# NiO-Based Electronic Flexible Devices
## 3 NiO-Based Flexible Devices
### Solar Cells  
Flexible inverted perovskite solar cells (PSCs) are achieved using a glass/ITO substrate as conductive transparent layer, and NiO\({}_{x}\) as an effective hole-transport material. Several variants were probed, with the purpose of achieving a high-power conversion efficiency (PCE) while keeping flexibility. Compared to normal PSCs, inverted flexible PSCs are light weighted, compatible with different surfaces, and portable, thus meeting the demands of the electronics industry [108,109]. Their further development, however, depends on the quality and characteristics of the hole transport layer (HTL). In this regard, NiO\({}_{x}\) is a promising candidate, owing to its low-temperature processing and good electron-blocking ability. In a relatively simple construction [110] NiO\({}_{x}\) nanofilms were deposited on ITO-PEN by spin coating from a high quality NiO\({}_{x}\) nanoparticle solution pre-synthesized by precipitation. Perovskite was deposited on top of the NiO\({}_{\rm x}\) layer by spin coating, after achieving a precursor by reacting PbI\({}_{2}\) and CH\({}_{3}\)NH\({}_{3}\)I in a mixture of DMSO and DMF, dripping chlorobenzene during spinning and baking at 100 \({}^{\circ}\)C. A thin layer of phenyl-C61-butyric acid methylester and a layer of Ag 150 nm thick completed the inverted planar heterojunction solar cells. The as-prepared NiO\({}_{\rm x}\) films had excellent hole extraction and electron blocking property, thus providing a good hole contact for perovskite solar cells. A maximum PCE of 16.47% (steady state PCE of 16.21%) was achieved for the NiO\({}_{\rm x}\)-based perovskite solar cell on the ITO-PEN glass substrate.  
In order to improve the band structure alignment between low-temperature-processed NiO\({}_{\rm x}\) and the perovskite layer, amino-functionalized graphene quantum dots (AGQDs) were included in the NiO\({}_{\rm x}\) film as a dual-role additive. In addition, the AGQDs can provide abundant N atoms at the modified NiO\({}_{\rm x}\) layer surface to enhance the crystallization of the perovskite film by a Lewis base-acid interaction. On the other hand, the AGQDs can optimize the band structure alignment between the NiO\({}_{\rm x}\) and perovskite layers, facilitating hole extraction at the NiO\({}_{\rm x}\)/perovskite interface [111]. The overall building up of AGQDs-aided NiO/peroskite flexible solar cells requires the separate synthesis of AGQDs, NiO\({}_{\rm x}\) ink and subsequent assembling on PEN/ITO substrate after etching. More in detail, AGQDs were achieved by RT reaction of graphene sheets with ammonia, followed by autoclave treatment at 150 \({}^{\circ}\)C for 5 h and dialysis of the cooled solution. NiO\({}_{\rm x}\) was prepared by reaction of Ni(NO\({}_{3}\))\({}_{2}\) with NaOH at pH 10, drying of the precipitate at 80 \({}^{\circ}\)C and calcination at 275 \({}^{\circ}\)C for 2 h. The pristine NiO\({}_{\rm x}\) ink was prepared by dispersing 20 mg of NiO\({}_{\rm x}\) powder in 1 mL of deionized water. AGQD-doped NiO\({}_{\rm x}\)inks were obtained by directly mixing AGQD aqueous solutions with the pristine NiO\({}_{\rm x}\) ink. The pristine and AGQD-doped NiO\({}_{\rm x}\)inks were spin-coated on the flexible substrates, heated at 120 \({}^{\circ}\)C for 15 min and transferred to a N\({}_{2}\)-filled glovebox to prepare the perovskite films. The resulting inverted flexible PSC was subjected to automatic bending and exhibited an efficiency of 18.10%, good air stability, retention of 88% of its initial efficiency after continuously bending 1000 times and a retention of 64% of the initial efficiency after 30 days.