Methods for Roll-to-Roll Vapor Deposition of Organic Semiconductors

Abstract

Organic semiconductors find their most successful commercial application as organic light emitting device (OLED) displays, and are extensively investigated in other applications, such as photovoltaic cells (OPVs) and thin film transistors (OTFTs). The key to take advantage of the low cost of organic semiconductors, is to rapidly fabricate thin film organic semiconductor devices on a continuous flexible roll. This production method, called roll-to-roll (R2R) manufacturing, can be implemented by solution processing or vapor deposition of organic thin films. Despite its advantages in high material purity and device reliability, the R2R vapor deposition of organic semiconductors is insufficiently investigated compared to solution processing due to the complex equipment and costly material consumption needed. This thesis aims to investigate critical methods that can be used to develop practical large scale R2R fabrication of organic semiconductors. The integration of two different vapor deposition methods, vacuum thermal evaporation (VTE) and organic vapor phase deposition (OVPD), into a R2R deposition system is demonstrated in a pilot tool that accommodates 10 cm-wide substrate rolls. The uniformity of deposition at various conditions and the tension and temperature control of the translating substrate are investigated. OPVs with a power conversion efficiency of 8.5 % and white OLEDs (WOLEDs) with an external quantum efficiency of 16 % are fabricated on translating flexible substrates in this tool. The method to achieve ultrahigh deposition rates required by R2R processing is demonstrated by OVPD. Uniform organic semiconductor thin films grown by OVPD at rates as high as 50 Å/s are achieved. Theoretical analysis and an experimental verification of strategies to avoid gas phase nucleation common to high rate OVPD are demonstrated. Green OLEDs comprising emission layers grown at high deposition rates exhibit efficiencies similar to devices grown by low speed VTE. The relationship between the deposition rate, thin film morphology, and OLED efficiency is established. A comprehensive numerical model that is capable of simulating complex, multilayer WOLED structures is developed to provide an alternative to experimental iterations of OLED design and tests. The model describes the charge and exciton distribution across broad multi-color emission layers in WOLEDs, and predicts the color change of a WOLED as a function of current density. The relative magnitude of different exciton decay channels is also studied. A simple method to reduce the WOLED spectral change inspired by the model is by insertion of a charge blocking layer between the WOLED transport layers. A cost estimate on the R2R production of WOLEDs for lighting is developed as an example to demonstrate the framework on cost modelling of large scale production of various organic semiconductor devices. Assuming a WOLED luminance of 10 klm/m2, the cost of a WOLED light engine is anticipated to be $12.5 /klm. With incremental reduction in material and driver costs and improved luminance, the cost of WOLED lighting can be reduced to $6.3 /klm in the near term, potentially positioning WOLEDs for use in numerous premium lighting applications.