Influence of P3HT:PCBM blend preparation on the active layer morphology and cell degradation
Introduction
Much work has been done in polymeric solar cells during the last years, trying to increase their efficiency, while maintaining low fabrication cost. Many factors have been reported to affect solar cells parameters, among which the active layer morphology, related to blend preparation and annealing, is very important [1], [2], [3], [4]. From another point of view, modeling of solar cells has also attracted much attention trying to predict the device behavior, as well as variations of its electrical parameters with device processing. Equivalent circuits, EC, that are generally used to describe solar cells can consider 1, 2 or more junctions, plus series and shunt resistances, where elements can be associated to physical mechanisms [5], [6].
Several polymers, methanofullerenes and their blends have been studied as the active layer of polymeric solar cells [7], [8], [9], [10], [11]. Among them, blends of poly(3-hexylthiophene), P3HT, and [6,6]-phenyl-C61-butyric acid methyl ester, PCBM, are providing among highest reported efficiencies, in the order of 5%. Cell parameters are very sensitive to small variations in blend preparation, as for example, ratio of P3HT to PCBM, solvent, temperature during preparation, film thickness and annealing [4]. For these reasons, characterizing the active layer morphology is of much importance for device improvement, in order to determine how it contributes to better charge dissociation and recollection.
In this work we analyze the behavior of solar cells based on P3HT:PCBM blends prepared under different conditions. Basic parameters are extracted from measured characteristics in dark and under illumination. Modeling is used to understand mechanisms involved in the device behavior and its degradation, after been left in ambient conditions for several days. Cells are compared regarding the mechanisms involved in the dissociation of charges, as well as regarding their series resistance, shunt resistance, open circuit voltage Voc and short circuit current Isc, as prepared and after several days in ambient conditions.
Section snippets
Experimental details
Solar cells were prepared by spin-coating a poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), PEDOT:PSS, layer on top of an Indium Tin Oxide, ITO, substrate, over which the active layer of a P3HT:PCBM blend was spin-coated, followed by annealing and thermal evaporation of Aluminum. The active layer blend was prepared in 2 ways:
Blend 1: 4.8 mg of PCBM were diluted in 0.6 ml of chlorobenzene, stirring during 2 h at room temperature, after which, 6 mg of P3HT were added to prepare a (1:0.8) w%
Results and discussion
Fig. 1 shows the electrical equivalent circuit used to describe dark and illuminated I–V characteristics, where D2 represents the P3HT:PCBM heterojunction, D1 represents a Schottky diode associated to regions of P3HT in contact with Aluminum, where a rectifying contact is formed and charge dissociation can also occur. Rs1 and Rs2 are series resistance associated to diode D1 and D2 respectively and Rsh2 is the overall shunt resistance. The element SCLC incorporates the effect of space charge
Conclusion
Combining measured I–V characteristics with modeling under dark and illuminated conditions, it is possible to relate variations in device behavior with the morphology of the active layer, as well as possible causes of degradation. It was observed that relatively small variations in the active layer preparation procedure can produce significant variations in the cell series resistance. For cells from both blends, the ideality factor of I–V characteristics under dark and illuminated for V > 0.5 V,
Acknowledgements
This work was supported by ‘‘HOPE” Project (CONSOLIDER Ingenio 2010, Ref:CSD2007-00007), AECID-A/024560/09, The Spanish Ministry of Science and Innovation (MICINN) under grant no. TEC2009-09551, CONACYT Project 56461 and PROMEP/103-5/09/1265 in Mexico.
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