (ITO), a hole transport layer (HTL) relying on MeO-2PACz, the perovskite absorber, an electron transport layer based on based phenyl-C61-butyric acid methyl ester (PCBM), and a silver (Ag) metal contact. “The perovskite layer was deposited via a two-step spin-coating procedure at 1000 and 5000 rpm for 12 and 27 s, respectively,” the researchers explained.
The group tested the performance of the device under standard illumination conditions and found it achieved a power conversion efficiency of 24%, an open-circuit voltage of 1.21 V, and a fill factor of 84%. The open-circuit voltage achieved by the device is described as the highest value reported to date for highly efficient perovskite photovoltaics.
“The inclusion of the 2D interfaces improved the device fill factor and open-circuit voltage ultimately enhancing its efficiency,” the scientists said referring to a comparison they made with the performance of a benchmark device without the 2D perovskites.
The analysis also showed that the novel cell retained more than 90% of its relative efficiency after over 1000 h, which compares to only 40% in the control device.
“As a result of the interface modification, the additional electrical field induced by spontaneous polarization in the ferroelectric 2D layer adds to the device built-in, driving interfacial charges apart and reducing their recombination,” the group stated. “This concept opens a new path for the advancement of perovskite materials and interface engineering by leveraging the electrostatic fields produced by polar building blocks, with immediate impact on the working mechanisms and performances of optoelectronic devices.”
“This approach can be easily translated to industrial scale module pushing device stability and efficiency and offering a new path for interface manipulation,” Grancini concluded.
The research team included scientists from the Chinese Academy of Sciences (CAS), Saudi Arabia's King Abdullah University of Science and Technology (KAUST), and the Imperial College London in the UK.
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