Karl Leo

PEDOT:PSS with embedded TiO2 nanoparticles as light trapping electrode for organic photovoltaics

The performance of organic optoelectronic devices can be improved by employing a suitable optical cavity design beyond the standard plane layer approach, e.g., by the inclusion of periodically or randomly textured structures which increase light incoupling or extraction. One of the simplest approaches is to add an additional layer containing light scattering particles into the device stack. Solution processed poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin films are promising for replacing the brittle and expensive indium tin oxide transparent electrode. We use a blend of 100u2009nm TiO2 nscattering particles in PEDOT:PSS solution to fabricate transparent electrode films which also functions as a scattering layer. When utilized in an organic photovoltaic device, a power conversion efficiency of 7.92% is achieved, which is an 8.6% relative improvement compared to a device with a neat PEDOT:PSS electrode without the nanoparticles. This improvement is caused by an increase in short-circuit current due to an improved photon harvesting in the 320u2009nm–700u2009nm spectral wavelength range.

Flexible, light trapping substrates for organic photovoltaics

Micro-structured organic photovoltaic (OPV) devices on polyethylene terephthalate substrates are produced using direct laser interference patterning (DLIP). The performance of organic solar cells on these substrates is improved by a factor of 1.16, and a power conversion efficiency of 7.70% is achieved. We show that a shorter spatial period of the pattern allows for a stronger light trapping effect in solar cell, as it leads to a longer light path. Moreover, since the patterned structures are located on the outside of the fully encapsulated OPV devices, there are no problems with the roughness induced shunts.

Beyond conventional organic transistors: novel approaches with improved performance and stability

Organic electronics hold the promise of enabling the field of flexible electronics. Several novel organic transistor concepts based on the technology of molecular doping are presented that open new directions to improve the performance of OFETs. The realization of doped organic transistors as well as organic junction field-effect transistors is demonstrated. Furthermore, vertical transistor concepts with channel lengths in the sub-micrometer regime are discussed.

Rubrene-based diodes for rectification applications (Conference Presentation)

The main focus of development in the field of organic electronics is on light-emitting diodes, solar cells, and various types of field-effect transistors(FET). However, for sophisticated electronic circuits, not only transistors but also high-performance diodes are required, serving as key elements in rectifiers and voltage stabilizers for various circuit applications. Such devices are essential for high-frequency signal processing (e.g. RFID systems) or power conversion. Several diode parameters, such as maximum driving-current, switch-on voltage, transition frequency, on/off-ratio and non-ideality, are relevant, depending on the specific application. nMost of these parameters are closely related to the mobility of the semiconductor materials in use. Due to the anisotropy of charge carrier transport in most high-mobility organic semiconductors, the mobility is far higher in lateral devices, such as FETs, than in vertical devices, such as diodes. Therefore, it is necessary to find material systems that offer high vertical mobilities.nWe present diodes targeted for rectification applications that are optimized for high current density, on/off-ratio, and switching speed. We employ a pin-diode design based on highly crystalline rubrene layers. Due to their crystallinity, these layers offer a high vertical mobility. We thoroughly studied the effect of variation in n- and p-doping on the layer properties and the resulting diodes. These devices show a very low turn-on voltage, stability of up to several 100 A cm−2 at constant power and on/off ratios of 1e6. These properties possibly allow for device operations beyond 1GHz which makes them ideal devices for signal rectification, needed for e.g. wireless communication.

Investigations on electrical performance and contact resistance in solution-processed vertical organic field-effect transistors (Conference Presentation)

