NIR sensitive organic dyes for tandem solar cells and transparent photodiodes

Due to advantages such as mechanical flexibility, light weight and the prospect to use low-cost roll-to-roll manufacturing processes, organic semiconductors have been widely investi-gated in many application areas as alternatives for their inorganic counterpart. In organic sem-iconductors, the rather weak Van der Waals interactions holding together the molecular build-ing blocks result in narrow absorption bands which endow organic electronics with important advantages for the development of smart functionalities. Transparent organic electronics (TOEs), for example, incorporate devices through which visible light is transmitted. Among other semiconducting devices, it is actually possible to construct sensors and photovoltaic de-vices that solely use ultraviolet (UV) and near infrared (NIR) light to produce electrical energy or signal. TOEs have been proposed for easy integration with other electronic devices. Among the different molecular materials, cyanine dyes stand out by sharp, intense absorption bands exhibiting the highest molar extinction coefficients. The absorption peak can be easily shifted into the NIR wavelength region by increasing the length of the conjugated polymethine chain. For example, NIR light absorbing heptamethine cyanine dyes (Cy7) are promising candidates as transparent and colorless photoactive film materials. In this thesis work, highly efficient TOE devices such as transparent solar cells and transparent photodetectors using NIR absorbing cyanine dyes as photosensitive materials have been successfully fabricated. To optimize these multilayer devices, various cyanine dyes were in-vestigated, device architecture and interfaces were engineered. Optical simulations of the stacked thin film structures allowed understanding and tuning device performance. Moreover, organic solar cells which are transparent in the visible range have been integrated into tandem and triple junction solar cells. Low bandgap materials that absorb NIR light were combined with cyanine cells which absorb visible light, thereby more sunlight could be harvested and power conversion efficiency was dramatically enhanced in such tandem solar cells. The photo-stability investigation of cyanine solar cells showed that cyanine dyes were photostable when illuminated in the absence of oxygen and water vapor. We found that the initial degradation of cyanine dye devices during operation was due to the photo-polymerization of the widely used electron acceptor material fullerene C60 and photo-chromism of the hole extraction interfacial layer molybdenum oxide (MoO3).

Dynamics of Electronic and Ionic Charges in Cyanine Organic Semiconductor Devices

Sandra Christina Jenatsch

Organic semiconductor materials have been widely applied in optoelectronic devices to replace their inorganic counterparts and explore new fields of applications. Due to their high extinction coefficients, chemical tunability and solubility, cyanine dyes are an interesting class of molecules to be employed in organic semiconductor devices. Their beneficial properties have been demonstrated in organic solar cells (OSC), photodetectors and recently in organic light-emitting electrochemical cells (OLEC). The focus of this thesis is to analyse different aspects regarding the dynamics of electronic and ionic charges in cyanine based devices. Models and conclusions are not restricted to cyanine based devices but can potentially be applied in various systems where combined ionic and electronic charges are present. In order to focus on electronic processes in OSCs, a combination of steady-state, transient and impedance analysis is used. It is resolved that trapped charges can be introduced at the dye/hole extracting layer interface by using specific solvents for the cyanine layer fabrication. These measurements also reveal the low hole mobility of cyanine films which in turn limits the possible upper layer thickness in OSCs. A frequently used way to enhance the conductivity of organic semiconductors is the addition of extra charges by chemical doping. This concept is employed for a near-infrared absorbing dye which is used as donor in a bilayer solar cell. The dependence of all figures of merit versus doping concentration and layer thickness is studied in detail. It is found that the performance of the optimised cell is not further improved by doping, but that it presents a good option if thicker layers are required, e.g. on rough substrates. Furthermore, the stability and degradation pathway of the p-doped species is analysed. This information is of general relevance as they also occur after charge transfer at the heterojunction interface of an OSC and during operation of OLECs. The dynamics of ionic charge carriers is separated into two contributions. In a first step, the steady-state diffusion profile of the counter ion within a cyanine/fullerene bilayer structure is scrutinised using different profiling techniques. An unexpected constant ion profile is found throughout the C60 layer. Simultaneously, the potential within the acceptor is measured by the Kelvin probe technique. A mechanism of combined electron and ion transfer from the C60 to the dye and vice versa is proposed which is in agreement with the experimental results. A prerequisite for OLEC operation is the mobility of ionic charge carriers. Their dynamics in cyanine salts under the effect of an external field is therefore studied in such devices. Several methods are developed and applied to characterise the dynamic p-i-n structure in detail, i.e. the position and width of the intrinsic region as well as the doping concentrations in p- and n-doped regions. The combination of all these methods helps to understand the transient behaviour of these cyanine devices but can also be adapted to other OLEC systems. A major drawback of cyanine dyes as emissive material is their low fluorescence quantum yield in solid films. To overcome this restriction and improve the OLEC efficiency, host-guest systems with various visibly emitting dyes are studied. The gain in fluorescence quantum yield in the film can directly be transferred to an increase in external quantum efficiency of the device.

