Organic semiconductors now offer the performance, cost and route to adoption, for foldable displays; from ultra-thin, conformal, wearables to truly foldable smartphones and tablets.
BY DR. MICHAEL COWIN, SmartKem Ltd, St Asaph, Wales
Buoyed by consumer demand for fresh innovation and fierce industry competition, the display industry exists in a cycle of continuous improvement.
Today a new breed of semiconductors – a key enabling component in the evolution of active matrix displays – are competing to offer manufacturers a route to the production of high performance curved, foldable and even roll-able displays.
There are two key factors that define the impact and adoption of any new enabling technology like this; namely how will it perform and what will be the cost.
This article demonstrates that the performance of organic thin-film transistors (OTFT) for display backplane application has reached a tipping point into market adoption. OTFTs are now equal and arguably greater than competitive technology solutions while also offering ultra-flexibility and a significant cost advantage in production and ownership over the more traditional inorganic equivalents. OTFTs are now a serious contender to fill a critical gap in the market for high performance, ultra-flexible TFT backplanes to drive the next generation of conformal displays.
At first, low-temperature polysilicon (LTPS) was considered the most likely solution to replace hydro-genated amorphous silicon (a-Si:H) as the TFT channel layer for rigid flat panel display backplanes, until the advent of indium gallium zinc oxide (IGZO). While the vastly superior mobility of LTPS gave uplift in mobility over traditional a-Si TFT, it came at a price of significantly higher manufacturing costs through high CAPEX, complicated processing and much lower yields, some of which were as low as 20% in early 2014.[1]
However, the recent aggressive drive to manufacture OLED, EPD and LCD display products with new form factors so they are lightweight, conformal or flexible has placed new challenging demands on the TFT material characteristics. This has allowed new technology platforms such as OTFTs to enter into the supply chain to compete head on with LTPS and IGZO as a TFT channel material based on the same metrics of performance and cost.
Electrical performance: It’s all about power
While a semiconductor technology’s cost of ownership outlines the market entry opportunities, no TFT platform will even be considered a viable alternative to incumbent semiconductors unless it meets, and surpasses key criteria. When defining these criteria it is vital that context to the end application and how this might improve the user experience is considered. Power consumption is one such aspect becoming critical in defining the battery life of mobile and wearable displays and any new TFT channel material, such as OTFT needs to demonstrate either equal or better performance to add value to the user experience in end product form.
The progression from a-Si semiconductors to alternative materials for rigid displays was originally driven by the charge carrier mobility bottleneck, as manufacturers tried to move to higher resolution active matrix LCD displays. The same requirement exists for AMOLED displays, and as such a parallel can be drawn to the arguments for and against the competing materials systems, but with the increasingly important necessity for physical flexibility.
Each semiconductor platform has its own advantages and disadvantages. For instance while LTPS has a very high carrier mobility it could be debated whether it’s necessary in the average pixel driver circuit for a high quality LCD or OLED display where a mobility of 5-10 cm2/V.s is more than adequate. Indeed IGZO and the latest generation of OTFTs meet this requirement with ease. In contrast TFT electrical (bias stress) stability is an issue with IGZO, usually resulting in more complexity in the TFT drive circuitry for each pixel to compensate for this short coming. From a general perspective each of the above mentioned three contenders are more than suitable as a channel semiconductor. However, these options also need to be considered in context; which of these offers the potential to add real uplift in the user experience at a price point the market will accept? Most displays today are mobile-enabled and are soon to become wearable with the advent of the smartwatch. The power consumption of these displays and its impact on battery life may well be a defining factor in the choice of TFT channel semiconductor for many manufacturers.
An important contribution to this argument was made by Sharp with the introduction to the market of IGZO. Sharp highlighted the importance of TFT leakage current which led to a clearer understanding of the mechanisms responsible for these leakage currents. The causes are found to be predominantly dependent on the smoothness of the interface between the insulator and the channel semiconductor.
So while LTPS has a rough polycrystalline surface its leakage current is higher; IGZO in contrast has smooth amorphous surfaces at this key interface and as such much lower leakage currents.
The context of lower leakage currents is that it will become a very desirable quality since less current is dissipated when the TFT is off and as such the TFT switch capacitor/s can retain an internal charge for a longer period of time. Thus the display refresh rate can be reduced which leads to a potentially dramatic reduction in power consumption – especially for displays that will have static images – ideal for wearable and mobile based displays. As such IGZO has a clear advantage over LTPS for this display based application.
However, recent advances in OTFT technology reported here for the first time show the potential for low leakage currents equivalent to IGZO; but achieved using OTFTs. By designing into solution based organic semiconductor ‘inks’ the preferred features of the single- crystal organic semiconductor combined with semiconducting polymers or ‘binders’ an amorphous semiconductor layer can be achieved. This material combination offers the high mobility of single crystals but with highly uniform processing charac- teristics required for device uniformity. Furthermore, the amorphous nature of these materials offers very smooth interfaces between the solution processed insulator and solution processed semiconductor.
The results in FIGURE 1 demonstrate that the low leakage current levels achieved by a single gate OTFT. This could be lowered further by use of a dual gate OTFT stack as with commercial IGZO TFTs.
FIGURE 1. TEM of copper hillocks
Therefore OTFTs represent serious competition to IGZO as a channel material in the context for application to wearable and mobile displays for extended battery life. Coupled with the further advantages of excellent bias stress stability and low temperature processing, the case for OTFT adoption rather than IGZO becomes more attractive from a performance perspective.
