Flexible electronics are often sold on a simple promise: bendable screens, lightweight solar cells or wearable devices that can bend and flex without breaking. But what does that 'flexibility' actually look like at the molecular scale, and how does it affect performance?
Researchers led by the University of Cambridge say they have taken a first step towards answering this question. Using ultra-sensitive atomic force microscopy – which analyses materials by 'feeling' them – the researchers were able to measure how stiff flexible semiconductor molecules are when packed together, down to the scale of just a few molecules.
Their findings, reported in the journal Nature Communications, provide the first experimental evidence that the mechanical stiffness of individual molecules contributes to the overall stiffness of a material. In future, the work could help researchers understand whether flexibility places a fundamental limit on the speed and efficiency of future flexible electronic devices.
Unlike silicon, which is rigid and crystalline, organic semiconductors are made from carbon-based molecules that assemble into soft, bendable solids. This flexibility is essential for rollable displays and lightweight devices – but it may come at a cost.
"Silicon electronics are fast partly because silicon is very stiff and orderly, so it's easy for electrical charges to move around," said Dr Deepak Venkateshvaran from Cambridge's Cavendish Laboratory, who led the research. "For decades, we've built flexible electronics without really understanding what flexibility means at the molecular scale, and whether it might have an impact on how well these materials can conduct electricity."
To explore that question, Venkateshvaran and his colleagues used atomic force microscopy (AFM), which uses a tiny 'needle' – around ten nanometres wide – to gently press on a surface and measure how much it resists deformation.
"It's a bit like feeling the ground with a stick," said Venkateshvaran. "If the ground is firm, it pushes back. If it's soft, it gives way. We're doing that, but on the scale of a few nanometres – about the size of a handful of molecules."
By carefully controlling the force applied to the organic semiconductors by the AFM 'needle', the researchers were able to map stiffness across thin films of organic semiconductors with unprecedented resolution.
The team focused on an organic semiconductor called DNTT, which is widely used in flexible transistors. They compared DNTT with several closely related molecules, each modified with different chemical 'side chains' attached to the same rigid molecular core.
These side chains act like molecular padding. Longer, more flexible chains increase the spacing between the rigid cores when the molecules pack together, changing both the structure and the mechanical response of the material.
The AFM measurements showed that materials with longer and more flexible side chains were softer when pressed perpendicular to the surface. Unsubstituted DNTT was the stiffest, while versions with long side chains were significantly softer.
"People have always assumed that adding flexible side chains would soften a material, but no one had ever measured that effect directly at the molecular level," said Venkateshvaran. "The effect is subtle, and you only see it if you're extremely careful."
The researchers then compared their experimental data with computer simulations. The calculations independently predicted the same reduction in stiffness when flexible side chains were introduced.
Venkateshvaran says the result can be thought of like a brick wall. "Traditionally, we've focused on the 'mortar': the weak forces that hold molecules together," he said. "But our work shows that the 'bricks' themselves also matter. We've been able to separate the contribution from individual molecules from the collective forces between them. That's never been done experimentally before."
This distinction opens the door to molecular-level design of mechanical properties. If scientists can tune the stiffness of individual molecules, they may be able to engineer materials with specific mechanical or electronic behaviours.
"Our result doesn't prove that stiffness controls electronic performance in organic semiconductors," said Venkateshvaran. "But it gives us the tools to ask that question properly for the first time."
The experiments reveal what flexibility looks like at the nanoscale and demonstrate that it can be measured reliably. In the longer term, the work could inform the design of faster, more efficient flexible electronics by identifying how much softness is too much.
"There may be a glass ceiling on how well flexible molecular materials can conduct electricity," said Venkateshvaran. "If we understand the relationship between stiffness and charge transport, we might find ways to push past it."
The research was supported in part by the Royal Society, the Wiener-Anspach Foundation and the European Union. Deepak Venkateshvaran is a Fellow of Selwyn College, Cambridge.
