Distinguishing Carrier Transport and Interfacial Recombination at Perovskite-Transport Layer Interfaces Using Ultrafast Spectroscopy and Numerical Simulation

TL;DR

This study combines ultrafast spectroscopy and numerical modeling to distinguish charge transport from interfacial recombination at perovskite-transport layer interfaces, providing insights for improving solar cell efficiency.

Contribution

It introduces a combined experimental and modeling approach to separately analyze charge extraction and recombination at perovskite interfaces, guiding targeted improvements.

Findings

Fullerenes enable fast electron extraction but have high recombination rates.

Spiro-OMeTAD shows slow hole extraction but low surface recombination.

Charge dynamics vary significantly among different transport layers.

Abstract

In perovskite solar cells, photovoltaic action is created by charge transport layers (CTLs) either side of the light-absorbing metal halide perovskite semiconductor. Hence, the rates for desirable charge extraction and unwanted interfacial recombination at the perovskite-CTL interfaces play a critical role for device efficiency. Here, the electrical properties of perovskite-CTL bilayer heterostructures are obtained using ultrafast THz and optical studies of the charge carrier dynamics after pulsed photoexcitation, combined with a physical model of charge carrier transport that includes the prominent Coulombic forces that arise after selective charge extraction into a CTL, and cross-interfacial recombination. The charge extraction velocity at the interface and the ambipolar diffusion coefficient within the perovskite are determined from the experimental decay profiles for heterostructures with three of the highest performing CTLs, namely C$_{60}$, PCBM and Spiro-OMeTAD. Definitive targets for the further improvement of devices are deduced: fullerenes deliver fast electron extraction, but suffer from a large rate constant for cross-interface recombination or hole extraction. Conversely, Spiro-OMeTAD exhibits slow hole extraction but does not increase the perovskite's surface recombination rate, likely contributing to its success in solar cell devices.