The polymer-fullerene interpenetrating network: one route to a solar cell approach

We analyse recent developments of thin film plastic solar cells, using polymer-fullerene bulk heterojunctions as an absorber, as well as peculiarities of the device behaviour at various temperature and light illumination intensities. To establish the potential of this type of cells in full scale and to identify the loss factors, we studied the electronic transport properties of the conjugated polymer/fullerene based solar cells by means of temperature and illumination intensity-dependent current voltage characteristics. A positive temperature coefficient of the short-circuit current at all light illumination intensities applied, i.e., from 0.1 to 100 mW/cm(2) (white light), has been established. An increase of the open-circuit voltage from 850 to 940 mV was observed, when cooling down the device from room temperature to 100 K. The incerase of the V-OC at low temperatures is in contrast to polymer absorbtion red-shift (due to lowering of HOMO-LUMO gap) as well as to the red-shift of external quantum yield spectrum at low temperatures. The fill factor depends strongly on temperature with a positive temperature coefficient in the same temperature range. In contrast, the light intensity dependence of the fill factor shows a maximum of 52% at intermediate illumination intensities (3 mW/cm(2)) and decreases subsequently, with increasing intensity. These observations, clearly formulate the requirements to utilise high-mobility polymers with reduced defect density as the absorber. The spectral matching of the active layer absorption to the solar spectrum needs to be improved, being essential for potential applications of these novel photo-voltaic devices. (C) 2002 Elsevier Science B.V. All rights reserved.