Pressure-induced giant emission enhancement, large band gap narrowing and rich polymorphism in two-dimensional 1,2,4-triazolium lead bromide perovskite.

Layered lead halide perovskites are attractive materials for optoelectronic applications. In this work, temperature-dependent photoluminescence (PL) as well as pressure-dependent Raman and PL studies of lead bromide comprising small disc shape 1,2,4-triazolium cations (Tz) are reported. Temperature-dependent studies reveal that at room-temperature (RT) TzPbBr exhibits narrow emission at 2.89 eV related to a free exciton (FE). At low temperature, three new narrow and weakly red-shifted bands as well as a very broad and strongly red-shifted band near 2.2 eV appear. The narrow and broad bands are attributed to self-trapped excitons (STEs) trapped by shallow and deep donors (acceptors), respectively. Pressure-dependent Raman studies revealed the presence of three pressure-induced phase transitions near 2, 6 and 8 GPa to phases II, III and IV, associated with increased distortion of the inorganic subnetwork and apperance of a static disorder in phase IV. These structural changes affect the excitonic emission, which changes from a strong red shift on compression in the ambient pressure I to a weaker red shift in phase II, negligible shift in phase III and blue shift in phase IV. Moreover, a new narrow and weakly red-shifted band appears in phases II-IV. Most importantly, PL intensity increases 16.7 times when pressure changes from ambient to 7.77 GPa but decreases in phase IV. The increase in PL intensity can be attributed to the increase in STE formation energy and activation energy for non-radiative recombination, while the decrease in intensity may be related to the formation of point defects, which are the source of non-radiative recombination. Overall, high-pressure PL data show that application of external pressure allows band gap engineering and giant enhancement of the PL efficiency of TzPbBr perovskite.