a b s t r a c tThe mobility is an important parameter for organic solar cell materials as it influences the charge extraction and recombination dynamics. In this study, the time of flight technique is used to investigate the charge mobility of the important organic photovoltaic materials PC 71 BM, PTB7 and their blend. The electron mobility of PC 71 BM is in the region of 1 Â 10 À3 cm 2 /Vs for the neat fullerene film, and has a positive electric field dependence. At room temperature the hole mobility of PTB7 is 1 Â 10 À3 cm 2 /Vs for the neat film and 2 Â 10 À4 cm 2 /Vs for their blend. The hole mobility of the blend reduces by a factor of a thousand when the sample is cooled from room temperature to 77 K. This finding is compared with the device performance of efficient PTB7:PC 71 BM solar cells for varying temperature. At 77 K the solar cell efficiency halved, due to losses in fill factor and short circuit current. Bimolecular and trap-assisted recombination increase at low mobility (low temperature) conditions, whereas at high mobility conditions the open circuit voltage reduces. The power conversion efficiency as a function of temperature has a maximum between 260 K and 295 K, revealing an optimized mobility at room temperature.

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Carbon quantum dots (CQDs) derived from biomass, a suggested green approach for nanomaterial synthesis, often possess poor optical properties and have low photoluminescence quantum yield (PLQY). This study employed an environmentally friendly, cost-effective, continuous hydrothermal flow synthesis (CHFS) process to synthesise efficient nitrogen-doped carbon quantum dots (N-CQDs) from biomass precursors (glucose in the presence of ammonia). The concentrations of ammonia, as nitrogen dopant precursor, were varied to optimise the optical properties of CQDs. Optimised N-CQDs showed significant enhancement in fluorescence emission properties with a PLQY of 9.6% compared to pure glucose derived-CQDs (g-CQDs) without nitrogen doping which have PLQY of less than 1%. With stability over a pH range of pH 2 to pH 11, the N-CQDs showed excellent sensitivity as a nano-sensor for the highly toxic highly-pollutant chromium (VI), where efficient photoluminescence (PL) quenching was observed. The optimised nitrogen-doping process demonstrated effective and efficient tuning of the overall electronic structure of the N-CQDs resulting in enhanced optical properties and performance as a nano-sensor.

X marks the spot! A fascinating packing motif is observed in a novel cruciform oligothiophene containing a Ge (violet; see picture) spiro center. Long‐range interchain interactions between molecules and strong electronic coherence between orthogonal chains within the molecules are present. Such ordering is advantageous for bulk charge transport, as demonstrated by high short‐circuit currents in bulk heterojunction organic solar cells.