Although most research in the field of organic bulk heterojunction solar cells has focused on combinations of a p-type conducting polymer as a donor and a fullerene-based acceptor, recent work has demonstrated the viability of solution-processed heterojunctions composed entirely of molecular solids. Molecular solids offer potential advantages over conjugated polymer systems in terms of easier purification, amenability to mass-scale production and better batch-to-batch reproducibility. This article reviews the major classes of molecular donors that have been reported in the literature in the past several years and highlights some of key considerations in molecular heterojunction design compared to polymer-based bulk heterojunctions.

Objectives We sought to relate imaging findings on PET to adverse cardiac events in patients referred for evaluation of known or suspected cardiac sarcoidosis (CS). Background Although cardiac positron emission tomography (PET) is commonly used to evaluate patients with suspected CS, the relationship between PET findings and clinical outcomes has not been reported. Methods We studied 118 consecutive patients with no history of CAD who were referred for PET using 18F-fluorodeoxyglucose (FDG) [to assess for inflammation] and 82Rubidium [to evaluate for perfusion defects (PD)] following a high fat / low carbohydrate diet to suppress normal myocardial glucose uptake. Blind reads of the PET data categorized cardiac findings as (a) normal; (b) positive PD or FDG; (c) positive PD and FDG. Images were also used to identify if findings for extra cardiac sarcoidosis were present. Adverse events (AE) -- death or sustained ventricular tachycardia (VT) -- were ascertained by electronic medical records, defibrillator interrogation, patient questionnaires and phone interviews. Results Among the 118 patients (age 52±11; males 57%, mean ejection fraction 47%±16%), 47 (40%) had normal and 71 (60%) abnormal cardiac PET findings. Over a median follow-up of 1.5 years, there were 31 (26%) adverse events (27 VT and 8 deaths). Cardiac PET findings were predictive of AE with the presence of both a PD and abnormal FDG (29% of patients) being associated with hazard ratio of 3.9 (p<0.01) and remaining significant after adjusting for left ventricular ejection fraction (LVEF) and clinical criteria. Extra-cardiac FDG uptake (26% of patients) was not associated with AE. Conclusions The presence of focal PD and FDG uptake on cardiac PET identifies patients at higher risk of death or VT. These findings offer prognostic value beyond Japanese clinical criteria, the presence of extra cardiac sarcoidosis and LVEF.

Charge transport and nongeminate recombination are investigated in two solution-processed small molecule bulk heterojunction solar cells consisting of diketopyrrolopyrrole (DPP)-based donor molecules, mono-DPP and bis-DPP, blended with [6,6]-phenyl-C71-butyric acid methyl ester (PCBM). While the bis-DPP system exhibits a high fi ll factor (62%) the mono-DPP system suffers from pronounced voltage dependent losses, which limit both the fi ll factor (46%) and short circuit current. A method to determine the average charge carrier density, recombination current, and effective carrier lifetime in operating solar cells as a function of applied bias is demonstrated. These results and light intensity measurements of the current-voltage characteristics indicate that the mono-DPP system is severely limited by nongeminate recombination losses. Further analysis reveals that the most signifi cant factor leading to the difference in fi ll factor is the comparatively poor hole transport properties in the mono-DPP system (2 × 10 − 5 cm 2 V − 1 s − 1 versus 34 × 10 − 5 cm 2 V − 1 s − 1 ). These results suggest that future design of donor molecules for organic photovoltaics should aim to increase charge carrier mobility thereby enabling faster sweep out of charge carriers before they are lost to nongeminate recombination.

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