Efficient polymer-fullerene photovoltaic devices require close proximity of the two materials to ensure photoexcited electron transfer from the semiconducting polymer to the fullerene acceptor. We describe studies in which a bilayer system consisting of spin-cast 2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene copolymer (MEH-PPV) and sublimed C60 is heated above the MEH-PPV glass transition temperature in an inert environment, inducing an interdiffusion of the polymer and the fullerene layers. With this process, a controlled, bulk, gradient heterojunction is created bringing the fullerene molecules within the exciton diffusion radius of the MEH-PPV throughout the film to achieve highly efficient charge separation. The interdiffused devices show a dramatic decrease in photoluminescence and concomitant increase in short circuit currents, demonstrating the improved interface.
ABSThAcT A simple diode model with a parallel photocurrent source is used to calculate the effects of detector series resistance on the electro-optical performance of infrared detectors. At low operating frequencies, where diode capacitance can be ignored, these calculations show that a saturation of the measurable short circuit current occurs. The flux levels at which this saturation occurs is dependent primarily on the series resistance and only logarithmically on detector junction impedance. The calculations also show that additional nonsaturated flux levels can be realized in the presence of a fixed series resistance by applying a reverse bias to the detector. At flux levels where low frequency signal saturation is not a problem, small series resistances can lead to premature rolloff of the frequency response. Calculations that consider the effects of detector capacitance and self biasing induced by the series resistance show an RC type time constant and frequency rolloff well before amplifier or intrinsic detector limitations. At higher flux levels, this rolloff is even more severe due to the bias dependent detector capacitance. A reverse bias can decrease the detector capacitance somewhat which can lead to a moderate increase in frequency response.
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