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Engineering the dielectric constant (ε) to lower the exciton binding energy of the light‐absorbing semiconductor can improve organic photovoltaic (OPV) device performance. Here, a series of materials are reported with 2‐(3‐oxo‐2,3‐dihydro‐1H‐inden‐1‐ylidene)malononitrile (INCN) acceptor end groups and a central glycolated bis(4H‐cyclopenta[2,1‐b:3,4‐b′]dithiophene) unit with large low‐frequency (εlf = 7.4–7.9 at 0.1–0.2 MHz) and optical‐frequency (εopt up to 6.6 at 2 × 1014 Hz) dielectric constants. The INCN end groups differed in whether they were protonated, chlorinated, or fluorinated, with the latter having the highest εopt. An εopt of 6.6 is predicted to lead to a low exciton binding energy of ≈0.04 eV. Time‐resolved microwave conductivity measurements showed a temperature‐dependent yield–mobility product, with it increasing linearly from 340 K. The onset temperature was near that required to overcome the calculated exciton binding energy and indicates increased free charge generation in a homojunction film. Room temperature transient absorption spectroscopy revealed that photoexcitation rapidly converted to a lower energy state that was consistent with the formation of polarons or a charge transfer state. This work provides experimental evidence of the importance of εopt for the generation of free charges, and a strategy for development of efficient single chromophore homojunction OPV devices. [ABSTRACT FROM AUTHOR]

A series of fluorophores with single-exponential fluorescence decays in liquid solution at 20 degrees C were measured independently by nine laboratories using single-photon timing and multifrequency phase and modulation fluorometry instruments with lasers as excitation source. The dyes that can serve as fluorescence lifetime standards for time-domain and frequency-domain measurements are all commercially available, are photostable under the conditions of the measurements, and are soluble in solvents of spectroscopic quality (methanol, cyclohexane, water). These lifetime standards are anthracene, 9-cyanoanthracene, 9,10-diphenylanthracene, N-methylcarbazole, coumarin 153, erythrosin B, N-acetyl-l-tryptophanamide, 1,4-bis(5-phenyloxazol-2-yl)benzene, 2,5-diphenyloxazole, rhodamine B, rubrene, N-(3-sulfopropyl)acridinium, and 1,4-diphenylbenzene. At 20 degrees C, the fluorescence lifetimes vary from 89 ps to 31.2 ns, depending on fluorescent dye and solvent, which is a useful range for modern pico- and nanosecond time-domain or mega- to gigahertz frequency-domain instrumentation. The decay times are independent of the excitation and emission wavelengths. Dependent on the structure of the dye and the solvent, the excitation wavelengths used range from 284 to 575 nm, the emission from 330 to 630 nm. These lifetime standards may be used to either calibrate or test the resolution of time- and frequency-domain instrumentation or as reference compounds to eliminate the color effect in photomultiplier tubes. Statistical analyses by means of two-sample charts indicate that there is no laboratory bias in the lifetime determinations. Moreover, statistical tests show that there is an excellent correlation between the lifetimes estimated by the time-domain and frequency-domain fluorometries. Comprehensive tables compiling the results for 20 (fluorescence lifetime standard/solvent) combinations are given.

Many molecular semiconductors show a pronounced polymorphism; i.e. they can adopt different crystal arrangements depending, e.g., on temperature, pressure, and selected solidification pathways. This renders reliable fabrication of molecular semiconductor devices challenging, as minute changes in processing can lead to numerous structures and, hence, optoelectronic responses. Here, we demonstrate using the example of p-DTS(FBTTh2)2 that spatial confinement at the nanoscale can be exploited to detect specific polymorphs and the conditions under they form. A new polymorph exhibiting a higher charge-carrier mobility compared to previously reported p-DTS(FBTTh2)2 crystal forms is found at elevated temperatures and high degree of confinement, illustrating the benefit of our approach and promising that spatial confinement will find wide-spread application to understand and control polymorph formation in organic semiconductors. [ABSTRACT FROM AUTHOR]

The possibility of hole transfer between a photoexcited semiconductor quantum dot and a pi-conjugated polymer is discussed. The result suggests that the polymer and the quantum dot form a relatively stable complex.

We analyze the random process of fluorescence reabsorption and trapping in solid-state optical materials in general and its influence on the efficiency of optical cooling of solids by anti-Stokes fluorescence in particular. Using the absorption and fluorescence spectra of Y[b.sup.3+]:Zr[F.sub.4]-Ba[F.sub.2]-La[F.sub.3]-Al[F.sub.3]-NaF (ZBLAN) as input data, we employ a random-walk model to test analytical approximations of the fluorescence escape efficiency and cooling efficiency, including reflections at the boundary. OCIS codes: 160.4670, 350.5340, 350.5500.

Photoenhancement effect in colloidal CdSe and (CdSe)ZnS core-shell quantum dots attached to a variety of surfactant molecules in aqueous and nonaqueous environment is observed. Results show that the effect occurs in dots suspended in a variety of organic or aqueous situations.

Femtosecond transient absorption spectroscopy and time-integrated photoluminescence (PL) measurements are used to study charge carrier relaxation in colloidal InP quantum dots (QDS). The results suggest that with increasing excitation energy, there arise relaxation pathways involving surface or external energy states exhibiting reduced radiative efficiency.

Research has been conducted on secoporphyrazines. The results of the photophysical evaluation and comparison of these compounds and their precursors are presented.

A comparison of the maximum absorbed intensity for a sample inside and outside a laser cavity was reported. This analysis was based on a comparison of absorbed intensities of radiation.