Printed Diodes: Materials Processing, Fabrication, and Applications

Abstract

Printing techniques for the fabrication of diodes have received increasing attention over the last decade due to their great potential as alternatives for high-throughput and cost-effective manufacturing approaches compatible with both flexible and rigid substrates. Here, the progress achieved and the challenges faced in the fabrication of printed diodes are discussed and highlighted, with a focus on the materials of significance (silicon, metal oxides, nanomaterials, and organics), the techniques utilized for ink deposition (gravure printing, screen printing, inkjet printing, aerosol jet printing, etc.), and the process through which the printed layers of diode are sintered after printing. Special attention is also given to the device applications within which the printed diodes have been successfully incorporated, particularly in the fields of rectification, light emission, energy harvesting, and displays. Considering the unmatched production scalability of printed diodes and their intrinsic suitability for flexible and wearable applications, significant improvement in performance and intensive research in development and applications of the printed diodes will continuously progress in the future.

Keywords: nanomaterials; organic light‐emitting diodes (OLEDs); printed diodes; printing technologies; radio frequency identifications (RFIDs).

Figure 1

Outline illustration of the review for printed diodes via different kinds of materials. The focus of the review is placed on materials processing, fabrication approaches, and broad applications of the diodes and their integrated devices.

Figure 2

Variation of device frequency characteristics and charge carrier mobility for different kinds of semiconducting materials. Reproduced with permission.1 Copyright 2017, IOP Publishing.

Figure 3

Phase‐diagram of the printing resolutions and speeds demonstrated by the major printing techniques. For each method, the minimum layer thickness and required ink viscosity are presented. Reproduced with permission.27 Copyright 2017, Royal Society of Chemistry.

Figure 4

Schematic illustration of the most popular printing techniques. a) Inkjet printing. b) Screen printing. Reproduced with permission.27 Copyright 2017, Royal Society of Chemistry. c) Gravure printing. Reproduced with permission.42 Copyright 2016, IOP Publishing. d) Aerosol jet printing. Reproduced with permission.191 Copyright 2017, IOP Publishing.

Figure 5

Printed diodes from silicon materials. a) Optical image of the finished PIN diodes arrays on a bent PET substrate. b) Microscopic image of the 80 mm² flexible microwave single‐crystalline SiNM PIN diode on plastic substrate (top) and Microscopic image of the 240 mm²‐PIN diode on plastic substrate (bottom). Reproduced with permission.64 Copyright 2011, Elsevier. c) Schematic illustration of fabrication process for nanotrench Si NM flexible RF TFTs by NIL. d) i) A microscope image of a bent array of TFTs and ring oscillators on a PET substrate. ii) A microscopic image of a single 5‐stage ring oscillator under a flat condition. e) Measured voltage–time characteristic of the 5‐stage ring oscillator showing a frequency of 165 MHz and a delay time of 0.59 ns. Reproduced with permission.65 Copyright 2016, Springer Nature Group.

Figure 6

Flexible diodes from silicon particles. Schematic illustration of a) the cross section and b) the lateral architecture of the Schottky diode made of two layers of microparticles (NbSi₂ and Si) bonded together by SU‐8 binder. c) Demonstration of e‐label application. The antenna–diode–display circuit is deposited onto a PET substrate. When the mobile phone is held close to the antenna, the display starts to switch on. Reproduced with permission.68 Copyright 2014, National Academia of Science. d) Illustration for the structure of the modified Schottky diode. e) SEM image of top NFC:Si film surface. f) Altered response of output DC voltage to signal frequency. g) The fabrication process of the diode: Peeling off the Si film; Attaching it to the substrate; Calendering; Peeling off the Ni/C double side adhesive tape; Attaching the Ni/C tape to the Si film; Calendering once more. Reproduced with permission.70 Copyright 2016, Springer Nature Group.

