The Sun is known to be the most powerful source of energy. Solar cells are devices that convert absorbed light into electricity and silicon solar cells are currently the most used technology to do that. However, it has its own drawbacks and other solar cell technologies have been developed. Perovskite solar cells (PSCs) have recently emerged as a promising technology for the further cost reduction of the energy production from the sunlight. However there are several obstacles that need to be addressed for it to see a widespread use. One of them is the use of expensive hole-transporting material (HTM) such as Spiro-OMeTAD to obtain efficient devices [1]. Therefore, there is vigorous research for cheaper and simpler methods for the synthesis of organic semiconductors. Carbazole derivatives are among the most popular structural building blocks, used for the construction of organic HTMs [2­‒4].

Five new star-shaped carbazole-based molecules are successfully synthesized from low cost, commercially available reagents via a simple one-step synthetic route. New π-electron starburst molecules are comprised of a 3,6-diaminocarbazole core with carbazole peripheral groups substituted at the 2- or 3- positions and various aliphatic side chains. The new molecules were evaluated as hole transport materials to replace 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) in perovskite solar cells. Amorphous, uniform thin films of these materials are readily fabricated by spin coating. Power conversion efficiencies of the devices with these carbazole hole-transporting layers reached 19.0%, comparable to 19.7% obtained with the Spiro-OMeTAD-based device. The thermal and operational stability the candidate molecules were found to depend on the side chain substituents. Compounds with short aliphatic chains showed exceptionally high glass transition temperatures over 200 °C, resulting in superior thermal stability compared to Spiro-OMeTAD. After heating at 120 °C in N₂ atmosphere, the efficiency of devices using Spiro-OMeTAD dropped to 9% while with the carbazole-based HTM, the device efficiency still remained at 60% of the initial value. Given their high efficiency, ease of fabrication, good stability, and favorable electronic properties, these simple and low-cost carbazole hole transport materials promise to be effective alternatives to Spiro-OMeTAD in commercial perovskite solar cells.