Conjugated polymers enable the production of flexible semiconductor devices that can be processed from solution at low temperatures. Over the past 25 years, device performance has improved greatly as ...a wide variety of molecular structures have been studied. However, one major limitation has not been overcome; transport properties in polymer films are still limited by pervasive conformational and energetic disorder. This not only limits the rational design of materials with higher performance, but also prevents the study of physical phenomena associated with an extended π-electron delocalization along the polymer backbone. Here we report a comparative transport study of several high-mobility conjugated polymers by field-effect-modulated Seebeck, transistor and sub-bandgap optical absorption measurements. We show that in several of these polymers, most notably in a recently reported, indacenodithiophene-based donor-acceptor copolymer with a near-amorphous microstructure, the charge transport properties approach intrinsic disorder-free limits at which all molecular sites are thermally accessible. Molecular dynamics simulations identify the origin of this long sought-after regime as a planar, torsion-free backbone conformation that is surprisingly resilient to side-chain disorder. Our results provide molecular-design guidelines for 'disorder-free' conjugated polymers.
A general semiconductor‐independent two‐dimensional character of the carrier distribution in top‐gate polymer field‐effect transistors is revealed by analysing temperature‐dependent transfer ...characteristics and the sub‐bandgap absorption tails of the polymer semiconductors. A correlation between the extracted width of the density of states and the Urbach energy is presented, corroborating the 2D accumulation layer and demonstrating an intricate connection between optical measurements concerning disorder and charge transport in transistors.
Thermal vibrations and the dynamic disorder they create can detrimentally affect the transport properties of van der Waals bonded molecular semiconductors. The low-energy nature of these vibrations ...makes it difficult to access them experimentally, which is why we still lack clear molecular design rules to control and reduce dynamic disorder. In this study we discuss the promising organic semiconductors rubrene, 2,7-dioctyl1benzothieno3,2-b1benzothio-phene and 2,9-di-decyl-dinaphtho-2,3-b:20,30-f-thieno-3,2-b-thiophene in terms of an exceptionally low degree of dynamic disorder. In particular, we analyse diffuse scattering in transmission electron microscopy, to show that small molecules that have their side chains attached along the long axis of their conjugated core are better encapsulated in their crystal structure, which helps reduce large-amplitude thermal motions. Our work provides a general strategy for the design of new classes of very high mobility organic semiconductors with a low degree of dynamic disorder.
Charge transport in conjugated polymer semiconductors has traditionally been thought to be limited to a low-mobility regime by pronounced energetic disorder. Much progress has recently been made in ...advancing carrier mobilities in field-effect transistors through developing low-disorder conjugated polymers. However, in diodes these polymers have to date not shown much improved mobilities, presumably reflecting the fact that in diodes lower carrier concentrations are available to fill up residual tail states in the density of states. Here, we show that the bulk charge transport in low-disorder polymers is limited by water-induced trap states and that their concentration can be dramatically reduced through incorporating small molecular additives into the polymer film. Upon incorporation of the additives we achieve space-charge limited current characteristics that resemble molecular single crystals such as rubrene with high, trap-free SCLC mobilities up to 0.2 cm
/Vs and a width of the residual tail state distribution comparable to k
T.
The exploration of a wide range of molecular structures has led to the development of high‐performance conjugated polymer semiconductors for flexible electronic applications including displays, ...sensors, and logic circuits. Nevertheless, many conjugated polymer field‐effect transistors (OFETs) exhibit nonideal device characteristics and device instabilities rendering them unfit for industrial applications. These often do not originate in the material's intrinsic molecular structure, but rather in external trap states caused by chemical impurities or environmental species such as water. Here, a highly efficient mechanism is demonstrated for the removal of water‐induced traps that are omnipresent in conjugated polymer devices even when processed in inert environments; the underlying mechanism is shown, by which small‐molecular additives with water‐binding nitrile groups or alternatively water–solvent azeotropes are capable of removing water‐induced traps leading to a significant improvement in OFET performance. It is also shown how certain polymer structures containing strong hydrogen accepting groups will suffer from poor performances due to their high susceptibility to interact with water molecules; this allows the design guidelines for a next generation of stable, high‐performing conjugated polymers to be set forth.
Removal of water‐induced traps that are omnipresent in conjugated polymer devices is demonstrated by a highly efficient mechanism. The underlying mechanism is shown, by which small‐molecular additives with water‐binding nitrile groups or alternatively water–solvent azeotropes are capable of completely removing water‐induced traps, leading to a significant improvement in performance of organic field‐effect transistors.
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