Ultra-precise photothermal measurements reveal near unity photoluminescence quantum yields of molecular emitters in solution.

Molecules with a photoluminescence quantum yield (PLQY) approaching unity enable new applications such as efficient luminescent solar concentrators and spectral redistributors. Moreover, they have the potential for thermally assisted photon upconversion and optical refrigeration, for which the slightest amount of non-radiative loss is detrimental. However, when the PLQY is within a few percent of 100%, it cannot be precisely determined using standard techniques. Here, we combine spectroscopic measurements with photothermal techniques to determine the photothermal threshold energy, i.e. the minimum photon energy at which the chromophores produce heat upon excitation. The PLQY is directly related to this energy and is determined for six fluorescent molecules in low concentration solutions with an unprecedented precision down to ±0.003 within 95% confidence intervals. Independent measurements based on photothermal-deflection spectroscopy and thermal lensing spectroscopy generally provide values within the margin of error, demonstrating the reliability of this measurement concept. Solutions of perylene red in carbon tetrachloride are found to have the highest PLQY of the measured series, being 0.994 ± 0.003. In addition, we observe phonon-assisted, optical upconversion when exciting perylene red within its optical gap at photon energies below its photothermal threshold. Similar measurements on perylene orange in chloroform reveal the presence of low energy sub-gap impurities, preventing upconversion when exciting at the photothermal threshold.

Electron-donating amine-interlayer induced n-type doping of polymer:nonfullerene blends for efficient narrowband near-infrared photo-detection.

Inherently narrowband near-infrared organic photodetectors are highly desired for many applications, including biological imaging and surveillance. However, they suffer from a low photon-to-charge conversion efficiencies and utilize spectral narrowing techniques which strongly rely on the used material or on a nano-photonic device architecture. Here, we demonstrate a general and facile approach towards wavelength-selective near-infrared phtotodetection through intentionally n-doping 500-600 nm-thick nonfullerene blends. We show that an electron-donating amine-interlayer can induce n-doping, resulting in a localized electric field near the anode and selective collection of photo-generated carriers in this region. As only weakly absorbed photons reach this region, the devices have a narrowband response at wavelengths close to the absorption onset of the blends with a high spectral rejection ratio. These spectrally selective photodetectors exhibit zero-bias external quantum efficiencies of ~20-30% at wavelengths of 900-1100 nm, with a full-width-at-half-maximum of ≤50 nm, as well as detectivities of >1012 Jones.

The Cost of Converting Excitons into Free Charge Carriers in Organic Solar Cells.

Efficient exciton dissociation and subsequent generation of free charge carriers at the organic donor-acceptor interface requires a number of electron-transfer processes. It is a common view that these steps result in an unavoidable energy loss in organic photovoltaic devices that is not present in other types of solar cells. The currently best performing organic solar cells with power conversion efficiencies over 16% challenge this view, and no interfacial charge-transfer states with energy significantly lower than the strongly absorbing singlet states are detected within the gap of the used donor and acceptor materials. This Perspective will discuss implications, the remaining sources of energy loss, and open questions to be solved to achieve power conversion efficiencies over 20%.