Karen Verena Strassel
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.EPFL