High performance transistors are indispensable building blocks for future flexible electronic circuits. In order to overcome performance limitations such as low switching speed and high driving voltage, it is necessary to shorten the channel length and use high mobility semiconductors. Vertical organic field-effect transistors (VOFET) may overcome these limitations since their channel length can be reduced to a few nanometer without being restricted by photolitographic resolution limits. Previously reported VOFETs consist of small molecules from vacuum deposition. However, these semiconductors do not show as high mobilities as achieved with new semiconductors deposited from solution and are less suited for low-cost applications because of cost considerations.nHere, we present our investigations on the electrical performance of solution-processed VOFETs with the polycrystalline organic semiconductor 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-Pentacene) which shows mobilities up to 2 cm²/Vs. One major problem hindering the VOFET performance is contact resistance due to non-ohmic source/drain contacts as well as a non-linear channel resistance.nThus, we discuss the VOFET performance with regard to both kinds of resistances and reveal that the gate voltage induced modulation of contact resistance governs the IV curve for these VOFETs. The findings are supported by device simulations helping to create an improved understanding of the electric field and charge carrier distribution. We also demonstrate our results on channel resistance and compare experimental results with theoretical simulations proving that substantial improvements regarding contact resistance need to be made in order to obtain higher cutoff frequencies in organic transistors.

Vapor-deposited vertical organic field-effect transistors with optimized geometry for unrivaled transition frequencies (Conference Presentation)

Future flexible consumer electronic devices require powerful organic field-effect transistors (OFETs) to fulfill the demanding performance targets for, e.g., flexible RFID tags or active-matrix display driving. To achieve the required switching speed and current density (e.g. for display driving), it is firstly important to improve the charge carrier mobility of the organic semiconductors but secondly, it is also essential to reduce the channel length. In this context, optimizing the geometry of OFETs led to the development of novel approachs for vertical organic field-effect transistors (VOFETs) [1] where the vertical channel can be scaled down to the range of <50 nm.n nHere we present VOFETs produced by a new, highly reliable integration process which allows to push the cutoff frequency, a main figure of merit, to new limits. In particular, geometrical aspects of the VOFET such as gate-source overlap and charge carrier injection/ extraction length are investigated to derive scaling laws allowing to predict dynamic device properties. Furthermore, we present an advanced patterning technique helping to maximize on/off ratio and leakage current. These improvements on device performance allow for alternating current operation above 10 MHz.nMoreover, to evaluate the role of the organic semiconductor in these highly contacted limited devices, we investigate how semiconductor properties such as HOMO-LUMO gap, intrinsic doping, and mobility affect the VOFET performance and in particular device stability. Combining these finding on the device scaling and the influence of the semiconductor properties, we can provide a roadmap for future device improvement strategies.

Chapter 10:Small Molecule Organic Solar Cells

In this chapter, we review recent progress in small-molecule organic solar cells. First, we introduce the p-i-n-structure which is realized by combining the intrinsic absorber zone with doped transport layers. The doping of the transport layers is realized by mixing the host transport material with dopant molecules which lead to charge transfer in the ground state, thus creating free carriers. Such doped layers offer a number of advantages when used in organic solar cells: they improve the built-in field, allow easy optimization of thin-film optics and thus absorption in the photovoltaically active layers, and lead to good ohmic contacts even when the work function of the electrodes does not fit well to the adjacent organic transport materials. The materials systems used for doped layers and their influence on the built-in field are discussed in detail. We then discuss various absorber materials leading to improved solar cell parameters and overall efficiencies. Among the many potential materials classes for small-molecule organic solar cells, thiophene derivatives have shown excellent properties. By variations of the electronic core and the alkyl side groups, it is possible to independently study the influence of the electronic levels of the molecule and the crystal packing. By photo-induced absorption, we study the exciton separation as a function of orbital energies and temperature. Finally, we discuss optimized cells. The p-i-n concept allows to easily stack cells on top of each other to realize multi-junction organic solar cells. The key challenges here are current matching and optical design optimization. Combining all technologies, efficiencies of 12% have been reached.

