Study of Charge Carrier Dynamics in Methylammonium Lead Iodide with Electron and Hole Transport Layer under Bias Light

Abstract

Perovskite solar cell s(PSC) has seen a huge increase in its efficiency, since its introduction in 2009, to 25.5% in 2021.d been introduced in 2009 and had seen a huge increase in their efficiency since then. Improving the perovskite–charge transport layer interface and charge extraction can increase the open circuit voltage (Voc) and thereby the efficiency of the PSC. Studying the charge carrier dynamics in the perovskite-charge transport layer interface can give insight into the various processes such as charge extraction, trapping, etc. taking place which can affect the Voc. In this work, the charge carrier dynamics in a bi-layer system comprising of the commonly used perovskite absorber methylammonium lead iodide (MAPI)/ Spiro-MeOTAD hole transport layer (HTL) and MAPI/ C60 electron transport layer (ETL) using time-resolved microwave conductivity (TRMC) is studied. The bi-layer system is excited by a 650 nm pulsed laser and the TRMC measurements are carried out with and without the presence of continuous bias illumination. The obtained TRMC traces are modeled by differential equations involving the various charge carrier dynamics in the bi-layer system to get quantitative information about various parameters like extraction rates, interfacial recombination, trap density, etc. before and after the continuous illumination.
An efficient charge extraction by C60 and Spiro-MeOTAD is observed in the absence of bias illumination. Under bias illumination, trap or defect states are created in MAPI. While the charge extraction by the C60 is not affected after the bias illumination, the hole extraction by the Spiro-MeOTAD decreases. Curve fitting of the TRMC traces reveals that a larger number of trap states are created in the MAPI/ Spiro-MeOTAD bi-layer system along with an increase in the interfacial recombination, indicating that bias illumination also creates trap states in the MAPI/ Spiro-MeOTAD interface affecting the charge kinetics. Lastly, TRMC measurements carried out on MAPI/ Spiro-MeOTAD bi-layer system at a low temperature of 200 K show that charge extraction and interfacial recombination are no longer affected by continuous illumination. This indicates that the defect states created in the MAPI- Spiro-MeOTAD interface is possibly due to mobile ions in MAPI which is prevented at the low temperature due to quenching of ion mobility.