Quantifying charge carrier density in organic solar cells by differential charging techniques

Kyohei Nakano,Yujiao Chen,Keisuke Tajima

doi:10.1063/1.5122709

Abstract

Accurate determination of charge carrier density in organic solar cells under light irradiation is essential because charge carrier density is directly related to the bimolecular recombination rate and open-circuit voltage of the cells. We investigate the robustness of transient photovoltage/current (TPV/C) and impedance spectroscopy (IS) to interference from the geometric capacitance of the cells (Cgeo) during quantification of the charge carrier density. TPV/C and IS accurately quantify the charge carrier density of bulk heterojunction cells with small Cgeo. For planar heterojunction cells with a larger Cgeo contribution, IS fails to separate the charge carriers in the organic layer from those in the electrodes. In contrast, TPV/C eliminates the effect of Cgeo and gives a reasonable estimation of the charge carrier density in the organic layer with the planar heterojunction, demonstrating that TPV/C is more robust than IS to interference from Cgeo of the cells.

Highlights

Power conversion efficiency of organic solar cells (OSCs) has been improved up to 16%,1 mainly driven by the development of new organic semiconductor materials
Differential charging methods have the advantage that they can evaluate the number of charge carriers in the organic layer separately from those stored in the electrodes
We examine the suitability of transient photovoltage/current (TPV/C) and impedance spectroscopy (IS) techniques for evaluating the charge carrier density in OSCs with bulk heterojunction (BHJ) and planar heterojunction (PHJ) structures

Summary

Introduction

Power conversion efficiency of organic solar cells (OSCs) has been improved up to 16%,1 mainly driven by the development of new organic semiconductor materials. Many charge carriers are stored in the electrodes, which makes quantifying the charge carriers in the organic layer by CEX difficult in some cases.16–18 Instead of counting all the charge carriers by CEX, differential charging techniques, such as transient photovoltage/current (TPV/C) and impedance spectroscopy (IS), can quantify the charge carrier density indirectly by using the differential capacitance (Cdiff).13,19 Cdiff is the voltage-dependent capacitance, defined as the ratio between small changes in the charge and voltage (ΔQ/ΔV), and it is measured by applying a small charge density perturbation to the cell.

Results

Conclusion