CORRELATIONS BETWEEN MECHANICAL AND ELECTRICAL PROPERTIES OF ORGANIC SEMICONDUCTORS
Brendan O’Connor1, Edwin Chan1, Christopher M. Stafford1, Lee J. Richter2, R. Joe Kline1, Christopher R. Soles, Dean M. Delongchamp1
1 Polymers Division, MSEL, NIST
2 Surface and Microanalysis Science Division, CSTL, NIST
A common advantage ascribed to organic electronics is the potential for flexible electronic devices. Flexibility offers several benefits including being light-weight, rugged, conformal, storable by rolling or folding, and amenable to low-cost fabrication methods such as roll-to-roll processing. Due to these characteristics, achieving high performing flexible organic electronics is expected to significantly advance a range of technologies including displays, solar power, imaging, and radio frequency identification tags among others. Accurately characterizing the mechanical properties of these films, and correlating structure to electrical properties is therefore critical in guiding the development, selection, and application of candidate materials for flexible organic electronics.
Here we consider common solution processable organic semiconductors, poly(3-hexylthiophene) (P3HT) and poly-(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophenes) (PBTTT), and show that improving the organic semiconductor’s field effect mobility correlates with increasing elastic modulus and brittle behavior. The elastic modulus is determined by using Strain Induced Elastomer Buckling Instability for Mechanical Measurements (SIEBIMM). The modulus is then compared to the field effect mobility of similarly cast films in a thin film transistor configuration. We show that for thermally annealed PBTTT the elastic modulus is 1.89 GPa, roughly 7 times greater than the modulus of P3HT measured to be 271 MPa. This is very similar to the difference in field effect mobility, where the mobility of PBTTT is 0.35 cm2/Vs and the mobility of P3HT is 0.045 cm2/Vs. In addition, strain induced cracking is investigated, showing that extremely large strains (> 100%) are possible for P3HT films without the onset of cracks while PBTTT cracks at strains below 2%. Previously, the increased mobility of PBTTT over P3HT has been attributed to improved packing of the conjugated backbone of the polymer, aided by superior crystallization in three dimensions. The increased crystallization improves electronic transport, but as we show it also gives rise to increased elastic modulus and brittle behavior.