Grain Boundaries Act as Solid Walls for Charge Carrier Diffusion in Large Crystal MAPI Thin Films

Area of Science:

• Materials Science
• Photovoltaics
• Solid-State Physics

Background:

• Methylammonium lead iodide (MAPI)-based thin-film solar cells achieve high power conversion efficiencies exceeding 20%.
• Understanding charge carrier transport mechanisms is crucial for further efficiency improvements.

Purpose of the Study:

• To investigate the impact of grain boundaries on charge carrier transport in large-crystal MAPI thin films.
• To determine if grain boundaries act as charge carrier loss channels in MAPI solar cells.

Main Methods:

• Time-resolved photoluminescence (PL) microscopy was employed to study charge carrier dynamics.
• Numerical model calculations were used to analyze experimental data and grain boundary effects.
• Spatially and time-resolved PL measurements were performed on large MAPI crystals (tens of micrometers).

Main Results:

• Diffusive charge carrier transport occurs within single MAPI crystals but is blocked at grain boundaries.
• Photoluminescence transients are dependent on crystal geometry and observation point, with slower decay near edges due to restricted diffusion.
• No evidence of charge quenching or additional loss channels attributable to grain boundaries was observed in the studied MAPI material.

Conclusions:

• Grain boundaries in MAPI thin films do not negatively impact charge carrier transport or device performance.
• The findings challenge some existing literature suggesting detrimental effects of grain boundaries in perovskite solar cells.
• Optimized MAPI thin-film solar cells can be fabricated without concern for grain boundary-induced performance losses.