Effect of subcrystalline domains, grain boundaries and heterointerfaces on the carrier transport in perovskite solar cells
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Abstract
The climate crisis and ever-rising energy demand require clean, sustainable and affordable solutions for electricity generation. Perovskite solar cells (PSC), based on the perovskite compound methylammonium lead iodide (MAPbI3), offer high power conversion efficiencies of 25.5% and low production costs. By reducing losses in the charge carrier extraction, the efficiency of PSCs can push towards the radiative efficiency limit. Particularly the impact of local morphological features in the absorber and heterointerfaces on charge carrier dynamics is of great interest to further optimize PSCs. In this work, I explored how boundary structures on different lengths scales – from subcrystalline domains to grain boundaries in polycrystalline MAPbI3 films to heterointerfaces in PSCs – affect the charge carrier transport.
First, I studied a periodic subcrystalline domain pattern within MAPbI3 grains via piezoresponse force microscopy (PFM) and 2D x-ray diffraction for in-depth structural characterization. In-situ PFM across MAPbI3’s phase transition temperature and during the application of mechanical stress revealed the ferroelastic nature of the domains, while simultaneously demonstrating routes for strain engineering. Finally, the correlation of the domain pattern with spatial and time-resolved photoluminescence (PL) microscopy showed that the photocarrier diffusion depends on the orientation of the domain walls.
Furthermore, I investigated the impact of grain boundaries on the charge carrier drift and diffusion via conductive atomic force microscopy (CAFM) and spatial and time-resolved PL microscopy. CAFM measurements allowed the determination of single grain boundary resistances. Moreover, CAFM and PL results indicate that grain boundaries act as semitransparent barriers to the inter-grain carrier transport. Here, the transparency appeared to depend on the boundary morphology, including density and contact area. I observed a similar effect for PL-photons that become trapped in the MAPbI3 film due to internal reflection: Dense grain boundaries with large contact areas seemed to facilitate photon propagation with minimal outcoupling as required for efficient light management in PSCs.
Lastly, I studied the influence of different electron transport layers (ETL) on the photocarrier extraction using Kelvin probe force microscopy (KPFM) on PSC cross sections with and without illumination. The charge distribution resolved via KPFM indicates that mobile iodide ions released at the MAPbI3 / ETL interface degrade the organic hole transport layer. This degradation likely increases the interfacial resistance and defect density, resulting in losses in the charge carrier extraction.
Ultimately, this work suggests that targeted control of boundary structures within the MAPbI3 absorber and stabilization of heterointerfaces will improve carrier extraction and thereby contribute to PSCs’ reaching their full potential.