Figure 7

Printed electronics from solution‐processed silicon. a) A TEM image of a solution‐processed poly‐Si film. The film was formed by spin‐coating and baking of the liquid silicon materials followed by laser crystallization. The inset TEM image highlights the atomic image of the silicon crystal. The grain size in the film is about 300 nm, which is comparable to that of conventional CVD‐formed poly‐Si film. b) SEM image of a TFT made from ink‐jet printed silicon film. Reproduced with permission.71 Copyright 2006, Springer Nature Group. c) Comparison of current density–voltage (JV) characteristic of printed and plasma‐enhanced chemical vapor deposition (PECVD) based a‐Si diode. Dashed and star curves denote freshly printed and air aged printed diodes, respectively. Reproduced with permission.73 Copyright 2008, Elsevier. d) Schematic of the solution‐based synthesis of microcrystalline silicon (µc‐Si) thin films. e) SEM images of a cyclohexasilane precursor‐based Si film after plasma hydrogenation treatment for 20 min. Each scale bar is 100 nm. f) Changes in grain size and roughness of the surface in (e) following plasma hydrogenation treatment of 0, 20, and 30 min (left to right). Reproduced with permission.75 Copyright 2015, American Chemical Society.

Figure 8

Flexible electronics from IGZO. a) Optical image and a schematic cross‐section of photoannealed IGZO TFTs and circuits on PAR. b) Optical image of a seven‐stage ring oscillator. Gate to source/drain overlap distance is 5 µm. c) Oscillation frequency (red) and per‐stage propagation delay (blue) of the ring oscillator as a function of supply voltage, V _DD. d) Output waveforms of the ring oscillator operating with supply voltages of 5 V (left panel) and 15 V (right panel), and oscillation frequencies of 45 and 341 kHz, respectively. Reproduced with permission.81 Copyright 2012, Springer Nature Group. e) AFM image and f) cross section SEM image of IGZO (In:Ga:Zn = 1:1:1) composition film, annealed at 400 °C for 2 h. g) I–V curves of IGZO based heterojunction didoes. Reproduced with permission.88 Copyright 2017, Elsevier.

Figure 9

Flexible transistors and diodes based on solution‐processed metal oxides. a) Photograph of an actual transistor array containing 35 flexible ZnO TFTs based on a UV‐grown bilayer AlOx/ZrOx gate dielectric and Al source–drain electrodes. b) Schematic of the device architecture used and c) TEM image of the SiO2/ZnO/Al cross‐section showing the ultrathin nature of the ZnO film. Reproduced with permission.89 Copyright 2013, Wiley‐VCH. d) Representative sets of transfer characteristics measured from transistors based on metal oxide quasi‐superlattices (QSL) channels. Reproduced with permission.90 Copyright 2015, Wiley‐VCH. e) Optical image of a flexible combustion‐processed In₂O₃ device on AryLite (30 nm Al gate electrode/41 nm a‐alumina dielectric, with 30 nm Al source and drain electrodes) and optical image of an inkjet printed In₂O₃ line on n++Si/41 nm a‐alumina (inset). f) Comparison of energetics of combustion synthesis‐based processes versus conventional approaches. g) schematic of top‐contact bottom‐gate TFT device structure in (e). Reproduced with permission.91 Copyright 2011, Springer Nature Group.

Figure 10

Printed diodes with metal oxides. a) Optical image of the R2R gravure printed rectenna on PET foil. The bottom schematics describe the components of rectenna: printed capacitor, printed diode, printed bottom Ag electrode. b) Cross‐sectional SEM images for the R2R gravure printed diode. c) Input–output electrical characteristics of rectifier at 13.56 MHz A, indicating a rectifying efficiency of about 90%. Reproduced with permission.93 Copyright 2012, IOP Publishing. d) Scheme for R2R gravure to completely print sensor‐Signage Tags on PET foils. e) R2R samples by combining R2R gravure printed rectenna and R2R coated electrochromic signage. Reproduced with permission.94 Copyright 2014, Springer Nature Group. f) Photograph of a 2 × 2 cm² substrate patterned with the use of the adhesion lithography (a‐Lith) technique. The substrate incorporates 72 discrete Al/Au nanogap diodes. g) Optical micrograph of a‐Lithfabricated interdigitated electrode structures. h) Current–voltage characteristics of Al/ZnO/Au nanogap diodes (with different widths) fabricated on the substrate shown in (f). Reproduced with permission.192 Copyright 2016, SPIE.

Figure 11

Schematic illustration of the major CNT synthesis techniques compatible with solution processing: a) arc‐discharge, b) laser ablation, and c) high‐pressure carbon monoxide growth. Reproduced with permission.105 Copyright 2004, Elsevier. The CNTs purification approaches for removing the mixed metallic CNTs: d) functionalization via polymer wrapping, e) chromatography and f) density‐gradient ultracentrifugation. Reproduced with permission.109 Copyright 2008, Springer Nature. The alignment methods for CNT applications in high performance devices: g) selective positioning via SEM/Raman spectroscopy. Reproduced with permission.112 Copyright 2009, American Chemical Society. h,i) Alternating current dielectrophoresis (DEP). Reproduced with permission.115 Copyright 2014, IEEE.

