Utilizing intermixing of conjugated polymer and fullerene from sequential solution processing for efficient polymer solar cells

Efficient polymer solar cells based on poly[2, 6-(4, 4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-fluorobenzothiadiazole)] (PCPDTFBT) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) are successfully fabricated by a sequential processing (SqP). With appropriate orthogonal solvent and thermal treatment, the SqP film can form an inter-diffused layer, and the SqP devices show efficient photovoltaic performance in both conventional and inverted layouts. The SqP inverted device was firstly demonstrated and the highest power conversion efficiency (PCE) of 5.84% with the enhanced J_sc of 16.4 mA cm^-2 was able to be achieved with the high internal quantum efficiency (IQE). Photoluminescence quenching shows the SqP films can provide efficient exciton quenching. X-ray photoemission spectroscopy (XPS) and ellipsometry analysis shows a polymer-rich surface in SqP films after thermal annealing. The charge mobilities in the SqP films were significantly enhanced as measured by space-charge-limited-current (SCLC) method. All these contribute to the improved photovoltaic performance in the inverted SqP device. We believe that these results inspire a new way of forming the active layer with controllable morphology, efficient charge separation and collection in polymer solar cells.