Polymer-facilitated low temperature fusing of spray-coated silver nanowire networks as transparent top and bottom electrodes in small molecule organic photovoltaics (Presentation Recording)

Networks of silver nanowires (AgNWs) are promising candidates for transparent conducting electrodes in organic photovoltaics (OPV), as they achieve similar performance as the commonly used indium tin oxide (ITO) at lower cost and increased flexibility. The initial sheet resistance (Rs) of AgNW electrodes typically needs to be reduced by a post-annealing step (90 min@200 °C), being detrimental for processing on polymeric substrates. We present novel low temperature-based methods to integrate AgNWs in organic small molecule-based photovoltaics, either as transparent and highly conductive bottom-electrode or, for the first time, as spray-coated AgNW top-electrode. The bottom-electrodes are prepared by organic matrix assisted low-temperature fusing. Here, selected polymers are coated below the AgNWs to increase the interaction between NWs and substrate. In comparison to networks without these polymeric sublayers, the Rs is reduced by two orders of magnitude. AgNW top-electrodes are realized by dispersing modified high-quality AgNWs in inert solvents, which do not damage small molecule layers. Accordingly, our AgNW dispersion can be spray-coated onto all kind of OPV devices. Both bottom- and top-electrodes show a Rs of <11 Ω/ at >87 % transparency directly after spray-coating at very low substrate temperatures of <80 °C. We also demonstrate the implementation of our AgNW electrodes in organic solar cells. The corresponding devices show almost identical performance compared to organic solar cells exploiting ITO as bottom or thermally evaporated thin-metal as top-electrode.

A solution-doped small molecule hole transport layer for efficient ITO-free organic solar cells (Presentation Recording)

Indium-tin-oxide-free (ITO-free) organic solar cells are an important, emerging research field because ITO transparent electrodes are a bottleneck for cheap large area devices on flexible substrates. Among highly conductive PEDOT:PSS and metal grids, percolation networks made of silver nanowires (AgNW) with a diameter in the nanoscale show a huge potential due to easy processing (e.g. spray coating), high aspect ratios and excellent electrical and optical properties like 15 Ohm/sq with a transmission of 83.5 % including the substrate. However, the inherent surface roughness of the AgNW film impedes the implementation as bottom electrode in organic devices, especially fully vacuum deposited ones, where often shunts are obtained. Here, we report about the solution processing of a small molecule hole transport layer (s-HTL) comprising N,N-((Diphenyl-N,N-bis)9,9,-dimethyl-fluoren-2-yl)-benzidine (BF-DPB, host material) and the proprietary NDP9 (p-dopant) deposited from tetrahydrofuran (THF) as non-halogenated, “green” solvent. We show, that the doping process already takes place in solution and that conductivities, achieved with this process at high doping efficiencies (4 * 10^-4 S/cm at 10 wt% doping concentration), are comparable to thermal co-evaporation of BF-DPB:NDP9 under high vacuum, which is the proven deposition method for doped small molecule films. Applying this s-HTL to AgNW films leads to well smoothened electrodes, ready for application in organic devices. Vacuum-deposited organic p-i-n solar cells with DCV2-5T-Me(3:3):C60 as active layer show a power conversion efficiency of 4.4% and 3.7% on AgNW electrode with 35nm and 90 nm wire diameter, compared to 4.1% on ITO with the s-HTL.

Exciton binding energy limitations in organic materials and potentials for improvements (Presentation Recording)

In current organic photovoltaic devices, the loss in energy caused by the inevitable charge transfer step leads to a low open circuit voltage, which is one of the main reasons for rather low power conversion efficiencies. A possible approach to avoid these losses is to tune the exciton binding energy below 25 meV, which would lead to free charges upon absorption of a photon, and therefore increase the power conversion efficiency towards the Shockley Queisser limit for inorganic solar cells. We determine the size of the excitons for different one-dimensional organic small molecules or polymers by electron energy loss spectroscopy (EELS) measurements and by DFT calculations. Using the measured dielectric constant and exciton extension, the exciton binding energy is calculated for the investigated molecules, leading to a lower limit of the exciton binding energy for ladder-type polymers. We discuss and propose potential ways to increase the ionic and electronic part of the dielectric function in order to further lower the limit of the exciton binding energy in organic materials. Furthermore, the influence of charge transfer states on the exciton size and its binding energy is calculated with DFT methods for the ladder-type polymer poly(benzimidazobenzophenanthroline) (BBL) in a dimer configuration.