Statistical Paradigm for Organic Optoelectronic Devices: Normal Force Testing for Adhesion of Organic Photovoltaics and Organic Light-Emitting Diodes

Abstract

In this study, we assess the utility of a normal force (pull-test) approach to measuring adhesion in organic solar cells and organic light-emitting diodes. This approach is a simple and practical method of monitoring the impact of systematic changes in materials, processing conditions, or environmental exposure on interfacial strength and electrode delamination. The ease of measurement enables a statistical description with numerous samples, variant geometry, and minimal preparation. After examining over 70 samples, using the Weibull modulus and the characteristic breaking strength as metrics, we were able to successfully differentiate the adhesion values between 8-tris(hydroxyquinoline aluminum) (Alq) and poly(3-hexyl-thiophene) and [6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) interfaces with Al and between two annealing times for the bulk heterojunction polymer blends. Additionally, the Weibull modulus, a relative measure of the range of flaw sizes at the fracture plane, can be correlated with the roughness of the organic surface. Finite element modeling of the delamination process suggests that the out-of-plane elastic modulus for Alq is lower than the reported in-plane elastic values. We suggest a statistical treatment of a large volume of tests be part of the standard protocol for investigating adhesion to accommodate the unavoidable variability in morphology and interfacial structure found in most organic devices.

Origin of macroscopic adhesion in organic light-emitting diodes analyzed at different length scales

“…In order to explore the adhesion properties of the OLEDs, the widely used peel test was employed as shown in Fig. 1(b) 1 , 13 , 24 – 26 . The peel test is a well-known technique for measuring macroscopically the adhesion properties of materials through the analysis of the maximum load because this maximum load is defined as the force measured when the layers completely detached from the fixed OLED.…”

“…Several methods are available to measure the adhesion properties of materials, including peel tests and contact angle measurements 13 , 19 – 21 . However, these conventional methods are limited to the analysis of macroscopic adhesion properties.…”

The Adhesive Force Measurement between Single μLED and Substrate Based on Atomic Force Microscope

“…Min sock Kim proposed a new instrument to measure the dynamic adhesion of the interaction surface on the flexible substrate, and proposed an optimal adhesion control strategy based on the analysis of adhesion [13]. In 2017, Lindsay Vasilak used strain gauge load cell to measure the normal adhesive force of OLED [14]. Chang-Dong Yeo used a high-resolution, high-dynamic bandwidth capacitive force transducer and two piezoelectric actuators to measure adhesive pull-off forces between nominally flat rough silicon surfaces under various dynamic conditions [15].…”