EPFL2017

Interface issues in organic semiconductor based devices

Mauro Castellani

The present doctoral thesis aims to contribute to the field of organic semiconductor physics and technology, both of which have become fast growing disciplines. Two technological applications are emerging from these research efforts: Organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs). OLEDs are already being fabricated on an industrial level today, while some fundamental issues still remain to be resolved for OFETs. During the last ten years, research interest has been continuously shifting from the bulk properties of organic semiconductors to their interfaces. Such issues are in fact addressed in this thesis work and include different interactions of thin organic layers with their environments. In the field of OLEDs, a multitude of issues have been investigated here. Most importantly and in contrast to most publications in this field, OLED's with complex device architectures that fully compete with standard devices performancewise have been fabricated and characterized. An oxygen plasma treatment of indium-tin-oxide anode has been shown to reduce the injection performance while enhancing the quantum efficiency. Two different mechanisms are proposed to explain this. Going a step further, the life-time behavior of such devices has been examined. Different aging mechanisms were identified: in short term and intermediate term aging, an oxygen plasma treatment of anode clearly reduces the operational voltage. Bulk degradation of the organic electroluminescent layer then becomes predominant for long term aging. Another important finding concerns the formation of exciplexes at organic/organic heterojunctions: In devices comprising a high hole injection barrier between the hole-transport and the electron-transport layer, the dominant formation of exciplexes is identified by a red-shifted shoulder in the electroluminescent spectrum and a low quantum yield. In contrast to that, the quantum efficiency is increased markedly by minimizing this hole barrier. In such devices, the formation of exciplexes is beneficial for device performance, since it facilitates hole transfer across the heterojunction. For the first time, the operational currents could be divided into different radiative and non-radiative recombination channels of exciplexes and excitons respectively. This quantification was further developed to simulate current-versus-voltage behavior and quantum efficiency. The work on pentacene based devices was initiated by fabricating ultrathin pentacene films on polished sapphire and by characterizing them with a temperature variable four probe setup. Oxygen plasma treatment of the substrate introduces oxygen radicals onto the surface and subsequent pentacene deposition leads to electron extraction from the pentacene molecules to defects at the oxide surface. Such an increase of charge carrier density within the semiconducting layer enhances its conductivity. Using a thick, aliphatic self-assembled monolayer (SAM) as an intermediate layer, both the activation energy and the film resistance increase by at least an order of magnitude compared to devices without SAM. Hence, charge transfer to the substrate is inhibited. The OFETs fabricated for this thesis work are all based on pentacene as a semiconductor and the gate interface was the focus of interest. Two different device types were fabricated: One device category comprises ultra-thin film OFETs (10nm). In such devices contact resistance is negligible. This gives rise to a dependence of device behavior on the gate interface. For this reason, the dielectric constant of the gate insulator was varied by changing the gate materials: the hole mobility is enhanced four-fold by using an aliphatic SAM at the gate interface instead of an oxygen plasma treated alumina (Al2O3). This effect has been interpreted in terms of self-trapping at the interface and its reduction by a low-k dielectric gate. In addition the on-off ratio was enhanced by a factor of 20, thanks to the SAM-inhibited pentacene doping due to a reduced semiconductor/substrate interaction. The second OFETdevice category was based on thicker (80nm) pentacene films. Their behavior is injection controlled. This finding originates in the application of a top contact configuration which implies that the carriers cross the granular material to reach the channel. Impedance spectroscopic measurements on different metal-insulator-semiconductor diodes unveiled a morphology dependent transveral mobility, which is linked to the degree of structural disorder in the film. In conclusion: Understanding of the various physical processes occurring at the different interfaces (anode and organic/organic interfaces in OLEDs, gate interface in OFETs) has considerably improved the performance of organic semiconductor based devices.

