Researchers at China's Tsinghua University, Zurich University of Applied Sciences and University of Ferrara have developed a perovskite solar cell with a new hole transport material that promises enhanced efficiency and stability while also ensuring a scalable fabrication technique.
The team explained that the new organic hole-transporting material, named T2, offers a performance advantage over conventional materials like spiro-OMeTAD as its characteristics, including unique electronic, structural, and chemical properties, synergistically enhance the efficiency of hole extraction and significantly reduce charge recombination at the interface with the perovskite layer.
The cell was built with thermally evaporated perovskite films. The fabrication process that yields these thermally evaporated perovskite films can be suitable for large-scale production, and is different than the spin-coating methods traditionally used.
The scientists synthesized the T2 using thiomethyl-substituted fluorene as the arm structure and spiro-[fluorene-9,9′-xanthene] as the core. This combination is said to offer a better band alignment and hole extraction than the more expensive spiro-OMeTAD.
They fabricated the cell with a substrate made of fluorine-doped tin oxide (FTO), an electron transport layer (ETL) based tin oxide (SnO2), a perovskite absorber, a hole transport layer (HTL) based on T2, and a gold (Au) metal contact.
Tested under standard illumination conditions, the device achieved a power conversion efficiency of 26.41%, an open-circuit voltage of 1.175 V, a short-circuit current density of 26.47 mA cm−2, and a fill factor of 84.94%. For comparison, a reference cell with an HTL based on spiro-OMeTAD reached an efficiency of 24.43%, an open-circuit voltage of 1.154 V, a short-circuit density of 25.94 mA cm−2, and a fill factor of 81.57%.
The group claimed the result represents the highest efficiency ever recorded for a perovskite solar cell using “alternative” HTLs. The cell based on T2 reportedly showed better stability under continuous illumination, thermal heating, and storage in ambient air compared with the cells with spiro-OMeTAD because of the suppressed ion migration and the resulting chemical reactions.
The researchers also said they were able to identify the atomistic origin of the improved hole extraction, which they attributed to a better energy level alignment and the overlapping partial local density of electronic states (LDOS) between the valence band maximum of perovskite absorber at the interface and T2 energy level.
Using this cell architecture, the team also built a mini solar module with a substrate size of 5 × 5 cm2, which was reportedly able to achieve an efficiency of 21.45%, an open-circuit voltage of 4.385 V, a short-circuit current density of 6.41 mA cm−2, and a fill factor of 76.31%.
