Starburst Carbazole Derivatives as Efficient Hole Transporting Materials for Perovskite Solar Cells

Five new star‐shaped carbazole‐based molecules are successfully synthesized from low‐cost, commercially available reagents via a simple one‐step synthesis route. All carbazole derivatives comprise a 3,6‐diaminocarbazole core with carbazole peripheral groups substituted at the 2‐ or 3‐positions and various aliphatic side chains. These molecules are evaluated as hole transporting materials to replace 2,2′,7,7′‐tetrakis‐( N , N ‐di‐ p ‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) in perovskite solar cells. Power conversion efficiencies of the devices with these carbazole hole transporting layers reach 19.0%, comparable with 19.7% obtained with the spiro‐OMeTAD‐based device. The thermal and operational stability of the candidate molecules are found to depend on the side chain substituents. Two candidate molecules with ethyl side chains show superior thermal stability compared with that of the reference solar cells prepared with spiro‐OMeTAD.