Charge Carrier Dynamics of Methylammonium Lead-Iodide Perovskite Solar Cells

TL;DR

This study combines transient opto-electrical measurements and numerical simulations to analyze charge carrier dynamics in methylammonium lead-iodide perovskite solar cells, revealing the influence of mobile ions and trapping effects on device operation.

Contribution

It introduces an integrated experimental and simulation approach to elucidate the complex charge dynamics and operating mechanisms of perovskite solar cells.

Findings

Charge response spans 9 orders of magnitude in time.

Mobile ions cause slow charge density changes at interfaces.

Mobility imbalance and hole trapping lead to dynamic doping effects.

Abstract

Transient opto-electrical measurements of methylammonium lead iodide (MALI) perovskite solar cells (PSCs) are performed and analyzed in order to elucidate the operating mechanisms. The current response to a light pulse or voltage pulse shows an extraordinarily broad dynamic range covering 9 orders of magnitude in time - from microseconds to minutes - until steady-state is reached. Evidence of a slowly changing charge density at the perovskite layer boundaries is found, which is most probably caused by mobile ions. Current-voltage curves (IV curves) are measured with very fast scan-rate after keeping the cell for several seconds at a constant voltage as proposed by Tress et al. Numerical drift-diffusion simulations reproduce the measured IV curves using different distributions of ions in the model. Analysing the band diagram of the simulation result sheds light on the operating mechanism. To further investigate the effects at short time scales (below milliseconds) photo-generated charge extraction by linearly increasing voltage (photo-CELIV) experiments are performed. We postulate that mobility imbalance in combination with deep hole trapping leads to dynamic doping causing effects from microseconds to milliseconds. Comprehensive transient drift-diffusion simulations of the photo-CELIV experiments strengthen this hypothesis. This advanced characterization approach combining dynamic response measurements and numerical simulations represents a key step on the way to a comprehensive understanding of device working mechanisms in emerging perovskite solar cells.