EPFL2006

Scanning Tunneling Luminescence of Pentacene Nanocrystals

Alexander Kabakchiev

Organic semiconductors are promising materials for future electronic and electroluminescence applications. A detailed understanding of organic layers and nano-sized crystals down to single molecules can address fundamental questions of contacting organic semiconductors at the nanometer limit and obtaining luminescence from them. In this thesis, electroluminescence spectra from pentacene, a policyclic hydrocarbon (acene), are discussed. The luminescence is induced by the current from the tip of a scanning tunneling microscope (STM). Pentacene is an organic semiconductor which gained a lot of attention in technology because of its electronic properties suitable for applications in thin film devices. Moreover, recent fundamental studies employed pentacene as a standard to demonstrate sub-molecular resolution by scanning probe techniques. This work reports the first observation of light emission from nanometer-sized pentacene crystals grown on an ultrathin insulating layer on noble metal surfaces. Different STM-techniques are combined to characterize the individual systems studied with respect to topography, crystal structure, electronic properties and work function changes. The initial step of the project was the implementation of an optical system for light collection from the tunnel-junction of a low-temperature STM. In the set-up, a novel approach based on the use of in situ adjustable lenses has been realized. Three lenses placed in the vicinity of the tip-apex allow light collection into three independent channels which can be used for versatile optical analysis. An important part in the characterization of the organic system was to clarify the interaction between adsorbed molecules and substrates. We investigated individual pentacene molecules on different ultrathin insulator-metal systems. With the STM tip in tunnel contact, the molecules are situated in a double barrier junction formed by the insulating layer on one side and the vacuum gap on the other side. The metal surface and the STM-tip form the electrodes. The electronic properties of pentacene in this configuration have been characterized. Insulator-metal-systems, which provide a good electronic decoupling for pentacene from the metal, have been chosen as substrates for the growth of pentacene nanocrystals. Using the sub-molecular resolution of the STM we resolved the structure of the top-layer of the pentacene nanocrystals and found that the crystal phase agrees with the pentacene bulk structure. Moreover, the comparison of charge injection barriers between individual molecules and nanocrystals of pentacene indicates a significant change of electronic properties after the formation of ordered structure from the individual building blocks. Optical spectroscopy of the emitted light reveals an excitonic emission from the nanocrystals, which is in very good agreement with photoemission spectra of macroscopic crystals. Although a highly localized current-injection by the STM tip is used for excitation, it can be concluded that the source of light emission is delocalized. In contrast to luminescence measurements reported for other organic materials in STM, the excitation is not localized on the individual molecule, into which a charge is injected by the STM-tip. Our study indicates the importance of inter-molecular coupling leading to a high mobility of the excited state which has to be considered in the framework of STM-induced luminescence. A further result is the observation of the Stark shift of pentacene luminescence, that has so far not been measured at large electric fields of the order of 1V/nm. The evaluation of the Stark shift provides information on dipole and polarizability change during the decay of an exciton. The measured large dipole change of the lowest singlet exciton in pentacene indicates a mixing of the Frenkel exciton (FE) with a small contribution of charge-transfer (CT). Finally, we developed a set-up for photon correlation measurements in the STM. The properties of photon statistics is a means which can unambiguously prove that the luminescence is due to a single-photon source, i.e. a light source which emits no more than one photon at a time. A typical example of such a source is the emission from a single molecule. We performed photon correlations measurements of the luminescence from pentacene and C60 nanocrystals. While the excitonic bulk emission of pentacene is not expected to represent a single photon emitter, we also did not observe anticorrelations in the case of C60 indicating the importance of the local environment in the STM-junction, e.g. the short distance to the metal electrodes.

EPFL2010