Amorphous indium-gallium-zinc-oxide as electron transport layer in organic photodetectors

Abstract

Amorphous indium-gallium-zinc-oxide (a-IGZO) is demonstrated as an electron transport layer (ETL) in a high-performance organic photodetector (OPD). Dark current in the range of 10 nA/cm2 at a bias voltage of −2 V and a high photoresponse in the visible spectrum were obtained in inverted OPDs with poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester active layer. The best results were obtained for the optimum a-IGZO thickness of 7.5 nm with specific detectivity of 3 × 1012 Jones at the wavelength of 550 nm. The performance of the best OPD devices using a-IGZO was shown to be comparable to state-of-the-art devices based on TiOx as ETL, with higher rectification achieved in reverse bias. Yield and reproducibility were also enhanced with a-IGZO, facilitating fabrication of large area OPDs. Furthermore, easier integration with IGZO-based readout backplanes can be envisioned, where the channel material can be used as photodiode buffer layer after additional treatment.

Color Determination from a Single Broadband Organic Photodiode

“…Figure 1e shows that these optical cavity modes cause the modeled spectral distribution of absorbed photons (calculated from the fraction of incident photons absorbed by each layer using the transfer matrix method, see the Experimental Section) to differ substantially from the thin film absorption spectrum. This difference in spectral distribution, not observed in a previous study on a similar device48 with much thinner extraction layers and thus far less optical confinement, is supported by the experimentally measured EQE spectrum, which approximately maps the features of the modeled distribution. Differences between the EQE are attributed the real device having a lower cavity finesse than the model, likely due to rough surfaces, along with wavelength dependent charge collection efficiency (discussed below).…”

“…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.…”

Demonstration of a Broadband Photodetector Based on a Two‐Dimensional Metal–Organic Framework

“…Another important parameter to evaluate the performance of the photodetector is NEP. It is defined as the detection limit of the photodetector and is a function of the detector's noise level, expressed as where R is the responsivity and is the root mean square of the total noise current . The fundamental noise sources in a photoconductor are Johnson noise ( I j ), generation–recombination (G–R) noise ( I gr ), and 1/ f noise ( I f ), the latter being dominant at low frequencies .…”

“…where R is the responsivity and 2 1/2 I n is the root mean square of the total noise current. [36,37] The fundamental noise sources in a photoconductor are Johnson noise (I j ), generation-recombination (G-R) noise (I gr ), and 1/f noise (I f ), the latter being dominant at low frequencies. [29,30] From the photocurrent measurements, the Johnson noise is calculated as [4kTΔf/R d ] 1/2 , where k is the Boltzmann constant, T is the absolute temperature, Δf is the bandwidth and R d is the resistance of the device.…”