In-Situ Monitoring of the Charge Carrier Dynamics of CH$_3$NH$_3$PbI$_3$ Perovskite Crystallization Process

TL;DR

This study introduces an in-situ femtosecond transient absorption spectroscopy method to monitor charge carrier dynamics during the crystallization of CH$_3$NH$_3$PbI$_3$ perovskite films, revealing how annealing conditions affect film quality and device performance.

Contribution

It presents a novel in-situ TAS approach to evaluate perovskite layer quality during crystallization, linking charge dynamics with structural and morphological features.

Findings

Annealing temperature influences optimal crystallization time.

Different hole transport layers affect charge carrier dynamics.

Higher annealing temperatures reduce required crystallization time.

Abstract

Although methylammonium lead iodide (CH$_3$NH$_3$PbI$_3$) perovskite has attracted enormous scientific attention over the last decade or so, important information on the charge extraction dynamics and recombination processes in perovskite devices is still missing. Herein we present a novel approach to evaluate the quality of CH$_3$NH$_3$PbI$_3$ layers, via in-situ monitoring of the perovskite layer charge carrier dynamics during the thermal annealing crystallization process, by means of time-resolved femtosecond transient absorption spectroscopy (TAS). In particular, CH$_3$NH$_3$PbI$_3$ films were deposited on two types of polymeric hole transport layers (HTL), poly(3,4-ethylenedioxythiophene)-poly-(styrenesulfonate) (PEDOT:PSS) and poly-(triarylamine) (PTAA), that are known to provide different carrier transport characteristics in perovskite solar cells. In order to monitor the evolution of the perovskite charge carrier dynamics during the crystallization process, the so-formed CH$_3$NH$_3$PbI$_3$/HTL architectures were studied in-situ by TAS at three different annealing temperatures, i.e. 90, 100 and 110 oC. It is revealed that the annealing time period required in order to achieve the optimum perovskite film quality in terms of the decay dynamics strongly depends on the annealing temperature, as well as, on the employed HTL. For both HTLs, the required period decreases as higher annealing temperature is used, while, for the more hydrophobic PTAA polymer, longer annealing periods were required in order to obtain the optimum charge carrier dynamics. The correlation of the TAS finding with the structural and morphological features of the perovskite films is analysed and provides useful insights on the charge extraction dynamics and recombination processes in perovskite optoelectronic devices.