Shortwave Infrared-Sensitive, Narrowband Organic Upconversion Devices with a Gain

Sensing and imaging of light in the shortwave infrared (SWIR) range is increasingly used in various fields, including bio-imaging, remote sensing, and semiconductor process control. SWIR-sensitive organic photodetectors (OPDs) are promising because organic molecules can be tailor-made and devices can be solution-processed, enabling large-area and low-cost light detection. So far, SWIR-absorbing organic semiconductors are scarce, and there is a growing interest in synthesizing organic dyes that absorb beyond the Si bandgap (1100 nm). Current inorganic image sensors require a readout integrated circuit to process the signal. An organic upconversion device (OUC), composed of a SWIR OPD and an organic light emitting diode (OLED) connected in series, converts SWIR light into visible light without the need for external electronics, enabling direct SWIR imaging.The thesis aims to develop efficient OUCs with sensitivity extending considerably in the SWIR range. As a first step, squaraine dyes with strong donor and acceptor are synthesized, which results in large bathochromic shifts and peak absorptions beyond 1000 nm. OUC optimization allows clear visualization of low-intensity SWIR images, with wavelength sensitivity extending out to 1200 nm. A challenge with OUCs is that a broadband-absorbing, SWIR-sensitive layer can reabsorb emitted visible light, which compromises device efficiency. To resolve this issue, a narrowband (full width at half maximum 130 nm) near-infrared-selective OUC is developed using a J-aggregated cyanine dye. To enhance efficiency, a phosphorescent OLED is integrated into the OUC and compared with a fluorescent OLED counterpart. The transient device response of the fluorescent and the phosphorescent OUC exhibit opposite trends with increasing voltage, with the speed increasing for the phosphorescent OLED and decreasing for the fluorescent OLED. Numerical simulations reveal that this discrepancy is caused by differences in electron mobility of the OLEDs.Another drawback of SWIR OPDs and OUCs is that a blend of donor and acceptor materials is commonly used to enable efficient photo-charge generation and extraction, which introduces a large space for performance optimization and morphological film instability. Thus, photo-charge generation using a single-component dye film would be advantageous for the facile manufacturing and optimization process. It is demonstrated that several neat squaraine dyes can efficiently photo-generate charges (peak EQE = 43% at -10V) in the absence of an acceptor component, with the efficiency increasing with a higher electric field. Numerical simulations confirm that the field dependency is due to better photo-generation of charges, and not due to better charge extraction at a higher field. An OUC based on a single-component OPD and a TADF OLED is demonstrated, characterized by a low turn-on voltage (1.5V) and a high luminance contrast (on-off ratio 16000).The low efficiency of SWIR OPDs is addressed by diluting squaraine dyes in a PCBM matrix, leading to trap-assisted photomultiplication. This approach utilizes the donor material as a charge trap instead of the commonly used acceptor traps, overcoming the challenge of finding suitable electron acceptors for SWIR-sensitive dyes. An EQE of 640% for an OPD and an efficiency of 13.9% for converting SWIR photons to visible photons for an OUC are realized using this concept.