Experimentally Calibrated Kinetic Monte Carlo Model Reproduces Organic Solar Cell Current–Voltage Curve

Sebastian Wilken,Ronald Österbacka,Gustav Persson,Staffan Dahlström,Eva Olsson,Armantas Melianas,Tanvi Upreti,Martijn Kemerink

doi:10.1002/solr.202000029

Abstract

Kinetic Monte Carlo (KMC) simulations are a powerful tool to study the dynamics of charge carriers in organic photovoltaics. However, the key characteristic of any photovoltaic device, its current–voltage (J–V) curve under solar illumination, has proven challenging to simulate using KMC. The main challenges arise from the presence of injecting contacts and the importance of charge recombination when the internal electric field is low, i.e., close to open‐circuit conditions. Herein, an experimentally calibrated KMC model is presented that can fully predict the J–V curve of a disordered organic solar cell. It is shown that it is crucial to make experimentally justified assumptions on the injection barriers, the blend morphology, and the kinetics of the charge transfer state involved in geminate and nongeminate recombination. All of these properties are independently calibrated using charge extraction, electron microscopy, and transient absorption measurements, respectively. Clear evidence is provided that the conclusions drawn from microscopic and transient KMC modeling are indeed relevant for real operating organic solar cell devices.

Highlights

This high carrier density is demanding from the computational point of view and challenging to correctly account for
The reason for choosing TQ1:phenyl-C71-butyric acid methyl ester (PC71BM) is that for this specific system a clear picture of the carrier dynamics has emerged from time-resolved measurements and previous modeling, which is summarized in a recent review article.[23]
Many of the parameters for the Kinetic Monte Carlo (KMC) model are already known; in particular, it has been shown that the charge extraction in thin devices with an active-layer thickness around 100 nm is strongly affected by nonequilibrium effects

Summary

Introduction

This high carrier density is demanding from the computational point of view and challenging to correctly account for. Kinetic Monte Carlo (KMC) simulations have successfully been though a few concepts exist how contacts can be implemented used to model the charge carrier dynamics in organic photovol- in KMC, literature studies have so far failed to fully describe taics (OPVs) on the ps to μs timescale. It was shown J–V data of real devices or are based on assumptions that are that in thin-film OPV devices, thermalization in the disorder-broad- not justified experimentally.[10,11,12]. The competition nation and its dependence on the morphology in slabs of between charge extraction and recombination has been demonmaterial, i.e., in absence of contacts.[5,6,7,8,9] it is still an strated to be the main determinant of the device fill factor.[14,15,16]

Objectives

Results

Conclusion