Densification-Related Optical and Photodetection Properties of Green-Synthesized MAPbI₃ and MAPbI₃@Graphite Powders

For photodetection applications using 3D hybrid perovskites (HPs), dense and thick films or compacted powders in wafer form are needed and generally require large amounts of HPs. HPs are also often combined with a graphene/carbon layer to improve their conductivity. Among HP synthesis methods, mechanosynthesis, a green synthesis method, provides a large amount of powders, which are furthermore easily densified in compact wafers due to their mechanical activation. Thus, methylammonium lead iodide (MAPI) and MAPI with 3 and 5 wt % graphite powders were synthesized, densified by uniaxial compaction, and their photoluminescent (PL) and photodetection properties were studied. MAPI wafers compacted under 100 and 500 MPa showed a PL blue shift compared to the MAPI powder, and photodetection measurements indicated that composite wafers exhibited an enhanced photoresponse with improved photocurrent generation due to the addition of graphite. However, they exhibited weaker photoswitching (on/off) sensitivity with high detected currents in comparison to the MAPI wafer. Such unexpected photodetection behavior with composite wafers were explained by characterizing their microstructural and optical properties. Microstructural characterizations showed no grain coalescence in all MAPI and composite wafers compacted at 100 MPa, but a preferential crystallographic orientation along the {002} plane was detected. Additionally, an unusual graphite segregation near the wafer surface was noticed in composite wafers, questioning the effect of graphite in photodetection performance. Furthermore, all optical measurements evidenced lattice distortions and a decrease of the PL reabsorption phenomenon as a function of the densification pressure, in agreement with the observation of a higher population of photoexcited charge carriers in MAPI compacted at 500 MPa, as observed in transient absorption spectroscopy. These findings demonstrate that green-synthesized MAPI and graphite composites can be easily shaped into dense wafers through simple low-pressure compaction and emphasize the importance of microstructural and optical characterizations to fully understand the resulting properties.