Impact of Unintentional Oxygen Doping on Organic Photodetectors

TL;DR

This paper investigates how oxygen plasma treatment unintentionally dopes organic photodetectors with oxygen, leading to reduced sensitivity and altered performance due to p-doping and trap states.

Contribution

It provides a detailed analysis of oxygen plasma's effects on organic photodetectors, revealing unintentional doping mechanisms and their impact on device performance.

Findings

Oxygen plasma causes significant p-doping of the active layer.

Doping leads to reduced device sensitivity and altered EQE.

Trap states are formed approximately 350 meV above HOMO.

Abstract

Oxygen plasma is a widely used treatment to change the surface properties of organic layers. This treatment is particularly interesting to enable the deposition from solution of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) on top of the active layer of organic solar cells or photodetectors. However, oxygen is known to be detrimental to organic devices, as the active layer is very sensitive to oxygen and photo-oxidation. In this study, we aim to determine the impact of oxygen plasma surface treatment on the performance of organic photodetectors (OPD). We show a significant reduction of the sensitivity as well as a change in the shape of the external quantum efficiency (EQE) of the device. Using hole density and conductivity measurements, we demonstrate the p-doping of the active layer induced by oxygen plasma. Admittance spectroscopy shows the formation of trap states approximately 350 meV above the highest occupied molecular orbital of the active organic semiconductor layer. Numerical simulations are carried out to understand the impact of p-doping and traps on the electrical characteristics and performance of the OPDs.