A High‐Resolution Thin‐Film Fingerprint Sensor Using a Printed Organic Photodetector

Abstract

However, it is not easy to process largearea, homogeneous thin OPD films by conventional coating techniques, such as spin-coating. Using slot-die coating of the active organic photodetector materials, we have developed a cost-effective, scalable photodetector array technology. We demonstrate its use in a high-resolution fingerprint detector with a thin form factor that ultimately would allow users to securely unlock their phone or open user-specific apps by touching anywhere on the display screen.The fingerprint scanner is based on three main building blocks (Figure 1a for a schematic of the fingerprint scanner, Figure 1b for the circuit diagram, and Figure S1, Supporting Information, for a full stack cross section). The backplane is a dual-gate self-aligned oxide thin-film transistor (DG-SA TFT) array, described and characterized elsewhere. [12] The second (bottom) gate is typically used to increase the transistor current for a given geometry and voltage. Here, it also acts as a light shield of the oxide semiconductor in case of backside exposure, thereby increasing device stability. [13] The DG-SA TFT exhibits a low off-current with values below 1 pA, lower than the off-currents of amorphous and low-temperature polycrystalline silicon transistors. This is due to the large bandgap of IGZO (≈3 eV) that results in a low leakage current. The field-effect mobility of our TFTs is 15 and 16 cm 2 V −1 s −1 for the top and bottom gate operation, respectively. In so-called dual gate mode, that is, V g top = V g bottom , the effective mobility is equivalent to 35 cm 2 V −1 s −1 . The values can be compared to a-Si TFTs (≈1 cm 2 V −1 s −1 ) and LTPS TFTs (≈100 cm 2 V −1 s −1 ). The frontplane consists of organic photodiodes (OPDs). The technology is based on a bulk heterojunction (BHJ) structure of a p-type (donor) polymer, polyPCDTBT, and an n-type (acceptor) fullerene, 6,6-phenyl-C61-butyric acid methyl ester, PCBM. The blend is deposited by slot-die coating, resulting in a 280 nm film. Semitransparent IGZO (bottom) and MoOx/Ag (top) electrodes are used. The a-IGZO is deposited on top of an opaque MoCr layer. This is done to prevent direct detection from the backlight, so only light that is reflected from objects on top of the imager array is measured by the OPD. The sensor is coupled with an optically transparent thin-film encapsulation to avoid moisture ingress that degrades the OPD materials. [14] The fabrication processes are compatible with flat panel display manufacturing lines. Details on the Organic photodetectors (OPDs) have attracted much attention in recent years, due to their promise in large-area light sensing applications. Here, high-resolution slot-die-coated large-area bulk heterojunction organic photodiode (OPD) arrays are reported. The OPD uses a novel electron transport layer, indium gallium zinc oxide in combination with a molybdenum oxide top-electrode. Together, these effectively reduce dark current densities to very low levels of ≈10 −7 mA cm −2 at −2 V. The OPDs show linear behavior in a wide rang...

High‐Accuracy Photoplethysmography Array Using Near‐Infrared Organic Photodiodes with Ultralow Dark Current

“…Nevertheless, the low J d in the order of 10 −6 mA cm −2 of our OPD is attributed to the charge blocking properties of a‐IGZO, reducing the injection of holes under reverse bias. [ 25,26 ] This J d value is comparable to previously reported dark current densities for organic pulse oximeters. [ 8,9 ] Regarding the OPD stability, the J – V characteristics show no visible signs of degradation after 2 weeks.…”

“…In fact, previously reported OPDs based on the same architecture were demonstrated to show no signs of degradation after more than 1 year. [ 26 ]…”

On-screen fingerprint sensor with optically and electrically tailored transparent electrode patterns for use on high-resolution mobile displays

“…However, the most convenient and intuitive location of the fingerprint sensor is on the front face. Various types of sensors that can recognize fingerprints on display screens have been developed [1][2][3][4][5][6][7][8][9][10][11] . Typical examples include capacitive, optical, and ultrasonic fingerprint sensors.…”

Color Determination from a Single Broadband Organic Photodiode

“…This is followed by a metal oxide layer that induces the wavelength variation in reverse‐bias J–V characteristics, here ≈30 nm of amorphous indium gallium zinc oxide (α‐IGZO) 43,44. IGZO is widely used for metal oxide thin film transistors (TFTs),37,43–47 and here forms an EEL since its −4.2 eV conduction band48,49 provides a downward energetic cascade (see energy level diagram50–53 in Figure 1b) for electrons under reverse bias (when the Au electrode is negative relative to the Ag electrode, see charge carrier directions in Figure 1b). 15 Furthermore, its deep (−7.5 eV) valence band43,48 suppresses unwanted hole injection, thus reducing dark current density J d 48,49.…”

“…IGZO is widely used for metal oxide thin film transistors (TFTs),37,43–47 and here forms an EEL since its −4.2 eV conduction band48,49 provides a downward energetic cascade (see energy level diagram50–53 in Figure 1b) for electrons under reverse bias (when the Au electrode is negative relative to the Ag electrode, see charge carrier directions in Figure 1b). 15 Furthermore, its deep (−7.5 eV) valence band43,48 suppresses unwanted hole injection, thus reducing dark current density J d 48,49. Crucially for this application, and in common with ZnO,35,39,40 the IGZO trap state density is influenced by the incident light color38,42 so can induce wavelength dependent electron extraction.…”