Unlock the performance of Organic Photodetectors by “tuning” the PEDOT layers

Printed electronics serves as an opportunity to create functional structures/devices, which can be realized via high-throughput printing/coating techniques. Given technologies enable to create functional features/structure, fabricated in high volume by low-cost production processes. Because the technology production process of given devices is designed, in most cases, from the roots, it is developed by respecting the low environmental impact and easy recyclability of end products.

Printed Electronics is created using printable materials based mostly on organic photodetectors and materials. By appropriate prepared stacks of layers, several types of devices like OPVs (Organic photovoltaics), OLEDs (Organic Light Emitting Diodes), OPDs (Organic photodetectors), sensors, OFETs (Organic field-effect transistors), batteries, etc. can be printed.

Nowadays, many of the devices are printed by a combination of several organic and inorganic layers, which allows appropriate charge transfer within its structure or its recombination, or other phenomena. Even if inorganic-based layers of devices are printed by means of nanodispersion or sol-gel of inorganic materials, organic-based materials still offer a unique opportunity from the point of view of structural tuning of the forming small molecules or/and polymers. By this synthetic “tuning”, organic materials can be optimized in terms of solubility, conductivity, HOMO/ LUMO energy level and spectral characteristics, which, generally, in the end, enhances the performance of mentioned functional structures.

In  printed electronics, conductive polymers are one of the most widely used organic-based conductive materials, being poly(3,4-ethylenedioxythiophene) (PEDOT) the most frequent one, both in research as well as in practical applications . Within the MADRAS project, the PEDOT:PSS conductive polymer is being applied either as HTL ( Hole Transport Layer) or as a TCL (Transparent Conductive Layer) in the fabrication of OPDs, which comprise the sensing part of TFT (thin-film transistor)-based fingerprint reader.

The OPD device consists in a stack of several layers – a TCL, an electron transporting layer (ETL), a PAL (photoactive layer), a HTL and a top electrode. For the high-performance capabilities of the OPD, a high-charge carrier mobility, appropriate work function (WF), and high conductivity are required for the comprising PEDOT layers. It is possible to modify given parameters by varying specific conditions of the synthesis of the PEDOT in two different approaches: i) by adjusting PEDOT:PSS ratio, where the PSS- serves as the primary doping counterion or ii) by optimising the composition of the final ink formulation.

Within the MADRAS project we are targeting a high charge carrier mobility of 0.7 cm2/V.s, which allows an effective transport of holes from the PAL and a high conductive PEDOT:PSS inks with target value of 500 – 900 S/cm, especially intended for transparent conductive layers. .

Alongside these parameters, and for an effective charge transfer in OPD, the proper WF of the PEDOT:PSS layer should be optimized in the context of energy levels of the underlying and next printed layer. It is possible to optimise all characteristics of conductive polymers by ink formulation composition, by PEDOT:PSS ratio or by using special kind of additives.

The PEDOT:PSS is synthetised in the form of water-based dispersion, which is ensured by COC. Pristine PEDOT:PSS dispersions are then tuned by the University of Pardubice (UPA) to the form of ink formulations, with the required rheology and surface tension properties. These parameters are set for a specific printing technique and in relation to PAL surface free energy value too. By balancing the solids of the ink formulation, solvents, and additives, the viscosity of the ink is optimised from low viscosity -ones of mPa.s- for coating techniques such as slot-die, or printing techniques as gravure, flexography up to high viscosity – tens of Pa.s – for technique as screen printing.

The conductivity is one of the most important parameters that can be significantly influenced by the ratio of PEDOT:PSS complex. The PEDOT:PSS layers with lower ratio has generally higher conductivity, where the one of the most conductive (Fig. 2) layer is with PEDOT:PSS ratio 1:1.4, which is close to nature molar stoichiometry. On the other side, the higher ratios have a generally positive effect to the film forming properties of the dispersion.

Together with the ratio of PEDOT:PSS complex, conductivity can also be tuned by addition of a secondary doping solvent – secondary dopant – which fixes the optimal morphology and internal arrangement of PEDOT:PSS layers. Our research made evident that the polarity of the solvent, its amount and its dielectric constant have strong boosting effects on the conductivity of PEDOT:PSS layer, leading to an increase of the specific conductivity by three orders of magnitude in comparison to the ink formulation without a given secondary dopant (Fig 3).

The optimisation of Surface Tension (ST) and the overall film forming properties of the ink is essential. These parameters are influenced by changing the type and the amount of surfactant or by using cosolvent/s. The amount of surfactant is crucial, as low amount does not ensure the excellent film forming/wetting behaviour of the ink and high amount has an effect on the conductivity (Fig. 4) or could cause charge carrier trapping.

The WF (Work Function) is also a very influential parameter to the performance of the OPD. One of the most influential parameter to the WF is primarily given by PEDOT:PSS complex ratio. Alongside the complex ratio optimization, within the MADRAS project the WF is being refined by using special kinds of solvents(Fig. 5).