Bilayer polymer solar cells with improved power conversion efficiency and enhanced spectrum coverage

We demonstrate the construction of an efficient bilayer polymer solar cell comprising of Poly(3-hexylthiophene) (P3HT) as a p-type semiconductor and asymmetric fullerene (C₇₀) as n-type counterparts. The bilayer configuration was very efficient compared to the individual layer perfornance and it behaved like a regular p-n junction device. The photovoltaic characteristic of the bilayers were studied under AM 1.5 solar radiation and the optimized device parameters are the following: Voc=0.5V, Jsc=10.1 mA/cm ², FF=0.60 and power conversion efficiency of 3.6 %. A high fill factor of ∼0.6 was achieved, which is only slightly reduced at very intense illumination. Balanced mobility between p-and n-layers is achieved which is essential for achieving high device performance. Correlation between the crystallinity, morphology and the transport properties of the active layers is established. The External quantum efficiency (EQE) spectral distribution of the bilayer devices with different processing solvents correlates well with the trends of short circuit current densities (J_sc) measured under illumination. Efficiency of the bilayer devices with rough P3HT layer was found to be about 3 times higher than those with a planar P3HT surface. Hence it is desirable to have a larger grains with a rough surface of P3HT layer for providing larger interfacial area for the exciton dissociation.