Organometal halide perovskite-based solar cells represent a major breakthrough in emerging photovoltaic technology as the reported power conversion efficiency has extended beyond 20%. However, the fundamental photophysics underlying this remarkable performance was, until now, poorly understood and a topic hotly debated. A key fundamental question limiting basic understanding of hybrid perovskite photophysics pertains to the nature of elementary photoexcitations that are generated immediately after the absorption of light, which can be either free charge carriers or bound electron-hole pairs (i.e., excitons). The intrinsic differences between these species leads to vastly different descriptions of excited state processes and energy flow inside a solar cell. Although exciting discoveries have been reported on these materials, nearly all of the optical based measurements average over the spatially heterogeneous morphology of these materials, thus obscuring essential electronic level details taking place in distinct crystalline grains, each with remarkably different size, shape, relative orientation, and level of defects. As such, until now, conflicting reports have surfaced that show evidence for the existence of excitons or charge carriers and have lead to ambiguities in the interpretation of experimental results.
This paper reports, for the first time, experimental evidence for the coexistence of both excitons and free charge carriers at spatially distinct locations in these perovskite thin films. This is accomplished using femtosecond transient absorption microscopy to acquire ultrafast snapshots of electronic excitations and their spatial distributions that describe relaxation dynamics evolving on the femtosecond (10-15) and picosecond (10-12) timescales. The approach had a typical temporal resolution of 300 fs and a spatial resolution of 350 nm that is sufficient to resolve ultrafast dynamics associated with individual grains, which have typical sizes ranging from 1-4 mm. Detailed analysis of the electronic relaxation dynamics acquired at distinct spatial locations in the thin films of both pristine perovskite and its composite structure formed with PCBM enabled us to separate signatures of excitons and free charge carriers. These observations help to settle the ongoing debate in the literature regarding the nature of the fundamental photoexcitation in these materials, and raises new questions as to what drives excited state heterogeneity in these material and how one can control it. Results further demonstrate that joint spatial and temporal resolution is essential to accurately describe the photophysics of spatially heterogeneous systems. Advances in describing such spatially dependent ultrafast dynamics can open up avenues to improve material synthesis and fabrication techniques aimed at achieving higher power conversion efficiencies.
