Synthesis of NIR-Absorbing Squaraine Dyes and their Use in Organic Upconversion Devices

Squaraines are organic dyes showing the advantages of a simple synthesis, high absorption coefficients and tunable bandgaps from the visible to the near-infrared (NIR) region. Their molecular structure is based on a polymethine chain and a donor-acceptor-donor structure that can be easily matched to the requirements of new application fields. This thesis focuses on the synthesis of squaraine dyes absorbing beyond 1000 nm and their use in organic upconversion devices. Organic dyes are widely studied and applied in optoelectronic devices such as organic solar cells, organic photodetectors (OPDs) and organic light-emitting devices (OLEDs). However, investigations so far were mainly focused on dyes absorbing and emitting in the visible range. With increasing interest in NIR-technology for biomedical and optoelectronic applications, there is a growing demand for organic NIR dyes. Here, a systematic chemical synthesis approach is used to tune the properties of benz[cd]indole substituted squaraine dyes. The donor part of the dye is modified by introducing Ï-conjugated substituents that increase the donor strength and enlarge the Ï-system. Additionally, a stronger acceptor part is introduced. A series of eight dyes is synthesized and characterized. The absorption maxima show bathochromic shifts with increasing conjugation, increasing donor strength and increasing acceptor strength. The beneficial properties of the NIR-absorbing squaraine dyes are demonstrated in organic upconversion devices with sensitivity beyond 1000 nm. The device is composed of a NIR-absorbing OPD possessing an external quantum efficiency of around 80% in the reverse bias and a visible OLED. The upconversion device converts NIR light into visible light without the need of intermediate electronics, allowing for direct NIR imaging. The organic semiconducting layers of the device stack absorb very little in the visible range. Combined with an optimized semitransparent metal top electrode, a device with an average visible transmittance of 65% is achieved. The advantages of visible transparent upconversion devices can be exploited e.g. in window-integrated electronic circuits. Another major benefit of organic electronics is the possibility to fabricate devices from solution, allowing low-cost and large-area production on flexible substrates. Here, the NIR-absorbing squaraine dye is used in upconversion devices consisting of five solution-processed layers. As emitting part, a fluorescent copolymer (super yellow)-based OLED or light-emitting electrochemical cell (LEC) is investigated. The solution-processed upconversion device converts NIR light with a wavelength of 980 nm efficiently to yellow light with a wavelength of around 575 nm, exhibiting a low turn-on voltage of 2.7 V. Replacing the OLED by a LEC allows for eliminating the sensitive calcium layer that acts as electron-injection layer. Due to the presence of ions, the LEC-based device shows a dynamic behavior, which is characterized in detail. A great deal of progress has been made with upconversion devices over the last years, but a number of challenges must be resolved to finally transform present-day prototype NIR imagers into a technology. Here, I address some of the scientific opportunities and challenges, including upconverters with sensitivity beyond 1000 nm, and visibly transparent and solution-processed devices with a narrow spectral response in the NIR.

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

Organic thin-film transistors

Frédéric Zanella

Organic molecules (i.e. carbon-based) have opened a new and rapidly-growing industrial field in the optoelectronic market bringing to this field a new dimension of thinness and flexibility. In this context, this thesis has focused on one particular building block of the vast and emerging field of organic electronics: the organic thin-film transistor (OTFT) which uses organic compounds as semiconductor. Whereas the OTFT-based circuits are not meant to compete with the silicon-based high-end industry (micro-processors...), their performance have already reached levels enabling their use in potential applications such as displays (e-paper, LCD, OLED) or radiofrequency identification (RFID) tags. The continuously growing number of available organic molecules exhibiting conductive, semi-conductive or insulating properties combined with the number of available deposition/patterning methods (e.g. gravure printing) gives more flexibility to the technology. These additional degrees of freedom raise two main questions: How to identify the most suitable OTFT platform for a given application and how to estimate its potential, as for instance in, of digital circuits? This thesis targets to answer to those questions. For this purpose, several OTFT platforms have been screened and their performance have been discussed and compared through standard figures of merit. The self-aligned nano-imprinted technology has demonstrated state-of-the-art sub-micrometer OTFTs on 4-inch flexible substrates. This made this platform the most suitable candidate for developing the potential evaluation framework. For that purpose, a static model suitable for the sub-micrometer OTFTs has been developed which embeds almost all known electrical aspects of OTFTs. Then the device-to-device discrepancy often observed in OTFTs has been studied and statistical modeling methods introduced. This allowed the simulation of sub-micrometer inverters performed with commercially available tools. Next, a statistical method has been developed to evaluate the potential of the sub-micrometer OTFTs for digital applications. Whereas the method concludes that these sub-micrometer OTFTs are not mature enough to make complex digital circuits, this methodology is technology-independent and may thus serve as a basis to characterize unipolar-logic printed electronics and be further extended to complementary-logic circuits. Last but not least, an automation effort has been undergone all along this thesis in order to increase the throughput for such demanding data analysis. The main outcome of this task is a user-friendly multi-analysis and parameter extraction platform.

EPFL2014

Versatile short-wave and mid-infrared sources based on wideband parametric conversion

Adrien Billat

The mid-infrared part of the optical spectrum is of high interest in a wide range of applications such as environmental gas monitoring, contaminant detection in the chemical, food or pharmaceutical industry, medical diagnosis, or defense and security. Relevant molecules can readily be identified through their mid-infrared absorption spectra, as the latter contains the fundamental resonances of a number of pollutant and toxic gases. Consequently, spectroscopic apparatus, light detection and ranging systems or free-space communication links all benefit from the progress accomplished by mid-infrared technologies over the last years. However some shortcomings in the light emitters capabilities are still to be addressed. In this research work, we aim at designing a mid-infrared laser as versatile as possible and based on nonlinear wavelength conversion. The conversion relies on third-order parametric effects in waveguides such as optical fibers made of various types of glass, or integrated semiconductor chips. The objective is to leverage mature communication-band components to generate and shape the seed optical signals, then mixed in the abovementioned waveguides to down-convert them towards midinfrared. The wavelength conversion is performed in two stages, and the first stage consists of a parametric source emitting in the short-wave infrared range. This thesis mostly focuses on the design and realization of this stage. As such, it is closely linked to the field of nonlinear fiber optics, where the use of silica is preponderant. We build on the research performed over the last years on parametric amplifiers, initially used for the re-amplification of communication signals, and we combine it with technologies dedicated to short-wave infrared fiber lasers. As such, we are able to build wavelength tunable and modulation-capable short-wave infrared sources, significantly more powerful and versatile than previous broadband parametric converter designs. The end of the dissertation is then dedicated to the solutions that are then envisioned to realize the second conversion stage, towards mid-infrared. Very promising numerical and experimental results indicate a successful outcome to the project, confirming the validity of the laser concept.

EPFL2017

Versatile short-wave and mid-infrared sources based on wideband parametric conversion

Adrien Billat

EPFL2017

Dynamics of Electronic and Ionic Charges in Cyanine Organic Semiconductor Devices

Sandra Christina Jenatsch

EPFL2017

Organic thin-film transistors

Frédéric Zanella

EPFL2014