Francesco Arca

Exploiting Equalization Techniques for Improving Data Rates in Organic Optoelectronic Devices for Visible Light Communications

This paper presents the use of equalization techniques in visible light communication (VLC) systems in order to increase the data rate. Here we investigate two VLC links a silicon (Si) light emitting diode (LED) and an organic photodetector (OPD), and an organic LED (OLED) plus an Si photodetector (PD), together with three equalization schemes of an RC high pass equalizer, a fractionally spaced zero-forcing equalizer (ZF) and an artificial neural network (ANN). In addition we utilize a pre-distortion scheme to enhance the performance of the digital equalizers. For both systems the bit rate achieved are 750 kb/s from a raw bandwidth (BW) of 30 kHz and 550 kb/s from a raw BW of 93 kHz.

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.

A 1-Mb/s Visible Light Communications Link With Low Bandwidth Organic Components

This letter presents new experimental results on a 1-Mb/s organic visible light communications system. These results are the first demonstration of a fully organic free space optical communications system. Due to low charge transport characteristics, organic devices are typically highly band limited in the hundreds of kilohertz region (up to 135 kHz in this letter). Therefore, an artificial neural network (ANN) equalizer is required to undo the effects of intersymbol interference. Without the ANN, the maximum link performance corresponds to a data rate of 350 kb/s; however, with the ANN, a transmission speed of 1.15 Mb/s could be supported.

Large Active Area Organic Photodiodes for Short-Pulse X-Ray Detection

Organic thin film light sensors are promising devices for X-ray imaging systems. Compared to crystalline silicon photodiodes (c-Si), organic sensors can be fabricated on large active area at low cost. Furthermore, organic semiconductors have the advantage of low X-ray absorption. Here, we show that despite the high diode capacitance of several nF/cm2, a single X-ray pulse detection as low as ~14 μGy can be detected in the ms regime. Such device properties match industrial requirements for X-ray sensing.

Enhancing Efficiency of Organic Bulkheterojunction Solar Cells by Using 1,8-Diiodooctane as Processing Additive

Organic solar cells (OSCs) are attractive as an alternative to inorganic devices for their easy fabrication and solution-processability. A major and unsolved problem with bulk heterojunction devices remains the optimization of the network morphology. Here, we discuss the influence of the 1,8-diiodooctane (DIO) solvent additive on the efficiency of OSCs and show that by selectively controlling the crystallization of the organic material, the power conversion efficiency (PCE) can be increased by about 30%. For P3HT:PCBM-based devices, the power conversion efficiency (PCE) was increased from 3.7% to 4.9% for PCPDTBT:P3HT:PCBM-based devices from 3.2% to 4.1%. This improvement is due to the higher Isc, which is in agreement with the higher external quantum efficiency (EQE) observed on the devices fabricated with DIO. We correlate this to an increase of the surface roughness observed with atomic force microscopy (AFM) analysis. We demonstrate that the effect of the DIO additive is equivalent to a high-temperature thermal annealing.

Visible light communications: 3.75 Mbits/s data rate with a 160 kHz bandwidth organic photodetector and artificial neural network equalization [Invited]

This paper presents an experimental demonstration of a visible light communications link with an light emitting diode and a low-bandwidth organic photodetector as transmitter and receiver, respectively, that achieves sub 4  Mbits/s speeds. An artificial neural network (ANN) equalizer is required in order to achieve such high data rates because of the influence of intersymbol interference. The digital modulation formats tested in this paper are nonreturn-to-zero on–off keying (OOK), and fourth-order pulse position modulation (4-PPM). Without equalization, data rates of 200 and 300  kbits/s can be achieved for 4-PPM and OOK, respectively. With ANN equalization, data rates of 2.8 and 3.75  Mbits/s can be achieved for the first time for OOK and 4-PPM, respectively.

Kinetic Monte Carlo modeling of low-bandgap polymer solar cells

Numerical simulations based on kinetic Monte Carlo (kMC) techniques provide a powerful and versatile tool to gain a deep understanding of nanoscale processes in bulk organic heterojunction (BHJ) solar cells and to guide their optimization. Low-bandgap polymer donor materials count as constituents for novel cells with improved efficiency, mainly because they extend the absorption to the infrared region. We developed a kMC model which is able to accurately reproduce the current-voltage (JV) characteristics of an organic solar cell consisting of a blend of low-bandgap polymer and fullerene active materials with different active layer thicknesses.

Optimization of Organic Solar Cells by Kinetic Monte Carlo Simulations

Simulations based on kinetic Monte Carlo (kMC) methods provide a powerful and versatile tool to gain a deep understanding of nanoscale processes in organic bulk-heterojunction (BHJ) solar cells and to guide their optimization. We developed a simulation algorithm to model a state-of-the-art solar cell device comprised of a polymer:fullerene active layer and show that we are able to reproduce the current-voltage (JV) characteristics of a fabricated device. A major influence on device performance is found to be the dielectric constant of the organic materials.