Page 46

EETE JULAUG 2013

FLEXIBLE ELECTRONICS A new class of flexible semiconductors enters the market By Mike Cowin Recent reports by HSBC Global Research and by IHS predicting global shipments of flexible displays to hit 800M units by 2020 coupled with recent announcements that LG plans to launch a flexible OLED product by Q4 2013 show the flexible display market is at a tipping point and 2014 is set to be an important year for the industry. It is clear that significant technology gaps existed in the evolving supply chain for flexible semiconductors in both inorganics and organic material offerings. Inorganic semiconductor materials pose serious concerns Fig. 1: Typical morphology of the SmartKem p-Flex. for manufacturers with the need for vacuum processes, high-temperature procedures - a key barrier for use with flexible plastic backplanes and most significantly unproven flexibility in final form. Historic concerns regarding organic semiconductor were low carrier mobility, temperature resistance and controllable uniformity in TFT (Thin-Film-Transistor) performance. Nevertheless the compelling benefits over inorganics offered by organics such as inherent flexibility and the potential for solution processing with a wide range of substrates, print processes and device architectures meant that if these prior art issues could be resolved a real enabling product would exist for the flexible market. To meet these requirements SmartKem’s approach was to develop a new class of semiconductors called p-FLEX. These semiconductors use high mobility small molecule materials with proprietary binder matrix materials to yield exceptionally high performance solution-based semiconductors that can be processed in air, offer highly uniform films, are stable up to and above 250°C and are fully compatible with a range of print processes. The development of organic semiconductors (OSCs) gene rally falls into two main categories of materials: polymeric and distinct molecular materials. A common feature is that both these types of materials are conjugated systems, that is to say they consist of alternating single and double bonds. Key consideration also has to be given to matching the highest electron energy level of the OSC to the work function of a metal contact to ensure efficient device operation. For high performance TFTs, high charge carrier mobility is required and this favours crystalline small molecule semiconductors. While close packed and regular arrangement of the molecule crystal lattice give rise to good π-overlap and efficient charge carrier mobility these materials on their own tend to demonstrate anisotropy. This new class of semiconductor materials eliminates such issues by designing into the inks the preferred features of chemically stable, high mobility, single-crystal organic semiconductors and combining this with amorphous semiconducting polymers and the uniform processing characteristics that these binders offer. When looking at prior art, solution printing of semiconductors has tended towards either solution printing of single-crystal films or printing small molecule semiconductors in polymer blends. Printing singlecrystal organic semiconductors such as substituted benzothienobenzothiophenes in solvents has managed to yield impressive carrier mobility’s of up to 30 cm2V-1s-1, but with the serious disadvantage of the yielding very poor device to device uniformity and mobility values ranging between 1 to 32 cm2V-1s-1 - great in the lab and for headline results but not practical for real world applications. Whereas the printed small molecule-polymer blend approach afford good device uniformities that are becoming industrially interesting, but with very low mobility’s of only around 1 cm2V-1s-1 and an inability to control the performance of the formulated blend in top gate and bottom gate TFTs. SmartKem’s semiconductor approach overcomes these technology roadblocks by carefully engineering chemical improvements into its material molecular framework. These inks incorporate polycrystalline small molecules that have been designed with the highest levels of pi-pi overlap and minimum inter-planar pi-pi distances when deposited with the optimum crystal packing in the printed semiconductor layer. The key to achieving this effect in printed layers is to use the small molecule in combination with a new class of patented “matched” Mike Cowin is Head of Product Development at SmartKem - www.smartkem.com Fig. 2: SmartKem’s formulation facility at Technium OpTIC in St Asaph, UK. 32 Electronic Engineering Times Europe July/August 2013 www.electronics-eetimes.com


EETE JULAUG 2013
To see the actual publication please follow the link above