Organic Electronics and Photovoltaics

Our goal is to develop an integrated suite of nondestructive measurement methods to evaluate organic-based electrical devices and tie both the electrical performance and interfacial morphology of the active molecules in the device to their chemical structure, fabrication methods, and processing parameters. By providing the measurement link for the structure - processing - performance paradigm, our methods will rationally accelerate product development, enable standard measurements, and provide a basis for quantitative comparisons in this emerging technology where device variability and optimization are still poorly understood.

Developing commercial products based on organic electronics requires materials that deliver predictable and reproducible performance. One of the advantages of these materials is their compatibility with versatile solution processing methods. However, this also leads to unpredictable performance and poor reproducibility because the critical microstructure of the material forms dynamically as the solution dries. Many parameters influence this drying process and the microstructure formation is often hard to control. This leads to variability in performance and makes is difficult to determine why new materials often under perform. To address these challenges, we develop quantitative methods to correlate chemical structure and processing variables to performance via microstructure measurements. A combination of spectroscopic tools (IR, Vis, and X-ray), diffraction, and scanning probe microscopy provides sufficiently detailed characterization to isolate the contributions of individual structure and processing variables. This integrated measurement platform provides a rational basis for evaluating current processing methods will further accelerate materials development by separating the molecular basis for electric performance from the process induced variability.

Conjugated, fused ring polymers with alkyl side chains constitute a general class of organic semiconductors, offering some of the best thin film charge mobilities to date. The detailed packing structure of these conjugated polymers that impacts their electrical performance, however, can be a challenge to determine.

This year we worked closely with Merck to characterize the microstructure evolution in their pBTTTs, a series of thiophene ring semiconductors with alkyl side chains and record-setting charge mobilities. We determined the substrate-relative tilts of the pBTTT conjugated plane, side chains, and backbones using our suite of spectroscopies. By combining orientation information with crystal spacings from X-ray diffraction and microscopy, we could discern important structure details. Notably, the conjugated planes have a preferred tilt, and the side chains are highly interdigitated.

Our measurements explain why pBTTT delivers exceptional charge mobility, recently displacing P3HT (poly(3-hexylthiophene) as the industry benchmark for performance. P3HT has a similar structure, but the side chains are disordered and do not interdigitate. The interdigitation promote registry between the crystalline layers, enhancing long range order and improving performance.

The primary factor determining side chain interdigitation is the spacing of the side chains along the polymer backbone. This spacing is controlled entirely by the synthetic structure. Our measurement established the side chain attachment density as a critical parameter in designing the next generation of high performance conjugated polymers.

Interface formation during processing is also critical to organic semiconductor microstructure and performance. By a combination of microscopy, spectroscopy, and diffraction, we found that the crystal nucleation density of pBTTT on silicon oxide is greater than that on a hydrophobic dielectric, causing a tenfold smaller lateral domain size and substantially lower performance. The morphology of the dielectric is also crucial. A series of hydrophobic dielectrics with controlled roughness revealed that a modest roughness of 0.8 nm not only reduces lateral pBTTT domain size but also disrupts order within domains. This disruption can cause up to a 500-fold decrease in performance on dielectrics with 3.0 nm roughness. These results provide robust practical guidelines for dielectric choice and design.

We reported our work via 19 invited lectures, including ACS and MRS meetings and other academic and industrial venues. In FY08, we will extend measurement development to address emerging organic circuit processing methods.

• Organic electronic devices are an emerging technology, with potential in displays, photovoltaics, sensors, logic, lighting, and radio-frequency identification tags. Their market is predicted to be $10 to $ 30 billion globally by 2010-2015.
• Our measurements provide critical structural insights that have helped guide to materials development at Merck and Corning.
• NIST partnered with iNEMI to lead the development of the Organic and Printed Electronics 2007 road map.
• NIST Organic Electronics collaborators include Merck Chemicals, IBM, Corning, Plextronics, H.C. Starck, ASU, U.C. Berkeley, Stanford, and Northwestern.