Deciphering capacitance frequency technique for performance limiting defect state parameters in energy harvesting perovskites

TL;DR

This paper develops a theoretical model to improve the accuracy of capacitance frequency technique in measuring trap state parameters in perovskite-based energy harvesting devices by accounting for electrostatic effects of trapped charges.

Contribution

It introduces a parabolic band approximation model that corrects for non-linear effects, enabling more precise trap density estimation in perovskite PIN devices.

Findings

Electrostatic effects cause non-linear energy bands, leading to underestimation of trap density.

The PBAEL model accurately predicts trap densities from CFT analysis.

Reducing perovskite thickness helps in better extracting trapped charge dynamics.

Abstract

With emerging thin film PIN based optoelectronics devices, a significant research thrust is focused on the passivation of trap states for performance enhancement. Among various methods, capacitance frequency technique (CFT) is often employed to quantify trap state parameters, however, the trapped charge induced electrostatic effect on the same is not yet established for such devices. Herein, we present a theoretical methodology to incorporate such effects in the CF characteristics of well-established carrier selective perovskite-based PIN devices. We show that the electrostatic effect of trapped charges leads to non-linear energy bands in perovskite layer which results in the underestimation of trap density from existing models of CFT. Consequently, a parabolic band approximation with effective length PBAEL model is developed which accurately predicts the trap density for shallow or deep states from CFT analysis. In addition, we demonstrate that the attempt to escape frequency, crucial for trapped charge dynamics with continuum energy bands, can be well extracted by eliminating non-linear effects at reduced perovskite thickness. We believe that our work provides a unified theoretical platform for CFT to extract trap state parameters for a broad class of organic and hybrid materials-based thin film devices for energy conversion applications such as solar cells, LEDs, etc.