Downscaling the Sample Thickness to Sub-Micrometers by Employing Organic Photovoltaic Materials as a Charge-Generation Layer in the Time-of-Flight Measurement.

Shun-Wei Liu,Yu-Hsuan Liu,Kuan-Ting Chen,Tsung-Hao Su,Bo-Yao Huang,Wen-Chang Chang,Chun-Feng Lin,Chih-Hsien Yuan,Wei-Cheng Su,Ya-Ze Li,Chih-Chien Lee,Yi-Sheng Shu

doi:10.1038/srep10384

Abstract

Time-of-flight (TOF) measurements typically require a sample thickness of several micrometers for determining the carrier mobility, thus rendering the applicability inefficient and unreliable because the sample thicknesses are orders of magnitude higher than those in real optoelectronic devices. Here, we use subphthalocyanine (SubPc):C70 as a charge-generation layer (CGL) in the TOF measurement and a commonly hole-transporting layer, N,N’-diphenyl-N,N’-bis(1,1’-biphenyl)-4,4’-diamine (NPB), as a standard material under test. When the NPB thickness is reduced from 2 to 0.3 μm and with a thin 10-nm CGL, the hole transient signal still shows non-dispersive properties under various applied fields, and thus the hole mobility is determined accordingly. Only 1-μm NPB is required for determining the electron mobility by using the proposed CGL. Both the thicknesses are the thinnest value reported to data. In addition, the flexibility of fabrication process of small molecules can deposit the proposed CGL underneath and atop the material under test. Therefore, this technique is applicable to small-molecule and polymeric materials. We also propose a new approach to design the TOF sample using an optical simulation. These results strongly demonstrate that the proposed technique is valuable tool in determining the carrier mobility and may spur additional research in this field.

Highlights

Time-of-flight (TOF) measurements typically require a sample thickness of several micrometers for determining the carrier mobility, rendering the applicability inefficient and unreliable because the sample thicknesses are orders of magnitude higher than those in real optoelectronic devices
The general geometry of field-effect transistors (FETs) in which the current flows parallel to the substrate may lead to an undesirable result for the organic photovoltaic (OPV) devices and the organic light-emitting diodes (OLEDs) because the electrical properties in these devices are primarily dominated by the currents perpendicular to the substrate[38,39]
The EQE decreased with the SubPc:C70 thickness, these values seem sufficient to allow the carriers for flowing through the material under test in the TOF measurement. These results indicate that the mixture of the SubPc and C70 is a promising candidate for the use in charge-generation layer (CGL), and its thickness allows room for modulating according to the tested material thickness

Summary

Introduction

Time-of-flight (TOF) measurements typically require a sample thickness of several micrometers for determining the carrier mobility, rendering the applicability inefficient and unreliable because the sample thicknesses are orders of magnitude higher than those in real optoelectronic devices. Based on the same materials system, Pandey et al demonstrated an improved device fabrication process to enhance the device efficiency[71], showing the potential of the combination of SubPc and C70 for the use in CGLs. In the current study, we determined the hole mobility of 0.3-μ m N,N′-diphenyl-N,N′-bis(1,1′-biphenyl)-4,4′-diamine (NPB) by using the proposed CGL, which provides a well-confined charge-generation width, leading to clear non-dispersive properties under various applied voltages. The proposed technique was applicable to determine the carrier mobility of polymeric materials

Methods

Results

Conclusion