Intramolecular and Interfacial Dynamics of Triarylamine-Based Hole Transport Materials

We present steady-state and time-resolved spectroscopic investigations combined with detailed kinetic modelling for the triarylamine derivatives X60 and PTAA which serve as hole transport materials (HTMs) in perovskite solar cells and represent model compounds for organic electronic materials. Photoexcitation of the spiro-fluorene-xanthene X60 populates its S1 state which decays via intersystem crossing (ISC) as well as fluorescence and internal conversion on a nanosecond time scale. Photoexcitation of the PTAA polymer leads to the formation of singlet excitons which relax and migrate in the time regime from a few to about a hundred picoseconds and decay to the ground state with a lifetime of ca. 900 ps. Both X60 and PTAA exhibit efficient photoinduced electron injection into mesoporous TiO2 thin films forming radical cations with characteristic spectral bands, as confirmed by spectroelectrochemistry in solution. Photoluminescence (PL) experiments performed for the HTMs on methylammonium lead iodide perovskite deposited on mesoporous TiO2 show that small molecular HTMs, such as X60, quench the PL much better than the PTAA polymer. Ultrafast transient absorption experiments on the other hand suggest that the hole transfer at the interface between the perovskite and these HTMs is very fast, regardless of the type of HTM. It is therefore concluded that small molecular HTMs infiltrate much better into mesoporous structures and therefore more efficiently accept holes from the perovskite on such thin film architectures due to the better interfacial contact compared with their polymer-based counterparts.