Figure 12

Methods for dispersing CNTs into a stable and uniform ink. a) Schematic diagram of the sonication procedure for CNTs. b) Plot of the decrease in CNT conductivity with increased physical mixing time. c) CNT length distribution after 1 h of sonication treatment (inset: SEM image of lightly damaged CNTs). Reproduced with permission.193 Copyright 2012, MDPI. d) Schematic of the functionalization procedure for CNTs. e) Cylindrical miscelle configuration of surfactants encapsulating a SWCNT. f) Hemimicellar adsorption configuration of surfactants on a SWCNT. g) Random adsorption configuration of surfactants on a SWCNT. h) Adsorption spectra of SWCNTs in an SDS surfactant suspension. Lines A–C represent SWCNTs of different diameters, while line D represents aggregated SWCNTs bundles. i) TEM cross‐sectional image of a SWCNT bundle. j) Separation procedure through which surfactants may disperse SWCNT bundles. Reproduced with permission.125 Copyright 2006, Elsevier.

Figure 13

CNT‐based Schottky diode. a) Illustration of a fabrication approach using the chemical forces between inks. i) Schematic description for inkjet printing transistors with small channel gaps via chemical forces. ii,iii) optical images of two parallel lines forming a chemical gap. The lines are electrically isolated. iv) SEM image of an electronically isolated gap. Reproduced with permission.133 Copyright 2017, Springer Nature. b) Illustration for a CNT based Schottky diode detector. c) Photomicrograph of a log‐periodic antenna with CNT Schottky diodes, d) AFM scan of the CNT channel region of the diode, and e) close‐up view of CNT rich region. e) I–V characteristics for the Schottky diode of (b) alongside its effective resistance. f) Output voltage of the diode at 18 GHz as a function of input power. Reproduced with permission.134 Copyright 2014, IEEE.

Figure 14

Current techniques in fabricating graphene nanosheets and Schottky diodes fabricated with graphene. a) Cascade centrifugation. Reproduced with permission.194 Copyright 2016, American Chemical Society. b) Density gradient ultracentrifugation Reproduced with permission.195 Copyright 2009, American Chemical Society. c) Photograph of a polymer PmPV/DCE solution with graphene nanoribbons stably suspended in the solution. d) Chemically derived graphene nanoribbons down to sub 30 nm width. e) Transfer characteristics of the transistor made of graphene nanoribbon with Pd contacts. (Inset) The AFM image of this device. Scale bar is 100 nm. Reproduced with permission.143 Copyright 2008, AAAS. f) SEM image of a Schottky diode fabricated on a graphene monolayer. Reproduced with permission.146 Copyright 2012, APS. g) Partially reduced graphene oxide‐based Schottky diodes on a PEEK substrate. h) Magnified SEM image of a single Schottky diode from (d). Reproduced with permission.147 Copyright 2013, IEEE.

Figure 15

LEDs from quantum dots. a) Electroluminescence image of a full‐color QD display. b) Flexible LED with RGB QDs patterned by transfer printing (Inset: Magnified image of emission layer rows). Reproduced with permission.196 Copyright 2011, Springer Nature. c) 3D printed QD‐LED on a 3D scanned onto a curvilinear substrate. d) Color coordinates of the green (0.323, 0.652) and orange‐red (0.612, 0.383) QD‐LEDs marked by stars on the Commission International de l′Eclairage (CIE) 1931 chromaticity diagram, with dashed lines representing the HD television color standard saturation windows as defined by the National Television System Committee. Reproduced with permission.41 Copyright 2014, American Chemical Society. e) Schematic diagram shows the structure and materials of an archetypical QD‐LED. f) A high‐resolution AFM micrograph of a close‐packed monolayer of QDs deposited on top of the CBP hole transporting layer. g) Electroluminescent red, green, and blue QD‐LED pixels with the device structure shown in (e). h) Chromaticity diagram shows the positions of red, green, and blue QD‐LED colors, an HDTV color triangle is shown for comparison. Reproduced with permission.154 Copyright 2008, American Chemical Society.

