Investigation of the factors affecting the power conversion efficiency of all-solution-processed 'bilayer' P3HT:PCBM solar cells

Organic solar cells containing poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) that are fabricated by a layer-by-layer solution-cast method can achieve comparable efficiency as that using conventional bulk heterojunction (BHJ) method. In this work, we found that the two methods have different processing requirements for achieving high efficiency. In contrast to the BHJ method, the layer-by-layer method showed a better efficiency if the P3HT layer was formed by rapid evaporation from a solution. The structural requirements of the P3HT layer for efficient device performances were studied. We found that poorly organized aggregation in the P3HT layer is likely to promote diffusion of the PCBM into the polymer to form the heterojunction. A poorly organized aggregation of the P3HT layer with enhanced density of nanocrystals can be achieved by accelerated drying of the P3HT film by heating. Using a P3HT film fabricated by this method, the power conversion efficiency can be improved by about 15% over that using a P3HT film formed by drying in a normal rate.