OPE Journal

FLEXIBLE DISPLAYS 14 No 40 | OCTOBER 2022 | OPE journal Active matrix addressed displays To couple an addressing transistor with each display pixel is efficient to avoid cross-talk effects when organising the pixels along the rows and columns of a matrix. The simplicity of the manufacturing process is important to maximise the manufacturing yield. It is therefore natural to rely on organic electro- chemical transistors in active-matrix displays. The transistors and the pixels take advantage of the conductivity switching and the colour changing property of PEDOT:PSS, respectively. But despite being two completely different components, they rely on identical materials and can therefore be manufactured simulta- neously in the same screen printing process (Adv. Eng. Mater. 2021, 23, 2000771). Passive matrix addressed displays The digital revolution has generated many dif- ferent electronic display technologies, includ- ing advanced screens with millions of pixels. Such high resolution cannot be obtained by printing techniques, and in addition to this, it would not be feasible to drive such display with a direct addressing protocol. The only realistic solution would be to use a matrix addressing drive protocol, to minimise the number of I/ Os in the addressing circuits, but this typically generates cross-talk effects in neighbouring pixels. An active-matrix approach could be used to avoid cross-talk, however, the increased number of components per display and the additional conductors for the gate electrodes would lower both fill factor and manufacturing yield. The ultimate goal is instead to drive the pixels by using a passive matrix addressing drive protocol. But this still adds the requirement of a non-linear pixel switching response. Recently, we demon- strated passive-matrix electrochromic displays manufactured at high yield (99.8%) by screen printing on flexible substrates, see Figure 3 (Sci. Rep. 2022, 12, 10959). These displays rely on a remarkably simple pixel architecture; the PEDOT:PSS-based electrochromic elec- trode and the carbon-based counter electrode are sandwiching an electrolyte, and all layers are deposited by screen printing. Despite the simplicity, the pixels exhibit the desired non- linearity in their switching response, which in turn enable unique addressability of each pixel in the matrix display. Additionally, the non- linear pixel switching response has proven to be independent of both pixel area and number of switch cycles, which allow for an endless number of electronic display applica- tions along with that the same drive protocol can be used throughout the lifetime of the matrix display. When the voltage exceeds the threshold voltage of the pixel, the coloration occurs linearly with increased voltage. This implies yet another feature; greyscale imag- ing. Besides the high manufacturing yield obtained by the screen printing process, the resulting passive-matrix displays show unprec- edented performances, exemplified by 50ms pixel switching times, greyscale images and fully dynamic QR codes easily interpretable by mobile phones. The combination of vapour phase polymerisation and screen printing Vapour phase polymerisation is a promis- ing technique to provide uniform layers of electrochromic polymers. As such, it is not a novel deposition method, but recently the possibility to use the combination of vapour phase polymerisation and screen printing for the manufacturing of electrochromic displays on large-area substrates was demonstrated (Adv. Mater. Technol. 2022, 7, 2200054) . The possibility to combine different electrochro- mic materials by using this method enables displays with a multitude of colours. Applications The Internet of Things (IoT) technology provides billions of sensor nodes for ubiq- uitous monitoring of almost any parameter. The result of the sensor monitoring is often communicated wirelessly, but there are still plenty of applications requiring an electronic display to visualise the result. The variety of electrochromic displays described herein, including their switching performances and high manufacturing yields, indicate a robust technology that can be manufactured via scal- able screen printing techniques, either sheet by sheet or roll to roll. The resulting displays, ranging from the simplest form of indicators to the presentation of arbitrary images with greyscale effects in passive-matrix displays, may be used within authentication, packag- ing, distributed healthcare, or any other IoT application, e.g., dynamic QR codes and large-area electronic billboards. Written by RISE: Peter Andersson Ersman, Jessica Åhlin, Kathrin Freitag, Marie Nilsson, Ulrika Boda, Robert Brooke, Valerio Beni, Mats Sandberg Image sources: Fig. 1: RISE Fig. 2a: John Wiley and Sons; https://onlinelibrary.wiley.com/doi/10.1002/ adpr.202200012 Fig. 2b: MDPI; Electron. Mater., Vol. 2, Issue 4 (Dec 2021) Fig. 3: Springer Nature; https://www.nature. com/articles/s41598-022-14792-9 Fig. 3: a) The pixel switching response that enables passive matrix addressing, including greyscale effects. b) Unique pixel addressability (chessboard pattern) and greyscaling (gradual decrease of the colour from row 1 to row 16) demonstrated in the same image; two features that enable subsequent presentation of different images in the same display

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