Physical performance: The foldable frontier
Recently there have been a number of commercial products launched based on curved AMOLED displays such as the Galaxy Round, LG G Flex and Galaxy Note Edge with curved features (and slight flex in the case of the G Flex), all based on LTPS TFT backplanes on plastic. When the user context is taken into account it could be suggested that these products have not offered much value differentiation from glass based equivalent devices.
As such the real ‘wow’ factor in the consumer experience or user value-add has yet to be achieved.
Next generation smart and wearable technology will come with the introduction of flexible and foldable devices such as wearables, smartphones and tablets; but this demands a semicon- ductor platform with entirely new physical properties and a form factor capability which in turn raises a unique set of challenges for traditional and new TFT technologies to overcome.
The current limiting factor is the inability of LTPS and IGZO technologies to offer robust and acute bend capability in TFT form. Even with the use of exotic and expensive strain management layering techniques the maximum bend radius of these technologies have hit a roadblock at around 5 mm.
To genuinely offer a differentiated product with a compelling value-add proposition to the consumer experience, manufacturers must turn to the use of material technologies that enable truly foldable mobile devices or fully bendable, robust and light- weight smartwatches (FIGURE 2). The solution to the limitations presented by LTPS and IGZO in bend capability is the use of OTFTs. It has long been understood that the polymeric nature of OTFTs is ideally suited for bendable applications, and it has widely been reported that products such as Smart- Kem’s tru-FLEX® can withstand 10,000 bends below 1mm with minimal effect on device performance. As such OTFT technology is now considered a key enabler for a wide range of highly robust bendable and foldable display based products; and the market timing could not be better with the recent upturn in demand for smartwatch based products.
FIGURE 2: Display form factor dependency on bend radius.
In the context of performance it may be suggested that while the initial market entrants in curved display products have been manufactured with LTPS, and that there is further development potential in the IGZO platform, a complete technology solution already exists – OTFT.
The OTFT technology platform offers the transistor performance for exciting new applica- tions while also holding two ‘aces’ when it comes to product-specific performance for this new generation of wearable and mobile displays; low leakage for significant battery life extension and ultra-flexibility for foldable mobile devices and bendable smart- watches.
How much will it cost?
Beyond the performance benefits of OTFTs, a commercially viable TFT channel semicon-
ductor must provide favourable characteristics for integration into a robust and cost-effective semiconductor manufacturing process. The savings in manufacturing costs compared with inorganic materials as well as the low risk approach of re-purposing existing a-Si production lines to pilot OTFT backplanes on plastic is an appealing prospect.
One of the major advantages of organic semiconductors comes from their ease of application. Solution based semiconductor inks can be applied to substrates through a range of additive processes and print production systems such as slot dye coating as well as low temperature process (FIGURE 3). Although modern organic semiconductors are stable up to 300°C the ease by which these solution-based materials can be processed at low temperatures offers manufacturers a wide range of cost effective stack materials and substrates, and easier bond/de-bond and inter-layer alignment due to less expansion and contraction. This all adds up to significantly improving production yield (over high temperature processing) and thereby reducing production costs over any area of substrate.
FIGURE 3. Commercial organic semiconductors, such as SmartKem’s tru-FLEX® material, offer a total technology solution, combining high performance mobility, low temperature processing and true flexibility.
An independent study has been commissioned by SmartKem comparing the cost of key features within the TFT stack that would show the maximum variance between technology platforms; the semiconductor and gate dielectric layer. This will ensure a complete understanding of the difference in the cost of ownership and cost of production for the alternate TFT channel materials for backplane manufacture for flexible displays.
The four technology platforms chosen for the TFT array devices were: a-Si, LTPS, IGZO and SmartKem’s OTFT semiconductor tru-FLEX®. The overall cost of TFT device manufacture included manufacturing overheads to produce the two layers, depreciation of equipment (amortized over five years of production of 1.8 million substrates) and the direct materials costs.
The CAPEX for each fabrication process is determined from the type and quantity of equipment needed for producing the semiconductor and gate insulator layers with an assumed input capacity of 30,000 substrates per month. In this study, the assumed equipment and materials are shown in Table 2. The summary findings of the on-going study have shown the cost of manufacturing TFT arrays with organic semiconductors is almost half that of LTPS and a third lower than a-Si and IGZO. The most significant findings (to be published in a white paper) were that the manufacturing overheads and depreciation costs for OTFT were ten times less than LTPS and four times less than a-Si and IGZO.
It was found that the depreciation cost of production for a ‘greenfield’ OTFT line is vastly smaller than competing technologies and could be further reduced by the re-purposing of an a-Si production line; OTFTs thus offer an easy route to adoption for the cost-down manufacture of superior performance flexible TFT backplanes.
The future is organic
The value proposition of organic semiconductors now makes sense to an industry eager for differentiated products that can be adopted and scaled with low risk. From a performance and cost perspective the immediate value-add to the consumer is longer battery life and fully foldable mobile displays. While the cost of production is reduced with OTFT, the extremely low cost of ownership offers a low risk industrialization strategy through the building of a ‘greenfield’ line or by the re-purposing of an existing a-Si line.
One of the most exciting and eagerly awaited outputs of this rapid evolution in material perfor- mance and cost is the advancement and commercialization of bendable and foldable displays. From ultra-thin, conformal, wearables to truly foldable smartphones and tablets, organic semiconductors now offers the performance, cost and route to adoption for the manufacture of a new generation of OLED, EPD and LCD displays with entirely new physical properties and form factors.
References
1. http://www.displaysearchblog.com/2014/08/waiting-for-the-apple-iwatch/
DR MICHAEL COWIN is Head of Strategic Marketing, SmartKem Ltd., St Asaph, Wales