| PCT PRINCIPLE OF OPERATION |
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For the first time, the PCT allows for detailed studies of defects in organic semiconductor devices without suffering from the above named drawbacks and limitations. The principle of photocurrent measurement is universal for organic semiconductors and thus the PCT is not limited to special applications, e.g. OLEDs or OPVs.
 For measuring the photocurrent the organic semiconductor is excited with a laser beam. In the organic material this optical excitation leads to formation of excitons (1). Part of these excitons dissociates (2) and due to the internal field or an external applied electric field the generated electrons and holes are transported to the electrodes (3). There the charge carriers are extracted from the organic material (4), resulting in an electric current that is measured with the PCT.
The fundamental processes (1)..(4) for photocurrent generation also determine the principle of operation of organic light emitting diodes, organic sensors or organic transistors. Thus, the PCT is suited for investigations of all kinds of application of organic semiconductors.
THE MOST IMPORTANT PARAMETERS FOR THE OPRATION OF AN ORGANIC SEMICONDUCTOR DEVICE AS A SOLAR CELL ARE:
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SHORT CIRCUIT CURRENT (Isc, RED CIRCLE):
Photocurrent in the absence of an external field. It is basically determined by the material properties of the organic semiconductor.
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OPEN CIRCUIT VOLTAGE (Uoc, BLUE CIRCLE):
Voltage for which the photocurrent is zero. It is determined by the organic material and the contacts of the device.
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BUILD-IN VOLTAGE (Ubi, GREEN CIRCLE):
Voltage at which the photocurrent-voltage-curves for different illumination intensity intersect. It is basically determined by the device contacts only.
 By combining measurements of these different characteristic parameters it is possible to discriminate between the influence of the organic material and the contacts on the device properties. This constitutes a first important step in the analysis of local performance variations and defects in organic semiconductors. |
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