BRANCHEDFLUORENYLIDENE DERIVATIVES WITH LOW IONIZATION POTENTIALS AS HOLE-TRANSPORTINGMATERIALS FOR PEROVSKITE SOLAR CELLS

Today, everyday appliances have become an essential part of life, with most of them relying on electricity to function. One of the primary methods of generating electricity is through non-renewable energy sources. However, these sources have significant drawbacks, as their byproducts are often non-biodegradable and can harm the environment. As a result, researchers have been exploring alternative energy generation methods using renewable sources such as wind, solar, and hydropower. Among these, the Sun is considered the most powerful energy source. Solar cells are devices that convert sunlight into electricity, with silicon-based solar cells being the most widely used technology. However, they come with limitations, leading to the development of more efficient alternatives. The evaluation of perovskite solar cell evaluation began more than ten years ago; nevertheless, expensive semiconductors, such as Spiro-OMeTAD are still used as a benchmark for hole-transporting layers. Studies have shown that adopting a similar molecular design can lead to good results in both efficiency and stability. [1,2] There are two main strategies developing materials similar to Spiro-OMeTAD: (1) using spirobifluorene as a central fragment while replacing 4-methoxyphenyl chromophores [3, 4], or (2) mimicking the Spiro-OMeTAD core by carefully selecting central fragments [5, 6]. The first method is generally considered as more reliable, simpler, and more successful in achieving high device performance. However, the second approach has also gained popularity due to its potential to produce more cost-effective and stable compounds. Based on previous study [7], it has been observed that the proper number of carbazolyl fragments can, in some cases, enhance device stability. Therefore, 9-ethyl-N-(4-methoxyphenyl)-9H-carbazol-3-amine was chosen as a side chromophore. In order to maintain similarity to Spiro-OMeTAD, fluorenylidene fragments were introduced into the molecular "central part" and linked through various aromatic moieties. In this work we synthesized new hole transporting materials that can be divided into two groups: molecules containing two fluorenylidene fragments, and compounds with three fluorenylidene moieties. Thermal and photoelectric properties were investigated and compared with Spiro-OMeTAD. All synthesized hole- transporting materials (HTMs) exhibit good thermal stability. In this study, no additional phase transitions were detected, confirming the complete amorphous nature of the materials. Consequently, these materials are likely to produce high-quality layers. The branched structure compounds were utilized as HTM layer in n-i-p architecture solar cells. The PCE of the most efficient n-i-p PSCs perovskite device containing carbazolyl-terminated Spiro-OMeTAD analogue V1387 has reached 22.13 %. Furthermore, solar cell containing V1387 exhibits good long-term stability and outperforms the device with Spiro-OMeTAD. The project "Technological and Physical Sciences Excellence Centre (TiFEC)" No. S-A-UEI-23-1 is funded by the Science Council of Lithuania and the Ministry of Education, Science and Sports of the Republic of Lithuania from the state budget under the programme "University Excellence Initiative".