The photophysical properties and solar cell performance of the classical donor–acceptor copolymer PCDTBT(poly(N‐9′‐heptadecanyl‐2,7‐carbazole‐alt ‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole))) in relation to unintentionally formed main chain defects are investigated. Carbazole–carbazole homocouplings (Cbz hc) are found to significant extent in PCDTBT made with a variety of Suzuki polycondensation conditions. Cbz hc vary between 0 and 8 mol% depending on the synthetic protocol used, and are quantified by detailed nuclear magnetic resonance spectroscopy including model compounds, which allows to establish a calibration curve from optical spectroscopy. The results are corroborated by extended time‐dependent density functional theory investigations on the structural, electronic, and optical properties of regularly alternating and homocoupled chains. The photovoltaic properties of PCDTBT:fullerene blend solar cells significantly depend on the Cbz hc content for constant molecular weight, whereby an increasing amount of Cbz hc leads to strongly decreased short circuit currents JSC. With increasing Cbz hc content, JSC decreases more strongly than the intensity of the low energy absorption band, suggesting that small losses in absorption cannot explain the decrease in JSC alone, rather than combined effects of a more localized LUMO level on the TBT unit and lower hole mobilities found in highly defective samples. Homocoupling‐free PCDTBT with optimized molecular weight yields the highest efficiency up to 7.2% without extensive optimization.

Efficient light detection in the near-infrared (NIR) wavelength region is central to emerging applications such as medical imaging and machine vision. An organic upconverter (OUC) consists of a NIR-sensitive organic photodetector (OPD) and an visible organic light-emitting diode (OLED), connected in series. The device converts NIR light directly to visible light, allowing imaging of a NIR scene in the visible. Here, we present an OUC composed of a NIR-selective squaraine dye-based OPD and a fluorescent OLED. The OPD has a peak sensitivity at 980 nm and an internal photon-to-current conversion efficiency of ∼100%. The OUC conversion efficiency (0.27%) of NIR to visible light is close to the expected maximum. The materials of the OUC multilayer stack absorb very little light in the visible wavelength range. In combination with an optimized semitransparent metal top electrode, this enabled the fabrication of transparent OUCs with an average visible transmittance of 65% and a peak transmittance of 80% at 620 nm. Visibly transparent OUCs are interesting for window-integrated electronic circuits or imaging systems that allow for the simultaneous detection of directly transmitted visible and NIR upconverted light.

The possibility for unselective C–H activation of a thiophene-based, donor–acceptor–donor monomer during direct arylation polycondensation is investigated.

Strongly coupled dye molecules are known to produce narrowband absorption in a large spectral range. Here we exploit this feature to achieve organic photodetectors with ultra-narrow full-width at half-maximum response at low bias voltage.

Bauer (2021): Shortwave infrared-absorbing squaraine dyes for all-organic optical upconversion devices, Science and Technology of Advanced Materials,

The growing interest in near-infrared (NIR) imaging is explained by the increasing number of applications in this spectral range, which includes process monitoring and medical imaging. NIR-to-visible optical upconverters made by integrating a NIR photosensitive unit with a visible emitting unit convert incident NIR light to visible light, allowing imaging of a NIR scene directly with the naked eye. Optical upconverters made entirely from organic and hybrid materialswhich include colloidal quantum dots, and metal-halide perovskitesenable low-cost and pixel-free NIR imaging. These devices have emerged as a promising addition to current NIR imagers based on inorganic semiconductor photodiode arrays interconnected with read-out integrated circuitry. Here, we review the recent progress in the field of optical upconverters made from organic and hybrid materials, explain their functionality and characterization, and identify open challenges and opportunities.

Imaging in the near-infrared (NIR) is getting increasingly important for applications such as machine vision or medical imaging. NIR-to-visible optical upconverters consist of a monolithic stack of a NIR photodetector and a visible light-emitting unit. Such devices convert NIR light directly to visible light and allow capturing a NIR image with an ordinary camera. Here, five-layer organic solution-processed upconverters (OUCs) are reported which consist of a squaraine dye NIR photodetector and a fluorescent poly­(para-phenylene vinylene) copolymer (super yellow)-based organic light-emitting diode (OLED) or light-emitting electrochemical cell (LEC), respectively. Both OLED–OUCs and LEC–OUCs convert NIR light at 980 nm to yellow light at around 575 nm with comparable device metrics of performance, such as a turn-on voltage of 2.7–2.9 V and a NIR-to-visible photon conversion efficiency of around 1.6%. Because of the presence of a salt in the emitting layer, the LEC–OUC is a temporally dynamic device. The LEC–OUC turn-on and relaxation behavior is characterized in detail. It is demonstrated that a particular ionic distribution and thereby the LEC–OUC status can be frozen by storing the device in the presence of a small voltage applied. This provides a test chart for quantitative measurements.

and are still cost prohibitive for most consumer and low-end applications. In addition, due to the broadband absorption of inorganic semiconductors, spectrally selective detection is not possible without attached optical filters.As an alternative approach, optical upconversion devices have been developed that directly convert NIR light into visible light. These devices are also denoted as upconversion photodetector, [9] upconversion display/imager, [10,11] or upconversion light-emitting diode. [12] The basic idea of any upconverter is the serial connection of a NIR photodetector with a visible lightemitting component. When NIR light is absorbed in the photodetector, a current is generated and directly converted into a visible image by the emitter element. Important advantages of this design concept are that no intermediate electronics for data processing and no external display for data visualization are required. We note that the functionality of an upconverter is different from the several known photon upconversion processes. Photon upconversion is a process that converts sequentially absorbed photons of low energy into a photon of higher energy.All-organic upconversion devices (OUCs) are composed of an organic NIR photodetector and an organic light-emitting diode (OLED). OUCs can be fabricated entirely from solution over large area using coating and printing processes. This potentially enables new and alternative NIR imaging applications Organic upconversion devices (OUCs) consist of an organic infrared photodetector and an organic visible light-emitting diode (OLED), connected in series. OUCs convert photons from the infrared to the visible and are of use in applications such as process control or imaging. Many applications require a fast OUC response speed, namely the ability to accurately detect in the visible a rapidly changing infrared signal. Here, high image-contrast, narrowband OUCs are reported that convert near-infrared (NIR) light at 980 and 976 nm with a full-width at half maximum of 130 nm into visible light. Transient photocurrent measurements show that the response speed decreases when lowering the NIR light intensity. This is contrary to conventional organic photodetectors that show the opposite speed-versus-light trend. It is further found that the response speed increases (when using a phosphorescent OLED) or decreases (for a fluorescent OLED) when increasing the driving voltage. To understand these surprising results, an analysis by numerical simulation is conducted. Results show that the response speed behavior is primarily determined by the electron mobility in the OLED. It is proposed that the low electron drift velocity in the emitter layer sets a fundamental limit to the response speed of OUCs.