To further understand the optoelectronic properties of metal halide perovskites, we investigate sub-bandgap absorption in methylammonium lead iodide (MAPbI 3 ) films. Charge carrier dynamics are studied using time-resolved microwave conductivity measurements using sub-bandgap excitation. From changes in the decay dynamics as a function of excitation energy and intensity, we have identified three regimes: (i) Band-like charge transport at photon energies above 1.48 eV; (ii) a transitional regime between 1.48 and 1.40 eV; and (iii) below 1.40 eV localized optically active defects (8 × 10 13 cm −3 ) dominate the absorption at low intensities, while two-photon absorption is observed at high intensities. We determined an Urbach energy of approximately 11.3 meV, indicative of a low structural and/or thermal disorder. Surprisingly, even excitation 120 meV below the bandgap leads to efficient charge transfer into electron (C60) or hole (spiro-OMeTAD) transport layers. Therefore, we conclude that for MAPbI 3 , the band tail states do not lead to nonradiative losses.

Despite intense research into the optoelectronic properties of metal halide perovskites (MHPs), sub-bandgap absorption in MHPs remains largely unexplored. Here we recorded two-photon absorption spectra of MHPs using the time-resolved microwave conductivity technique. A two-step upward trend is observed in the two-photon absorption spectrum for methylammonium lead iodide, and some analogues, which implies that the commonly used scaling law is not applicable to MHPs. This aspect is further confirmed by temperature-dependent conductivity measurements. Using an empirical multiband tight binding model, spectra for methylammonium lead iodide were calculated by integration over the entire Brillouin zone, showing compelling similarity with experimental results. We conclude that the second upward trend in the two-photon absorption spectrum originates from additional optical transitions to the heavy and light electron bands formed by the strong spin-orbit coupling. Hence, valuable insight can be obtained in the opto-electronic properties of MHPs by sub-bandgap spectroscopy, complemented by modelling.

Hydroxyapatite–doxorubicin nanotheranostic agents are used for tumor-targeted chemotherapy, providing a platform for ratiometric real-time tracking of drug release.

Abstract3D hybrid perovskites (APbX3) have made a significant impact on the field of optoelectronic materials due to their excellent performance combined with facile solution deposition and up‐scalable device fabrication. Nonetheless, these materials suffer from environmental instability. To increase material stability, the organic cation (A) is substituted by the non‐volatile cesium cation. However, the desired photoactive cesium lead(II) iodide black phase is metastable in ambient conditions and spontaneously converts into the photo‐inactive yellow δ‐phase. In this work, the black phase is stabilized by the formation of a quasi‐2D perovskite containing a benzothieno[3,2‐b]benzothiophene (BTBT) large organic ammonium cation. Thermal analysis shows that degradation of the butylammonium (BA)‐based quasi‐2D perovskite (BA)2CsPb2I7 sets in at ≈130 °C, while (BTBT)2CsPb2I7 is phase‐stable until ≈230 °C. Additionally, the (BTBT)2CsPb2I7 film does not show any sign of degradation after exposure to 77% Relative Humidity in the dark for 152 days, while (BA)2CsPb2I7 degrades in a single day. Photoconductor‐type detectors based on (BTBT)2CsPb2I7 demonstrate an increased external quantum efficiency and a similar specific detectivity compared to the BA‐based reference detectors. The results demonstrate the utility of employing a BTBT cation within the organic layer of quasi‐2D perovskites to significantly enhance the stability while maintaining the optoelectronic performance.