The widespread use of solar cells as a renewable source of energy is seriously held back by the high cost of the existing crystalline silicon-based technology. The prospect of cheap reel-to-reel processing makes organic semiconducting mate-

Organic photodetectors (OPDs) have gained increasing interest as they offer cost-effective fabrication methods using low temperature processes, making them particularly attractive for large area image detectors on lightweight flexible plastic substrates. Moreover, their photophysical and optoelectronic properties can be tuned both at a material and device level. Visible-light OPDs are proposed for use in indirect-conversion X-ray detectors, fingerprint scanners, and intelligent surfaces for gesture recognition. Near-infrared OPDs find applications in biomedical imaging and optical communications. For most applications, minimizing the OPD dark current density (J d ) is crucial to improve important figures of merits such as the signal-to-noise ratio, the linear dynamic range, and the specific detectivity (D*). Here, a quantitative analysis of the intrinsic dark current processes shows that charge injection from the electrodes is the dominant contribution to J d in OPDs. J d reduction is typically addressed by fine-tuning the active layer energetics and stratification or by using charge blocking layers. Yet, most experimental J d values are higher than the calculated intrinsic limit. Possible reasons for this deviation are discussed, including extrinsic defects in the photoactive layer and the presence of trap states. This provides the reader with guidelines to improve the OPD performances in view of imaging applications.

The morphology of composite thin films consisting of a conjugated polymer (poly[2-methoxy-5-(3‘,7‘-dimethyloctyloxy)-1,4-phenylenevinylene], MDMO-PPV) and methanofullerene ([6,6]-phenyl C61 butyric acid methyl ester, PCBM), which are used as the active layer in polymer photovoltaic devices, has been extensively studied using transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). Composite MDMO-PPV:PCBM films have been prepared with PCBM concentrations varying from 20 to 90 wt %. PCBM-rich clusters are clearly observed in TEM bright-field mode when the PCBM concentration is increased to ca. 75 wt % in the composite film. The SAED analysis shows that these clusters consist of many PCBM nanocrystals with random crystallographic orientations. Furthermore, we show that these nanocrystals are also present in the homogeneous matrix at PCBM concentrations below 75 wt %. Annealing of the blend films has been performed at temperatures between 60 and 130 °C for different times. In all cases, but especially when the annealing temperature is above the glass transition temperature of MDMO-PPV (∼80 °C), PCBM molecules show high diffusion mobility, resulting in accelerated phase segregation and in the formation of large PCBM single crystals in the film. The observed phase segregation, even at temperatures as low as 60 °C, indicates that the thermal stability of MDMO-PPV:PCBM films will likely limit the long-term performance of solar cells based on these materials.

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