Maria Sramek

Interface Trap States in Organic Photodiodes

Organic semiconductors are attractive for optical sensing applications due to the effortless processing on large active area of several cm2, which is difficult to achieve with solid-state devices. However, compared to silicon photodiodes, sensitivity and dynamic behavior remain a major challenge with organic sensors. Here, we show that charge trapping phenomena deteriorate the bandwidth of organic photodiodes (OPDs) to a few Hz at low-light levels. We demonstrate that, despite the large OPD capacitances of ~10 nF cm−2, a frequency response in the kHz regime can be achieved at light levels as low as 20 nW cm−2 by appropriate interface engineering, which corresponds to a 1000-fold increase compared to state-of-the-art OPDs. Such device characteristics indicate that large active area OPDs are suitable for industrial sensing and even match medical requirements for single X-ray pulse detection in the millisecond range.

Near-Infrared Organic Photodiodes

Organic photodiodes (OPDs) are attractive as solution-processed devices for sensing applications. Industrial and medical sensors often have the requirement to operate in the near-infrared (NIR) spectrum between 650 and 900 nm and are ideally visible-blind. Due to the tailored spectral sensitivity of the organic semiconductors, OPDs are attractive as filter-free solid-state alternative. In addition, the large active areas of the OPDs potentially allow fabricating lens-free light-barrier and reflective sensors. In this paper, we discuss different approaches toward NIR sensitive OPDs with a large active area up to 1 cm2 applying polymers and small molecules as light absorbers. We demonstrate that with layer stacks optimized to the solution-processed semiconductor properties photodiodes with bulk heterojunctions with a minimum external quantum efficiency peak in the NIR and a rectification ratio of ~105 can be achieved, which match industrial sensing requirements.

Modeling and Simulation of Organic Photodetectors for Low Light Intensity Applications

In this paper, we investigate the dynamic response of two different bulk heterojunction organic photodetectors over a large illumination and frequency range. To our knowledge, there is no similar study that includes the nW/cm2 regime. Photocurrent transient measurements reveal that the interlayer at the hole-extracting electrode is critical for the device performance under ultralow illumination. Furthermore, we observe a nonlinear cutoff frequency behavior over the illumination range, which we attribute to interface-related phenomena. We perform a detailed simulation study of the transient response for the measured samples. Making use of a drift diffusion model that also takes into account charge trapping and detrapping effects, both in bulk and at material interfaces, we are able to successfully reproduce the measured transients. Based on our simulations, we propose an explanation for this effect: it can be attributed to the interplay between the potential landscape seen by the charge carriers and to the presence of a large concentration of interface trap states, as well as of fixed interface charges. The importance of smart interface engineering as a key factor for device optimization is also highlighted.