## 4 DNA for Other Optical Electronic Devices  
Alongside OTFTs and OTFT-based memory, OLEDs have also received huge attention in the flexible printed electronics [34; 35; 36; 37; 38; 39]. In the past decades, OLEDs have been actively explored for the low cost (through roll to roll processing), high contrast ratio, and wide color fullfillness. OLEDs have become a part of the heart in the display technology and have been used in wide applications like smart phones and music players. Since in 1987, [PERSON] et al. improved the device performance through applying TPD, Alq\({}_{3}\), and Mg:Ag as the hole transport layer, luminous layer, cathode layer, respectively. The electromechanical luminescent devices exhibit the low driving voltage (~10 V), high luninence (>1000 cd/m\({}^{2}\)), high efficiency (1.5 Im\(\cdot\)W\({}^{-1}\)) [40]. In 1990, the [PERSON] developed the electroluminescent device with polymers [41]. The research created the infinite possibility for the choice of materials and also opened new avenues for the preparation of large area electroluminescent devices. Among the various polymers, the DNA and the nucleic acid bases were explored as hole blocking layers (HBL) or effective electron blocking layers (EBL) in Bio Organic light-emitting diodes (BioLEDs) [42; 43; 44; 45], which enhanced the blue and green light emission effectively. As shown in Figure 6, [PERSON] et al. [46] demonstrated that the BioLEDs show a maximum luminous efficiency of about 8 and 1 cdA\({}^{-1}\), respectively. The operating voltages and turn-on voltages of these two devices were demonstrated as 10-25 V and 4-5 V, respectively. The green and blue BioLEDs exhibits the highest luminance of 21,000 cdm\({}^{-2}\) and 1500 cdm\({}^{-2}\), respectively. These values indicated that the luminance improved about 10 times compared with the polymethyl methacrylate or polyvinyl carbazole based device. In addition, significant advances have also been obtained in the phosphor based bio-hybrid light emitting diodes (Bio-HLEDs). Such devices exhibit high brightness and, most importantly, are ecofriendly and low cost. [PERSON] et al. [47] have demonstrated Bio-HLEDs through applying the pure DNA-curcumin-based phosphors as the chromophore material, as shown in Figure 7. The device shows a lower luminous efficiency drop rate and a bright luminescence of 62% and 0.0551 s\({}^{-1}\), respectively. Such results envision large-scale industrial applications of low-cost and ecofriendly illumination devices. Moreover, DNA can be even used in color-tunable OLEDs, which are of great importance for the good control of pixels in large-screen based display with enhanced resolution and color quality. [PERSON] et al. [48] developed the DNA/Pan/Ru(pypy)\({}_{3}\)\({}^{2+}\) based color tunable OLEDs for the first time. The resulting OLEDs exhibited multicolor emission, namely, the color of the device change from green to yellow to red when applying the voltage of 10, 14, 18, 20 V, respectively. The device structure and characteristics are shown in Figure 8.  
Figure 6: (**a–d**) Photographs of green-emitted (Alq3) and blue emitting (NPB) based BioLEDs devices in operation. (**e**) The energy-level diagrams for two color based BioLEDs (Reproduced with permission from [46], AIP Publishing, 2010). Interestingly, DNA also exhibits the possibility of application in photovoltaic devices because of its wide bandgap, adjustable length, and so on. [PERSON] et al. [49] reported DNA and DNA-PEDOT:PSS coatings could be applied as hole-transport materials in the preparation of dye sensitized solar cell and promoted an increase in the device conversion efficiency. [PERSON] et al. [50] integrated DNA nanolayers in polymer solar cells, which resulted in power conversion efficiencies increased by \(\sim\)110% and rectifying ratios by two orders of magnitude. [PERSON] et al. [51] investigated perovskite solar cells with DNA hole-transport layer and observed that the device lifetime significantly improved. Besides using DNA as the nanolayer in solar cells, [PERSON] et al.