Additives-free indolo[3,2-b]carbazole-based hole-transporting materials for perovskite solar cells with three yeses: Stability, efficiency, simplicity

Indolo[3,2-b]carbazole-based hole transporting materials (HTM1-3) are developed for dopant-free pemvskite solar cells (PSCs). The newly synthesized compounds are studied as alternatives of conventional hole-transporting materials which typically require additives, are characterized by low resistivity to penetration of water, complicated synthesis and purification. The influence of substituents of derivatives of indolo[3,2-b]carbazole on their physical properties, e.g. ionization potentials, hole mobilities, the temperatures of thermal transitions, is investigated using experimental and theoretical tools. Ionization potentials in the order HTM2 < HTM1 < HTM3 indicate good energy level alignment with the valence band maximum of the pemvskite layer. Time-of-flight hole mobilities in the order HTM3 (5.26 x 10(-3) cm(2)V(-1)s(-1)) > HTM1 (1.1 x 10(-3) cm(2)V(-1)s(-1)) > HTM2 (0.55 x 10(-3) cm(2)V(-1)s(-1)) without additives indicate good hole transporting properties, principally stemming from their small degrees of energetic disorder following the order HTM3 (73.4 meV) similar to HTM2 (73.2 meV) > HTM1 (59.5 meV). The influence of different combinations of these parameters results in the different power conversion efficiencies of the developed dopant-free PSCs: [19.45% for the device containing HTM2] similar to [18.75% for PCS containing HTM3] > [14.46% for the device containing HTM1]. The devices demonstrate considerably higher stability and practically comparable efficiency as additives-containing reference PSCs with conventional hole-transporting material spiro-OMeTAD.