Correlations between Mechanical and Electrical Properties of Polythiophenes

Abstract

The elastic moduli of polythiophenes, regioregular poly(3-hexylthiophene) (P3HT) and poly-(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (pBTTT), are compared to their field effect mobility showing a proportional trend. The elastic moduli of the films are measured using a buckling-based metrology, and the mobility is determined from the electrical characteristics of bottom contact thin film transistors. Moreover, the crack onset strain of pBTTT films is shown to be less than 2.5%, whereas that of P3HT is greater than 150%. These results show that increased long-range order in polythiophene semiconductors, which is generally thought to be essential for improved charge mobility, can also stiffen and enbrittle the film. This work highlights the critical role of quantitative mechanical property measurements in guiding the development of flexible organic semiconductors.

Structure and design of polymers for durable, stretchable organic electronics

“…97 A system of various fused and non-fused thiophenes exhibit a clear trend; the greater the content of fused rings, the higher the modulus (notably PT2T (4) and PBTTT (6)). 69,91,98 This trend is also evident from PDPP2T-2T (12) and PDPP2T-TT (8). The non-fused thiophene ring allows for greater freedom of movement, resulting in a lower modulus for PDPP2T-2T (12) (0.74 GPa) than for PDPP2T-TT (8) (0.99 GPa).…”

“…This effect is due to the intercalation of PCBM (5) in PT2T (4) (Figure 9), embrittling the blend and hindering the formation of independent crystalline domains of PT2T (4) and PCBM (5) for efficient charge transport. 83 This effect is also seen with larger spacing, such as PBTTT (6), 69,94,95 and PQT (7). 89 It is important to note that this is dependent on the side chains as well; long, branching side chains can block intercalation in PBTTT (6), limiting these effects.…”

“…[66][67][68] It is commonly understood that in many cases the device efficiency increases with the modulus of the BHJ material. 69 This increasing modulus results in brittle devices with severe property degradation even with minimal elongation. Beyond modulus, it is well known that both the morphology and crystallinity have a strong part in the device efficiency in polymer solar cells, and that there is significant interplay between the two.…”

“…78 This is largely attributed to a reduction in glass transition temperature, T g . 69 This extended chain disrupts crystallization, which also has the effect of reducing charge carrier mobility. 79 As such devices made from poly(3-octylthiophene) (P3OT) have very low PCE of only 0.68%, in spite of chemical and band gap similarities to P3HT.…”

Direct imaging of defect formation in strained organic flexible electronics by Scanning Kelvin Probe Microscopy

“…The interdigitated binding structure of the bulky sidechains in the crystal structure provides interactions in all spatial directions with comparable surface energies for different crystal planes42. These interactions combined with the long-range order of the crystal render the material susceptible to crack formation thus limiting the strain-interval in which reversible elastic response to tensile strain occurs47. Beyond a critical value of the range of ε ~1% irreversible changes occur due to the onset of crack formation and transport in crystals gets reduced.…”