Figure 16

Electronics applications of the organic small molecules. a) Photograph of 8 bit RFID transponder chips on a plastic foil, based on a blend of TIPS‐pentacene and the polymer polystyrene. Each transponder has a footprint of 34 mm². b) Output oscillations for an 8‐bit RFID transponder, with ink‐jet printed TIPS‐PEN:PS blend as the active layer. Reproduced with permission.163 Copyright 2013, Elsevier. c) BCBG device architecture and d) blade‐coating set‐up. e) Transfer characteristics of OTFTs prepared using neat diF‐TES‐ADT, diF‐TES‐ADT:PαMS (100 kDa) and diF‐TES‐ADT:PS (123 kDa) blends at a blade speed of 1.5 mm s⁻¹ and a stage temperature of 70 °C. We employed V _ds = −10 V for blends and V _ds = −20 V for neat OSC. f) Hole mobility of OTFTs fabricated by blade‐coating diF‐TES‐ADT:PS blends using different Mw of PS both in different blade speeds. Reproduced with permission.165 Copyright 2015, Springer Nature. g) Schematic illustration of OFETs with the dual gate geometry made of TIPS‐pentacene/polymer blend film. h) TIPS‐pentacene/polymer blend films morphologies of PTAA blend layers processed from tetralin. i) Transfer characteristics of TIPS‐pentacene/poly‐(triarylamine) (PTAA) blend OFETs with top gate geometry processed from tetralin; CYTOP (700 nm) single layer gate dielectric. Reproduced with permission.166 Copyright 2012, Royal Society of Chemistry.

Figure 17

Electronics made of pentacene and C₆₀. a) Structural schematic of a pentacene diode with self‐assembled monolayers of 2,3,4,5,6‐pentafluorobenzenethiol (PFBT) coated on the Au anode. b) The current density response of the structure in (a) to various biasing voltages (Inset: Corresponding diode rectification ratios). Reproduced with permission.168 Copyright 2016, Wiley. c) Optical micrograph image of zone‐cast TIPS‐pentacene crystals on a self‐aligned structure. d) Recorded mobility for zone‐cast and spin‐coated TFT devices with the structure of (c). Reproduced with permission.169 Copyright 2015, Wiley. e) Saturation mobility of C₆₀:C₇₀ films as a function of the weight ratio (Inset: Images of C₆₀:C₇₀ film (above, full coverage granted) and C₆₀ film (below, incomplete coverage)). f) Ternary phase diagram for C₆₀:C₇₀:O‐DCB with a blue‐shaded single‐phase region and a two‐phase region corresponding to transmission optical images seen to the top left and right, respectively. g) Diagram of C₆₀:C₇₀ solubility as a function of the weight ratio. Reproduced with permission.171 Copyright 2015, Wiley. h) Saturation and linear carrier mobilities of diF‐TES‐ADT:PTAA and diF‐TES‐ADT:PF‐TAA blend OTFTs. Reproduced with permission.172 Copyright 2012, Wiley. i) Transfer characteristics of a C₁₀‐DNBDT transistor. Reproduced with permission.173 Copyright 2015, Wiley.

Figure 18

Schottky diode from organic polymers. a) Schottky diode structure including a PEDOT:PSS hole injection layer. b) Fully functional flexible wireless power transmission sheet with PQT‐12‐based Schottky diodes fabricated via spin‐coating with the structure of (a). c) Current density as a function of bias voltage for the diodes with three polymer varieties. Reproduced with permission.175 Copyright 2011, Elsevier. d) Photograph of gravure printed full‐wave circuit including four PTAA‐based Schottky diodes. e) Output voltage as a function of operation frequency for various rectifier setups with the diodes of (d). Reproduced with permission.11 Copyright 2013, IEEE. f) C16IDT‐BT‐based diode structure. g) Output voltage as a function of frequency for half‐wave rectifiers with spin‐coated Schottky diodes based on the design of (f). h) Image of the diode on a flexible substrate and the I–V characteristics for the diode both with and without a PEIE interlayer separating the cathode and semiconductor. Reproduced with permission.177 Copyright 2017, Wiley. i) Fully inkjet‐printed PTAA‐based MIS diode architecture. j) Plot of current density as a function of voltage and the number of MMAcoMAA layers for the device structure in (i). Reproduced with permission.178 Copyright 2017, IOP Publishing.