Perovskite solar cells (PSCs) require high efficiency and good long-term stability that must come at low costs to achieve the commercial viability. Counting more than 20 years, spiro-OMeTAD now dominates the field of PSCs and is the hole-transporting material routinely employed as highly efficient reference material for the research interests despite its high price (~250 $/g) [1]. Although PSCs have recently achieved certified power conversion efficiency (PCE) exceeding 25%, one major bottleneck of stabilizing the performance is the lack of stable HTMs, which extract positive charges from the active perovskite light absorber and transmit them to the electrode [2].

Here, four HTMs containing well-established spirobifluorene core with introduced enamine arms were synthesized by a simple condensation reaction that does not require expensive catalysts, inert reaction conditions and time-consuming sophisticated product purification. Therefore, it may result in significantly reduced synthesis costs.

Ionization potential values of V1305, V1306 that have one and two enamine arms, respectively, tetra-substituted V1307 and trans-configurated V1308 were found to be 5.33, 5.37, 5.46, and 5.46 eV, respectively, which is significantly stabilized comparing to that of spiro-OMeTAD (5.00 eV). Based on the solid-state optical gap and ionization potential values electron affinities were calculated to be 2.43, 2.53, 2.63, and 2.68 eV for V1305, V1306, V1307 and V1308, respectively. It is important that the electron affinities are smaller comparing with the conduction band (CB) energy of the perovskite (-4.10 eV), therefore effective electron blocking from the perovskite to the electrode should be ensured. Xerographic time of flight (XTOF) technique was used to determine the charge mobility of investigated HTMs layers. V1308 and V1307 exhibited the highest zero-field hole drift mobility among the series having the values of 9.4 × 10^-4 and 6.4 × 10^-4 cm²/Vs, respectively, both outperforming that of spiro-OMeTAD (1.3×10^-4 cm²/Vs).

n-i-p solar cells were fabricated with the device layout FTO/c-TiO₂/m-TiO₂/SnO₂/perovskite/HTM/Au to test novel materials as HTMs. The most efficient devices containing V1305, V1307 and V1308 presented similar power conversion efficiency values of 19.0, 19.2 and 19.1%, respectively. Data analysis confirms the correlation between hole drift mobility and device performance, being most efficient the devices containing the HTM with higher hole mobility values following the order: V1306˂V1305˂V1308˂=V1307. This work shows that simple enamine condensation protocol could be used as universal approach achieving HTMs for highly efficient and stable PSCs.